climate change research paper
https://www.jstor.org/stable/44809341
Evaluating severity of leaf spot of lettuce, caused by Allophoma tropica, under a climate change scenario on JSTOR
2022-11-02
Maria Lodovica GULLINO; Giovanna GILARDI; Angelo GARIBALDI
<div><div><div><div><p> journal article</p></div><div><div><p>Phytopathologia Mediterranea</p><div><p>, pp. 235-241 (7 pages)</p></div><p>Published By: Firenze University Press</p></div><div></div></div><div><p>/stable/44809341</p></div><p></p><div><div></div></div></div><div><div><div><p></p></div></div><div><div><p>Climate changes, particularly increases in temperature and CO₂, are seriously challenging agriculture, and are one of the main factors that should be considered in the emergence of new diseases and their potential spread. Six trials were carried out to evaluate the effects of increased temperature and CO₂ on the severity of leaf spot of lettuce, caused by Allophoma tropica (syn. Phoma tropica), a pathogen that was first observed on lettuce in northern Italy in 2011. Temperature, CO₂ and their interactions were significant factors (P&lt;0.0001) influencing incidence and severity of leaf spot on lettuce. Temperatures between 22 and 26°C were the most favourable to the pathogen, and increased disease incidence and severity. Reductions in disease incidence and severity were observed at lower (18-22°C) and higher (26-30°C) temperatures. Concentrations of CO₂ ranging from 800 to 850 ppm increased disease incidence and severity at all the temperature ranges tested, and these effects were greatest at 22-26°C. Analysis of these results could be useful for mid-term agricultural planning at a regional scale, so that crops and their varieties can be adapted to anticipated future climate trends.</p></div><div><p>Phytopathologia Mediterranea is an international journal edited by the Mediterranean Phytopathological Union. The journal’s mission is the promotion of plant health for Mediterranean crops, climate and regions, safe food production, and the transfer of new knowledge on plant diseases and their sustainable management.
The journal deals with all areas of plant pathology, including etiology, epidemiology, disease control, biochemical and physiological aspects, and utilization of molecular technologies. All types of plant pathogens are covered, including fungi, oomycetes, nematodes, protozoa, bacteria, phytoplasmas, viruses, and viroids. The journal also gives a special attention to research on mycotoxins, biological and integrated management of plant diseases, and the use of natural substances in disease and weed control. The journal focuses on pathology of Mediterranean crops grown throughout the world.
 </p></div><div><p>
Firenze University Press publishes the results of academic research, primarily conducted by the University but also by the national scientific community, and certified by the Scientific Committee that steers its choices, disseminating its products within the academic circuits in Italy and abroad. 
This is why FUP invests in digital technologies, so that each publication is initially published in electronic version, which may then be paralleled by a paper version.
 
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 journal article
 Evaluating severity of leaf spot of lettuce, caused by Allophoma tropica, under a climate change scenario

 Maria Lodovica GULLINO, Giovanna GILARDI and Angelo GARIBALDI

 Phytopathologia Mediterranea
 Vol. 56, No. 2 (August, 2017), pp. 235-241 (7 pages)
 Published By: Firenze University Press
 https://www. jstor .org /stable/44809341
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 Abstract

 Climate changes, particularly increases in temperature and CO₂, are seriously challenging agriculture, and are one of the main factors that should be considered in the emergence of new diseases and their potential spread. Six trials were carried out to evaluate the effects of increased temperature and CO₂ on the severity of leaf spot of lettuce, caused by Allophoma tropica (syn. Phoma tropica), a pathogen that was first observed on lettuce in northern Italy in 2011. Temperature, CO₂ and their interactions were significant factors (P<0.0001) influencing incidence and severity of leaf spot on lettuce. Temperatures between 22 and 26°C were the most favourable to the pathogen, and increased disease incidence and severity. Reductions in disease incidence and severity were observed at lower (18-22°C) and higher (26-30°C) temperatures. Concentrations of CO₂ ranging from 800 to 850 ppm increased disease incidence and severity at all the temperature ranges tested, and these effects were greatest at 22-26°C. Analysis of these results could be useful for mid-term agricultural planning at a regional scale, so that crops and their varieties can be adapted to anticipated future climate trends.

 Journal Information

 Phytopathologia Mediterranea is an international journal edited by the Mediterranean Phytopathological Union. The journal’s mission is the promotion of plant health for Mediterranean crops, climate and regions, safe food production, and the transfer of new knowledge on plant diseases and their sustainable management. The journal deals with all areas of plant pathology, including etiology, epidemiology, disease control, biochemical and physiological aspects, and utilization of molecular technologies. All types of plant pathogens are covered, including fungi, oomycetes, nematodes, protozoa, bacteria, phytoplasmas, viruses, and viroids. The journal also gives a special attention to research on mycotoxins, biological and integrated management of plant diseases, and the use of natural substances in disease and weed control. The journal focuses on pathology of Mediterranean crops grown throughout the world.

 Publisher Information

 Firenze University Press publishes the results of academic research, primarily conducted by the University but also by the national scientific community, and certified by the Scientific Committee that steers its choices, disseminating its products within the academic circuits in Italy and abroad. This is why FUP invests in digital technologies, so that each publication is initially published in electronic version, which may then be paralleled by a paper version.

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 Phytopathologia Mediterranea © 2017 Mediterranean Phytopathological Union

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journal article    Phytopathologia Mediterranea  , pp. 235-241 (7 pages)  Published By: Firenze University Press      /stable/44809341                  Climate changes, particularly increases in temperature and CO₂, are seriously challenging agriculture, and are one of the main factors that should be considered in the emergence of new diseases and their potential spread. Six trials were carried out to evaluate the effects of increased temperature and CO₂ on the severity of leaf spot of lettuce, caused by Allophoma tropica (syn. Phoma tropica), a pathogen that was first observed on lettuce in northern Italy in 2011. Temperature, CO₂ and their interactions were significant factors (P&lt;0.0001) influencing incidence and severity of leaf spot on lettuce. Temperatures between 22 and 26°C were the most favourable to the pathogen, and increased disease incidence and severity. Reductions in disease incidence and severity were observed at lower (18-22°C) and higher (26-30°C) temperatures. Concentrations of CO₂ ranging from 800 to 850 ppm increased disease incidence and severity at all the temperature ranges tested, and these effects were greatest at 22-26°C. Analysis of these results could be useful for mid-term agricultural planning at a regional scale, so that crops and their varieties can be adapted to anticipated future climate trends.   Phytopathologia Mediterranea is an international journal edited by the Mediterranean Phytopathological Union. The journal’s mission is the promotion of plant health for Mediterranean crops, climate and regions, safe food production, and the transfer of new knowledge on plant diseases and their sustainable management.
The journal deals with all areas of plant pathology, including etiology, epidemiology, disease control, biochemical and physiological aspects, and utilization of molecular technologies. All types of plant pathogens are covered, including fungi, oomycetes, nematodes, protozoa, bacteria, phytoplasmas, viruses, and viroids. The journal also gives a special attention to research on mycotoxins, biological and integrated management of plant diseases, and the use of natural substances in disease and weed control. The journal focuses on pathology of Mediterranean crops grown throughout the world.
    
Firenze University Press publishes the results of academic research, primarily conducted by the University but also by the national scientific community, and certified by the Scientific Committee that steers its choices, disseminating its products within the academic circuits in Italy and abroad. 
This is why FUP invests in digital technologies, so that each publication is initially published in electronic version, which may then be paralleled by a paper version.
 
     This item is part of a JSTOR Collection.  For terms and use, please refer to our
Evaluating severity of leaf spot of lettuce, caused by Allophoma tropica, under a climate change scenario on JSTOR
https://www.jstor.org/stable/44809341
http://unipub.lib.uni-corvinus.hu/1012/
Cost-benefit analysis of climate change – A methodological overview of recent studies
2012-11-19
Szijártó; Norbert
<div><div><p>
 
(2012)
<em>Cost-benefit analysis of climate change – A methodological overview of recent studies.</em>
 
 In: 
 Price of unsustainability. Economic impacts of climate change.
 
 
 Aula, Budapest, pp. 71-87.
 . ISBN 978-963-339-029-0
 
 
 
</p><br/><div><h2>Abstract</h2><p>Global warming16 has already begun. Climate change has become a self-propelling
and self-reinforcing process as a result of the externality associated with greenhouse-
gas (GHG) emissions. Although it is an externality related to humankind,
according to a number of unique features we should distinguish it from other externalities.
Climate change is a global phenomenon in its causes and consequences. The long-term and persistent impacts of climate change will likely continue over centuries without further anthropogenic mechanism. The preindustrial (equilibrium) level of GHG concentration in the atmosphere cannot be restored since it is irreversible, but if we do not stabilise the actual level of atmospheric concentration, the situation will become much worse than it is now. Assessing the impacts of climate change requires careful considerations because of the pervasive uncertainties and
risks associated with it.</p></div><table><tbody><tr><th>Item Type:</th><td>Book Section</td></tr><tr><th>Uncontrolled Keywords:</th><td>cost-benefit analysis</td></tr><tr><th>Divisions:</th><td>Faculty of Economics &gt; Department of World Economy</td></tr><tr><th>Subjects:</th><td><a href="https://unipub.lib.uni-corvinus.hu/view/subjects/152.html">Ecology</a><br/><a href="https://unipub.lib.uni-corvinus.hu/view/subjects/16.html">Environmental economics</a></td></tr><tr><th>Projects:</th><td>TAMOP 4.2.1/B-09/1/KMR-2010-0005</td></tr><tr><th>ID Code:</th><td>1012</td></tr><tr><th>Deposited By:</th><td>
</td></tr><tr><th>Deposited On:</th><td>19 Nov 2012 15:24</td></tr><tr><th>Last Modified:</th><td>19 Nov 2012 15:24</td></tr></tbody></table><p>Repository Staff Only: <a href="https://unipub.lib.uni-corvinus.hu/cgi/users/home?screen=EPrint::View&amp;eprintid=1012">item control page</a></p></div></div>||||I|||| * Home
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 Cost-benefit analysis of climate change – A methodological overview of recent studies              
               
 Szijártó, Norbert (2012) Cost-benefit analysis of climate change – A methodological overview of recent studies. In: Price of unsustainability. Economic impacts of climate change. Aula, Budapest, pp. 71-87. . ISBN 978-963-339-029-0           
               
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 Abstract               
               
 Global warming16 has already begun. Climate change has become a self-propelling and self-reinforcing process as a result of the externality associated with greenhouse- gas (GHG) emissions. Although it is an externality related to humankind, according to a number of unique features we should distinguish it from other externalities. Climate change is a global phenomenon in its causes and consequences. The long-term and persistent impacts of climate change will likely continue over centuries without further anthropogenic mechanism. The preindustrial (equilibrium) level of GHG concentration in the atmosphere cannot be restored since it is irreversible, but if we do not stabilise the actual level of atmospheric concentration, the situation will become much worse than it is now. Assessing the impacts of climate change requires careful considerations because of the pervasive uncertainties and risks associated with it.
               
 Item Type: Book Section              
 Uncontrolled Keywords: cost-benefit analysis              
 Divisions: Faculty of Economics > Department of World Economy              
 Subjects: Ecology              
 Environmental economics              
 Projects: TAMOP 4.2.1/B-09/1/KMR-2010-0005              
 ID Code: 1012              
 Deposited By: Ádám Hoffmann              
 Deposited On: 19 Nov 2012 15:24              
 Last Modified: 19 Nov 2012 15:24              
               
               
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(2012)
Cost-benefit analysis of climate change – A methodological overview of recent studies.
 
 In: 
 Price of unsustainability. Economic impacts of climate change.
 
 
 Aula, Budapest, pp. 71-87.
 . ISBN 978-963-339-029-0
 
 
 
   Abstract Global warming16 has already begun. Climate change has become a self-propelling
and self-reinforcing process as a result of the externality associated with greenhouse-
gas (GHG) emissions. Although it is an externality related to humankind,
according to a number of unique features we should distinguish it from other externalities.
Climate change is a global phenomenon in its causes and consequences. The long-term and persistent impacts of climate change will likely continue over centuries without further anthropogenic mechanism. The preindustrial (equilibrium) level of GHG concentration in the atmosphere cannot be restored since it is irreversible, but if we do not stabilise the actual level of atmospheric concentration, the situation will become much worse than it is now. Assessing the impacts of climate change requires careful considerations because of the pervasive uncertainties and
risks associated with it.     Item Type: Book Section   Uncontrolled Keywords: cost-benefit analysis   Divisions: Faculty of Economics &gt; Department of World Economy   Subjects:  Ecology  Environmental economics    Projects: TAMOP 4.2.1/B-09/1/KMR-2010-0005   ID Code: 1012   Deposited By: 
   Deposited On: 19 Nov 2012 15:24   Last Modified: 19 Nov 2012 15:24    Repository Staff Only: item control page
Cost-benefit analysis of climate change – A methodological overview of recent studies
http://unipub.lib.uni-corvinus.hu/1012/
https://rgs-ibg.onlinelibrary.wiley.com/doi/10.1111/tran.12319
Hidden carbon costs of the “everywhere war”: Logistics, geopolitical ecology, and the carbon boot‐print of the US military
2020-03-01
Oliver Belcher
<div><div>
<main>
<div>
<div>
<article><div><div><div>
<p></p>
<div><p></p></div>
<div><p></p>
<p></p></div></div><div><nav><div><p></p><div><ul><li><ul><li><a href="#pane-pcw-references"><h2>References</h2></a></li><li><a href="#pane-pcw-related"><h2>Related</h2></a></li><li><a href="#pane-pcw-details"><h2>Information</h2></a></li></ul></li><li></li></ul></div></div></nav></div><div><div>
<section>
<h3>Abstract</h3>
<p>This paper examines the US military's impact on climate by analysing the geopolitical ecology of its global logistical supply chains. Our geopolitical ecology framework interrogates the material-ecological metabolic flows (hydrocarbon-based fuels, water, sand, concrete) that shape geopolitical and geoeconomic power relations. We argue that to account for the US military as a major climate actor, one must understand the logistical supply chain that makes its acquisition and consumption of hydrocarbon-based fuels possible. Our paper focuses on the US Defense Logistics Agency – Energy (DLA-E), a large yet virtually unresearched sub-agency within the US Department of Defense. The DLA-E is the primary purchase-point for hydrocarbon-based fuels for the US military, as well as a powerful actor in the global oil market. After outlining our geopolitical ecology approach, we detail the scope of the DLA-E's operations, its supply chain, bureaucratic practices, and the physical infrastructure that facilitates the US military's consumption of hydro-based carbons on a global scale. We show several “path dependencies” – warfighting paradigms, weapons systems, bureaucratic requirements, and waste – that are put in place by military supply chains and undergird a heavy reliance on carbon-based fuels by the US military for years to come. The paper, based on comprehensive records of bulk fuel purchases we have gathered from DLA-E through Freedom of Information Act requests, represents a partial yet robust picture of the geopolitical ecology of American imperialism.</p>
</section>
<section>
<h3>Graphical Abstract</h3>
<div>
<p>This paper examines the US military's impact on climate by analysing the geopolitical ecology of its global logistical supply chains. Our geopolitical ecology framework examines how material–ecological metabolic flows shape geopolitical and geoeconomic power relations. We argue that to account for the US military as a major climate actor, one must understand the logistical supply chain that makes its acquisition and consumption of hydrocarbon-based fuels possible.
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<div><a href="https://rgs-ibg.onlinelibrary.wiley.com/cms/asset/b6f9ce85-500c-40d9-9bff-651f6bd54ea6/tran12319-toc-0001-m.jpg"></a>
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<p><a href="https://rgs-ibg.onlinelibrary.wiley.com/toc/14755661/2020/45/1"></a></p>
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<p><a href="https://rgs-ibg.onlinelibrary.wiley.com/doi/toc/10.1111/(ISSN)1475-4959.borders_borderlands_bordering"></a>
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</main>
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 Transactions of the Institute of British Geographers
 Volume 45, Issue 1 p. 65-80
 REGULAR PAPER

 Hidden carbon costs of the “everywhere war”: Logistics, geopolitical ecology, and the carbon boot-print of the US military

 Oliver Belcher ,

 Corresponding Author

 Oliver Belcher

 * oliver.belcher@durham.ac.uk
 * orcid.org/0000-0003-0239-8705

 Department of Geography, Durham University, Durham, UK

 Correspondence

 Oliver Belcher

 Email: oliver.belcher@durham.ac.uk

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 Patrick Bigger ,

 Patrick Bigger

 * orcid.org/0000-0002-0022-6822

 Lancaster Environmental Centre, Lancaster University, Lancaster, UK

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 Ben Neimark ,

 Ben Neimark

 Lancaster Environmental Centre, Lancaster University, Lancaster, UK

 Search for more papers by this author
 Cara Kennelly ,

 Cara Kennelly

 Lancaster Environmental Centre, Lancaster University, Lancaster, UK

 Search for more papers by this author
 Oliver Belcher ,

 Corresponding Author

 Oliver Belcher

 * oliver.belcher@durham.ac.uk
 * orcid.org/0000-0003-0239-8705

 Department of Geography, Durham University, Durham, UK

 Correspondence

 Oliver Belcher

 Email: oliver.belcher@durham.ac.uk

 Search for more papers by this author
 Patrick Bigger ,

 Patrick Bigger

 * orcid.org/0000-0002-0022-6822

 Lancaster Environmental Centre, Lancaster University, Lancaster, UK

 Search for more papers by this author
 Ben Neimark ,

 Ben Neimark

 Lancaster Environmental Centre, Lancaster University, Lancaster, UK

 Search for more papers by this author
 Cara Kennelly ,

 Cara Kennelly

 Lancaster Environmental Centre, Lancaster University, Lancaster, UK

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 First published: 19 June 2019
 https://doi.org/10.1111/tran.12319
 Citations: 63
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 Abstract

 This paper examines the US military's impact on climate by analysing the geopolitical ecology of its global logistical supply chains. Our geopolitical ecology framework interrogates the material-ecological metabolic flows (hydrocarbon-based fuels, water, sand, concrete) that shape geopolitical and geoeconomic power relations. We argue that to account for the US military as a major climate actor, one must understand the logistical supply chain that makes its acquisition and consumption of hydrocarbon-based fuels possible. Our paper focuses on the US Defense Logistics Agency – Energy (DLA-E), a large yet virtually unresearched sub-agency within the US Department of Defense. The DLA-E is the primary purchase-point for hydrocarbon-based fuels for the US military, as well as a powerful actor in the global oil market. After outlining our geopolitical ecology approach, we detail the scope of the DLA-E's operations, its supply chain, bureaucratic practices, and the physical infrastructure that facilitates the US military's consumption of hydro-based carbons on a global scale. We show several “path dependencies” – warfighting paradigms, weapons systems, bureaucratic requirements, and waste – that are put in place by military supply chains and undergird a heavy reliance on carbon-based fuels by the US military for years to come. The paper, based on comprehensive records of bulk fuel purchases we have gathered from DLA-E through Freedom of Information Act requests, represents a partial yet robust picture of the geopolitical ecology of American imperialism.

 Graphical Abstract

 This paper examines the US military's impact on climate by analysing the geopolitical ecology of its global logistical supply chains. Our geopolitical ecology framework examines how material–ecological metabolic flows shape geopolitical and geoeconomic power relations. We argue that to account for the US military as a major climate actor, one must understand the logistical supply chain that makes its acquisition and consumption of hydrocarbon-based fuels possible.

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 Volume 45 , Issue 1

 March 2020

 Pages 65-80

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 * carbon
 * Defense Logistics Agency
 * geopolitical ecology
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 Publication History

 * Issue Online: 06 February 2020
 * Version of Record online: 19 June 2019
 * Manuscript accepted: 03 May 2019
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References     Related     Information             
Abstract
This paper examines the US military's impact on climate by analysing the geopolitical ecology of its global logistical supply chains. Our geopolitical ecology framework interrogates the material-ecological metabolic flows (hydrocarbon-based fuels, water, sand, concrete) that shape geopolitical and geoeconomic power relations. We argue that to account for the US military as a major climate actor, one must understand the logistical supply chain that makes its acquisition and consumption of hydrocarbon-based fuels possible. Our paper focuses on the US Defense Logistics Agency – Energy (DLA-E), a large yet virtually unresearched sub-agency within the US Department of Defense. The DLA-E is the primary purchase-point for hydrocarbon-based fuels for the US military, as well as a powerful actor in the global oil market. After outlining our geopolitical ecology approach, we detail the scope of the DLA-E's operations, its supply chain, bureaucratic practices, and the physical infrastructure that facilitates the US military's consumption of hydro-based carbons on a global scale. We show several “path dependencies” – warfighting paradigms, weapons systems, bureaucratic requirements, and waste – that are put in place by military supply chains and undergird a heavy reliance on carbon-based fuels by the US military for years to come. The paper, based on comprehensive records of bulk fuel purchases we have gathered from DLA-E through Freedom of Information Act requests, represents a partial yet robust picture of the geopolitical ecology of American imperialism.
Graphical Abstract
This paper examines the US military's impact on climate by analysing the geopolitical ecology of its global logistical supply chains. Our geopolitical ecology framework examines how material–ecological metabolic flows shape geopolitical and geoeconomic power relations. We argue that to account for the US military as a major climate actor, one must understand the logistical supply chain that makes its acquisition and consumption of hydrocarbon-based fuels possible.
Hidden carbon costs of the “everywhere war”: Logistics, geopolitical ecology, and the carbon boot‐print of the US military
https://rgs-ibg.onlinelibrary.wiley.com/doi/10.1111/tran.12319
https://academic.oup.com/bioscience/article/70/1/8/5610806
World Scientists’ Warning of a Climate Emergency
2020-01-01
Ripple; William J; Wolf; Christopher; Newsome; Thomas M; Barnard; Phoebe; Moomaw; William R
<div><div>
<p>Scientists have a moral obligation to clearly warn humanity of any catastrophic threat and to “tell it like it is.” On the basis of this obligation and the graphical indicators presented below, we declare, with more than 11,000 scientist signatories from around the world, clearly and unequivocally that planet Earth is facing a climate emergency.</p><p>Exactly 40 years ago, scientists from 50 nations met at the First World Climate Conference (in Geneva 1979) and agreed that alarming trends for climate change made it urgently necessary to act. Since then, similar alarms have been made through the 1992 Rio Summit, the 1997 Kyoto Protocol, and the 2015 Paris Agreement, as well as scores of other global assemblies and scientists’ explicit warnings of insufficient progress (Ripple et al. 2017). Yet greenhouse gas (GHG) emissions are still rapidly rising, with increasingly damaging effects on the Earth's climate. An immense increase of scale in endeavors to conserve our biosphere is needed to avoid untold suffering due to the climate crisis (IPCC 2018).</p><p>Most public discussions on climate change are based on global surface temperature only, an inadequate measure to capture the breadth of human activities and the real dangers stemming from a warming planet (Briggs et al. 2015). Policymakers and the public now urgently need access to a set of indicators that convey the effects of human activities on GHG emissions and the consequent impacts on climate, our environment, and society. Building on prior work (see <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">supplemental file S2</a>), we present a suite of graphical vital signs of climate change over the last 40 years for human activities that can affect GHG emissions and change the climate (figure 1), as well as actual climatic impacts (figure 2). We use only relevant data sets that are clear, understandable, systematically collected for at least the last 5 years, and updated at least annually.</p> <div><p>Figure 1.</p><div><p></p></div><p>Change in global human activities from 1979 to the present. These indicators are linked at least in part to climate change. In panel (f), annual tree cover loss may be for any reason (e.g., wildfire, harvest within tree plantations, or conversion of forests to agricultural land). Forest gain is not involved in the calculation of tree cover loss. In panel (h), hydroelectricity and nuclear energy are shown in <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">figure S2</a>. The rates shown in panels are the percentage changes per decade across the entire range of the time series. The annual data are shown using gray points. The black lines are local regression smooth trend lines. Abbreviation: Gt oe per year, gigatonnes of oil equivalent per year. Sources and additional details about each variable are provided in <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">supplemental file S2</a>, including <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">table S2</a>.</p></div> <div><p>Figure 2.</p><div><p></p></div><p>Climatic response time series from 1979 to the present. The rates shown in the panels are the decadal change rates for the entire ranges of the time series. These rates are in percentage terms, except for the interval variables (d, f, g, h, i, k), where additive changes are reported instead. For ocean acidity (pH), the percentage rate is based on the change in hydrogen ion activity, a<sub>H+</sub> (where lower pH values represent greater acidity). The annual data are shown using gray points. The black lines are local regression smooth trend lines. Sources and additional details about each variable are provided in <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">supplemental file S2</a>, including <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">table S3</a>.</p></div><p>The climate crisis is closely linked to excessive consumption of the wealthy lifestyle. The most affluent countries are mainly responsible for the historical GHG emissions and generally have the greatest per capita emissions (<a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">table S1</a>). In the present article, we show general patterns, mostly at the global scale, because there are many climate efforts that involve individual regions and countries. Our vital signs are designed to be useful to the public, policymakers, the business community, and those working to implement the Paris climate agreement, the United Nations’ Sustainable Development Goals, and the Aichi Biodiversity Targets.</p><p>Profoundly troubling signs from human activities include sustained increases in both human and ruminant livestock populations, per capita meat production, world gross domestic product, global tree cover loss, fossil fuel consumption, the number of air passengers carried, carbon dioxide (CO<sub>2</sub>) emissions, and per capita CO<sub>2</sub> emissions since 2000 (figure 1, <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">supplemental file S2</a>). Encouraging signs include decreases in global fertility (birth) rates (figure 1b), decelerated forest loss in the Brazilian Amazon (figure 1g), increases in the consumption of solar and wind power (figure 1h), institutional fossil fuel divestment of more than US$7 trillion (figure 1j), and the proportion of GHG emissions covered by carbon pricing (figure 1m). However, the decline in human fertility rates has substantially slowed during the last 20 years (figure 1b), and the pace of forest loss in Brazil's Amazon has now started to increase again (figure 1g). Consumption of solar and wind energy has increased 373% per decade, but in 2018, it was still 28 times smaller than fossil fuel consumption (combined gas, coal, oil; figure 1h). As of 2018, approximately 14.0% of global GHG emissions were covered by carbon pricing (figure 1m), but the global emissions-weighted average price per tonne of carbon dioxide was only around US$15.25 (figure 1n). A much higher carbon fee price is needed (IPCC 2018, section 2.5.2.1). Annual fossil fuel subsidies to energy companies have been fluctuating, and because of a recent spike, they were greater than US$400 billion in 2018 (figure 1o).</p><p>Especially disturbing are concurrent trends in the vital signs of climatic impacts (figure 2, <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">supplemental file S2</a>). Three abundant atmospheric GHGs (CO<sub>2</sub>, methane, and nitrous oxide) continue to increase (see <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">figure S1</a> for ominous 2019 spike in CO<sub>2</sub>), as does global surface temperature (figure 2a–2d). Globally, ice has been rapidly disappearing, evidenced by declining trends in minimum summer Arctic sea ice, Greenland and Antarctic ice sheets, and glacier thickness worldwide (figure 2e–2h). Ocean heat content, ocean acidity, sea level, area burned in the United States, and extreme weather and associated damage costs have all been trending upward (figure 2i–2n). Climate change is predicted to greatly affect marine, freshwater, and terrestrial life, from plankton and corals to fishes and forests (IPCC 2018, 2019). These issues highlight the urgent need for action.</p><p>Despite 40 years of global climate negotiations, with few exceptions, we have generally conducted business as usual and have largely failed to address this predicament (figure 1). The climate crisis has arrived and is accelerating faster than most scientists expected (figure 2, IPCC 2018). It is more severe than anticipated, threatening natural ecosystems and the fate of humanity (IPCC 2019). Especially worrisome are potential irreversible climate tipping points and nature's reinforcing feedbacks (atmospheric, marine, and terrestrial) that could lead to a catastrophic “hothouse Earth,” well beyond the control of humans (Steffen et al. 2018). These climate chain reactions could cause significant disruptions to ecosystems, society, and economies, potentially making large areas of Earth uninhabitable.</p><p>To secure a sustainable future, we must change how we live, in ways that improve the vital signs summarized by our graphs. Economic and population growth are among the most important drivers of increases in CO<sub>2</sub> emissions from fossil fuel combustion (Pachauri et al. 2014, Bongaarts and O’Neill 2018); therefore, we need bold and drastic transformations regarding economic and population policies. We suggest six critical and interrelated steps (in no particular order) that governments, businesses, and the rest of humanity can take to lessen the worst effects of climate change. These are important steps but are not the only actions needed or possible (Pachauri et al. 2014, IPCC 2018, 2019).</p> <h2>Energy</h2>
<p>The world must quickly implement massive energy efficiency and conservation practices and must replace fossil fuels with low-carbon renewables (figure 1h) and other cleaner sources of energy if safe for people and the environment (<a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s2.pdf?Expires=1684333346&amp;Signature=z90imkDXlaW~j7Asi2TP~r59m~RZZl2Stnli52HARrlox38rFopCRi2v2mdHUgZFzsvS0PvW0KlnwSH2J8CNmsnkdJmG-cz2n-vdj8tBYFj~UA0tcNvAGoOJAI-2yEH76GWxFPPA~LOft4M8MPK9aM4RHxo52y4Nxy83mscO3sa3OJI2McIlCHgq-hwdzeF3ohe9qnK86g44qg5aRHeTh9nRq4xtoQLaErEtmXjkOxbYkX94Z7WiCtUwBLF43pdzfVHrFRy-Vq3BDONfP7nbYiqwlxXVXuT8ZyGNsIj6yK-mOyc2lMM~MmeftIrXJhBh~T0S9hHkHdBoHX6Vhi~IPA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">figure S2</a>). We should leave remaining stocks of fossil fuels in the ground (see the timelines in IPCC 2018) and should carefully pursue effective negative emissions using technology such as carbon extraction from the source and capture from the air and especially by enhancing natural systems (see “Nature” section). Wealthier countries need to support poorer nations in transitioning away from fossil fuels. We must swiftly eliminate subsidies for fossil fuels (figure 1o) and use effective and fair policies for steadily escalating carbon prices to restrain their use.</p> <h2>Short-lived pollutants</h2>
<p>We need to promptly reduce the emissions of short-lived climate pollutants, including methane (figure 2b), black carbon (soot), and hydrofluorocarbons (HFCs). Doing this could slow climate feedback loops and potentially reduce the short-term warming trend by more than 50% over the next few decades while saving millions of lives and increasing crop yields due to reduced air pollution (Shindell et al. 2017). The 2016 Kigali amendment to phase down HFCs is welcomed.</p> <h2>Nature</h2>
<p>We must protect and restore Earth's ecosystems. Phytoplankton, coral reefs, forests, savannas, grasslands, wetlands, peatlands, soils, mangroves, and sea grasses contribute greatly to sequestration of atmospheric CO<sub>2</sub>. Marine and terrestrial plants, animals, and microorganisms play significant roles in carbon and nutrient cycling and storage. We need to quickly curtail habitat and biodiversity loss (figure 1f–1g), protecting the remaining primary and intact forests, especially those with high carbon stores and other forests with the capacity to rapidly sequester carbon (proforestation), while increasing reforestation and afforestation where appropriate at enormous scales. Although available land may be limiting in places, up to a third of emissions reductions needed by 2030 for the Paris agreement (less than 2°C) could be obtained with these natural climate solutions (Griscom et al. 2017).</p> <h2>Food</h2>
<p>Eating mostly plant-based foods while reducing the global consumption of animal products (figure 1c–d), especially ruminant livestock (Ripple et al. 2014), can improve human health and significantly lower GHG emissions (including methane in the “Short-lived pollutants” step). Moreover, this will free up croplands for growing much-needed human plant food instead of livestock feed, while releasing some grazing land to support natural climate solutions (see “Nature” section). Cropping practices such as minimum tillage that increase soil carbon are vitally important. We need to drastically reduce the enormous amount of food waste around the world.</p> <h2>Economy</h2>
<p>Excessive extraction of materials and overexploitation of ecosystems, driven by economic growth, must be quickly curtailed to maintain long-term sustainability of the biosphere. We need a carbon-free economy that explicitly addresses human dependence on the biosphere and policies that guide economic decisions accordingly. Our goals need to shift from GDP growth and the pursuit of affluence toward sustaining ecosystems and improving human well-being by prioritizing basic needs and reducing inequality.</p> <h2>Population</h2>
<p>Still increasing by roughly 80 million people per year, or more than 200,000 per day (figure 1a–b), the world population must be stabilized—and, ideally, gradually reduced—within a framework that ensures social integrity. There are proven and effective policies that strengthen human rights while lowering fertility rates and lessening the impacts of population growth on GHG emissions and biodiversity loss. These policies make family-planning services available to all people, remove barriers to their access and achieve full gender equity, including primary and secondary education as a global norm for all, especially girls and young women (Bongaarts and O’Neill 2018).</p> <h2>Conclusions</h2>
<p>Mitigating and adapting to climate change while honoring the diversity of humans entails major transformations in the ways our global society functions and interacts with natural ecosystems. We are encouraged by a recent surge of concern. Governmental bodies are making climate emergency declarations. Schoolchildren are striking. Ecocide lawsuits are proceeding in the courts. Grassroots citizen movements are demanding change, and many countries, states and provinces, cities, and businesses are responding.</p><p>As the Alliance of World Scientists, we stand ready to assist decision-makers in a just transition to a sustainable and equitable future. We urge widespread use of vital signs, which will better allow policymakers, the private sector, and the public to understand the magnitude of this crisis, track progress, and realign priorities for alleviating climate change. The good news is that such transformative change, with social and economic justice for all, promises far greater human well-being than does business as usual. We believe that the prospects will be greatest if decision-makers and all of humanity promptly respond to this warning and declaration of a climate emergency and act to sustain life on planet Earth, our only home.</p> <h2>Contributing reviewers</h2>
<p>Franz Baumann, Ferdinando Boero, Doug Boucher, Stephen Briggs, Peter Carter, Rick Cavicchioli, Milton Cole, Eileen Crist, Dominick A. DellaSala, Paul Ehrlich, Iñaki Garcia-De-Cortazar, Daniel Gilfillan, Alison Green, Tom Green, Jillian Gregg, Paul Grogan, John Guillebaud, John Harte, Nick Houtman, Charles Kennel, Christopher Martius, Frederico Mestre, Jennie Miller, David Pengelley, Chris Rapley, Klaus Rohde, Phil Sollins, Sabrina Speich, David Victor, Henrik Wahren, and Roger Worthington.</p> <h2>Funding</h2>
<p>The Worthy Garden Club furnished partial funding for this project.</p> <h2>Project website</h2>
<p>To view the Alliance of World Scientists website or to sign this article, go to <a href="https://scientistswarning.forestry.oregonstate.edu/">https://scientistswarning.forestry.oregonstate.edu</a>.</p> <h2>Supplemental material</h2>
<p>A list of the signatories appears in <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s1.pdf?Expires=1684333346&amp;Signature=3kiMZpSAaFFKOwzQkJ5dGiGhZWhcCjP7X~CmLvx0NDyBPuA6LAQa1BDSn6F1U6ctaIXppDnlu2K-WdHlEO6zVi334lFgnOFc25c50DVc1CD82TXV1fZrhh2YR7xXgAgTTgytBLUpMiUh~HwDU-OTO1fAWFfCwZAaDh9pjhwpZ69t1f3m4jPhzw4mBm~HcrmbHh2ow8X0~R01nKAQ~JlBw8bYkLtbvrYM1CHHKdc7iTpVF1KjRpoodqBhH1QC3rvzA5034eAFuGb7xiH8g0TIUpPNUrYtcSh~aB3ypBn94trgkvzZWRkjnx32Va8bmArQzuJDf-TQ3WRymfWKb954dA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">supplemental file S1</a>.</p> <h3>Author Biographical</h3>
<p>William J. Ripple (<a href="mailto:bill.ripple@oregonstate.edu">bill.ripple@oregonstate.edu</a>) and Christopher Wolf (<a href="mailto:christopher.wolf@oregonstate.edu">christopher.wolf@oregonstate.edu</a>) are affiliated with the Department of Forest Ecosystems and Society at Oregon State University, in Corvallis and contributed equally to the work. Thomas M. Newsome is affiliated with the School of Life and Environmental Sciences at The University of Sydney, in Sydney, New South Wales, Australia. Phoebe Barnard is affiliated with the Conservation Biology Institute, in Corvallis, Oregon, and with the African Climate and Development Initiative, at the University of Cape Town, in Cape Town, South Africa. William R. Moomaw is affiliated with The Fletcher School and the Global Development and Environment Institute, at Tufts University, in Medford, Massachusetts</p><p>11,258 scientist signatories from 153 countries (list in <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bioscience/70/1/10.1093_biosci_biz088/1/biz088_supplemental_file_s1.pdf?Expires=1684333346&amp;Signature=3kiMZpSAaFFKOwzQkJ5dGiGhZWhcCjP7X~CmLvx0NDyBPuA6LAQa1BDSn6F1U6ctaIXppDnlu2K-WdHlEO6zVi334lFgnOFc25c50DVc1CD82TXV1fZrhh2YR7xXgAgTTgytBLUpMiUh~HwDU-OTO1fAWFfCwZAaDh9pjhwpZ69t1f3m4jPhzw4mBm~HcrmbHh2ow8X0~R01nKAQ~JlBw8bYkLtbvrYM1CHHKdc7iTpVF1KjRpoodqBhH1QC3rvzA5034eAFuGb7xiH8g0TIUpPNUrYtcSh~aB3ypBn94trgkvzZWRkjnx32Va8bmArQzuJDf-TQ3WRymfWKb954dA__&amp;Key-Pair-Id=APKAIE5G5CRDK6RD3PGA">supplemental file S1</a>)</p> <h2>References cited</h2>
<div><div><p>Briggs</p> <p>S</p><p>, <p>Kennel</p> <p>CF</p>, <p>Victor</p> <p>DG</p>. </p><p>2015</p><p>. </p><p>Planetary vital signs</p><p>. </p><p>Nature Climate Change</p> <p>5</p><p>: </p><p>969</p><p>.</p></div><div><p>Bongaarts</p> <p>J</p><p>, <p>O’Neill</p> <p>BC</p></p><p>2018</p><p>. </p><p>Global warming policy: Is population left out in the cold?</p> <p>Science</p> <p>361</p><p>: </p><p>650</p><p>–</p><p>652</p><p>.</p></div><div><p>Griscom</p> <p>BW</p><p> et al. . </p><p>2017</p><p>. </p><p>Natural climate solutions</p><p>. </p><p>Proceedings of the National Academy of Sciences</p> <p>114</p><p>: </p><p>11645</p><p>–</p><p>11650</p><p>.</p></div><div><p>[IPCC] Intergovernmental Panel on Climate Change</p><p>. </p><p>2018</p><p>. </p><p>Global Warming of 1.5°C: An IPCC Special Report</p><p>. </p><p>IPCC</p><p>.</p></div><div><p>[IPCC] Intergovernmental Panel on Climate Change</p><p>. </p><p>2019</p><p>. </p><p>Climate Change and Land</p><p>. </p><p>IPCC</p><p>.</p></div><div><p>Pachauri</p> <p>RK</p><p> et al. . </p><p>2014</p><p>. </p><p>Climate Change 2014: Synthesis Report</p><p>. </p><p>Intergovernmental Panel on Climate Change.</p></div><div><p>Ripple</p> <p>WJ</p><p>, <p>Smith</p> <p>P</p>, <p>Haberl</p> <p>H</p>, <p>Montzka</p> <p>SA</p>, <p>McAlpine</p> <p>C</p>, <p>Boucher</p> <p>DH</p>. </p><p>2014</p><p>. </p><p>Ruminants, climate change, and climate policy</p><p>. </p><p>Nature Climate Change</p> <p>4</p><p>: </p><p>2</p><p>–</p><p>5</p><p>.</p></div><div><p>Ripple</p> <p>WJ</p><p>, <p>Wolf</p> <p>C</p>, <p>Newsome</p> <p>TM</p>, <p>Galetti</p> <p>M</p>, <p>Alamgir</p> <p>M</p>, <p>Crist</p> <p>E</p>, <p>Mahmoud</p> <p>MI</p>, <p>Laurance</p> <p>WF</p>. </p><p>2017</p><p>. </p><p>World scientists’ warning to humanity: A second notice</p><p>. </p><p>BioScience</p> <p>67</p><p>: </p><p>1026</p><p>–</p><p>1028</p><p>.</p></div><div><p>Shindell</p> <p>D</p><p>, <p>Borgford-Parnell</p> <p>N</p>, <p>Brauer</p> <p>M</p>, <p>Haines</p> <p>A</p>, <p>Kuylenstierna</p> <p>J</p>, <p>Leonard</p> <p>S</p>, <p>Ramanathan</p> <p>V</p>, <p>Ravishankara</p> <p>A</p>, <p>Amann</p> <p>M</p>, <p>Srivastava</p> <p>L</p>. </p><p>2017</p><p>. </p><p>A climate policy pathway for near- and long-term benefits</p><p>. </p><p>Science</p> <p>356</p><p>: </p><p>493</p><p>–</p><p>494</p><p>.</p></div><div><p>Steffen</p> <p>W</p><p> et al. . </p><p>2018</p><p>. </p><p>Trajectories of the Earth System in the Anthropocene</p><p>. </p><p>Proceedings of the National Academy of Sciences</p> <p>115</p><p>: </p><p>8252</p><p>–</p><p>8259</p><p>.</p></div></div> 
 
 
 
 <h2>Author notes</h2>
<p><p>William J. Ripple and Christopher Wolf contributed equally to the work.</p></p><p>© The Author(s) 2019. Published by Oxford University Press on behalf of the American Institute of Biological Sciences.</p>
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 Volume 70
 Issue 1
 January 2020

 Article Contents

 * Energy
 * Short-lived pollutants
 * Nature
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 World Scientists’ Warning of a Climate Emergency

 William J Ripple ,
 William J Ripple
 Department of Forest Ecosystems and Society at Oregon State University
 , Corvallis
 E-mail: bill.ripple@oregonstate.edu
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 Christopher Wolf ,
 Christopher Wolf
 Department of Forest Ecosystems and Society at Oregon State University
 , Corvallis
 E-mail: christopher.wolf@oregonstate.edu
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 Thomas M Newsome ,
 Thomas M Newsome
 School of Life and Environmental Sciences at The University of Sydney
 , Sydney, New South Wales,
 Australia
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 Phoebe Barnard ,
 Phoebe Barnard
 Conservation Biology Institute, Corvallis, Oregon, and with the African Climate and Development Initiative, University of Cape Town
 , Cape Town,
 South Africa
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 William R Moomaw
 William R Moomaw
 The Fletcher School and the Global Development and Environment Institute, Tufts University
 , Medford, Massachusetts
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 William J. Ripple and Christopher Wolf contributed equally to the work.

 Author Notes
 BioScience, Volume 70, Issue 1, January 2020, Pages 8–12, https://doi.org/10.1093/biosci/biz088
 Published:
 05 November 2019
 A correction has been published: BioScience , Volume 70, Issue 1, January 2020, Page 100, https://doi.org/10.1093/biosci/biz152
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 William J Ripple, Christopher Wolf, Thomas M Newsome, Phoebe Barnard, William R Moomaw, World Scientists’ Warning of a Climate Emergency, BioScience, Volume 70, Issue 1, January 2020, Pages 8–12, https://doi.org/10.1093/biosci/biz088

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 Scientists have a moral obligation to clearly warn humanity of any catastrophic threat and to “tell it like it is.” On the basis of this obligation and the graphical indicators presented below, we declare, with more than 11,000 scientist signatories from around the world, clearly and unequivocally that planet Earth is facing a climate emergency.

 Exactly 40 years ago, scientists from 50 nations met at the First World Climate Conference (in Geneva 1979) and agreed that alarming trends for climate change made it urgently necessary to act. Since then, similar alarms have been made through the 1992 Rio Summit, the 1997 Kyoto Protocol, and the 2015 Paris Agreement, as well as scores of other global assemblies and scientists’ explicit warnings of insufficient progress (Ripple et al. 2017). Yet greenhouse gas (GHG) emissions are still rapidly rising, with increasingly damaging effects on the Earth's climate. An immense increase of scale in endeavors to conserve our biosphere is needed to avoid untold suffering due to the climate crisis (IPCC 2018).

 Most public discussions on climate change are based on global surface temperature only, an inadequate measure to capture the breadth of human activities and the real dangers stemming from a warming planet (Briggs et al. 2015). Policymakers and the public now urgently need access to a set of indicators that convey the effects of human activities on GHG emissions and the consequent impacts on climate, our environment, and society. Building on prior work (see supplemental file S2), we present a suite of graphical vital signs of climate change over the last 40 years for human activities that can affect GHG emissions and change the climate (figure 1), as well as actual climatic impacts (figure 2). We use only relevant data sets that are clear, understandable, systematically collected for at least the last 5 years, and updated at least annually.

 Figure 1.
 Open in new tabDownload slide

 Change in global human activities from 1979 to the present. These indicators are linked at least in part to climate change. In panel (f), annual tree cover loss may be for any reason (e.g., wildfire, harvest within tree plantations, or conversion of forests to agricultural land). Forest gain is not involved in the calculation of tree cover loss. In panel (h), hydroelectricity and nuclear energy are shown in figure S2. The rates shown in panels are the percentage changes per decade across the entire range of the time series. The annual data are shown using gray points. The black lines are local regression smooth trend lines. Abbreviation: Gt oe per year, gigatonnes of oil equivalent per year. Sources and additional details about each variable are provided in supplemental file S2, including table S2.

 Figure 2.
 Open in new tabDownload slide

 Climatic response time series from 1979 to the present. The rates shown in the panels are the decadal change rates for the entire ranges of the time series. These rates are in percentage terms, except for the interval variables (d, f, g, h, i, k), where additive changes are reported instead. For ocean acidity (pH), the percentage rate is based on the change in hydrogen ion activity, aH+ (where lower pH values represent greater acidity). The annual data are shown using gray points. The black lines are local regression smooth trend lines. Sources and additional details about each variable are provided in supplemental file S2, including table S3.

 The climate crisis is closely linked to excessive consumption of the wealthy lifestyle. The most affluent countries are mainly responsible for the historical GHG emissions and generally have the greatest per capita emissions (table S1). In the present article, we show general patterns, mostly at the global scale, because there are many climate efforts that involve individual regions and countries. Our vital signs are designed to be useful to the public, policymakers, the business community, and those working to implement the Paris climate agreement, the United Nations’ Sustainable Development Goals, and the Aichi Biodiversity Targets.

 Profoundly troubling signs from human activities include sustained increases in both human and ruminant livestock populations, per capita meat production, world gross domestic product, global tree cover loss, fossil fuel consumption, the number of air passengers carried, carbon dioxide (CO2) emissions, and per capita CO2 emissions since 2000 (figure 1, supplemental file S2). Encouraging signs include decreases in global fertility (birth) rates (figure 1b), decelerated forest loss in the Brazilian Amazon (figure 1g), increases in the consumption of solar and wind power (figure 1h), institutional fossil fuel divestment of more than US$7 trillion (figure 1j), and the proportion of GHG emissions covered by carbon pricing (figure 1m). However, the decline in human fertility rates has substantially slowed during the last 20 years (figure 1b), and the pace of forest loss in Brazil's Amazon has now started to increase again (figure 1g). Consumption of solar and wind energy has increased 373% per decade, but in 2018, it was still 28 times smaller than fossil fuel consumption (combined gas, coal, oil; figure 1h). As of 2018, approximately 14.0% of global GHG emissions were covered by carbon pricing (figure 1m), but the global emissions-weighted average price per tonne of carbon dioxide was only around US$15.25 (figure 1n). A much higher carbon fee price is needed (IPCC 2018, section 2.5.2.1). Annual fossil fuel subsidies to energy companies have been fluctuating, and because of a recent spike, they were greater than US$400 billion in 2018 (figure 1o).

 Especially disturbing are concurrent trends in the vital signs of climatic impacts (figure 2, supplemental file S2). Three abundant atmospheric GHGs (CO2, methane, and nitrous oxide) continue to increase (see figure S1 for ominous 2019 spike in CO2), as does global surface temperature (figure 2a–2d). Globally, ice has been rapidly disappearing, evidenced by declining trends in minimum summer Arctic sea ice, Greenland and Antarctic ice sheets, and glacier thickness worldwide (figure 2e–2h). Ocean heat content, ocean acidity, sea level, area burned in the United States, and extreme weather and associated damage costs have all been trending upward (figure 2i–2n). Climate change is predicted to greatly affect marine, freshwater, and terrestrial life, from plankton and corals to fishes and forests (IPCC 2018, 2019). These issues highlight the urgent need for action.

 Despite 40 years of global climate negotiations, with few exceptions, we have generally conducted business as usual and have largely failed to address this predicament (figure 1). The climate crisis has arrived and is accelerating faster than most scientists expected (figure 2, IPCC 2018). It is more severe than anticipated, threatening natural ecosystems and the fate of humanity (IPCC 2019). Especially worrisome are potential irreversible climate tipping points and nature's reinforcing feedbacks (atmospheric, marine, and terrestrial) that could lead to a catastrophic “hothouse Earth,” well beyond the control of humans (Steffen et al. 2018). These climate chain reactions could cause significant disruptions to ecosystems, society, and economies, potentially making large areas of Earth uninhabitable.

 To secure a sustainable future, we must change how we live, in ways that improve the vital signs summarized by our graphs. Economic and population growth are among the most important drivers of increases in CO2 emissions from fossil fuel combustion (Pachauri et al. 2014, Bongaarts and O’Neill 2018); therefore, we need bold and drastic transformations regarding economic and population policies. We suggest six critical and interrelated steps (in no particular order) that governments, businesses, and the rest of humanity can take to lessen the worst effects of climate change. These are important steps but are not the only actions needed or possible (Pachauri et al. 2014, IPCC 2018, 2019).

 Energy

 The world must quickly implement massive energy efficiency and conservation practices and must replace fossil fuels with low-carbon renewables (figure 1h) and other cleaner sources of energy if safe for people and the environment (figure S2). We should leave remaining stocks of fossil fuels in the ground (see the timelines in IPCC 2018) and should carefully pursue effective negative emissions using technology such as carbon extraction from the source and capture from the air and especially by enhancing natural systems (see “Nature” section). Wealthier countries need to support poorer nations in transitioning away from fossil fuels. We must swiftly eliminate subsidies for fossil fuels (figure 1o) and use effective and fair policies for steadily escalating carbon prices to restrain their use.

