WEBVTT 00:00:00.000 --> 00:00:13.884 rc3 prerol music 00:00:13.884 --> 00:00:18.068 Herald angel: Greeting creatures im Neuland. In 2015 governments from around 00:00:18.068 --> 00:00:22.814 the world met in Paris and agreed to attempt to limit anthropogenic climate 00:00:22.814 --> 00:00:28.336 change to well below two degrees. Unfortunately, it seems that since then we 00:00:28.336 --> 00:00:32.764 have not done enough and the climate crisis has only gotten more urgent. Our 00:00:32.764 --> 00:00:38.152 next speaker, Stefan Rahmstorf, has more accolades than I have time to tell. He's 00:00:38.152 --> 00:00:42.440 published more than 100 papers, including in the journals Nature and Science, co- 00:00:42.440 --> 00:00:46.875 authored four books and won the Climate Communication Prize from the American 00:00:46.875 --> 00:00:52.955 Geophysical Union, the first European to do so. Please welcome him. And heed his 00:00:52.955 --> 00:00:55.246 advice. Here's Stefan. 00:01:04.616 --> 00:01:10.560 Stefan Rahmstorf: Hi, everyone, my name is Stefan Rahmstorf, and I'm thrilled to be 00:01:10.560 --> 00:01:17.680 invited to give a talk at the Chaos Computer Club's remote chaos experience 00:01:17.680 --> 00:01:26.720 2020. I want to give you an overview of climate tipping points, a very exciting 00:01:26.720 --> 00:01:34.400 subject that I will try to shed some light on. But let's first start with some 00:01:34.400 --> 00:01:40.160 background on climate change. You probably know this image. It shows the global 00:01:40.160 --> 00:01:46.720 temperature evolution since the year 1880. Every line is one year. This is the more 00:01:46.720 --> 00:01:54.080 conventional way of viewing this time series. And the last seven years have been 00:01:54.080 --> 00:02:04.028 the hottest seven years since record keeping began in the 19th century. We know 00:02:04.028 --> 00:02:08.560 the reason for this warming: it's the increase of carbon dioxide, which you can 00:02:08.560 --> 00:02:13.924 see here for the last ten thousand years. And if you just look at the end of the 00:02:13.924 --> 00:02:21.602 curve, how the increase has accelerated in ever shorter time spans, we have seen an 00:02:21.602 --> 00:02:30.788 ever greater increase in the amount of carbon dioxide in our planet's atmosphere. 00:02:30.788 --> 00:02:37.822 This increase causes what we call a radiative forcing that is a kind of 00:02:37.822 --> 00:02:44.678 heating in terms of energy release per square meter of Earth's surface. And the 00:02:44.678 --> 00:02:51.295 increase in CO2 in the atmosphere until now is causing heating at a rate of two 00:02:51.295 --> 00:02:59.360 Watts per square meter surface. We understand the energy budget of our planet 00:02:59.360 --> 00:03:04.320 pretty well. On the left here in this diagram, you can see the incoming solar 00:03:04.320 --> 00:03:09.600 radiation in yellow. Part of that is reflected already in the atmosphere by the 00:03:09.600 --> 00:03:15.760 clouds, for example. Another part is reflected by the bright surfaces, that's 00:03:15.760 --> 00:03:22.960 the snow and ice surfaces primarily, and the rest is absorbed. On the right hand 00:03:22.960 --> 00:03:28.640 side, and let's zoom into that, you see in orange the long wave radiation, which is 00:03:28.640 --> 00:03:36.080 clearly distinct from the incoming short wave solar radiation by its wavelength and 00:03:36.080 --> 00:03:42.240 this thick arrow of long wave radiation leaving the Earth's surface basically to a 00:03:42.240 --> 00:03:47.600 large extent gets absorbed by the atmosphere. And the atmosphere itself 00:03:47.600 --> 00:03:53.840 emits like anything, any substance, any matter depending on its surface 00:03:53.840 --> 00:03:59.600 temperature, sorry, depending on its temperature, emits also infrared 00:03:59.600 --> 00:04:07.920 radiation. And one thing that few people realize is that the back radiation coming 00:04:07.920 --> 00:04:13.680 down from the atmosphere through the greenhouse effect, the greenhouse gases, 00:04:13.680 --> 00:04:20.240 is actually twice as large at the Earth's surface as the absorbed solar radiation. 