Climate Tipping Points
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Rahmstorf, Stefan
Politicians, economists and even some natural scientists have tended to assume that tipping points in the Earth system — such as the loss of the Amazon rainforest or the West Antarctic ice sheet — are of low probability and little understood. Yet evidence is mounting that these events could be more likely than was thought, have high impacts and are interconnected across different biophysical systems, potentially committing the world to long-term irreversible changes. This talk summarizes evidence on the threat of exceeding tipping points and explores the effects of such large-scale changes, how quickly they might unfold and whether we still have any control over them. The Intergovernmental Panel on Climate Change (IPCC) introduced the idea of tipping points two decades ago. At that time, these ‘large-scale discontinuities’ in the climate system were considered likely only if global warming exceeded 5 °C above pre-industrial levels. Information summarized in the two most recent IPCC Special Reports (published in 2018 and in September 2019) suggests that tipping points could be exceeded even between 1 and 2 °C of warming (see ‘Too close for comfort’). If current national pledges to reduce greenhouse-gas emissions are implemented — and that’s a big ‘if’ — they are likely to result in at least 3 °C of global warming. This is despite the goal of the 2015 Paris agreement to limit warming to well below 2 °C. Some economists, assuming that climate tipping points are of very low probability (even if they would be catastrophic), have suggested that 3 °C warming is optimal from a cost–benefit perspective. However, if tipping points are looking more likely, then the ‘optimal policy’ recommendation of simple cost–benefit climate-economy models aligns with those of the recent IPCC report. In other words, warming must be limited to 1.5 °C. This requires an emergency response. In our view, the clearest emergency would be if we were approaching a global cascade of tipping points that led to a new, less habitable, ‘hothouse’ climate state. Interactions could happen through ocean and atmospheric circulation or through feedbacks that increase greenhouse-gas levels and global temperature. Alternatively, strong cloud feedbacks could cause a global tipping point. This talk will give an overview on how these different scenarios might play out, how likely they are, and what we need to do to avoid them.
 Greetings, creatures in New Zealand. In 2015, governments from around the world met in Paris and agreed to attempt to limit anthropogenic climate change to well below 2 degrees. Unfortunately, it seems that since then we have not done enough and the climate crisis has only gotten more urgent. Our next speaker, Stefan Rammstorf, has more accolades than I have time to tell. He's published more than 100 papers, including in the journal's Nature and Science, co-authored four books, and won the Climate Communication Prize from the American Geophysical Union, the first European to do so. Please welcome him and heed his advice. Here's Stefan. Hi everyone. My name is Stefan Rammstorf and I'm thrilled to be invited to give a talk at the Chaos Computer Club's Remote Chaos Experience 2020. I want to give you an overview of a climate tipping point, a very exciting subject that I will try to shed some light on. But let's first start with some background on climate change. You probably know this image. It shows the global temperature evolution since the year 1880. Every line is one year. This is the more conventional way of viewing this time series. And the last seven years have been the hottest seven years since record keeping began in the 19th century. We know the reason for this warming. It's the increase of carbon dioxide, which you can see here for the last 10,000 years. And if you just look at the end of the curve, how the increase has accelerated in ever shorter time spans, we have seen an ever greater increase in the amount of carbon dioxide in our planet's atmosphere. This increase causes what we call a radiator forcing that is a kind of heating in terms of energy released per square meter of Earth's surface. And the increase in CO2 in the atmosphere until now is causing a heating at a rate of two watts per square meter of surface. We understand the energy budget of our planet pretty well. On the left here in this diagram, you can see the incoming solar radiation in yellow. Part of that is reflected already in the atmosphere by the clouds, for example. Another part is reflected by the bright surfaces. That's the snow and ice surfaces primarily. And the rest is absorbed. On the right hand side, and let's zoom into that, you see in orange the long wave radiation, which is clearly distinct from the incoming shortwave solar radiation by its wavelength. And this thick arrow of long wave radiation leaving the Earth's surface basically to a large extent gets absorbed by the atmosphere. And the atmosphere itself emits like anything, any substance, any matter at a, depending on its surface temperature, on its, sorry, depending on its temperature, emits also infrared radiation. And one thing that few people realize is that the back radiation coming down from the atmosphere through the greenhouse effect, the greenhouse gases, is actually twice as large at the Earth's surface as the absorbed solar radiation. So heating by the greenhouse effect by the long wave radiation is twice as big as the absorbed solar radiation at the Earth's surface. And so it's little wonder that if we are increasing this natural greenhouse effect, which actually makes our planet livable in the first place, if we are increasing this effect, that it is going to get warmer. We can also quantify this effect. And if you add in not just the CO2 increase, but other human-caused greenhouse gases, and also cooling effects caused by humans, then you see that the total human-caused warming that we see here in the orange bar is to within uncertainty as big as the observed global warming since the 1950s. And that means that about 100% of the observed global warming over the past 70 years is human-caused. And the best estimates of the human-caused warming is actually even slightly more than the observed warming, which has to do partly or is consistent with the fact that solar activity has gone down. So the decrease in solar activity has compensated a small part of the human-caused global warming. It's also very interesting and especially to me as a paleo-climatologist who studies natural climate variations in Earth history and has done so for more than 25 years, how the modern warming compares with the changes throughout the Holocene and before that since the last Ice Age. And this is what we see here based on decades of paleo-climat research, as sediment cores taken at the sea bottom ice cores on the big ice sheets and so on. We have enough data now to form meaningful global average temperatures. And you can see here the warming from the height of the last Ice Age into the Holocene, the Holocene optimum, the warmest period about until about 5000 years before present. And since then we have seen a very slow cooling trend, which we have bent around due to human activities and we have within a hundred years more than undone 5000 years of natural cooling trend, which normally would have very slowly continued. These natural variations by the way are due to the Earth orbital cycles, the so-called Milankovitch cycles. You can easily read up on those, for example, at Wikipedia. Now let's come to the famous much feared tipping points in the climate system. What is a tipping point? That has been described in a seminal paper, which I'm proud of having been a part of from 2008 by Tim Lenton and colleagues. And this is called tipping elements in the Earth's climate system. And it says that the term tipping point commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. And the different parts of the Earth's system which can undergo such a transition, they are called the tipping elements. This whole concept is illustrated in the red line that's shown here. In the horizontal axis we see a control parameter and that could be the greenhouse gas content of atmosphere. It could be the temperature. It could be, if you talk about natural climate changes, for example, those orbital changes, what we call the Milankovitch forcing, which drives changes. And on the vertical axis you see the response. And if you imagine the control parameter changing from left to right in this diagram, you would march along that upper part of the red curve here, the branch, until you come close to a threshold and at that threshold the system will undergo a major change and reach then this lower part of the curve, a different kind of equilibrium state. So it's basically a small change in the driver causing a very big systemic response. That is what defines the tipping point. If we want to be very accurate here, we can distinguish two different types of tipping points. The first one is what I just showed you is repeated here on the left side and it is characterized by the fact that this red equilibrium line has one state for every point on the x-axis. So every amount of forcing corresponds to one particular system state and this system state just makes a major transition in a small range of the driving parameter around this threshold. Now a second even more drastic or non-linear type of tipping point is shown in the right hand side where the equilibrium states are somewhat more complex than the single red line on the left. You can see here that there is again an upper stable branch and there is also a lower stable branch but they overlap. So there is a region that is shaded here where two stable equilibria exist and it depends on the initial conditions on which of these branches you are. Now there is what is called a bifurcation structure underlying this with a bifurcation point. There is an unstable branch which separates the basins of attraction of the two stable branches. So if you're in the bistable regime and you start kind of away from an equilibrium but above the dashed line you will fall up onto that upper stable branch. If you start out below the dashed line you will fall down onto a lower branch. That actually is pretty standard non-linear dynamics. It's a whole branch of physics which investigates exactly this type of behavior in many different physical systems. So the second type of tipping point, the right hand side one, is corresponding to multiple equilibrium states, in this case two stable equilibria. That's why this error range here is called bistability, two stable equilibria. It is coming with irreversibility. So basically if you march to the right here on that upper stable branch at that bifurcation point you fall off down onto the lower stable branch but you can't just go back up from there. You have to go all the way to the left to that second lower blue point there until you can go back onto that stable branch. The second type is actually as an everyday system that behaves like that. It can be easily compared to a kayak. If you're sitting in a kayak and you lean a little bit to one side then you experience a counter force. So the kayak is trying to upright itself. It's resisting you tipping it but if you move further and further and further eventually you will reach a tipping point. This is the point where the kayak stops resisting you're further leaning over and instead it starts tipping over further by itself and then it flips right over until it's upside down and you're falling out. So I have done this quite a few times. I have a