 Short-lived pollutants

 We need to promptly reduce the emissions of short-lived climate pollutants, including methane (figure 2b), black carbon (soot), and hydrofluorocarbons (HFCs). Doing this could slow climate feedback loops and potentially reduce the short-term warming trend by more than 50% over the next few decades while saving millions of lives and increasing crop yields due to reduced air pollution (Shindell et al. 2017). The 2016 Kigali amendment to phase down HFCs is welcomed.

 Nature

 We must protect and restore Earth's ecosystems. Phytoplankton, coral reefs, forests, savannas, grasslands, wetlands, peatlands, soils, mangroves, and sea grasses contribute greatly to sequestration of atmospheric CO2. Marine and terrestrial plants, animals, and microorganisms play significant roles in carbon and nutrient cycling and storage. We need to quickly curtail habitat and biodiversity loss (figure 1f–1g), protecting the remaining primary and intact forests, especially those with high carbon stores and other forests with the capacity to rapidly sequester carbon (proforestation), while increasing reforestation and afforestation where appropriate at enormous scales. Although available land may be limiting in places, up to a third of emissions reductions needed by 2030 for the Paris agreement (less than 2°C) could be obtained with these natural climate solutions (Griscom et al. 2017).

 Food

 Eating mostly plant-based foods while reducing the global consumption of animal products (figure 1c–d), especially ruminant livestock (Ripple et al. 2014), can improve human health and significantly lower GHG emissions (including methane in the “Short-lived pollutants” step). Moreover, this will free up croplands for growing much-needed human plant food instead of livestock feed, while releasing some grazing land to support natural climate solutions (see “Nature” section). Cropping practices such as minimum tillage that increase soil carbon are vitally important. We need to drastically reduce the enormous amount of food waste around the world.

 Economy

 Excessive extraction of materials and overexploitation of ecosystems, driven by economic growth, must be quickly curtailed to maintain long-term sustainability of the biosphere. We need a carbon-free economy that explicitly addresses human dependence on the biosphere and policies that guide economic decisions accordingly. Our goals need to shift from GDP growth and the pursuit of affluence toward sustaining ecosystems and improving human well-being by prioritizing basic needs and reducing inequality.

 Population

 Still increasing by roughly 80 million people per year, or more than 200,000 per day (figure 1a–b), the world population must be stabilized—and, ideally, gradually reduced—within a framework that ensures social integrity. There are proven and effective policies that strengthen human rights while lowering fertility rates and lessening the impacts of population growth on GHG emissions and biodiversity loss. These policies make family-planning services available to all people, remove barriers to their access and achieve full gender equity, including primary and secondary education as a global norm for all, especially girls and young women (Bongaarts and O’Neill 2018).

 Conclusions

 Mitigating and adapting to climate change while honoring the diversity of humans entails major transformations in the ways our global society functions and interacts with natural ecosystems. We are encouraged by a recent surge of concern. Governmental bodies are making climate emergency declarations. Schoolchildren are striking. Ecocide lawsuits are proceeding in the courts. Grassroots citizen movements are demanding change, and many countries, states and provinces, cities, and businesses are responding.

 As the Alliance of World Scientists, we stand ready to assist decision-makers in a just transition to a sustainable and equitable future. We urge widespread use of vital signs, which will better allow policymakers, the private sector, and the public to understand the magnitude of this crisis, track progress, and realign priorities for alleviating climate change. The good news is that such transformative change, with social and economic justice for all, promises far greater human well-being than does business as usual. We believe that the prospects will be greatest if decision-makers and all of humanity promptly respond to this warning and declaration of a climate emergency and act to sustain life on planet Earth, our only home.

 Contributing reviewers

 Franz Baumann, Ferdinando Boero, Doug Boucher, Stephen Briggs, Peter Carter, Rick Cavicchioli, Milton Cole, Eileen Crist, Dominick A. DellaSala, Paul Ehrlich, Iñaki Garcia-De-Cortazar, Daniel Gilfillan, Alison Green, Tom Green, Jillian Gregg, Paul Grogan, John Guillebaud, John Harte, Nick Houtman, Charles Kennel, Christopher Martius, Frederico Mestre, Jennie Miller, David Pengelley, Chris Rapley, Klaus Rohde, Phil Sollins, Sabrina Speich, David Victor, Henrik Wahren, and Roger Worthington.

 Funding

 The Worthy Garden Club furnished partial funding for this project.

 Project website

 To view the Alliance of World Scientists website or to sign this article, go to https://scientistswarning.forestry.oregonstate.edu.

 Supplemental material

 A list of the signatories appears in supplemental file S1.

 Author Biographical

 William J. Ripple (bill.ripple@oregonstate.edu) and Christopher Wolf (christopher.wolf@oregonstate.edu) are affiliated with the Department of Forest Ecosystems and Society at Oregon State University, in Corvallis and contributed equally to the work. Thomas M. Newsome is affiliated with the School of Life and Environmental Sciences at The University of Sydney, in Sydney, New South Wales, Australia. Phoebe Barnard is affiliated with the Conservation Biology Institute, in Corvallis, Oregon, and with the African Climate and Development Initiative, at the University of Cape Town, in Cape Town, South Africa. William R. Moomaw is affiliated with The Fletcher School and the Global Development and Environment Institute, at Tufts University, in Medford, Massachusetts

 11,258 scientist signatories from 153 countries (list in supplemental file S1)

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 Author notes

 William J. Ripple and Christopher Wolf contributed equally to the work.

 © The Author(s) 2019. Published by Oxford University Press on behalf of the American Institute of Biological Sciences.
 This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
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Scientists have a moral obligation to clearly warn humanity of any catastrophic threat and to “tell it like it is.” On the basis of this obligation and the graphical indicators presented below, we declare, with more than 11,000 scientist signatories from around the world, clearly and unequivocally that planet Earth is facing a climate emergency. Exactly 40 years ago, scientists from 50 nations met at the First World Climate Conference (in Geneva 1979) and agreed that alarming trends for climate change made it urgently necessary to act. Since then, similar alarms have been made through the 1992 Rio Summit, the 1997 Kyoto Protocol, and the 2015 Paris Agreement, as well as scores of other global assemblies and scientists’ explicit warnings of insufficient progress (Ripple et al. 2017). Yet greenhouse gas (GHG) emissions are still rapidly rising, with increasingly damaging effects on the Earth's climate. An immense increase of scale in endeavors to conserve our biosphere is needed to avoid untold suffering due to the climate crisis (IPCC 2018). Most public discussions on climate change are based on global surface temperature only, an inadequate measure to capture the breadth of human activities and the real dangers stemming from a warming planet (Briggs et al. 2015). Policymakers and the public now urgently need access to a set of indicators that convey the effects of human activities on GHG emissions and the consequent impacts on climate, our environment, and society. Building on prior work (see supplemental file S2), we present a suite of graphical vital signs of climate change over the last 40 years for human activities that can affect GHG emissions and change the climate (figure 1), as well as actual climatic impacts (figure 2). We use only relevant data sets that are clear, understandable, systematically collected for at least the last 5 years, and updated at least annually.  Figure 1.     Change in global human activities from 1979 to the present. These indicators are linked at least in part to climate change. In panel (f), annual tree cover loss may be for any reason (e.g., wildfire, harvest within tree plantations, or conversion of forests to agricultural land). Forest gain is not involved in the calculation of tree cover loss. In panel (h), hydroelectricity and nuclear energy are shown in figure S2. The rates shown in panels are the percentage changes per decade across the entire range of the time series. The annual data are shown using gray points. The black lines are local regression smooth trend lines. Abbreviation: Gt oe per year, gigatonnes of oil equivalent per year. Sources and additional details about each variable are provided in supplemental file S2, including table S2.   Figure 2.     Climatic response time series from 1979 to the present. The rates shown in the panels are the decadal change rates for the entire ranges of the time series. These rates are in percentage terms, except for the interval variables (d, f, g, h, i, k), where additive changes are reported instead. For ocean acidity (pH), the percentage rate is based on the change in hydrogen ion activity, aH+ (where lower pH values represent greater acidity). The annual data are shown using gray points. The black lines are local regression smooth trend lines. Sources and additional details about each variable are provided in supplemental file S2, including table S3.  The climate crisis is closely linked to excessive consumption of the wealthy lifestyle. The most affluent countries are mainly responsible for the historical GHG emissions and generally have the greatest per capita emissions (table S1). In the present article, we show general patterns, mostly at the global scale, because there are many climate efforts that involve individual regions and countries. Our vital signs are designed to be useful to the public, policymakers, the business community, and those working to implement the Paris climate agreement, the United Nations’ Sustainable Development Goals, and the Aichi Biodiversity Targets. Profoundly troubling signs from human activities include sustained increases in both human and ruminant livestock populations, per capita meat production, world gross domestic product, global tree cover loss, fossil fuel consumption, the number of air passengers carried, carbon dioxide (CO2) emissions, and per capita CO2 emissions since 2000 (figure 1, supplemental file S2). Encouraging signs include decreases in global fertility (birth) rates (figure 1b), decelerated forest loss in the Brazilian Amazon (figure 1g), increases in the consumption of solar and wind power (figure 1h), institutional fossil fuel divestment of more than US$7 trillion (figure 1j), and the proportion of GHG emissions covered by carbon pricing (figure 1m). However, the decline in human fertility rates has substantially slowed during the last 20 years (figure 1b), and the pace of forest loss in Brazil's Amazon has now started to increase again (figure 1g). Consumption of solar and wind energy has increased 373% per decade, but in 2018, it was still 28 times smaller than fossil fuel consumption (combined gas, coal, oil; figure 1h). As of 2018, approximately 14.0% of global GHG emissions were covered by carbon pricing (figure 1m), but the global emissions-weighted average price per tonne of carbon dioxide was only around US$15.25 (figure 1n). A much higher carbon fee price is needed (IPCC 2018, section 2.5.2.1). Annual fossil fuel subsidies to energy companies have been fluctuating, and because of a recent spike, they were greater than US$400 billion in 2018 (figure 1o). Especially disturbing are concurrent trends in the vital signs of climatic impacts (figure 2, supplemental file S2). Three abundant atmospheric GHGs (CO2, methane, and nitrous oxide) continue to increase (see figure S1 for ominous 2019 spike in CO2), as does global surface temperature (figure 2a–2d). Globally, ice has been rapidly disappearing, evidenced by declining trends in minimum summer Arctic sea ice, Greenland and Antarctic ice sheets, and glacier thickness worldwide (figure 2e–2h). Ocean heat content, ocean acidity, sea level, area burned in the United States, and extreme weather and associated damage costs have all been trending upward (figure 2i–2n). Climate change is predicted to greatly affect marine, freshwater, and terrestrial life, from plankton and corals to fishes and forests (IPCC 2018, 2019). These issues highlight the urgent need for action. Despite 40 years of global climate negotiations, with few exceptions, we have generally conducted business as usual and have largely failed to address this predicament (figure 1). The climate crisis has arrived and is accelerating faster than most scientists expected (figure 2, IPCC 2018). It is more severe than anticipated, threatening natural ecosystems and the fate of humanity (IPCC 2019). Especially worrisome are potential irreversible climate tipping points and nature's reinforcing feedbacks (atmospheric, marine, and terrestrial) that could lead to a catastrophic “hothouse Earth,” well beyond the control of humans (Steffen et al. 2018). These climate chain reactions could cause significant disruptions to ecosystems, society, and economies, potentially making large areas of Earth uninhabitable. To secure a sustainable future, we must change how we live, in ways that improve the vital signs summarized by our graphs. Economic and population growth are among the most important drivers of increases in CO2 emissions from fossil fuel combustion (Pachauri et al. 2014, Bongaarts and O’Neill 2018); therefore, we need bold and drastic transformations regarding economic and population policies. We suggest six critical and interrelated steps (in no particular order) that governments, businesses, and the rest of humanity can take to lessen the worst effects of climate change. These are important steps but are not the only actions needed or possible (Pachauri et al. 2014, IPCC 2018, 2019). Energy
The world must quickly implement massive energy efficiency and conservation practices and must replace fossil fuels with low-carbon renewables (figure 1h) and other cleaner sources of energy if safe for people and the environment (figure S2). We should leave remaining stocks of fossil fuels in the ground (see the timelines in IPCC 2018) and should carefully pursue effective negative emissions using technology such as carbon extraction from the source and capture from the air and especially by enhancing natural systems (see “Nature” section). Wealthier countries need to support poorer nations in transitioning away from fossil fuels. We must swiftly eliminate subsidies for fossil fuels (figure 1o) and use effective and fair policies for steadily escalating carbon prices to restrain their use. Short-lived pollutants
We need to promptly reduce the emissions of short-lived climate pollutants, including methane (figure 2b), black carbon (soot), and hydrofluorocarbons (HFCs). Doing this could slow climate feedback loops and potentially reduce the short-term warming trend by more than 50% over the next few decades while saving millions of lives and increasing crop yields due to reduced air pollution (Shindell et al. 2017). The 2016 Kigali amendment to phase down HFCs is welcomed. Nature
We must protect and restore Earth's ecosystems. Phytoplankton, coral reefs, forests, savannas, grasslands, wetlands, peatlands, soils, mangroves, and sea grasses contribute greatly to sequestration of atmospheric CO2. Marine and terrestrial plants, animals, and microorganisms play significant roles in carbon and nutrient cycling and storage. We need to quickly curtail habitat and biodiversity loss (figure 1f–1g), protecting the remaining primary and intact forests, especially those with high carbon stores and other forests with the capacity to rapidly sequester carbon (proforestation), while increasing reforestation and afforestation where appropriate at enormous scales. Although available land may be limiting in places, up to a third of emissions reductions needed by 2030 for the Paris agreement (less than 2°C) could be obtained with these natural climate solutions (Griscom et al. 2017). Food
Eating mostly plant-based foods while reducing the global consumption of animal products (figure 1c–d), especially ruminant livestock (Ripple et al. 2014), can improve human health and significantly lower GHG emissions (including methane in the “Short-lived pollutants” step). Moreover, this will free up croplands for growing much-needed human plant food instead of livestock feed, while releasing some grazing land to support natural climate solutions (see “Nature” section). Cropping practices such as minimum tillage that increase soil carbon are vitally important. We need to drastically reduce the enormous amount of food waste around the world. Economy
Excessive extraction of materials and overexploitation of ecosystems, driven by economic growth, must be quickly curtailed to maintain long-term sustainability of the biosphere. We need a carbon-free economy that explicitly addresses human dependence on the biosphere and policies that guide economic decisions accordingly. Our goals need to shift from GDP growth and the pursuit of affluence toward sustaining ecosystems and improving human well-being by prioritizing basic needs and reducing inequality. Population
Still increasing by roughly 80 million people per year, or more than 200,000 per day (figure 1a–b), the world population must be stabilized—and, ideally, gradually reduced—within a framework that ensures social integrity. There are proven and effective policies that strengthen human rights while lowering fertility rates and lessening the impacts of population growth on GHG emissions and biodiversity loss. These policies make family-planning services available to all people, remove barriers to their access and achieve full gender equity, including primary and secondary education as a global norm for all, especially girls and young women (Bongaarts and O’Neill 2018). Conclusions
Mitigating and adapting to climate change while honoring the diversity of humans entails major transformations in the ways our global society functions and interacts with natural ecosystems. We are encouraged by a recent surge of concern. Governmental bodies are making climate emergency declarations. Schoolchildren are striking. Ecocide lawsuits are proceeding in the courts. Grassroots citizen movements are demanding change, and many countries, states and provinces, cities, and businesses are responding. As the Alliance of World Scientists, we stand ready to assist decision-makers in a just transition to a sustainable and equitable future. We urge widespread use of vital signs, which will better allow policymakers, the private sector, and the public to understand the magnitude of this crisis, track progress, and realign priorities for alleviating climate change. The good news is that such transformative change, with social and economic justice for all, promises far greater human well-being than does business as usual. We believe that the prospects will be greatest if decision-makers and all of humanity promptly respond to this warning and declaration of a climate emergency and act to sustain life on planet Earth, our only home. Contributing reviewers
Franz Baumann, Ferdinando Boero, Doug Boucher, Stephen Briggs, Peter Carter, Rick Cavicchioli, Milton Cole, Eileen Crist, Dominick A. DellaSala, Paul Ehrlich, Iñaki Garcia-De-Cortazar, Daniel Gilfillan, Alison Green, Tom Green, Jillian Gregg, Paul Grogan, John Guillebaud, John Harte, Nick Houtman, Charles Kennel, Christopher Martius, Frederico Mestre, Jennie Miller, David Pengelley, Chris Rapley, Klaus Rohde, Phil Sollins, Sabrina Speich, David Victor, Henrik Wahren, and Roger Worthington. Funding
The Worthy Garden Club furnished partial funding for this project. Project website
To view the Alliance of World Scientists website or to sign this article, go to https://scientistswarning.forestry.oregonstate.edu. Supplemental material
A list of the signatories appears in supplemental file S1. Author Biographical
William J. Ripple (bill.ripple@oregonstate.edu) and Christopher Wolf (christopher.wolf@oregonstate.edu) are affiliated with the Department of Forest Ecosystems and Society at Oregon State University, in Corvallis and contributed equally to the work. Thomas M. Newsome is affiliated with the School of Life and Environmental Sciences at The University of Sydney, in Sydney, New South Wales, Australia. Phoebe Barnard is affiliated with the Conservation Biology Institute, in Corvallis, Oregon, and with the African Climate and Development Initiative, at the University of Cape Town, in Cape Town, South Africa. William R. Moomaw is affiliated with The Fletcher School and the Global Development and Environment Institute, at Tufts University, in Medford, Massachusetts 11,258 scientist signatories from 153 countries (list in supplemental file S1) References cited
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 Author notes
 William J. Ripple and Christopher Wolf contributed equally to the work.  © The Author(s) 2019. Published by Oxford University Press on behalf of the American Institute of Biological Sciences.
World Scientists’ Warning of a Climate Emergency
https://academic.oup.com/bioscience/article/70/1/8/5610806
https://www.nature.com/articles/nature01286
A globally coherent fingerprint of climate change impacts across natural systems - Nature
2003-01-27
Parmesan; Camille; Yohe; Gary
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 1. nature
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 * Published: 02 January 2003

 A globally coherent fingerprint of climate change impacts across natural systems

 * Camille Parmesan1 &
 * Gary Yohe2

 Nature volume 421, pages 37–42 ( 2003 )Cite this article

 * 74k Accesses

 * 6918 Citations

 * 420 Altmetric

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 Abstract

 Causal attribution of recent biological trends to climate change is complicated because non-climatic influences dominate local, short-term biological changes. Any underlying signal from climate change is likely to be revealed by analyses that seek systematic trends across diverse species and geographic regions; however, debates within the Intergovernmental Panel on Climate Change (IPCC) reveal several definitions of a ‘systematic trend’. Here, we explore these differences, apply diverse analyses to more than 1,700 species, and show that recent biological trends match climate change predictions. Global meta-analyses documented significant range shifts averaging 6.1 km per decade towards the poles (or metres per decade upward), and significant mean advancement of spring events by 2.3 days per decade. We define a diagnostic fingerprint of temporal and spatial ‘sign-switching’ responses uniquely predicted by twentieth century climate trends. Among appropriate long-term/large-scale/multi-species data sets, this diagnostic fingerprint was found for 279 species. This suite of analyses generates ‘very high confidence’ (as laid down by the IPCC) that climate change is already affecting living systems.

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 Figure 1: Probabilistic model based on parameter estimates from a review of the literature.

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 Acknowledgements

 This paper was stimulated by discussion during meetings of the Intergovernmental Panel on Climate Change, particularly with Q. K. Ahmad, N. Leary, R. Leemans, R. Moss, J. Price, T. L. Root, C. Rosenzweig, S. Schneider, R. Tol, F. Toth and R. Warrick. We thank L. Kaila, J. Kullberg, J. J. Lennon, N. Ryrholm, C. D. Thomas, J. A. Thomas and M. Warren for use of their raw data for analyses. We also thank C. Krebs, J. Matthews, R. Plowes, J. A. Pounds, R. Sagarin, M. C. Singer and B. Wee. Writing was facilitated by the Centre National de la Recherche Scientifique (CEFE) and by the National Science Foundation of the United States through its support of the Center for Integrated Assessment of the Human Dimensions of Global Change at Carnegie Mellon University.

 Author information

 Authors and Affiliations

 1. Integrative Biology, Patterson Laboratories 141, University of Texas, Austin, Texas, 78712, USA

 Camille Parmesan

 2. John E. Andrus Professor of Economics, Wesleyan University, 238 Public Affairs Center, Middletown, Connecticut, 06459, USA

 Gary Yohe

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 Parmesan, C., Yohe, G. A globally coherent fingerprint of climate change impacts across natural systems. Nature 421, 37–42 (2003). https://doi.org/10.1038/nature01286

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 * Received : 05 March 2002

 * Accepted : 22 October 2002

 * Issue Date : 02 January 2003

 * DOI : https://doi.org/10.1038/nature01286

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References    Intergovernmental Panel on Climate Change Third Assessment Report Climate Change 2001: Impacts, Adaptation, and Vulnerability (eds McCarthy, J. J., Canziani, O. F., Leary, N. A., Dokken, D. J. &amp; White, K. S.) (Cambridge Univ. Press, Cambridge, 2001)  
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A globally coherent fingerprint of climate change impacts across natural systems - Nature
https://www.nature.com/articles/nature01286
https://journals.ametsoc.org/view/journals/bams/77/3/1520-0477_1996_077_0437_tnyrp_2_0_co_2.xml
The NCEP/NCAR 40-Year Reanalysis Project
1996-03-01
E Kalnay; M Kanamitsu; R Kistler; W Collins; D Deaven; L Gandin; M Iredell; S Saha; G White; J Woollen; Y Zhu; M Chelliah; W Ebisuzaki; W Higgins; J Janowiak; K C Mo; C Ropelewski; J Wang; A Leetmaa; R Reynolds; Roy Jenne; Dennis Joseph
<div><section><p>The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system.</p><p>The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by different countries and organizations. The system has been designed with advanced quality control and monitoring components, and can produce 1 mon of reanalysis per day on a Cray YMP/8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a “quick look” CD-ROM (one per year) with six tropospheric and stratospheric fields available twice daily, as well as surface, top-of-the-atmosphere, and isentropic fields. Reanalysis information and selected output is also available on-line via the Internet (<a href="https://journals.ametsoc.org/view/journals/bams/77/3/http//:nic.fb4.noaa.gov:8000">http//:nic.fb4.noaa.gov:8000</a>). A special CDROM, containing 13 years of selected observed, daily, monthly, and climatological data from the NCEP/NCAR Reanalysis, is included with this issue. Output variables are classified into four classes, depending on the degree to which they are influenced by the observations and/or the model. For example, “C” variables (such as precipitation and surface fluxes) are completely determined by the model during the data assimilation and should be used with caution. Nevertheless, a comparison of these variables with observations and with several climatologies shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predictability studies and for monitoring the quality of the observing systems.</p><p>The 40 years of reanalysis (1957–96) should be completed in early 1997. A continuation into the future through an identical Climate Data Assimilation System will allow researchers to reliably compare recent anomalies with those in earlier decades. Since changes in the observing systems will inevitably produce perceived changes in the climate, parallel reanalyses (at least 1 year long) will be generated for the periods immediately after the introduction of new observing systems, such as new types of satellite data.</p><p>NCEP plans currently call for an updated reanalysis using a state-of-the-art system every five years or so. The successive reanalyses will be greatly facilitated by the generation of the comprehensive database in the present reanalysis.</p></section></div>||||I||||Jump to Content
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 Online Publication: 01 Mar 1996 Print Publication: 01 Mar 1996 DOI: https://doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2 Page(s): 437–472
 Article History
 Received: 05 Sep 1995 Published Online: 01 Mar 1996

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 The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system.

 The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by different countries and organizations. The system has been designed with advanced quality control and monitoring components, and can produce 1 mon of reanalysis per day on a Cray YMP/8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a “quick look” CD-ROM (one per year) with six tropospheric and stratospheric fields available twice daily, as well as surface, top-of-the-atmosphere, and isentropic fields. Reanalysis information and selected output is also available on-line via the Internet (http//:nic.fb4.noaa.gov:8000). A special CDROM, containing 13 years of selected observed, daily, monthly, and climatological data from the NCEP/NCAR Reanalysis, is included with this issue. Output variables are classified into four classes, depending on the degree to which they are influenced by the observations and/or the model. For example, “C” variables (such as precipitation and surface fluxes) are completely determined by the model during the data assimilation and should be used with caution. Nevertheless, a comparison of these variables with observations and with several climatologies shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predictability studies and for monitoring the quality of the observing systems.

 The 40 years of reanalysis (1957–96) should be completed in early 1997. A continuation into the future through an identical Climate Data Assimilation System will allow researchers to reliably compare recent anomalies with those in earlier decades. Since changes in the observing systems will inevitably produce perceived changes in the climate, parallel reanalyses (at least 1 year long) will be generated for the periods immediately after the introduction of new observing systems, such as new types of satellite data.

 NCEP plans currently call for an updated reanalysis using a state-of-the-art system every five years or so. The successive reanalyses will be greatly facilitated by the generation of the comprehensive database in the present reanalysis.

 *Environmental Modeling Center, National Centers for Environmental Prediction, Washington, D.C.

 +Climate Prediction Center, National Centers for Environmental Prediction, Washington, D.C.

 #National Center for Atmospheric Research, Boulder, Colorado

 Corresponding author address: Dr. Eugenia Kalnay, director, Environmental Modeling Center, W/NMCZ, Room 204 WWB, Washington, DC 20233. E-mail: WD23EK@SUNl.WWB.NOAA.GOV

 The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system.

 The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by different countries and organizations. The system has been designed with advanced quality control and monitoring components, and can produce 1 mon of reanalysis per day on a Cray YMP/8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a “quick look” CD-ROM (one per year) with six tropospheric and stratospheric fields available twice daily, as well as surface, top-of-the-atmosphere, and isentropic fields. Reanalysis information and selected output is also available on-line via the Internet (http//:nic.fb4.noaa.gov:8000). A special CDROM, containing 13 years of selected observed, daily, monthly, and climatological data from the NCEP/NCAR Reanalysis, is included with this issue. Output variables are classified into four classes, depending on the degree to which they are influenced by the observations and/or the model. For example, “C” variables (such as precipitation and surface fluxes) are completely determined by the model during the data assimilation and should be used with caution. Nevertheless, a comparison of these variables with observations and with several climatologies shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predictability studies and for monitoring the quality of the observing systems.

 The 40 years of reanalysis (1957–96) should be completed in early 1997. A continuation into the future through an identical Climate Data Assimilation System will allow researchers to reliably compare recent anomalies with those in earlier decades. Since changes in the observing systems will inevitably produce perceived changes in the climate, parallel reanalyses (at least 1 year long) will be generated for the periods immediately after the introduction of new observing systems, such as new types of satellite data.

 NCEP plans currently call for an updated reanalysis using a state-of-the-art system every five years or so. The successive reanalyses will be greatly facilitated by the generation of the comprehensive database in the present reanalysis.

 *Environmental Modeling Center, National Centers for Environmental Prediction, Washington, D.C.

 +Climate Prediction Center, National Centers for Environmental Prediction, Washington, D.C.

 #National Center for Atmospheric Research, Boulder, Colorado

 Corresponding author address: Dr. Eugenia Kalnay, director, Environmental Modeling Center, W/NMCZ, Room 204 WWB, Washington, DC 20233. E-mail: WD23EK@SUNl.WWB.NOAA.GOV
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The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system. The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by different countries and organizations. The system has been designed with advanced quality control and monitoring components, and can produce 1 mon of reanalysis per day on a Cray YMP/8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a “quick look” CD-ROM (one per year) with six tropospheric and stratospheric fields available twice daily, as well as surface, top-of-the-atmosphere, and isentropic fields. Reanalysis information and selected output is also available on-line via the Internet (http//:nic.fb4.noaa.gov:8000). A special CDROM, containing 13 years of selected observed, daily, monthly, and climatological data from the NCEP/NCAR Reanalysis, is included with this issue. Output variables are classified into four classes, depending on the degree to which they are influenced by the observations and/or the model. For example, “C” variables (such as precipitation and surface fluxes) are completely determined by the model during the data assimilation and should be used with caution. Nevertheless, a comparison of these variables with observations and with several climatologies shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predictability studies and for monitoring the quality of the observing systems. The 40 years of reanalysis (1957–96) should be completed in early 1997. A continuation into the future through an identical Climate Data Assimilation System will allow researchers to reliably compare recent anomalies with those in earlier decades. Since changes in the observing systems will inevitably produce perceived changes in the climate, parallel reanalyses (at least 1 year long) will be generated for the periods immediately after the introduction of new observing systems, such as new types of satellite data. NCEP plans currently call for an updated reanalysis using a state-of-the-art system every five years or so. The successive reanalyses will be greatly facilitated by the generation of the comprehensive database in the present reanalysis.
The NCEP/NCAR 40-Year Reanalysis Project
https://journals.ametsoc.org/view/journals/bams/77/3/1520-0477_1996_077_0437_tnyrp_2_0_co_2.xml
https://arxiv.org/abs/2104.05506
Global temperature goals should determine the time horizons for greenhouse gas emission metrics
2021-04-12
Abernethy; Sam; Jackson; Robert B
<div><div>
 
 
 
 
 
 
 <p><a href="/pdf/2104.05506">Download PDF</a></p><blockquote>
 Abstract: Emission metrics, a crucial tool in setting effective equivalences between
greenhouse gases, currently require a subjective, arbitrary choice of time
horizon. Here, we propose a novel framework that uses a specific temperature
goal to calculate the time horizon that aligns with scenarios achieving that
temperature goal. We analyze the Intergovernmental Panel on Climate Change
Special Report on Global Warming of 1.5 C Scenario Database to find that time
horizons that align with the 1.5 and 2 C global warming goals of the Paris
Agreement are 24 [90% prediction interval: 7, 41] and 58 [90% PI: 41, 74] years
respectively. We then use these time horizons to quantify time-dependent
emission metrics with methane as our main example. We find that the Global
Warming Potential values that align with the 1.5 and 2 C goals are GWP1.5 C =
75 [90% PI: 54, 107] and GWP2 C = 42 [90% PI: 35, 54]; for the Global
Temperature change Potential they are GTP1.5 C = 41 [90% PI: 16, 102] and GTP2
C = 9 [90% PI: 7, 16]. The most commonly used time horizon, 100 years,
underestimates methane emission metrics by 34-38% relative to the values we
calculate that align with the 2 C goal and 63-87% relative to the 1.5 C goal.
To best align emission metrics with the 1.5 C goal of the Paris Agreement, we
recommend a 24-year time horizon, using 2045 as the endpoint time, with its
associated GWP1.5 C = 75 and GTP1.5 C = 41.
 </blockquote>
 
 
 </div><div>
 <h2>Submission history</h2><p> From: Sam Abernethy [<a href="/show-email/3db1d7d3/2104.05506">view email</a>]
 <br />
 <strong><a href="/abs/2104.05506v1">[v1]</a></strong>
 Mon, 12 Apr 2021 14:39:58 UTC (1,863 KB)<br /><strong>[v2]</strong>
Tue, 16 Nov 2021 19:44:36 UTC (1,228 KB)<br /></p></div>||||I|||| Skip to main content
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 Physics > Atmospheric and Oceanic Physics

 arXiv:2104.05506 (physics)
 [Submitted on 12 Apr 2021 (v1), last revised 16 Nov 2021 (this version, v2)]

 Title: Global temperature goals should determine the time horizons for greenhouse gas emission metrics

 Authors: Sam Abernethy, Robert B. Jackson
 Download PDF
 Abstract: Emission metrics, a crucial tool in setting effective equivalences between greenhouse gases, currently require a subjective, arbitrary choice of time horizon. Here, we propose a novel framework that uses a specific temperature goal to calculate the time horizon that aligns with scenarios achieving that temperature goal. We analyze the Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5 C Scenario Database to find that time horizons that align with the 1.5 and 2 C global warming goals of the Paris Agreement are 24 [90% prediction interval: 7, 41] and 58 [90% PI: 41, 74] years respectively. We then use these time horizons to quantify time-dependent emission metrics with methane as our main example. We find that the Global Warming Potential values that align with the 1.5 and 2 C goals are GWP1.5 C = 75 [90% PI: 54, 107] and GWP2 C = 42 [90% PI: 35, 54]; for the Global Temperature change Potential they are GTP1.5 C = 41 [90% PI: 16, 102] and GTP2 C = 9 [90% PI: 7, 16]. The most commonly used time horizon, 100 years, underestimates methane emission metrics by 34-38% relative to the values we calculate that align with the 2 C goal and 63-87% relative to the 1.5 C goal. To best align emission metrics with the 1.5 C goal of the Paris Agreement, we recommend a 24-year time horizon, using 2045 as the endpoint time, with its associated GWP1.5 C = 75 and GTP1.5 C = 41.
 Comments: 15 pages, 3 figures  
 Subjects: Atmospheric and Oceanic Physics (physics.ao-ph) ; Data Analysis, Statistics and Probability (physics.data-an); Physics and Society (physics.soc-ph)
 Cite as: arXiv:2104.05506 [physics.ao-ph]  
 (or arXiv:2104.05506v2 [physics.ao-ph] for this version)  
 https://doi.org/10.48550/arXiv.2104.05506  
 Focus to learn more  
 arXiv-issued DOI via DataCite  
 https://doi.org/10.1088/1748-9326/ac4940  
 Related DOI: Focus to learn more  
 DOI(s) linking to related resources  
 

 Submission history

 From: Sam Abernethy [view email]
 [v1] Mon, 12 Apr 2021 14:39:58 UTC (1,863 KB)
 [v2] Tue, 16 Nov 2021 19:44:36 UTC (1,228 KB)
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Global temperature goals should determine the time horizons for greenhouse gas emission metrics
S Abernethy sabernet@stanford.edu 
Department of Applied Physics
Stanford University
Department of Earth System Science
Stanford University
R B Jackson 
Department of Earth System Science
Stanford University
Woods Institute for the Environment and Precourt Institute for Energy
Stanford University
94305StanfordUSA
Global temperature goals should determine the time horizons for greenhouse gas emission metrics
1 * Corresponding author:
Emission metrics, a crucial tool in setting effective equivalences between greenhouse gases, currently require a subjective, arbitrary choice of time horizon. Here, we propose a novel framework that uses a specific temperature goal to calculate the time horizon that aligns with scenarios achieving that temperature goal. We analyze the Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5 °C Scenario Database to find that time horizons that align with the 1.5 and 2 ℃ global warming goals of the Paris Agreement are 24 [90% prediction interval: 7, 41] and 58 [90% PI: 41, 74] years respectively. We then use these time horizons to quantify time-dependent emission metrics with methane as our main example. We find that the Global Warming Potential values that align with the 1.5 and 2 ℃ goals are GWP1.5 ℃ = 75 [90% PI: 54, 107] and GWP2 ℃ = 42 [90% PI: 35, 54]; for the Global Temperature change Potential they are GTP1.5 ℃ = 41 [90% PI: 16, 102] and GTP2 ℃ = 9 [90% PI: 7, 16]. The most commonly used time horizon, 100 years, underestimates methane emission 2 metrics by 34-38% relative to the values we calculate that align with the 2 ℃ goal and 63-87%relative to the 1.5 ℃ goal. To best align emission metrics with the 1.5 °C goal of the Paris Agreement, we recommend a 24-year time horizon, using 2045 as the endpoint time, with its associated GWP1.5 ℃ = 75 and GTP1.5 ℃ = 41.
Introduction
Emission metrics, the "exchange rates" between greenhouse gases, are necessary for policymakers and decision-makers to compare the relative effects of different gases to carbon dioxide, the generally accepted baseline [1][2][3][4][5][6][7][8][9][10] . Emission metrics underpin the Nationally Determined Contributions (NDCs) of the Paris Agreement and form the basis of so-called "carbon dioxide equivalent" emissions by allowing greenhouse gases to be compared on the same scale. Here, we develop a framework to quantify emission metrics that align with the Paris Agreement goal to keep global warming well below 2 °C and ideally to 1.5 °C, compared to preindustrial levels.
Currently, the Intergovernmental Panel on Climate Change (IPCC) uses 20-and 100-year time horizons for emission metrics 1 while simultaneously acknowledging that these arbitrary values "should not be considered as having any special significance" 11 . Our framework derives time horizons of special significance that align with specific temperature goals.
Time horizons are required for most emission metrics because greenhouse gases with shorter atmospheric lifetimes impact the climate much more over the short-term compared to the more constant effects of carbon dioxide over centuries 12 . We focus on methane because it is the second-most important greenhouse gas after carbon dioxide and its atmospheric lifetime of only 11.8 years means that its emission metrics depend strongly on the time horizon 1,13 . The extension of our method to other short-lived climate forcers would be straightforward.
The IPCC's use of two reference time horizons, 20 and 100 years, has been criticized for being arbitrary and unjustified 14 , with the choice between the two often based on political perspectives or vested interests 15 . Quantitative justifications for specific time horizons are surprisingly lacking in the literature. One rare example comes from Sarofim and Giordano, who argued that time horizons could be justified based on their equivalent economic discount rate, and showed that the 100-year time horizon aligned implicitly with a 3.3% discount rate (consistent with many climate and economic impact analyses) 15 . Here, we instead define "justified time horizons" as those that align with the predicted timing of peak warming for the specific temperature goals of the Paris Agreement. We then use these justified time horizons to quantify time-dependent emission metrics.
What is the justification for aligning emission metrics with temperature goals? We believe that greenhouse gases should be weighted based on their contribution to achieving a given climate goal. We acknowledge that emission metrics are used in a wide range of applications, not all of which relate to temperature stabilization goals, meaning that the criteria for selecting time horizons is not universal. Potential policy goals, beyond temperature stabilization, include intergenerational equity 8,9,16 , air quality 17 , and cost-effectiveness 18,19 . However, since the main goal of the Paris Agreement is to limit the temperature peak well below 2 °C, the impact of emissions on achieving this specific temperature goal should dictate the relative importance of different greenhouse gases. We use 1.5 °C and 2 °C as representative examples of the temperature goals of the Paris Agreement since they are the two values explicitly mentioned. Throughout this work we use "justified" as short-hand for "justified in its alignment with the Paris Agreement temperature goals," while noting that other justifications could and should be developed for different applications.
5
Our framework is primarily designed for times before peak temperature; we make no claim about optimal emission metrics after the time of peak temperature, but simply aim to align emission metrics with the current goal of avoiding the exceedance of a given temperature limit. We hypothesize that as a temperature peak is approached, new climate goals will be developed (likely regarding stabilization at a certain temperature) and argue that future time horizons should be calculated that align with those goals.
We recognize the value in having an agreed upon emission metric to standardize the weighting of different greenhouse gases and support the narrowing of the broad range of possible emission metrics. The use of the Global Warming Potential with a 100-year time horizon (GWP100) as the common metric in the implementation of the Paris Agreement, as agreed upon in the 24th
Conference of the Parties (COP24) in 2019, is a step in the right direction towards standardization 19 . However, it has been shown that pathways with identical carbon dioxide equivalent emissions calculated using GWP100 can vary in temperature by as much as 0.17 °C 20 .
One proposed solution is simply to report both GWP20 and GWP100 in all cases, as is often done for city-highway fuel economy, for example. However, this practice still leaves the choice between the two as a subjective, arbitrary decision 21 . Another approach involves bounding the range for emission metrics using a "do no harm" principle 10 , but this leads to a large range of potential values. Given the openness of the IPCC to revising emission metrics in future Assessment Reports, we believe our framework for selecting time horizons could help create more justified, but still standardized, emission metrics.
Generalizing, temperature changes near the peak temperature are more important than those that happen long before or after, in part because they introduce the possibility of passing climate tipping points 22 . With justified time horizons, as we define them, greenhouse gases would be weighted by their impact on achieving specific temperature goals at the time at which temperature peaks. To illustrate our point, consider an extreme hypothetical temperature goal to limit global warming to 4 °C (a threshold unlikely to be met for several decades, if ever). If a 10year time horizon was used, this would drastically overvalue current emissions of short-lived climate forcers like methane and suboptimally distribute resources towards reducing them, thereby lowering the likelihood of achieving the temperature goal. Higher temperature goals are associated with longer time horizons for achieving them, and emission metrics should reflect this fact. Generally speaking, the more that rapid action towards stringent temperature goals (such as 1.5 °C) proves feasible, the higher weight we should give to short-lived climate forcers such as methane.
Our framework builds on the work of Manne and Richels 18 , who established the timedependence of emission metrics based on price ratios and claimed that the optimal emission metrics for methane depended on both the time and the temperature goal. We also incorporate the "combined target and metric approach" of Shine et al. 23 and Berntsen et al. 24 , who clearly outlined the importance of aligning temperature goals with emission metric time horizons. They established that lower target temperatures imply an earlier target year-and emission metrics should reflect this fact. However, their work was limited because (in the words of Berntsen et al.)
"a unique relationship between the target level and the target year [had] not been derived in the existing literature." 24 Using the current availability of scenarios from numerous Integrated Assessment Models (IAMs) 25 , we derive the first relationship of this type. The works closest to ours are those of Tanaka et al. 19,26,27 . They first examined time-dependent emission metrics using one IAM and three scenarios for stabilization at 2, 3, and 4 °C 26 . They then presented a framework for choosing time horizons using cost-effective socioeconomic metrics, this time with five scenarios but again only one IAM 19 . We take a different approach by: 1) considering climate only, rather than socioeconomics, 2) aggregating ten IAMs instead of a single model for robustness, and 3) analyzing 213 mitigation pathway scenarios instead of five to generalize the framework for use with a general temperature goal and associated time horizon.
We emphasize the importance of increasing NDCs such that temperature stabilization can be achieved; all greenhouse gases must be reduced rapidly. Climate policy using our framework would first set limits on carbon dioxide-equivalent emissions to ensure that the long-term temperature goal is achievable 23 . Indeed, the emission metrics we present are only valid if a temperature peak occurs. Linking the time horizon with the temperature goal means that increased emphasis on methane would not reduce the importance of carbon dioxide, since it would simply make the overall climate target stricter 24 . The emission metrics we present, along with the future costs of different greenhouse gas mitigation and removal approaches, would then dictate the portfolio of emission reductions needed to most efficiently achieve the specified temperature goal.
Emission metrics
Depending on the choice of emission metric, the implied severity of methane relative to carbon dioxide ranges from 4 to 200, although most estimates are between 10 and 80 28 . Our framework 8 narrows this range by incorporating temperature goals and the temporal proximity to the peak temperatures that meet those goals.
The most commonly used metric for comparisons between greenhouse gases, the Global Warming Potential (GWP), measures the radiative forcing of a pulse of gas relative to the radiative forcing of a pulse of carbon dioxide with equal mass, both integrated over a given time horizon. However, GWP has been criticized widely, including that it is inappropriate for the temperature goals of the Paris Agreement because it quantifies radiative forcing rather than temperature 18,23,29-33 .
The Global Temperature change Potential (GTP) is a more policy-relevant metric that quantifies the temperature impact of a pulse of gas relative to the temperature impact of a pulse of carbon dioxide with equal mass, both evaluated at an endpoint time horizon 23,32 . Limiting the instantaneous peak temperature is the explicit goal of the Paris Agreement due in part to the significance of instantaneous elevated temperature in causing heat waves and extreme events 34 .
However, the policy relevance of GTP comes at the expense of increased uncertainty because climate modelling is required to calculate it, unlike the purely physical GWP 23,35,36 .
Extended periods of time at elevated temperature directly impact humanity and the climate, including through sea-level rise and the potential passing of tipping points in the climate system such as melting sea-ice and permafrost thawing. The integrated Global Temperature change Potential (iGTP) quantifies the duration of elevated temperature by integrating the temperature impact over time (and as such is based on the same climate modelling as GTP) 37,38 .
9
A growing body of research suggests that pulse emissions of carbon dioxide should not be equated with pulse emissions of short-lived climate forcers like methane, but rather with sustained changes ('steps') in their emission rates 1,23,39 . This approach, using 'step-pulse' emission metrics like CombinedGTP 40 , captures the contrasting nature of carbon dioxide as a stock pollutant and methane as a flow pollutant and also reduces the sensitivity of emission metrics to the time horizon-but selecting a time horizon is still required. Another recent emission metric is GWP*, which equates pulses of carbon dioxide with emission rates of methane over a preceding time range (often 20 years) 10,29,30,33,41 . However, GWP* has been criticized for its unfairness on equity grounds at the country level 9,16 and for its potential incompatibility with the Paris Agreement due to the requirement to take preceding decades into account 19 . As is currently done by the IPCC, we focus on metrics that equate pulses with pulses, but note that our framework is also compatible with CombinedGTP (see Methods for details and an example).
Keeping global warming well below 2 °C is the main goal of the Paris Agreement, but other considerations beyond temperature are important and should be incorporated into decisionmaking processes. The social cost of methane, for example, is much larger than temperaturebased emission metrics alone. This is because increased methane concentrations cause increased surface ozone which negatively impacts human health and crop productivity, among other secondary effects 12,17 . Two other emission metrics, the Global Damage Potential 42 and the Global Cost Potential 43 , come from a cost-benefit and a cost-effectiveness framing of climate change, respectively. Economic metrics such as these are further along the climate change cause-effect 10 chain, meaning that they are more descriptive of the actual damages caused, but at the expense of higher uncertainty due to the required modelling of the socioeconomic impacts of climate change 28 . Since our work focuses on temperature goals, we limit our focus to physical metrics (radiative forcing and temperature) rather than socioeconomic metrics. Therefore, we present GWP and GTP as representative emission metrics to outline our framework.
Results
We start by screening the 21 st century mitigation pathway scenarios in the IPCC Special Report on Global Warming of 1.5 °C (SR1.5) Scenario Database 44 to extract the 213 scenarios with a temperature peak. We then aggregate all 213 scenarios that come from ten IAMs (Figure 1) to form the basis of our dataset. All scenarios are weighted equally. Next, we plot the peak temperature ( !"#$ ) against the time at which it occurs ( !"#$ ) to derive a relationship between these variables given by
!"#$ = !"#$ + ,(1)
where = 68 ± 4 years/℃ and = 1943 ± 7 years ( Figure 2). These parameters are calculated by performing linear least squares on the raw temperature peak data (Figures 1 and 2). Temperature peaks are identified with black circles.
Equation 1, on its own, is simply a description of the emergent linear relationship between the peak temperature and the time at which it occurs ( Figure 2). To understand the applicability of this relationship, we use the "very likely" nomenclature adopted by the IPCC to calculate 90% prediction intervals (PI), the range in which we predict 90% of future scenarios will fall. The 90% PI for the relationship between the temperature peak and its time of occurrence has a range of approximately 33 years for a given temperature goal ( Figure 2  We then calculate time-dependent emission metrics using the established "combined metric and target approach" 24 where the time horizon is the proximity to the time of peak temperature ( Figure 3a) 26 . Methane's emission metrics increase with proximity to the time of peak temperature, meaning that the values we present in this paper are specific to 2021 (and will increase in the future). Closer to the time of peak temperature, climate goals may change. For example, a goal to ensure long-term stability at a certain temperature would lead to emphasizing carbon dioxide, whereas a goal to rapidly return to preindustrial temperatures would emphasize 13 short-lived climate forcers. We focus solely on times before the peak temperature where the explicit end goal (a temperature limit in the Paris Agreement) has been formalized; times beyond the peak are addressed in the Discussion but are generally beyond the scope of this paper.
Combining these established time-metric relationships (Equations 2-4 and Figure 3a) with our novel temperature-time relationship (Equation 1 and Figure 2), we derive a primary result: a relationship between temperature goals and emission metrics (Figure 3b). We then use our framework to quantify emission metrics for the specific temperature goals of the Paris  The 100-year time horizon underestimates emission metrics that align with the 1.5 ℃ goal even more drastically: by 63% for GWP and 87% for GTP. 
Discussion
We present recommended time horizons for emission metrics but cannot address which emission metric should be used in all cases. This choice, or multiple choices based on different goals, must be made based on the outcome of interest, the decision between endpoint and integrated, and the tradeoff between the usefulness of temperature-based or socioeconomic emission metrics with the added uncertainty that comes with the modelling required to calculate them 6,28 . We believe (i)GTP is more applicable than GWP when considering temperature goals, but recognize the potential for our framework to be used for other goals. For radiative forcing targets, GWP would be better suited; if policy decisions between pulse emissions of carbon dioxide and sustained changes in methane emission rates were being considered, a step-pulse metric such as CombinedGTP 40 might be the metric of choice. Aiming for net zero emissions is also an explicit goal of the Paris Agreement, but it has been shown that the definition of net zero emissions depends heavily on the metric used 31,46 . Regardless of the chosen metric, however, our framework avoids the subjective, arbitrary choice in time horizon (often either 20 or 100 years) by determining the time horizon that aligns with a given climate goal.
The relationship between peak temperature and the time at which it occurs ( Figure 2) is to our knowledge the first of its kind but comes with several limitations. First, the assumption of linearity in the temperature-time relationship (Equation 1) is not motivated by an underlying physical mechanism. The outliers that fall outside of the prediction interval (roughly between 1.6 and 1.9 ℃) are primarily above this interval, suggesting that a fit with a negative second derivative may better capture this relationship. However, fitting the data in Figure 2 with a quadratic or logarithmic function changes the predicted time of peak warming by less than 2 years for all temperature goals relevant to the Paris Agreement (1.5-2 ℃). Therefore, we make no claim about the applicability of this relationship to temperature goals extrapolated beyond these temperature goals, but note that the functional form of the fitted relationship has only a minor influence on the predicted timing of peak temperature within this range. Second, the set of scenarios in SR1.5 is not exhaustive (and indeed there are arguments calling for IAMs to be supplemented with other modelling approaches 47 ). SR1.5 does not include scenarios with temperature peaks higher than 2.3 ℃, thereby missing a broad class of likely scenarios (based on current global NDCs) that may reach 3 ℃ warming 48 . The SR1.5 scenarios are also primarily 'peak and decline' scenarios, whereas 'stabilization' scenarios may follow a qualitatively different temperature trajectory 46 . Finally, the SR1.5 scenarios end in 2100, thereby potentially biasing the prediction interval downwards (earlier). To address these concerns and increase the robustness of our results, we encourage the extension of the scenario database to include higher temperature peaks, scenarios that continue past 2100, and more extensive modelling of the full space of potential future scenarios (ideally weighted based on their likelihood).
One potential criticism of our framework (and other work upon which it builds 18,23,24,46 ) is that it is valid only until the time of peak temperature. Furthermore, it implicitly ignores temperature impacts that occur after the peak. This criticism is valid, but we argue that it is best aimed toward the creation and modification of climate goals, rather than at the specifics of our framework.
From a cost-effectiveness perspective, it has been shown that emission metrics stabilize asymptotically as temperature stabilizes 19 , meaning that the resolution of this criticism may be the simplest solution: to take our recommended emission metric at the time of peak for all times thereafter. One could also develop a framework similar to the one we present where the goal is to minimize time at elevated temperatures above 2 ℃, for example, or to minimize the rate of temperature increase to allow ecosystems more time to adapt 34 . Whether to use the time horizons we present that align with the temperature peak goals, or to have different emission metrics before and after the temperature peak, is a policy-specific question beyond the scope of this work. We welcome future extensions of our framework to align with alternative climate goals, particularly those that incorporate times after peak temperature. 
21
The GWP for a non-carbon dioxide greenhouse gas is the ratio of the absolute Global Warming Potential (AGWPX) of gas X divided by the AGWP of carbon dioxide. For most greenhouse gases, including methane, the AGWPX at a time horizon is given by
% ( ) = % ∫ & ' ( % ( ) ,(2)
where % is the radiative efficiency of gas X, and ( % ( ) is the climate impulse response function, values and equations for which can be found in Appendices C and D of Gasser et al. 52 .
The GWP for gas X (GWPX) is then given by AGWPX / AGWPCO2.
The GTP for a non-carbon dioxide greenhouse gas is calculated as the ratio of the absolute Global Temperature change Potential (AGTPX) of gas X divided by the AGTP of carbon dioxide.
For most greenhouse gases, the AGTPX at time horizon is given by
% ( ) = % ∫ & ' ( % ( ′) ) ( − ′) ′,(3)
where ( − ′), the climate response function, is given analytically in Appendix C of Gasser et al. 52 . The GTPX is then given by AGTPX / AGTPCO2.
The integrated Global Temperature Change potential (iGTPX) is the ratio found by integrating the AGTPX of a non-carbon dioxide gas X divided by the integrated AGTP of carbon dioxide 37 :
% ( ) = % ( ) / +, 2 ( ) = ∫ & 0 % ( ′) ′ / ∫ & 0 +, 2 ( ′) ′.(4)
The CombinedGTP (CGTP) 40 is the ratio of the temperature impact caused by a sustained increase in emission rate of a non-carbon dioxide gas divided by the temperature impact of a pulse of carbon dioxide of mass equal to the annual change in emission rate. This equating of a step function in non-carbon dioxide greenhouse gas emission rate with a pulse of carbon dioxide 22 makes CGTP a 'step-pulse' emission metric with units of years. As outlined by Collins et al. 40 , the order of integration can be switched and CGTP can be expressed as % ( ) = % ( ) / +, 2 ( ).
To derive the relationships shown in Figure 3b, we simply inserted Equation 1 (the relationship between time and peak temperature) into Equations 2-5. This means that for any temperature goal ( !"#$ ), Equation 1 gives a corresponding time of peak ( !"#$ ) and associated time horizon from present day, which can then be inserted into the equations for AGWP, AGTP, and iAGTP for methane and carbon dioxide and then divided to give GWP, GTP, iGTP, and CGTP.
To calculate uncertainties for the parameters in Equation 1 and prediction intervals throughout, we used the Python package statsmodels. Implementation details can be found in the Python code that we have made available online (see Code availability).
Data availability
The data that support the findings of this study are openly available on the IPCC SR1.5 Scenario Database download page 44 and the IPCC AR6 data repository 1,45 . Our code repository (below) also contains the raw data required to replicate our analysis. Processed data used in the analysis and generation of figures and tables is available directly from the corresponding author on request or can be generated by running the provided Jupyter Python notebook (see Code availability).
Code availability
The Jupyter Python notebook used for the analysis, figures, and tables presented in this work can 23 be downloaded from Zenodo at https://doi.org/10.5281/zenodo.4609302.
11 Figure 1 :
111All 213 scenarios with a temperature peak from the IPCC SR1.5 Scenario Database 44 , categorized by IAM used. The number of scenarios for each IAM (n) is shown in each panel.
Figure 2 :Figure 1 .
21The relationship between peak temperature and the time the peak temperature is reached for all 213 scenarios and all 10 IAMs. Individual scenarios are shown as points, as in Overlaid on this raw data is the line of best fit (black line) and 90% prediction interval (gray shaded area).
Figure 3 :
3Time dependent emission metrics (GWP in green and GTP in blue) for methane. Panel a) The "combined target and metric approach" 24 where the time horizon is the number of years before peak temperature. Panel b) Combining this approach with the results of Figure 2 to show a relationship between temperature goal and emission metrics. Shaded area is the 90% prediction interval. See Methods for details and analytic equations 1,45 .The time horizon we calculate to align with scenarios peaking at 1.5 ℃, 24 years, is close to the commonly used 20-year time horizon. Using our framework, therefore, the use of a 20-year time horizon can roughly be thought of as implicitly aligning with the 1.5 ℃ temperature goal. On the other hand, the commonly used 100-year time horizon is far beyond the time horizon we calculate that aligns with the 2 ℃ goal (58 years). In fact, if Equation 1 (the linear relationship shown inFigure 2) could be extrapolated to higher temperature peaks, a 100-year time horizon would roughly align with a 2.6 ℃ temperature stabilization. (We address the question of whether a linear relationship is the best choice for fitting and extrapolating this data in the Discussion.)When the 100-year time horizon is applied to a 2 ℃ goal, instead of our proposed 58-year time horizon, methane emission metrics are significantly underestimated relative to those we find that align with the 2 ℃ goal: 28 vs. 42 for GWP (34% lower) and 5.4 vs. 8.7 for GTP (38% lower).
).Our estimates for when the temperature goals of the Paris Agreement will be reached are 2045 
[90% PI: 2028, 2062] for 1.5 ℃ and 2079 [90% PI: 2062, 2095] for 2 ℃. These results 
correspond to time horizons of 24 and 58 years, respectively, from present day (2021). The 100-
year time horizon most commonly used by the IPCC lies far beyond these 24-and 58-year time 
12 
horizons that align with their specific temperature goals. Our estimate for the time of reaching a 
2 ℃ peak temperature, 2079, is 15 years beyond Tanaka et al.'s estimate of 2064 26 , thereby 
revising their emission metrics for methane downwards by roughly 20% for GWP and iGTP 
(integrated metrics) and by 40% for GTP (an endpoint metric). 
Agreement(Table 1). We propose the use of subscripting the temperature goal for an emission metric. For example, GWP2 ℃ is the value for GWP that aligns with scenarios satisfying a 2 ℃ temperature goal and, for methane, is equal to 42 [90% PI:35, 54].
Table 1 :
1Summary of the time horizons and emission metrics for specific temperature goals of the Paris Agreement. 90% prediction intervals are shown in brackets.1.5 ℃ 
2 ℃ 
Time horizon (years) 
24 [7, 41] 
58 [41, 74] 
GWP 
75 [54, 107] 
42 [35, 54] 
GTP 
41 [16, 102] 
9 [7, 16] 
iGTP 
78 [56, 109] 
43 [36, 56] 
CGTP (years) 
1850 [552, 2436] 
2738 [2436, 2929] 
year time horizon, using 2045 as the endpoint time, with its associated GWP1.5 ℃ = 75 and GTP1.5 ℃ = 41.MethodsRaw scenario data were taken from the IPCC SR1.5 Scenario Database44 . These scenarios are "model-based climate change mitigation pathways" used in the IPCC's Special Report on Global Warming of 1.5 °C 51 . Scenarios were categorized by IAM, aggregating those with different versions of the same IAM (such as IMAGE 3.0.1 and IMAGE 3.0.2 being denoted simply 'IMAGE'). Ten IAMs were analyzed: GCAM, IMAGE, MERGE, MESSAGE, REMIND, WITCH, POLES, IEA, C-ROADS, and AIMS. The scenarios analyzed across these IAMs span a wide range of potential future pathways. These include emphases on bioenergy, sustainable development, negative emissions, carbon dioxide removal, and transport. While it is true that not all possible scenarios are compiled here, this set of scenarios forms the basis of the international climate community's current best understanding of realistic potential pathways. Median temperatures were taken from the variable "AR5 climate diagnostics|Temperature|Global Mean|MAGICC6|MED".Emission metrics were calculated using data from the IPCC's Sixth Assessment Report 1,45 . These metrics use the most recent carbon-climate feedback impulse response functions 52 and radiative efficiencies 53 . Details on the calculation of GWP, GTP, iGTP, and CGTP are given below, based on the formulation in Gasser et al.52 . In our work, we use the exact dataset used in AR6 (see Data Availability) for consistency with theirTable 7.15. We incorporate the carbon cycle feedback but make no corrections for the carbon dioxide effects of different methane sources. Equivalent results can be found for specific types of methane (fossil or biogenic) in the Jupyter notebook (see Code Availability).The application of our results in climate policy is complicated by the fact that we consider time-
dependent emission metrics (where, for methane, they strictly increase as the time horizon 
decreases). While such a dynamic approach may have inhibited the early adoption of time-
dependent emission metrics 18 , and may prove politically difficult to implement, we believe that 
dynamic carbon pricing already signals an ability for government agencies to use time-dependent 
emission metrics for policy applications. More research is needed into the political feasibility of 
updating emission metrics over time, such as with the global stocktake every five years from 
2023 onwards. 
Although NDCs and long-term national pledges are currently insufficient to keep warming below 
2 °C, let alone 1.5 °C 48-50 , the time horizon used for emission metrics should nevertheless be 
consistent with that central goal of the Paris Agreement. We therefore support the use of the 20-
year time horizon over the 100-year version, when binary choices between these two must be 
made, due to the better alignment of the former with the temperature goals of the Paris 
Agreement. However, to best align emission metrics with the Paris Agreement, we recommend 
19 
the use of the 24-20 
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Framing and Context. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty. M Allen, In PressAllen, M. et al. Framing and Context. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty. (In Press, 2018).
Accounting for the climate-carbon feedback in emission metrics. T Gasser, Earth Syst. Dyn. 8Gasser, T. et al. Accounting for the climate-carbon feedback in emission metrics. Earth Syst. Dyn. 8, 235-253 (2017).
Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing: Greenhouse Gas Radiative Forcing. M Etminan, G Myhre, E J Highwood, K P Shine, Geophys. Res. Lett. 43Etminan, M., Myhre, G., Highwood, E. J. & Shine, K. P. Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing: Greenhouse Gas Radiative Forcing. Geophys. Res. Lett. 43, 12,614-12,623 (2016).
Global temperature goals should determine the time horizons for greenhouse gas emission metrics
https://arxiv.org/abs/2104.05506
https://arxiv.org/abs/1910.00583
The Physics of Climate Variability and Climate Change
2019-10-01
Ghil; Michael; Lucarini; Valerio
<div><div>
 