00:04:20.240 --> 00:04:26.080 So heating by the greenhouse effect by the long wave radiation is twice as big as the 00:04:26.080 --> 00:04:32.240 absorbed solar radiation at the Earth's surface. And so it's little wonder that if 00:04:32.240 --> 00:04:38.800 we are increasing this natural greenhouse effect, which actually makes our planet 00:04:38.800 --> 00:04:43.680 livable in the first place, if we are increasing this effect that it is going to 00:04:43.680 --> 00:04:54.000 get warmer. We can also quantify this effect. And if you add in not just the CO2 00:04:54.000 --> 00:04:58.640 increase, but other human caused greenhouse gases and also cooling effects 00:04:58.640 --> 00:05:05.040 caused by humans, then you see that the total human caused warming that we see in 00:05:05.040 --> 00:05:12.880 the orange bar is to, within uncertainty, as big as the observed global warming 00:05:12.880 --> 00:05:21.440 since the 1950s. And that means that about 100% of the observed global warming over 00:05:21.440 --> 00:05:28.320 the past 70 years is human caused, and the best estimates of the human caused warming 00:05:28.320 --> 00:05:34.560 is actually even slightly more than the observed warming, which has to do partly 00:05:34.560 --> 00:05:40.800 or is consistent with the fact that solar activity has gone down. So the decrease in 00:05:40.800 --> 00:05:49.840 solar activity has compensated a small part of the human caused global warming. 00:05:49.840 --> 00:05:55.840 It's also very interesting, and especially to me as a paleoclimatologist who studies 00:05:55.840 --> 00:06:01.840 natural climate variations in Earth's history and has done so for more than 25 00:06:01.840 --> 00:06:10.720 years, how the modern warming compares with the changes throughout the Holocene, 00:06:10.720 --> 00:06:17.360 and before that, since the last Ice Age. And this is what we see here based on 00:06:17.360 --> 00:06:23.360 decades of paleoclimate research, countless sediment cores taken at the sea 00:06:23.360 --> 00:06:29.680 bottom, ice cores on the big ice sheets and so on. We have enough data now to form 00:06:29.680 --> 00:06:35.440 meaningful global average temperatures. And you can see here the warming from the 00:06:35.440 --> 00:06:40.880 height of the last ice age into the Holocene, the Holocene optimum, the 00:06:40.880 --> 00:06:46.960 warmest period about until about five thousand years before present. And since 00:06:46.960 --> 00:06:55.200 then, we have seen a very slow cooling trend, which we have bent around due to 00:06:55.200 --> 00:07:02.720 human activities. And we have within 100 years more than undone 5000 years of 00:07:02.720 --> 00:07:08.640 natural cooling trend, which normally would have very slowly continued. These 00:07:08.640 --> 00:07:14.510 natural variations, by the way, are due to the Earth orbital cycles, these so-called 00:07:14.510 --> 00:07:24.320 Milankovitch cycles. You can easily read up on those, for example, at Wikipedia. 00:07:24.320 --> 00:07:33.023 Now let's come to the famous, much feared tipping points in the climate system. What 00:07:33.023 --> 00:07:40.164 is a tipping point? That has been described in a seminal paper which I'm 00:07:40.164 --> 00:07:50.336 proud of having been a part of from 2008 by Tim Lenton and colleagues. And this is 00:07:50.336 --> 00:07:56.011 called tipping elements in the Earth's climate system. And it says that the term 00:07:56.011 --> 00:08:01.503 tipping point commonly refers to a critical threshold at which a tiny 00:08:01.503 --> 00:08:08.719 perturbation can qualitatively alter the state or development of a system and the 00:08:08.719 --> 00:08:13.702 different parts of the Earth's system, which can undergo such a transition, they 00:08:13.702 --> 00:08:21.120 are called the tipping