kayak that is quite narrow where it easily happens if you don't take care that you flip over. Now this kayak also has a range of bi-stability so once it's flipped over it's also in a stable state and it takes considerable effort to turning upright again into the other stable state when it's vertical upright rather than upside down. Now the whole point is that systems like this exist also in the climate system. The kind of first type on the left hand side corresponds for example to sea eyes and on the right hand side this type of tipping element compares to refers to the Greenland ice sheets or continent lye sheets also Antarctica or the Atlantic ocean circulation. In terms of the transient behavior that means if you kind of go through a global warming phase you're moving from left to right in these diagrams then in that sense they don't differ very much because in either case you follow a line like this green line so on the left hand side the green line more or less follows more or less closely the red equilibrium line with a certain delay depending on how sluggish the system responds so that's why the green arrows are not exactly on top of the red line here and in the right hand side case you have a similar thing you're kind of in theory in equilibrium you'd fall off the cliff at this bifurcation point but in praxis the system has some inertia it takes some time so if you gradually move on the right towards the right there you will also follow a green line which is very similar to the one in the left so in practical terms if you're not trying to go back but you're just going forward progressive global warming the difference isn't all that big and the main difference comes from the intrinsic timescale of the system obviously sea ice can respond much more quickly to being just a few meters thick compared to continental ice sheets like Greenland ice which is about 3000 meters thick and that just takes a very long time to melt now here's an overview of different tipping elements in the climate system a few examples you can see starting on the left here the boreal forests that are the kind of northern forests which typically like ecosystems do have a tipping point a point of collapse the whole idea of these tipping points and and the system collapse is very strongly linked actually to ecosystem research and the boreal forests they have a point where they get too dry that fires and pests are weakening the forest so much that in a hot summer like last year in Siberia they go up in flames lit by lightning or the amazon rainforest this is also a tipping element has been shown in many vegetation dynamics models which is partly linked to the fact that such a forest generates its own rain to an extent by storing water in the soil keeping it there and then bringing it up again through evapotranspiration as we call it the tree brings up water to the leaves from where it then enters the atmosphere again and then it moves with the winds and maybe 50 100 kilometers downwind it falls again as rain so it's a kind of perpetual rain recycling system which keeps the whole forest nice and moist but if you stress that too far and reduce the first of all you cut down forest you make it smaller and also you make it more drought prone by warming up the climate which leads to faster loss of moisture etc greater moisture requirements by the trees then you can stress it up to the point where it gets so dry that even the amazon rainforest can go up in flames another example of you see the top right is the permafrost thawing this is when it gets too warm there is a very simple threshold namely the freezing point of course that is a tipping point in a sense a freezing point of water when the permafrost thaws then there is methane gas escaping to the atmosphere which then also can enhance the further warming which then leads to more permafrost thawing and so on typically these tipping points are associated with such amplifying feedbacks i will discuss three of these in a little bit more detail the greenland ice sheet which is undergoing accelerated ice loss the atlantic over turning circulation or often called gulf stream system and the third one is a coral reefs which are suffering from large-scale die-off which also as a typical ecosystem the response have a critical threshold these examples are discussed in our paper climate tipping points to risky Tibet against which we published in nature about one year ago and they are also some of these tipping points interact they're interlinked and one of our quotes there is that the clearest emergency would be if we were approaching a global cascade of tipping points that is a situation where one tipping element is triggering the next one in a kind of domino effect this is what we feel most now let's have a look at the greenland ice sheet this is a nasa video showing based on gray satellite data where the ice sheet is losing mass you can see an increasing blue colors here that the greenland ice sheet is indeed losing mass you can look up at the nasa vital signs website which is very good indicators of various vital signs of our planet including the data on greenland ice loss constantly updated now the point with the greenland ice sheet is that it does have a stability diagram like the schematic one that i showed you earlier with the bi-stable range and this is shown i think it was shown for the first time by my colleagues karlow and ganapolsky in 2005 in this article where they used a three-dimensional ice sheet model coupled inside a global climate model with ocean atmosphere and so on and on the x-axis there's basically increasing amount of heating going on in this case because they were interested in the paleo climate question it is this driving force by the orbital cycles the milankovic cycles you don't need to understand the numbers but on the vertical axis you see the response of the ice sheet the size of the ice sheet in million cubic kilometers and you can see that upper branch in the blue line we're actually moving towards the right here in this model simulation experiment and you can see you stay on that upper branch until you reach this value on the x-axis of around about 500 and this is where the tipping point is there the ice mass declines melt survey away very quickly and you then end up at that lower branch with no ice on greenland and they played this game they ran this simulation out to more than 550 watts per square meter and the light blue line is what happens when they return you know when they turn down the heat again you move towards the left on this diagram but you don't go back up the same way as a dark blue line you have to go to much lower radiation values until the ice sheet starts to grow again and comes back the dots by the way are points where this has to has been run for many thousands of years really into an equilibrium just to show that there are really for the same value on the x-axis two very different equilibrium states with and without greenland ice sheet and the fact that we now and in the holocene in the last 10 000 years have a greenland ice sheet and it actually is stable in the holocene climate is only because of the initial condition because we came out of an ice age if you took away the greenland ice sheet now then in the current climate or the holocene or pre-industrial climate it would never grow back what is the positive feedback the more positive we don't mean that it's good there's actually quite bad a positive feedback we mean an amplifying feedback and the key amplifying feedback here is what is called the ice elevation feedback the greenland ice sheet does not melt because it's very cold at the surface mostly below freezing and why is it so cold because it is very high up in the atmosphere this ice sheet is 3000 meters thick after all so it's like in a high mountain area where it is quite cold if you took away that ice sheet though the surface then would be down at sea level or even below if you did this quickly because the bedrock is depressed but the surface would come up to sea level but down there it's much warmer than up at 3000 meters altitude in the atmosphere and there it is actually too warm to keep any snow on the ground year round which would be required to regrow a new greenland ice sheet and that's why you'd have to go back to a much colder climate than the holocene to get the greenland ice sheet back once it were lost this is a typical example of this amplifying feedback which leads to a self stabilizing system it can either self stabilize in the upper branch here when you start there or itself stabilizes in the lower branch with no ice when you start there this is what makes it a bi-stable system to summarize the greenland ice sheet is melting as the NASA data the gray satellite show but also other data sets it has a tipping point due to the ice elevation feedback now what I haven't shown but it's come out in in study with many climate model simulation experiments going through more than 200 000 years of simulation from the past through the emion interglacial period where we know how much the ice sheet shrank back and we could use those data from the past behavior of greenland to calibrate the model and so we know the tipping point for the complete loss of the greenland ice sheet is somewhere between one degree and three degree global warming we're already at one point two degrees global warming so we have started to enter the danger zone where we cross that tipping point it doesn't mean that it suddenly starts to melt very fast or so because it has its own intrinsic slow response time but what that crossing that tipping point means is that even without further warming the greenland ice sheet is doomed it will continue to melt until it's gone and this will lead to seven meters of global sea level rise drowning most of our big coastal cities and many island nations here is a look at the future from model simulations from ashwanden from nasa and you can see the nice view of what the surface looks like and here's what the what it looks like in the ice sheet model you can see the ice flowing you can see it retreating so in purple that's bedrock that is exposed where the ice sheet has was drawn in this simulation and so this as much as ice of ice that you would lose in the coming 300 years a substantial fraction of the greenland ice sheet now let's look at another kind of tipping element and that is the Gulfstream system or the north atlantic current and i i can't really introduce this topic is one of my favorite topics which i have studied since the early 90s without showing a clip from the famous hollywood blockbuster the day after tomorrow what about the north atlantic current what about it the current depends upon a delicate balance of salt and fresh water we all know that yes but no one is taken into account how much fresh water has been dumped into the ocean because of melting polar ice i think we've hit a critical desalination point yeah now that statement about the critical desalination point is a completely correct description of the bifurcation point of the atlantic circulation i'll show it in a minute and the the statement that nobody has taken into account the melt water from the greenland ice sheet is also was completely correct when the movie appeared in 2004 until then the typical climate simulations that you could see in the ipcc reports actually until quite a few years later still had not taken account greenland melt water because basically at that point in time the models almost all climate models were just ocean atmosphere models plus land surface but they didn't have continental ice sheet models coupled into them and so in the meantime