 
 
 
 
 
 <p><a href="/pdf/1910.00583">Download PDF</a></p><blockquote>
 Abstract: The climate system is a forced, dissipative, nonlinear, complex and
heterogeneous system that is out of thermodynamic equilibrium. The system
exhibits natural variability on many scales of motion, in time as well as
space, and it is subject to various external forcings, natural as well as
anthropogenic. This paper reviews the observational evidence on climate
phenomena and the governing equations of planetary-scale flow, as well as
presenting the key concept of a hierarchy of models as used in the climate
sciences. Recent advances in the application of dynamical systems theory, on
the one hand, and of nonequilibrium statistical physics, on the other, are
brought together for the first time and shown to complement each other in
helping understand and predict the system's behavior. These complementary
points of view permit a self-consistent handling of subgrid-scale phenomena as
stochastic processes, as well as a unified handling of natural climate
variability and forced climate change, along with a treatment of the crucial
issues of climate sensitivity, response, and predictability.
 </blockquote>
 
 
 </div><div>
 <h2>Submission history</h2><p> From: Valerio Lucarini [<a href="/show-email/9fad0e77/1910.00583">view email</a>]
 <br />
 <strong><a href="/abs/1910.00583v1">[v1]</a></strong>
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Wed, 26 Feb 2020 15:45:48 UTC (11,541 KB)<br /></p></div>||||I|||| Skip to main content
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 arXiv:1910.00583 (physics)
 [Submitted on 1 Oct 2019 (v1), last revised 26 Feb 2020 (this version, v2)]