elements. This whole concept is illustrated in the red 00:08:21.120 --> 00:08:26.366 line that's shown here: In the horizontal axis, we see a control parameter and that 00:08:26.366 --> 00:08:32.153 could be the greenhouse gas content of our atmosphere, it could be the temperature, 00:08:32.153 --> 00:08:37.565 it could be, if you talk about natural climate changes, for example, those 00:08:37.565 --> 00:08:43.692 orbital changes, the what we call the Milankovitch forcing, which drives 00:08:43.692 --> 00:08:48.886 changes. And on the vertical axis, you see the response. And if you imagine the 00:08:48.886 --> 00:08:54.840 control parameter changing from left to right in this diagram, you would march 00:08:54.840 --> 00:09:01.702 along that upper part of the red curve here, the branch, until you come close to 00:09:01.702 --> 00:09:07.278 a threshold. And at that threshold, the system will undergo a major change and 00:09:07.278 --> 00:09:17.070 reach then this lower part of the curve, a different kind of equilibrium state. So 00:09:17.070 --> 00:09:24.493 it's basically a small change in the driver causing a very big systemic 00:09:24.493 --> 00:09:31.336 response. That is what defines a tipping point. If we want to be very accurate 00:09:31.336 --> 00:09:36.422 here, we can distinguish two different types of tipping points. The first one is 00:09:36.422 --> 00:09:41.560 what I just showed you, is repeated here on the left side, and it is characterized 00:09:41.560 --> 00:09:49.210 by the fact that this red equilibrium line has one state for every point on the x 00:09:49.210 --> 00:09:55.003 axis. So every amount of forcing corresponds to one particular system 00:09:55.003 --> 00:10:02.130 state. And this is some state just makes a major transition in a smaller range of the 00:10:02.130 --> 00:10:12.310 driving parameter around this threshold. Now, a second, even more drastic or non- 00:10:12.310 --> 00:10:18.069 linear type of tipping point is shown in the right hand side, where the equilibrium 00:10:18.069 --> 00:10:24.040 states are somewhat more complex than the single red line on the left. You can see 00:10:24.040 --> 00:10:30.270 here that there is, again, an upper stable branch and there is also a lower stable 00:10:30.270 --> 00:10:37.415 branch, but they overlap. So there is a region that is shaded here where two 00:10:37.415 --> 00:10:44.955 stable equilibria exist. And it depends on the initial conditions on which of these 00:10:44.955 --> 00:10:55.474 branches you are. Now, there is what is called a bifurcation structure underlying 00:10:55.474 --> 00:11:03.403 this with a bifurcation point. There is an unstable branch which separates the basins 00:11:03.403 --> 00:11:10.050 of attraction of the two stable branches. So if you're in the bi-stable regime and 00:11:10.050 --> 00:11:15.390 you start kind of away from an equilibrium but above the dashed line, you will fall 00:11:15.390 --> 00:11:19.891 up onto that upper stable branch; if you start out below the dash line, you will 00:11:19.891 --> 00:11:25.056 fall down on the lower branch. That actually is pretty standard non-linear 00:11:25.056 --> 00:11:30.719 dynamics. It's a whole branch of physics which investigates exactly this type of 00:11:30.719 --> 00:11:37.996 behavior in many different physical systems. So the second type of tipping 00:11:37.996 --> 00:11:43.791 point, the right hand side one, is corresponding to multiple equilibrium 00:11:43.791 --> 00:11:48.302 states, in this case two stable equilibria. That's why this error range 00:11:48.302 --> 00:11:54.893 here is called bistability, two stable equilibria. It is coming with 00:11:54.893 --> 00:11:59.790 irreversibility, so basically, if you march to the right here on that upper 00:11:59.790 --> 00:12:06.150 stable branch at that bifurcation point, you fall off down onto the lower stable 00:12:06.150 --> 00:12:11.543 branch, but you can't just go back up from there. You have to go all