of course we have better models that include experiments either with artificially added greenland melt water from data estimates or fully coupled with ice sheet models and from that an example here being that nature article by claus burning and colleagues we know that the melt water input from greenland has a non negligible effect on the north atlantic overturn and it's probably not the dominant effect but it adds to various factors that weaken this north atlantic current and we also know that this system has a well-defined tipping point actually i described that in a nature article in 1996 due to assault transport feedback the basic idea behind that has actually been known since the late 1950s or early 60s since work by the famous american oceanographer henry stommer but what i showed in my nature article in 96 is that it actually works that way in a complex three-dimensional global ocean circulation model not just in very simplified models and since then this has been shown for a whole range of different climate models the salt transportation feedback is also one of these amplifying feedbacks and it's easy to explain the overturning circulation of the atlantic is called overturning because it's really a vertical overturning where water sinks down from the surface to great depth of two to three kilometers in the atlantic because this water is heavy and it spreads then in the deeper ocean until it rises up in other parts mainly around and arctica in the um and arctic circumpolar current area and comes back at the surface so basically the whole ocean is overturned with deep water being renewed and then coming back to the surface on very long timescale of about a thousand to two thousand years for complete overturning there now the whole system is driven by the fact that that the water sinks down where it has the highest density and that's in the northern atlantic and around arctica around the arctic continent and it has the highest density there not only because it's very cold but also quite salty this is why you don't have deep water formation in the north pacific in in the northern hemisphere you only have that in the north atlantic and that's and that's because the north atlantic waters are quite salty and this is because this north atlantic current exists and brings salty water from the subtropics up to the high latitudes where normally it isn't very salty because it gets diluted by excess rainfall whereas the subtropics have excess evaporation and that's why they're salty and so it's like a chicken and an egg situation the northern atlantic is salty because you have this overturning circulation and you have this overturning circulation because it's salty there and so you can see the self-amplifying feedback there again which means it is a self-stabilizing system up to a certain breaking point a tipping point which can be reached if you add too much fresh water diluting the northern atlantic and the stability diagram again looks like that second one you've seen that for the greenland ice sheet and as I said this has been verified in detailed model simulations with many different models that it really works like that in a complex 3d situation where you have depending on how much fresh water you add into the northern atlantic this is the control parameter here you can move along that upper stable branch with the overturning to circulation until that stumble bifurcation point and there this overturning breaks down and you fall down onto that lower branch without this overturning it's labeled here nadw flow that nadw stands for north atlantic deep water it's uh yeah I would say one of the favorite water masses of the oceanographers now let's look at the Gulfstream the surface circulation in a climate model this is the cm2.6 global coupled climate model ocean atmosphere by the geophysical fluid dynamics laboratory in princeton you can beautifully see the Gulfstream and dark red here because it's warm leaving the coasts of the united states at Cape Hatteras they're starting to meander breaking up into these eddies etc and it actually means the meets the cold waters coming down inshore from the north which are shown in blue here and so this is what this the surface part of the circulation looks in a global climate model and if you add carbon dioxide to that climate model's atmosphere the climate warms of course but it does show a peculiar pattern of sea surface temperature change which you see here and this actually shows the sea surface temperature change relative to the global mean so everything that is blue has either warmed less than the global average or even cooled which is actually the case south of greenland and everything that is orangey or red has a warmed substantially more than the global average sea surface and you see a very strong pattern in the northern atlantic with this big cold blob the blue blob south of greenland and a very warm region inshore of the Gulfstream along the coast of north america and in the climate model of course we are a bit like gods in that sense that we have complete information about what's going on there if we store all the data at every grid point we know exactly everything that's happening and we can analyze the reasons and the reason for this funny pattern in the northern atlantic actually is a slowdown of the north atlantic overturning circulation that means that less heat is transported to the sub-corner ocean south of green and there that blue area which makes it cool down and the Gulfstream proper at the surface moves inshore there is complicated dynamical reasons for that but there is already long before this was shown in this model a theoretical underpinning for this it has to do with the vorticity dynamics on a rotating sphere too technical to go into in such a talk but it's a well understood phenomenon and so we know that this slowdown of the Gulfstream system is the reason behind this peculiar temperature pattern and this pattern is predicted by this climate model for a global warming situation and my phd student live get Caesar who was the first author on this nature paper from 2018 she looked at all the available measurements of sea surface temperature since the beginning of the 20th century and of course because we have only limited ocean temperature measurements we have only a fuzzy picture here not a sharp one like in the climate model but you can see a similar pattern in the north atlantic in the observations compared to what the model predicts in response to a slowdown of the overturning circulation and our conclusion here is that we are actually observing this slowdown of the circulation why do we take indirect evidence for this like this because we don't of course have measurements going back 100 years or more about the strength of that overturning circulation we have actually only started to measure this regularly in 2004 with a so-called rapid array at 26 degrees north in the atlantic and what we reconstructed about the evolution of this current for the last period where we do have the direct measurements agrees well with what the direct measurements show we concluded that the overturning circulation has declined since at least the mid 20th century by about 15 percent so far there are of course other indirect types of measurements you can use sediment data of various kinds and with various methodologies to reconstruct the strength of this atlantic overturning and a number of different studies are compiled here in this diagram and even though of course they differ somewhat in the detail they all tend to agree in this overall picture that the atlantic overturning circulation has been quite stable for the previous thousand years or so before the 20th century but and then in the 20th century has showed a clear declining signature and one example of the media coverage of this is that washington post article here which if you can see the small print of the most red articles there on that day actually made it to number three of the most red washington post articles there is definitely an interest in science and climate change science by the readers in the newspapers so far we've talked about a slowdown and not so much about where this tipping point is one reason is we don't know really we know there is a tipping point that is a robust result of many different studies and model experiments and theory but we don't know how far away we are from this that is very typical for these tipping points because they involve highly non-linear dynamics that means they can depend very sensitively on the exact conditions for example in this case the exact salinity distribution in the atlantic and the exact circulation pattern and models get these things kind of approximately right but not exactly right and if you have a situation where the question of where the tipping point is is very sensitive to the exact conditions then you have a large uncertainty about where the tipping point is and so there is discussion in the literature I just point out to one study here in science advances that try to correct for the inaccuracies in how we can reproduce the salinity in the atlantic waters and found that if you correct for that the circulation is actually a lot more sensitive than in other models and maybe that model is more correct of course it has other weaknesses as well we don't know which of the models is correct but should we cross this tipping point then the north atlantic circulation system would break down and you get a temperature pattern like the one shown here the cold blob in the atlantic that is now only over the ocean it exists right it's the only part of the world that has cooled since the beginning of the 20th century but it hasn't affected any land areas but if the circulation would break down all together and not only weaken by 15% this cold would expand greatly and affect Great Britain, Scandinavia, Iceland as you can see here which would then get a much colder climate whereas the rest of the globe continues to have a warmer climate this is really distinct from an ice age and so this is also really distinct from that Hollywood movie the day after tomorrow where the earth goes into a huge ice age instant freeze that of course is totally unrealistic and the screenwriter and the director they knew this they actually told me that if they were in the business of making a movie for a few million viewers they would stick to the laws of physics but since they make movies for a few hundred million viewers they stick to the laws of Hollywood drama but you would get a substantial regional cooling with major impact on ecosystems on human society now let me come to the third type of tipping point that i want to discuss today this is the coral reefs coral reefs like many ecosystems do have critical thresholds coral reefs are very important even though they only cover a very small percentage of the earth's surface they support a quarter of all marine life 40 percent coral cover of the world has already been lost 100 countries depend quite substantially on corals there's a 800 billion total global asset of coral reefs so it does have a major impact on people now corals when they are about to die they bleach they are abandoned by their algae that provides them with nutrition and that's why they lose their color and then after a while they they die they get covered by other by seaweed non symbiotic algae and they die and they do have a temperature threshold there's a critical warming threshold where this bleaching happens but an additional factor not yet the most important factor is the acidification of water it's a direct chemical chemical effect of adding carbine dioxide to the atmosphere