 Title: The Physics of Climate Variability and Climate Change

 Authors: Michael Ghil, Valerio Lucarini
 Download PDF
 Abstract: The climate system is a forced, dissipative, nonlinear, complex and heterogeneous system that is out of thermodynamic equilibrium. The system exhibits natural variability on many scales of motion, in time as well as space, and it is subject to various external forcings, natural as well as anthropogenic. This paper reviews the observational evidence on climate phenomena and the governing equations of planetary-scale flow, as well as presenting the key concept of a hierarchy of models as used in the climate sciences. Recent advances in the application of dynamical systems theory, on the one hand, and of nonequilibrium statistical physics, on the other, are brought together for the first time and shown to complement each other in helping understand and predict the system's behavior. These complementary points of view permit a self-consistent handling of subgrid-scale phenomena as stochastic processes, as well as a unified handling of natural climate variability and forced climate change, along with a treatment of the crucial issues of climate sensitivity, response, and predictability.
 Comments: 86 pages, 52 figures   
 Subjects: Atmospheric and Oceanic Physics (physics.ao-ph) ; Statistical Mechanics (cond-mat.stat-mech); Chaotic Dynamics (nlin.CD); Fluid Dynamics (physics.flu-dyn); Geophysics (physics.geo-ph)
 Cite as: arXiv:1910.00583 [physics.ao-ph]   
 (or arXiv:1910.00583v2 [physics.ao-ph] for this version)  
 https://doi.org/10.48550/arXiv.1910.00583   
 Focus to learn more   
 arXiv-issued DOI via DataCite   
 Journal reference: Rev. Mod. Phys. 92, 35002 (2020)   
 https://doi.org/10.1103/RevModPhys.92.035002   
 Related DOI: Focus to learn more   
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 From: Valerio Lucarini [view email]
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 [v2] Wed, 26 Feb 2020 15:45:48 UTC (11,541 KB)
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The Physics of Climate Variability and Climate Change
Michael Ghil 
Valerio Lucarini 
Geosciences Department
Department of Atmospheric and Oceanic Sciences
Laboratoire de Météorologie Dynamique (CNRS and IPSL)
Ecole Normale Supérieure and PSL Research University
F-75231, Cedex 05ParisFrance
Department of Mathematics and Statistics
University of California
90095-1565Los AngelesCAUSA
Centre for the Mathematics of Planet Earth
University of Reading
RG66AXReadingUK
CEN -Institute of Meteorology
University of Reading
RG66AXReadingUK
University of Hamburg
20144HamburgGermany
The Physics of Climate Variability and Climate Change
(Dated: October 3, 2019)numbers: 9205Df9230Bc9260Ry9270Np0570Ln0545?a CONTENTS
The climate system is a forced, dissipative, nonlinear, complex and heterogeneous system that is out of thermodynamic equilibrium. The system exhibits natural variability on many scales of motion, in time as well as space, and it is subject to various external forcings, natural as well as anthropogenic. This paper reviews the observational evidence on climate phenomena and the governing equations of planetary-scale flow, as well as presenting the key concept of a hierarchy of models as used in the climate sciences. Recent advances in the application of dynamical systems theory, on the one hand, and of nonequilibrium statistical physics, on the other, are brought together for the first time and shown to complement each other in helping understand and predict the system's behavior. These complementary points of view permit a self-consistent handling of subgrid-scale phenomena as stochastic processes, as well as a unified handling of natural climate variability and forced climate change, along with a treatment of the crucial issues of climate sensitivity, response, and predictability.
I. Introduction and
The climate system is a forced, dissipative, chaotic system that is out of equilibrium and whose complex natural variability arises from the interplay of positive and negative feedbacks, instabilities and saturation mechanisms. These processes span a broad range of spatial and temporal scales and include many chemical species and all physical phases. The system's heterogeneous phenomenology includes the mycrophysics of clouds, cloud-radiation interactions, atmospheric and oceanic boundary layers, and several scales of turbulence (Ghil, 2019); it evolves, furthermore, under the action of large-scale agents that drive and modulate its evolution, mainly differential solar heating and the Earth's rotation and gravitation.
As is often the case, the complexity of the physics is interwoven with the chaotic character of the dynamics. Moreover, the climate system's large natural variability on different time scales is strongly affected by relatively small changes in the forcing, anthropogenic as well as natural (Ghil and Childress, 1987;Lucarini et al., 2014;Peixoto and Oort, 1992) On the macroscopic level, climate is driven by differences in the absorption of solar radiation throughout the depth of the atmosphere, as well as in a narrow surface layer of the ocean and of the soil; the system's actual governing equations are given in Sect. II.C below. The prevalence of absorption at the surface and in the atmosphere's lower levels leads, through several processes, to compensating vertical energy fluxes -most notably, fluxes of infrared radiation throughout the atmosphere and convective motions in the troposphere; see Fig. 1.
More solar radiation is absorbed in the low latitudes, leading to horizontal energy fluxes as well. The atmosphere's large-scale circulation is, to first order, a result of these horizontal and vertical fluxes arising from the gradients in solar radiation absorption, in which the hydrological cycle plays a key role as well. The ocean circulation, in turn, is set into motion by surface or nearsurface exchanges of mass, momentum and energy with the atmosphere: the so-called wind-driven component of the circulation is due mainly to the wind stress and the thermohaline one is due mainly to buoyancy fluxes (Dijkstra, 2005;Dijkstra and Ghil, 2005). The coupled atmospheric and oceanic circulation reduces the temperature differences between tropics and polar regions with respect to that on an otherwise similar planet with no horizontal energy transfers (Held, 2001;Lorenz, 1967;Lucarini and Ragone, 2011;Peixoto and Oort, 1992). At steady state, the convergence of enthalpy transported by the atmosphere and by the oceans compensates the radiative imbalance at the top of the atmosphere; see Fig. 2.
The classical theory of the general circulation of the atmosphere (Lorenz, 1967) describes in further detail how the mechanisms of energy generation, conversion and dissipation produce the observed circulation, which deviates substantially from the highly idealized, zonally symmetric picture sketched so far. According to Lorenz (1955), atmospheric large-scale flows result from the conversion of available potential energy -which is produced by the atmosphere's differential heating -into kinetic energy, and the Lorenz (1967) energy cycle is completed by energy cascading to smaller scales to be eventually dissipated. McWilliams (2019) provides an up-to-date criticism of and further perspective on this theory.
Overall, the climate system can be seen as a thermal engine capable of transforming radiative heat into mechanical energy with a given, highly suboptimal efficiency, given the many irreversible processes that make it less than ideal (Kleidon and Lorenz, 2005;Lucarini, 2009b;Lucarini et al., 2014;Pauluis and Held, 2002). This conversion occurs through genuinely threedimensional (3-D) baroclinic instabilities (Charney, 1947;Eady, 1949) that are triggered by large temperature gradients and would break zonal symmetry even on a socalled aqua-planet, with no topographic or thermal asymmetries at its surface. These instabilities give rise to a negative feedback, as they tend to reduce the temperature gradients they feed upon by favoring the mixing between masses of fluids at different temperatures.
Note that, while these baroclinic and other large-scale instabilities do act as negative feedbacks, they cannot be treated as diffusive, Onsager (1931)-like processes. Faced with the Earth system's complexity discussed herein and illustrated by Fig. 3, the closure of the coupled thermodynamical equations governing the general circulation of the atmosphere and oceans would provide a self-consistent theory of climate. Such a theory should able to connect instabilities and large-scale stabilizing processes on longer spatial and temporal scales, and to predict its response to a variety of forcings, both natural and anthropogenic (Ghil, 2015;Ghil and Childress, 1987;Lucarini, 2009b;Lucarini et al., 2014). This goal is being actively pursued but is still out of reach at the time of this writing (e.g., Ghil, 2019, and references); see also Sects. IV and V herein. The observed persistence of spatial gradients in chemical concentrations and temperatures, as well as the associated mass and energy fluxes, are a basic signature of the climate system's intrinsic disequilibrium. Figure 3 emphasizes, moreover, that the fluid and the solid parts of the Earth system are coupled on even longer time scales, on which geochemical processes become of paramount importance (Kleidon, 2009;Rothman et al., 2003). In contrast, closed, isolated systems cannot maintain disequilibrium and have to evolve towards homogenous thermodynamical equilibrium, as a result of the second law of thermodynamics (Prigogine, 1961).
Studying the climate system's entropy budget provides a good global perspective on this system. The Earth as a whole absorbs shortwave radiation carried by lowentropy solar photons at T Sun 6000 K and emits infrared radiation to space via high-entropy thermal pho-  (1985)(1986)(1987)(1988)(1989). (b) Inferred meridional enthalpy transport from ERBE observations (solid line) and estimate of the atmospheric enthalpy transport from two reanalysis datasets (ECMWF and NCEP). Reproduced from Trenberth and Caron (2001) c American Meteorological Society; used with permission.
tons at T Earth 255 K (Lucarini et al., 2014;Peixoto and Oort, 1992). Besides the viscous dissipation of kinetic energy, many other irreversible processes-such as turbulent diffusion of heat and chemical species, irreversible phase transitions associated with various hydrological processes, and chemical reactions involved in the biogeochemistry of the planet-contribute to the total material entropy production (Goody, 2000;Kleidon, 2009). These and other important processes appear in the schematic diagram of Fig. 3. In general, in a forced dissipative system, entropy is continuously produced by irreversible processes and at steady state, this production is balanced by a net outgoing flux of entropy at the system's boundaries (de Groot and Mazur, 1984;Prigogine, 1961); in the case at hand, this flux leaves mainly through the top of the atmosphere (Goody, 2000;Lucarini, 2009b). Thus, on average, the climate system's entropy budget is balanced, just like its energy budget.
The phenomenology of the climate system is commonly approached by focusing on distinct and complementary aspects that include:
• wave-like features such as Rossby waves or equatorially trapped waves (e.g., Gill, 1982), which play a key role in the transport of energy, momentum, and water vapor, as well as in the study of atmospheric, oceanic and coupled-system predictability;
• particle-like features such as hurricanes, extratropical cyclones or oceanic vortices (e.g., McWilliams, 2019;Salmon, 1998), which strongly affect the local properties of the climate system and of its subsystems and subdomains;
• turbulent cascades, which are of crucial importance in the development of large eddies through the mechanism of geostrophic turbulence (Charney, 1971), as well as in mixing and dissipation within the planetary boundary layer (Zilitinkevich, 1975).
Each of these points of view is useful and they do overlap and complement each other Lucarini et al., 2014) but neither by itself provides a comprehensive understanding of the properties of the climate system. It is a key objective of this review paper to provide the interested and motivated reader with the tools for achieving such a comprehensive understanding with predictive potential.
FIG. 3: Schematic diagram representing forcings, dissipative and mixing processes, gradients of temperature and chemical species, and coupling mechanisms across the Earth system.
Bluish (reddish) colors refer to the fluid (solid) Earth. Reproduced from Kleidon (2010) with permission.
While much progress has been achieved (Ghil, 2019), understanding and predicting the dynamics of the cli-mate system faces -on top of all the difficulties that are intrinsic to any nonlinear, complex system out of equilibrium -the following additional obstacles that make it especially hard to grasp fully:
• the presence of well-defined subsystems -the atmosphere, the oceans, the cryosphere -characterized by distinct physical and chemical properties and widely differing time and space scales;
• the complex processes coupling these subsystems;
• the continuously varying set of forcings that result from fluctuations in the incoming solar radiation and in the processes, both natural and anthropogenic, that alter the atmospheric composition;
• the lack of scale separation between different processes, which requires a profound revision of the standard methods for model reduction, and calls for unavoidably complex parametrization of subgridscale processes in numerical models;
• the lack of detailed, homogeneous, high-resolution and long-lasting observations of climatic fields that leads to the need for combining direct and indirect measurements when trying to reconstruct past climate states preceding the industrial era; and, last but not least,
• the fact that we only have one realization of the processes that give rise to climate evolution in time.
For all these reasons, it is far from trivial to separate the climate system's response to various forcings from its natural variability in the absence of time-dependent forcings. In simpler words, and as noted already by Lorenz (1979), it is hard to separate forced and free climatic fluctuations (Lucarini et al., 2014Lucarini and Sarno, 2011). This difficulty is a major stumbling block on the road to a unified theory of climate evolution (Ghil, 2015(Ghil, , 2017 but some promising ideas for overcoming it are emerging and will be addressed in Sects. IV and V herein; see also Ghil (2019).
B. More Than "Just" Science
The Intergovernmental Panel on Climate Change
Besides the strictly scientific aspects of climate research, much of the recent interest in it has been driven by the accumulated observational and modeling evidence on the ways humans influence the climate system. In order to review and coordinate the research activities carried out by the scientific community in this respect, the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO) established in 1988 the Intergovernmental Panel on Climate Change (IPCC); its assessments reports (ARs) are issued every 4-6 years. By compiling systematic reviews of the scientific literature relevant to climate change, the ARs summarize the scientific progress, the open questions, and the bottlenecks regarding our ability to observe, model, understand and predict the climate system's evolution.
More specifically, it is the IPCC Working Group I that focuses on the physical basis of climate change; see IPCC (2001, 2007, 2014a) for the three latest reports -AR3, AR4 and AR5 -in this area. Working Groups II and III are responsible for the reports that cover the advances in the interdisciplinary areas of adapting to climate change and of mitigating its impacts; see IPCC (2014b,c) for the contributions of Working Groups II and III, respectively, to AR5. AR6 is currently in preparation, cf. https: //www.ipcc.ch/assessment-report/ar6/.
Moreover, the IPCC supports the preparation of special reports on themes which are of interest across two of the working groups, e.g. climatic extremes (IPCC, 2012), or across all three of them. The IPCC experience and working group structure is being replicated for addressing climate change at regional level, as in the case for the Hindu Kush Himalaya region, called sometimes the "third pole" (Wester et al., 2019).
The IPCC reports are based on the best science available and are policy-relevant but not policy-prescriptive. Their multi-stage review is supposed to guarantee neutrality but they are still inherently official, UNsanctioned documents and have to bear the imprimatur of the IPCC's 195 member countries. Their release thus leads to considerable and often adversarial debates involving a variety of stakeholders from science, politics, civil society, and business; they also affect media production, cinema, videogames, and art at large, and are reflected more and more by them.
Climate change has thus become an increasingly central topic of discussion in the public arena, involving all levels of decision making from local through regional and on to global. In recent years, climate services have emerged as a new area at the intersection of science, technology, policy making and business. They emphasize tools to enable climate change adaptation and mitigation strategies, and have benefited from large public investments like the European Union's Copernicus Programme, https://climate.copernicus.eu.
The lack of substantial progress in national governments' and international bodies' tackling climate change has recently led to the rapid growth of global, youngpeople-driven grassroot movements like Extinction Rebellion, https://rebellion.earth/, and FridaysFor-Future, https://www.fridaysforfuture.org. Some countries, like the United Kingdom, have declared a state of climate emergency, cf. https://www.bbc.co.uk/ news/uk-politics-48126677, and some influential media outlets have started to use the expression climate crisis instead of climate change, https://tinyurl.com/ y2v2jwzy. While of great consequence, the present paper does not further dwell on such socio-economic and political issues. Mann et al. (1999, Fig. 3(a)) produced a temperature reconstruction from proxy data, cf. Sect. II.A, for the last 1000 yr, shown as part of the blue curve in Fig. 4. This curve was arguably the most striking, and hence controversial, scientific result contained in AR3 (IPCC, 2001) and it was dubbed, for obvious reasons, the hockey stick. The AR3 report combined into one figure - Fig. 1(b) of the Summary for Policy Makers (SPM) -this blue curve and the red curve shown in Fig. 4 here, which was based on instrumental data over the last century-and-one-half; see Fig. 1(a) of the SPM. This superposition purported to demonstrate that recent temperature increase was unprecedented over the last two millennia, in both values attained and rate of change. Figure 1(b) of the AR3's SPM received an enormous deal of attention from the social and political forces wishing to underscore the urgency of tackling anthropogenic climate change. For the opposite reasons, the paper and its authors were the subject of intense political and judicial scrutiny and attack by other actors in the controversy, claiming that the paper was both politically motivated and scientifically unsound. McIntyre and McKitrick (2005), among others, strongly criticized the results of Mann et al. (1999), claiming that the statistical procedures used for smoothly combining the diverse proxy records used -including tree rings, coral records, ice cores, and long historical records, with their diverse sources and ranges of uncertainty -into a single multiproxy record, and the latter with instrumental records, were marred by biases and underestimation of the actual statistical uncertainty. Later papers criticized in turn the statistical methods of McIntyre and McKitrick (2005) and confirmed the overall correctness of the hockey stick reconstruction (e.g., Huybers, 2005;Mann et al., 2008;PAGES, 2013;Taricco et al., 2009). NRC (2006) provided an excellent review of the state of our knowledge concerning the last two millennia of climate change and variability. This controversy included the notorious "Climategate" incident, in which data hacked from the computer of a well-known UK scientist were used to support the thesis that scientific misconduct and data falsification had been routinely used to support the hockey stick reconstruction. These claims were later dismissed but they did lead to an important change in the relationship between the climate sciences, society, and politics, and in the way climate scientists interact among themselves and with the public. In certain countries -e.g., the UK and  (PAGES, 2013), while the red curve shows the global mean temperature, according to HadCRUT4 data from 1850 onwards; the original "hockey stick" of Mann et al. (1999) is plotted in dark blue and its uncertainty range in light blue. Graph by Klaus Bitterman.
Hockey Stick Controversy and Climate Blogs
the USA -stringent rules have been imposed to ascertain that scientists working in governmental institutions have to publicly reveal the data they use in the preparation of scientific work, if formally requested to do so. Faced with the confusion generated by the polemics, several leading scientists started blogse.g., www.realclimate.org, www.ClimateAudit.org, www.climate-lab-book.ac.uk, and judithcurry.com -in which scientific literature and key ideas are presented for a broader audience and debated outside the traditional media of peer-reviewed journals or public events, such as conferences and workshops. Most contributions are of high quality, but sometimes arguments appear to sink to the level of a bitter strife between those in favor and those against the reality of climate change and of the anthropogenic contribution to its causes.
C. This Review
The main purpose of this review paper is to bring together a substantial body of literature published over the last few decades in the geosciences, as well as in mathematical and physical journals, and provide a comprehensive picture of climate dynamics. Moreover, this picture should appeal to a readership of physicists, and help stimulate interdisciplinary research activities.
For decades meteorology and oceanography, on the one side, and physics, on the other, have had a relatively low level of interaction, with by-and-large separate scientific gatherings and scholarly journals. Recent developments in dynamical systems theory, both finite-and infinite-dimensional, as well as in random processes and statistical mechanics, have created a common language that makes it possible, at this time, to achieve a higher level of communication and mutual stimulation.
The key aspects of the field that we want to tackle here are the natural variability of the climate system, the deterministic and random processes that contribute to this variability, its response to perturbations, and the relations between internal and external causes of observed changes in the system. Moreover, we will present tools for the study of critical transitions in the climate system, which can help understand and possibly predict the potential for catastrophic climate change.
In Sect. II, we provide an overview for non-specialists of the way climate researchers collect and process information on the state of the atmosphere, the land surface, and the oceans. Next, the relevant conservation laws and the equations that govern climatic processes are introduced.
An important characteristic of the climate system is the already mentioned coexistence and nonlinear interaction of multiple subsystems, processes and scales of motion. This state of affairs entails two important consequences that are also addressed in Sect. II. First is the need for scale-dependent filtering: on the positive side, this filtering leads to simplified equations; on the negative one, it calls for so-called parametrization of unresolved processes, i.e. for the representation of subgridscale processes in terms of the resolved, larger-scale ones. Second is the fact that no single model can encompass all the subsystems, processes and scales, hence the need for resorting to a hierarchy of models. The section ends by discussing present-day standard protocols for climate modeling and the associated problem of evaluating the models' performance in a coherent way.
Section III treats in greater depth climate variability. We describe the most important modes of climate variability and provide an overview of the coexistence of several equilibria in the climate system, and of their dependence on parameter values. While the study of bifurcations and exchange of stability in the climate system goes back to the work of E. N. Lorenz, H. M. Stommel andG. Veronis in the 1960s (e.g., Dijkstra, 2013;Ghil, 2019;Ghil and Childress, 1987), a strikingly broadened interest in these matters has been stimulated by the borrowing from the social sciences of the term "tipping points" (Gladwell, 2000;Lenton et al., 2008).
Proceeding beyond multiple equilibria, we show next how complex processes give rise to the system's internal variability by successive instabilities setting in, competing and eventually leading to the quintessentially chaotic nature of the evolution of climate. This section concludes by addressing the need for using random processes to model the faster and smaller scales of motion in multiscale systems, and by discussing markovian and nonmarkovian approximations for the representation of the neglected degrees of freedom. We will also discuss top-down vs. data-driven approaches.
Section IV delves into the analysis of climate response. The response to external forcing of a physico-chemical system out of equilibrium is the overarching concept we use in clarifying the mathematical and physical bases of climate change. We critically appraise climate models as numerical laboratories and review ways to test their skill at simulating past and present changes, as well as at predicting future ones. The classical concept of equilibrium climate sensitivity is critically presented first, and we discuss its merits and limitations.
We present next the key concepts and methods of nonautonomous and random dynamical systems, as a framework for the unified understanding of intrinsic climate variability and forced climate change, and emphasize the key role of pullback attractors in this framework. These concepts have been introduced only quite recently into the climate sciences, and we show how pullback attractors and the associated dynamical-systems machinery provide an excellent setting for studying the statistical mechanics of the climate system as an open system.
This system is subject to variations in the forcing and in its boundary conditions on all time scales. Such variations include, on different time scales, the incoming solar radiation, the position of the continents, and the sources of aerosols and greenhouse gases. We further introduce time-dependent invariant measures on a parameterdependent pullback attractor, and the Wasserstein distance between such measures, as the main ingredients for a more geometrical treatment of climate sensitivity in the presence of large and sudden changes in the forcings.
We then outline, in the context of non-equilibrium statistical mechanics, Ruelle's response theory as an efficient and flexible tool for calculating climate response to small and moderate natural and anthropogenic forcings, and reconstruct the properties of the pullback attractor from a suitably defined reference background state. The response of a system near a tipping point is studied, and we emphasize the link between properties of the autocorrelation of the unperturbed system and its vicinity to the critical transition, along with their implications in terms of tell-tale properties of associated time series.
Section V is devoted to discussing multistability in the climate system, and the critical transitions that occur in the vicinity of tipping points in systems possessing multiple steady states. The corresponding methodology is then applied to the transitions between a fully frozen, so-called snowball state of our planet and its warmer states. These transitions have played a crucial role in modulating the appearance of complex life forms. We introduce the concept of an edge state, a dynamical object that has helped explain bistability in fluid mechanical systems, and argue that such states will also yield a more complete picture of tipping points in the climatic context. Finally, we present an example of a more exotic chaos-to-chaos critical transition that occurs in a delay differential equation model for the Tropical Pacific.
In Section VI, we briefly summarize this review paper's main ideas and introduce complementary research lines that have not been discussed herein. Appendix A contains two tables of scientific and institutional acronyms used throughout the paper.
II. A BRIEF INTRODUCTION TO CLIMATE DYNAMICS
A. Climate Observations: Direct and Indirect
A fundamental difficulty in the climate sciences arises from humanity's insufficient ability to collect data of standardized quality, with sufficient spatial detail and of sufficient temporal coverage. Instrumental data sets have substantial issues of both synchronic and diachronic coherence. Moreover, such data sets only extend, at best, for about one-to-two centuries into the past. In this section, we cover first instrumental data sets and then socalled historical and proxy data sets, which use indirect evidence on the value of meteorological observables before the industrial era.
Instrumental Data and Reanalyses
Since the establishment of the first meteorological stations in Europe and in North America in the 19 th century, the extent and quality of the network of observations and the technology supporting the collection and storage of data have rapidly evolved. Still, at any given time, the spatial density of data changes dramatically across the globe, with much sparser observations over the oceans and over land areas characterized by low population density or a low degree of technological development (e.g., Ghil and Malanotte-Rizzoli, 1991, Fig. 1 Starting in the late 1960s, polar orbiting and geostationary satellites have led to a revolution in collecting weather, land surface and ocean surface data. Spaceborne instruments are now remotely sensing many climatic variables from the most remote areas of the globe; for instance, they measure the overall intensity and spectral features of emitted infrared and reflected visible and ultraviolet radiation, and complex algorithms relate their raw measurements to the actual properties of the atmosphere, such as temperature and cloud cover. Figure 5 represents schematically the evolution of the observational network for climatic data, while Fig. 6 portrays the instruments that today comprise the Global Observing System of the Word Meteorological Organization (WMO), the United Nations agency that coordinates the collection and quality check of weather and climate data over the entire globe. Since the early 20 th century, the daily measurements have grown in number by many orders of magnitude and cover now more regularly the entire globe, even though large swaths of the Earth still feature relatively sparse observations. Figures 7 and 8 illustrate the coverage and variety of the observing systems available at present to individual researchers and practitioners, as well as to environmental and civil-protection agencies. Note that the so-called conventional network of ground-based weather stations and related observations has evolved since the Global Weather Experiment in the late 1970s, cf. Fig. 1 in Bengtsson et al. (1981) but only marginally so: it is the remote-sensing observations that have increased tremendously in number, variety and quality.
The number and quality of oceanographic observations was several orders of magnitude smaller than that of meteorological ones in the 1980s (Ghil and Malanotte-Rizzoli, 1991;Munk and Wunsch, 1982). Here also the advent of space-borne altimetry for sea surface heights (SSHs), scatterometry for surface winds, and other remote-sensing methods has revolutionized the field (e.g., Robinson, 2010). This number, however, is still smaller by at least one order of magnitude than that of atmospheric observations since, as pointed out by Munk and Wunsch (1982), the oceans' interior is not permeable to exploration by electromagnetic waves. This is a fundamental barrier hindering our ability to directly observe the deep ocean.
Observational data for the atmosphere and oceans are at any rate sparse, irregular, and of different degrees of accuracy, while in many applications one has to obtain the best estimate, with known error bars, of the state of the atmosphere or oceans at a given time and with a given, uniform spatial resolution. More often than not, this estimate also needs to include meteorological, oceanographic or coupled-system variables, such as vertical wind velocity or surface heat fluxes, that can only be observed very poorly or not at all.
The very active field of data assimilation has developed to bridge the gap between the observations that are, typically, discrete in both time and space, and the continuum of the atmospheric and oceanic fields. Data assimilation -as distinct from polynomial interpolation, statistical regression or the inverse methods used in solid-earth geophysics -first arose in the late 1960s from the needs of numerical weather prediction (NWP), on the one hand, and the appearance of time-continuous data streams from satellites, on the other (Charney et al., 1969;Ghil et al., 1979). NWP is, essentially an initial-value problem for the partial differential equations (PDEs) governing largescale atmospheric flows (Richardson, 1922) that needed a complete and accurate initial state every 12 or 24 hours.
Data assimilation combines partial and inaccurate observational data with a dynamic model, based on physical laws, that governs the evolution of the continuous medium under study in order to provide the best estimates of the state of the medium. This model also is subject to errors, due to incomplete knowledge of the smaller-scale processes, numerical discretization errors and other factors. Given these two sources of information, observational and physico-mathematical, there are three types of problems that can be formulated and solved, given measurements over a time interval {t 0 ≤ t ≤ t 1 }: filtering, smoothing and prediction; see Fig. 9.
Filtering involves obtaining a best-possible estimate of the state X(t) at t = t 1 , smoothing at all times t 0 ≤ t ≤ t 1 , and prediction at times t > t 1 . Filtering and prediction are typically used in NWP, and can be considered as the generation of short "video loops," while smoothing is typically used in climate studies, and resembles the generation of long "feature movies" (Ghil and Malanotte-Rizzoli, 1991). Figure 10 illustrates a so-called forecast-assimilation cycle, as used originally in NWP: at evenly spaced, preselected times {t k : k = 1, 2, . . . , K}, one obtains an analysis of the state X(t k ) by combining the observations over some interval preceding the time t k with the forecast from the previous state X(t k−1 ) (Bengtsson et al., 1981;Kalnay, 2003). Many variations on this relatively simple scheme have been introduced in adapting it to oceanographic data (Ghil and Malanotte-Rizzoli, 1991, and ref- erences therein) and to space plasmas (e.g., Merkin et al., 2016) or to actually using the time-continuous stream of remote-sensing data (Ghil et al., 1979).
Analyses are routinely used for numerical weather forecasts and take advantage of the continuous improvements of models and observations. But climate studies require data of consistent spatial resolution and accuracy over long time intervals, over which an operational NWP center might have changed its numerical model or its data assimilation scheme, as well as its raw data sources. To satisfy this need, several NWP centers have started in the 1990s to produce so-called reanalyses that use the archived data over multidecadal time intervals, typically since World War II, as well as the best model and data assimilation method available at the time of the reanalysis project. For obvious reasons of computational cost, reanalyses are often run at a lower spatial resolution than the latest version in operational use at that time.
Some leading examples of such diachronically coherent reanalyses for the atmosphere are those produced by the European Centre for Medium-range Weather Forecasts (ECMWF: Dee and Coauthors, 2011), the NCEP-NCAR Reanalysis produced in collaboration by the U.S. National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR: Kistler and Coauthors, 2001), and the JRA-25 reanalysis produced by the Japan Meteorological Agency (JMA: Onogi and Coauthors, 2007). While these reanalyses agree pretty well for fields that are relatively well observed, such as the geopotential field (see Sect. II.C herein) over the continents of the Northern Hemisphere, substantial differences persist in their fields over the Southern Hemisphere or those that are poorly observed or not at all (Dell'Aquila et al., 2005, 2007Kharin et al., 2005;Kim and Kim, 2013;Marques et al., 2010Marques et al., , 2009).
Recently, Compo et al. (2011) produced a centennial reanalysis from 1871 to the present by assimilating only surface pressure reports and using observed monthly sea-FIG. 9: Schematic diagram of filtering (F), smoothing (S) and prediction (P); green filled circles are observations. Based on Wiener (1949).
FIG. 10: Schematic diagram of a forecast-assimilation cycle that is used for constructing the best estimates of the state of the atmosphere, oceans or both through the procedure of data assimilation: observational data are dynamically interpolated using the a meteorological, oceanographic or coupled model to yield the analysis products. The red arrow corresponds to a longer forecast, made only from time to time. Greater detail for the case of operational weather prediction appears in Ghil (1989, Fig . 1).
surface temperature and sea-ice distributions as boundary conditions, while Poli et al. (2016) provided a similar product for the time interval 1899-2010, where the surface pressure and the surface winds were assimilated. These enterprises are motivated by the need to provide a benchmark for testing the performance of climate models for the late 19 th and the 20 th century. A similar need has become manifest for the oceans: on the one hand, several much more substantial data sources have become available through remote sensing, prompting detailed modeling of the oceans; on the other, the study of the climate as a coupled system requires a more uniform data set, albeit less accurate than for the atmosphere. Thus the equivalent of a reanalysis for the oceans had to be produced, in spite of the fact that the equivalent of NWP for the oceans did not exist. A good example of a diachronically coherent data set for the global ocean is the Simple Ocean Data Assimilation (SODA: Carton and Giese, 2008). More recently, the community of ocean modelers and observation specialists has delivered several ocean reanalyses able to provide a robust estimate of the state of the ocean (Balmaseda et al., 2015;Lee et al., 2009).
Finally, by relying on recent advances in numerical methods and in the increased availability of observational data, as well as of increased performance of computing and storage capabilities, coupled atmosphereocean data assimilation systems have been constructed (e.g., Penny and Hamill, 2017). Vannitsem and Lucarini (2016) provide a theoretical rationale for the need of coupled data assimilation schemes to be able to deal effectively with the climate system's multiscale instabilities. These coupled systems play a key role in efforts to produce seamless weather, subseasonal-to-seasonal (S2S), seasonal, and interannual climate predictions (Palmer et al., 2008;Robertson and Vitart, 2018); they have already been used for constructing climate reanalyses (e.g., Karspeck et al., 2018;Laloyaux et al., 2018).
Proxy Data
As mentioned already repeatedly, and discussed in greater detail in Sect. II.B below, climate variability covers a vast range of time scales, and the information we can garner from the instrumental record is limited to the last century or two. Even so-called historical records only extend to the few millennia of a literate humanity (Lamb, 1972). In order to extend our reach beyond this eyeblink of the planet's life, it is necessary to resort to indirect measures of past climatic conditions able to inform us on its state thousands or even millions of years ago.
Climate proxies are physical, chemical or biological characteristics of the past that have been preserved in various natural repositories and that can be correlated with the local or global state of the atmosphere, oceans or cryosphere at that time. Paleoclimatologists and geochemists currently take into consideration multiple proxy records, including coral records (Boiseau et al., 1999;Karamperidou and Coauthors, 2015) and tree rings (Esper et al., 2002) for the last few millennia, as well as marine-sediment (Duplessy and Shackleton, 1985;Taricco et al., 2009) and ice-core (Andersen and Coauthors, 2004;Jouzel et al., 1991) records for the last two million years of Earth history, the Quaternary. Glaciation cycles, i.e. an alternation of warmer and colder climatic episodes, dominated the latter era. The chemical and physical characteristics, along with the accumulation rate of the samples, require suitable calibration and dating, and are used thereafter to infer some of the properties of the climate of the past (e.g., Cronin, 2010;Ghil, 1994, and references therein).
Proxies differ enormously in terms of precision, uncertainties in the values and dating, and spatio-temporal extent, nor do they cover homogeneously the Earth. It is common practice to combine and cross-check many different sources of data to have a more precise picture of the past (Cronin, 2010;Imbrie and Imbrie, 1986). Recently, data assimilation methods have started to be applied to this problem as well, using simple models and addressing the dating uncertainties in particular (e.g., Roques et al., 2014). Combining the instrumental and proxy datawith their very different characteristics of resolution and accuracy -is a complex, and sometimes controversial, exercise in applied statistics. An important example is that of estimating the globally averaged surface air temperature record well before the industrial era, cf. Fig. 4 and previous discussion on the hockey stick controversy in Sect. I.B.2.
B. Climate Variability on Multiple Time Scales
The presence of multiple scales of motions -in space and in time -in the climate system can be summarized through so-called Stommel diagrams. Figure 11a presents the original Stommel (1963) diagram, in which a somewhat idealized spectral density associated with the oceans' variability was plotted in logarithmic space and time scales, while identifying characteristic oceanic phenomena whose variance exceeds the background level. Stommel diagrams describe the spatial-temporal variability in a climatic subdomain by associating different, phenomenologically well-defined dynamical features -such as cyclones and long waves in the atmosphere or meanders and eddies in the ocean -with specific ranges of scales; they emphasize relationships between spatial and temporal scales of motion. Usually, specific dynamical features are associated with specific approximate balances governing the properties of the evolution equations of the geophysical fluids, cf. Sect. II.C.2.
In Figs. 11b and 11c, a qualitative Stommel diagram portrays today's estimates of the main range of spatial and temporal scales in which variability is observed for the oceans and the atmosphere, respectively. One immediately notices that larger spatial scales are typically associated with longer temporal scales, in both the atmosphere and oceans. The two plots clearly show that, for both geophysical fluids, a "diagonal" of high spectral density in the wavelength-frequency plane predominates. As the diagonal reaches the size of the planet in space, the variability can no longer maintain this proportionality of scales and it keeps increasing in time scales, which are not bounded, while the spatial ones are.
Both extratropical cyclones in the atmosphere and eddies in the ocean are due to baroclinic instability, but their characteristic spatial extent in the ocean is ten times smaller and their characteristic duration is one hundred times longer than in the atmosphere. In Fig. 11c, three important meteorological scales are explicitly mentioned: the microscale (small-scale turbulence), the mesoscale (e.g. thunderstorms and frontal structures), and the synoptic scale (e.g. extratropical cyclones).
Given the different dynamical variability ranges in space and time, different classes of numerical models, based on different dynamical balances, can simulate explicitly only one or a few such dynamical ranges. The standard way of modeling processes associated with a particular range of scales is to "freeze" processes on slower time scales or to prescribe their slow, quasiadiabatic effect on the variability being modeled, in order to handle the processes that are too large or too slow in scale to be included in the model.
As for the faster processes, these are "parametrized," i.e. one attempts to model their net effect on the variability of interest. Such parametrizations have been, until fairly recently, purely deterministic but have started over the last decade or so to be increasingly stochastic (Palmer and Williams, 2009, and references therein). We will discuss the mathematics behind parametrizations and provide a few examples in Sect. III.G.
To summarize, there are about 15 orders of magnitude in space and in time that are active in the climate system, from continental drift at millions of years to cloud processes at hours and shorter. The presence of such a wide range of scales in the system provides a formidable challenge for its direct numerical simulation. There is no numerical model that can include all the processes that are active on the various spatial and temporal scales and can run for 10 7 simulated years. Ockham's razor and its successors, including Poincaré's parsimony principle (Poincaré, 1902), suggest that, if we had one, it wouldn't necessarily be such a good tool for developing scientific insight, rather than just a gigantic simulator not helping scientists to distinguish the forest from the trees.
A Survey of Climatic Time Scales
Combining proxy and instrumental data allows one to gather information not only on the mean state of the climate system, but also on its variability on many different scales. An artist's rendering of climate variability on all time scales is provided in Fig. 12a. The first version of this figure was produced by Mitchell (1976) and many versions thereof have circulated since. The figure is meant to provide semi-quantitative information on the spectral power S = S(ω), where the angular frequency ω is 2π times the inverse of the oscillation period; S(ω) is supposed to give the amount of variability in a given frequency band for a generic climatic variable, although one has typically in mind the globally averaged surface air temperature. Unlike in the Stommel diagrams of Fig. 11, there is no information on the spatial scales of interest.
This power spectrum is not computed directly by spectral analysis from a time series of a given climatic quantity, such as (local or global) temperature; indeed, there  to the present. Each peak in the spectrum is tentatively attributed to a physical mechanism; see Plaut et al. (1995) for details. From Ghil (2002). c John Wiley and Sons, Ltd.
Reproduced with permission.
is no single time series that is 10 7 years long and has a sampling interval of hours, as the figure would suggest. Figure 12a includes, instead, information obtained by analyzing the spectral content of many different time series, for example, the spectrum of the 335-year long record of Central England temperatures in Fig. 12b. This time series is the longest instrumentally measured record of temperatures; see, though, Kondrashov et al. (2005a) for Nile River water levels. Given the lack of earlier instrumental records, one can imagine, but cannot easily confirm, that the higher-frequency spectral features might have changed, in amplitude, frequency or both, over the course of climatic history.
With all due caution in its interpretation, Fig. 12a reflects three types of variability: (i) sharp lines that correspond to periodically forced variations, at one day and one year; (ii) broader peaks that arise from internal modes of variability; and (iii) a continuous portion of the spectrum that reflects stochastically forced variations, as well as deterministic chaos (Ghil, 2002).
Between the two sharp lines at 1 day and 1 year lies the synoptic variability of mid-latitude weather systems, concentrated at 3-7 days, as well as intraseasonal variability, i.e. variability that occurs on the time scale of 1-3 months. The latter is also called low-frequency atmospheric variability, a name that refers to the fact that this variability has lower frequency, or longer periods, than the life cycle of weather systems. Intraseasonal variability comprises phenomena such as the Madden-Julian oscillation of winds and cloudiness in the tropics or the alternation between episodes of zonal and blocked flow in mid-latitudes (Ghil and Childress, 1987;Ghil and Mo, 1991;Haines, 1994;Molteni, 2002).
Immediately to the left of the seasonal cycle in Fig. 12a lies interannual, i.e. year-to-year, variability. An important component of this variability is the El Niño phenomenon in the Tropical Pacific: once about every 2-7 years, the sea-surface temperatures (SSTs) in the Eastern Tropical Pacific increase by one or more degrees over a time interval of about one year. This SST variation is associated with changes in the trade winds over the Tropical Pacific and in sea level pressures (Bjerknes, 1969;Philander, 1990); an East-West seesaw in the latter is called the Southern Oscillation. The combined El Niño-Southern Oscillation (ENSO) phenomenon arises through large-scale interaction between the equatorial Pacific and the atmosphere above. Equatorial wave dynamics in the ocean plays a key role in setting ENSO's time scale (Cane and Zebiak, 1985;Dijkstra and Burgers, 2002;Neelin et al., 1998Neelin et al., , 1994.
The greatest excitement among climate scientists, as well as the public, is more recently being generated by interdecadal variability, i.e. climate variability on the time scale of a few decades, the time scale of an individual human's life cycle (Martinson et al., 1995). Figure 12b represents a "blow-up" of the interannual-to-interdecadal portion of Fig. 12a. The broad peaks are due to the climate system's internal processes: each spectral component can be associated, at least tentatively, with a mode of interannual or interdecadal variability (Plaut et al., 1995). Thus the rightmost peak, with a period of 5.5 years, can be attributed to the remote effect of ENSO's low-frequency mode Ghil and Robertson, 2000), while the 7.7-year peak captures a North Atlantic mode of variability that arises from the Gulf Stream's interannual cycle of meandering and intensification , and references therein). The two interdecadal peaks, near 14 and 25 years, are also present in global records, instrumental as well as pa-leoclimatic (Delworth and Mann, 2000;Kushnir, 1994;Mann et al., 1998;Moron et al., 1998).
Finally, the leftmost part of Fig. 12a represents paleoclimatic variability. The information summarized here comes exclusively from proxy indicators of climate; see Sect. II.A.2.
The presence of near-cyclicity is manifest, in this range, in the broad peaks present in Fig. 12a between roughly 1 kyr and 1 Myr. The two peaks at about 20 kyr and 40 kyr reflect variations in Earth's orbit, while the dominant peak at 100 kyr remains to be convincingly explained (Ghil, 1994;Gildor and Tziperman, 2001;Imbrie and Imbrie, 1986). Quaternary glaciation cycles provide a fertile testing ground for theories of climate variability for two reasons: (i) they represent a wide range of climatic conditions; and (ii) they are much better documented than earlier parts of paleoclimatic history.
Within these glaciation cycles, there are higherfrequency oscillations prominent in North Atlantic paleoclimatic records. These are the Heinrich (1988) events, with a near-periodicity of 6-7 kyr, and the Dansgaard-Oeschger cycles (Dansgaard et al., 1993) that provide the peak at around 1-2.5 kyr in Fig. 12a. Rapid changes in temperature, of up to one half of the amplitude of a typical glacial-interglacial temperature difference, occurred during Heinrich events and somewhat smaller ones over a Dansgaard-Oeschger cycle. Progressive cooling through several of the latter cycles followed by an abrupt warming defines a Bond cycle (Bond et al., 1995). In North Atlantic sediment cores, the coldest part of each Bond cycle is marked by a so-called Heinrich layer that is rich in icerafted debris. None of these higher-frequency phenomena can be directly connected to orbital or other cyclic forcings.
In summary, climate variations range from the largeamplitude climate excursions of past millennia to smalleramplitude fluctuations on shorter time scales. Several spectral peaks of variability can be clearly related to forcing mechanisms; others cannot. In fact, even if the external forcing were constant in time -i.e., if no systematic changes in insolation or atmospheric composition, such as trace gas or aerosol concentrations, would occurthe climate system would still display variability on many time scales. This statement is clearly true for interannual ENSO variability in the equatorial Pacific, as discussed above. We will understand multiscale climate variability better in Sects. III and IV , where we will look in greater detail at natural climate variability and climate response to forcings, respectively.
Atmospheric Variability in Mid-latitudes
Mid-latitude atmospheric variability during boreal winter -when the winds are stronger and the variability is larger in the Northern Hemisphere -has long been a major focus of dynamic meteorology. The intent of this section is to motivate the reader to appreciate the complexity of large-scale atmospheric dynamics by focusing on a relatively well understood aspect thereof. We shall see that fairly diverse processes contribute to the spectral features discussed in connection with Fig. 12.
The synoptic disturbances that are most closely associated with mid-latitude weather have characteristic time scales of the order of 3-10 days, with a corresponding spatial scale of the order of 1 000-2 000 km (Holton and Hakim, 2013). They roughly correspond to the familiar eastward-propagating cyclones and anticyclones, and emerge as a result of the process of baroclinic instability, which converts available potential energy of the zonal flow into eddy kinetic energy; this conversion occurs through the lowering of the center of mass of the atmospheric system undergoing an unstable development, as a result of the negative correlation between vertical velocity and anomalies of the atmospheric density. Baroclinic instability (Charney, 1947;Holton and Hakim, 2013;Vallis, 2006) is active when the meridional temperature gradient or, equivalently (see below), the vertical wind shear are strong enough. These conditions are more readily verified in the winter season, which features a large equatorto-pole temperature difference and a strong mid-latitude jet (Holton and Hakim, 2013;Speranza, 1983).
The space-time spectral analysis introduced by Hayashi (1971) and refined by Pratt (1976) and by Fraedrich and Bottger (1978) builds upon the idea of the Stommel diagrams in Fig. 11. In addition, it provides information about the direction and speed at which the atmospheric eddies move and associates to each range of spatial and temporal scales a corresponding weight in terms of spectral power. This information may be obtained, in the first instance, by Fourier analysis of a onedimensional spatial field and it allows one to reconstruct the propagation of atmospheric waves. This analysis is usually carried out in the so-called zonal, i.e. west-to-east direction; see Sect. II.C below.
Next, one can compute the power spectrum in the frequency domain for each spatial Fourier component, and then average the results across consecutive winters to derive a climatology of winter atmospheric waves. The difficulty here lies in the fact that straightforward spacetime decomposition will not distinguish between standing and traveling waves: a standing wave will give two spectral peaks corresponding to waves that travel east-and westward at the same speed and with the same phase. This problem can only be circumvented by making assumptions regarding a given wave's nature. For instance, we may assume complete coherence between the eastward and westward components of standing waves and attribute the incoherent part of the spectrum to actual traveling waves. Figure 13 shows the spectral properties of the winter 500-hPa geopotential height field meridionally aver-aged across the mid-latitudes of the northern hemisphere (specifically, between 30 • and 75 • N) for the time interval . The properties of all the waves, as well as the standing, eastward-and westward-traveling waves appear in panels (a)-(d), respectively. As discussed later, the 500-hPa height field provides a synthetic yet comprehensive picture of the atmosphere's synoptic to large-scale dynamics. Figure 13c clearly shows that the eastwardpropagating waves are dominated by synoptic variability, concentrated between 3-12-day periods and zonal wavenumbers 5-8; note that a single cyclone or anticyclone counts for half-a-wavelength. In addition, the slanting high-variability ridge in H E indicates the existence of a statistically defined dispersion relation that relates frequency and wavenumber, in agreement with the basic tenets of baroclinic instability theory (Holton and Hakim, 2013).
When looking at the westward propagating variance in Fig. 13d, one finds that the dominant portion of the variability is associated with low-frequency, planetary-scale Rossby waves. Finally, Fig. 13b shows the contribution to the variance given by standing waves, which correspond to large-scale, geographically locked and persistent phenomena like blocking events. Note that westward propagating and stationary waves provide the lion's share of the overall variability of the atmospheric field; see also Kimoto and Ghil (1993a, Fig. 7).
The dynamics and energetics of planetary waves are still under intensive scrutiny. Descriptions and explanations of several highly nonlinear aspects thereof are closely interwoven with those of blocking events, identified as persistent, large-scale deviations from the zonally symmetric general circulation (Benzi et al., 1986a;Benzi and Speranza, 1989;Charney and DeVore, 1979;Ghil and Childress, 1987;Kimoto and Ghil, 1993a;Legras and Ghil, 1985). Persistent blocking events affect strongly the weather for up to a month over continental-size areas. Such persistence offers some hope for extended predictability of large-scale flows, and of the associated synoptic-scale weather, beyond the usual predictability of the latter, which is believed not to exceed 10-15 days (Lorenz, 1996, and references therein).
Today, the most highly resolved and physically detailed NWP models are reasonably good at predicting the persistence of a blocking event, once the model's initial state is within that event, but not at predicting the onset of such an event or its collapse (Ferranti et al., 2015). Likewise, the capability of necessarily lower-resolution climate models to simulate the spatio-temporal statistics of such events is far from perfect; in fact, relatively limited improvement has been realized in the last two decades (Davini and D'Andrea, 2016). Weeks et al. (1997) reproduced successfully in the laboratory key features of the dynamics and statistics of blocking events.  reviewed several schematic descriptions of the mid-latitude atmosphere's lowfrequency variability (LFV) as jumping between a zonal regime and a blocked one or, more generally, a small number of such regimes. This coarse-graining of the LFV's phase space and Markov chain representation of the dynamics continues to inform current efforts at understanding what atmospheric phenomena can be predicted beyond 10-15 days and how. Recently, analyses based on dynamical systems theory have associated blocked flow configurations with higher instability of the atmosphere as a whole (Faranda et al., 2017;Lucarini and Gritsun, 2019;Schubert and Lucarini, 2016), as predicted by Legras and Ghil (1985). The effect of global warming on the statistics of blocking events has become recently a matter of considerable controversy. The sharper increase of near-surface temperatures in the Arctic than near the Equator is fairly well understood (e.g., Ghil, 1976, Fig. 7) and has been abundantly documented in recent observations (e.g., Walsh, 2014, Fig. 8). This decrease of pole-toequator temperature difference ∆T is referred to as polar amplification of global warming. Francis and Vavrus (2012) and Liu et al. (2012) have suggested that reduced ∆T slows down the prevailing westerlies and increases the north-south meandering of the subtropical jet stream, resulting in more frequent blocking events. This suggestion seems to agree with fairly straightforward arguments of several authors on the nature of blocking (Charney and DeVore, 1979;Ghil and Childress, 1987;Legras and Ghil, 1985), as illustrated, for instance, in   Fig. 2): The driving jet ψ * A in the figure is proportional to ∆T , in accordance with standard quasi-geostrophic flow theory, 1 and lower jet speeds ψ A favor blocking. But considerable evidence against this apparently straightforward argument has accumulated, too (Barnes and Screen, 2015;Hassanzadeh et al., 2014, and references therein). The existence of a nontrivial relation between intensity of the jet and probability of occurrence of blocking events has been also presented by Ruti et al. (2006), who used as theoretical framework the theory of low-frequency variability of the atmosphere proposed by Benzi et al. (1986a).
C. Basic Properties and Fundamental Equations
Governing Equations
The evolution of the atmosphere, the oceans, the soil, and the ice masses can be described by using the continuum approximation, in which these subsystems are represented by field variables that depend on three spatial dimensions and time. For each of the climatic subdomains, we consider the following field variables: the density ρ and the heat capacity at constant volume C, with the specific expression for ρ and C defining the thermodynamics of the medium; the concentration of the chemical species {ξ k : 1 ≤ k ≤ K} contained in the medium, and present in different phases, e.g. the salt dissolved in the oceans or the water vapor in the atmosphere; the three components {v i : 1 ≤ i ≤ 3} of the velocity vector; the temperature T ; the pressure p; the heating rate J; and the gravitational potential Φ.
Note that, by making the thin-shell approximation H/R 1, where H is the vertical extent of the geophysical fluid and R is the radius of the Earth, we can assume that the gravitational potential at the local sea level is zero and we can thus safely use the approximation Φ = gz, where Φ is then called the geopotential, g is the gravity at the surface of the Earth, and z is the geometric height above sea level. Moreover, one has to take into account that the climate system is embedded in a noninertial frame of reference that rotates with an angular velocity Ω with components {Ω i : 1 ≤ i ≤ 3}.
The PDEs that govern the evolution of the field variables are based on the budget of mass, momentum and energy. When the fluid contains several chemical species, their separate budgets also have to be accounted for (Vallis, 2006). In order to have a complete picture of the Earth system, one should in principle also consider the evolution of biological species. Doing so, however, is well beyond our scope here, even though present Earth system models do attempt to represent biological processes, albeit in a simplified way; see later discussion in Sect. II.D.
The mass budget equations for the constituent species can be written as follows:
∂ t (ρξ k ) = −∂ i (ρξ k v i ) + D ξ k + L ξ k + S ξ k .(1)
Here ∂ t is the partial derivative in time and ∂ i in the x i -direction; D ξ k , L ξ k and S ξ k are the diffusion operator, phase changes, and local mass budget due to other chemical reactions that are associated with k. The momentum budget's i th component is written:
∂ t (ρv i ) = − ∂ j (ρv j v i ) − ∂ i p + ρ∂ i Φ − 2ρ ijk Ω j v k + T i + F i .(2)
Here the Levi-Civita antisymmetric tensor ijk has been used to write the Coriolis force; T i indicates direct mechanical forcings, e.g. those resulting from luni-solar tidal effects; F i = −∂ j τ ij corresponds to friction, with {τ ij } the stress tensor; and summation over equal indices is used. Equation (2) is just a forced version of the momentum equation in the Navier-Stokes equations (NSEs), written in a rotating frame of reference. In general, one can write the specific energy of the climate system as the sum of the specific internal, potential, and kinetic energies, taking into account also the contributions coming from chemical species in different phases. In order to get manageable formulas, some approximations are necessary (see, e.g., Peixoto and Oort, 1992).
Neglecting reactions other than the phase changes between the liquid and gas phases of water, the expression of the specific energy in the atmosphere is
e = c v T + Φ + v j v j /2 + Lq,
where c v is the specific heat at constant volume for the gaseous atmospheric mixture, L is the latent heat of evaporation, and q is the specific humidity. In this formula, we neglect the heat content of the liquid and solid water and the heat associated with the phase transition between solid and liquid water. Instead, the approximate expression for the specific energy in the oceans is
e = c W T + Φ + v j v j /2,
where c W is the specific heat at constant volume of water, while neglecting the effects of salinity and of pressure. Finally, for the specific energy of soil or ice, we can take e = c {S,I} T + Φ, respectively.
After some nontrivial calculations, one derives the following general equation for the local energy budget:
∂ t (ρe) = −∂ j (ρεv j ) − ∂ j Q SW j − ∂ j Q LW j − ∂ j J SH j − ∂ j J LH j − ∂ j (v i τ ij ) + v i T i ,(3)
where e is the energy per unit mass and ε = e + p/ρ is the enthalpy per unit mass. The energy source and sink terms can be written as the sum of the work done by the mechanical forcing v i T i and of the respective divergences of the shortwave (solar) and longwave (terrestrial) components of the Poynting vector, Q SW j and Q LW j ; of the turbulent sensible and latent heat fluxes, J SH j and J LH j ; and of the scalar product of the velocity field with the stress tensor v i τ ij .
Equation (3) is written in a conservative form, with the right-hand side containing only the sum of flux divergences, except for the last term, which is negligible. By taking suitable volume integrals of Eq. (3) and assuming steady-state conditions, one derives meridional enthalpy transports from the zonal budgets of energy fluxes (Ghil and Childress, 1987;Lucarini et al., 2014;Lucarini and Ragone, 2011;Peixoto and Oort, 1992);recall Figs. 2(a,b). The presence of inhomogeneous absorption of shortwave radiation due to the geometry of the Sun-Earth system and of the physico-chemical properties of the climatic subdomains determines the presence of nonequilibrium conditions for the climate system, as already discussed in Sect. I.A.
Various versions of Eqs.
(1)-(3) have been studied for over a century, while well-established thermodynamical and chemical laws describe accurately the phase transitions and reactions of the climate system's constituents. Finally, quantum mechanics allows one to calculate in detail the interaction between matter and radiation.
Still, despite the fact that climate dynamics is governed by well-known evolution equations, it is way beyond our scientific abilities to gain a complete picture of the mathematical properties of the solutions. In fact, many fundamental questions are still open regarding the basic NSEs in a homogeneous fluid, without phase transitions and rotation (Temam, 1984(Temam, , 1997. In particular, even for the basic NSEs, providing analytical, closed-form solutions is only possible in some highly simplified cases that are either linear or otherwise separable (Batchelor, 1974).
As already discussed above, Eqs.
(1)-(3) can simulate a range of phenomena that spans many orders of magnitude in spatial and temporal scales. Hence, it is virtually impossible to construct numerical codes able to represents explicitly all the ongoing processes, at the needed resolution in space and time. It is thus necessary to parametrize the processes that occur at subgrid scales and cannot, therewith, be directly represented. Among the most important processes of this type are cloudradiation interactions and turbulent diffusion. We will discuss the theoretical framework behind the formulation of parametrizations in Sect. III.G.
Approximate Balances and Filtering
Equations (1)-(3) are too general and contain too many wave propagation speeds and instabilities in order to properly focus on certain classes of phenomena of interest. For instance, the fluid's compressibility only plays a role in the propagation of very fast acoustic waves, whose energy is negligible, compared to that of the winds and currents we are interested in here.
Therefore, starting with Charney (1947), more or less systematic approximations have been introduced to filter out undesirable waves from the equations of motion and to study particular classes of phenomena and processes. Depending on the climate subsystem and the range of scales of interest, different approximations can be adopted. For example, if one considers ice sheets and bedrock on the time scale of millennia, it is reasonable to assume that v j 0, i.e., to remove the flow field from the evolution equation (Ghil, 1994;Saltzman, 2001, and references therein). Obviously, this approximation is not valid if one wants to describe explicitly the motion of the ice sheets on shorter time scales (Paterson, 1981).
More precisely, the filtering process consists in applying a set of mathematical approximations into the fundamental governing equations -usually considered to be a suitable generalization of the classical NSEs, like Eqs. (1)-(3). The purpose of this filtering is to exclude from the filtered system certain physical processes that are heuristically assumed to play only a minor role in its dynamics at the time and space scale under investigation. The magnitudes of all the terms in the governing equations for a particular type of motion are estimated by dimensional analysis (Barenblatt, 1987), and various classes of simplified equations have been derived by considering distinct asymptotic regimes in the associated scales (Ghil and Childress, 1987;Klein, 2010;McWilliams, 2006;Pedlosky, 1987).
The approximations adopted rely on assuming that the continuous medium, whether fluid or solid, obeys suitably defined time-independent relations -or undergoes only small departures from such relations -and that these relations result from the balance of two or more dominating forces. Imposing such balances leads to reducing the number of independent field variables of the system that obey a set of evolution equations.
In meteorological terminology, a prognostic variable whose tendency, i.e. time derivative, appears in the full equations, may thus become a diagnostic variable, which only appears in a nondifferentiated form in the filtered equations. Additionally, the imposition of dynamical balances leads to removing specific wave motions from the range of allowed dynamical processes. Below we shall see two of the most important examples of filtering, which, among other things, are essential for the practical implementation of numerical models of the atmosphere and oceans (Cushman-Roisin and Beckers, 2011a; Holton and Hakim, 2013;Vallis, 2006). a. Hydrostatic Balance. A classical example of filtering is the hydrostatic approximation. In a local Cartesian coordinate system (x, y, z), we define byẑ= −∇Φ/|∇Φ| the direction perpendicular to a geopotential isosurface Φ ≡ const. An obvious stationary solution of the NSEs is given in these coordinates by the time-independent hydrostatic balance equation:
ρ h g = −∂ z p h ,(4)
where the subscript 'h' denotes this particular solution.
On sufficiently large spatial and temporal scales, many geophysical flows -e.g., atmospheric and oceanic, as well as continental surface and ground water -are near hydrostatic equilibrium. In general, stable hydrostatic equilibrium is achieved when fluid with lower specific entropy lies below fluid with higher specific entropy; in this case, Eq. (4) is obeyed for all (x, y, z) within the domain occupied by the fluid.
When this condition is broken because of external forcing or internal heating, say, the stratification is not stable and the fluid readjusts so as to reestablish the hydrostatic equilibrium. This readjustment is achieved by vertical convective motions, in which available potential energy is transformed into kinetic energy responsible for vertical motions that can be locally much faster than the largescale flow. Violations of hydrostatic balance thus exist only on short time and space scales.
A large class of models used in studying, simulating and attempting to predict climate variability and climate change is based on a particular simplification of the full set of Eqs. (1)-(3). This simplification is called the primitive equations, which filter out nonhydrostatic motions. As a result, sound waves are excluded from the solutions, a fact that greatly facilitates the numerical implementation of such models (Cushman-Roisin and Beckers, 2011a; Washington and Parkinson, 2005).
The primitive equations are derived by assuming that the time-independent hydrostatic balance given in Eq. (4) does apply at all times, even when the flow is time dependent. One thus assumes that the vertical acceleration of the fluid is everywhere much smaller than gravity. Neglecting the vertical acceleration altogether leads to a monotonic relationship between the vertical coordinate z and pressure p. 2 One can hence replace z by p as a vertical coordinate, and rewrite the modified Eqs. (1)-(3) using a new coordinate system in which one also replaces, typically, the horizontal coordinates (x, y) by the longitude λ and the latitude φ, respectively. In such a coordinate system, v 3 ≡ ω ≡ ∂ t p is the change of pressure with time, the three-dimensional (3-D) velocity field is nondivergent, ∂ j v j = 0, and the density is automatically constant and set to 1 (Holton and Hakim, 2013;Vallis, 2006).
Despite the use of primitive equations, climate models aim at simulating a system that nonetheless features nonhydrostatic motions. Additionally, geophysical flows obeying primitive equations can lead to unstable vertical configurations of the fluids. Typical climate models are formulated so as to quickly eliminate local nonhydrostatic conditions and to parametrize the balanced condition that is thus recovered in terms of variables that are explicitly represented in the model, following the idea of convective adjustment originally proposed by Manabe and Strickler (1964). Finding optimal ways to parametrize the effect of convection on hydrostatic climate models has become a key research area in climate dynamics; see Emanuel (1994) for a classical treatise on the problem of atmospheric convection and Plant and Yano (2016) for a recent overview.
The primitive equations were at the core of both climate models and weather prediction models for three decades, from about the 1970s through the 1990s. Nonhydrostatic models -constructed through discretization of the full NSEs with forcing and rotation -have become increasingly available over the last decade, first as limited-area models and, more recently, even as global ones (e.g., Marshall et al., 1997).
These models require extremely high resolution in space and time and are computationally quite expensive; as they start to enter operational use, they require stateof-the-art hard-and software, in terms of both computing and storage, as well as sophisticated post-processing of the output. Moreover, for any practical purpose, sufficiently detailed initial and boundary data are not available and methodological problems reappear in connection with formulating such data. Still, the use of nonhydrostatic models allows, in principle, for bypassing the key problem of parametrizing convection.
b. Geostrophic Balance. Another important example of filtering is the one associated with time-independent purely horizontal balanced flows, in which the horizontal components of the pressure gradient force and the Coriolis force cancel out in the NSEs. Such flows are termed geostrophic; etymologically, this means Earthturning. The nondimensional parameter that determines the validity of this approximation is the Rossby number Ro = U/f 0 L, where U is a characteristic horizontal velocity, L a characteristic horizontal extent, and f 0 = 2Ω sin φ 0 ; here Ω is the modulus of Earth's angular frequency of rotation around its axis, and φ 0 is a reference latitude.
The geostrophic approximation provides a rather good diagnostic description of the flow when Ro 1. Such small Ro-values prevail for the atmosphere on synoptic and planetary scales, such as those of extratropical weather systems -say poleward of 30 • , where f 0 is large enough -and in the free atmosphere, i.e. above the planetary boundary layer, where frictional forces can be comparable to the Coriolis force. In the oceans, this approximation is extremely accurate everywhere except very close to the equator. The smallness of Rossby number in the oceans arises from oceanic currents being orders of magnitude slower than atmospheric winds, so that Ro nears unity only where f 0 is extremely small.
Purely geostrophic flow fields are obtained via a zerothorder expansion in Ro of the NSEs. Fluid motion is introduced by considering small perturbations ρ g and p g that break the translation symmetry of the basic, purely at a given z-level, the pressure gradient force (upward-pointing arrow), and Coriolis force (downward-pointing arrow) cancel out and the flow (horizontal arrow) is parallel to the isobars. (b) Synoptic conditions for the 500-hPa level over the United States at 0000 Greenwich mean time (GMT) on 12 July 2019. The dark gray lines indicate the isolines of geopotential height z = Φ/g (in units of 10 m), and the barbed blue arrows indicate the direction of the wind; the barbs indicate the wind speed, each short (long) barb corresponding to a speed of 5 (10) knots, with 2 knots = 1.03 ms −1 . Reproduced with permission from the NOAA-National Weather Service https://www.spc.noaa.gov/obswx/maps/.
hydrostatic density and pressure fields, ρ h and p h , but preserve the geostrophic balance, i.e. one sets ρ = ρ h +ρ g and p = p h + p g , respectively.
Lettingx be locally the zonal direction andŷ be locally the meridional direction, one can write (Holton and Hakim, 2013) geostrophic balance at each height z as:
ρ h uf 0 = −∂ y p g ,(5a)ρ h vf 0 = ∂ x p g ,(5b)
where the derivatives are taken at constant z, (u, v) = (v 1 , v 2 ) and, furthermore, the geostrophic perturbation itself is in hydrostatic equilibrium, i.e. gρ g = −∂ z p g .
The combined hydrostatic and geostrophic balance constrain atmospheric and oceanic motions so that the velocity field is uniquely determined by the pressure field. In such doubly balanced flows, the winds or currents are parallel to the isobars at a given geopotential height z, as shown in Fig. 14a, rather than perpendicular to the isobars, as in nonrotating fluids. Moreover, the vertical component of the velocity field vanishes, w = v 3 ≡ 0.
Using the pressure coordinate system (x, y, p) described above, where p plays the role of the vertical coordinate, it is possible to express the geostrophic balance as follows (Holton and Hakim, 2013):
f 0 u g = −∂ y Φ,(6a)f 0 v g = ∂ x Φ,(6b)
where, this time, derivatives are taken at constant p. The vertical derivative ∂ p Φ can, furthermore, be expressed in terms of ρ g . For the atmosphere, one can make the simplifying assumption of dry conditions, for which the equation of state is described by the gas constant R and the heat capacity at constant pressure C p . Assuming, moreover, a horizontally homogeneous background state with T h = T h (p), introducing the potential temperature Θ h = T h (p/p s ) R/Cp , with p s = const. the reference surface pressure, and letting σ := −RT h d ln Θ h /dp, which defines the stratification of the background state, one obtains:
∂ p Φ = − Rp σ T g .(7)
Thus, in the geostrophic approximation, the scalar field Φ provides complete diagnostic information on the state of the fluid: its horizontal derivatives give us the velocity field, while the vertical derivative gives us the geopotential, or mass field, via perturbations with respect to the background temperature field (Holton and Hakim, 2013).
Essentially, geophysical fluid dynamics (GFD) is the study of large-scale flows dominated by the combined hydrostatic and geostrophic balances. The former is a consequence of the shallowness of planetary flows, the latter of their rotation (Cushman-Roisin and Beckers, 2011a; Ghil and Childress, 1987;McWilliams, 2006;Pedlosky, 1987). The next subsection will introduce the more complex situation of finite-but-small perturbations from this combined balance.
Although modern computers allow a fully ageostrophic (beyond geostrophic approximation) description of most geophysical fluids, geostrophic balance remains a fundamental tool of theoretical research, as well as being used for practical applications in everyday weather prediction. Figure 14b illustrates a typical mid-latitude synoptic situation: at a given pressure level, the winds blow, to a very good approximation, parallel to the isolines of geopotential height, and the speed of the wind is higher where the gradient of geopotential height is stronger. An appendix in Ghil et al. (1979) describes the extent to which diagnostic relations based on the geostrophic approximation are still used in the analysis and prediction of mid-latitude weather, as simulated by advanced numerical models. Indeed, geostrophic approximation is implicit in the day-to-day reading of weather maps on synoptic-to-planetary scales.