the way to the 00:12:11.543 --> 00:12:16.702 left to that second lower blue point there until you can go back onto that stable 00:12:16.702 --> 00:12:24.529 branch. The second type is actually as an everyday system that behaves like that it 00:12:24.529 --> 00:12:29.577 can be easily compared to a kayak: if you're sitting in a kayak and you lean a 00:12:29.577 --> 00:12:35.867 little bit to one side, then you experience a counterforce. So the kayak is 00:12:35.867 --> 00:12:41.720 trying to upright itself, it's resisting you tipping it. But if you move further 00:12:41.720 --> 00:12:47.241 and further and further, eventually you will reach a tipping point. This is the 00:12:47.241 --> 00:12:53.820 point where the kayak stops resisting your further leaning over and instead it starts 00:12:53.820 --> 00:12:59.991 tipping over further by itself and then it flips right over until it's upside down 00:12:59.991 --> 00:13:05.941 and you're falling out. So I have I have done this quite a few times. So I have a 00:13:05.941 --> 00:13:11.262 kayak that is quite narrow where it easily happens if you don't take care, that you 00:13:11.262 --> 00:13:18.232 flip over. Now, this kayak also has a range of bistability, so once it's flipped 00:13:18.232 --> 00:13:23.326 over, it's also in a stable state and it takes considerable effort to turn it 00:13:23.326 --> 00:13:28.935 upright again into the other stable state when it's vertical, upright rather than 00:13:28.935 --> 00:13:37.168 upside down. Now, the whole point is that systems like this exist also in the 00:13:37.168 --> 00:13:45.120 climate system. The kind of first type on the left hand side corresponds, for 00:13:45.120 --> 00:13:51.920 example, to sea ice and on the right hand side this type of tipping element compares 00:13:51.920 --> 00:13:58.640 to refers to the Greenland ice sheet or continental ice sheets, also Antarctica or 00:13:58.640 --> 00:14:06.640 the Atlantic Ocean circulation. In terms of the trends in behavior, and that means 00:14:06.640 --> 00:14:12.160 if you if you kind of go through a global warming phase, you're moving from left to 00:14:12.160 --> 00:14:19.440 right in these diagrams, then in that sense, they don't differ very much because 00:14:19.440 --> 00:14:26.000 in either case, you follow a line like this green line. So on the left hand side, 00:14:26.000 --> 00:14:32.400 the green line more or less follows more or less closely the red equilibrium line 00:14:32.400 --> 00:14:37.280 with a certain delay, depending on how sluggish the system responds. So that's 00:14:37.280 --> 00:14:44.880 why the green arrows are not exactly on top of the red line here. And in the 00:14:44.880 --> 00:14:50.560 right hand side case, you have a similar thing. You are kind of, in theory, in 00:14:50.560 --> 00:14:56.320 equilibrium, you would fall off the cliff at this bifurcation point. But in praxis, 00:14:56.320 --> 00:15:01.440 the system has some inertia, it takes some time. So if you gradually move on the 00:15:01.440 --> 00:15:05.920 right towards the right there, you will also follow a green line, which is very 00:15:05.920 --> 00:15:10.960 similar to the one in the left. So in practical terms, if you're not trying to 00:15:10.960 --> 00:15:16.160 go back, but you just going forward, progressive global warming, the difference 00:15:16.160 --> 00:15:20.960 isn't all that big. And the main difference comes from the intrinsic 00:15:20.960 --> 00:15:27.200 timescale of the system. Obviously, sea ice can respond much more quickly to being 00:15:27.200 --> 00:15:32.320 just a few meters thick compared to continental ice sheet like Greenland ice, 00:15:32.320 --> 00:15:36.720 which is about three thousand meters thick. And that just takes a very long 00:15:36.720 --> 00:15:45.466 time to melt. Now, here's an overview of different tipping elements in the climate 00:15:45.466 --> 00:15:55.840 system. A few examples you can see starting on the left here, the