which then goes partly into the oceans and acidifies the ocean waters but the main effect until now is the marine heat waves which cross more and more frequently the temperature tolerance threshold of coral reefs and here you can see that for the Great Barrier Reef a huge fantastic world wonder that you can see from space and you can see here the bleaching in the years 2016 2017 and 2020 three major bleaching events which affected in each case the red area here with the most severe bleaching you can see that by now large is a very large part of the Great Barrier Reef has bleached in these three events it's very tragic and you can see here for example the March the 2016 bleaching event in March the coral was bleached by May it was already overgrown by seaweed and just in 2015 and 2016 we actually had worldwide coral reef bleaching not only at the Great Barrier Reef in Australia only the blue ones out of these hundred reefs that were observed in this study only the blue ones escaped bleaching so we are actually in the midst of a great worldwide coral die-off event which is another prediction of climate science coming through if you look at the latest IPCC report it states that with two degrees warming virtually all coral reefs will be lost more than 99 percent 1.5 degree warming if we manage to limit the warming to 1.5 degrees we can save between 10 percent and 30 percent of the corals that is really depressing now let me talk briefly about what can we do a major success is of course the Paris Accord the biggest failure of which is that it hasn't come 20 years earlier after all the world community already in 1992 decided to stop global warming at the Rio Earth Summit the nation signed the United Nations framework convention on climate change and it took a full 25 years of further negotiations to finally reach the Paris Accord now you can see here that the goal of this is to hold the increase in the global average temperature to well below two degrees above pre-industrial level so it's not two degrees it's well below two degrees that's very important point many countries would not have signed up if it simply had said two degrees which was an older goal but it has shown to be insufficient and and to sorry and to pursue efforts to limit the temperature increase to 1.5 degrees above pre-industrial level so that is a more stringent Paris goal but at least the nations have committed to pursue efforts so my view is that every person should ask their own government what you are doing here is this a credible effort to try and limit warming to 1.5 degrees we might not make it but at least we should try to limit the warming to 1.5 to avoid the risk of destabilization of the Greenland ice sheet almost complete coral die-off and many further risks so what does this entail that is an important point if you want to limit global warming to some value whatever it is 1.5 2 3 whatever you choose it means you can only emit a limited amount of carbon dioxide that is because the amount of global warming is to good extent proportional to the total amount of CO2 that we have ever emitted so to the cumulative emissions it's like filling a bathtub with water if you want to draw the line at any level and say no further than here you can only add a limited amount of water and if you want to limit global warming to some value you can only add a limited amount of CO2 to the atmosphere and this is shown here for two different examples two different amounts this is actually the numbers here are emissions from the year 2016 so it's don't take these numbers from now we have already had four more years of emissions the solid lines show three scenarios with 600 billion tons of CO2 and they all have the same amount of emission so they all three solid lines get the same amount of warming this is about actually these lines correspond to about a 50 percent chance of ending up at 1.5 degrees and so they all get you the same amount of warming but with different times of when the peak emissions are reached so 2016 went past without us getting over the peak of the emissions 2020 maybe we still have a chance emissions have dropped a bit in 2020 but not for structural change mostly but due to corona but we still we have a chance that maybe next year they are lower still and what this shows is that the longer you wait the steeper your reductions have to be not only because you're starting later but also because you have to reach zero earlier at the end notice how all these three lines the later you start with reducing the earlier you have to reach zero emissions because the surface area under these curves is what counts for the climate goal the dashed line is a more generous goal which would end us had about 1.75 degrees or so best estimate this is kind of the weaker peris goal of well below two degrees which would allow us to gradually reduce emissions to zero by 2050 this is not counting in any negative emissions afterwards by the way this is the net emissions if you like so we have to reach net zero emissions in 2050 but of course if we wait five more years until emissions start to decline then they'll have to be at zero five years earlier so this is why it's so important to start now this by the way is from an article by christiana figueras et al in nature published 2017 where i was a co-author as well now final point can tipping points maybe help us and i'm talking here about societal tipping points and there are also some interesting studies on that the basic idea is that shown in the top right here we are in a kind of stable equilibrium where the red ball is now and we are stuck there it's it's hard to get out of this but there is a better equilibrium a more stable one further off to the right and the question is how do we get over the hill into that beneficial equilibrium of a sustainable global economy a sustainable energy system a stable climate and so on complete decarbonization that means no more fossil fuel use and these this green addition there that is added there is there are just some examples of how we can make this transition earlier easier and the hill that we have to get over smaller so we can make this current status quo that we're in a little bit less comfortable by putting a price on carbon we can make the transition easier by subsidizing renewable energies there are there is a greening of values there is a tipping point in thinking in society there are many co-benefits of this transformation in terms of avoided air pollution for example millions of people die every year from outdoor air pollution which would to a large extent go away if we stop fossil fuel use and we have seen a massive movement by the young people fridays for future here's Greta Thunberg talking to me at our institute she came last year to visit us there here is a fridays demonstration in berlin where i took this photo this is really changing the society's values and it's changing election results and it could be a tipping point towards a sustainable global society and with that hopeful message i want to end and i thank you very much for your attention if you want to read more there's a couple of books of mine that have also come out in english uh you can follow me on the blogs and of course in social media preferably a twitter but also the scientist for future logo there because many thousands of scientists are engaged there to try and stop the climate crisis this is really a matter of a survival of civilization thank you very much for listening you should stick to science and leave policy to us well we tried that approach you didn't want to hear about the science when it could have made a difference thank you so much chevron for your talk now we have some questions from the internets see the first question which additional tipping points will be triggered at two degrees three degrees and so on that is actually a difficult question to answer because of the uncertainty that i mentioned in my talk about where these tipping points are there is one in Antarctica the wilk space and that is a part of the Antarctic ice sheet that that could be triggered say below three degrees there are others like the ocean circulation where you probably at least we hope you have to go beyond three degrees to really trigger a collapse of the Gulfstream system but the truth is that there are very large uncertainty ranges and the main fact is that with every bit of extra warming we increase the risk of crossing more tipping points and are there some of these tipping points that are interrelated or correlated for instance could we save some tipping points if we are able to save others for instance the collapse of the Gulfstream yes there are these interconnections for example if the Gulfstream system collapses it will affect the atmospheric circulation the monsoon systems and can shift the tropical rainfall belts this is not just theoretical we see that in paleo climate where we have seen these collapses of the North Atlantic circulation and the paleo climatic proxy data show that it comes with shifts in the tropical rainfall belts that could then in this way trigger a major drought in the amazon region if the Gulfstream system collapses and so it would be very wise to prevent these tipping points especially when it comes to the ocean circulation or atmospheric circulation because that's really going to mess up the the weather patterns in a major way. How long have we known about human caused climate change? Well in principle in the 19th century Alexander von Humboldt actually wrote in 1843 if I remember correctly that humans are changing the climate by cutting down forests and emitting large amounts of gases at the centers of industry that's almost a little literal quote by Alexander von Humboldt we've known about how sensitive the climate is to a change in CO2 since the Swedish Nobel laureates van der Arenis remotely related to Greta Thunberg by the way in studied the effect of CO2 doubling he wasn't worried by that because he thought global warming would be great you know bring it on let's just die no it's back you can see my picture still yeah don't know what happened there and so he suggested you know burning a lot of coal to enhance global warming I guess he came from Sweden and thought coal is bad without thinking this through properly but the first real expert report warning the US government Lyndon B. Johnson of the coming global warming due to fossil fuel use was a rebel report in 1965 exactly 50 years half a century before finally the Paris agreement was reached will you be publishing your slides from the talk yes I will uploading the slides what is or what should be the ultimate goal of the climate change mitigation for instance is it saving lives saving other species well I think the the ultimate goal is of course preserving human civilization as we know it because I think if we let this run we will not only destroy a lot of ecosystems and biodiversity but we will probably cause major hunger crisis which you know with big droughts like the the one in Syria before the unrest in Syria started in 2011 the country went through the biggest drought in history and according to sediment data from the eastern Mediterranean it was the worst drought in at least 900 years and then you know I think especially in some unstable conflicted countries this can really turn them into failed states that is what happened in Syria and it's what a German report for the German government actually warned in 2009 it was called climate change as a security risk I was actually one of the co-authors of that report because I was in the German government's advisory panel on global change at the time and I think we will see increasing hunger crisis failed states and all the effects that that has on international politics if we cannot keep global warming below two degrees and finally is there a specific call to action for the chaos community is there anything that we can do with our mindset and our skills that's a good question that I haven't thought about but maybe you can know yourself the best thing what you can do I think the key is really to keep up the pressure on the political world like Fridays for Future has been doing go on the streets protest vote with climate as a priority I think these are the key things that everyone should be doing and specifically in whatever profession they are they will see some ways of how you can help to reduce emissions in your company put sustainability at the top of the agenda and so on chef and thanks so much for taking the time to join us today it's a great pleasure and honor and honor always welcome and now the news you