Quasi-Geostrophy and Weather Forecasting
The diagnostic, i.e. time-independent nature of the geostrophic balance implies that the ageostrophic terms, although relatively small, are important for the time evolution of the flow. A planetary flow that evolves slowly in time compared to 1/f 0 can be described using quasigeostrophic theory, namely a perturbative theory that expands the NSEs in Ro and truncates at first order.
The use of the quasi-geostrophic approximation effectively filters out solutions that correspond to higherspeed atmospheric or oceanic inertia-gravity waves; the latter, also called Poincaré waves, are gravity waves modified by the presence of rotation (Cushman-Roisin and Beckers, 2011a; Ghil and Childress, 1987;Holton and Hakim, 2013;McWilliams, 2006;Pedlosky, 1987;Vallis, 2006). This approximation breaks down, though, near frontal discontinuities and in other situations in which the ageostrophic component of the velocity field plays an important advective role, and it has to be improved upon by retaining higher-order terms.
In quasi-geostrophic theory, a fundamental role is played by the quasi-geostrophic potential vorticity 3 :
q = 1 f 0 ∆ 2 Φ + f + ∂ p f 0 σ ∂ p Φ .(8)
Here ∆ 2 is the horizontal Laplacian in p-ccordinates, the first term ζ g = ∆ 2 Ψ/f 0 is the relative vorticity of the geostrophic flow, f is the planetary vorticity, and the last term is the stretching vorticity; the static stability σ, which measures the stratification of the fluid was introduced in Eq. (7). The potential vorticity q thus combines dynamical and thermodynamical information on the fluid flow. In more abstract terms, one can write:
q − f = LΦ,(9)
where L is a modified three-dimensional Laplacian operator. Using suitable boundary conditions, one can invert the expression above and derive the mass field from the vorticity field,
Φ = L −1 (q − f ).(10)
In the absence of forcing and dissipation, q is conserved by geostrophic motions, so that:
∂ t q + J Φ f 0 , q = 0,(11)
where
J(A, B) = ∂ x A∂ y B − ∂ y A∂ x B is the Jacobian op- erator, so that J(Φ/f 0 , q) = (1/f 0 )(∂ x Φ/∂ y q − ∂ y Φ∂ x q)
describes vorticity advection by the geostrophic velocity field (u g , v g ), cf. Eq. (5). Note that, in the limit σ → ∞, i.e. if one assumes infinitely stable stratification, the third term on the right-hand side of Eq. (8) drops out, and Eq. (11) describes the conservation of the absolute vorticity in a barotropic, two-dimensional flow.
Quasi-geostrophic theory, as introduced by Charney (1947), arguably provided a crucial advance for the understanding of the dynamics of planetary flows and provided the foundation for the successful start of NWP (Charney et al., 1950a). The filtering associated with this theory was, in particular, instrumental in eliminating the numerical instability that marred the pioneering weather forecast experiment of Richardson (1922). The inconclusive result of his one-step, 6-hour numerical experiment was received by the meteorological community at the time as proof that NWP, as proposed by Bjerknes (1904), was not possible. The success, three decades later, of several 24-hour numerical forecasts by Charney et al. (1950a), on the other hand, paved the way for the theory's successive applications to physical oceanography and climate dynamics as a whole (Dijkstra, 2005;Ghil and Childress, 1987;Pedlosky, 1987).
We outline herewith how the quasi-geostrophic potential-vorticity equations can be used as a suitable framework for performing numerical weather forecasts. Assume that, at time t = t 0 , we have information on the field Φ 0 = Φ(t 0 ).
•
Step 0: Using Eq. (9), one computes the corresponding potential vorticity field q 0 = q(t 0 );
• Step A: Equation (11) is used next to predict the potential vorticity field at time t 1 as follows:
q 1 = q(t 1 ) = q(t 0 ) + J(Φ 0 /f 0 , q 0 )∆t, where ∆t = t 1 − t 0 ;
• Step B: Equation (10) is used now to infer Φ 1 ;
• Step C: Go to Step A and predict q 2 , and so on.
In effect, J. G. Charney, R. Fjørtoft and J. von Neumann performed the first successful numerical weather forecasts on the ENIAC computer in Princeton by using essentially the procedure detailed above for the simpler case of barotropic, two-dimensional flow, where the standard vorticity, given by the sum of relative and planetary vorticity, plays a role analogous to potential vorticity in a quasi-geostrophic three-dimensional flow. The procedure was adapted to describe the evolution of the 500-hPa field, which deviates the least from the behavior of an idealized atmosphere of homogeneous density with no vertical velocities. The simulation was performed on a limited domain that covered North America and the adjacent ocean areas. The authors investigated the issues associated with horizontal boundary conditions, as well as with the numerical stability of the integration (Charney et al., 1950a). The saga of this scientific and technological breakthrough was told many times; see Lynch (2008) for a good recent account.
D. Climate Prediction and Climate Model Performance
A key area of interest in the climate sciences is the development and testing of numerical models used for simulating the past, present, and future of the climate system. As discussed later in the paper, climate models differ enormously in terms of scientific scope, computational cost, and flexibility, so that one has to consider systematically a hierarchy of climate models, rather than one model that could incorporate all subsystems, processes, and scales of motion (Ghil, 2001;Ghil and Robertson, 2000;Held, 2005;Lucarini, 2002). Figure 15a shows the so-called Bretherton horrendogram (NAC, 1986), which displays the full range of subsystems one needs to deal with when addressing the Earth system as a whole, along with some of the many interactions among these subsystems. The climate modeling community has slowly caught up with the complexities illustrated by F. Bretherton and his colleagues in this diagram, three decades ago.
Models of different levels of complexity and detail are suited for addressing different kinds of questions, according, in particular, to the main spatial and temporal scales of interest. At the top of the hierarchy, one finds global climate models, known in the 1970s and '80s as general circulation models. Both these phrases share the acronym GCMs, with the change in name reflecting a change in emphasis from understanding the planetaryscale circulation -first of the atmosphere, then of the oceans -to simulating and predicting the global climate. GCMs aim to represent, at the highest computationally achievable resolution, the largest number of physical, chemical, and biological processes of the Earth system; see an early overview in Randall (2000).
The atmosphere and oceans, whose modeling relies on the equations discussed in Sect. II.C.1 above, are still at the core of the Earth system models being developed today. Following improvements in basic scientific knowledge, as well as in computing and data storage capabilities, the latter models include an increasing number of physical, chemical and biological processes. They also rely on a much higher spatial and temporal resolution of the fields of interest. Figure 15b illustrates schematically the process of model development across the last three decades, from the first to the fourth assessment report: FAR-SAR-TAR-AR4. The graph does not cover the development of the last 5-10 years, which has largely dealt with the inclusion of eco-biological modules and the above-mentioned nonhydrostatic effects. Currently available state-of-theart models result from the cumulative efforts of generations of climate scientists and coding experts, with hundreds of individual researchers contributing to different parts of the code. As in the early stages of post-WWII NWP, climate modelling exercises now are some of the heaviest users of civilian High Performance Computing.
Predicting the State of the System
In spite of its remarkable progresses, climate modeling and prediction face several kinds of uncertainties: First, uncertainties in predicting the state of the system at a certain lead time, given the (uncertain) knowledge of its state at the present time. Geophysical flows are typically chaotic, as are other processes in the system. Hence, as discussed in depth in the next two sections, the climate system depends has sensitively on its initial data, as suggested already by Poincaré (1908) and recognized more fully by Lorenz (1963). Following up on terminology introduced by Lorenz (1976), these are uncertainties of the first kind, namely small errors in the initial data that can lead at later times to large errors in the flow pattern.
In a nearly linear regime, in which the phase-space distance between the system's orbits is small, their divergence rate is described by the spectrum of Lyapunov exponents and, in particular, by the algebraically largest ones. A chaotic system has at least one positive Lyapunov exponent, and physical instabilities that act on distinct spatial and temporal scales are related to distinct positive exponents (Eckmann and Ruelle, 1985). These instabilities can be described by using the formalism of covariant Lyapunov vectors (Ginelli et al., 2007).
Addressing the uncertainties of the first kind and providing good estimates for the future state of the system is the classical goal of NWP. After the initially exponential increase of errors, nonlinear effects kick in, and the orbits in the ensemble populate the attractor of the system, so that any predictive skill is lost. In such an ensemble, several simulations using the same model are started with slightly perturbed initial states, and the ensemble of orbits produced is used to provide a probabilistic estimate of how the system will actually evolve (Palmer, 2017). Figure 16a illustrates the main phases of error growth from an ensemble of initial states in the Lorenz (1963) model and Fig. 16b outlines how an ensemble forecast system actually works. Obvious limitations are related to the computational difficulties of running a sufficient number of ensemble members in order to obtain an accurate estimate, as well as with the fact that an initial ellipsoid of states tends to become flattened out in time, making the error estimate more laborious. Thus, care has to be exercised in the choice of the initial states, which is done in practice by taking advantage of the reanalyses described earlier in Sect. II.A.1.
Recently, similar methods are being used on longer time scales to perform experimental climate predictions, seen as an initial value problem (Eade et al., 2014). These predictions are performed on seasonal (Doblas-Reyes et al., 2013) to decadal (Meehl et al., 2014) time scales; they aim not only to reach a better understanding of the multiscale dynamics of the climate system, but also to help achieve the many potential benefits of skillful medium-to-long-term predictions. One interesting example of such an application is the wind energy market (Torralba et al., 2017).
The World Climate Research Programme (WCRP) has created an encompassing project focusing on the challenges of subseasonal-to-seasonal (S2S) prediction; see https://www.wcrp-climate.org/s2s-overview and Robertson and Vitart (2018).
We will delve more specifically into extended-range forecasting in Sect. III.E.1 and see how its performance depends on a model's ability to capture the natural modes of variability of the climate system on different time scales.
Predicting the System's Statistical Properties
A key goal of climate modeling is to capture the system's statistical properties -i.e., its mean state and its variability -and its response to forcings of different nature. These problems will be treated thoroughly in the next two sections but are rendered quite difficult by a second set of uncertainties. Uncertainties in model formulation, as well as an unavoidably limited knowledge of the external forcings are referred to as uncertainties of the second kind (Lorenz, 1967(Lorenz, , 1976Peixoto and Oort, 1992) and limit intrinsically the possibility of providing realistic simulations of the statistical properties of the climate system; they affect, in particular, severely the modeling of abrupt climate changes and of the processes that may lead to such changes.
These deficiencies are related to uncertainties in many key parameters of the climate system, as well as to the fact that each model may represent incorrectly certain processes that are relevant on the temporal and spatial scales of interest or that it may miss them altogether. These types of uncertainty are termed parametric and structural uncertainty, respectively (Lucarini, 2013).
A growing number of comprehensive climate models are available to the international scientific community for studying the properties of the climate system and predicting climate change; currently there are about 50 such models (IPCC, 2014a). Still, many of these models have in common a substantial part of their numerical code, as they originate from a relatively small number of models, atmospheric and oceanic, that were originally developed in the 1960s and 1970s. Figure 17 illustrates this family tree for the case of atmospheric GCMs. Hence the widely noted fact that the climate simulations produced by this large number of models often fall into classes that bear marked similarities and do not necessarily resemble to a desired extent the observed climate evolution over the last century or so. IPCC's evolution over three decades has led to a coordination and restructuring of modeling activities around the world. In order to improve comparisons among distinct models and the replicability of investigations aimed at climate change, the Program for Climate Model Diagnostics and Intercomparison (PCMDI), through its climate model intercomparison projects (CMIPs), defines standards for the modeling exercises to be performed by research groups that wish to participate in a given assessment report (AR) and provide projections of future climate change; see http://cmip-pcmdi.llnl. gov/cmip5/data_portal.html for CMIP5 and https: //www.wcrp-climate.org/wgcm-cmip/wgcm-cmip6 for CMIP6, respectively.
The PCMDI's CMIPs have also supported a single website that gathers climate model outputs contributing to the IPCC-initiated activities, thereby providing a unique opportunity to evaluate the state-of-the-art capabilities of climate models in simulating the climate system's past, present and future behavior. A typical IPCC-style package of experiments includes simulating the climate system under various conditions, such as:
• a reference state, e.g., a statistically stationary preindustrial state with fixed parameters;
• industrial era and present-day conditions, including known natural and anthropogenic forcings;
• future climate projections, performed by using a set of future scenarios of greenhouse gas and aerosol emissions and land-use change with some degree of realism, as well as idealized ones (e.g. instantaneous doubling of CO 2 concentration).
The latter changes of greenhouse gas, aerosol concentrations and land use follow a prescribed evolution over a given time window, and are then fixed at a certain value to observe the relaxation of the system to a new stationary state. Each such evolution of the forcing is defined a "scenario" in ARs 1-4 (IPCC, 2001(IPCC, , 2007, and a Representative Concentration Pathway (RCP) in AR5 (IPCC, 2014a). Note that each AR has involved an increasing number of models, people -reaching many hundreds by the latest AR -and hence acronyms.
Each scenario or RCP is a representation of the expected greenhouse gas and aerosol concentrations resulting from a specific path of industrialization and change in land use, as provided by Working Groups II and III to Working Group I; see Fig. 18 for AR5's simulations of the 20 th and 21 st century with two extreme RCPs. At the same time, the attribution of unusual climatic conditions to specific climate forcing is far from being a trivial matter (Allen, 2003;Hannart et al., 2016a).
In the most recent CMIP exercise, CMIP6, the scope of model intercomparisons has grown beyond just test-.
FIG. 17: Family tree of the main atmospheric general circulation models. The tree clearly shows that many of today's state-of-the-art models share a -smaller or larger -portion of their genes, i.e., of the basic ideas and parameter values that went into the coding. Reproduced with permission from Edwards (2010).
ing future climate response to specific forcing scenarios or RCPs. In order to test more comprehensively the performance of climate models, various standardized intercomparison projects focus on the analysis of specific subsystems, processes or time scales; see https: //search.es-doc.org. Eyring et al. (2016a) provide a useful summary of the strategy for PCMDI's CMIP6 project and of the scientific questions it addresses.
Metrics for Model Validation
The standardization of climate model outputs promoted by the CMIPs has also helped address a third major uncertainty in climate modeling: what are the best metrics for analyzing a model's outputs and evaluating its skill? Note that, in this context, the term metric does not have its usual mathematical meaning of a distance in function space but refers instead to a statistical estimator of model performance, whether quadratic or not.
The validation, or auditing -i.e., the overall evaluation of the faithfulness -of a set of climate models, is a delicate operation, which can be decomposed into two distinct but related procedures. The first one is model intercomparison, which aims to assess the consistency of different models in the simulation of certain physical phenomena in a certain time frame. The second procedure is model verification, whose goal is to compare a model's outputs with corresponding observed or reconstructed scalar quantities or fields (Lucarini, 2008b). In principle, there virtually infinite ways to construct a metric buy taking any reasonable function of the variables included in a climate model. Nevertheless, even if several observables are mathematically well defined, their physical relevance and robustness can differ widely. There is no a priori valid criterion for selecting a good climatic observable, even though taking into account basic physical properties of the climate system can provide useful guidance.
There is thus no unique recipe for testing climate models, a situation that is in stark contrast with more traditional areas of physics. For instance, in high-energy physics, the variables mass, transition probability or cross-section are suggested by the very equations that one tries to solve or to study experimentally.
In the absence of dissipation and of sources and sinks, certain scalars -such as the atmosphere's or the oceans' total mass, energy and momentum -are also conserved by the fundamental equations of Sect. II.C.1. But it is often more local quantities, like total rain over India during the summer monsoon or the intensity of the subtropical jet over North America during a given winter month, which truly matter in evaluating the skill of a climate model. Note that current climate models still have sub-stantial difficulties in simulating the statistics of major climatic processes, such as ENSO in the Tropical Pacific (Bellenger et al., 2014;Lu et al., 2018), the Indian Monsoon (Boos and Hurley, 2013;Hasson et al., 2013;Turner and Annamalai, 2012), and mid-latitude LFV associated with blocking (Davini and D'Andrea, 2016;Woollings et al., 2018).
From the end user's point of view, it is important to check how realistic the modeled fields of practical interest are. But if the aim is to define strategies for the radical improvement of model performance -beyond incremental advances often obtained at the price of large increases in computer power -it is crucial to fully understand the differences among models in their representation of the climate system, and to decide whether specific physical processes are correctly simulated by a given model.
Additional issues, practical as well as epistemological, emerge when we consider the actual process of comparing theoretical and numerical investigations with observational or reanalysis data. Model results and approximate theories can often be tested only against observational data from a sufficiently long past history, which may pose problems of both accuracy and coverage, as mentioned in Sect. II.A.1.
To summarize, difficulties emerge in evaluating climate model performance because (i) we always have to deal with three different kinds of attractors: the attractor of the real climate system, its reconstruction from observations, and the attractors of the climate models; and (ii) because of the high dimensionality of both the phase space and the parameter space of these attractors.
In order to address these issues, multi-variable metrics are currently used to try to assess the skill of available climate models. Thus, Fig. 19 shows a diagram describing the performance of the 42 models participating in PCMDI's CMIP5. Even superficial analysis of the diagram indicates that no model is the best for all the variables under consideration. An improved assessment package is given by PCMDI's Metrics Package (Gleckler et al., 2016) and by the ESMValTool package (Eyring et al., 2016b). Recently, Lembo et al. (2019) released TheDiato, a flexible diagnostic tool able to evaluate comprehensively the energy, entropy and water budgets, and their transports for climate models. This package will become part of the second generation of the ESMValTool package.
Finally, in order to describe synthetically and comprehensively the outputs of a growing number of climate models, it has become common to consider multi-model ensembles and focus the attention on the ensemble mean and the ensemble spread. Mean and spread have been taken as the, possibly weighted, first two moments of the models' outputs for the metric under study (Tebaldi and Knutti, 2007); see Fig. 18. This approach merges information from different attractors and the resulting statistical estimators cannot be interpreted in the standard way, with the mean approximating the true field and the standard deviation describing its uncertainty. Such a naive interpretation relies on an implicit assumption that the set is a probabilistic ensemble formed by equivalent realizations of a given process, and that the underlying probability distribution is unimodal; see Parker (2010) for a broader epistemological discussion of the issues.
While the models in such an "ensemble of opportunity" may be related to each other, as shown in Fig. 17, they are by no means drawn from the same distribution. A number of alternative approaches for uncertainty quantification in climate modeling have been proposed but they go beyond the scope of the present review.
III. CLIMATE VARIABILITY AND THE MODELING HIERARCHY
A. Radiation Balance and Energy-Balance Models (EBMs)
The concepts and methods of the theory of deterministic dynamical systems (Andronov and Pontryagin, 1937;Arnold, 1983;Guckenheimer and Holmes, 1983) have been applied first to simple models of atmospheric and oceanic flows, starting about half-a-century ago (Lorenz, 1963;Stommel, 1961;Veronis, 1963). More powerful computers now allow their application to fairly realistic and detailed models of the atmosphere, ocean, and the coupled atmosphere-ocean system. We start herewith by presenting such a hierarchy of models.
This presentation is interwoven with that of the successive bifurcations that lead from simple to more complex solution behavior for each climate model. Useful tools for comparing model behavior across the hierarchy and with observations are provided by ergodic theory (Eckmann and Ruelle, 1985;Ghil et al., 2008). Among these tools, advanced methods for the analysis and prediction of uniand multivariate time series play an important role (Ghil et al., 2002, and references therein).
The concept of a modeling hierarchy in climate dynamics was introduced by Schneider and Dickinson (1974). Several authors have discussed lately in greater detail the role of such a hierarchy in the understanding and prediction of climate variability Ghil and Robertson, 2000;Held, 2005;Lucarini, 2002;Saltzman, 2001). At present, the best-developed hierarchy is for atmospheric models. These models were originally developed for weather simulation and prediction on the time scale of hours to days. Currently they serve -in a stand-alone mode or coupled to oceanic and other models -to address climate variability on all time scales.
The first rung of the modeling hierarchy for the atmosphere is formed by zero-dimensional (0-D) models, where the number of dimensions, from zero to three, refers to the number of independent space variables used to describe the model domain, i.e. to physical-space dimensions. Such 0-D models essentially attempt to follow the evolution of the globally averaged air temperature at surface as a result of changes in global radiative balance:
c dT dt = R i − R o ,(12a)R i = µQ 0 {1 − α(T )},(12b)R o = σm(T )(T ) 4 .(12c)
Here R i and R o are incoming solar radiation and outgoing terrestrial radiation. The heat capacity c is that of the global atmosphere, plus that of the global ocean or some fraction thereof, depending on the time scale of interest: one might only include in c the ocean mixed layer when interested in subannual time scales but the entire ocean when studying paleoclimate (e.g., Saltzman, 2001).
The rate of change ofT with time t is given by dT /dt, while Q 0 is the solar radiation received at the top of the atmosphere, also called the "solar constant," σ is the Stefan-Boltzmann constant, and µ is an insolation parameter equal to unity for present-day conditions. To have a closed, self-consistent model, the planetary reflectivity or albedo α and grayness factor m have to be expressed as functions ofT ; m = 1 for a perfectly black body and 0  0 the two branches X = ±µ 1/2 inherit its stability.
In the 2-D THC problem, the left cell dominates on one of the two branches, while the right cell dominates on the other: for a given value of µ, the two stable steadystate solutions -on the {X = +µ 1/2 } branch and on the {X = −µ 1/2 } branch, respectively -are mirror images of each other. The idealized THC in Fig. 22, with the North Atlantic Deep Water extending to the Southern Ocean's polar front, corresponds to one of these two branches. In theory, therefore, a mirror-image circulation, with the Antarctic Bottom Water extending to the North Atlantic's polar front, is equally possible. Bryan (1986) was the first to document transition from a two-cell to a one-cell circulation in a simplified ocean GCM with idealized, symmetric forcing. In Sect. III.B atmospheric GCMs confirmed essentially the EBM results. Results of coupled ocean-atmosphere GCMs, however, have led to questions about the realism of more than one stable THC equilibrium. The situation with respect to [After Quon and Ghil (1992) with permission from Cambridge University Press.] the THC's pitchfork bifurcation (16) is thus subtler than it was with respect to Fig. 21 for radiative equilibria. Dijkstra (2007) showed, however, by comparing observational and reanalysis data with high-end ocean GCMs, that the current Atlantic Ocean's THC is actually in its multiple-equilibria regime. Climate models of intermediate complexity did indeed find the Atlantic Ocean to be multistable (e.g., Rahmstorf et al., 2005), while several of the GCMs used in recent IPCC ARs did not show such a behavior, with an exception coming from the study by Hawkins et al. (2011); see Fig. 24. Thus a failure of highend models in the hierarchy to confirm results obtained on the hierarchy's lower rungs does not necessarily imply it is the simpler models that are wrong. To the contrary,   Ghil (1994).
Bifurcation Diagrams for GCMs
such a failure might well indicate that the high-end models, no matter how detailed, may still be rather imperfect; see also Ghil (2015) for a summary.
Internal variability of the THC -with smaller and more regular excursions than the huge and totally irregular jumps associated with bistability -was studied intensively in the late 1980s and the 1990s. These studies placed themselves on various rungs of the modeling hierarchy, from box models through 2-D models and all the way to ocean GCMs. A summary of the different kinds of oscillatory variability found in the latter appears in Table I 6 . Such oscillatory behavior seems to match more closely the instrumentally recorded THC variability, as well as the paleoclimatic records for the recent geological past, than bistability.
The (multi)millennial oscillations interact with variability in the surface features and processes shown in Fig. 22. Chen and Ghil (1996), in particular, studied some of the interactions between atmospheric processes and the THC. They used a so-called hybrid coupled model, namely a 2-D EBM, coupled to a rectangularbox version of the North Atlantic rendered by a lowresolution ocean GCM. This hybrid model's regime diagram is shown in Fig. 25(a). A steady state is stable for higher values of the coupling parameter λ ao or of the EBM's diffusion parameter d. Interdecadal oscillations with a period of 40-50 years are self-sustained and stable for lower values of these parameters.
The self-sustained THC oscillations in question are characterized by a pair of vortices of opposite sign that grow and decay in quadrature with each other in the ocean's upper layers. Their centers follow each other anti-clockwise through the northwestern quadrant of the model's rectangular domain. Both the period and the spatio-temporal characteristics of the oscillation are thus rather similar to those seen in a fully coupled GCM with realistic geometry (Delworth et al., 1993). The transition from a stable equilibrium to a stable limit cycle, via Hopf bifurcation, in this hybrid coupled model, is shown 6 Anomalies in the table are defined as the difference between the monthly mean value of a variable and its climatological mean.
in Fig. 25(b), and we will further elaborate upon it in Sect. III.D.2 below.
D. Bistability, Oscillations and Bifurcations
In Sects. III.A, III.B and III.C.1, we have introduced bistability of steady-state solutions via saddle-node and pitchfork bifurcations, while in Sect. III.C.2, we mentioned oscillatory solutions and Hopf bifurcation as the typical way the latter are reached as a model parameter changes. We start this subsection by summarizing the bifurcations of steady-state solutions, often referred to as equilibria, and will then introduce and discuss the normal forms of Hopf bifurcation, both supercritical and subcritical.
Bistability and Steady-State Bifurcations
Section III.B introduced EBMs and explained how the present climate and a totally ice-covered Earth can result as coexisting stable steady states over a certain parameter range. The normal forms of a supercritical and a subcritical saddle-node bifurcation were given as Eqs. (14) and (15), respectively. Here the criticality is defined as the existence of the two equilibria, stable an unstable, to the right or the left of the critical or bifurcation point.
An S-shaped bifurcation curve, such as the one that appears in Fig. 21 for the 1-D EBM of Eq. (13), can be obtained easily by soldering smoothly together the backto-back saddle-node bifurcations of Eqs. (14) and (15). While there is no generic normal form for such a curve, here is a simple example:
X = µ − X 2 ,(17a)X = (µ − µ 0 ) + (X − X 0 ) 2 ,(17b)
with µ 0 = 1 and X 0 = −1/2. Note that both the sub-and supercritical saddle-node bifurcations are structurally stable, i.e. they persist as the system of evolution equations, be it a system of ordinary or partial differential equations, is smoothly perturbed (Andronov and Pontryagin, 1937;Arnold, 2003;Guckenheimer and Holmes, 1983;Temam, 1997) by a small amount. This robustness is the reason why saddle-node bifurcations -and other elementary bifurcations called of co-dimension one, i.e., depending on a single parameter, like the Hopf bifurcation -are so important and can, in practice, be a guide through the hierarchy of models, in the climate sciences and elsewhere. A striking example was provided in Sect. III.B for the supercritical saddlenode bifurcation that can be found in the 3-D GCM of Wetherald and Manabe (1975, Fig. 5), as well as the 1-D EBM reproduced in Fig. 21 herein. The next kind of bifurcation that leads to bistability of stationary states is the pitchfork bifurcation introduced in Sect. III.C.1, in connection with the THC, and whose normal form is given by Eq. (16). This bifurcation, however, only arises in the presence of a mirror symmetry in the model under study, such as seen in Fig. 23a. One suspects, therefore, that it is nor structurally stable with respect to perturbations of the dynamics that do not preserve this symmetry.
A simple example is given by the following perturbed pitchfork bifurcation:
X = f (X; µ, ) = X(µ − X 2 ) + ,(18)
where is a small parameter. The bifurcation diagrams for = 0 and = +0.1 are given in Figs. 26(a,b), respectively. Clearly, a nonzero value of breaks the X → −X symmetry of Eq. (16), since it is no longer the case that f (X) = f (−X); hence the pitchfork breaks up into a continuous uppermost branch that is stable for all X-values, and a supercritical saddle-node bifurcation, whose lower branch is stable. If  0, i.e., it contains the possibility of an oscillatory instability, while the cubic term corresponds to an increasing modification of this simple rotation away from the origin. The parsimonious complex notation above follows Arnold (1983) and Ghil and Childress (1987) and is, we believe, more suggestive and transparent than the more common one that uses separately the two real variables x and y (e.g., Guckenheimer and Holmes, 1983). The advantages of the former are apparent when introducing polar variables via z = ρ 1/2 e iθ , with ρ = zz ≥ 0. One can then separate the flow induced by Eq. [From Chen and Ghil (1996) with permission from the American Meteorological Society.] rotation with angular velocity ω and a radius r = ρ 1/2 that either increases or decreases asρ ≷ 0, according tȯ
ρ = 2ρ(µ + cρ),(20a)θ = ω.(20b)
Equation (20a) is quadratic in ρ and has the two roots ρ = 0 and ρ = −µ/c. The former corresponds to a fixed point at the origin z = 0 in Eq. (19), while the latter only exists when cµ  0, is plotted in Fig. 27b: it corresponds to subcritical Hopf bifurcation. Here, in fact, the stability of the fixed point is also lost at µ = 0 but there is no oscillatory solution for µ > 0 at all, since the higher-order terms in either Eq. (19) or (20a) cannot stabilize the linearly unstable oscillatory solution, which spirals out to infinity. It is not just that the parabolas in Figs. 27a and 27b point in opposite directions, as for the saddle-node bifurcations in Eqs. (17a) and (17b) or Eqs. (14) and (15), but the two figures are topologically distinct.
E. Main Modes of Variability
The atmosphere, ocean, and the coupled oceanatmosphere climate system have many modes of variability, as initially discussed in Sects. II.A-II.B-II.C. We review here some of the most important ones, and comment on the general features of such modes.
Modes of Variability and Extended-Range Prediction
Several large-scale spatial patterns of atmospheric covariability have been studied, starting in the second half of the 19 th century. Lorenz (1967) and Wallace and Gutzler (1981) provided good reviews of the earlier studies. The earliest work tended to emphasize "centers of action," where the variability is strongest (Teisserenc de Bort, 1881), while more recently, it is so-called teleconnections between such centers of action that have been emphasized. J. Namias (1910Namias ( -1997 played a key role in developing the interest in such teleconnections, by applying systematically the use of their spatio-temporal properties to the development of operational extended-range weather forecasting (e.g., Namias, 1968).
Returning to the discussion of prediction in Sect. II.D, it is important to recall John von Neumann's (1903Neumann's ( -1957 important distinction (Von Neumann, 1960) between weather and climate prediction. To wit, short-term NWP is the easiest -i.e., it is a pure initial-value problem; long-term climate prediction is next easiest -it corresponds to studying the system's asymptotic behav- ior; while intermediate-term prediction is hardest -both initial and boundary values are important. Von Neumann's role in solving the NWP problem by integrating the discretized equations that govern large-scale atmospheric motions (Bjerknes, 1904;Charney et al., 1950b;Richardson, 1922) is well known. In fact, he also played a key role in convening the conference on the "Dynamics of Climate" that was held at Princeton's Institute for Advanced Studies in October 1955, and whose proceedings (Pfeffer, 1960) were finally published three years after Von Neumann's untimely death. Today, routine NWP is quite skillful for several days, but we also know that detailed prediction of the weather is limited in theory by the exponential growth of small errors (Lorenz, 1963) and by their turbulent cascading from small to large scales (Leith and Kraichnan, 1972;Lorenz, 1969a,b;Thompson, 1957). The theoretical limit of detailed prediction -in the sense of predicting future values of temperature, wind and precipitation at a certain point, or within a small volume, in time and space -is of the order of 10-15 days (Epstein, 1988;Tribbia and Anthes, 1987).
In the sense of Von Neumann (1960), short-term prediction is being improved by meteorologists and oceanographers through more-and-more accurate discretization of the governing equations, increased horizontal and vertical resolution of the numerical models, improved observations and data assimilation methodologies, and improved parametrization of subgrid-scale processes.
Important strides in solving the theoretical problems of the climate system's asymptotic behavior are being taken by the use of idealized models -either by simplifying the governing equations in terms of the number of subsystems and of active physical processes, e.g., by eliminating phase transitions or chemical processes in the atmosphere -or by systematic model reduction to small or intermediate-size sets of ordinary differential or stochastic differential equations (e.g., Chang et al., 2015;Palmer and Williams, 2009). We shall return to the latter in Sects. IV and V.
What, then, if anything, and how, and how accurately can climate-related scalars or fields be predicted beyond the limits of NWP? In other words, what can be done about the gap between short-term and asymptotic prediction of climate? These issues have been actively pursued for the last three decades (e.g., Epstein, 1988;Tribbia and Anthes, 1987, and many others). Atmospheric, oceanic and coupled modes of variability play an important role in extended-and long-range forecasting.
The key idea is that a mode that is stationary or oscillatory can, by its persistence or periodicity, contribute a fraction of variance that is predictable for the mode's characteristic time or, at least, for a substantial fraction thereof. We start therefore with a brief review of some of the most promising modes of variability.
Coupled Atmosphere-Ocean Modes of Variability
The best known of all these modes is the El Niño-Southern Oscillation (ENSO: Dijkstra, 2005;Philander, 1990, and references therein), mentioned earlier in Sect. II.B. The ENSO phenomenon is particularly strong over the Tropical Pacific, but it affects temperatures and precipitations far away and over a large area of subcritical Hopf bifurcation for c = +1. Solid lines indicate stable solutions (s), and dashed lines indicate unstable ones (u); it is common to plot |z| = −µ 1/2 along with |z| = µ 1/2 , in order to emphasize that one is actually projecting onto the (|z|, µ)-plane a paraboloid-shaped, one-parameter family of limit cycles with given r = |z| = µ 1/2 and 0 ≤ θ  0 and γ is real. We are interested in x, which is slow, and want to parameterize r, which is fast.
Proceeding for (39) as we did for the system (36), but now for a fixed realization ω ∈ Ω, we get
r(t; ω) = e −αt r 0 + γ t 0 e −α(t−s) x(s)ds + W t (ω),
where r(t 0 ; ω) = r 0 . Substituting this back into (39a) yields the following randomly forced integro-differential equation, (40) which is the analog of Eq. (37) in the stochastic-dynamic context of (39). Kondrashov et al. (2015) proved rigorously that the EMR (38) is equivalent to a suitable generalization of the GLE (40). Thus, the EMR methodology can be seen as an efficient implementation of the MZ formalism, i.e., an efficient solution of the associated GLE, even in the absence of large-scale separation. This result explains the remarkable success of EMR in producing reduced models that capture the multimodality as well as the nontrivial power spectrum of phenomena merely known from time series of observations or of high-end model simulations.
x = f (x) + e −αt r 0 + γ t 0 e −α(t−s) x(s)ds + W t (ω),
In the remainder of this subsection, we give two examples of this success and further references to many more. Alternative approaches to efficient solutions of the GLE can be found, for instance, in Chorin et al. (2002).
d. EMR applications. We choose here an EMR model to simulate Northern Hemisphere mid-latitude flow (Kravtsov et al., 2005) and a real-time ENSO prediction model (Kondrashov et al., 2005b). Further examples of successful application of the methodology appear in Kravtsov et al. (2009Kravtsov et al. ( , 2011 and elsewhere. Kravtsov et al. (2005) introduced the EMR methodology and illustrated it at first with quadratically nonlinear models of the general form given here in Eq. (38). The applications were to the Lorenz (1963) convection model, the classical double-well potential in one space dimension, and a triple-well potential in two dimensions with an exponential shape for the wells. More challenging was a real-data application to geopotential height data for 44 boreal winters (1 December 1949-31 March 1993. The data set consisted of 44 × 90 = 3960 daily maps of winter data, defined as 90-day sequences starting on 1 December of each year. The best EMR fit for the data required the use of nine principal components and of L = 3 levels.
The probability density functions (PDFs) for the observed and the EMR model-generated data sets are plotted in Fig. 32. The EMR clearly captures quite well the three modes obtained with a Gaussian mixture model, cf. Smyth et al. (1999) and . These three modes correspond to three clusters found by by Cheng and Wallace (1993) using very different methods on a somewhat different data set. The maps of the corresponding centroids appear as Fig. 1 in  and are discussed therein; they agree quite well with those of Cheng andWallace (1993), cf. Smyth et al. (1999, Fig . 9). Kondrashov et al. (2005b) fitted the global SST field between 30 • S-60 • N over the time interval January 1950-September 2003 by using linear and quadratic EMR models with one and two noise levels, L = 1, 2, based on monthly SST anomaly maps and allowing a seasonal dependence of the dissipative terms in Eq. (38). Their results when using L = 2 were much better, for either a linear or a quadratic model, which clearly shows the role of memory effects in EMR modeling and the connection FIG. 32: Multimodal probability density function (PDF) for the Northern Hemisphere's geopotential height anomalies of 44 boreal winters; see Smyth et al. (1999) for details of the data set and of the mixture model methodology for computing the PDFs. (a) PDF of the observed height anomalies; and (b) of the anomalies given by the EMR model. Modified from Kravtsov et al. (2005), with the permission of the American Meteorological Society.
with the MZ formalism that was explained above. The use of the EMR models in prediction was tested by so-called hindcasting or retrospective forecasting, i.e., a protocol -also called "no look-ahead" -in which the data available past a certain time instant are eliminated when constructing the model to be used in the forecast to be evaluated. The results of these tests are plotted in Fig. 33, for L = 2 and a linear vs. a quadratic model. The light-black rectangle in the Eastern Tropical Pacific corresponds to the region (5 • S-5 • N, 150 • -90 • W) over which SST anomalies are averaged to obtain the Niño-3 index, already mentioned in Sect. III.E.2.
Climate forecast skill is measured mainly via rootmean-square errors and anomaly correlations. The former skill scores are given in Kondrashov et al. (2005b, Fig. 2d) and clearly indicate the superiority of the quadratic model. The latter apeear in Fig. 33 here, too.
Anomaly correlations of roughly 0.5-0.6 (red in the figure) are considered quite useful in climate prediction, and the red area is substantially larger in panel (b) here, covering most of the Tropical Pacific and Indian Oceans. Concerning actual real-time forecasts, Barnston et al. (2012) have found that, over the 2002-2011 interval of their evaluation, the EMR-based forecasts of UCLA's Theoretical Climate Dynamics (TCD) group were at the top of the eight statistical models being evaluated and exceeded in skill by only a few of the 12 dynamical, high-end models in the group that participated in the IRI ENSO Forecast plume https://iri.columbia.edu/ our-expertise/climate/forecasts/enso/current/ ?enso_tab=enso-sst_table.
e. Explicit derivation of the parametrized equations. The EMR methodology is data-driven, so that it allows to construct bottom-up an effective dynamics able to ac- count for the observed data. Lucarini (2012, 2013), showed that the MZ formalism can be used also to derive the parametrizations in a top-down manner in a rather general way. Assume the system of interest is described by the following evolution equations:
X = f X (X) + Ψ X (X, Y), (41a) Y = f Y (Y) + Ψ Y (X, Y);(41b)
here, again, X ∈ R m , Y ∈ R n , X is the set of large-scale, energetic variables of interest and, typically, m n. We assume, furthermore, that is a small parameter describing the strength of the coupling between the two sets of variables, and that, if = 0, the dynamics is chaotic for both the X and Y variables. By expanding the MZ projection operator, it is possible to derive the following expression for the projected dynamics on the X variables, which is valid up to order O( 3 ):
X = f X (X)+ M (X)+ S(X)+ 2 K(X, t−s)ds. (42)
This equation provides the explicit expression of the mean-field, deterministic term M ; the time-correlation properties of the stochastic term S that is, in general, multiplicative; and of the integration kernel K, which defines the non-Markovian contribution. Figure 34 provides a schematic diagram of the three terms of the parametrization: the M -term comes from time averaging of the effects of the Y variables on the X variables; the S-term results from the fluctuations of the forcing of the Y variables on the X variables; and the non-Markovian contribution represents the selfinteraction of the X variables on themselves at a later time, mediated by the Y variables.
These terms are derived using the statistical properties of the uncoupled dynamics of the Y variables, at = 0. Note that, in the limit of infinite time scale separation between the X and Y variables, the non-Markovian term drops out and the stochastic term becomes a white-noise contribution, where one needs to use the Stratonovich definition of the stochastic integral. If, instead, Ψ Y = 0 in the master-slave system (41), the non-Markovian term is identically zero, as expected. term S; and (c) memory term with kernel K. The symbols (τ, τ 1 , τ 2 ) denote the delays involved in these effects. Reproduced with permission from Wouters and Lucarini (2013).
A surprising property of the surrogate dynamics given by Eq. (42) is that the expectation value of any observable φ(X) is the same as for the full dynamics governed by Eq. (41), up to third order in (Wouters and Lucarini, 2013). This property is due to the fact that one can derive Eq. (42) by treating the weak coupling using Ruelle response theory (Ruelle, 1998(Ruelle, , 1999(Ruelle, , 2009); see Sect. IV.E.3 below. As a result, the average model error due to the use of parametrized dynamics is well under control; see discussion in Hu et al. (2019). Moreover, Vissio and Lucarini (2018a,b) showed by explicit examples that this top-down approach can, in some cases, help derive scale-adaptive parametrizations, while Demaeyer and Vannitsem (2017) demonstrated its effectiveness in an intermediate complexity climate model.
IV. CLIMATE SENSITIVITY AND RESPONSE
A central goal of the climate sciences is to predict the impact of changes in the system's internal or external parameters -such as the greenhouse gas (GHG) concentration or the solar constant -on its statistical properties.
A key concept in doing so is climate sensitivity, which aims to measure the response of the climate system to external perturbations of Earth's radiative balance. As we shall see below, this measure is being used for projecting, for instance, mean temperature changes over the coming century as a response to increasing concentrations of atmospheric GHGs. While a good start, accurate and flexible predictions of climate changes require, though, more sophisticated concepts and methods.
A. A Simple Framework for Climate Sensitivity
In order to illustrate the main ideas, let us consider the simple energy balance model (EBM) introduced in Eqs. (12) of Sect. III.A where the net radiation R = R i − R o at the top of the atmosphere is related to the corresponding average temperature T near the Earth's surface by R = R(T ). This simple, 0-D EBM includes both longwave and shortwave processes, so that cdT /dt = R(T ), as in Eq. 12a.
Following Peixoto and Oort (1992) and Zaliapin and Ghil (2010), we assume, furthermore, that there are N climatic variables {α k = α k (T ), k = 1, . . . , N } that are, to a first approximation, directly affected by the temperature change only and which can, in turn, affect the radiative balance. Hence, one can write R = R (T, α 1 (T ), . . . , α N (T )). Let us assume, furthermore, that, for a certain reference temperature T = T 0 one has R(T 0 ) = 0, which corresponds to steady-state conditions. The simplest framework for climate sensitivity is to think of the difference in global annual mean surface air temperature ∆T between two statistical steady states, which have distinct CO 2 concentration levels. We then assume that changing the CO 2 concentration corresponds to applying an extra net radiative forcing ∆R to the system, and look for the corresponding change ∆T in the average temperature, so that R(T 0 + ∆T ) + ∆R = 0.
For small ∆T and smooth R = R(T ), the Taylor expansion yields
∆R = − dR dT T =T0 ∆T + O (∆ T ) 2 = ∂R ∂T T =T0 ∆ T − N k=1 ∂R ∂α k ∂α k ∂T T =T0 ∆T + O (∆ T ) 2 .(43)
Here, O(x) is a function such that O(x) ≤ C x as soon as 0  1, the system is unstable.
Referring again to Eqs. (12), the feedback associated with the dependence of α on the temperature in Eq. (12b) is traditionally the ice-albedo feedback, while the dependence of the emissivity m on the temperature in Eq. (12c) is associated with the changes in the atmospheric opacity. For the latter one, the standard, reference sensitivity, associated with deviations from Planck's law for black-body radiation, is λ 0 (T 0 ) = −∂R/∂T | T =T0 = 4σm(T 0 )T 3 0 . More specifically, feedbacks that can contribute to changes in reflectivity in Eq. (12b) include, aside from the incremental presence of snow and ice, also the climatevegetation feedback (e.g., Rombouts and Ghil, 2015;Watson and Lovelock, 1983;Zeng and Neelin, 2000, and references therein). The feedbacks that can affect the sensitivity of emitted radiation in Eq. (12c) include atmospheric alteration in water vapor content and cloud cover, as well as in GHGs and aerosol concentration.
In climate studies, different measures of climate sensitivity are used. The so-called equilibrium climate sensitivity (ECS) denotes the globally and annually averaged surface air temperature increase that would result from sustained doubling of the concentration of carbon dioxide in Earth's atmosphere vs. that of the reference state, after the climate system had reached a new steady-state equilibrium . The ECS was used extensively by the IPCC's first three assessment reports, up to IPCC (2001). Taking the linear approximation in Eqs. (43)-(46) above, one has:
ECS = Λ(T 0 )∆R 2×CO2 = λ 0 ((T 0 )) 1 − N k=1 f k (T 0 ) ∆R 2×CO2 .(48)
Note that the radiative forcing is, to a good approximation, proportional to the logarithm of the CO 2 concentration. Hence, in the linear-response regime, and for a given reference state T 0 , the long-term globally averaged surface air temperature change resulting from a quadrupling of the CO 2 concentration is twice as large as the ECS; see Fig. 35. The concept of climate sensitivity can be generalized to describe the linear dependence of the long-term average of any climatic observable with respect to the radiative forcing due to changes in CO 2 or in other GHGs, as well as to changes in solar radiation, aerosol concentration or any other sudden changes in the forcing (Ghil, 1976(Ghil, , 2015von der Heydt et al., 2016;Lucarini et al., 2010b).
B. Climate Sensitivity: Uncertainties and Ambiguities
The ECS is widely considered to be the most important indicator in understanding climate response to natural and anthropogenic forcings. It is usually estimated from instrumental data coming from the industrial age, from proxy paleoclimatic data, and from climate models of different levels of complexity. In climate models, the ECS results from a nontrivial combination of several model parameters that enter the feedback factors {f k } in Eq. (44b) above, and it requires careful tuning. Despite many years of intense research, major uncertainties still exist in estimating it from past climatic data, as well as substantial discrepancies among different climate models (IPCC, 2001(IPCC, , 2007(IPCC, , 2014a. In fact,  estimated the ECS uncertainty as 1.5-4.5 K for CO 2 doubling and this range of uncertainties has increased rather than decreased over the four intervening decades. The basic reason for these uncertainties lies in the high sensitivity of the ECS to the strength of the feedbacks f k , as a result of the factor 1/{1 − N k=1 f k (T 0 )} in Eq. (47). Efforts to reduce the uncertainty in ECS values for the current climate include adopting ultra-high resolution GCMs (Satoh et al., 2018), in which one may better account for feedbacks that act on a larger range of scales, to applying relations that are rigorously valid for simple stochastic models to the climate data (Cox et al., 2018), where one hopes to take advantage of general, and possibly universal, relationships between climatic variables. In particular, the largest uncertainty in defining the ECS for the current climate state is associated with the difficulty in estimating correctly the strength of the two main feedbacks associated with clouds (Bony et al., 2015;Schneider et al., 2017). A warmer climate leads to increased presence of water vapor in the atmosphere, and, in turn, to more clouds. An increased cloud cover leads, on the one side, to an increase in the climate system's albedo (cooling effect) and, on the other side, to a more efficient trapping of longwave radiation near the surface (warming effect). The balance between the two feedbacks changes substantially according to the type of cloud, with the cooling effect dominant for low-lying clouds, while the warming effect is dominant for high-altitude ones. This is a striking example of the multiscale nature of the climate system: an extremely small-scale, short-lived dynamical process -cloud formation -has a substantial effect on the planet's global and long-term energy budget.
Despite the highly simplified description above, it should be clear that the ECS is a state-dependent indicator. This state dependence is further supported by the evidence in Figs. 36 and 37. In particular:
• Both the Planck response and the strength of the feedbacks that determine the gain factor Λ in Eq. (47) depend on the reference state T 0 . As an example, in warmer climates where sea ice is absent, the positive ice-albedo feedback is greatly reduced, thus contributing to a smaller climate sensitivity.
On the other hand, in warmer climates the atmosphere is more opaque as a result of the presence of more water vapor, leading to a strong enhancement of the greenhouse effect.
• The radiative forcing is only approximately linear with the logarithm of the CO 2 concentration, so that ∆R 2×CO2 depends on the concentration's reference value. In fact, this dependence is weak across a large range of CO 2 concentrations, but it is greatly strengthened by optical saturation effects in the CO 2 absorption bands;
• Near the moist greenhouse threshold, which corresponds to a tipping point of the Earth system, the ECS is greatly strengthened. Figure 36 shows that for a solar irradiance comparable or stronger than the present one, the peak in the value of the ECS is realised at a surface temperature of about 320 K, which corresponds to a lower CO 2 concentration in the case of weaker irradiance; see the discussion in Gómez-Leal et al. (2018). Note that reaching the moist greenhouse threshold for lower values of the solar irrandiance requires exceedingly high CO 2 concentrations.
While useful, the ECS concept faces practical difficulties because its definition assumes that, after the forcing is applied, the climate reaches a new steady state after all transients have died out. Since the climate is multiscale in both time and space, it is extremely non-trivial to define an effective cut-off time scale able to include all transient behavior. Thus, a time scale of 100 years is long compared to atmospheric processes, but short with respect to oceanic ones that involve the deep ocean. While a time scale of 5 000 years is long compared to oceanic processes, but short with respect to cryospheric ones that involve the dynamics of the Antarctic ice sheets. Therefore, one needs to associate each ECS estimate from observational or model data to a reference time scale; see Fig. 38 for an illustrative cartoon, and its discussion by Members et al. (2012) andvon der Heydt et al. (2016).
C. Transient Climate Response (TCR)
Transient climate response (TCR) has recently gained popularity in the study of climate change and climate variability because of its ability to help capture the time dimension of climate change. TCR is defined as the change in the globally averaged surface air temperature recorded at the time at which CO 2 has doubled due to an increase at a 1% annual rate, i.e. roughly after 70 years, having started at a given reference value T 0 (Otto et al., 2013). The advantage of TCR over ECS lies in the fact that it is operationally well defined, fits very well with the standard IPCC-like simulation protocols; it has the merit of addressing the transient, rather than asymptotic, response of the climate system to perturbations in the CO 2 concentration. Therefore, it is better suited to test model outputs with observational data sets from the industrial era. As shown in Fig. 39, the TCR is found to be lower than the ECS for a long time, because of the climate system's thermal intertia, which is dominated by the oceans' heat capacity. A smaller effective heat capacity c in Eq. (12a), and hence a shorter relaxation time, would result in the TCR catching up much faster with the ECS, as is the case in regular diffusion process; see Fig. 40a. Assuming linearity in the response, a relationship must clearly exist between ECS and TCR. So far, this inference has been based, by-and-large, on heuristic arguments of time scale separation between climate feedbacks rather than being rigorously derived (Otto et al., 2013); it will be derived more systematically in Sect. IV.E herein.
D. Beyond Climate Sensitivity
The standard viewpoint on climate sensitivity discussed above is associated with the idea that the climate is in equilibrium, in the absence of external perturbations. In the setting of deterministic, autonomous dynamical systems, this view can be described by the change in the position of a fixed point, X 0 = X 0 (µ), as a function of a parameter µ.
We illustrate in Fig. 40 the difference between the ways that a change in a parameter can affect a climate model's behavior in the case of equilibrium solutions, cf. panel (a), vs. more complex dynamical behavior, cf. panels (b) and (c). Assume that the climate state is periodic, i.e., lies on a limit cycle, rather than being a fixed point, FIG. 39: Transient climate response (TCR) estimates from two climate models: (i) a coupled atmosphere-ocean GCM (red curve) and (ii) a simple illustrative model with no energy exchange with the deep ocean (green curve). Time on abscissa from start of CO 2 concentration increase at pre-industrial levels, with change in global mean temperature on the ordinate. The "additional warming commitment" corresponds to temperature stabilization at a given CO 2 level, i.e., at 2 × CO 2 or at 4 × CO 2 . Reproduced with permission from IPCC (2001, Fig. 9.1).
as in panel (a). In this case, climate sensitivity can no longer be defined by a single scalar, ∂T /∂µ, but needs four scalars -the sensitivity of the mean temperature along with that of the limit cycle's frequency, amplitude, and phase -or more, e.g., the orbit's ellipticity, too. But the internal climate variability can be better described in terms of strange attractors than by fixed points or limit cycles. Moreover, the presence of time-dependent forcing, deterministic as well as stochastic, introduces additional complexities into the proper definition of climate sensitivity. It is thus apparent that a rigorous definition of climate sensitivity requires considerably more effort. Ghil (2015) proposed to measure the change in the overall properties of the attractor before and after the change in forcing by computing the Wasserstein distance d W between the two invariant measures. The Wasserstein distance or "earth mover's distance" d W quantifies the minimum "effort" in morphing one measure into another one of equal mass on a metric space, like an n-dimensional Euclidean space (Dobrushin, 1970). Monge (1781) originally introduced this distance to study a problem of military relevance. Roughly speaking, d W represents the total work needed to move the "dirt" (i.e., the measure) from a trench you are digging to another one you are filling, over the distance between the two trenches. In general, the shape of the two trenches and the depth along the trench -i.e., the support of the measure and its density -can differ. Robin et al. (2017) and Vissio and Lucarini (2018b) recently showed the ef-fectiveness of applying this idea to climate problems.
E. A General Framework for Climate Response
A major use of state-of-the-art climate models is to produce projections of climate change taking into account different possible future scenarios of emission of greenhouse gases and pollutants like aerosols, as well as changes in land use, which has substantial impacts on the terrestrial carbon cycle. Projections are needed not only for quantities like the global average surface air temperature, but on spatially and temporally detailed information is needed for a multitude of practical needs; see, for instance, Fig. 18.
The ECS concept is well suited for describing the properties of equilibrium solutions of heuristically simplified equations of the climate system, like Eq. 12a, and has clear intuitive appeal, as in Fig. 40a. But it also has basic scientific limitations:
• it only addresses long term-climatic changes and no detailed temporal information, an issue only partially addressed by TCR information;
• it only addresses changes in the globally averaged surface air temperature and no spatial information at the regional scale and at different levels of the atmosphere, of the ocean, and of the soil;
• it cannot discriminate between radiative forcings resulting from different physical and chemical processes, e.g. differences resulting from changes in aerosol vs. GHG concentration; the two impact quite differently shortwave and longwave radiation, and different atmospheric levels.
We will thus try to address these shortcomings by taking the complementary points of view of nonequilibrium statistical mechanics and dynamical systems theory. The setting of nonautonomous and of stochastically forced dynamical systems allows one to examine the interaction of internal climate variability with the forcing, whether natural or anthropogenic; it also helps provide a general definition of climate response that takes into account the climate system's nonequilibrium behavior, its timedependent forcing, and its spatial patterns.
Pullback Attractor (PBA)
The climate system experiences forcings that vary on many different time scales (e.g., Saltzman, 2001), and its feedbacks also act on multiple time scales (e.g., Ghil and Childress, 1987). Hence, defining rigorously what climate response to forcing, vs. intrinsic variability, actually is requires some care: Observed variations can be related to the presence of natural periodicities -such as the daily and the seasonal cycle, and orbital forcings; to rapid, impulsive forcings, such as volcanic eruptions; or to slow modulations to the parameters of the system, as in the case of anthropogenic climate change.
For starters, consider a dynamical system in continuous time,ẋ
= F (x, t) (49) on a compact manifold Y ⊂ R d ; here x(t) = φ(t, t 0 )x(t 0 ), with initial state x(t 0 ) = x 0 ∈ Y.
The evolution operator φ(t, t 0 ) is assumed to be defined for all t ≥ t 0 , with φ(s, s) = 1, and it thus generates a two-parameter semigroup. In the autonomous case, time-translational invariance reduces the latter to a one-parameter semigroup since, ∀t ≥ s, φ(t, s) = φ(t − s). In the nonautonomous case, in other terms, there is an absolute clock. We are interested in forced and dissipative systems such that, with probability one, initial states in the remote past are attracted at time t towards A(t), a timedependent family of geometrical sets that define the system's pullback attractor (PBA). In the autonomous case, A(t) ≡ A 0 is the time-independent attractor of the system, and it is known to support, under suitable conditions, a physical measure µ(dx) (Eckmann and Ruelle, 1985;Ledrappier and Young, 1988). 7 Such a PBA can also be constructed when random forcing is present (e.g., Arnold, 1988, and references therein),
dx = F (x, t)dt + g(x)dη,(50)
where η = η(t; ω) is a Wiener process, while ω labels the particular realization of this random process, and dη(t) is commonly referred to as "white noise." The noise can be multiplicative, and one then uses the Itô calculus for the integration of Eq. (50).
In the random case, the PBA A(t; ω) is commonly referred to as a random attractor. A more detailed and mathematically rigorous discussion of these concepts appears in Carvalho et al. (2013);Chekroun et al. (2011) and Ghil et al. (2008, Appendix A). Careful numerical applications of PBAs to explain the wind-driven circulation and the THC are now available (Pierini et al., 2018(Pierini et al., , 2016Sevellec and Fedorov, 2015).
In the purely deterministic case, the theory of nonautonomous dynamical systems goes back to the skewproduct flows of Sell (1967). A concept that is closely related to PBAs is that of snapshot attractors; it was introduced in a more intuitive and less rigorous manner into . The construction is exact in the limit τ → ∞; for a random attractor, see Ghil et al. (2008, Figs. A.1 and A.2). Adapted from Sevellec and Fedorov (2015), with permission.
the physical literature by Romeiras et al. (1990). Snapshot attractors have also been used for studying timedependent problems of climatic relevance (Bódai et al., 2013(Bódai et al., , 2011Bódai and Tél, 2012;Drótos et al., 2015). Note that, in the most recent IPCC reports (IPCC, 2001(IPCC, , 2007(IPCC, , 2014a and according to the standard protocols described in Sect. II.D.2 , future climate projections are virtually always performed using as initial states the final states of sufficiently long simulations of historical climate conditions. As a result, it is reasonable to assume that the pullback time τ , as defined in Fig. 41, is large enough, and that the covariance properties of the associated A(t) sets are therefore well approximated.
Fluctuation Dissipation and Climate Change
The fluctuation-dissipation theorem (FDT) has its roots in the classical theory of many-particle systems in thermodynamic equilibrium. The idea is very simple: the system's return to equilibrium will be the same whether the perturbation that modified its state is due to a small external force or to an internal, random fluctuation. The FDT thus relates natural and forced fluctuations of a system (e,g., Kubo, 1957Kubo, , 1966; it is a cornerstone of statistical mechanics and has applications in many areas (Marconi et al., 2008, and references therein).
We have emphasized already in Sect. III.B that, even when the climate system is in a steady state, it is not at all in thermodynamic equilibrium (e.g., Ghil, 2019;Ragone, 2011). Still, Leith (1975) showed that FDT applies to a 2-D or QG turbulent flow with two integral invariants, kinetic energy E and enstrophy Z, under some additional assumptions of normal distribution of the realizations and stationarity. Soon thereafter, Bell (1980) showed that the FDT still seemed to work for a highly truncated version of such a model, even in the presence of dissipative terms that invalidate the thermodynamic equilibrium assumption.
The FDT has been applied to the output of climate models to predict the climate response to a step-like increase of the solar irradiance (North et al., 1993) as well as to increases in atmospheric CO 2 concentration (Cionni et al., 2004;Langen and Alexeev, 2005), while Gritsun and Branstator (2007) and Gritsun et al. (2008) used it to predict the response of an atmospheric model to localised heating anomalies. Most recently, Cox et al. (2018) tried to reduce the uncertainty in the ECS discussed in Sect. IV.B by a systematic FDT application to an ensemble of model outputs, as well as to the observed instrumental climate variability.
The FDT-based response of the system to perturbations in the above examples reproduces the actual changes at a good qualitative, rather than strictly quantitative level. An important limitation of these insightful studies is the use of a severely simplified version of the FDT that is heuristically constructed by taking a gaussian approximation for the invariant measure of the unperturbed system. This approximation amounts to treating the climate as being in thermodynamic equilibrium; see also Majda et al. (2009), who specifically address the applicability of the FDT in a reduced phase space.
In order to address the problem of gaussian approximation, Cooper and Haynes (2011) proposed to construct a kernel-based approximation of the actual invariant measure of the unperturbed system and then use Eq. (55) below to construct the system's Green function. This approach has been applied successfully in a very lowdimensional system, but its robustness may be limited by the kernel's arbitrary cut-off not being able to account for the smaller scales of the invariant measure's fine structure.
Ruelle Response Theory
FDT generalizations to systems out of equilibrium have been developed since the early 1950s (e.g., Kubo, 1966). But a particularly fruitful change in point of view was provided by D. Ruelle (Ruelle, 1998(Ruelle, , 1999(Ruelle, , 2009, who considered the problem in the setting of dynamical systems theory, rather than that of statistical mechanics. The former point of view is justified in this context by the so-called chaotic hypothesis (Gallavotti and Cohen, 1995), which states, roughly, that chaotic systems with many degrees of freedom possess a physically relevant invariant measure, as discussed in Sect. IV.E.1.
It is common in practice to assume that a timedependent measure µ t (dx) associated with the evolution of the dynamical system given by Eq. (49) does exist. Still, computing the expectation value of measurable observables with respect to this measure is in general far from trivial and requires setting up a large ensemble of initial states in the Lebesgue measurable set B mentioned before. Moreover, PBAs and the physical measures they support only set the stage for predicting the system's sensitivity to small changes in the forcing or the dynamics.
Ruelle's response theory allows one to compute the change in the measure µ(dx) of an autonomous Axiom A system due to a weak perturbations of intensity applied to the dynamics, in terms of the unperturbed system's properties. The basic idea behind it is that µ(dx) = µ(dx; ) of such a system, even though supported on a strange attractor, is differentiable with respect to . Lucarini (2016) discusses the radius of convergence in of the theory and Baladi (2008) summarizes its extensions and a different point of view on it.
The nonautonomous version of the Ruelle response theory allows one to calculate the time-dependent measure µ t (dx) on the PBA by computing the time-dependent corrections to it with respect to a reference state x(t) = x(t). This provides a fairly general formulation for the climate system's response to perturbations.
Let us assume that we can writė
x = F (x, t) = F (x) + X(x, t)(51)
where, ∀t ∈ R and ∀x ∈ Y ⊂ R d , | X(x, t)| |F (x)|. Hence, we can take F (x) as the background dynamics and X(x, t) as a perturbation. We only treat here the case of deterministic dynamics; stochastic perturbations are treated by Lucarini (2012). As shown by , we can restrict our analysis without loss of generality to the separable case of F (x, t) = F (x) + X(x)T (t).
To evaluate the expectation value Ψ (t) of a measurable observable Ψ(x) with respect to the measure µ t (dx) of the system governed by Eq. (49), one writes: (52) here Ψ 0 = Ψ(x)μ(dx) is the expectation value of Ψ with respect to the SRB invariant measureμ(dx) of the autonomous dynamical systemẋ = F (x). We will restrict ourselves here to the linear correction term, which can be written as:
Ψ (t) = Ψ(x)µ t (dx) = Ψ 0 + ∞ j=1 j Ψ (j) 0 (t);Ψ (1) 0 (t) = ∞ 0 ΛS τ 0 Ψ(x)T (t − τ )dτμ(dx); = ∞ 0 G (1) Ψ,X (τ )T (t − τ )dτ.(53)
The Green's function G
Ψ,X (τ ) above is given by
G (1) Ψ,X (τ ) = Θ(τ )ΛS τ 0 Ψ(x)μ(dx),(54)
where Λ(•) = X · ∇(•) and S t 0 (•) = exp(tF · ∇)(•) is the semigroup of unperturbed Koopman operators 8 , (·) denotes the inner product in Y, and the Heaviside distribution Θ(τ ) enforces causality. If the unperturbed invariant measure dμ(x) is smooth with respect to the standard Lebesgue measure, one hasμ(dx) =μ(x)dx, withμ(x) the density, and the Green's function can be writen as follows:
G (1) Ψ,X (τ ) = Θ(τ ) −∇ · (μ(x)X) µ(x) S τ 0 Ψ(x)μ(x)dx = Θ(τ )C(Φ, S τ 0 Ψ);(55)
here Φ = − (∇ · (μ(x)X))/μ(x) and C(A, S τ 0 B) is the τlagged correlation between the variables A and B, and the average of Φ vanishes. Note that Eq. (55) is the appropriate generalization of the FDT for the nonautonomous, out-of-equilibrium system (51).
Given any specific choice of the forcing's time dependency T (t) in Eqs. (51)-(53) and measuring the linear correction term Ψ
(1) 0 (t) from a set of experiments, the same equations allow one to derive the appropriate Green's function. Therefore, using the output of a specific set of experiments or of GCM simulations, we achieve predictive power for any temporal pattern of the forcing X(x).
Consider now the Fourier transform of Eq. (53):
Ψ (1) 0 (ω) = χ (1) Ψ,X (ω)T (ω),(56)
where we have introduced the susceptibility χ Ψ,X is causal is equivalent to saying that its susceptibility obeys the so-called Kramers-Kronig relations (Lucarini and Sarno, 2011;Ruelle, 2009); these provide integral constraints that link the real and imaginary parts, so that χ
(1) Ψ,X (ω) = iP(1/ω) χ (1) Ψ,X (ω), where i = √
−1, indicates the convolution product, and P stands for integration by parts. Extensions to the case of higher-order susceptibilities are also available (Lucarini, 2008a(Lucarini, , 2009aLucarini and Colangeli, 2012;Lucarini et al., 2005b).
Instead of studying merely individual trajectories, one can study the evolution of ensembles of trajectories to obtain probabilistic estimates. The evolution of the measure ρ driven by the dynamical systemẋ = F (x) is described by the transfer or Perron-Frobenius operator L t 0 (Baladi, 2000;Chekroun et al., 2014;Villani, 2009), which is the adjoint of the Koopman or composition operator S t 0 ; it is defined as follows:
L t 0 ρ(x)Ψ(x)dx = ρ(x)S t 0 Ψ(x)dx.(57)
Assuming that no degenaries are present, the generator of the semigroup {L t 0 } is the so-called Liouville operator L and it satisfies ∂ t ρ = −∇(ρF ) = Lρ.
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Stochastic perturbations to dynamical systems: A response theory approach. V Lucarini, 10.1007/s10955-012-0422-0Journal of Statistical Physics. 1464Lucarini, V (2012), "Stochastic perturbations to dynamical systems: A response theory approach," Journal of Statisti- cal Physics 146 (4), 774-786.
Modeling complexity: the case of climate science. V Lucarini, Models, Simulations, and the Reduction of Complexity. U Gähde, S Hartmann, and JH WolfDe GruyterLucarini, V (2013), "Modeling complexity: the case of cli- mate science," in Models, Simulations, and the Reduction of Complexity, edited by U Gähde, S Hartmann, and JH Wolf (De Gruyter) pp. 229-254.
Response operators for Markov processes in a finite state space: Radius of convergence and link to the response theory for Axiom A systems. V Lucarini, 10.1007/s10955-015-1409-4Journal of Statistical Physics. 1622Lucarini, V (2016), "Response operators for Markov processes in a finite state space: Radius of convergence and link to the response theory for Axiom A systems," Journal of Sta- tistical Physics 162 (2), 312-333.
Revising and extending the linear response theory for statistical mechanical systems: Evaluating observables as predictors and predictands. V Lucarini, 10.1007/s10955-018-2151-5Journal of Statistical Physics. 1736Lucarini, V (2018), "Revising and extending the linear re- sponse theory for statistical mechanical systems: Evaluat- ing observables as predictors and predictands," Journal of Statistical Physics 173 (6), 1698-1721.
Mathematical and physical ideas for climate science. V Lucarini, R Blender, C Herbert, F Ragone, S Pascale, J Wouters, 10.1002/2013RG000446Rev. Geophys. 524Lucarini, V, R. Blender, C. Herbert, F. Ragone, S. Pascale, and J. Wouters (2014), "Mathematical and physical ideas for climate science," Rev. Geophys. 52 (4), 809-859.
Edge states in the climate system: exploring global instabilities and critical transitions. V Lucarini, T Bódai, Nonlinearity. 30732Lucarini, V, and T. Bódai (2017), "Edge states in the climate system: exploring global instabilities and critical transi- tions," Nonlinearity 30 (7), R32.
Transitions across melancholia states in a climate model: Reconciling the deterministic and stochastic points of view. V Lucarini, T Bódai, 10.1103/PhysRevLett.122.158701Phys. Rev. Lett. 122158701Lucarini, V, and T. Bódai (2019a), "Transitions across melancholia states in a climate model: Reconciling the de- terministic and stochastic points of view," Phys. Rev. Lett. 122, 158701.
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Destabilization of the thermohaline circulation by transient changes in the hydrological cycle. V Lucarini, S Calmanti, V Artale, 10.1007/s00382-004-0484-zClimate Dynamics. 242Lucarini, V, S. Calmanti, and V. Artale (2005a), "Desta- bilization of the thermohaline circulation by transient changes in the hydrological cycle," Climate Dynamics 24 (2), 253-262.
Experimental mathematics: Dependence of the stability properties of a two-dimensional model of the atlantic ocean circulation on the boundary conditions. V Lucarini, S Calmanti, V Artale, 10.1134/S1061920807020124Russian Journal of Mathematical Physics. 142Lucarini, V, S. Calmanti, and V. Artale (2007), "Experimen- tal mathematics: Dependence of the stability properties of a two-dimensional model of the atlantic ocean circulation on the boundary conditions," Russian Journal of Mathe- matical Physics 14 (2), 224-231.
Beyond the linear fluctuation-dissipation theorem: the role of causality. V Lucarini, M Colangeli, Journal of Statistical Mechanics: Theory and Experiment. 2012055013Lucarini, V, and M. Colangeli (2012), "Beyond the lin- ear fluctuation-dissipation theorem: the role of causality," Journal of Statistical Mechanics: Theory and Experiment 2012 (05), P05013.
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Thermodynamic analysis of snowball earth hysteresis experiment: Efficiency, entropy production, and irreversibility. V Lucarini, K Fraedrich, F Lunkeit, Q. J. Royal Met. Soc. 136Lucarini, V, K. Fraedrich, and F. Lunkeit (2010a), "Thermo- dynamic analysis of snowball earth hysteresis experiment: Efficiency, entropy production, and irreversibility," Q. J. Royal Met. Soc. 136, 2-11.
Thermodynamics of climate change: generalized sensitivities. V Lucarini, K Fraedrich, F Lunkeit, 10.5194/acp-10-9729-2010Atmospheric Chemistry and Physics. 1020Lucarini, V, K. Fraedrich, and F. Lunkeit (2010b), "Ther- modynamics of climate change: generalized sensitivities," Atmospheric Chemistry and Physics 10 (20), 9729-9737.
A new mathematical framework for atmospheric blocking events. V Lucarini, A Gritsun, arXiv:1902.08464arXiv e-printsLucarini, V, and A. Gritsun (2019), "A new mathematical framework for atmospheric blocking events," arXiv e-prints , arXiv:1902.08464.
Towards a general theory of extremes for observables of chaotic dynamical systems. V Lucarini, T Kuna, D Faranda, J Wouters, Journal of Statistical Physics. 154Lucarini, V, T. Kuna, D. Faranda, and J. Wouters (2014b), "Towards a general theory of extremes for observables of chaotic dynamical systems," Journal of Statistical Physics 154.
Habitability and multistability in earth-like planets. V Lucarini, S Pascale, V Boschi, E Kirk, N Iro, 10.1002/asna.201311903Astronomische Nachrichten. 3346Lucarini, V, S. Pascale, V. Boschi, E. Kirk, and N. Iro (2013), "Habitability and multistability in earth-like planets," As- tronomische Nachrichten 334 (6), 576-588.
Energetics of climate models: Net energy balance and meridional enthalpy transport. V Lucarini, F Ragone, 10.1029/2009RG000323Rev. Geophys. 491001Lucarini, V, and F Ragone (2011), "Energetics of climate models: Net energy balance and meridional enthalpy trans- port," Rev. Geophys. 49, RG1001.
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Understanding our planet to benefit humankind
 