boreal 00:15:55.840 --> 00:16:02.080 forest, that are the kind of northern forests, which typically, like ecosystems, 00:16:02.080 --> 00:16:10.480 do have a tipping point, a point of collapse. The whole idea of these tipping 00:16:10.480 --> 00:16:15.360 points and system collapse is very strongly linked actually to ecosystem 00:16:15.360 --> 00:16:21.280 research and the boreal forests, They have a point where they get too dry, that fires 00:16:21.280 --> 00:16:27.040 and pests are weakening the forest so much that in a hot summer like last year in 00:16:27.040 --> 00:16:34.240 Siberia, they go up in flames lit by lightning. Or the Amazon rain forest. This 00:16:34.240 --> 00:16:43.760 is also a tipping element, has been shown in many vegetation dynamics models, which 00:16:43.760 --> 00:16:51.120 is partly linked to the fact that such a forest generates its own rain to an extent 00:16:51.120 --> 00:16:56.480 by storing water in the soil, keeping it there and then bringing it up again 00:16:56.480 --> 00:17:02.640 through evapotranspiration, as we call it, the tree brings up water to the leaves 00:17:02.640 --> 00:17:08.560 then into the atmosphere again, and then it moves with the winds and maybe 50, 100 00:17:08.560 --> 00:17:14.720 kilometers downwind, it falls again as rain. So it's a kind of perpetual rain 00:17:14.720 --> 00:17:20.880 recycling system which keeps the whole forest nice and moist. But if you stress 00:17:20.880 --> 00:17:27.680 that too far and reduce the first of all, you cut down forests, you make it smaller, 00:17:27.680 --> 00:17:34.880 and also you make it more drought prone by warming up the climate, which leads to 00:17:34.880 --> 00:17:40.160 faster loss of moisture, etc. greater moisture requirements by the trees. Then 00:17:40.160 --> 00:17:45.000 you can stress it up to the point where it gets so dry that even the Amazon rain 00:17:45.000 --> 00:17:53.440 forest can go up in flames. Another example of how you see the top right is 00:17:53.440 --> 00:17:59.120 the permafrost thawing. This is when it gets too warm. There is a very simple 00:17:59.120 --> 00:18:02.960 threshold, namely the freezing point. Of course, that is a tipping point in the 00:18:02.960 --> 00:18:11.040 sense of freezing point of water. When the permafrost thaws, then there is methane 00:18:11.040 --> 00:18:19.360 gas escaping to the atmosphere, which then also can enhance the further warming, 00:18:19.360 --> 00:18:25.200 which then leads to more permafrost thawing and so on. Typically, these 00:18:25.200 --> 00:18:34.640 tipping points are associated with such amplifying feedbacks. I will discuss three 00:18:34.640 --> 00:18:41.040 of these in a little bit more detail. The Greenland ice sheet, which is undergoing 00:18:41.040 --> 00:18:47.840 accelerated ice loss, the Atlantic overturning circulation, often called Gulf 00:18:47.840 --> 00:18:54.480 Stream system. And the third one is the coral reefs, which are suffering from 00:18:54.480 --> 00:18:59.520 large scale die-off, which also as a typical ecosystem response, have a 00:18:59.520 --> 00:19:06.880 critical threshold. These examples are discussed in our paper 'Climate tipping 00:19:06.880 --> 00:19:12.720 points - too risky to bet against' which we published in Nature about one year ago. 00:19:12.720 --> 00:19:20.080 And they are also some of these tipping points interact, they are interlinked. And 00:19:20.080 --> 00:19:24.960 one of our quotes there is that the clearest emergency would be if we were 00:19:24.960 --> 00:19:30.080 approaching a global cascade of tipping points. That is a situation where one 00:19:30.080 --> 00:19:36.400 tipping element is triggering the next one in a kind of domino effect. This is what 00:19:36.400 --> 00:19:44.960 we fear most. Now, let's have a look at the Greenland ice sheet. This is a NASA 00:19:44.960 --> 00:19:50.480 video showing based on GRACE satellite data where the ice sheet is losing mass. 00:19:50.480 --> 00:19:56.800 You can see increasing blue colors here that the Greenland ice sheet is indeed 00:19:56.800 --> 00:20:03.280 losing mass. You can look up at the NASA Vital Signs website, which has very good 00:20:03.280 --> 00:20:09.200 indicators of various vital signs of our planet, including the data on Greenland 00:20:09.200 --> 00:20:17.760 ice loss, constantly updated. Now, the point with the Greenland ice sheet is that 00:20:17.760 --> 00:20:24.000 it does have a stability diagram like the schematic one that I showed you earlier 00:20:24.000 --> 00:20:29.920 with the bi-stable range. And this is shown, I think it was shown for the first 00:20:29.920 --> 00:20:38.720 time by my colleagues, Calov and Ganopolski in 2005 in this article where 00:20:38.720 --> 00:20:43.440 they used the three dimensional ice sheet model coupled inside a global climate 00:20:43.440 --> 00:20:49.840 model with ocean atmosphere and so on and on the x axis is basically increasing 00:20:49.840 --> 00:20:54.240 amount of heating going on, in this case because they were interested in the 00:20:54.240 --> 00:21:01.360 paleoclimate question, it is this driving force by the orbital cycles and 00:21:01.360 --> 00:21:06.640 Milankovitch cycles. You don't need to understand the numbers, but on the 00:21:06.640 --> 00:21:11.600 vertical axis, you see the response of the ice sheet, the size of the ice sheet, in 00:21:11.600 --> 00:21:17.840 million cubic kilometers. And you can see that upper branch in the blue line, we're 00:21:17.840 --> 00:21:24.080 actually moving towards the right here in this model simulation experiment. And you 00:21:24.080 --> 00:21:28.720 can see you stay on that upper branch until you reach this value on the x axis 00:21:28.720 --> 00:21:35.760 of around about five hundred. And this is where the tipping point is. There the ice 00:21:35.760 --> 00:21:41.280 mass declines, melts away, away very quickly. And you then end up at that lower 00:21:41.280 --> 00:21:47.120 branch with no ice on Greenland. And they played this game. They ran the simulation 00:21:47.120 --> 00:21:52.640 out to more than 550 watts per square meter. And the light blue line is what 00:21:52.640 --> 00:21:58.960 happens when they return, when they turn down the heat again. You move towards the 00:21:58.960 --> 00:22:03.600 left on this diagram, but you don't go back up the same way as the dark blue 00:22:03.600 --> 00:22:09.360 line. You have to go to much lower radiation values until the ice sheet 00:22:09.360 --> 00:22:15.520 starts to grow again and comes back. The dots, by the way, are points where this 00:22:15.520 --> 00:22:21.600 has to has been run for many thousands of years really into an equilibrium just to 00:22:21.600 --> 00:22:27.520 show that there are really for the same value on the x axis, two very different 00:22:27.520 --> 00:22:34.960 equilibrium states with and without Greenland ice sheet. And the fact that we 00:22:34.960 --> 00:22:39.600 now and in the Holocene in the last ten thousand years have the Greenland ice 00:22:39.600 --> 00:22:45.280 sheet and it actually is stable in the Holocene climate is only because of the 00:22:45.280 --> 00:22:50.400 initial condition, because we came out of an ice age. If you took away the Greenland 00:22:50.400 --> 00:22:55.920 ice sheet now, then in the current climate or the Holocene or pre-industrial climate, 00:22:55.920 --> 00:23:01.680 it would never grow back. What is the positive feedback? The most positive? We 00:23:01.680 --> 00:23:06.560 don't mean that it's good. That's actually quite bad and positive feedback. We mean 00:23:06.560 --> 00:23:12.000 and amplifying feedback and the key amplifying feedback here is what is called 00:23:12.000 --> 00:23:18.480 the ice elevation feedback. The Greenland ice sheet does not melt because it's very 00:23:18.480 --> 00:23:24.640 cold at the surface, mostly below freezing. And why is it so cold? Because 00:23:24.640 --> 00:23:28.800 it