 Greetings, creatures in New Zealand. In 2015, governments from around the world met in Paris  and agreed to attempt to limit anthropogenic climate change to well below 2 degrees. Unfortunately,  it seems that since then we have not done enough and the climate crisis has only gotten  more urgent. Our next speaker, Stefan Rammstorf, has more accolades than I have time to tell.  He's published more than 100 papers, including in the journal's Nature and Science, co-authored  four books, and won the Climate Communication Prize from the American Geophysical Union,  the first European to do so. Please welcome him and heed his advice. Here's Stefan.  Hi everyone. My name is Stefan Rammstorf and I'm thrilled to be invited to give a talk  at the Chaos Computer Club's Remote Chaos Experience 2020. I want to give you an overview  of a climate tipping point, a very exciting subject that I will try to shed some light  on. But let's first start with some background on climate change. You probably know this  image. It shows the global temperature evolution since the year 1880. Every line is one year.  This is the more conventional way of viewing this time series. And the last seven years  have been the hottest seven years since record keeping began in the 19th century. We know  the reason for this warming. It's the increase of carbon dioxide, which you can see here  for the last 10,000 years. And if you just look at the end of the curve, how the increase  has accelerated in ever shorter time spans, we have seen an ever greater increase in the  amount of carbon dioxide in our planet's atmosphere. This increase causes what we call a radiator  forcing that is a kind of heating in terms of energy released per square meter of Earth's  surface. And the increase in CO2 in the atmosphere until now is causing a heating at a rate of  two watts per square meter of surface. We understand the energy budget of our planet  pretty well. On the left here in this diagram, you can see the incoming solar radiation in  yellow. Part of that is reflected already in the atmosphere by the clouds, for example.  Another part is reflected by the bright surfaces. That's the snow and ice surfaces primarily.  And the rest is absorbed. On the right hand side, and let's zoom into that, you see in  orange the long wave radiation, which is clearly distinct from the incoming shortwave solar  radiation by its wavelength. And this thick arrow of long wave radiation leaving the Earth's  surface basically to a large extent gets absorbed by the atmosphere. And the atmosphere itself  emits like anything, any substance, any matter at a, depending on its surface temperature,  on its, sorry, depending on its temperature, emits also infrared radiation. And one thing  that few people realize is that the back radiation coming down from the atmosphere through the  greenhouse effect, the greenhouse gases, is actually twice as large at the Earth's surface  as the absorbed solar radiation. So heating by the greenhouse effect by the long wave  radiation is twice as big as the absorbed solar radiation at the Earth's surface. And  so it's little wonder that if we are increasing this natural greenhouse effect, which actually  makes our planet livable in the first place, if we are increasing this effect, that it  is going to get warmer. We can also quantify this effect. And if you add in not just the  CO2 increase, but other human-caused greenhouse gases, and also cooling effects caused by  humans, then you see that the total human-caused warming that we see here in the orange bar  is to within uncertainty as big as the observed global warming since the 1950s. And that means  that about 100% of the observed global warming over the past 70 years is human-caused. And  the best estimates of the human-caused warming is actually even slightly more than the observed  warming, which has to do partly or is consistent with the fact that solar activity has gone  down. So the decrease in solar activity has compensated a small part of the human-caused  global warming. It's also very interesting and especially to me as a paleo-climatologist  who studies natural climate variations in Earth history and has done so for more than 25 years,  how the modern warming compares with the changes throughout the Holocene and before that since  the last Ice Age. And this is what we see here based on decades of paleo-climat research,  as sediment cores taken at the sea bottom ice cores on the big ice sheets and so on.  We have enough data now to form meaningful global average temperatures. And you can see  here the warming from the height of the last Ice Age into the Holocene, the Holocene optimum,  the warmest period about until about 5000 years before present. And since then we have  seen a very slow cooling trend, which we have bent around due to human activities and we  have within a hundred years more than undone 5000 years of natural cooling trend, which  normally would have very slowly continued. These natural variations by the way are due  to the Earth orbital cycles, the so-called Milankovitch cycles. You can easily read up  on those, for example, at Wikipedia. Now let's come to the famous much feared tipping points  in the climate system. What is a tipping point? That has been described in a seminal paper,  which I'm proud of having been a part of from 2008 by Tim Lenton and colleagues. And this  is called tipping elements in the Earth's climate system. And it says that the term  tipping point commonly refers to a critical threshold at which a tiny perturbation can  qualitatively alter the state or development of a system. And the different parts of the  Earth's system which can undergo such a transition, they are called the tipping elements. This  whole concept is illustrated in the red line that's shown here. In the horizontal axis we  see a control parameter and that could be the greenhouse gas content of atmosphere. It could  be the temperature. It could be, if you talk about natural climate changes, for example,  those orbital changes, what we call the Milankovitch forcing, which drives changes. And on the  vertical axis you see the response. And if you imagine the control parameter changing from  left to right in this diagram, you would march along that upper part of the red curve here,  the branch, until you come close to a threshold and at that threshold the system will undergo a  major change and reach then this lower part of the curve, a different kind of equilibrium state.  So it's basically a small change in the driver causing a very big systemic response. That is  what defines the tipping point. If we want to be very accurate here, we can distinguish two  different types of tipping points. The first one is what I just showed you is repeated here on the  left side and it is characterized by the fact that this red equilibrium line has one state for  every point on the x-axis. So every amount of forcing corresponds to one particular system  state and this system state just makes a major transition in a small range of the driving parameter  around this threshold. Now a second even more drastic or non-linear type of tipping point is shown  in the right hand side where the equilibrium states are somewhat more complex than the single red line  on the left. You can see here that there is again an upper stable branch and there is also a lower  stable branch but they overlap. So there is a region that is shaded here where two stable  equilibria exist and it depends on the initial conditions on which of these branches you are.  Now there is what is called a bifurcation structure underlying this with a bifurcation point.  There is an unstable branch which separates the basins of attraction of the two stable branches.  So if you're in the bistable regime and you start kind of away from an equilibrium but above the  dashed line you will fall up onto that upper stable branch. If you start out below the dashed  line you will fall down onto a lower branch. That actually is pretty standard non-linear dynamics.  It's a whole branch of physics which investigates exactly this type of behavior in many different  physical systems. So the second type of tipping point, the right hand side one, is corresponding to  multiple equilibrium states, in this case two stable equilibria. That's why this error range  here is called bistability, two stable equilibria. It is coming with irreversibility. So basically if  you march to the right here on that upper stable branch at that bifurcation point you fall off  down onto the lower stable branch but you can't just go back up from there. You have to go all  the way to the left to that second lower blue point there until you can go back onto that stable branch.  The second type is actually as an everyday system that behaves like that. It can be easily compared  to a kayak. If you're sitting in a kayak and you lean a little bit to one side then you experience  a counter force. So the kayak is trying to upright itself. It's resisting you tipping it but if you  move further and further and further eventually you will reach a tipping point. This is the point  where the kayak stops resisting you're further leaning over and instead it starts tipping over  further by itself and then it flips right over until it's upside down and you're falling out.  So I have done this quite a few times. I have a kayak that is quite narrow where it  easily happens if you don't take care that you flip over. Now this kayak also has a range of  bi-stability so once it's flipped over it's also in a stable state and it takes considerable effort  to turning upright again into the other stable state when it's vertical upright rather than upside  down. Now the whole point is that systems like this exist also in the climate system.  The kind of first type on the left hand side corresponds for example to sea eyes and on the  right hand side this type of tipping element compares to refers to the Greenland ice sheets or  continent lye sheets also Antarctica or the Atlantic ocean circulation.  