 
What Is Climate Change?
Evidence
How Do We Know Climate Change Is Real?
There is unequivocal evidence that Earth is warming at an unprecedented rate. Human activity is the principal cause.
 
more
 
Causes
Why Is Climate Change Happening?
Human activities are driving the global warming trend observed since the mid-20th century.
 
more
 
Effects
What Are the Effects of Climate Change?
The effects of human-caused global warming are happening now, are irreversible on the timescale of people alive today, and will worsen in the decades to come.
 
more
 
Solutions
What Is Being Done to Solve Climate Change?
NASA provides the robust scientific data needed to understand climate change.
 
more
Home – Climate Change: Vital Signs of the Planet
https://climate.nasa.gov/
https://www.ipcc.ch/
The Intergovernmental Panel on Climate Change
2023-03-13
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<p>Through its assessments, the IPCC determines the state of knowledge on climate change. It identifies where there is agreement in the scientific community on topics related to climate change, and where further research is needed. The reports are drafted and reviewed in several stages, thus guaranteeing objectivity and transparency.  The IPCC does not conduct its own research. IPCC reports are neutral, policy-relevant but not policy-prescriptive.  The assessment reports are a key input into the international negotiations to tackle climate change. Created by the United Nations Environment Programme (UN Environment) and the World Meteorological Organization (WMO) in 1988, the IPCC  has 195 Member countries. In the same year, the UN General Assembly <a href="https://www.ipcc.ch/site/assets/uploads/2019/02/UNGA43-53.pdf">endorsed the action by WMO and UNEP in jointly establishing the IPCC</a>.</p>
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 IPCC Workshop on the Use of Scenarios in the Sixth Assessment Report and Subsequent Assessments

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Through its assessments, the IPCC determines the state of knowledge on climate change. It identifies where there is agreement in the scientific community on topics related to climate change, and where further research is needed. The reports are drafted and reviewed in several stages, thus guaranteeing objectivity and transparency.  The IPCC does not conduct its own research. IPCC reports are neutral, policy-relevant but not policy-prescriptive.  The assessment reports are a key input into the international negotiations to tackle climate change. Created by the United Nations Environment Programme (UN Environment) and the World Meteorological Organization (WMO) in 1988, the IPCC  has 195 Member countries. In the same year, the UN General Assembly endorsed the action by WMO and UNEP in jointly establishing the IPCC.
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https://www.wired.com/story/think-climate-change-is-messy-wait-until-geoengineering/
Think Climate Change Is Messy? Wait Until Geoengineering
2021-11-30
Condé Nast; Matt Simon
<div><div><div><p>Here’s the thing about the stratosphere, the region between six and 31 miles up in the sky: If you really wanted to, you could turn it pink. Or green. Or what have you. If you sprayed some colorant up there, stratospheric winds would blow the material until it wrapped around the globe. After a year or two, it would fade, and the sky would go back to being blue. Neat little prank.</p><p>This is the idea behind a <a href="https://www.wired.com/story/geoengineering-is-the-only-solution-to-our-climate-calamities/">solar geoengineering technique</a> known as stratospheric aerosol injection, only instead of a pigment, engineers would spray a sulfate that bounces some of the sun’s radiation back into space, an attempt at cooling the planet. It’s the same principle behind a supervolcano loading the stratosphere with aerosols and blocking out the sun. And it, too, would rely on those winds distributing the material evenly. “If you do it in one place, it's going to affect the whole planet,” says climate scientist Kate Ricke, who studies the intersection of geoengineering, human behavior, and economics at Scripps Institution of Oceanography. “Not just because you've cooled down and changed the global energy balance, but because the particles spread out.”</p><p>While it’s not likely that someone will colorize the atmosphere anytime soon, it's getting increasingly likely that someone will decide it’s time for stratospheric aerosol injection. Emissions are not declining at anywhere near the rate needed to keep global temperatures from rising 1.5 degrees Celsius above preindustrial levels, and <a href="https://www.wired.com/story/its-time-to-delete-carbon-from-the-atmosphere-but-how/">the climate crisis is worsening</a>. </p><p>But the science <a href="https://www.wired.com/story/more-scientists-now-think-geoengineering-may-be-essential/">isn’t ready</a>. This anthropogenic <a href="https://www.wired.com/story/geoengineering-climate-fine-print/">geoengineering</a> might trigger unintended effects, like droughts in certain regions and massive storms in others. Plus, if engineers abruptly stopped spraying aerosols in the atmosphere, temperatures would swing back to where they started, potentially <a href="https://www.wired.com/story/how-engineering-earths-climate-could-seriously-imperil-life/">imperiling crops and species</a>. </p><p>Still, stratospheric aerosol injection would be fairly cheap. And there’s nothing stopping countries from unilaterally deciding to spray their airspace, even though those materials would ultimately spread around the globe. “I just have a hard time seeing with the economics of it how it doesn't happen,” says Ricke. “To me, that means that it's really urgent to do more research.” </p><p>WIRED sat down with Ricke to talk about the allure and <a href="https://www.wired.com/story/lets-talk-geoengineering/">potential pitfalls of geoengineering</a>, what makes it so politically perilous, and how scientists can make sense of it—for the good of humanity and the planet. The conversation has been condensed and edited for clarity.</p><p><strong>WIRED: Can you give me an idea of the scale that we'd be talking about with solar geoengineering—both spatial scales and timescales?</strong></p><p>Kate Ricke: Let's say you want to start geoengineering today to stabilize global temperatures where we are, or maybe bring it down a little bit. You basically need a fleet of airplanes that can reach the stratosphere. We're talking on the scale of maybe tens to hundreds of airplanes, and the capability to spray aerosol precursors. </p><p>But the way that the stratosphere works is that once you get up there, stratospheric winds take things around the planet relatively quickly in bands of latitude. And then slowly over time, on the timescale of months, things sort of migrate in general from equatorial regions up toward the poles, and then particles fall out near the poles. So you wouldn't need to be flying through the whole stratosphere spraying stuff. The stratosphere does a lot of work to spread it out. And that's part of the reason why you can't do stratospheric geoengineering over just one area. </p></div><div><p><strong>WIRED: Would we notice this? Visually, would we see anything?</strong> </p><p>KR: Yes, on an absolute scale. It changes the ratio of direct and diffuse radiation. So the idea is the sky would on average become a little bit whiter, and, for example, sunsets would become a bit more vivid. It's definitely much smaller than the difference between going from the desert in California to the city. The white skies thing is also not, in my opinion, probably the biggest problem.</p><p><strong>WIRED: What about any concerns about toxicology? Is this stuff benign to living creatures on Earth?</strong> </p><p>KR: It's not benign—it's the same stuff that comes out of power plants. Large concentrations of it in one area makes people and crops sick. But, in terms of the scale, the amount you need in the stratosphere is way, way smaller than what we emit from power plants, and it's spread out over the planet. </p><p>People have done some studies on this, too, and it seems like probably the biggest risk from the particles would be to sort of sensitive high-latitude ecosystems—so polar ecosystems that don't get very much exposure to urban pollution right now, but would get more from this. Especially because the particles move towards the poles, generally, before they precipitate out of the stratosphere. </p><p><strong>WIRED: Say a country unilaterally says, ‘We're going to do this.’ They want to cool down their own country by spraying the stratosphere, and it doesn't matter if it's going to wrap around the planet.</strong></p><p>KR: Legally, it's complicated, because countries own their airspace up to space, basically. It's a little ambiguous. So people could spray stuff over their country, and it would go everywhere. And then [the particles] stay in the atmosphere for on average about a year and a half. They spread out and the radiative effects take effect immediately. That's why after a large volcanic eruption, you see a dip in the global temperature immediately that persists for about a year to two years and then drops off again. So you wouldn't need to be spraying stuff every day, necessarily. If you stopped doing it for two years, the effect would go away.</p><div><p></p><div><p>The world is getting warmer, the weather is getting worse. Here's everything you need to know about what humans can do to stop wrecking the planet.</p></div></div><p>I'm having a hard time seeing how we're <em>not</em> going to do it at this point, actually, because it's so inexpensive. Already the impacts of climate change are looking to be so disruptive that I don't see in this world how such a low-expense solution doesn't get implemented by someone. There's just nothing else in the world that can cool the planet as quickly. Even if we started rapidly decarbonizing and <a href="https://www.wired.com/story/its-time-to-delete-carbon-from-the-atmosphere-but-how/">taking CO2 out of the atmosphere</a>, it's still a decade timescale for consequences. Whereas blocking sunlight, the climate response starts right away. </p><p><strong>WIRED: I've seen some modeling that if you were to suddenly stop solar geoengineering, you'd have a problem with temperatures</strong> <a href="https://www.wired.com/story/how-engineering-earths-climate-could-seriously-imperil-life/"><strong>dramatically climbing and imperiling species</strong></a><strong>.</strong></p><p>KR: If the program got disrupted, and we were blocking a lot of warming with stratospheric geoengineering, you would get this really rapid warming if someone stopped doing it. I mean, it would be catastrophic if we stopped treating our drinking water too, right? There's things that humans do that we need to keep doing, or it's catastrophic. </p></div><div><p>The technology's not so complicated that we would need just the person who developed the technology to be the one to keep doing it. And so I'm a little skeptical about that argument being the biggest issue, because we already basically know how to do it. It's within the grasp of a medium-sized country or something. The resources are substantial enough that it would be hard for a single individual, or a very small country to do it. But it's not like nuclear weapons or something like that. </p><p><strong>WIRED: Are we getting to the point where the science is sound enough that we can begin to make these decisions? And is that even going to be possible, given the general lack of cooperation on an international scale?</strong></p><p>KR: There might be some technical experts, like me or other people who have worked on this, that would say: ‘Yes, I've seen enough to believe it.’ But in order to have collective decision-making at the global scale, you need science that's viewed as legitimate by everyone. Not <em>everyone</em>, but a lot of people. And we're not there, by a longshot, with geoengineering.</p><p>But that's why we need more research. And we need more diversity of who's doing research and where, because the results are going to need to be viewed as legitimate by a much broader group of people. They're not right now. That's definitely not true. </p><p><strong>WIRED: Why not?</strong></p><p>KR: Because it's been a small group of mostly elite university white dudes in North America and Europe who've done all the research. And people just don't automatically trust a small group of elites like that. It's actually important that the ministry of the environment in Bangladesh has someone who's Bangladeshi talking to them about geoengineering science. So that, I think, is the biggest problem with the science right now. You can look at certain areas of climate science and you see we're saying the same thing over and over and over again. But there has been some value to that, too—replication and repetition. It builds consensus, and it builds trust in the science.</p><p><strong>WIRED: Country-scale commitments to reducing emissions</strong> <a href="https://www.wired.com/story/its-time-to-delete-carbon-from-the-atmosphere-but-how/"><strong>are one thing</strong></a><strong>, but this involves everybody simultaneously because we share one atmosphere. Is there going to be agreement on that?</strong></p><p>KR: We're not there where we can have global consensus about geoengineering, not by a longshot. But I would guess it's more likely that this will happen <em>not</em> with a global consensus. Certainly, there are some actors that, if they did it, would be constrained by more powerful actors. But there are definitely other major actors in the world that already exist that could do geoengineering and get away with it. Because the alternative is: Is it bad enough for you that you're willing to go to war over it?</p><p><strong>WIRED: What about the moral hazard? Wouldn't geoengineering make it less urgent to reduce emissions?</strong></p><p>KR: The moral hazard is a totally valid concern, and it's a big one. In terms of the existing empirical research, the results are very mixed. It doesn't seem like [for] individual humans, when you put them in behavioral experiments, that a moral hazard around geoengineering exists. Telling people about geoengineering in a controlled way tends to make people want to mitigate greenhouse gases <em>more</em>, because people think geoengineering is kind of nuts and scary. They see it as an indicator that climate change is a big problem.</p><p>This is me editorializing about my fellow climate scientists, but I think most climate scientists don't like the idea of geoengineering. And the reason they still don't like it the most is because of the moral hazard. They think we’ve got to tell people ‘This is a bad idea’ as long as possible because of that. And they're probably right. But the risk is that if things get bad enough with climate change, people are going to do geoengineering anyway, and we're not going to be ready to do it.</p><hr /><p>More Great WIRED Stories</p><ul><li>📩 The latest on tech, science, and more: <a href="https://www.wired.com/newsletter?sourceCode=BottomStories">Get our newsletters</a>!</li><li><a href="https://www.wired.com/story/sci-fi-icon-neal-stephenson-global-warming/?itm_campaign=BottomRelatedStories&amp;itm_content=footer-recirc">Neal Stephenson</a> finally takes on global warming</li><li>A cosmic ray event pinpoints <a href="https://www.wired.com/story/a-cosmic-ray-event-pinpoints-the-viking-landing-in-canada/?itm_campaign=BottomRelatedStories&amp;itm_content=footer-recirc">the Viking landing in Canada</a></li><li>How to <a href="https://www.wired.com/story/how-to-delete-your-facebook-account/?itm_campaign=BottomRelatedStories&amp;itm_content=footer-recirc">delete your Facebook account</a> forever</li><li>A look inside <a href="https://www.wired.com/story/plaintext-inside-apple-silicon/?itm_campaign=BottomRelatedStories&amp;itm_content=footer-recirc">Apple's silicon playbook</a></li><li>Want a better PC? 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 Photograph: Citizen of the Planet/Getty Images

 Matt Simon

 Science
 Nov 30, 2021 8:00 AM

 Think Climate Change Is Messy? Wait Until Geoengineering

 Someone's bound to hack the atmosphere to cool the planet. So we urgently need more research on the consequences, says climate scientist Kate Ricke.
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 Here’s the thing about the stratosphere, the region between six and 31 miles up in the sky: If you really wanted to, you could turn it pink. Or green. Or what have you. If you sprayed some colorant up there, stratospheric winds would blow the material until it wrapped around the globe. After a year or two, it would fade, and the sky would go back to being blue. Neat little prank.

 This is the idea behind a solar geoengineering technique known as stratospheric aerosol injection, only instead of a pigment, engineers would spray a sulfate that bounces some of the sun’s radiation back into space, an attempt at cooling the planet. It’s the same principle behind a supervolcano loading the stratosphere with aerosols and blocking out the sun. And it, too, would rely on those winds distributing the material evenly. “If you do it in one place, it's going to affect the whole planet,” says climate scientist Kate Ricke, who studies the intersection of geoengineering, human behavior, and economics at Scripps Institution of Oceanography. “Not just because you've cooled down and changed the global energy balance, but because the particles spread out.”

 While it’s not likely that someone will colorize the atmosphere anytime soon, it's getting increasingly likely that someone will decide it’s time for stratospheric aerosol injection. Emissions are not declining at anywhere near the rate needed to keep global temperatures from rising 1.5 degrees Celsius above preindustrial levels, and the climate crisis is worsening.

 But the science isn’t ready. This anthropogenic geoengineering might trigger unintended effects, like droughts in certain regions and massive storms in others. Plus, if engineers abruptly stopped spraying aerosols in the atmosphere, temperatures would swing back to where they started, potentially imperiling crops and species.

 Still, stratospheric aerosol injection would be fairly cheap. And there’s nothing stopping countries from unilaterally deciding to spray their airspace, even though those materials would ultimately spread around the globe. “I just have a hard time seeing with the economics of it how it doesn't happen,” says Ricke. “To me, that means that it's really urgent to do more research.”

 WIRED sat down with Ricke to talk about the allure and potential pitfalls of geoengineering, what makes it so politically perilous, and how scientists can make sense of it—for the good of humanity and the planet. The conversation has been condensed and edited for clarity.

 WIRED: Can you give me an idea of the scale that we'd be talking about with solar geoengineering—both spatial scales and timescales?

 Kate Ricke: Let's say you want to start geoengineering today to stabilize global temperatures where we are, or maybe bring it down a little bit. You basically need a fleet of airplanes that can reach the stratosphere. We're talking on the scale of maybe tens to hundreds of airplanes, and the capability to spray aerosol precursors.

 But the way that the stratosphere works is that once you get up there, stratospheric winds take things around the planet relatively quickly in bands of latitude. And then slowly over time, on the timescale of months, things sort of migrate in general from equatorial regions up toward the poles, and then particles fall out near the poles. So you wouldn't need to be flying through the whole stratosphere spraying stuff. The stratosphere does a lot of work to spread it out. And that's part of the reason why you can't do stratospheric geoengineering over just one area.

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 * 

 WIRED: Would we notice this? Visually, would we see anything?

 KR: Yes, on an absolute scale. It changes the ratio of direct and diffuse radiation. So the idea is the sky would on average become a little bit whiter, and, for example, sunsets would become a bit more vivid. It's definitely much smaller than the difference between going from the desert in California to the city. The white skies thing is also not, in my opinion, probably the biggest problem.

 WIRED: What about any concerns about toxicology? Is this stuff benign to living creatures on Earth?

 KR: It's not benign—it's the same stuff that comes out of power plants. Large concentrations of it in one area makes people and crops sick. But, in terms of the scale, the amount you need in the stratosphere is way, way smaller than what we emit from power plants, and it's spread out over the planet.

 People have done some studies on this, too, and it seems like probably the biggest risk from the particles would be to sort of sensitive high-latitude ecosystems—so polar ecosystems that don't get very much exposure to urban pollution right now, but would get more from this. Especially because the particles move towards the poles, generally, before they precipitate out of the stratosphere.

 WIRED: Say a country unilaterally says, ‘We're going to do this.’ They want to cool down their own country by spraying the stratosphere, and it doesn't matter if it's going to wrap around the planet.

 KR: Legally, it's complicated, because countries own their airspace up to space, basically. It's a little ambiguous. So people could spray stuff over their country, and it would go everywhere. And then [the particles] stay in the atmosphere for on average about a year and a half. They spread out and the radiative effects take effect immediately. That's why after a large volcanic eruption, you see a dip in the global temperature immediately that persists for about a year to two years and then drops off again. So you wouldn't need to be spraying stuff every day, necessarily. If you stopped doing it for two years, the effect would go away.

 The WIRED Guide to Climate Change
 The world is getting warmer, the weather is getting worse. Here's everything you need to know about what humans can do to stop wrecking the planet.

 By Katie M. Palmer and Matt Simon

 I'm having a hard time seeing how we're not going to do it at this point, actually, because it's so inexpensive. Already the impacts of climate change are looking to be so disruptive that I don't see in this world how such a low-expense solution doesn't get implemented by someone. There's just nothing else in the world that can cool the planet as quickly. Even if we started rapidly decarbonizing and taking CO2 out of the atmosphere, it's still a decade timescale for consequences. Whereas blocking sunlight, the climate response starts right away.

 WIRED: I've seen some modeling that if you were to suddenly stop solar geoengineering, you'd have a problem with temperatures dramatically climbing and imperiling species.

 KR: If the program got disrupted, and we were blocking a lot of warming with stratospheric geoengineering, you would get this really rapid warming if someone stopped doing it. I mean, it would be catastrophic if we stopped treating our drinking water too, right? There's things that humans do that we need to keep doing, or it's catastrophic.

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 * 

 The technology's not so complicated that we would need just the person who developed the technology to be the one to keep doing it. And so I'm a little skeptical about that argument being the biggest issue, because we already basically know how to do it. It's within the grasp of a medium-sized country or something. The resources are substantial enough that it would be hard for a single individual, or a very small country to do it. But it's not like nuclear weapons or something like that.

 WIRED: Are we getting to the point where the science is sound enough that we can begin to make these decisions? And is that even going to be possible, given the general lack of cooperation on an international scale?

 KR: There might be some technical experts, like me or other people who have worked on this, that would say: ‘Yes, I've seen enough to believe it.’ But in order to have collective decision-making at the global scale, you need science that's viewed as legitimate by everyone. Not everyone, but a lot of people. And we're not there, by a longshot, with geoengineering.

 But that's why we need more research. And we need more diversity of who's doing research and where, because the results are going to need to be viewed as legitimate by a much broader group of people. They're not right now. That's definitely not true.

 WIRED: Why not?

 KR: Because it's been a small group of mostly elite university white dudes in North America and Europe who've done all the research. And people just don't automatically trust a small group of elites like that. It's actually important that the ministry of the environment in Bangladesh has someone who's Bangladeshi talking to them about geoengineering science. So that, I think, is the biggest problem with the science right now. You can look at certain areas of climate science and you see we're saying the same thing over and over and over again. But there has been some value to that, too—replication and repetition. It builds consensus, and it builds trust in the science.

 WIRED: Country-scale commitments to reducing emissions are one thing, but this involves everybody simultaneously because we share one atmosphere. Is there going to be agreement on that?

 KR: We're not there where we can have global consensus about geoengineering, not by a longshot. But I would guess it's more likely that this will happen not with a global consensus. Certainly, there are some actors that, if they did it, would be constrained by more powerful actors. But there are definitely other major actors in the world that already exist that could do geoengineering and get away with it. Because the alternative is: Is it bad enough for you that you're willing to go to war over it?

 WIRED: What about the moral hazard? Wouldn't geoengineering make it less urgent to reduce emissions?

 KR: The moral hazard is a totally valid concern, and it's a big one. In terms of the existing empirical research, the results are very mixed. It doesn't seem like [for] individual humans, when you put them in behavioral experiments, that a moral hazard around geoengineering exists. Telling people about geoengineering in a controlled way tends to make people want to mitigate greenhouse gases more, because people think geoengineering is kind of nuts and scary. They see it as an indicator that climate change is a big problem.

 This is me editorializing about my fellow climate scientists, but I think most climate scientists don't like the idea of geoengineering. And the reason they still don't like it the most is because of the moral hazard. They think we’ve got to tell people ‘This is a bad idea’ as long as possible because of that. And they're probably right. But the risk is that if things get bad enough with climate change, people are going to do geoengineering anyway, and we're not going to be ready to do it.

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 * Neal Stephenson finally takes on global warming
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 Matt Simon is a science journalist at WIRED, where he covers biology, robotics, cannabis, and the environment. He’s also the author of Plight of the Living Dead: What Real-Life Zombies Reveal About Our World—And Ourselves, and The Wasp That Brainwashed the Caterpillar, which won an Alex Award.
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 Topics climate change geoengineering climate Emissions carbon emissions Earth Science environment
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Here’s the thing about the stratosphere, the region between six and 31 miles up in the sky: If you really wanted to, you could turn it pink. Or green. Or what have you. If you sprayed some colorant up there, stratospheric winds would blow the material until it wrapped around the globe. After a year or two, it would fade, and the sky would go back to being blue. Neat little prank. This is the idea behind a solar geoengineering technique known as stratospheric aerosol injection, only instead of a pigment, engineers would spray a sulfate that bounces some of the sun’s radiation back into space, an attempt at cooling the planet. It’s the same principle behind a supervolcano loading the stratosphere with aerosols and blocking out the sun. And it, too, would rely on those winds distributing the material evenly. “If you do it in one place, it's going to affect the whole planet,” says climate scientist Kate Ricke, who studies the intersection of geoengineering, human behavior, and economics at Scripps Institution of Oceanography. “Not just because you've cooled down and changed the global energy balance, but because the particles spread out.” While it’s not likely that someone will colorize the atmosphere anytime soon, it's getting increasingly likely that someone will decide it’s time for stratospheric aerosol injection. Emissions are not declining at anywhere near the rate needed to keep global temperatures from rising 1.5 degrees Celsius above preindustrial levels, and the climate crisis is worsening.  But the science isn’t ready. This anthropogenic geoengineering might trigger unintended effects, like droughts in certain regions and massive storms in others. Plus, if engineers abruptly stopped spraying aerosols in the atmosphere, temperatures would swing back to where they started, potentially imperiling crops and species.  Still, stratospheric aerosol injection would be fairly cheap. And there’s nothing stopping countries from unilaterally deciding to spray their airspace, even though those materials would ultimately spread around the globe. “I just have a hard time seeing with the economics of it how it doesn't happen,” says Ricke. “To me, that means that it's really urgent to do more research.”  WIRED sat down with Ricke to talk about the allure and potential pitfalls of geoengineering, what makes it so politically perilous, and how scientists can make sense of it—for the good of humanity and the planet. The conversation has been condensed and edited for clarity.  WIRED: Can you give me an idea of the scale that we'd be talking about with solar geoengineering—both spatial scales and timescales?  Kate Ricke: Let's say you want to start geoengineering today to stabilize global temperatures where we are, or maybe bring it down a little bit. You basically need a fleet of airplanes that can reach the stratosphere. We're talking on the scale of maybe tens to hundreds of airplanes, and the capability to spray aerosol precursors.  But the way that the stratosphere works is that once you get up there, stratospheric winds take things around the planet relatively quickly in bands of latitude. And then slowly over time, on the timescale of months, things sort of migrate in general from equatorial regions up toward the poles, and then particles fall out near the poles. So you wouldn't need to be flying through the whole stratosphere spraying stuff. The stratosphere does a lot of work to spread it out. And that's part of the reason why you can't do stratospheric geoengineering over just one area.     WIRED: Would we notice this? Visually, would we see anything?  KR: Yes, on an absolute scale. It changes the ratio of direct and diffuse radiation. So the idea is the sky would on average become a little bit whiter, and, for example, sunsets would become a bit more vivid. It's definitely much smaller than the difference between going from the desert in California to the city. The white skies thing is also not, in my opinion, probably the biggest problem.  WIRED: What about any concerns about toxicology? Is this stuff benign to living creatures on Earth?  KR: It's not benign—it's the same stuff that comes out of power plants. Large concentrations of it in one area makes people and crops sick. But, in terms of the scale, the amount you need in the stratosphere is way, way smaller than what we emit from power plants, and it's spread out over the planet.  People have done some studies on this, too, and it seems like probably the biggest risk from the particles would be to sort of sensitive high-latitude ecosystems—so polar ecosystems that don't get very much exposure to urban pollution right now, but would get more from this. Especially because the particles move towards the poles, generally, before they precipitate out of the stratosphere.   WIRED: Say a country unilaterally says, ‘We're going to do this.’ They want to cool down their own country by spraying the stratosphere, and it doesn't matter if it's going to wrap around the planet.  KR: Legally, it's complicated, because countries own their airspace up to space, basically. It's a little ambiguous. So people could spray stuff over their country, and it would go everywhere. And then [the particles] stay in the atmosphere for on average about a year and a half. They spread out and the radiative effects take effect immediately. That's why after a large volcanic eruption, you see a dip in the global temperature immediately that persists for about a year to two years and then drops off again. So you wouldn't need to be spraying stuff every day, necessarily. If you stopped doing it for two years, the effect would go away.     The world is getting warmer, the weather is getting worse. Here's everything you need to know about what humans can do to stop wrecking the planet.   I'm having a hard time seeing how we're not going to do it at this point, actually, because it's so inexpensive. Already the impacts of climate change are looking to be so disruptive that I don't see in this world how such a low-expense solution doesn't get implemented by someone. There's just nothing else in the world that can cool the planet as quickly. Even if we started rapidly decarbonizing and taking CO2 out of the atmosphere, it's still a decade timescale for consequences. Whereas blocking sunlight, the climate response starts right away.   WIRED: I've seen some modeling that if you were to suddenly stop solar geoengineering, you'd have a problem with temperatures  dramatically climbing and imperiling species  .  KR: If the program got disrupted, and we were blocking a lot of warming with stratospheric geoengineering, you would get this really rapid warming if someone stopped doing it. I mean, it would be catastrophic if we stopped treating our drinking water too, right? There's things that humans do that we need to keep doing, or it's catastrophic.    The technology's not so complicated that we would need just the person who developed the technology to be the one to keep doing it. And so I'm a little skeptical about that argument being the biggest issue, because we already basically know how to do it. It's within the grasp of a medium-sized country or something. The resources are substantial enough that it would be hard for a single individual, or a very small country to do it. But it's not like nuclear weapons or something like that.   WIRED: Are we getting to the point where the science is sound enough that we can begin to make these decisions? And is that even going to be possible, given the general lack of cooperation on an international scale?  KR: There might be some technical experts, like me or other people who have worked on this, that would say: ‘Yes, I've seen enough to believe it.’ But in order to have collective decision-making at the global scale, you need science that's viewed as legitimate by everyone. Not everyone, but a lot of people. And we're not there, by a longshot, with geoengineering. But that's why we need more research. And we need more diversity of who's doing research and where, because the results are going to need to be viewed as legitimate by a much broader group of people. They're not right now. That's definitely not true.   WIRED: Why not?  KR: Because it's been a small group of mostly elite university white dudes in North America and Europe who've done all the research. And people just don't automatically trust a small group of elites like that. It's actually important that the ministry of the environment in Bangladesh has someone who's Bangladeshi talking to them about geoengineering science. So that, I think, is the biggest problem with the science right now. You can look at certain areas of climate science and you see we're saying the same thing over and over and over again. But there has been some value to that, too—replication and repetition. It builds consensus, and it builds trust in the science.  WIRED: Country-scale commitments to reducing emissions  are one thing  , but this involves everybody simultaneously because we share one atmosphere. Is there going to be agreement on that?  KR: We're not there where we can have global consensus about geoengineering, not by a longshot. But I would guess it's more likely that this will happen not with a global consensus. Certainly, there are some actors that, if they did it, would be constrained by more powerful actors. But there are definitely other major actors in the world that already exist that could do geoengineering and get away with it. Because the alternative is: Is it bad enough for you that you're willing to go to war over it?  WIRED: What about the moral hazard? Wouldn't geoengineering make it less urgent to reduce emissions?  KR: The moral hazard is a totally valid concern, and it's a big one. In terms of the existing empirical research, the results are very mixed. It doesn't seem like [for] individual humans, when you put them in behavioral experiments, that a moral hazard around geoengineering exists. Telling people about geoengineering in a controlled way tends to make people want to mitigate greenhouse gases more, because people think geoengineering is kind of nuts and scary. They see it as an indicator that climate change is a big problem. This is me editorializing about my fellow climate scientists, but I think most climate scientists don't like the idea of geoengineering. And the reason they still don't like it the most is because of the moral hazard. They think we’ve got to tell people ‘This is a bad idea’ as long as possible because of that. And they're probably right. But the risk is that if things get bad enough with climate change, people are going to do geoengineering anyway, and we're not going to be ready to do it.  More Great WIRED Stories  📩 The latest on tech, science, and more: Get our newsletters!  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Think Climate Change Is Messy? Wait Until Geoengineering
https://www.wired.com/story/think-climate-change-is-messy-wait-until-geoengineering/
https://www.vox.com/science-and-health/22807575/venus-hot-hellscape-climate-change-earth
Venus could have been a paradise but turned into a hellscape. Earthlings, pay attention.
2021-12-01
Brian Resnick
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 <p>900 degrees Fahrenheit, crushing pressure, and acid clouds. Venus, what the hell happened? </p>
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<p>“Hellscape” is the most appropriate word to describe the surface of Venus, the second planet from the sun. At 900 degrees Fahrenheit, it’s the hottest planet in the solar system, thanks to an atmosphere that’s almost entirely made up of carbon dioxide. Clouds made of highly corrosive sulfuric acid are draped over a volcanic landscape of razor-sharp lava flows. Most crushingly, the pressure on the surface of Venus is about 92 times the pressure you’d feel at sea level on Earth. </p>
<p>“It’s really almost entertainingly, comically horrible, like some sort of cosmic deity had a really, really grumpy day and just went ‘Nope, I’m gonna ruin this planet,’” <a href="https://twitter.com/SquigglyVolcano?ref_src=twsrc%5Egoogle%7Ctwcamp%5Eserp%7Ctwgr%5Eauthor">Robin George Andrews</a>, a science journalist and volcanologist, says. Andrews compares it to being in a pressure cooker a mile underwater. “If you stood on the surface, you would be pancaked and you would melt,” he says. Your eyes would explode due to the pressure — “which would be gross,” he adds.</p>
<p>Yet as Andrews relays in his new book, <a href="https://www.amazon.com/Super-Volcanoes-Reveal-Worlds-Beyond/dp/0393542068/"><em>Super Volcanoes: What They Reveal about Earth and the Worlds Beyond</em></a>, some scientists suspect Venus was once much like Earth, with a liquid water ocean like the ones that support life on our planet. For Andrews, the question of what happened to ruin Venus is captivating and even existential. </p>
<p>“Venus and Earth are planetary siblings,” he says. “They were made at the same time and made of the same stuff, yet Venus is apocalyptic and awful in every possible way. Earth is a paradise. So why do we have a paradise next to a paradise lost?”</p>
<p>Scientists know <em>something</em> on Venus triggered truly catastrophic levels of climate change, causing surface temperatures to shoot up hundreds of degrees. But they don’t know exactly what. I spoke to Andrews for an episode of <a href="https://www.vox.com/unexplainable"><em>Unexplainable</em></a>, Vox’s science podcast about unanswered questions, about what could have triggered Venus’s apocalypse and why we should care about it. Our conversation has been edited for length and clarity. </p>
<p>The podcast episode also features a discussion with Sara Seager, an MIT planetary scientist and an expert on exoplanets, or planets outside our solar system, <a href="https://www.vox.com/science-and-health/2020/9/18/21443635/phosphine-on-venus-discovery-life-venus-clouds">who raises the tantalizing question</a>: What if, despite the cataclysm, something is still alive on Venus? </p>
<h3>The origins of Venus could tell us a lot about our place in the universe</h3>
 <figure>
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 <figcaption>On the left, Venus as it is today. On the right, an artist’s interpretation of where oceans could have lain on the surface in the past.</figcaption>
 
 
 <cite>NASA/Jet Propulsion Laboratory-Caltech (left) and NASA (right)</cite>
 
 
 
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<h4>Brian Resnick</h4>
<p>You say the question of “what killed Venus” is existential. What makes it so? </p>
<h4>Robin George Andrews</h4>
<p>It really is a question about why are we here. Answering it will help us answer the question: How lonely are we? Are there other Venuses or Earths out there? </p>
<p>If Earth is the odd one out, how lucky are we to exist? If Venus is the odd one out, then maybe we’re not so special after all.</p>
<h4>Brian Resnick</h4>
<p>So Earth and Venus started off as similar planets, and then went down different paths. You want to know what path is more common out there in the cosmos? </p>
<h4>Robin George Andrews</h4>
<p>When you hear in the news that scientists have discovered an Earth-like exoplanet, they might as well be saying, we found a Venus-like exoplanet. We don’t know if this is like a habitable world by our human, surface-dwelling standards, or if it’s gone through this sort of apocalyptic climate change like Venus. A good way to work out what may be more common in the cosmos is to study Earth and Venus, because they are siblings. </p>
<h4>Brian Resnick</h4>
<p>How do scientists know, or suspect, Venus used to be more pleasant, habitable even? </p>
<h4>Robin George Andrews</h4>
<p>So even though Venus today looks and is apocalyptic in every meaning of the word, probes have looked at its atmosphere and <a href="https://www.vanderbilt.edu/AnS/physics/astrocourses/AST101/readings/water_on_venus.html">found</a> there’s a lot of “heavy water” in the atmosphere. Heavy water is exactly what it sounds like. The water we’re used to, this classic H2O, which is found commonly everywhere on Earth, is a more common type of water throughout the cosmos. Heavy water just kind of switches out that hydrogen for something called deuterium, which is like a heavier version of hydrogen.</p>
<h4>Brian Resnick</h4>
<p>What does finding heavy water mean?</p>
<h4>Robin George Andrews</h4>
<p>If you measure how much heavy water exists somewhere, you can make a reasonable guess as to how much classic water there is or was on that planet. It suggests that there once was a lot of classic water on Venus, at least an ocean’s worth of water on Venus. If that water existed in liquid form, what are the odds that Venus was habitable at some point? It’s not unlikely, even though today it looks impossible. </p>
<h4>Brian Resnick</h4>
<p>How long ago must this have been, this habitable Venus? </p>
<h4>Robin George Andrews</h4>
<p>I think there is a possibility that water was always steam, and it might have never been liquid water. But if it was, then it could have been habitable for billions of years. Maybe right up until the last billion years.</p>
<h4>Brian Resnick</h4>
<p>So that’s what we’re talking about when we say Venus used to be “alive.” What do we mean by “killed”?</p>
<h4>Robin George Andrews</h4>
<p>Death, in this case, is runaway climate change. Absolutely irreversible, world-ending climate change. The average temperature of the planet shot up by something like several hundred degrees Fahrenheit. It really sort of cooked itself to death.</p>
<h3>Suspect No. 1 for the death of Venus: The sun</h3>
 <figure>
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 <figcaption>Plasma spewing out of the surface of the sun.</figcaption>
 
 
 <cite>NASA/Solar Dynamics Observatory</cite>
 
 
 
</figure>
<h4>Brian Resnick</h4>
<p>So the question, I imagine, is how Venus cooked itself.</p>
<h4>Robin George Andrews</h4>
<p>There’s kind of two leading theories as to what killed Venus. Option No. 1 is the sun. From what we know of our sun and other stars, when they’re kind of in their teenage years, they get hyper-excitable, and they get hotter and brighter quite quickly. If the sun actually gets brighter and hotter <em>too</em> quickly, even though you might have that water sitting on the surface, the sun can boil it off. And that is a fate that lots of exoplanets, or planets outside our galaxy, are presumed to have gone through.</p>
<h4>Brian Resnick</h4>
<p>And once Venus’s water gets vaporized...</p>
<h4>Robin George Andrews</h4>
<p>That steam is a greenhouse gas that would have kicked up the greenhouse gas effect. And then the carbon dioxide coming out of Venus’s embryonic volcanoes would have just sealed the deal. That could explain why we see Venus as it is today. </p>
<h4>Brian Resnick</h4>
<p>If this young, very excitable sun is what killed Venus, the Earth would have been fine, right? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>Yeah, the Earth would have been fine. It seems that Earth was spared the worst of it.</p>
<h3>Suspect No. 2 for the death of Venus: Tectonic plate-breaking volcanoes</h3>
 <figure>
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 <figcaption>An artist’s depiction of a volcanically active Venus.</figcaption>
 
 
 <cite>NASA/JPL-Caltech/Peter Rubin</cite>
 
 
 
</figure>
<h4><strong>Brian Resnick</strong></h4>
<p>So that’s one suspect in our whodunit. What is the other suspect? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>The other suspect just happens to be my favorite thing: volcanoes. </p>
<h4><strong>Brian Resnick</strong></h4>
<p>How could your fave, volcanoes, kill a planet? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>Two hundred and fifty-two million years ago, Earth experienced its worst mass extinction, something called the <a href="https://www.theguardian.com/environment/2018/dec/06/global-warming-extinction-report-the-great-dying">Great Dying</a>. At least 90 percent of all life was wiped out, and the primary suspects were these volcanic fissures that opened up in Siberia. It produced a continent-sized flood of lava that took about 2 million years to erupt. </p>
<p>So it causes giant explosions and also unleashed all these greenhouse gases into the atmosphere, and it created a global warming effect that raised the temperature <a href="https://www.theguardian.com/environment/2018/dec/06/global-warming-extinction-report-the-great-dying">by a dozen degrees</a> [Celsius]. That caused 90 percent of all the life on Earth to die. Earth had to kind of reset itself. And the idea is, well, what if that happened on Venus, but worse? </p>
<h4><strong>Brian Resnick</strong></h4>
<p>That could kill the planet? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>If you just have one [Great Dying-scale] eruption, it might be okay, because Venus had plate tectonics. Plate tectonics is essentially <a href="https://www.scientificamerican.com/article/earths-tectonic-activity-may-be-crucial-for-life-and-rare-in-our-galaxy/#:~:text=Scientists%20think%20plate%20tectonics%2C%20which,it%20to%20retain%20more%20heat.">a planet’s thermostat</a>.</p>
<h4><strong>Brian Resnick</strong></h4>
<p>Plate tectonics — that’s how continents kind of float around and smoosh into each other. How do those act like a thermostat? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>Carbon dioxide can get soaked up in the ocean, and that filters down to these tectonic plates. If tectonic plates dive down beneath each other, then you’re burying carbon [and slowing the greenhouse effect].</p>
<h4><strong>Brian Resnick</strong></h4>
<p>Okay, but how do you break this carbon-burying system? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>You have <a href="https://www.scientificamerican.com/article/nasa-just-broke-the-venus-curse-heres-what-it-took/">two of these epic</a>, Great Dying-like eruptions at the same time.</p>
<p>That will immediately trigger quite an intense period of global warming. And the oceans will just start to boil off.</p>
<p>Now, plate tectonics could bury carbon for a while. But if you boil off your oceans, that carbon dioxide has nothing to dissolve into. And if you get rid of that water, plate tectonics itself shuts down. If you dehydrate [tectonic plates], you make them brittle — they can’t bury the carbon anymore. That’s essentially game over. If you break plate tectonics you’ve broken the world.</p>
<h3>The scientific jury is still out</h3>
<div> <figure>
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 <figcaption>Venus’s clouds captured in infared.</figcaption>
 
 
 <cite>JAXA/ISAS/DARTS/Damia Bouic</cite>
 
 
 
</figure>
</div>
<h4><strong>Brian Resnick</strong></h4>
<p>So we have our two suspects here. We have the sun and we have massive eruptions on Venus that broke Venus. Which one should we sentence here? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>The volcanoes seem more likely because we can see how that’s happened on Earth — just to a slightly lesser extent. But if this was put into a court of law, they would both be presumed not guilty, just because there isn’t a telltale bit of evidence yet.</p>
<h4><strong>Brian Resnick</strong></h4>
<p>So how do we solve this mystery? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>There’s basically a fleet of missions that is going to unravel the geologic makeup of Venus today. If it looks bone-dry and it was always bone-dry, then maybe the sun did it, because it’s been dehydrated for a long time. </p>
<p>But if it looks like there’s still some dehydration going on, that means you still have water somewhere. If Venus was still kind of soggy on the inside, and it’s still kind of belching water out, the volcanoes probably killed Venus. </p>
<h4><strong>Brian Resnick</strong></h4>
<p>Could the Earth pull a Venus one day? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>[Laughs.] Yeah, it could. Let’s not panic — I mean, these sorts of eruptions are like millions of years in timescales. So it’s not like no one would see this coming and we’d instantly be doomed. But it could happen. And the question is: Is it normal for a planet to have just <em>one</em> of these really epic, game-changer eruptions at one time? Or is it just a fluke? </p>
<p>The fact that no one knows the answer to this is weirdly, perversely exciting to me. We don’t know how often it is that volcanoes decide to trash the planets they’re on. </p>
<h4><strong>Brian Resnick </strong></h4>
<p>People sometimes bring up Venus in the context of climate change. It’s an example of how bad a planet can become when greenhouse gases start to accumulate in an atmosphere. Could we humans potentially be the volcano? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>The pace at which we’re putting carbon dioxide into the sky is worse than what was happening during the Great Dying, in terms of the amount of carbon per year. </p>
<h4><strong>Brian Resnick </strong></h4>
<p>But we would need to keep this up for millions of years to match, right? </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>Right.</p>
<h4><strong>Brian Resnick </strong></h4>
<p>This is weirdly reassuring that humans are unlikely to break the Earth completely. </p>
<h4><strong>Robin George Andrews</strong></h4>
<p>Yeah. I think it would be a terrible idea to pay homage to what happened to Venus.</p>
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 Courtesy of NASA/JPL

 Venus could have been a paradise but turned into a hellscape. Earthlings, pay attention.

 900 degrees Fahrenheit, crushing pressure, and acid clouds. Venus, what the hell happened?

 By Brian Resnick @B_resnick Dec 1, 2021, 1:30pm EST

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 “Hellscape” is the most appropriate word to describe the surface of Venus, the second planet from the sun. At 900 degrees Fahrenheit, it’s the hottest planet in the solar system, thanks to an atmosphere that’s almost entirely made up of carbon dioxide. Clouds made of highly corrosive sulfuric acid are draped over a volcanic landscape of razor-sharp lava flows. Most crushingly, the pressure on the surface of Venus is about 92 times the pressure you’d feel at sea level on Earth.

 “It’s really almost entertainingly, comically horrible, like some sort of cosmic deity had a really, really grumpy day and just went ‘Nope, I’m gonna ruin this planet,’” Robin George Andrews, a science journalist and volcanologist, says. Andrews compares it to being in a pressure cooker a mile underwater. “If you stood on the surface, you would be pancaked and you would melt,” he says. Your eyes would explode due to the pressure — “which would be gross,” he adds.

 Yet as Andrews relays in his new book, Super Volcanoes: What They Reveal about Earth and the Worlds Beyond, some scientists suspect Venus was once much like Earth, with a liquid water ocean like the ones that support life on our planet. For Andrews, the question of what happened to ruin Venus is captivating and even existential.

 “Venus and Earth are planetary siblings,” he says. “They were made at the same time and made of the same stuff, yet Venus is apocalyptic and awful in every possible way. Earth is a paradise. So why do we have a paradise next to a paradise lost?”

 Scientists know something on Venus triggered truly catastrophic levels of climate change, causing surface temperatures to shoot up hundreds of degrees. But they don’t know exactly what. I spoke to Andrews for an episode of Unexplainable, Vox’s science podcast about unanswered questions, about what could have triggered Venus’s apocalypse and why we should care about it. Our conversation has been edited for length and clarity.

 The podcast episode also features a discussion with Sara Seager, an MIT planetary scientist and an expert on exoplanets, or planets outside our solar system, who raises the tantalizing question: What if, despite the cataclysm, something is still alive on Venus?

 The origins of Venus could tell us a lot about our place in the universe

 On the left, Venus as it is today. On the right, an artist’s interpretation of where oceans could have lain on the surface in the past.
 NASA/Jet Propulsion Laboratory-Caltech (left) and NASA (right)

 Brian Resnick

 You say the question of “what killed Venus” is existential. What makes it so?

 Robin George Andrews

 It really is a question about why are we here. Answering it will help us answer the question: How lonely are we? Are there other Venuses or Earths out there?

 If Earth is the odd one out, how lucky are we to exist? If Venus is the odd one out, then maybe we’re not so special after all.

 Brian Resnick

 So Earth and Venus started off as similar planets, and then went down different paths. You want to know what path is more common out there in the cosmos?

 Robin George Andrews

 When you hear in the news that scientists have discovered an Earth-like exoplanet, they might as well be saying, we found a Venus-like exoplanet. We don’t know if this is like a habitable world by our human, surface-dwelling standards, or if it’s gone through this sort of apocalyptic climate change like Venus. A good way to work out what may be more common in the cosmos is to study Earth and Venus, because they are siblings.

 "“We don’t know how often it is that volcanoes decide to trash the planets they’re on”"

 Brian Resnick

 How do scientists know, or suspect, Venus used to be more pleasant, habitable even?

 Robin George Andrews

 So even though Venus today looks and is apocalyptic in every meaning of the word, probes have looked at its atmosphere and found there’s a lot of “heavy water” in the atmosphere. Heavy water is exactly what it sounds like. The water we’re used to, this classic H2O, which is found commonly everywhere on Earth, is a more common type of water throughout the cosmos. Heavy water just kind of switches out that hydrogen for something called deuterium, which is like a heavier version of hydrogen.

 Brian Resnick

 What does finding heavy water mean?

 Robin George Andrews

 If you measure how much heavy water exists somewhere, you can make a reasonable guess as to how much classic water there is or was on that planet. It suggests that there once was a lot of classic water on Venus, at least an ocean’s worth of water on Venus. If that water existed in liquid form, what are the odds that Venus was habitable at some point? It’s not unlikely, even though today it looks impossible.

 Brian Resnick

 How long ago must this have been, this habitable Venus?

 Robin George Andrews

 I think there is a possibility that water was always steam, and it might have never been liquid water. But if it was, then it could have been habitable for billions of years. Maybe right up until the last billion years.

 Brian Resnick

 So that’s what we’re talking about when we say Venus used to be “alive.” What do we mean by “killed”?

 Robin George Andrews

 Death, in this case, is runaway climate change. Absolutely irreversible, world-ending climate change. The average temperature of the planet shot up by something like several hundred degrees Fahrenheit. It really sort of cooked itself to death.

 Suspect No. 1 for the death of Venus: The sun

 Plasma spewing out of the surface of the sun.
 NASA/Solar Dynamics Observatory

 Brian Resnick

 So the question, I imagine, is how Venus cooked itself.

 Robin George Andrews

 There’s kind of two leading theories as to what killed Venus. Option No. 1 is the sun. From what we know of our sun and other stars, when they’re kind of in their teenage years, they get hyper-excitable, and they get hotter and brighter quite quickly. If the sun actually gets brighter and hotter too quickly, even though you might have that water sitting on the surface, the sun can boil it off. And that is a fate that lots of exoplanets, or planets outside our galaxy, are presumed to have gone through.

 Brian Resnick

 And once Venus’s water gets vaporized...

 Robin George Andrews

 That steam is a greenhouse gas that would have kicked up the greenhouse gas effect. And then the carbon dioxide coming out of Venus’s embryonic volcanoes would have just sealed the deal. That could explain why we see Venus as it is today.

 Brian Resnick

 If this young, very excitable sun is what killed Venus, the Earth would have been fine, right?

 Robin George Andrews

 Yeah, the Earth would have been fine. It seems that Earth was spared the worst of it.

 Suspect No. 2 for the death of Venus: Tectonic plate-breaking volcanoes

 An artist’s depiction of a volcanically active Venus.
 NASA/JPL-Caltech/Peter Rubin

 Brian Resnick

 So that’s one suspect in our whodunit. What is the other suspect?

 Robin George Andrews

 The other suspect just happens to be my favorite thing: volcanoes.

 Brian Resnick

 How could your fave, volcanoes, kill a planet?

 Robin George Andrews

 Two hundred and fifty-two million years ago, Earth experienced its worst mass extinction, something called the Great Dying. At least 90 percent of all life was wiped out, and the primary suspects were these volcanic fissures that opened up in Siberia. It produced a continent-sized flood of lava that took about 2 million years to erupt.

 So it causes giant explosions and also unleashed all these greenhouse gases into the atmosphere, and it created a global warming effect that raised the temperature by a dozen degrees [Celsius]. That caused 90 percent of all the life on Earth to die. Earth had to kind of reset itself. And the idea is, well, what if that happened on Venus, but worse?

 Brian Resnick

 That could kill the planet?

 Robin George Andrews

 If you just have one [Great Dying-scale] eruption, it might be okay, because Venus had plate tectonics. Plate tectonics is essentially a planet’s thermostat.

 Brian Resnick

 Plate tectonics — that’s how continents kind of float around and smoosh into each other. How do those act like a thermostat?

 Robin George Andrews

 Carbon dioxide can get soaked up in the ocean, and that filters down to these tectonic plates. If tectonic plates dive down beneath each other, then you’re burying carbon [and slowing the greenhouse effect].

 Listen to Unexplainable

 Unexplainable is a weekly science podcast about everything we don’t know. For stories about great scientific mysteries, follow us wherever you listen to podcasts.

 Brian Resnick

 Okay, but how do you break this carbon-burying system?