is very high up in the atmosphere, this ice sheet of three thousand meters thick 00:23:28.800 --> 00:23:35.440 after all. So it's like in a high mountain area where it is quite cold. If you took 00:23:35.440 --> 00:23:41.040 away that ice sheet, though, the surface then would be down at sea level or even 00:23:41.040 --> 00:23:46.000 below if you did this quickly because the the bedrock is depressed, but the surface 00:23:46.000 --> 00:23:51.440 would come up to sea level, but down there it's much warmer than up at three thousand 00:23:51.440 --> 00:23:57.120 meters altitude in the atmosphere. And there it is actually too warm to keep any 00:23:57.120 --> 00:24:02.640 snow on the ground year round, which would be required to regrow a new Greenland ice 00:24:02.640 --> 00:24:06.800 sheet. And that's why you'd have to go back to a much colder climate than the 00:24:06.800 --> 00:24:12.560 Holocene to get the Greenland ice sheet back once it were lost. This is a typical 00:24:12.560 --> 00:24:18.400 example of this amplifying feedback, which leads to a self stabilizing system. It can 00:24:18.400 --> 00:24:23.600 either self stabilize in the upper branch here when you start there or it self- 00:24:23.600 --> 00:24:28.720 stabilizes in the lower branch with no ice when you start there. This is what makes 00:24:28.720 --> 00:24:37.360 it a bi-stable system. To summarize, the Greenland ice sheet is melting as another 00:24:37.360 --> 00:24:43.200 data the great satellites show, but also other data sets. It has a tipping point 00:24:43.200 --> 00:24:48.800 due to the ice elevation feedback. What I haven't shown, but it's come out in study 00:24:48.800 --> 00:24:55.600 with many climate models, simulation experiments going through more than two 00:24:55.600 --> 00:25:00.480 hundred thousand years of simulations from the past through the Eemian interglacial 00:25:00.480 --> 00:25:05.600 period where we know how much the ice sheets shrank back. And we could use those 00:25:05.600 --> 00:25:13.440 data from the past behavior of Greenland to calibrate the model. And so we know the 00:25:13.440 --> 00:25:17.920 tipping point for the complete loss of the Greenland ice sheet is somewhere between 00:25:17.920 --> 00:25:22.556 one degree and three degree global warming. We're already at one point two 00:25:22.556 --> 00:25:27.507 degrees global warming. So we have started to enter the danger zone where we crossed 00:25:27.507 --> 00:25:34.271 that tipping point. It doesn't mean that it suddenly starts to melt very fast also 00:25:34.271 --> 00:25:39.235 because it has its own intrinsic slow response time. But what that crossing, 00:25:39.235 --> 00:25:44.187 that tipping point means is that even without further warming, the Greenland ice 00:25:44.187 --> 00:25:49.120 sheet is doomed and will continue to melt until it's gone, and this will lead to 00:25:49.120 --> 00:25:56.171 seven meters of global sea level rise, drowning most of our big coastal cities 00:25:56.171 --> 00:26:04.047 and to many island nations. Here is a look at the future from models, simulations 00:26:04.047 --> 00:26:11.513 from Ashmont and from NASA. And you can see a nice view of what the surface looks 00:26:11.513 --> 00:26:15.088 like. And here's what the what it looks like in the ice sheet model. You can see 00:26:15.088 --> 00:26:20.452 the ice flowing. You can see it retreating. So in purple, that's bedrock 00:26:20.452 --> 00:26:27.000 that is exposed where the ice sheet has withdrawn in this simulation. And so it's 00:26:27.000 --> 00:26:32.686 as much as ice of ice that you would lose in the coming three hundred years, a 00:26:32.686 --> 00:26:43.180 substantial fraction of the Greenland ice sheet. Now, let's look at another kind of 00:26:43.180 --> 00:26:46.850 tipping element, and that is the Gulf Stream system or the North Atlantic 00:26:46.850 --> 00:26:52.184 current. And I can't really introduce this topic is one of my favorite topics, which 