In terms of the transient behavior that means if you kind of go through a global warming phase  you're moving from left to right in these diagrams then in that sense they don't differ very much  because in either case you follow a line like this green line so on the left hand side the green line  more or less follows more or less closely the red equilibrium line with a certain delay depending  on how sluggish the system responds so that's why the green arrows are not exactly on top  of the red line here and in the right hand side case you have a similar thing you're kind of in  theory in equilibrium you'd fall off the cliff at this bifurcation point but in praxis the system  has some inertia it takes some time so if you gradually move on the right towards the right  there you will also follow a green line which is very similar to the one in the left so  in practical terms if you're not trying to go back but you're just going forward progressive  global warming the difference isn't all that big and the main difference comes from the intrinsic  timescale of the system obviously sea ice can respond much more quickly to being just a few  meters thick compared to continental ice sheets like Greenland ice which is about 3000 meters  thick and that just takes a very long time to melt now here's an overview of different tipping  elements in the climate system a few examples you can see starting on the left here the boreal  forests that are the kind of northern forests which typically like ecosystems do have a tipping  point a point of collapse the whole idea of these tipping points and and the system collapse is very  strongly linked actually to ecosystem research and the boreal forests they have a point where they  get too dry that fires and pests are weakening the forest so much that in a hot summer like last  year in Siberia they go up in flames lit by lightning or the amazon rainforest this is also  a tipping element has been shown in many vegetation dynamics models which is partly linked to the fact  that such a forest generates its own rain to an extent by storing water in the soil keeping it  there and then bringing it up again through evapotranspiration as we call it the tree brings  up water to the leaves from where it then enters the atmosphere again and then it moves with the  winds and maybe 50 100 kilometers downwind it falls again as rain so it's a kind of perpetual  rain recycling system which keeps the whole forest nice and moist but if you stress that too far  and reduce the first of all you cut down forest you make it smaller and also you make it more  drought prone by warming up the climate which leads to faster loss of moisture etc greater  moisture requirements by the trees then you can stress it up to the point where it gets so dry  that even the amazon rainforest can go up in flames another example of you see the top right  is the permafrost thawing this is when it gets too warm there is a very simple threshold namely the  freezing point of course that is a tipping point in a sense a freezing point of water when the  permafrost thaws then there is methane gas escaping to the atmosphere which then also can enhance the  further warming which then leads to more permafrost thawing and so on typically these tipping points  are associated with such amplifying feedbacks i will discuss three of these in a little bit more  detail the greenland ice sheet which is undergoing accelerated ice loss the atlantic over turning  circulation or often called gulf stream system and the third one is a coral reefs which are  suffering from large-scale die-off which also as a typical ecosystem the response have a critical  threshold these examples are discussed in our paper climate tipping points to risky  Tibet against which we published in nature about one year ago and they are also some of these tipping  points interact they're interlinked and one of our quotes there is that the clearest emergency  would be if we were approaching a global cascade of tipping points that is a situation where one  tipping element is triggering the next one in a kind of domino effect this is what we feel most  now let's have a look at the greenland ice sheet this is a nasa video showing based on gray  satellite data where the ice sheet is losing mass you can see an increasing blue colors here  that the greenland ice sheet is indeed losing mass you can look up at the nasa vital signs  website which is very good indicators of various vital signs of our planet including the data on  greenland ice loss constantly updated now the point with the greenland ice sheet is that it  does have a stability diagram like the schematic one that i showed you earlier with the bi-stable  range and this is shown i think it was shown for the first time by my colleagues  karlow and ganapolsky in 2005 in this article where they used a three-dimensional ice sheet model  coupled inside a global climate model with ocean atmosphere and so on and on the x-axis  there's basically increasing amount of heating going on in this case because they were interested  in the paleo climate question it is this driving force by the orbital cycles the milankovic cycles  you don't need to understand the numbers but on the vertical axis you see the response of the  ice sheet the size of the ice sheet in million cubic kilometers and you can see that upper branch  in the blue line we're actually moving towards the right here in this model simulation experiment  and you can see you stay on that upper branch until you reach this value on the x-axis of around  about 500 and this is where the tipping point is there the ice mass declines melt survey away  very quickly and you then end up at that lower branch with no ice on greenland and they played  this game they ran this simulation out to more than 550 watts per square meter and the light blue  line is what happens when they return you know when they turn down the heat again you move towards  the left on this diagram but you don't go back up the same way as a dark blue line you have to go  to much lower radiation values until the ice sheet starts to grow again and comes back  the dots by the way are points where this has to has been run for many thousands of years really  into an equilibrium just to show that there are really for the same value on the x-axis two very  different equilibrium states with and without greenland ice sheet and the fact that we now and in the  holocene in the last 10 000 years have a greenland ice sheet and it actually is stable in the holocene  climate is only because of the initial condition because we came out of an ice age if you took  away the greenland ice sheet now then in the current climate or the holocene or pre-industrial  climate it would never grow back what is the positive feedback the more positive we don't mean  that it's good there's actually quite bad a positive feedback we mean an amplifying feedback  and the key amplifying feedback here is what is called the ice elevation feedback the greenland  ice sheet does not melt because it's very cold at the surface mostly below freezing  and why is it so cold because it is very high up in the atmosphere this ice sheet is 3000 meters  thick after all so it's like in a high mountain area where it is quite cold if you took away that  ice sheet though the surface then would be down at sea level or even below if you did this quickly  because the bedrock is depressed but the surface would come up to sea level but down there it's  much warmer than up at 3000 meters altitude in the atmosphere and there it is actually too warm  to keep any snow on the ground year round which would be required to regrow a new greenland  ice sheet and that's why you'd have to go back to a much colder climate than the holocene to get  the greenland ice sheet back once it were lost this is a typical example of this amplifying  feedback which leads to a self stabilizing system it can either self stabilize in the upper branch  here when you start there or itself stabilizes in the lower branch with no ice when you start there  this is what makes it a bi-stable system to summarize the greenland ice sheet is melting  as the NASA data the gray satellite show but also other data sets it has a tipping point due to the  ice elevation feedback now what I haven't shown but it's come out in in study with many climate  model simulation experiments going through more than 200 000 years of simulation from the past  through the emion interglacial period where we know how much the ice sheet shrank back and we  could use those data from the past behavior of greenland to calibrate the model and so we know  the tipping point for the complete loss of the greenland ice sheet is somewhere between one degree  and three degree global warming we're already at one point two degrees global warming so we have  started to enter the danger zone where we cross that tipping point it doesn't mean that it suddenly  starts to melt very fast or so because it has its own intrinsic slow response time but what that  crossing that tipping point means is that even without further warming the greenland ice sheet  is doomed it will continue to melt until it's gone and this will lead to seven meters of global  sea level rise drowning most of our big coastal cities and many island nations  here is a look at the future from model simulations from ashwanden from nasa and you can see  the nice view of what the surface looks like and here's what the what it looks like in the ice sheet  model you can see the ice flowing you can see it retreating so in purple that's bedrock that is  exposed where the ice sheet has was drawn in this simulation and so this as much as ice of ice that  you would lose in the coming 300 years a substantial fraction of the greenland ice sheet now let's look  at another kind of tipping element and that is the Gulfstream system or the north atlantic current  and i i can't really introduce this topic is one of my favorite topics which i have studied since  the early 90s without showing a clip from the famous hollywood blockbuster the day after tomorrow  what about the north atlantic current what about it the current depends upon a delicate  balance of salt and fresh water we all know that yes but no one is taken into account how much  fresh water has been dumped into the ocean because of melting polar ice i think we've  hit a critical desalination point yeah now that statement about the critical desalination point  is a completely correct description of the bifurcation point of the atlantic circulation i'll show it  in a minute and the the statement that nobody has taken into account the melt water from the  greenland ice sheet is also was completely correct when the movie appeared in 2004 until then the  typical climate simulations that you could see in the ipcc reports actually until quite a few  years later still had not taken account greenland melt water because basically at that point in  time the models almost all climate models were just ocean atmosphere models plus land surface  but they didn't have continental ice sheet models coupled into them and so in the meantime of course  we have better models that include experiments either with artificially added greenland melt  water from data estimates or fully coupled with ice sheet models and from that an example here  being that nature article by claus burning and colleagues we know that the melt water input from  greenland has a non negligible effect on the north atlantic overturn and it's probably not the  dominant effect but it adds to various factors that weaken this north atlantic current and we  also know that this system has a well-defined tipping point actually i described that in a  nature article in 1996 due to assault transport feedback the basic idea behind that has actually  been known since the late 1950s or early 60s since work by the famous american oceanographer  henry stommer but what i showed in my nature article in 96 is that it actually works that way  in a complex three-dimensional global ocean circulation model not just in very simplified  models and since then this has been shown for a whole range of different climate models  the salt transportation feedback is also one of these amplifying feedbacks and it's easy to explain  the overturning circulation of the atlantic is called overturning because it's really a  vertical overturning where