 Robin George Andrews

 You have two of these epic, Great Dying-like eruptions at the same time.

 That will immediately trigger quite an intense period of global warming. And the oceans will just start to boil off.

 Now, plate tectonics could bury carbon for a while. But if you boil off your oceans, that carbon dioxide has nothing to dissolve into. And if you get rid of that water, plate tectonics itself shuts down. If you dehydrate [tectonic plates], you make them brittle — they can’t bury the carbon anymore. That’s essentially game over. If you break plate tectonics you’ve broken the world.

 The scientific jury is still out

 Venus’s clouds captured in infared.
 JAXA/ISAS/DARTS/Damia Bouic

 Brian Resnick

 So we have our two suspects here. We have the sun and we have massive eruptions on Venus that broke Venus. Which one should we sentence here?

 Robin George Andrews

 The volcanoes seem more likely because we can see how that’s happened on Earth — just to a slightly lesser extent. But if this was put into a court of law, they would both be presumed not guilty, just because there isn’t a telltale bit of evidence yet.

 Brian Resnick

 So how do we solve this mystery?

 Robin George Andrews

 There’s basically a fleet of missions that is going to unravel the geologic makeup of Venus today. If it looks bone-dry and it was always bone-dry, then maybe the sun did it, because it’s been dehydrated for a long time.

 But if it looks like there’s still some dehydration going on, that means you still have water somewhere. If Venus was still kind of soggy on the inside, and it’s still kind of belching water out, the volcanoes probably killed Venus.

 Brian Resnick

 Could the Earth pull a Venus one day?

 Robin George Andrews

 [Laughs.] Yeah, it could. Let’s not panic — I mean, these sorts of eruptions are like millions of years in timescales. So it’s not like no one would see this coming and we’d instantly be doomed. But it could happen. And the question is: Is it normal for a planet to have just one of these really epic, game-changer eruptions at one time? Or is it just a fluke?

 The fact that no one knows the answer to this is weirdly, perversely exciting to me. We don’t know how often it is that volcanoes decide to trash the planets they’re on.

 Brian Resnick

 People sometimes bring up Venus in the context of climate change. It’s an example of how bad a planet can become when greenhouse gases start to accumulate in an atmosphere. Could we humans potentially be the volcano?

 Robin George Andrews

 The pace at which we’re putting carbon dioxide into the sky is worse than what was happening during the Great Dying, in terms of the amount of carbon per year.

 Brian Resnick

 But we would need to keep this up for millions of years to match, right?

 Robin George Andrews

 Right.

 Brian Resnick

 This is weirdly reassuring that humans are unlikely to break the Earth completely.

 Robin George Andrews

 Yeah. I think it would be a terrible idea to pay homage to what happened to Venus.

 Will you support Vox’s explanatory journalism?

 Millions turn to Vox to educate themselves, their family, and their friends about what’s happening in the world around them, and to learn about things that spark their curiosity. Financial contributions from our readers are a critical part of supporting our resource-intensive work and help us keep our journalism free for all. Please consider making a one-time contribution to Vox today.

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Courtesy of NASA/JPL
 
 
 
 
 
 
 
 
 
 900 degrees Fahrenheit, crushing pressure, and acid clouds. Venus, what the hell happened? 
 
 
 
 By
 
 
 
 
 
 
 Dec 1, 2021, 1:30pm EST
 
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
“Hellscape” is the most appropriate word to describe the surface of Venus, the second planet from the sun. At 900 degrees Fahrenheit, it’s the hottest planet in the solar system, thanks to an atmosphere that’s almost entirely made up of carbon dioxide. Clouds made of highly corrosive sulfuric acid are draped over a volcanic landscape of razor-sharp lava flows. Most crushingly, the pressure on the surface of Venus is about 92 times the pressure you’d feel at sea level on Earth. 
“It’s really almost entertainingly, comically horrible, like some sort of cosmic deity had a really, really grumpy day and just went ‘Nope, I’m gonna ruin this planet,’” Robin George Andrews, a science journalist and volcanologist, says. Andrews compares it to being in a pressure cooker a mile underwater. “If you stood on the surface, you would be pancaked and you would melt,” he says. Your eyes would explode due to the pressure — “which would be gross,” he adds.
Yet as Andrews relays in his new book,  Super Volcanoes: What They Reveal about Earth and the Worlds Beyond , some scientists suspect Venus was once much like Earth, with a liquid water ocean like the ones that support life on our planet. For Andrews, the question of what happened to ruin Venus is captivating and even existential. 
“Venus and Earth are planetary siblings,” he says. “They were made at the same time and made of the same stuff, yet Venus is apocalyptic and awful in every possible way. Earth is a paradise. So why do we have a paradise next to a paradise lost?”
Scientists know something on Venus triggered truly catastrophic levels of climate change, causing surface temperatures to shoot up hundreds of degrees. But they don’t know exactly what. I spoke to Andrews for an episode of  Unexplainable , Vox’s science podcast about unanswered questions, about what could have triggered Venus’s apocalypse and why we should care about it. Our conversation has been edited for length and clarity. 
The podcast episode also features a discussion with Sara Seager, an MIT planetary scientist and an expert on exoplanets, or planets outside our solar system, who raises the tantalizing question: What if, despite the cataclysm, something is still alive on Venus? 
The origins of Venus could tell us a lot about our place in the universe
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 On the left, Venus as it is today. On the right, an artist’s interpretation of where oceans could have lain on the surface in the past.
 
 
 NASA/Jet Propulsion Laboratory-Caltech (left) and NASA (right)
 
 
 
Brian Resnick
You say the question of “what killed Venus” is existential. What makes it so? 
Robin George Andrews
It really is a question about why are we here. Answering it will help us answer the question: How lonely are we? Are there other Venuses or Earths out there? 
If Earth is the odd one out, how lucky are we to exist? If Venus is the odd one out, then maybe we’re not so special after all.
Brian Resnick
So Earth and Venus started off as similar planets, and then went down different paths. You want to know what path is more common out there in the cosmos? 
Robin George Andrews
When you hear in the news that scientists have discovered an Earth-like exoplanet, they might as well be saying, we found a Venus-like exoplanet. We don’t know if this is like a habitable world by our human, surface-dwelling standards, or if it’s gone through this sort of apocalyptic climate change like Venus. A good way to work out what may be more common in the cosmos is to study Earth and Venus, because they are siblings. 
Brian Resnick
How do scientists know, or suspect, Venus used to be more pleasant, habitable even? 
Robin George Andrews
So even though Venus today looks and is apocalyptic in every meaning of the word, probes have looked at its atmosphere and found there’s a lot of “heavy water” in the atmosphere. Heavy water is exactly what it sounds like. The water we’re used to, this classic H2O, which is found commonly everywhere on Earth, is a more common type of water throughout the cosmos. Heavy water just kind of switches out that hydrogen for something called deuterium, which is like a heavier version of hydrogen.
Brian Resnick
What does finding heavy water mean?
Robin George Andrews
If you measure how much heavy water exists somewhere, you can make a reasonable guess as to how much classic water there is or was on that planet. It suggests that there once was a lot of classic water on Venus, at least an ocean’s worth of water on Venus. If that water existed in liquid form, what are the odds that Venus was habitable at some point? It’s not unlikely, even though today it looks impossible. 
Brian Resnick
How long ago must this have been, this habitable Venus? 
Robin George Andrews
I think there is a possibility that water was always steam, and it might have never been liquid water. But if it was, then it could have been habitable for billions of years. Maybe right up until the last billion years.
Brian Resnick
So that’s what we’re talking about when we say Venus used to be “alive.” What do we mean by “killed”?
Robin George Andrews
Death, in this case, is runaway climate change. Absolutely irreversible, world-ending climate change. The average temperature of the planet shot up by something like several hundred degrees Fahrenheit. It really sort of cooked itself to death.
Suspect No. 1 for the death of Venus: The sun
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Plasma spewing out of the surface of the sun.
 
 
 NASA/Solar Dynamics Observatory
 
 
 
Brian Resnick
So the question, I imagine, is how Venus cooked itself.
Robin George Andrews
There’s kind of two leading theories as to what killed Venus. Option No. 1 is the sun. From what we know of our sun and other stars, when they’re kind of in their teenage years, they get hyper-excitable, and they get hotter and brighter quite quickly. If the sun actually gets brighter and hotter too quickly, even though you might have that water sitting on the surface, the sun can boil it off. And that is a fate that lots of exoplanets, or planets outside our galaxy, are presumed to have gone through.
Brian Resnick
And once Venus’s water gets vaporized...
Robin George Andrews
That steam is a greenhouse gas that would have kicked up the greenhouse gas effect. And then the carbon dioxide coming out of Venus’s embryonic volcanoes would have just sealed the deal. That could explain why we see Venus as it is today. 
Brian Resnick
If this young, very excitable sun is what killed Venus, the Earth would have been fine, right? 
 Robin George Andrews 
Yeah, the Earth would have been fine. It seems that Earth was spared the worst of it.
Suspect No. 2 for the death of Venus: Tectonic plate-breaking volcanoes
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 An artist’s depiction of a volcanically active Venus.
 
 
 NASA/JPL-Caltech/Peter Rubin
 
 
 
 Brian Resnick 
So that’s one suspect in our whodunit. What is the other suspect? 
 Robin George Andrews 
The other suspect just happens to be my favorite thing: volcanoes. 
 Brian Resnick 
How could your fave, volcanoes, kill a planet? 
 Robin George Andrews 
Two hundred and fifty-two million years ago, Earth experienced its worst mass extinction, something called the Great Dying. At least 90 percent of all life was wiped out, and the primary suspects were these volcanic fissures that opened up in Siberia. It produced a continent-sized flood of lava that took about 2 million years to erupt. 
So it causes giant explosions and also unleashed all these greenhouse gases into the atmosphere, and it created a global warming effect that raised the temperature by a dozen degrees [Celsius]. That caused 90 percent of all the life on Earth to die. Earth had to kind of reset itself. And the idea is, well, what if that happened on Venus, but worse? 
 Brian Resnick 
That could kill the planet? 
 Robin George Andrews 
If you just have one [Great Dying-scale] eruption, it might be okay, because Venus had plate tectonics. Plate tectonics is essentially a planet’s thermostat.
 Brian Resnick 
Plate tectonics — that’s how continents kind of float around and smoosh into each other. How do those act like a thermostat? 
 Robin George Andrews 
Carbon dioxide can get soaked up in the ocean, and that filters down to these tectonic plates. If tectonic plates dive down beneath each other, then you’re burying carbon [and slowing the greenhouse effect].
 Brian Resnick 
Okay, but how do you break this carbon-burying system? 
 Robin George Andrews 
You have two of these epic, Great Dying-like eruptions at the same time.
That will immediately trigger quite an intense period of global warming. And the oceans will just start to boil off.
Now, plate tectonics could bury carbon for a while. But if you boil off your oceans, that carbon dioxide has nothing to dissolve into. And if you get rid of that water, plate tectonics itself shuts down. If you dehydrate [tectonic plates], you make them brittle — they can’t bury the carbon anymore. That’s essentially game over. If you break plate tectonics you’ve broken the world.
The scientific jury is still out
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Venus’s clouds captured in infared.
 
 
 JAXA/ISAS/DARTS/Damia Bouic
 
 
 
 Brian Resnick 
So we have our two suspects here. We have the sun and we have massive eruptions on Venus that broke Venus. Which one should we sentence here? 
 Robin George Andrews 
The volcanoes seem more likely because we can see how that’s happened on Earth — just to a slightly lesser extent. But if this was put into a court of law, they would both be presumed not guilty, just because there isn’t a telltale bit of evidence yet.
 Brian Resnick 
So how do we solve this mystery? 
 Robin George Andrews 
There’s basically a fleet of missions that is going to unravel the geologic makeup of Venus today. If it looks bone-dry and it was always bone-dry, then maybe the sun did it, because it’s been dehydrated for a long time. 
But if it looks like there’s still some dehydration going on, that means you still have water somewhere. If Venus was still kind of soggy on the inside, and it’s still kind of belching water out, the volcanoes probably killed Venus. 
 Brian Resnick 
Could the Earth pull a Venus one day? 
 Robin George Andrews 
[Laughs.] Yeah, it could. Let’s not panic — I mean, these sorts of eruptions are like millions of years in timescales. So it’s not like no one would see this coming and we’d instantly be doomed. But it could happen. And the question is: Is it normal for a planet to have just one of these really epic, game-changer eruptions at one time? Or is it just a fluke? 
The fact that no one knows the answer to this is weirdly, perversely exciting to me. We don’t know how often it is that volcanoes decide to trash the planets they’re on. 
 Brian Resnick  
People sometimes bring up Venus in the context of climate change. It’s an example of how bad a planet can become when greenhouse gases start to accumulate in an atmosphere. Could we humans potentially be the volcano? 
 Robin George Andrews 
The pace at which we’re putting carbon dioxide into the sky is worse than what was happening during the Great Dying, in terms of the amount of carbon per year. 
 Brian Resnick  
But we would need to keep this up for millions of years to match, right? 
 Robin George Andrews 
Right.
 Brian Resnick  
This is weirdly reassuring that humans are unlikely to break the Earth completely. 
 Robin George Andrews 
Yeah. I think it would be a terrible idea to pay homage to what happened to Venus.
 
 
 
 
 
 
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Venus could have been a paradise but turned into a hellscape. Earthlings, pay attention.
https://www.vox.com/science-and-health/22807575/venus-hot-hellscape-climate-change-earth
https://www.epa.gov/climate-change
Climate Change | US EPA
2013-08-12
None
<div><div>
 <div>
 
 
 <article>
 
 
 
 
 
<div><p>slide 1 of 4</p><div><ul>
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<div>
 <h2>
 Predicting an Early Spring?
 </h2>
 <p>No matter what the groundhog says, annual springtime events are creeping earlier across the country. For instance, peak springtime <a href="/climate-indicators/climate-change-indicators-streamflow">streamflow dates</a> have gotten earlier in several regions.</p>
 </div>
 </li>
 
 
 
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 <p>Understanding and addressing climate change is critical to EPA's mission of protecting human health and the environment. EPA tracks and reports greenhouse gas emissions, leverages <a href="/climatechange-science">sound science</a>, and works to reduce emissions to combat climate change. <a href="/climate-change/climate-change-epa">Learn more</a> about the objectives of the EPA Climate Change website.</p>
<p>EPA is committed to advancing the goals of <a href="/environmentaljustice">environmental justice</a> for all Americans, including those historically marginalized, overburdened, underserved, and living with the legacy of structural racism. Additional web content about climate change and environmental justice is currently under development.</p>
 
 
 
 
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 <h2>Addressing Climate Change</h2>
 
 
 
 
 
 
 
 
<figure><p></p>
 </figure><ul><li><a href="/climate-change/what-epa-doing-about-climate-change">What EPA Is Doing</a>
	<ul><li><a href="/climate-change/climate-change-regulatory-actions-and-initiatives">Regulatory Actions and Initiatives</a></li>
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 </figure><ul><li><a href="/energy">Energy and the Environment</a></li>
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 Climate Change

 slide 1 of 4

 * Predicting an Early Spring?

 No matter what the groundhog says, annual springtime events are creeping earlier across the country. For instance, peak springtime streamflow dates have gotten earlier in several regions.

 * Impacts of Climate Change

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 * Snow Season Is Here

 As the climate has changed, so have snowfall patterns across the United States. Since the first half of the 20th century, snowfall has decreased in many parts of the country.

 * Predicting an Early Spring?

 No matter what the groundhog says, annual springtime events are creeping earlier across the country. For instance, peak springtime streamflow dates have gotten earlier in several regions.

 1234

 Understanding and addressing climate change is critical to EPA's mission of protecting human health and the environment. EPA tracks and reports greenhouse gas emissions, leverages sound science, and works to reduce emissions to combat climate change. Learn more about the objectives of the EPA Climate Change website.

 EPA is committed to advancing the goals of environmental justice for all Americans, including those historically marginalized, overburdened, underserved, and living with the legacy of structural racism. Additional web content about climate change and environmental justice is currently under development.

 The Biden-Harris administration has taken executive actions to tackle the climate crisis at home and abroad, protect public health and the environment and restore science to tackle the climate crisis, and rejoin the Paris Climate Agreement. Learn more about The President’s approach to addressing climate change.

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slide 1 of 4  
 
 
 
 
 Predicting an Early Spring?
 
 No matter what the groundhog says, annual springtime events are creeping earlier across the country. For instance, peak springtime streamflow dates have gotten earlier in several regions.
 
 
 
 
 
   
 
 
 
 
 
 
 
 
 
 
 
 
 Understanding and addressing climate change is critical to EPA's mission of protecting human health and the environment. EPA tracks and reports greenhouse gas emissions, leverages sound science, and works to reduce emissions to combat climate change. Learn more about the objectives of the EPA Climate Change website.
EPA is committed to advancing the goals of environmental justice for all Americans, including those historically marginalized, overburdened, underserved, and living with the legacy of structural racism. Additional web content about climate change and environmental justice is currently under development.
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Climate Change Facts
 
 
 
 
 
 
 
 
  
    Climate Change Science 
	 Greenhouse Gas Emissions 
	 Climate Change Indicators 
	 Climate Change Impacts
	  Climate Change and Human Health 
	 Impacts by Sector 
	 Climate Equity 
	 
 
 
 
 
 
 
 
 
 Addressing Climate Change
 
 
 
 
 
 
 
 
  
    What EPA Is Doing
	  Regulatory Actions and Initiatives 
	 Partnership Programs 
	 International Climate Partnerships 
	 Bipartisan Infrastructure Law 
	 Inflation Reduction Act 
	 
	 What You Can Do 
	 Climate Change Resources for Educators and Students 
 
 
 
 
 
 
 
 
 Climate Connections
 
 
 
 
 
 
 
 
  
    Energy and the Environment 
	 State and Local Climate and Energy Program 
	 Climate and Transportation 
	 Climate and Waste 
	 Climate and Water 
	 Climate and Indoor Air Quality 
	 Climate Change Research 
	 Climate Adaptation 
	 Tribal Air and Climate Resources 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  Contact Us to ask a question, provide feedback, or report a problem. 
Last updated on February 2, 2023
Climate Change | US EPA
https://www.epa.gov/climate-change
https://www.un.org/en/climatechange/what-is-climate-change
What Is Climate Change? | United Nations
2021-08-09
United Nations
<div><div>
	<p>Climate change refers to long-term shifts in temperatures and weather patterns. These shifts may be natural, such as through variations in the solar cycle. But since the 1800s, <a href="https://www.ipcc.ch/2021/08/09/ar6-wg1-20210809-pr/">human activities have been the main driver of climate change</a>, primarily due to burning fossil fuels like coal, oil and gas.</p>
	<p>Burning fossil fuels generates greenhouse gas emissions that act like a blanket wrapped around the Earth, trapping the sun’s heat and raising temperatures.</p>
	<p>Examples of greenhouse gas emissions that are causing climate change include carbon dioxide and methane. These come from using gasoline for driving a car or coal for heating a building, for example. Clearing land and forests can also release carbon dioxide. Landfills for garbage are a major source of methane emissions. Energy, industry, transport, buildings, agriculture and land use are among the <a href="https://www.unep.org/interactive/six-sector-solution-climate-change/">main emitters</a>.</p>
	<p></p>
	<h2>Greenhouse gas concentrations are at their highest levels in 2 million years</h2>
	<p>And<a href="https://wedocs.unep.org/bitstream/handle/20.500.11822/36991/EGR21_ESEN.pdf"> emissions continue to rise</a>. As a result, <a href="https://www.ipcc.ch/2021/08/09/ar6-wg1-20210809-pr/">the Earth is now about 1.1°C warmer</a> than it was in the late 1800s. The <a href="https://public.wmo.int/en/media/press-release/2020-was-one-of-three-warmest-years-record">last decade (2011-2020) was the warmest on record</a>.</p>
	<p>Many people think climate change mainly means warmer temperatures. But temperature rise is only the beginning of the story. Because the Earth is a system, where everything is connected, changes in one area can influence changes in all others.</p>
	<p>The <a href="https://www.ipcc.ch/report/ar6/wg2/resources/press/press-release">consequences of climate change</a> now include, among others, intense droughts, water scarcity, severe fires, rising sea levels, flooding, melting polar ice, catastrophic storms and declining biodiversity.</p>
	<p></p>
	<h2>People are experiencing climate change in diverse ways</h2>
	<p>Climate change can affect our health, ability to grow food, housing, safety and work. Some of us are already more vulnerable to climate impacts, such as people living in small island nations and other developing countries. Conditions like sea-level rise and saltwater intrusion have advanced to the point where whole communities have had to relocate, and protracted droughts are putting people at risk of famine. In the future, the number of “climate refugees” is expected to rise.</p>
	<h2>Every increase in global warming matters</h2>
	<p>In a series of <a href="https://www.ipcc.ch/assessment-report/ar6/">UN reports</a>, thousands of scientists and government reviewers agreed that limiting global temperature rise to no more than 1.5°C would help us avoid the worst climate impacts and maintain a livable climate. Yet policies currently in place point to a <a href="https://www.unep.org/resources/emissions-gap-report-2022">2.8°C temperature rise</a> by the end of the century.</p>
	<p>The emissions that cause climate change come from every part of the world and affect everyone, but<a href="https://www.climatewatchdata.org/ghg-emissions?end_year=2018&amp;source=CAIT&amp;start_year=1990"> some countries produce much more than others</a>. The 100 least-emitting countries generate 3 per cent of total emissions. The 10 countries with the largest emissions contribute 68 per cent. Everyone must take climate action, but people and countries creating more of the problem have a greater responsibility to act first.</p>
	<p></p>
	<h2>We face a huge challenge but already know many solutions</h2>
	<p>Many climate change solutions can deliver economic benefits while improving our lives and protecting the environment. We also have global frameworks and agreements to guide progress, such as the <a href="https://www.un.org/en/climatechange/17-goals-to-transform-our-world">Sustainable Development Goals</a>, the <a href="https://unfccc.int/process-and-meetings/the-convention/what-is-the-united-nations-framework-convention-on-climate-change">UN Framework Convention on Climate Change</a> and the <a href="https://www.un.org/en/climatechange/paris-agreement">Paris Agreement</a>. Three broad categories of action are: cutting emissions, adapting to climate impacts and financing required adjustments.</p>
	<p>Switching energy systems from fossil fuels to<a href="https://iea.blob.core.windows.net/assets/5ae32253-7409-4f9a-a91d-1493ffb9777a/Renewables2021-Analysisandforecastto2026.pdf"> renewables like solar or wind</a> will reduce the emissions driving climate change. But we have to start right now. While a growing coalition of countries is committing to <a href="https://www.un.org/en/climatechange/net-zero-coalition">net zero emissions</a> by 2050, about <a href="https://www.ipcc.ch/report/ar6/wg3/resources/press/press-release">half of emissions cuts must be in place by 2030</a> to keep warming below 1.5°C.<a href="https://www.unep.org/news-and-stories/press-release/worlds-governments-must-wind-down-fossil-fuel-production-6-year"> Fossil fuel production must decline</a> by roughly 6 per cent per year between 2020 and 2030.</p>
	<p></p>
	<p>Adapting to climate consequences protects people, homes, businesses, livelihoods, infrastructure and natural ecosystems. It covers current impacts and those likely in the future. Adaptation will be required everywhere, but must be prioritized now for the most vulnerable people with the fewest resources to cope with climate hazards. The rate of return can be high. Early warning systems for disasters, for instance, save lives and property, and can deliver benefits up to 10 times the initial cost.</p>
	<h2>We can pay the bill now, or pay dearly in the future</h2>
	<p>Climate action requires significant <a href="https://www.un.org/en/climatechange/raising-ambition/climate-finance">financial investments</a> by governments and businesses. But climate inaction is vastly more expensive. One critical step is for industrialized countries to fulfil their commitment to provide $100 billion a year to developing countries so they can adapt and move towards greener economies.</p>
	<p></p>
	</div><div>
	<div><p><a href="https://www.un.org/en/climatechange/cop27"></a></p><h3>COP27</h3>
	<p>Learn more about the 2022 UN Climate Conference in Sharm el-Sheikh, Egypt.</p>
	</div>
	<div><p><a href="https://www.un.org/en/climatechange/science/causes-effects-climate-change"></a></p><h3>Causes and Effects</h3>
	<p>Fossil fuels are by far the largest contributor to the greenhouse gas emissions that cause climate change, which poses many risks to all forms of life on Earth. <a href="https://www.un.org/en/climatechange/science/causes-effects-climate-change">Learn more</a>.</p>
	</div>
	<div><p><a href="https://www.un.org/en/climatechange/speeches"></a></p><h3>From the Secretary-General</h3>
	</div>
	<div><p><a href="https://www.un.org/en/climatechange/net-zero-coalition"></a></p><h3>Net zero</h3>
	<p>What is net zero? Why is it important? Our <a href="https://www.un.org/en/climatechange/net-zero-coalition">net-zero page</a> explains why we need steep emissions cuts now and what efforts are underway.</p>
	</div>
	<div><p><a href="https://www.un.org/en/climatechange/raising-ambition/renewable-energy"></a></p><h3>Renewable energy – powering a safer future</h3>
	<p>What is renewable energy and why does it matter? Learn more about why the shift to renewables is our only hope for a brighter and safer world.</p>
	</div>
	<div><p><a href="https://www.un.org/en/climatechange/raising-ambition/renewable-energy-transition"></a></p><h3>Five ways to jump-start the renewable energy transition now</h3>
	<p>UN Secretary-General outlines <a href="https://www.un.org/en/climatechange/raising-ambition/renewable-energy-transition"> five critical actions</a> the world needs to prioritize now to speed up the global shift to renewable energy.</p>
	</div>
	<div><p><a href="https://www.un.org/en/climatechange/climate-adaptation"></a></p><h3>Adaptation</h3>
	<p>What is climate adaptation? Why is it so important for every country?<a href="https://www.un.org/en/climatechange/climate-adaptation"> Find out</a> how we can protect lives and livelihoods as the climate changes.</p>
	</div>
	<div><p><a href="https://www.un.org/en/climatechange/17-goals-to-transform-our-world"></a></p><h3>Sustainable Development Goals</h3>
	<p>Climate action is a global goal. And it contributes to all other SDGs.<a href="https://www.un.org/en/climatechange/17-goals-to-transform-our-world"> Learn more.</a></p>
	</div>
	</div>||||I|||| Skip to main content
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 What Is Climate Change?

 Climate change refers to long-term shifts in temperatures and weather patterns. These shifts may be natural, such as through variations in the solar cycle. But since the 1800s, human activities have been the main driver of climate change, primarily due to burning fossil fuels like coal, oil and gas.

 Burning fossil fuels generates greenhouse gas emissions that act like a blanket wrapped around the Earth, trapping the sun’s heat and raising temperatures.

 Examples of greenhouse gas emissions that are causing climate change include carbon dioxide and methane. These come from using gasoline for driving a car or coal for heating a building, for example. Clearing land and forests can also release carbon dioxide. Landfills for garbage are a major source of methane emissions. Energy, industry, transport, buildings, agriculture and land use are among the main emitters.

 Greenhouse gas concentrations are at their highest levels in 2 million years

 And emissions continue to rise. As a result, the Earth is now about 1.1°C warmer than it was in the late 1800s. The last decade (2011-2020) was the warmest on record.

 Many people think climate change mainly means warmer temperatures. But temperature rise is only the beginning of the story. Because the Earth is a system, where everything is connected, changes in one area can influence changes in all others.

 The consequences of climate change now include, among others, intense droughts, water scarcity, severe fires, rising sea levels, flooding, melting polar ice, catastrophic storms and declining biodiversity.

 People are experiencing climate change in diverse ways

 Climate change can affect our health, ability to grow food, housing, safety and work. Some of us are already more vulnerable to climate impacts, such as people living in small island nations and other developing countries. Conditions like sea-level rise and saltwater intrusion have advanced to the point where whole communities have had to relocate, and protracted droughts are putting people at risk of famine. In the future, the number of “climate refugees” is expected to rise.

 Every increase in global warming matters

 In a series of UN reports, thousands of scientists and government reviewers agreed that limiting global temperature rise to no more than 1.5°C would help us avoid the worst climate impacts and maintain a livable climate. Yet policies currently in place point to a 2.8°C temperature rise by the end of the century.

 The emissions that cause climate change come from every part of the world and affect everyone, but some countries produce much more than others. The 100 least-emitting countries generate 3 per cent of total emissions. The 10 countries with the largest emissions contribute 68 per cent. Everyone must take climate action, but people and countries creating more of the problem have a greater responsibility to act first.

 We face a huge challenge but already know many solutions

 Many climate change solutions can deliver economic benefits while improving our lives and protecting the environment. We also have global frameworks and agreements to guide progress, such as the Sustainable Development Goals, the UN Framework Convention on Climate Change and the Paris Agreement. Three broad categories of action are: cutting emissions, adapting to climate impacts and financing required adjustments.

 Switching energy systems from fossil fuels to renewables like solar or wind will reduce the emissions driving climate change. But we have to start right now. While a growing coalition of countries is committing to net zero emissions by 2050, about half of emissions cuts must be in place by 2030 to keep warming below 1.5°C. Fossil fuel production must decline by roughly 6 per cent per year between 2020 and 2030.

 Adapting to climate consequences protects people, homes, businesses, livelihoods, infrastructure and natural ecosystems. It covers current impacts and those likely in the future. Adaptation will be required everywhere, but must be prioritized now for the most vulnerable people with the fewest resources to cope with climate hazards. The rate of return can be high. Early warning systems for disasters, for instance, save lives and property, and can deliver benefits up to 10 times the initial cost.

 We can pay the bill now, or pay dearly in the future

 Climate action requires significant financial investments by governments and businesses. But climate inaction is vastly more expensive. One critical step is for industrialized countries to fulfil their commitment to provide $100 billion a year to developing countries so they can adapt and move towards greener economies.

 Learn more about…

 COP27

 Learn more about the 2022 UN Climate Conference in Sharm el-Sheikh, Egypt.

 The science

 See the latest climate reports from the United Nations as well as climate action facts.

 Causes and Effects

 Fossil fuels are by far the largest contributor to the greenhouse gas emissions that cause climate change, which poses many risks to all forms of life on Earth. Learn more.

 From the Secretary-General

 Net zero

 What is net zero? Why is it important? Our net-zero page explains why we need steep emissions cuts now and what efforts are underway.

 Renewable energy – powering a safer future

 What is renewable energy and why does it matter? Learn more about why the shift to renewables is our only hope for a brighter and safer world.

 Five ways to jump-start the renewable energy transition now

 UN Secretary-General outlines five critical actions the world needs to prioritize now to speed up the global shift to renewable energy.

 Finance

 How will the world foot the bill? We explain the issues and the value of financing climate action.

 Adaptation

 What is climate adaptation? Why is it so important for every country? Find out how we can protect lives and livelihoods as the climate changes.

 Sustainable Development Goals

 Climate action is a global goal. And it contributes to all other SDGs. Learn more.

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Climate change refers to long-term shifts in temperatures and weather patterns. These shifts may be natural, such as through variations in the solar cycle. But since the 1800s, human activities have been the main driver of climate change, primarily due to burning fossil fuels like coal, oil and gas.
	Burning fossil fuels generates greenhouse gas emissions that act like a blanket wrapped around the Earth, trapping the sun’s heat and raising temperatures.
	Examples of greenhouse gas emissions that are causing climate change include carbon dioxide and methane. These come from using gasoline for driving a car or coal for heating a building, for example. Clearing land and forests can also release carbon dioxide. Landfills for garbage are a major source of methane emissions. Energy, industry, transport, buildings, agriculture and land use are among the main emitters.
	 
	Greenhouse gas concentrations are at their highest levels in 2 million years
	And emissions continue to rise. As a result, the Earth is now about 1.1°C warmer than it was in the late 1800s. The last decade (2011-2020) was the warmest on record.
	Many people think climate change mainly means warmer temperatures. But temperature rise is only the beginning of the story. Because the Earth is a system, where everything is connected, changes in one area can influence changes in all others.
	The consequences of climate change now include, among others, intense droughts, water scarcity, severe fires, rising sea levels, flooding, melting polar ice, catastrophic storms and declining biodiversity.
	 
	People are experiencing climate change in diverse ways
	Climate change can affect our health, ability to grow food, housing, safety and work. Some of us are already more vulnerable to climate impacts, such as people living in small island nations and other developing countries. Conditions like sea-level rise and saltwater intrusion have advanced to the point where whole communities have had to relocate, and protracted droughts are putting people at risk of famine. In the future, the number of “climate refugees” is expected to rise.
	Every increase in global warming matters
	In a series of UN reports, thousands of scientists and government reviewers agreed that limiting global temperature rise to no more than 1.5°C would help us avoid the worst climate impacts and maintain a livable climate. Yet policies currently in place point to a 2.8°C temperature rise by the end of the century.
	The emissions that cause climate change come from every part of the world and affect everyone, but some countries produce much more than others. The 100 least-emitting countries generate 3 per cent of total emissions. The 10 countries with the largest emissions contribute 68 per cent. Everyone must take climate action, but people and countries creating more of the problem have a greater responsibility to act first.
	 
	We face a huge challenge but already know many solutions
	Many climate change solutions can deliver economic benefits while improving our lives and protecting the environment. We also have global frameworks and agreements to guide progress, such as the Sustainable Development Goals, the UN Framework Convention on Climate Change and the Paris Agreement. Three broad categories of action are: cutting emissions, adapting to climate impacts and financing required adjustments.
	Switching energy systems from fossil fuels to renewables like solar or wind will reduce the emissions driving climate change. But we have to start right now. While a growing coalition of countries is committing to net zero emissions by 2050, about half of emissions cuts must be in place by 2030 to keep warming below 1.5°C. Fossil fuel production must decline by roughly 6 per cent per year between 2020 and 2030.
	 
	Adapting to climate consequences protects people, homes, businesses, livelihoods, infrastructure and natural ecosystems. It covers current impacts and those likely in the future. Adaptation will be required everywhere, but must be prioritized now for the most vulnerable people with the fewest resources to cope with climate hazards. The rate of return can be high. Early warning systems for disasters, for instance, save lives and property, and can deliver benefits up to 10 times the initial cost.
	We can pay the bill now, or pay dearly in the future
	Climate action requires significant financial investments by governments and businesses. But climate inaction is vastly more expensive. One critical step is for industrialized countries to fulfil their commitment to provide $100 billion a year to developing countries so they can adapt and move towards greener economies.
	 
	 
	     COP27
	Learn more about the 2022 UN Climate Conference in Sharm el-Sheikh, Egypt.
	
	     Causes and Effects
	Fossil fuels are by far the largest contributor to the greenhouse gas emissions that cause climate change, which poses many risks to all forms of life on Earth. Learn more.
	
	     From the Secretary-General
	
	     Net zero
	What is net zero? Why is it important? Our net-zero page explains why we need steep emissions cuts now and what efforts are underway.
	
	     Renewable energy – powering a safer future
	What is renewable energy and why does it matter? Learn more about why the shift to renewables is our only hope for a brighter and safer world.
	
	     Five ways to jump-start the renewable energy transition now
	UN Secretary-General outlines  five critical actions the world needs to prioritize now to speed up the global shift to renewable energy.
	
	     Adaptation
	What is climate adaptation? Why is it so important for every country? Find out how we can protect lives and livelihoods as the climate changes.
	
	     Sustainable Development Goals
	Climate action is a global goal. And it contributes to all other SDGs. Learn more.
What Is Climate Change? | United Nations
https://www.un.org/en/climatechange/what-is-climate-change
https://www.worldbank.org/en/topic/climatechange
Climate Change
2019-10-02
None
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Climate Stories
 
 
 
 
 
 
 Our Climate Stories series highlights the World Bank Group's support for investments, solutions and innovations that have made a real difference in the lives of people, communities, and economies. 
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Climate Explainer Series
 
 
 
 
 
 
 Learn about the Word Bank Group’s plans, policies, and tools for fighting climate change from our experts.
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Sustainable Development Series
 
 
 
 
 
 
 This series offers insights into innovative and state-of-the-art solutions that can guide countries to build more inclusive and sustainable economies that are resilient in the face of pandemics, climate change and other ...
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 IDA and Climate Change
 
 
 
 
 
 
 IDA helps the poorest nations adapt to climate change by building their resilience to disasters, and promoting sustainable development to minimize their vulnerability.
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Carbon Pricing Leadership Coalition (CPLC)
 
 
 
 
 
 
 The Carbon Pricing Leadership Coalition brings together leaders from across government, the private sector and civil society to share experience working with carbon pricing and to expand the evidence base for the most ...
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 IFC Climate Business
 
 
 
 
 
 
 IFC invests in the private sector in clean energy, sustainable cities, climate-smart agriculture, energy efficiency, green buildings and green finance.
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Climate Biz Podcast
 
 
 
 
 
 
 The podcast where sustainability meets smart business. Join co-hosts Shari Friedman and Esohe Denise Odaro as they explore how to increase sustainability and reduce the impact of climate change through private sector and ...
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Connect4Climate
 
 
 
 
 
 
 Connect4Climate is a global partnership program dedicated to driving climate action with creative communications, by amplifying youth voices and generating advocacy campaigns centered on Film, Fashion, Music and Sports.
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Climate Investment Funds
 
 
 
 
 
 
 The $8.3 billion Climate Investment Funds (CIF) is providing 72 developing and middle income countries with urgently needed resources to manage the challenges of climate change and reduce their greenhouse gas emissions.
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Forest Carbon Partnership Facility
 
 
 
 
 
 
 The Forest Carbon Partnership Facility is focused on reducing emissions from deforestation and forest degradation, forest carbon stock conservation, the sustainable management of forests, and the enhancement of forest ...
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Carbon Pricing Dashboard
 
 
 
 
 
 
 This interactive dashboard provides an up-to-date overview of carbon pricing initiatives around the world and allows users to navigate through the visuals and data of the annual State and Trends of Carbon Pricing report ...
Climate Change
https://www.worldbank.org/en/topic/climatechange
https://www.amazon.com/How-Avoid-Climate-Disaster-Breakthroughs/dp/059321577X
How to Avoid a Climate Disaster: The Solutions We Have and the Breakthroughs We Need (Random House Large Print) Paperback – Large Print, February 23, 2021
2021-02-23
Bill Gates
<div><div> <h3>Amazon.com Review</h3> <p><a href="//www.amazon.com/b/?node=17143709011">An Amazon Best Book of February 2021:</a> Sad news: Electric cars won’t save the world. But Bill Gates has some pretty good ideas as to what will. Logical, compelling, and ultimately optimistic, this book is a rousing call from an innovator who believes we have the right stuff to change our fate, and his fresh way of thinking is desperately needed to clear away the overheated emotions this subject ignites. –Adrian Liang, Amazon Book Review </p> <h3>Review</h3> <p>“Gates gathers advice from experts while laying out his vision for technological innovations that could reduce greenhouse gases and stop the warming of the planet. If even some of his plans work, this might be the most important book of the year.” —CNN<p> “One of the most accessible, practical, and interesting books on the topic to emerge since Al Gore’s </p>An Inconvenient Truth.” —Oprah Daily<p>“The most comprehensible explanation for what’s driving our warming planet; how to measure the impact of the myriad contributions to this staggering and seemingly incalculable problem; and ultimately how to go about finding more effective approaches to each of them. It’s the closest thing I’ve seen to a how-to guide for addressing the climate crisis.” —</p>Clinton Leaf, Fortune“How to Avoid a Climate Disaster presents ideas with the methodical approach of a college textbook . . . Remarkably, given the state of the world, it is an optimistic, can-do sort of book, chock-full of solutions.” —Christina Binkley, The Wall Street Journal Magazine<p> “The most refreshing aspect of this book is its bracing mix of cold-eyed realism and number-crunched optimism . . . Ultimately his book is a primer on how to reorganise the global economy so that innovation focuses on the world’s gravest problems. It is a powerful reminder that if mankind is to get serious about tackling them, it must do more to harness the one natural resource available in infinite quantity—human ingenuity.” —</p>The Economist<p> “The author’s enthusiasm and curiosity about the way things work is infectious. He walks us through not just the basic science of global warming, but all the ways that our modern lives contribute to it . . . Gates seems energized by the sheer size and complexity of the challenge. That’s one of the best things about the book—the can-do optimism and conviction that science in partnership with industry are up to the task.” —</p>Richard Schiffman, The Christian Science Monitor<p> “In this wonklike and persuasive book, Gates takes his environmental activism a bridge further, laying out an ambitious plan to reduce greenhouse emissions to zero by 2050. Drawing on data from researchers, policymakers, and engineers, Gates advocates for solutions both scientific (like developing alternative fuels) and personal (like increasing civic engagement in environmental justice issues). If you feel a radical shred of hope reading these galvanizing pages, dare to let in—without hope, we’ll get nowhere.” </p>—Esquire<br />“With the help of experts in fields such as physics, engineering, chemistry, finance and politics, the technologist and philanthropist offers a practical and accessible plan for getting the world to zero greenhouse gas emissions and averting climate catastrophe.” —Barbara VanDenburgh, USA Today“How to Avoid a Climate Disaster is clear, concise on a colossal subject, and intelligently holistic in its approach to the problem. Gates may not be the perfect messenger, but he has written a fine primer on how to get ourselves out of this mess.” —Adama Vaughan, New Scientist<br /> <br /> “Bill Gates has a plan to save the world . . . While acknowledging that the challenge is daunting, and how we make things, grow things, move around, keep cool and stay warm will all need to fundamentally change, Gates argues that wholesale transformation is possible while maintaining lifestyles in high income countries and continuing to lift billions out of poverty.” —Greg Williams, Wired<br /> <br /> “His expertise . . . is apparent in the book’s lucid explanations of the scientific aspects of climate change. The solutions he outlines are pragmatic and grounded in forward-thinking economic reasoning. Although he does not avoid the hard truths we must face as our climate changes, Gates remains optimistic and believes that we have the ability to avoid a total climate disaster.” —Miriam R. Aczel, Science<br /> <br /> “Concise, straightforward . . . Gates has crafted a calm, reasoned, well-sourced explanation of the greatest challenge of our time and what we must change to avoid cooking our planet.” —Jeff Rowe, Associated Press<p>   “A persuasive, optimistic strategy for reducing greenhouse emissions to zero by midcentury . . . Though Gates doesn’t shy away from acknowledging the daunting challenges ahead, his narrative contains enough confidence—and hard science and economics—to convince many readers that his blueprint is one of the most viable yet . . . supremely authoritative and accessible.” —</p>Kirkus Reviews (starred review)<p>   “Those looking for an accessible review of how global warming can be countered will find this a handy—and maybe even hope-inspiring—guide.” —</p>Publishers Weekly<p>   “Gates has put his considerable wealth behind global health, educational, and economic initiatives and now turns his laser-like attention to this most existential of issues . . . He provides illuminating contexts for [his] perspectives and offers a treatise that is imperative, approachable, and useful.” —</p>Booklist </p> </div>||||I|||| Skip to main content
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 How to Avoid a Climate Disaster: The Solutions We Have and the Breakthroughs We Need (Random House Large Print) Paperback – Large Print, February 23, 2021

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 #1 NEW YORK TIMES BEST SELLER • In this urgent, authoritative book, Bill Gates sets out a wide-ranging, practical—and accessible— plan for how the world can get to zero greenhouse gas emissions in time to avoid a climate catastrophe.

 Bill Gates has spent a decade investigating the causes and effects of climate change. With the help of experts in the fields of physics, chemistry, biology, engineering, political science, and finance, he has focused on what must be done in order to stop the planet's slide to certain environmental disaster. In this book, he not only explains why we need to work toward net-zero emissions of greenhouse gases, but also details what we need to do to achieve this profoundly important goal.

 He gives us a clear-eyed description of the challenges we face. Drawing on his understanding of innovation and what it takes to get new ideas into the market, he describes the areas in which technology is already helping to reduce emissions, where and how the current technology can be made to function more effectively, where breakthrough technologies are needed, and who is working on these essential innovations. Finally, he lays out a concrete, practical plan for achieving the goal of zero emissions—suggesting not only policies that governments should adopt, but what we as individuals can do to keep our government, our employers, and ourselves accountable in this crucial enterprise.

 As Bill Gates makes clear, achieving zero emissions will not be simple or easy to do, but if we follow the plan he sets out here, it is a goal firmly within our reach.
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 Editorial Reviews

 Amazon.com Review

 An Amazon Best Book of February 2021: Sad news: Electric cars won’t save the world. But Bill Gates has some pretty good ideas as to what will. Logical, compelling, and ultimately optimistic, this book is a rousing call from an innovator who believes we have the right stuff to change our fate, and his fresh way of thinking is desperately needed to clear away the overheated emotions this subject ignites. –Adrian Liang, Amazon Book Review

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 “Gates gathers advice from experts while laying out his vision for technological innovations that could reduce greenhouse gases and stop the warming of the planet. If even some of his plans work, this might be the most important book of the year.” —CNN

 “One of the most accessible, practical, and interesting books on the topic to emerge since Al Gore’s An Inconvenient Truth. ” — Oprah Daily

 “The most comprehensible explanation for what’s driving our warming planet; how to measure the impact of the myriad contributions to this staggering and seemingly incalculable problem; and ultimately how to go about finding more effective approaches to each of them. It’s the closest thing I’ve seen to a how-to guide for addressing the climate crisis.” — Clinton Leaf, Fortune

 “How to Avoid a Climate Disaster presents ideas with the methodical approach of a college textbook . . . Remarkably, given the state of the world, it is an optimistic, can-do sort of book, chock-full of solutions.” — Christina Binkley, The Wall Street Journal Magazine

 “The most refreshing aspect of this book is its bracing mix of cold-eyed realism and number-crunched optimism . . . Ultimately his book is a primer on how to reorganise the global economy so that innovation focuses on the world’s gravest problems. It is a powerful reminder that if mankind is to get serious about tackling them, it must do more to harness the one natural resource available in infinite quantity—human ingenuity.” — The Economist

 “The author’s enthusiasm and curiosity about the way things work is infectious. He walks us through not just the basic science of global warming, but all the ways that our modern lives contribute to it . . . Gates seems energized by the sheer size and complexity of the challenge. That’s one of the best things about the book—the can-do optimism and conviction that science in partnership with industry are up to the task.” — Richard Schiffman, The Christian Science Monitor

 “In this wonklike and persuasive book, Gates takes his environmental activism a bridge further, laying out an ambitious plan to reduce greenhouse emissions to zero by 2050. Drawing on data from researchers, policymakers, and engineers, Gates advocates for solutions both scientific (like developing alternative fuels) and personal (like increasing civic engagement in environmental justice issues). If you feel a radical shred of hope reading these galvanizing pages, dare to let in—without hope, we’ll get nowhere.” —Esquire
 “With the help of experts in fields such as physics, engineering, chemistry, finance and politics, the technologist and philanthropist offers a practical and accessible plan for getting the world to zero greenhouse gas emissions and averting climate catastrophe.” — Barbara VanDenburgh, USA Today

 “How to Avoid a Climate Disaster is clear, concise on a colossal subject, and intelligently holistic in its approach to the problem. Gates may not be the perfect messenger, but he has written a fine primer on how to get ourselves out of this mess.” — Adama Vaughan, New Scientist

 “Bill Gates has a plan to save the world . . . While acknowledging that the challenge is daunting, and how we make things, grow things, move around, keep cool and stay warm will all need to fundamentally change, Gates argues that wholesale transformation is possible while maintaining lifestyles in high income countries and continuing to lift billions out of poverty.” — Greg Williams, Wired

 “His expertise . . . is apparent in the book’s lucid explanations of the scientific aspects of climate change. The solutions he outlines are pragmatic and grounded in forward-thinking economic reasoning. Although he does not avoid the hard truths we must face as our climate changes, Gates remains optimistic and believes that we have the ability to avoid a total climate disaster.” — Miriam R. Aczel, Science

 “Concise, straightforward . . . Gates has crafted a calm, reasoned, well-sourced explanation of the greatest challenge of our time and what we must change to avoid cooking our planet.” — Jeff Rowe, Associated Press

 “A persuasive, optimistic strategy for reducing greenhouse emissions to zero by midcentury . . . Though Gates doesn’t shy away from acknowledging the daunting challenges ahead, his narrative contains enough confidence—and hard science and economics—to convince many readers that his blueprint is one of the most viable yet . . . supremely authoritative and accessible.” — Kirkus Reviews (starred review)

 “Those looking for an accessible review of how global warming can be countered will find this a handy—and maybe even hope-inspiring—guide.” — Publishers Weekly

 “Gates has put his considerable wealth behind global health, educational, and economic initiatives and now turns his laser-like attention to this most existential of issues . . . He provides illuminating contexts for [his] perspectives and offers a treatise that is imperative, approachable, and useful.” — Booklist















































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 Product details

 * Publisher ‏ : ‎ Diversified Publishing; Large type / Large print edition (February 23, 2021)
 * Language ‏ : ‎ English
 * Paperback ‏ : ‎ 384 pages
 * ISBN-10 ‏ : ‎ 059321577X
 * ISBN-13 ‏ : ‎ 978-0593215777
 * Item Weight ‏ : ‎ 14.4 ounces
 * Dimensions ‏ : ‎ 6.07 x 0.95 x 9.15 inches
 * Best Sellers Rank: #400,027 in Books ( See Top 100 in Books )
 + #401 in Climatology
 + #515 in Environmental Economics (Books)
 + #1,006 in Environmental Science (Books)
 * Customer Reviews:
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 Bill Gates is a technologist, business leader, and philanthropist. In 1975, he cofounded Microsoft with his childhood friend Paul Allen; today he is cochair of the Bill & Melinda Gates Foundation. He also launched Breakthrough Energy, an effort to commercialize clean energy and other climate-related technologies.

