The carbon footprint of astronomical research is an increasingly topical issue. From a comparison of existing literature we infer an annual per capita carbon footprint of several tens of tonnes of CO2 equivalents for an average person working in astronomy. Astronomical observatories contribute significantly to the carbon footprint of astronomy and we examine the related sources of greenhouse gas emissions as well as lever arms for their reduction. Comparison with other scientific domains illustrates that astronomy is not the only field that needs to accomplish significant carbon footprint reductions of their research facilities. We show that limiting global warming to 1.5C or 2C implies greenhouse gas emission reductions that can only be reached by a systemic change of astronomical research activities and we argue that a new narrative for doing astronomical research is needed if we want to keep our planet habitable.
['Astrophysics - Instrumentation and Methods for Astrophysics']
The carbon footprint of astronomical observatories
{'Jürgen Knödlseder 1': '- 1 , Institut de Recherche en Astrophysique en Planétologie Toulouse, France; jknodlseder@irap.omp.eu', 'Abstract.': 'The carbon footprint of astronomical research is an increasingly topical issue. From a comparison of existing literature, we infer an annual per capita carbon footprint of several tens of tonnes of CO2 equivalents for an average person working in astronomy. Astronomical observatories contribute significantly to the carbon footprint of astronomy, and we examine the related sources of greenhouse gas emissions as well as lever arms for their reduction. Comparison with other scientific domains illustrates that astronomy is not the only field that needs to accomplish significant carbon footprint reductions of their research facilities. We show that limiting global warming to 1.5 · Cor 2 · C implies greenhouse gas emission reductions that can only be reached by a systemic change of astronomical research activities, and we argue that a new narrative for doing astronomical research is needed if we want to keep our planet habitable.', '1. Introduction': "Like many years in the recent past, 2022 was another example for record-breaking climate extremes that have made the headlines. Heat waves with temperatures exceeding 40 · C for prolonged periods hit many places, leading to wildfires that were especially devastating this year in Spain and Portugal (Pratt 2022). At the same time, heavy rainfall led to widespread flooding that cost thousands of lives and created millions of displacements, this year particularly disastrous in Pakistan and Nigeria (Trenberth 2022). While some countries flood, other regions are parched. The Greater Horn of Africa is bracing for a fifth consecutive failed rainy season, with over 50 million people su ff ering from acute food security (ICPAC 2022).  \nThese events are examples of observed impacts from climate change, as summarised in the sixth assessment report of working group II of the Intergovernmental Panel on Climate Change (IPCC) (IPCC 2022a). According to the report, increased global warming will 'cause unavoidable increases in multiple climate hazards and present multiple risks to ecosystems and humans'. Recognising these risks, 192 countries have signed the Paris Climate Agreement that aims 'holding the increase in the global average temperature to well below 2 · C above pre-industrial levels and pursuing e ff orts to limit the temperature increase to 1.5 · Cabove pre-industrial levels'. In order to reach this goal, the agreement prescribes 'to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century'. In other terms, humanity needs to reach net-zero emissions by 2050, i.e. in less than 30 years from now.", 'Knödlseder': "Near-term greenhouse gas (GHG) emission reduction goals are country specific. For example, the European climate law creates a legal obligation to reduce by 2030 GHG emissions in the European Union by at least 55%, compared to 1990 levels (EC 2021). The United States have set a comparable target of 50-52% below 2005 levels in 2030 (USA 2022). Other high-income countries have declared similar targets. Adjusting from the reference years' to today's GHG emissions, these targets translate into reductions of about ∼ 40% between today and 2030, corresponding to reductions of about ∼ 7% per year. This level is comparable to, but slightly larger than, the emission reductions that were reported due to COVID-19 lockdowns in 2020 (Jackson et al. 2022). And this level needs to be achieved continuously in high-income countries over the next three decades, with each year's reduction coming on top of those achieved in the year before.  \nIn their sixth assessment report, the working group III of the IPCC emphasises that limiting warming to 1.5 · C or 2 · C 'involve[s] rapid and deep and in most cases immediate GHG emission reductions in all sectors' (IPCC 2022b). Astronomy, as we will see, is a sector that contributes to global warming, and consequently it is concerned by this imperative. The IPCC reminds that 'doing less in one sector needs to be compensated by further reductions in other sectors if warming is to be limited' (IPCC 2022b). Eventually, policy makers and the society at large may set di ff erent reduction e ff orts for di ff erent activity sectors, yet until this is done, a reasonable assumption is that astronomy needs to reduce GHG emissions at a typical rate of ∼ 7% per year over the next three decades.", '2. The carbon footprint of astronomy': "In view of the challenge described above, several astronomical institutes and communities have assessed their carbon footprints with the aim to identify lever arms for GHG emission reductions. Stevens et al. (2020) estimated the GHG emissions of Australian astronomers, including powering o ffi ce buildings, business flights, supercomputer usage, and electricity consumption for operations of some ground-based observatories. Jahnke et al. (2020) estimated the GHG emissions of astronomers at the Max Planck Institute for Astronomy (MPIA) in Germany, including business flights, commuting, electricity, heating, computer purchases, paper use, and cafeteria meat consumption. Van der Tak et al. (2021) estimated the GHG emissions arising from professional astronomy activities in the Netherlands, including business flights, commuting, electricity, heating and supercomputing. Martin et al. (2022) estimated the GHG emissions of the Institut de Recherche en Astrophysique et Planétologie (IRAP), which is the largest astrophysics laboratory in France, including running the o ffi ce buildings (electricity, heating, water, air conditioning, waste), food, commuting, professional travelling, purchase of goods and services, external computing and use of observational data. The later work is the most comprehensive carbon footprint estimate performed to date, and the only study that employed a formal carbon accounting methodology.  \nThe results of these studies, expressed as annual per capita GHG emissions for a person working at an astronomy institute, are summarised in Fig. 1. The results were grouped by emission sources, where 'O ffi ce' combines all GHG emissions related to running the o ffi ce building (electricity, heating, water, air conditioning, waste) and 'Employee' covers food and commuting. Jahnke et al. (2020) presented a similar figure to compare the MPIA carbon footprint to the one of Australia, and like for their  \nFigure 1. Literature estimates of the annual average carbon footprint of a person working in astronomy. The numbers indicate the sta ff personal used to compute the per capita footprint.  \n<!-- image -->  \ncomparison, we multiplied the air travel footprint quoted by Stevens et al. (2020) for Australia by a factor of two to account for non-CO 2 radiative forcing of the same level as in all other studies. In their Fig. 1, Jahnke et al. (2020) compared the carbon footprint per researcher despite the fact that the definition of 'researcher' di ff ered among the studies (PhD students were included for Australia but excluded for MPIA). A more relevant denominator is the total number of employees since they all contribute to the carbon footprint of a given research institute, and consequently, we use the carbon footprint per employee for our comparison.  \nThe most striking feature of Fig. 1 are the huge di ff erences between the various carbon footprint estimates, which are partially explained by the carbon intensity of electricity production in a given country. For example, the largest contributor to the GHG emissions in Australia is computing with 16.2 tCO 2 e yr -1 per employee. Would the same amount of computing be performed in France, which has a carbon intensity for electricity production that is about 14 times lower than in Australia, computing would only account for 1.2 tCO 2 e yr -1 per employee. The relatively modest o ffi ce footprint in Australia with respect to the other countries comes therefore as surprise, since with the aforementioned large di ff erences in carbon intensity we would expect an o ffi ce footprint that is one order of magnitude larger for Australia compared to France. Note that Jahnke et al. (2020) used in their study for MPIA the quoted carbon intensity on the electricity bill, and Van der Tak et al. (2021) took into account green certificats for electricity purchase, putting the related GHG emissions to zero. Yet even with green certificats, electricity production is not emission free, and it has been argued that discounting on the basis of green certificats fails to provide accurate or relevant information in GHG reports (Brander et al. 2018). Consequently, carbon accounting methods either prescribe discounting of green certificats (Bilan Carbone © ) or impose dual reporting (GHG protocol, Carbon Disclosure project). Using the so called location-based method that applies grid average carbon intensities to electricity consumption, and that were used by Martin et al. (2022) for IRAP and Stevens et al. (2020) for Australia, would have led to larger o ffi ce-related footprints for MPIA and the Netherlands.  \nThe travel footprint, which is largely dominated by air travelling, varies among the di ff erent studies with the smallest footprint of ∼ 2 tCO 2 e yr -1 per employee estimated for the Netherlands and the largest footprint of ∼ 9 tCO 2 e yr -1 per employee estimated for Australia. While the latter can plausibly be explained by the size and remote loca-  \nof Australia (Stevens et al. 2020), the former may be related to the large number of PhD students that are among the employees in the Netherlands, as studies have revealed that students travel less compared to senior scientists (e.g. Pargman et al. 2022).  \nMost of the studies ignored some potentially important GHG emission sources (or considered them only partially), such as purchase of goods and services and use of observational data. Only the IRAP estimate provides a comprehensive assessment of all astronomy-related GHG emissions (Martin et al. 2022), resulting in a per-capita footprint of ∼ 28 tCO 2 e yr -1 . Since France has with 60 gCO 2 e kWh -1 a rather low carbon intensity for electricity production (compared to 415 gCO 2 e kWh -1 for the Netherlands, 460 gCO 2 e kWh -1 for Germany, and 840 gCO 2 e kWh -1 for Australia, see Breitenstein 2021), the IRAP estimate presents likely a lower limit to the average carbon footprint of a person working in astronomy. Adding the IRAP estimates for purchase and use of observational data on top of the other estimates in Australia would lead to a per-capita footprint of about ∼ 50 tCO 2 e yr -1 , which may be typical for a country that has a large carbon intensity for electricity production and that relies heavily on air travelling. While reducing the carbon intensity for electricity production (for example by switching to solar panels or wind turbines) and reducing air travelling (for example by using videoconferencing) will have a direct impact on the carbon footprint of astronomy, the lever arms to reduce the footprint of astronomical observatories are less obvious. We therefore will examine the carbon footprint of astronomical observatories now in more detail.", '3. The carbon footprint of astronomical observatories': "The annual carbon footprint of astronomical observatories was assessed by Knödlseder et al. (2022) to 1 . 2 ± 0 . 2 MtCO 2 e yr -1 for the year 2019, which is the reference year for all values quoted in this section. To derive this estimate, the authors used a monetary method that relates construction and operating costs for ground-based observatories and mission cost for space-based facilities to GHG emissions, complemented by an assessment based on payload launch mass for space missions. Applying this method to a list of 85 astronomical facilities, and extrapolating the results to the world-fleet of astronomical observatories led to the annual carbon footprint quoted above. By comparing the number of authors from IRAP to all authors in the world that published refereed articles citing a given astronomical facility, Martin et al. (2022) attributed a footprint of 4.1 ktCO 2 e yr -1 to IRAP, corresponding to 0 . 3% of the total annual carbon footprint of all facilities. This results in a per-capita footprint of 15 . 6 tCO 2 e yr -1 related to astronomical observatories at IRAP. Dividing the total annual carbon footprint of all facilities by this per-capita footprint suggests that around 77 , 000 people are working in astronomy world-wide, which is about twice the number of 30 , 000 astronomers with PhD degree in the world, as estimated by Knödlseder et al. (2022).  \nTo estimate the carbon footprint of space missions, Knödlseder et al. (2022) derived a carbon intensity of 140 tCO 2 e per M e of mission cost from the literature. Compared to carbon intensities for other activity sectors this is a relatively small value, reflecting the fact that a non-negligible fraction of the GHG emissions from the space sector are arising from o ffi ce work, which has a relatively small carbon intensity compared to other, more material-intensive activities (Chanoine et al. 2017). Consequently, decarbonising the powering and heating of o ffi ce buildings is an important lever arm for carbon footprint reductions of space facilities.  \nIt is estimated that between 50% to 70% of the carbon footprint of a space mission is related to the launcher (Maury 2019), putting the decarbonisation of the launcher in the focus of the space sector. Harris & Landis (2019) estimate the cradle-to-use carbon footprint of a Falcon 9 rocket and a Falcon Heavy rocket to about ∼ 10 ktCO 2 e and ∼ 15 ktCO 2 e, respectively. Owing to the large carbon footprint of propellant production for an Ariane 5 rocket, and based on information published by Chanoine et al. (2017) and Schabedoth (2020), we estimate its footprint to about twice that of a Falcon Heavy rocket. Dividing these footprints by the launcher costs results in a comparable carbon intensity of about ∼ 180 tCO 2 e M e -1 for all three launchers, suggesting that a reduction of launch cost leads to a reduction of carbon footprint. Given that the reduction of launch costs is pursued to facilitate access to space, it is likely that the per launcher footprint reductions will be quickly outpaced by the increasing number of launches, suggesting the appearance of a classical rebound e ff ect that will prevent a reduction in GHG emissions in the space sector. Probably more promising is the decarbonisation of propellant production, as planned for example by the Hyguane project for Europe's spaceport Kourou in French Guiana (ESA 2022). Using methane as alternative propellant for launchers also provides potential for carbon footprint reductions (Kurela & Noir 2020).  \nOn top of the launcher footprint comes the carbon footprint of the space centres from which the rockets are launched, for example ∼ 100 ktCO 2 e yr -1 for the Kennedy Space Center (Monteith 2020) and ∼ 40 ktCO 2 e yr -1 for Kourou (Serfass-Denis 2019), with electricity consumption being the most significant source of GHG emissions in both cases. Additional contributions come from the payload production, including the production of the astronomical relevant probes or telescopes, the test campaigns and the associated infrastructures, integration activities, and ground operations following the launch. For example, Sydnor et al. (2011) estimate the carbon footprint of NASA's ground test facilities in 2008-2010 to 137 ktCO 2 e yr -1 , with electricity consumption and natural gas being the most significant contributors. For all these contributions, moving away from fossil fuels towards renewable energies will have the potential to considerably reduce the GHG emissions.  \nCombining all contributions, aggregated lifecycle carbon footprints for space missions range from a few tens of ktCO 2 e for Small Explorer (SMEX) missions like the Nuclear Spectroscopic Telescope Array (NuStar) or the Galaxy Evolution Explorer (GALEX) to over one million tonnes of CO 2 e for flagship missions like the James Webb Space Telescope (JWST) (Knödlseder et al. 2022). While decarbonising electricity production for all lifecycle stages of a space mission has a significant decarbonisation potential, the current increase in the number of new space missions, favoured by the 'newspace' boom that accompanies the transition from government-funded to commercial space activities, has the potential to annihilate the e ff orts.  \nThe situation is not much di ff erent for ground-based observatories. Knödlseder et al. (2022) estimated carbon intensities for construction and operations of groundbased observatories to 240 tCO 2 e M e -1 and 250 tCO 2 e M e -1 , respectively. While detailed lifecycle analyses for ground-based observatories are still lacking, according to Gallo & Bachelet (2019), who assessed the carbon footprint of the European Extremely Large Telescope (E-ELT), the construction GHG emissions seem to arise primarily from concrete and steel (like in a typical building construction project), as well as from installations, mechanisms and freight. For operations, GHG emissions arise primarily from electricity consumption, followed by purchase of goods and services,  \nbusiness travels, commuting and freight (Gallo & Bachelet 2019). Options for carbon footprint reductions include use of low carbon concrete, recycled steel and bio sourced materials such as wood for construction, while a shift to renewable energies can help to decarbonise operations. Yet these decarbonisation e ff orts will only be e ff ective if no additional sources of GHG emissions are added.  \n12000  \nFigure 2. Past and predicted annual carbon footprint of electricity consumption at the ESO observatory sites in La Silla, Paranal and Armazones (data from Filippi et al. 2022).  \n<!-- image -->  \nYear  \nThis is illustrated in Fig. 2 that shows the past and predicted annual carbon footprint of electricity consumption at the ESO observatory sites in Chile. While between 2016 and 2022 a reduction of GHG emissions from electricity consumption by ∼ 50% was achieved (by swapping at Paranal from liquid petrol gas generators to a grid connection in 2018 and adding photovoltaic power plants in 2022), the additional electricity needs of E-ELT will have annihilated all the reductions by the end of this decade; despite important e ff orts, the GHG emissions due to electricity consumption will exceed in 2030 those of 2016. This illustrates an obvious but inconvenient truth: it is extremely di ffi cult to decarbonise while ramping up! ESO is so far the only organisation that provides public information on carbon footprint estimates and reduction plans, exposing the organisation obviously to be used as a case-study. There are no reasons to believe that the situation is di ff erent for other organisations, at least as long as they continue to expand.", '4. Other scientific domains': "The situation for astronomy is now clear, but what's the situation for other scientific domains that rely heavily on large-scale research infrastructures? To shed some light on this question, estimates of the carbon footprint of research infrastructures that are currently under development in Europe in di ff erent scientific domains are shown in Fig. 3.  \nConstruction  \nFigure 3. Estimated construction (top) and annual operations (bottom) carbon footprint of projects and landmarks in the ESFRI 2021 infrastructure roadmap, aggregated by scientific domain (data from ESFRI 2021). Percentages give the contribution of a scientific domain to the total carbon footprint.  \n<!-- image -->  \nWe based the figure on the 2021 update of the ESFRI infrastructure roadmap (ESFRI 2021), and distinguished infrastructures in the preparation phase (called 'projects' in the ESFRI roadmap) and infrastructures that were implemented or reached an advanced implementation phase (called 'landmarks'). We estimated the carbon footprints by converting construction and operating costs as quoted in the ESFRI roadmap into GHG emissions using the same emission factors that were derived by Knödlseder et al. (2022) for ground-based astronomical observatories. For the latter, we assume that all infrastructures listed in the ESFRI roadmap will be operating at some point contemporaneously, so that their footprints will add up. For comparison of astronomy with other scientific domains, we split the physics domain that includes astronomy in the ESFRI roadmap into astronomy infrastructures and other physics infrastructures.  \nAccording to this analysis, the total carbon footprint for construction of planned research infrastructures in Europe amounts to about ∼ 6 MtCO 2 e with an expected associated annual operations footprint of ∼ 420 ktCO 2 e yr -1 , suggesting that the annual operations footprint amounts to ∼ 7% of the construction footprint (for comparison, the annual GHG emissions of Europe in 2019 amounted to about ∼ 4000 MtCO 2 e, see EEA 2022). The construction of astronomical research infrastructures contribute to 24% to the total construction carbon footprint in Europe, while for operations the contribution is estimated to 12%. Research infrastructures in the domain of physics account for 33% of the construction footprint, raising to 57% when astronomy is included. The share in the combined physics operations footprint is comparable, suggesting that research infrastructures for physics account for more than half of the total carbon footprint in Europe. Examples of physics infrastructures with a large estimated construction carbon footprints include the European Spallation Source ( ∼ 720 ktCO 2 e), the European X-Ray Free-Electron Laser Facility ( ∼ 370 ktCO 2 e) and the High-Luminosity Large Hadron Collider ( ∼ 340 ktCO 2 e). All three facilities have estimated annual operation footprints of ∼ 35 ktCO 2 e yr -1 , in excess of those of the largest astronomical facili-  \net al. 2022). Obviously, astronomy is not the only domain that has an important GHG emission reduction burden, particle physics is certainly in a similar situation. An assessment of the global carbon footprint of particle physics is still lacking, yet some first estimates of specific GHG emission sources are appearing in the literature (Bloom et al. 2022; Janot & Blondel 2022).", '5. Towards sustainable astronomy': "Coming back to astronomy, the question we have to face now in our field is how we can bring our activities in line with the imperative to reduce GHG emissions at a rate of 7% per year over the next three decades. Or to be more specific: what will we change in 2023 to reduce GHG emissions by 7% next year? The required amplitude of GHG emission reductions is significant, and they are not achievable by fiddling on the margins. Reductions of 7% are superior to what was achieved following the COVID-19 lockdowns in 2020 which had important impacts on human activities with still visible economic aftermaths today. And as these aftermaths illustrate, reductions of 7% are not possible without systemic changes, giving up the way we operate currently and inventing a new narrative for doing astronomical research in the future.  \nA first step would be to pull the breaks and focus on using what we already have instead of aiming for new facilities. Implementing technical decarbonisation solutions will take some time and will need investments, making them solutions for the mid- or long-term, yet unlikely for the immediate future. An example of short-term solutions would be to shift activities away from building new research infrastructures towards a more extended and deeper exploitation of existing facilities, and the analysis of existing astronomical data archives. It is well recognised that archives are valuable resources for astronomy, and a significant fraction of discoveries is made by exploring already existing data (e.g. White et al. 2009; De Marchi 2022). Use of archival data shall be actively promoted and be considered the baseline for astronomical research.  \nAnext step would be to base resource allocation decisions on their associated carbon footprint, having in mind that remaining carbon budgets that keep global warming below 1.5 · C or 2 · C shrink rapidly. Today, no funding agency is investing significantly into decarbonising research infrastructures; tomorrow, decarbonising existing facilities must become their funding priority! This also means that less money will be available to build new infrastructures, yet is this really a problem? Stoehr et al. (2015) argue that, in the future, observatories will compete for astronomers to work with their data, which if true seems to indicate that we may have already too many facilities. There is no requirement per se on the deployment pace of new facilities or missions, and slowing down the current pace will lead to less GHG emissions, free resources for investing into decarbonisation and give more time for in-depth analyses of existing data.  \nAnother measure is moving away from competition towards more collaboration. If we really believe that astronomers are working for mankind, there is no need to build the same kind of facility several times on the globe. For example, one 40-m class telescope in the world should be su ffi cient to make the discoveries to be made with such an instrument. And there is no scientific justification for having a new space race towards the Moon or the planets, a few well-coordinated international missions should be su ffi cient to gain the knowledge we are after.  \nOf course, astronomy is not the root cause of climate change, nor can astronomy alone fix it, but astronomy with its significant per capita GHG emissions must be exem-  \nplary and take its fair share, leading the way towards a sustainable future for humanity on Earth. It's time to act now, before we are forced in an uncontrolled and likely very violent way to respect the boundaries of our planet.  \nAcknowledgments. The author would like to thank R. Arsenault, S. Brau-Nogué, M. Coriat, P. Garnier, A. Hughes, P. Martin, A. Stevens and L. Tibaldo for useful discussions. 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