water sinks down from the surface to great depth of two to three kilometers  in the atlantic because this water is heavy and it spreads then in the deeper ocean until it rises  up in other parts mainly around and arctica in the um and arctic circumpolar current area  and comes back at the surface so basically the whole ocean is overturned with deep water being  renewed and then coming back to the surface on very long timescale of about a thousand to two  thousand years for complete overturning there now the whole system is driven by the fact that  that the water sinks down where it has the highest density and that's in the northern atlantic  and around arctica around the arctic continent and it has the highest density there not only  because it's very cold but also quite salty this is why you don't have deep water formation in the  north pacific in in the northern hemisphere you only have that in the north atlantic and that's  and that's because the north atlantic waters are quite salty and this is because this north atlantic  current exists and brings salty water from the subtropics up to the high latitudes where normally  it isn't very salty because it gets diluted by excess rainfall whereas the subtropics have excess  evaporation and that's why they're salty and so it's like a chicken and an egg situation  the northern atlantic is salty because you have this overturning circulation and you have this  overturning circulation because it's salty there and so you can see the self-amplifying feedback  there again which means it is a self-stabilizing system up to a certain breaking point a tipping  point which can be reached if you add too much fresh water diluting the northern atlantic  and the stability diagram again looks like that second one you've seen that for the greenland  ice sheet and as I said this has been verified in detailed model simulations with many different  models that it really works like that in a complex 3d situation where you have depending on how much  fresh water you add into the northern atlantic this is the control parameter here you can move  along that upper stable branch with the overturning to circulation until that stumble bifurcation point  and there this overturning breaks down and you fall down onto that lower branch without  this overturning it's labeled here nadw flow that nadw stands for north atlantic deep water  it's uh yeah I would say one of the favorite water masses of the oceanographers now let's look at the  Gulfstream the surface circulation in a climate model this is the cm2.6 global coupled climate  model ocean atmosphere by the geophysical fluid dynamics laboratory in princeton you can beautifully  see the Gulfstream and dark red here because it's warm leaving the coasts of the united states at  Cape Hatteras they're starting to meander breaking up into these eddies etc and it actually means  the meets the cold waters coming down inshore from the north which are shown in blue here  and so this is what this the surface part of the circulation looks in a global climate model  and if you add carbon dioxide to that climate model's atmosphere the climate warms of course  but it does show a peculiar pattern of sea surface temperature change which you see here  and this actually shows the sea surface temperature change relative to the global mean so everything  that is blue has either warmed less than the global average or even cooled which is actually the case  south of greenland and everything that is orangey or red has a warmed substantially more than the  global average sea surface and you see a very strong pattern in the northern atlantic with  this big cold blob the blue blob south of greenland and a very warm region inshore of the Gulfstream  along the coast of north america and in the climate model of course we are a bit like gods in that  sense that we have complete information about what's going on there if we store all the data at  every grid point we know exactly everything that's happening and we can analyze the reasons  and the reason for this funny pattern in the northern atlantic actually is a slowdown of the  north atlantic overturning circulation that means that less heat is transported to the  sub-corner ocean south of green and there that blue area which makes it cool down  and the Gulfstream proper at the surface moves inshore there is complicated dynamical reasons  for that but there is already long before this was shown in this model a theoretical  underpinning for this it has to do with the vorticity dynamics on a rotating sphere  too technical to go into in such a talk but it's a well understood phenomenon  and so we know that this slowdown of the Gulfstream system is the reason behind this peculiar  temperature pattern and this pattern is predicted by this climate model for a global warming situation  and my phd student live get Caesar who was the first author on this nature paper from 2018  she looked at all the available measurements of sea surface temperature since the beginning of  the 20th century and of course because we have only limited ocean temperature measurements we  have only a fuzzy picture here not a sharp one like in the climate model but you can see a similar  pattern in the north atlantic in the observations compared to what the model predicts in response  to a slowdown of the overturning circulation and our conclusion here is that we are actually  observing this slowdown of the circulation why do we take indirect evidence for this like this  because we don't of course have measurements going back 100 years or more about the strength of that  overturning circulation we have actually only started to measure this regularly in 2004 with a  so-called rapid array at 26 degrees north in the atlantic and what we reconstructed about the  evolution of this current for the last period where we do have the direct measurements agrees well  with what the direct measurements show we concluded that the overturning circulation  has declined since at least the mid 20th century by about 15 percent so far there are of course  other indirect types of measurements you can use sediment data of various kinds and with various  methodologies to reconstruct the strength of this atlantic overturning and a number of different  studies are compiled here in this diagram and even though of course they differ somewhat in the detail  they all tend to agree in this overall picture that the atlantic overturning circulation has been  quite stable for the previous thousand years or so before the 20th century but and then in the  20th century has showed a clear declining signature and one example of the media coverage of this is  that washington post article here which if you can see the small print of the most red articles  there on that day actually made it to number three of the most red washington post articles  there is definitely an interest in science and climate change science by the readers in the  newspapers so far we've talked about a slowdown and not so much about where this tipping point is  one reason is we don't know really we know there is a tipping point that is a robust result of  many different studies and model experiments and theory but we don't know how far away we are from  this that is very typical for these tipping points because they involve highly non-linear dynamics  that means they can depend very sensitively on the exact conditions for example in this case the  exact salinity distribution in the atlantic and the exact circulation pattern and models get these  things kind of approximately right but not exactly right and if you have a situation where the question  of where the tipping point is is very sensitive to the exact conditions then you have a large  uncertainty about where the tipping point is and so there is discussion in the literature I just  point out to one study here in science advances that try to correct for the inaccuracies in how we  can reproduce the salinity in the atlantic waters and found that if you correct for that the circulation  is actually a lot more sensitive than in other models and maybe that model is more correct of  course it has other weaknesses as well we don't know which of the models is correct but should we  cross this tipping point then the north atlantic circulation system would break down and you get  a temperature pattern like the one shown here the cold blob in the atlantic that is now only over  the ocean it exists right it's the only part of the world that has cooled since the beginning of  the 20th century but it hasn't affected any land areas but if the circulation would break down  all together and not only weaken by 15% this cold would expand greatly and affect Great Britain,  Scandinavia, Iceland as you can see here which would then get a much colder climate whereas  the rest of the globe continues to have a warmer climate this is really distinct from an ice age  and so this is also really distinct from that Hollywood movie the day after tomorrow where the  earth goes into a huge ice age instant freeze that of course is totally unrealistic and the  screenwriter and the director they knew this they actually told me that if they were in the business  of making a movie for a few million viewers they would stick to the laws of physics but since  they make movies for a few hundred million viewers they stick to the laws of Hollywood drama  but you would get a substantial regional cooling with major impact on ecosystems on human society  now let me come to the third type of tipping point that i want to discuss today  this is the coral reefs coral reefs like many ecosystems do have critical thresholds  coral reefs are very important even though they only cover a very small percentage of the earth's  surface they support a quarter of all marine life 40 percent coral cover of the world has  already been lost 100 countries depend quite substantially on corals there's a 800 billion  total global asset of coral reefs so it does have a major impact on people  now corals when they are about to die they bleach they are abandoned by their algae that  provides them with nutrition and that's why they lose their color and then after a while they  they die they get covered by other by seaweed non symbiotic algae and they die  and they do have a temperature threshold there's a critical warming threshold where this bleaching  happens but an additional factor not yet the most important factor is the acidification of water  it's a direct chemical chemical effect of adding carbine dioxide to the atmosphere  which then goes partly into the oceans and acidifies the ocean waters but the main effect  until now is the marine heat waves which cross more and more frequently the temperature tolerance  threshold of coral reefs and here you can see that for the Great Barrier Reef a huge fantastic  world wonder that you can see from space and you can see here the bleaching in the years 2016 2017  and 2020 three major bleaching events which affected in each case the red area here with the most  severe bleaching you can see that by now large is a very large part of the Great Barrier Reef has  bleached in these three events it's very tragic and you can see here for example the March the  2016 bleaching event in March the coral was bleached by May it was already overgrown by seaweed  and just in 2015 and 2016 we actually had worldwide coral reef bleaching not only at the Great Barrier  Reef in Australia only the blue ones out of these hundred reefs that were observed in this study  only the blue ones escaped bleaching so we are actually in the midst of a great worldwide coral  die-off event which is another prediction of climate science coming