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  5.0 out of 5 stars Standing on the shoulders of Bill Gates
  Reviewed in the United States on February 17, 2021
  I've received this book this morning and went thru a third way into the book. It's hard to ignore when someone like Bill Gates makes a loud noise about something. As someone who pays a close attention to various investments he has been making in the recent years, I noticed that he's been making significant investment into clean energy and biotechnology. Therefore, I had to buy this book to get a full picture of his perspective on where we are headed towards to in terms of climate change. Some people may dismiss book by saying "He is not an expert on this matter so it's not worth reading this book." With all due respect, I disagree with them wholeheartedly. Elon Musk may not know everything about electric vehicles or rocket science, but he runs a few of the most innovative/futuristic companies of our generation. Same thing - Bill Gates has a substantial stake in this field in which there are many smart people working for him. We should most certainly listen in to what he has to say and I appreciate Bill Gates taking actions on this front. I may update this review after I finish the book but I certainly recommend this book for anyone who is interested not only in climate change but where to invest as well.
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  G. C. Carter
  5.0 out of 5 stars Well-written climate change treatise (with recommended actions) is well-worth purchasing
  Reviewed in the United States on March 13, 2021
  Verified Purchase
  The book entitled: “How to Avoid a Climate Disaster: The Solutions We Have and the Breakthroughs We Need” by Bill Gates is a well written treatise on climate change worth purchasing and reading including actions needed. As an entrepreneur and capitalist now focused on giving his accumulated wealth to important world charities, Gates, with the aid of many acknowledged contributors writes a compelling outline of what the private sector and government should do in a coordinated manner to combat climate change.
  In a perfect world, one could speculate that the book should have been written by John Kerry, the new Climate czar but it still could be used as a road map for aggressive coordinated actions to be taken to control man-made climate change. Short of that, the book provides great technical detail and insight into a balanced approach to moving forward with both policy and technical actions.
  The main theme of the book is: “Fifty-one billion is how many tons of greenhouse gases the world typically adds to the atmosphere every year. … Zero is what we need to aim for.” The author states: “It’s hard to say exactly how much of America’s electricity will come from renewables in the end, but what we do know is that between now and 2050 we have to build them much faster—on the order of 5 to 10 times faster—than we’re doing right now.” The book also has a rational balanced approach stating for example: “In other words, fossil fuels are everywhere. Take oil as just one example: The world uses more than 4 billion gallons every day. When you’re using any product at that kind of volume, you can’t simply stop overnight.” Gates goes on to state: “Eventually it sank in. The world needs to provide more energy so the poorest can thrive, but we need to provide that energy without releasing any more greenhouse gases.” This book contains endorsements for several highly recommended readings that this reviewer has not yet read but suspects that the interested individual may wish to pursue reading them.
  The reader of Gate’s most comprehensive excellent book is forewarned and should be wary of several potentially misleading points or subtleties. Often the author himself points these out. Perhaps because he sees climate change as such a critical world issue, he has invested his own resources in not only writing this book but also investing in companies to contribute to solving the climate change problems; one therefore might be suspect of the potential for conflict of interest. Another more nuanced observation is that the book often slips from facts based upon past data to predictions about what the future holds. And while the book includes extensive notes at the end, the book does not make it apparent to the casual reader in the main text what is true and what is predicted, leaving the reader to trust the excellent salesmanship of the author to believe the points that he is making or sometimes trying to make.
  Gates writes a little about the Covid-19 pandemic (including our failure to prepare for it despite warnings) and how, while it did reduce greenhouse gases, it was not as much as one might have expected. To provide historical context, Gates states: “During the last ice age, the average temperature was just 6 degrees Celsius lower than it is today. During the age of the dinosaurs, when the average temperature was perhaps 4 degrees Celsius higher than today, there were crocodiles living above the Arctic Circle.” In terms of climate change challenges, the author goes on to remind us that: “Carbon dioxide is the most common greenhouse gas, but there are a handful of others, such as nitrous oxide and methane.”
  Gates goes on to state: “The 51 billion tons I keep mentioning is the world’s annual emissions in carbon dioxide equivalents. You may see numbers like 37 billion elsewhere—that’s just carbon dioxide, without the other greenhouse gases—or 10 billion, which is just the carbon itself. … Greenhouse gas emissions have increased dramatically since the 1850s due to human activity, such as burning fossil fuels. … So that’s the first part of the answer to the question “Why do we have to get to zero?”—because every bit of carbon we put into the atmosphere adds to the greenhouse effect. … There’s no getting around physics. Carbon dioxide emissions are on the rise, and so is the global temperature.”
  As illustrative of what is fact and what Gates predicts based upon the available scientific knowledge, Gates states: “What We Do and Don’t Know … One problem is that computer models are far from perfect. … The earth is warming, it’s warming because of human activity, and the impact is bad and will get much worse. … We’ve already raised the temperature at least 1 degree Celsius since preindustrial times, and if we don’t reduce emissions, we’ll probably have between 1.5 and 3 degrees Celsius of warming by mid-century, and between 4 and 8 degrees Celsius by the end of the century. … A hotter climate means there will be more frequent and destructive wildfires. … California is a dramatic example of what’s going on. Wildfires now occur there five times more often than in the 1970s, largely because the fire season is getting longer and the forests there now contain much more dry wood that’s likely to burn.”
  The author goes on to state: “If the temperature rises by 2 degrees Celsius, coral reefs could vanish completely, destroying a major source of seafood for more than a billion people. … In the worst drought ever recorded in Syria—which lasted from 2007 to 2010—some 1.5 million people left farming areas for the cities, helping to set the stage for the armed conflict that started in 2011. That drought was made three times more likely by climate change. By 2018, roughly 13 million Syrians had been displaced. … In the next decade or two, the economic damage caused by climate change will likely be as bad as having a COVID-sized pandemic every 10 years. And by the end of the 21st century, it will be much worse if the world remains on its current emissions path.”
  Gates, in a chapter entitled: “THIS WILL BE HARD” states “… fossil fuels are everywhere. Take oil as just one example: The world uses more than 4 billion gallons every day. When you’re using any product at that kind of volume, you can’t simply stop overnight. … the amount of energy used per person will go up, and so will the amount of greenhouse gases emitted per person. … The global population is headed toward 10 billion by the end of the century, … We need to get to zero—producing even more energy than we do today, but without adding any carbon to the atmosphere—.”
  The author explains: “Another argument you often hear goes like this: Yes, climate change is real, and its effects will be bad, and we have everything we need to stop it. Between solar power, wind power, hydropower, and a few other tools, we’re good. It’s simply a matter of having the will to deploy them. Chapters 4 through 8 explain why I don’t buy that notion. We have some of what we need, but far from all of it.
  No single country wants to pay to mitigate its emissions unless everyone else will too. That’s why the Paris Agreement, in which more than 190 countries signed up to eventually limit their emissions, was such an achievement. Not because the current commitments will make a huge dent in emissions— but because it was a starting point that proved global cooperation is possible.”
  In order to separate the forest from the trees and focus on important issues, Gates goes on to articulate “Five Questions to Ask in Every Climate Conversation” and goes on to introduce the concept of “These additional costs are what I call Green Premiums.” as a method of identifying the short term costs of “going green” and goes on to state “You can imagine Green Premiums high enough that the United States is willing and able to pay them but India, China, Nigeria, and Mexico are not. We need the premiums to be so low that everyone will be able to decarbonize.” Writing in 2021 illustrative of the challenge, Gates states: “All told, fossil fuels provide two-thirds of the world’s electricity. Solar and wind, meanwhile, account for 7 percent.”
  In terms of options, the author writes that: “Small-scale solar can be an option for people in poor, rural areas who need to charge their cell phones and run lights at night. But that kind of solution is never going to deliver the massive amounts of cheap, always-available electricity these countries need to jump-start their economies. They’re looking to do what China did: grow their economies by attracting industries like manufacturing and call centers—the types of businesses that demand far more (and far more reliable) power than small-scale renewables can provide today. … If these countries opt for coal plants, as China and every rich country did, it’ll be a disaster for the climate. But right now, that’s their most economical option.”
  Gates states: “Germany produced about 10 times more solar in June 2018 than it did in December 2018. In fact, at times during the summer, Germany’s solar and wind plants generate so much electricity that the country can’t use it all. When that happens, it ends up transmitting some of the excess to neighboring Poland and the Czech Republic, whose leaders have complained that it’s straining their own power grids and causing unpredictable swings in the cost of electricity. ... completely decarbonizing America’s power grid by 2050 will require adding around 75 gigawatts of capacity every year for the next 30 years. … But more efficient panels and turbines aren’t enough …”
  In terms of “Making Carbon-Free Electricity and Nuclear fission” Gates writes,
  The United States gets around 20 percent of its electricity from nuclear plants; France has the highest share in the world, getting 70 percent of its electricity from nuclear. … In 2018, researchers at the Massachusetts Institute of Technology analyzed nearly 1,000 scenarios for getting to zero in the United States; all the cheapest paths involved using a power source that’s clean and always available—that is, one like nuclear power. … High-profile accidents at Three Mile Island in the United States, Chernobyl in the former U.S.S.R., and Fukushima in Japan put a spotlight on all these risks.” But goes on to state: “Nuclear power kills far, far fewer people than cars do. For that matter, it kills far fewer people than any fossil fuel. … Nevertheless, we should improve it, just as we did with cars, by analyzing the problems one by one and setting out to solve them with innovation.”
  The author explains in considerable detail that materials like steel and concrete generate greenhouse gases and how getting to zero requires attention to these processes. Gates explains: “In short, we make materials that have become just as essential to modern life as electricity is. We’re not going to give them up. … how we can keep producing these materials without making the climate unlivable. For the sake of brevity, we’ll focus on three of the most important materials: steel, concrete, and plastic.” Going on to provide some technical details, Gates states: “To make steel, you need to separate the oxygen from the iron and add a tiny bit of carbon. You can accomplish both at the same time by melting iron ore at very high temperatures … A bit of the carbon bonds with the iron, forming the steel we want, and the rest of the carbon grabs onto the oxygen, forming a by-product we don’t want: carbon dioxide. Quite a bit of carbon dioxide, in fact. Making 1 ton of steel produces about 1.8 tons of carbon dioxide. … To make cement, you need calcium. To get calcium, you start with limestone—which contains calcium plus carbon and oxygen—and burn it in a furnace along with some other materials. … Make a ton of cement, and you’ll get a ton of carbon dioxide.”
  Gates then moves on to discuss the significant tons of carbon equivalent gases created by agriculture, having earlier in the book explained some of the nuances of methane versus carbon dioxide gases. Consistent with part of this theme to tap into innovation, Gates points out: “In 1968, an American biologist named Paul Ehrlich published a best-selling book called The Population Bomb, … Ehrlich wrote. “In the 1970s and 1980s hundreds of millions of people will starve to death … None of this came to pass. In the time since The Population Bomb came out, India’s population has grown by more than 800 million people—it’s now more than double what it was in 1968—but India produces more than three times as much wheat and rice as it did back then, and its economy has grown by a factor of 50. Why? What did Ehrlich and other doomsayers miss? They didn’t factor in the power of innovation. They didn’t account for people like Norman Borlaug, the brilliant plant scientist … Starvation plummeted, and today Borlaug is widely credited with saving a billion lives. He won the Nobel Peace Prize in 1970,”
  Illustrative of the incredible technical detail that Gates pulls together in this book: “All told, fertilizers were responsible for roughly 1.3 billion tons of greenhouse gas emissions in 2010, and the number will probably rise to 1.7 billion tons by mid-century...
  the American-style diet is responsible for almost as many emissions as all the energy Americans use in generating electricity, manufacturing, transportation, and buildings.”
  In terms of another important area of focus for reducing carbon, Gates writes: in “CHAPTER 7 HOW WE GET AROUND” 16 percent of 51 billion tons a year (total emissions to be eliminated) “Gas contains an amazing amount of energy—In the United States, gasoline is also remarkably cheap, right—gallon for gallon, gasoline is cheaper than Two Buck Chuck (Wine). … keep these two facts about gasoline in mind: It packs a punch, and it’s cheap. … The twin concepts of energy delivered per unit of fuel and per dollar spent are going to matter a lot as we look for ways to decarbonize our transportation system.” The author discusses at length electric vehicles, batteries and the success (or lack) in investing in battery technology.” He goes on to report: “The city of Shenzhen, China—home to 12 million people—has electrified its entire fleet of more than 16,000 buses and nearly two-thirds of its taxis.” And from a systems point of view, Gates states: “a typical truck running on diesel can go more than 1,000 miles without refueling. … Although electricity is a good option when you need to cover short distances, it’s not a practical solution for heavy, long-haul trucks.”
  Providing examples of government policies that worked, Gates reminds the reader that: “What’s now known as the Great Smog of London killed at least 4,000 people. …the 1950s and 1960s marked the arrival of air pollution as a major cause of public concern in the United States and Europe, and policy makers responded quickly.
  The next year, the British government enacted the Clean Air Act, which created smoke-control zones throughout the country where only cleaner-burning fuels could be used. Seven years later, America’s Clean Air Act established the modern regulatory system for controlling air pollution in the United States; … The U.S. Clean Air Act did what it was supposed to do—get poisonous gases out of the air—and since 1990 the level of nitrogen dioxide in American emissions has dropped by 56 percent, carbon monoxide by 77 percent, and sulfur dioxide by 88 percent.” And goes on to state: “we need the government to play a huge role in creating the right incentives and making sure the overall system will work for everyone.”
  The author summarizes: “Science tells us that in order to avoid a climate catastrophe, rich countries should reach net-zero emissions by 2050. … 2030 is not realistic. … there’s simply no way we’ll using them widely within a decade. … What we can do—and need to do—in the next 10 years is adopt the policies that will put us on a path to deep decarbonization by 2050.”
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  Simon Lack
  5.0 out of 5 stars Not Just Another Billionaire With A Plan
  Reviewed in the United States on February 22, 2021
  Verified Purchase
  Bill Gates readily concedes that the world isn’t short of “rich men with big ideas” in How To Avoid A Climate Disaster. He brings an intellectual honesty to the climate change debate that is absent from Democrat policy prescriptions, and often ignored by Republicans. Gates has done his homework, producing a book spilling over with facts and insights. The climate impact of each human activity (use of power, making things, moving around) is presented, along with its contribution to the 51 billion tons of Greenhouse Gases (GHGs) emitted annually. He presents the issues in an easily understood framework that many readers should find engaging and accessible.

  Although Bill Gates is surely a Democrat, it’s unlikely progressives will welcome his contribution. He dismisses Democrat orthodoxy by showing that the energy transition will be very expensive. He feels we should be motivated by the moral obligation to counter planetary warming because it will harm poorer countries the most. Rich countries can afford to manage rising sea levels (see Netherlands).

  Since the Gates Foundation is focused on disease and malnutrition in the developing world, his altruistic view isn’t surprising. And the moral argument is a respectable one. But it exposes the enormous political challenge in gaining popular support for higher domestic energy prices to stop, say, Bangladesh flooding. Last week Joe Biden rejected a French proposal to redirect 5% of our Covid vaccines to poorer countries until all Americans have been vaccinated. It was a minor acknowledgment of political reality. Few have the means or inclination to dedicate themselves to solving poor countries’ problems before their own.

  How To Avert A Climate Disaster reaches positive conclusions because it argues that current technology and innovation make solutions within reach. It provides policy prescriptions but deliberately avoids the politics. In many cases Gates calculates a “green premium”, reflecting the cost of converting transport, power generation, cement or steel production to be emission-free. Not surprisingly he favors a carbon tax to create price signals that fully reflect the externalities of burning fossil fuels.

  Renewables figure far less prominently than in the Green New Deal (see The Bovine Green Dream), a document Gates would likely view as fantasy if he didn’t studiously avoid such engagement. He illustrates the fundamental problem of solar and wind intermittency by considering the battery back-up a Tokyo 100% reliant on windmills would require to maintain power during a not-uncommon three-day typhoon. Even with optimistic assumptions about improved technology, the cost would be prohibitive. Gates concedes to have, “…lost more money on start-up battery companies than I ever imagined.” He knows a bit about the subject.

  Although efforts to curb emissions around the world generate enormous energy and press coverage, any actual improvements to date have come mostly from coal-to-gas switching for power generation (i.e. the U.S.) or last year’s drop in global economic activity due to Covid. U.S. energy costs haven’t risen noticeably, although California’s energy policies have managed to combine high costs with unreliability (see California Dreamin’ of Reliable Power).

  Gates believes poor countries should be allowed to increase emissions, since energy consumption is inextricably linked to improved economic well-being. “We can’t expect poor people to stay poor because too many rich countries emitted too many greenhouse gases” he argues from the lonely moral high ground.

  Reaching zero emissions by 2050 requires western democracies imposing substantial new regulation and costs on economic activity for decades. Meanwhile, the world’s building stock will grow mostly in poorer countries, requiring cement, steel and all the other emission-producing byproducts of human advancement. This construction will add the equivalent of another New York City every month for decades.

  Gates asserts that climate change will inevitably cost – inaction will lower GDP, and action will take lots of money. It’s well he doesn’t consider how governments will sell this to voters if an honest discussion ever occurs, because by comparison the technical challenges are more easily solved.

  Recognizing the political impracticality, Joe Biden instead disingenuously talks about “…tackling climate change and creating good union jobs here” (his emphasis).

  The technology already exists to capture the carbon dioxide emitted by burning fossil fuels, whether to generate electricity (27% of global GHGs) or produce steel (manufactures also including cement and plastics in total are 31% of GHGs).

  Gates estimates that emission-free power in the U.S., to include gas and coal with carbon capture, would raise prices by 15%. The green premium to make ethylene (plastic), steel and cement without CO2 emissions would raise prices by around 12%, 23% and 110% respectively. We could already start implementing such policies if the support was there. But political leaders avoid such talk, recognizing that voters’ concern about climate change doesn’t include much tolerance for higher prices.

  An energy investor today has to assess plausible outcomes, ignoring the shrill rhetoric of climate extremists unburdened by the costs, facts and technological challenges Gates lays out.

  Emerging economies will continue to grow, feeding all the increase in global energy demand as they seek OECD living standards. Their GHG emissions will rise. How tolerant will western democracies be of rising costs for virtually everything while we save the planet and allow poorer countries to catch up?

  Bill Gates is an unfailing optimist – invariably the most pleasant posture for living. But your blogger found the comprehensive list of what needs to be done dauntingly improbable. Mitigants of the results of global warming are probably a better investment than betting on decades of extended selflessness by 1.3 billion OECD citizens, the rich world whose collective actions Gates believes will save all 7.6 billion of us.

  There are already bold options available. We could phase out coal. We could require carbon capture on industrial use of fossil fuels. We could use more nuclear, whose safety record per unit of power generated is unmatched. Instead, more solar and wind is the climate extremists’ mantra in spite of intermittency and the NIMBY challenges of building transmission lines to move power from sparsely populated solar and wind farms to population centers (see Review Of Russell Gold’s Superpower for an example of how hard this is).

  Burning less coal, carbon capture and compensating for renewables’ unreliability all support growth in natural gas demand. Gates argues against a shift to natural gas for power generation. He fears the 30-year life of a typical combined cycle power plant would embed its CO2 emissions for too long. It would show progress to 2030, while putting zero by 2050 out of reach. But if tangible results within the timeframe of election cycles are needed, it’s hard to see a better way.

  If in a decade that’s how things have turned out, Gates the pragmatic optimist will hail it as success. We should too.
  Read more
  25 people found this helpful
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 Top reviews from other countries

  Adam Bowie
  5.0 out of 5 stars Practical steps the planet can take
  Reviewed in the United Kingdom on February 17, 2021
  Verified Purchase
  A year or so ago I read The Uninhabitable Earth by David Wallace-Wells, which laid out in often horrific details, the kinds of things that would happen to the planet if we didn’t change our ways.

  Bill Gates goes for the much more practical, “So what can we do about it now?” approach. This book is basically his plan to cut our carbon emissions from 51 billion tonnes to zero by 2050. That’s it in a nutshell. If we don’t then, like a bath, with even just a slow dripping tap, we’ll end up overflowing and facing some of the consequences that Wallace-Wells laid out in his earlier work.

  To do this, in this enormously readable book, Gates takes us through where the emissions come from, and examines ways to work through each of those sectors, which he breaks down into making things, plugging in, growing things, getting around and keeping warm and cool.

  He tackles each in turn, although he notes that we spend a lot of time thinking about “getting around” – aka transport – which accounts for 16% of net emissions, but not so much on making steel and concrete. The food industry also gets a good look-in.

  Gates is putting his money where his mouth is. Throughout the book he talks about the various companies he’s invested in. This could sound a bit, “Aren’t I clever?” but it doesn’t. He’s just being practical. It comes from the work he and his wife Melinda have carried out through their Gates Foundation in doing very practical things like fighting malaria. So he’s constantly referring to people he’s met, and businesses he’s keenly following.
  And throughout, he is very pragmatic. Only a few of us are willing – or even able – to pay a “Green Premium” for some of life’s essentials. He readily acknowledges that the lower the income you are in, the bigger a proportion of your overall costs something like transport will be. So paying even a small premium is simply not affordable. He’s also very aware that the big growth in greenhouse gases is likely to come from developing parts of the world where billions are coming to expect the same kinds of middle-class lifestyles that Americans and Europeans have experienced.

  So, what are the solutions? Well, this isn’t really a list of things that you or I can do directly – assuming neither of us is a world leader. There are some of those things, but this is more about policy as well as corporate and governmental support and investment. When we buy the cheapest concrete or steel, there is no carbon-cost attached to it. There’s no incentive to use the greener materials.

  And where there are financial incentives, they don’t necessarily help. The energy industry is rife with them, but they protect the enormously cheap fossil fuel industry. On the other hand, laws might make it ridiculously hard to build things like windfarms (a particular problem, seemingly, in the US).

  There are things which make you raise your eyebrows a bit. Gates doesn’t believe that just planting lots of trees will fix things. He’s got nothing against trees but I think sees them as a too simplistic solution that will require ongoing care to payback their investment over centuries. He is a big proponent of nuclear fuel, pointing out that while wind and solar energy are fantastic, they don’t provide consistent power. And even though at heart, Gates is a technologist through and through, he doesn’t see battery technology meaningfully moving on, which causes difficulties if you need to store vast amounts of power to even out supply on windless or cloudy days.

  Some will look at Gates, flying around in his private jet and wonder if he really practices what he preaches? He acknowledges his own shortcomings, but I think this book shows that he is indeed putting his money where his mouth is.

  Getting to net zero will not be easy, as he repeats throughout, but it’s achievable and he’s laid out a plan to get us there.
  Read more
  81 people found this helpful
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  Ruben
  5.0 out of 5 stars Excellent - Must Read Book - 5 *
  Reviewed in the United Kingdom on February 16, 2021
  Verified Purchase
  Excellent book. Bill Gates, really outlines the ins and outs of something most of the society ignores. Such optimism on his innovative perception and will on solving this global issue. I highly recommend.
  68 people found this helpful
  Report abuse
  S Hussain
  5.0 out of 5 stars Exceptional book written by an exceptional individual.
  Reviewed in the United Kingdom on February 16, 2021
  Verified Purchase
  Thought provoking. Insightful.
  52 people found this helpful
  Report abuse
  Dr W
  1.0 out of 5 stars No wonder he's a billionaire
  Reviewed in the United Kingdom on April 12, 2021
  Verified Purchase
  This book demonstrates clearly why Bill Gates is a billionaire. Despite my better instincts I was drawn in and bought this book with the Gates remedy for climate change. The core of the book is a very long advertisement for many of the companies that Bill Gates has invested in or owns (related to climate change). The discussion here is very familiar to anyone even vaguely interested in climate issues and is better explained and laid out in other work such as Smil (which Gates borrows heavily from). There is a neoliberal twist mind you - Gates wants the hugely expensive research to be publicly funded UNTIL it becomes profitable - then commercial interests can take over.
  So the billionaire Gates persuaded me to pay £20 for a long advert for Bill Gates. A fool and his money are easily parted.
  33 people found this helpful
  Report abuse
  Amazon Customer
  5.0 out of 5 stars Allows normal people to understand the necessary battle on climate change.
  Reviewed in the United Kingdom on February 17, 2021
  Verified Purchase
  The book is written in a way that's easy to read, not too long, and deeply impactful throughout. In only a few hours I was able to dramatically further my understanding of the larger picture on climate change. This included the breakup of challenges that we face to avoid a climate disaster (which I personally found somewhat surprising), along with the current and future measures needed to tackle it (some of which I already knew more about than others).
  33 people found this helpful
  Report abuse
  See all reviews


 
 

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Amazon.com Review  An Amazon Best Book of February 2021: Sad news: Electric cars won’t save the world. But Bill Gates has some pretty good ideas as to what will. Logical, compelling, and ultimately optimistic, this book is a rousing call from an innovator who believes we have the right stuff to change our fate, and his fresh way of thinking is desperately needed to clear away the overheated emotions this subject ignites. –Adrian Liang, Amazon Book Review  Review “Gates gathers advice from experts while laying out his vision for technological innovations that could reduce greenhouse gases and stop the warming of the planet. If even some of his plans work, this might be the most important book of the year.” —CNN “One of the most accessible, practical, and interesting books on the topic to emerge since Al Gore’s An Inconvenient Truth.” —Oprah Daily“The most comprehensible explanation for what’s driving our warming planet; how to measure the impact of the myriad contributions to this staggering and seemingly incalculable problem; and ultimately how to go about finding more effective approaches to each of them. It’s the closest thing I’ve seen to a how-to guide for addressing the climate crisis.” —Clinton Leaf, Fortune“How to Avoid a Climate Disaster presents ideas with the methodical approach of a college textbook . . . Remarkably, given the state of the world, it is an optimistic, can-do sort of book, chock-full of solutions.” —Christina Binkley, The Wall Street Journal Magazine “The most refreshing aspect of this book is its bracing mix of cold-eyed realism and number-crunched optimism . . . Ultimately his book is a primer on how to reorganise the global economy so that innovation focuses on the world’s gravest problems. It is a powerful reminder that if mankind is to get serious about tackling them, it must do more to harness the one natural resource available in infinite quantity—human ingenuity.” —The Economist “The author’s enthusiasm and curiosity about the way things work is infectious. He walks us through not just the basic science of global warming, but all the ways that our modern lives contribute to it . . . Gates seems energized by the sheer size and complexity of the challenge. That’s one of the best things about the book—the can-do optimism and conviction that science in partnership with industry are up to the task.” —Richard Schiffman, The Christian Science Monitor “In this wonklike and persuasive book, Gates takes his environmental activism a bridge further, laying out an ambitious plan to reduce greenhouse emissions to zero by 2050. Drawing on data from researchers, policymakers, and engineers, Gates advocates for solutions both scientific (like developing alternative fuels) and personal (like increasing civic engagement in environmental justice issues). If you feel a radical shred of hope reading these galvanizing pages, dare to let in—without hope, we’ll get nowhere.” —Esquire“With the help of experts in fields such as physics, engineering, chemistry, finance and politics, the technologist and philanthropist offers a practical and accessible plan for getting the world to zero greenhouse gas emissions and averting climate catastrophe.” —Barbara VanDenburgh, USA Today“How to Avoid a Climate Disaster is clear, concise on a colossal subject, and intelligently holistic in its approach to the problem. Gates may not be the perfect messenger, but he has written a fine primer on how to get ourselves out of this mess.” —Adama Vaughan, New Scientist  “Bill Gates has a plan to save the world . . . While acknowledging that the challenge is daunting, and how we make things, grow things, move around, keep cool and stay warm will all need to fundamentally change, Gates argues that wholesale transformation is possible while maintaining lifestyles in high income countries and continuing to lift billions out of poverty.” —Greg Williams, Wired  “His expertise . . . is apparent in the book’s lucid explanations of the scientific aspects of climate change. The solutions he outlines are pragmatic and grounded in forward-thinking economic reasoning. Although he does not avoid the hard truths we must face as our climate changes, Gates remains optimistic and believes that we have the ability to avoid a total climate disaster.” —Miriam R. Aczel, Science  “Concise, straightforward . . . Gates has crafted a calm, reasoned, well-sourced explanation of the greatest challenge of our time and what we must change to avoid cooking our planet.” —Jeff Rowe, Associated Press   “A persuasive, optimistic strategy for reducing greenhouse emissions to zero by midcentury . . . Though Gates doesn’t shy away from acknowledging the daunting challenges ahead, his narrative contains enough confidence—and hard science and economics—to convince many readers that his blueprint is one of the most viable yet . . . supremely authoritative and accessible.” —Kirkus Reviews (starred review)   “Those looking for an accessible review of how global warming can be countered will find this a handy—and maybe even hope-inspiring—guide.” —Publishers Weekly   “Gates has put his considerable wealth behind global health, educational, and economic initiatives and now turns his laser-like attention to this most existential of issues . . . He provides illuminating contexts for [his] perspectives and offers a treatise that is imperative, approachable, and useful.” —Booklist
How to Avoid a Climate Disaster: The Solutions We Have and the Breakthroughs We Need (Random House Large Print) Paperback – Large Print, February 23, 2021
https://www.amazon.com/How-Avoid-Climate-Disaster-Breakthroughs/dp/059321577X
https://www.bradford.gov.uk/environment/climate-change/how-to-prepare-for-climate-change/
How to prepare for climate change
2023-07-11
Bradford Metropolitan District Council
<div><div>
<h2>What is climate change adaptation?</h2>
<p>Even if we slow down climate change by reducing our carbon emissions we will still see some impact in the Bradford District. Adaptation is how we adjust to the different climate and the process of preparing for extreme weather.</p>
<p>The main weather events likely to impact on the Bradford district are:</p>
<ul>
<li>floods </li>
<li>droughts </li>
<li>heat waves </li>
<li>severe winds </li>
<li>severe snowfall</li>
</ul>
<h2>Are you ready for climate change?</h2>
<p>We need to prepare for the impacts of climate change, to reduce the risks to you, your property and the District.</p>
<p>The weather in Bradford is changing. The summers will become warmer and drier and the winters wetter and milder. Adapting to climate change must become a part of our life. We must adapt our property, communities and lifestyles for more extremes of weather.</p>
<p><strong>Be prepared and take action - climate change is happening</strong></p>
<p>We can all take simple steps to make sure we are ready for the extremes in weather:</p>
<h2>Adapt your home and property</h2>
<h3>Flooding</h3>
<p>You don’t have to live near a river for your property to flood, in Bradford surface water flooding is a big issue this is when rain cannot enter the drainage system so flows over the surface, this can affect property even at the top of a hill.</p>
<ul>
<li>flood proof your property</li>
<li>keep valuables, precious items and documents on higher ground</li>
<li>make sure you have insurance cover for your property and contents</li>
<li>avoid tarmac or paving over the garden, as this prevents rain draining away</li>
</ul>
<p>More advice on flood proofing your home is available from the <a href="https://www.gov.uk/prepare-for-a-flood">Environment Agency</a>. Further information can also be found on our <a href="https://www.bradford.gov.uk/emergencies/flooding/flooding-information-and-support/">flooding pages.</a></p>
<h3>Heatwaves and drought</h3>
<p>Temperatures are likely to rise and we are likely to have more heat waves. The hot temperatures can affect our health, particularly young and elderly people. It may also affect the availability of water.</p>
<ul>
<li>install blinds at the windows</li>
<li>create an area of shade in your garden where you can sit to cool off</li>
</ul>
<p>Try to conserve water:</p>
<ul>
<li>install a water butt, it's a good idea to connect it to a drainpipe. The rainwater collected can be used to water plants and wash the car</li>
<li>reuse bath water to water plants</li>
<li>use mulch on the garden, a protective layer on the soil prevents moisture loss, you can also use pebbles, gravel, chipped bark, grass clippings</li>
<li>take shorter showers, you could even fit a low flow shower head</li>
<li>don't let water run while washing your face - brush your teeth while waiting for the hot water then fill a basin to wash or shave</li>
<li>operate dishwashers and clothes washers only when they are fully loaded, or properly set the water level for the size of load you are using</li>
<li>stop drips - a dripping tap can waste more than 60 litres a week, thousands in a year. Turn off all your taps fully and fix leaky taps around the home</li>
</ul>
<p><strong>Better to act now – otherwise the impacts can be both costly and timely</strong></p>
<h2>Prepare yourself for extreme weather</h2>
<p>Be aware of the impact the weather may have on you, people you know and the community.</p>
<p>Things to consider:</p>
<ul>
<li>in a heat wave wear sun screen and a hat and drink plenty of water</li>
<li>snowfall and icy weather can make people more prone to falls so take care</li>
<li>weather can affect transport networks, making it difficult to get around</li>
<li>services may be disrupted in times of extreme weather, winds can bring down power lines and water pipes may freeze during cold periods</li>
<li>be aware of vulnerable members of your community they may need extra help</li>
<li>be aware of the needs of any elderly or sick relatives</li>
<li>have a list of emergency contacts and numbers</li>
<li>be prepared for an emergency have a plan and an emergency kit bag</li>
</ul>
<p>It is important to identify</p>
<ol>
<li>what the risks are </li>
<li>who is most vulnerable </li>
<li>have a plan of action ready</li>
</ol>
<p>Our website has more information about <a href="https://www.bradford.gov.uk/emergencies/what-to-do-in-an-emergency/preparing-for-emergencies-what-you-can-do/">preparing for emergencies</a>.</p>
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 How to prepare for climate change

 What is climate change adaptation?

 Even if we slow down climate change by reducing our carbon emissions we will still see some impact in the Bradford District. Adaptation is how we adjust to the different climate and the process of preparing for extreme weather.

 The main weather events likely to impact on the Bradford district are:

 * floods
 * droughts
 * heat waves
 * severe winds
 * severe snowfall

 Are you ready for climate change?

 We need to prepare for the impacts of climate change, to reduce the risks to you, your property and the District.

 The weather in Bradford is changing. The summers will become warmer and drier and the winters wetter and milder. Adapting to climate change must become a part of our life. We must adapt our property, communities and lifestyles for more extremes of weather.

 Be prepared and take action - climate change is happening

 We can all take simple steps to make sure we are ready for the extremes in weather:

 Adapt your home and property

 Flooding

 You don’t have to live near a river for your property to flood, in Bradford surface water flooding is a big issue this is when rain cannot enter the drainage system so flows over the surface, this can affect property even at the top of a hill.

 * flood proof your property
 * keep valuables, precious items and documents on higher ground
 * make sure you have insurance cover for your property and contents
 * avoid tarmac or paving over the garden, as this prevents rain draining away

 More advice on flood proofing your home is available from the Environment Agency. Further information can also be found on our flooding pages.

 Heatwaves and drought

 Temperatures are likely to rise and we are likely to have more heat waves. The hot temperatures can affect our health, particularly young and elderly people. It may also affect the availability of water.

 * install blinds at the windows
 * create an area of shade in your garden where you can sit to cool off

 Try to conserve water:

 * install a water butt, it's a good idea to connect it to a drainpipe. The rainwater collected can be used to water plants and wash the car
 * reuse bath water to water plants
 * use mulch on the garden, a protective layer on the soil prevents moisture loss, you can also use pebbles, gravel, chipped bark, grass clippings
 * take shorter showers, you could even fit a low flow shower head
 * don't let water run while washing your face - brush your teeth while waiting for the hot water then fill a basin to wash or shave
 * operate dishwashers and clothes washers only when they are fully loaded, or properly set the water level for the size of load you are using
 * stop drips - a dripping tap can waste more than 60 litres a week, thousands in a year. Turn off all your taps fully and fix leaky taps around the home

 Better to act now – otherwise the impacts can be both costly and timely

 Prepare yourself for extreme weather

 Be aware of the impact the weather may have on you, people you know and the community.

 Things to consider:

 * in a heat wave wear sun screen and a hat and drink plenty of water
 * snowfall and icy weather can make people more prone to falls so take care
 * weather can affect transport networks, making it difficult to get around
 * services may be disrupted in times of extreme weather, winds can bring down power lines and water pipes may freeze during cold periods
 * be aware of vulnerable members of your community they may need extra help
 * be aware of the needs of any elderly or sick relatives
 * have a list of emergency contacts and numbers
 * be prepared for an emergency have a plan and an emergency kit bag

 It is important to identify

 1. what the risks are
 2. who is most vulnerable
 3. have a plan of action ready

 Our website has more information about preparing for emergencies.

 Stay connected

 Sign up for our email updates about climate action

 Email:

 Contact details

 Bradford Council

 Email : BEEnergy@bradford.gov.uk

 Documents

 * Get ready for the unexpected leaflet (1 Mb)

 Related pages

 * Cold weather advice
 * Advice for hot weather and heat waves
 * What to do if it floods

 Other pages in this section

 * What is climate change and how will it affect the UK?
 * Greenhouse Gas Emissions Report
 * HECA Report
 * Electric vehicle charging points
 * Reducing your emissions
 * Fuel Poverty - Framework for Action
 * Climate emergency and green economy
 * Take Climate Action: Planting a Tree for Every Child
 * Tree planting - how we can work together
 * The Energy Company Obligation Scheme
 * Bradford Council Corporate utilities

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 © 2024 City of Bradford Metropolitan District Council

What is climate change adaptation?
Even if we slow down climate change by reducing our carbon emissions we will still see some impact in the Bradford District. Adaptation is how we adjust to the different climate and the process of preparing for extreme weather.
The main weather events likely to impact on the Bradford district are:
floods 
droughts 
heat waves 
severe winds 
severe snowfall
Are you ready for climate change?
We need to prepare for the impacts of climate change, to reduce the risks to you, your property and the District.
The weather in Bradford is changing. The summers will become warmer and drier and the winters wetter and milder. Adapting to climate change must become a part of our life. We must adapt our property, communities and lifestyles for more extremes of weather.
 Be prepared and take action - climate change is happening 
We can all take simple steps to make sure we are ready for the extremes in weather:
Adapt your home and property
Flooding
You don’t have to live near a river for your property to flood, in Bradford surface water flooding is a big issue this is when rain cannot enter the drainage system so flows over the surface, this can affect property even at the top of a hill.
flood proof your property
keep valuables, precious items and documents on higher ground
make sure you have insurance cover for your property and contents
avoid tarmac or paving over the garden, as this prevents rain draining away
More advice on flood proofing your home is available from the Environment Agency. Further information can also be found on our flooding pages. 
Heatwaves and drought
Temperatures are likely to rise and we are likely to have more heat waves. The hot temperatures can affect our health, particularly young and elderly people. It may also affect the availability of water.
install blinds at the windows
create an area of shade in your garden where you can sit to cool off
Try to conserve water:
install a water butt, it's a good idea to connect it to a drainpipe. The rainwater collected can be used to water plants and wash the car
reuse bath water to water plants
use mulch on the garden, a protective layer on the soil prevents moisture loss, you can also use pebbles, gravel, chipped bark, grass clippings
take shorter showers, you could even fit a low flow shower head
don't let water run while washing your face - brush your teeth while waiting for the hot water then fill a basin to wash or shave
operate dishwashers and clothes washers only when they are fully loaded, or properly set the water level for the size of load you are using
stop drips - a dripping tap can waste more than 60 litres a week, thousands in a year. Turn off all your taps fully and fix leaky taps around the home
 Better to act now – otherwise the impacts can be both costly and timely 
Prepare yourself for extreme weather
Be aware of the impact the weather may have on you, people you know and the community.
Things to consider:
in a heat wave wear sun screen and a hat and drink plenty of water
snowfall and icy weather can make people more prone to falls so take care
weather can affect transport networks, making it difficult to get around
services may be disrupted in times of extreme weather, winds can bring down power lines and water pipes may freeze during cold periods
be aware of vulnerable members of your community they may need extra help
be aware of the needs of any elderly or sick relatives
have a list of emergency contacts and numbers
be prepared for an emergency have a plan and an emergency kit bag
It is important to identify
what the risks are 
who is most vulnerable 
have a plan of action ready
Our website has more information about preparing for emergencies.
How to prepare for climate change
https://www.bradford.gov.uk/environment/climate-change/how-to-prepare-for-climate-change/
https://www.wusa9.com/article/tech/science/environment/climate-crisis-is-here-how-should-you-prepare-dc/65-79db0e2c-bd9a-44a8-8c86-6f7077feb442
A U.N. report says the climate crisis is here. How should you prepare?
2021-11-08
Anna Liss-Roy
<div><p>"[Individuals] can lead what happens and we can accelerate what happens," said climate expert Dr. Scott Sklar.</p><div>
<p>WASHINGTON — A new U.N. report warns of disaster if greenhouse gas emissions are not reduced. </p>
<p>It's "unequivocal" that humans have warmed the planet and that "widespread and rapid" changes in Earth's systems have already taken place, according to the 234 scientists who wrote the report.</p>
<p>We're not just talking about milder winters here. The data points to a changing climate that will massively affect our access to food and water.</p>
<p>"The real issue is, as temperatures increase, the soil dries out," said Dr. Scott Sklar. He's a professor of sustainable energy at George Washington University and director of GWU Solar Institute.</p>
<p>This is already happening in the western part of the United States, Sklar said, where soil is drying out to levels not seen in modern history. This creates fires and immense water shortages. It also makes it harder to grow crops that feed millions of households.</p>
<p>With all forms of extreme weather, people lose their homes, businesses and entire communities. People of color, indigenous people and those with low incomes are affected first and worst. </p>
<p>The way to limit future changes, according to the report, is to minimize greenhouse gas emissions as much as possible. These include gases like carbon dioxide and methane. </p>
<p>To a large extent, governments and corporations have that power. </p>
<p>But communities and individuals can play a huge role in mitigating emissions. They can also set the pace when it comes to preparing for big changes ahead, Sklar said.  </p>
<p>"Individual actions are always more important than government actions. We can lead what happens and we can accelerate what happens," Sklar said.</p>
<p>Transportation and electricity usage are two areas you can really make a difference. That's because those are the two most polluting industries, according to the U.S. Environmental Protection Agency.</p>
<div>
<p>
</p>
</div>
<p>"Those are the two things that, in fact, we as citizens have some control over," Sklar said.</p>
<p>Changes can be simple enough: walk, bike or use public transit instead of taking the car as much as you can. When you do need to use a car, try to run multiple errands in the same trip. When your car dies, get a more fuel efficient vehicle if you can.</p>
<p>"This is not something that's draconian," Sklar said. "It's just sort of smarter thinking."</p>
<p>When it comes to buildings, there are lots of steps you can take. Use L.E.D. lighting (these bulbs use up to 80% less electricity than the incandescent ones and can last up to 25x longer, according to the E.P.A.), turn the lights off when you leave the room and use power strips that have a switch on them. That way, when you're not using your appliances, you're not using power. </p>
<p>The next time you're buying an air conditioner, check out the energy efficiency ratings - this is called the SEER rating. The more energy efficient unit may be more costly up front, but you're likely to get that money back on your energy bills.</p>
<p>If you can get involved in community initiatives, here are some areas Sklar suggests starting:</p>
<div>
<ul><li>Programs for green investment</li><li>Subsidies for renewable and energy-efficient HVAC systems</li><li>Energy incentives for low income people</li></ul> </div>
<p>If you can, support community gardens and local farmers instead of relying on produce from across the country. </p>
<p>That will also help prepare your community for unreliable food supply chains, as the planet continues to heat up.</p>
<p>To prepare your home, equip it to survive severe weather. That could mean treating your windows to be able to handle heavy winds or repairing leaks and maintaining your building's foundation. </p>
<p>If you are a renter whose building owner is not on top of these things, research programs in your area that can facilitate those repairs and pass them along.</p>
<p>Preparations for climate change are a community endeavor. Institutions that have emerged during the COVID-19 pandemic, like the creation of mutual aid networks, will be helpful as the climate emergency escalates, Sklar said.</p>
<p>When it comes to limiting the extent of the crisis, people with the time and resources to implement one or more of these changes will be helping not only themselves, but also everyone else in their community who can't. </p>
<p>"We can't do business as usual," Sklar said. "Or the consequences are going to be stark."</p>
<div>
<p></p>
</div>
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 Environment

 A U.N. report says the climate crisis is here. How should you prepare?

 "[Individuals] can lead what happens and we can accelerate what happens," said climate expert Dr. Scott Sklar.

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 * 
 Author: Anna Liss-Roy (WUSA9)
 Published: 6:44 PM EDT August 11, 2021
 Updated: 6:44 PM EDT August 11, 2021

 WASHINGTON — A new U.N. report warns of disaster if greenhouse gas emissions are not reduced.

 It's "unequivocal" that humans have warmed the planet and that "widespread and rapid" changes in Earth's systems have already taken place, according to the 234 scientists who wrote the report.

 We're not just talking about milder winters here. The data points to a changing climate that will massively affect our access to food and water.

 RELATED: 'Code red for humanity' | UN climate change report issues dire warning

 "The real issue is, as temperatures increase, the soil dries out," said Dr. Scott Sklar. He's a professor of sustainable energy at George Washington University and director of GWU Solar Institute.

 This is already happening in the western part of the United States, Sklar said, where soil is drying out to levels not seen in modern history. This creates fires and immense water shortages. It also makes it harder to grow crops that feed millions of households.

 With all forms of extreme weather, people lose their homes, businesses and entire communities. People of color, indigenous people and those with low incomes are affected first and worst.

 RELATED: Here's how to help someone experiencing homelessness during a heatwave

 The way to limit future changes, according to the report, is to minimize greenhouse gas emissions as much as possible. These include gases like carbon dioxide and methane.

 To a large extent, governments and corporations have that power.

 But communities and individuals can play a huge role in mitigating emissions. They can also set the pace when it comes to preparing for big changes ahead, Sklar said.

 RELATED: Senate Dems unveil $3.5T budget for social, climate efforts

 "Individual actions are always more important than government actions. We can lead what happens and we can accelerate what happens," Sklar said.

 Transportation and electricity usage are two areas you can really make a difference. That's because those are the two most polluting industries, according to the U.S. Environmental Protection Agency.

 Credit: Environmental Protection Agency

 "Those are the two things that, in fact, we as citizens have some control over," Sklar said.

 Changes can be simple enough: walk, bike or use public transit instead of taking the car as much as you can. When you do need to use a car, try to run multiple errands in the same trip. When your car dies, get a more fuel efficient vehicle if you can.

 "This is not something that's draconian," Sklar said. "It's just sort of smarter thinking."

 When it comes to buildings, there are lots of steps you can take. Use L.E.D. lighting (these bulbs use up to 80% less electricity than the incandescent ones and can last up to 25x longer, according to the E.P.A.), turn the lights off when you leave the room and use power strips that have a switch on them. That way, when you're not using your appliances, you're not using power.

 RELATED: The impact of Joe Biden's new fuel economy rules

 The next time you're buying an air conditioner, check out the energy efficiency ratings - this is called the SEER rating. The more energy efficient unit may be more costly up front, but you're likely to get that money back on your energy bills.

 If you can get involved in community initiatives, here are some areas Sklar suggests starting:

 * Programs for green investment
 * Subsidies for renewable and energy-efficient HVAC systems
 * Energy incentives for low income people

 If you can, support community gardens and local farmers instead of relying on produce from across the country.

 That will also help prepare your community for unreliable food supply chains, as the planet continues to heat up.

 To prepare your home, equip it to survive severe weather. That could mean treating your windows to be able to handle heavy winds or repairing leaks and maintaining your building's foundation.

 If you are a renter whose building owner is not on top of these things, research programs in your area that can facilitate those repairs and pass them along.

 Preparations for climate change are a community endeavor. Institutions that have emerged during the COVID-19 pandemic, like the creation of mutual aid networks, will be helpful as the climate emergency escalates, Sklar said.

 RELATED: VERIFY: Yes, carbon credits are legitimate and some companies really do use them to offset carbon usage

 When it comes to limiting the extent of the crisis, people with the time and resources to implement one or more of these changes will be helping not only themselves, but also everyone else in their community who can't.

 "We can't do business as usual," Sklar said. "Or the consequences are going to be stark."

 Related Articles

 * Top Dem sees tough pathway for $3.5T social, climate plans
 * Yes, 'botanical sexism' has led to more pollen in American cities
 * Here's how to help someone experiencing homelessness during a heatwave
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"[Individuals] can lead what happens and we can accelerate what happens," said climate expert Dr. Scott Sklar. 
WASHINGTON — A new U.N. report warns of disaster if greenhouse gas emissions are not reduced. 
It's "unequivocal" that humans have warmed the planet and that "widespread and rapid" changes in Earth's systems have already taken place, according to the 234 scientists who wrote the report.
We're not just talking about milder winters here. The data points to a changing climate that will massively affect our access to food and water.
"The real issue is, as temperatures increase, the soil dries out," said Dr. Scott Sklar. He's a professor of sustainable energy at George Washington University and director of GWU Solar Institute.
This is already happening in the western part of the United States, Sklar said, where soil is drying out to levels not seen in modern history. This creates fires and immense water shortages. It also makes it harder to grow crops that feed millions of households.
With all forms of extreme weather, people lose their homes, businesses and entire communities. People of color, indigenous people and those with low incomes are affected first and worst. 
The way to limit future changes, according to the report, is to minimize greenhouse gas emissions as much as possible. These include gases like carbon dioxide and methane. 
To a large extent, governments and corporations have that power. 
But communities and individuals can play a huge role in mitigating emissions. They can also set the pace when it comes to preparing for big changes ahead, Sklar said.  
"Individual actions are always more important than government actions. We can lead what happens and we can accelerate what happens," Sklar said.
Transportation and electricity usage are two areas you can really make a difference. That's because those are the two most polluting industries, according to the U.S. Environmental Protection Agency.
"Those are the two things that, in fact, we as citizens have some control over," Sklar said.
Changes can be simple enough: walk, bike or use public transit instead of taking the car as much as you can. When you do need to use a car, try to run multiple errands in the same trip. When your car dies, get a more fuel efficient vehicle if you can.
"This is not something that's draconian," Sklar said. "It's just sort of smarter thinking."
When it comes to buildings, there are lots of steps you can take. Use L.E.D. lighting (these bulbs use up to 80% less electricity than the incandescent ones and can last up to 25x longer, according to the E.P.A.), turn the lights off when you leave the room and use power strips that have a switch on them. That way, when you're not using your appliances, you're not using power. 
The next time you're buying an air conditioner, check out the energy efficiency ratings - this is called the SEER rating. The more energy efficient unit may be more costly up front, but you're likely to get that money back on your energy bills.
If you can get involved in community initiatives, here are some areas Sklar suggests starting:
 Programs for green investment Subsidies for renewable and energy-efficient HVAC systems Energy incentives for low income people  
If you can, support community gardens and local farmers instead of relying on produce from across the country. 
That will also help prepare your community for unreliable food supply chains, as the planet continues to heat up.
To prepare your home, equip it to survive severe weather. That could mean treating your windows to be able to handle heavy winds or repairing leaks and maintaining your building's foundation. 
If you are a renter whose building owner is not on top of these things, research programs in your area that can facilitate those repairs and pass them along.
Preparations for climate change are a community endeavor. Institutions that have emerged during the COVID-19 pandemic, like the creation of mutual aid networks, will be helpful as the climate emergency escalates, Sklar said.
When it comes to limiting the extent of the crisis, people with the time and resources to implement one or more of these changes will be helping not only themselves, but also everyone else in their community who can't. 
"We can't do business as usual," Sklar said. "Or the consequences are going to be stark."
A U.N. report says the climate crisis is here. How should you prepare?
https://www.wusa9.com/article/tech/science/environment/climate-crisis-is-here-how-should-you-prepare-dc/65-79db0e2c-bd9a-44a8-8c86-6f7077feb442