through if you look at the  latest IPCC report it states that with two degrees warming virtually all coral reefs will be lost  more than 99 percent 1.5 degree warming if we manage to limit the warming to 1.5 degrees we  can save between 10 percent and 30 percent of the corals that is really depressing  now let me talk briefly about what can we do a major success is of course the Paris Accord  the biggest failure of which is that it hasn't come 20 years earlier after all the world community  already in 1992 decided to stop global warming at the Rio Earth Summit the nation signed the  United Nations framework convention on climate change and it took a full 25 years of further  negotiations to finally reach the Paris Accord now you can see here that the goal of this is to  hold the increase in the global average temperature to well below two degrees above pre-industrial  level so it's not two degrees it's well below two degrees that's very important point many  countries would not have signed up if it simply had said two degrees which was an older goal  but it has shown to be insufficient and  and to sorry and to pursue efforts to limit the temperature increase to 1.5  degrees above pre-industrial level so that is a more stringent Paris goal but at least the  nations have committed to pursue efforts so my view is that every person should ask their own  government what you are doing here is this a credible effort to try and limit warming to 1.5  degrees we might not make it but at least we should try to limit the warming to 1.5 to avoid  the risk of destabilization of the Greenland ice sheet almost complete coral die-off and many  further risks so what does this entail that is an important point if you want to limit global  warming to some value whatever it is 1.5 2 3 whatever you choose it means you can only emit  a limited amount of carbon dioxide that is because the amount of global warming is to good extent  proportional to the total amount of CO2 that we have ever emitted so to the cumulative emissions  it's like filling a bathtub with water if you want to draw the line at any level and say no  further than here you can only add a limited amount of water and if you want to limit global warming  to some value you can only add a limited amount of CO2 to the atmosphere  and this is shown here for two different examples two different amounts this is actually the numbers  here are emissions from the year 2016 so it's don't take these numbers from now we have already  had four more years of emissions the solid lines show three scenarios with 600 billion tons of CO2  and they all have the same amount of emission so they all three solid lines get the same amount  of warming this is about actually these lines correspond to about a 50 percent chance of ending  up at 1.5 degrees and so they all get you the same amount of warming but with different times of when  the peak emissions are reached so 2016 went past without us getting over the peak of the emissions  2020 maybe we still have a chance emissions have dropped a bit in 2020 but not for structural  change mostly but due to corona but we still we have a chance that maybe next year they are lower  still and what this shows is that the longer you wait the steeper your reductions have to be  not only because you're starting later but also because you have to reach zero earlier at the  end notice how all these three lines the later you start with reducing the earlier you have to reach  zero emissions because the surface area under these curves is what counts for the climate goal  the dashed line is a more generous goal which would end us had about 1.75 degrees or so best  estimate this is kind of the weaker peris goal of well below two degrees which would allow us to  gradually reduce emissions to zero by 2050 this is not counting in any negative emissions afterwards  by the way this is the net emissions if you like so we have to reach net zero emissions in 2050 but  of course if we wait five more years until emissions start to decline then they'll have to be at zero  five years earlier so this is why it's so important to start now this by the way is from an article  by christiana figueras et al in nature published 2017 where i was a co-author as well  now final point can tipping points maybe help us and i'm talking here about societal tipping  points and there are also some interesting studies on that the basic idea is that shown in the top  right here we are in a kind of stable equilibrium where the red ball is now and we are stuck there  it's it's hard to get out of this but there is a better equilibrium a more stable one further off  to the right and the question is how do we get over the hill into that beneficial equilibrium  of a sustainable global economy a sustainable energy system a stable climate and so on complete  decarbonization that means no more fossil fuel use and these this green addition there that is  added there is there are just some examples of how we can make this transition earlier  easier and the hill that we have to get over smaller so we can make this current status quo  that we're in a little bit less comfortable by putting a price on carbon we can make the transition  easier by subsidizing renewable energies there are there is a greening of values there is a tipping  point in thinking in society there are many co-benefits of this transformation in terms of  avoided air pollution for example millions of people die every year from outdoor air pollution  which would to a large extent go away if we stop fossil fuel use and we have seen a massive movement  by the young people fridays for future here's Greta Thunberg talking to me at our institute  she came last year to visit us there here is a fridays demonstration in berlin where i took  this photo this is really changing the society's values and it's changing election results and it  could be a tipping point towards a sustainable global society and with that hopeful message i want to  end and i thank you very much for your attention if you want to read more there's a couple of books  of mine that have also come out in english uh you can follow me on the blogs and of course in social  media preferably a twitter but also the scientist for future logo there because many thousands of  scientists are engaged there to try and stop the climate crisis this is really a matter of a survival  of civilization thank you very much for listening you should stick to science and leave policy to us  well we tried that approach you didn't want to hear about the science when it could have made a difference  thank you so much chevron for your talk now we have some questions from the internets  see the first question which additional tipping points will be triggered at two degrees three  degrees and so on that is actually a difficult question to answer because of the uncertainty  that i mentioned in my talk about where these tipping points are there is one in Antarctica the  wilk space and that is a part of the Antarctic ice sheet that that could be triggered say below  three degrees there are others like the ocean circulation where you probably at least we hope  you have to go beyond three degrees to really trigger a collapse of the Gulfstream system  but the truth is that there are very large uncertainty ranges and the main fact is that  with every bit of extra warming we increase the risk of crossing more tipping points  and are there some of these tipping points that are interrelated or correlated for instance could  we save some tipping points if we are able to save others for instance the collapse of the Gulfstream  yes there are these interconnections for example if the Gulfstream system collapses it will affect  the atmospheric circulation the monsoon systems and can shift the tropical rainfall belts this is  not just theoretical we see that in paleo climate where we have seen these collapses of the North  Atlantic circulation and the paleo climatic proxy data show that it comes with shifts in  the tropical rainfall belts that could then in this way trigger a major drought in the amazon region  if the Gulfstream system collapses and so it would be very wise to prevent these tipping points  especially when it comes to the ocean circulation or atmospheric circulation because that's really  going to mess up the the weather patterns in a major way. How long have we known about human  caused climate change? Well in principle in the 19th century Alexander von Humboldt actually  wrote in 1843 if I remember correctly that humans are changing the climate by cutting down forests  and emitting large amounts of gases at the centers of industry that's almost a little  literal quote by Alexander von Humboldt we've known about how sensitive the climate is to a  change in CO2 since the Swedish Nobel laureates van der Arenis remotely related to Greta Thunberg  by the way in studied the effect of CO2 doubling he wasn't worried by that because he thought global  warming would be great you know bring it on let's just die no it's back you can see my picture still  yeah don't know what happened there and so he suggested you know burning a lot of coal to  enhance global warming I guess he came from Sweden and thought coal is bad without thinking this  through properly but the first real expert report warning the US government Lyndon B. Johnson  of the coming global warming due to fossil fuel use was a rebel report in 1965  exactly 50 years half a century before finally the Paris agreement was reached  will you be publishing your slides from the talk yes I will uploading the slides  what is or what should be the ultimate goal of the climate change mitigation for instance  is it saving lives saving other species  well I think the the ultimate goal is of course preserving human civilization as we know it because  I think if we let this run we will not only destroy a lot of ecosystems and biodiversity  but we will probably cause major hunger crisis which you know with big droughts like the the one  in Syria before the unrest in Syria started in 2011 the country went through the biggest  drought in history and according to sediment data from the eastern Mediterranean it was the  worst drought in at least 900 years and then you know I think especially in some unstable  conflicted countries this can really turn them into failed states that is what happened in Syria  and it's what a German report for the German government actually warned in 2009  it was called climate change as a security risk I was actually one of the co-authors of that  report because I was in the German government's advisory panel on global change at the time  and I think we will see increasing hunger crisis failed states and all the effects that that has  on international politics if we cannot keep global warming below two degrees  and finally is there a specific call to action for the chaos community is there anything that we  can do with our mindset and our skills that's a good question that I haven't thought about but  maybe you can know yourself the best thing what you can do I think the key is really to keep up  the pressure on the political world like Fridays for Future has been doing go on the streets protest  vote with climate as a priority I think these are the key things that everyone should be doing  and specifically in whatever profession they are they will see some ways of how you can help to  reduce emissions in your company put sustainability at the top of the agenda and so on  chef and thanks so much for taking the time to join us today it's a great pleasure and honor  and honor always welcome and now the news  you
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