00:26:52.184 --> 00:26:59.323 I have studied since the early 90s, without showing a clip from the famous 00:26:59.323 --> 00:27:05.217 Hollywood blockbuster The Day After Tomorrow. What about the North Atlantic 00:27:05.217 --> 00:27:10.680 current? What about it? The current depends upon a delicate balance of salt 00:27:10.680 --> 00:27:16.100 and fresh water. We all know that, yes. But no one is taking into account how much 00:27:16.100 --> 00:27:21.140 fresh water has been dumped into the ocean because of melting polar ice. I think 00:27:21.140 --> 00:27:32.100 we've hit a critical desalinization point. Yeah, now that statement about the 00:27:32.100 --> 00:27:37.027 critical desalination point is a completely correct description of the 00:27:37.027 --> 00:27:41.915 bifurcation point of the Atlantic circulation, I'll show it in a minute. And 00:27:41.915 --> 00:27:48.910 the statement that nobody has taken into account the meltwater from the Greenland 00:27:48.910 --> 00:27:55.922 ice sheet is also was completely correct when the movie appeared in 2004. Until 00:27:55.922 --> 00:28:01.670 then, the typical climate simulations that you could see in the IPCC reports, 00:28:01.670 --> 00:28:07.266 actually until quite a few years later, still had not taken account Greenland melt 00:28:07.266 --> 00:28:12.614 water because basically at that point in time, the models, almost all climate 00:28:12.614 --> 00:28:18.032 models were just ocean-atmosphere models plus land surface, but they didn't have 00:28:18.032 --> 00:28:24.821 continental ice sheet models coupled into them. And so in the meantime, of course, 00:28:24.821 --> 00:28:30.160 we have better models that include experiments either with artificially added 00:28:30.160 --> 00:28:36.838 Greenland meltwater from data estimates or fully coupled with ice sheet models. And 00:28:36.838 --> 00:28:41.168 from that, an example here being that nature article by Claus Boening and 00:28:41.168 --> 00:28:48.500 colleagues. We know that the meltwater input from Greenland has a non-negligible 00:28:48.500 --> 00:28:52.992 effect on the North Atlantic overturning. It's probably not the dominant effect, but 00:28:52.992 --> 00:29:00.900 it adds to various factors that weaken this North Atlantic current. And we also 00:29:00.900 --> 00:29:07.020 know that this system has a well-defined tipping point. Actually, I described that 00:29:07.020 --> 00:29:17.440 in a nature article in 1996 due to a salt transport feedback. The basic idea behind 00:29:17.440 --> 00:29:25.054 that has actually been known since the late 1950s or early 60s since work by the 00:29:25.054 --> 00:29:29.960 famous American oceanographer Henry Stommel. But what I showed in my Nature 00:29:29.960 --> 00:29:35.750 article in 96 is that it actually works that way in a complex, three dimensional 00:29:35.750 --> 00:29:43.045 global ocean circulation model, not just in very simplified models. And since then, 00:29:43.045 --> 00:29:49.775 this has been shown for a whole range of different climate models. The sole 00:29:49.775 --> 00:29:54.578 transportation feedback is also one of these amplifying feedbacks, and it's easy 00:29:54.578 --> 00:30:01.744 to explain. The overturning circulation of the Atlantic is called overturning because 00:30:01.744 --> 00:30:08.772 it's really a vertical overturning where water sinks down from the surface to great 00:30:08.772 --> 00:30:16.192 depth of two to three kilometers in the Atlantic because this water is heavy and 00:30:16.192 --> 00:30:21.177 it spreads thin in the deep ocean until it rises up in other parts, mainly around 00:30:21.177 --> 00:30:29.483 Antarctica in the Antarctic circumpolar current area and comes back at the 00:30:29.483 --> 00:30:35.733 surface. So basically the whole ocean is overturned with deep water being renewed 00:30:35.733 --> 00:30:45.008 and then coming back to the surface on very long timescale of about 1000 to 2000 00:30:45.008 --> 00:30:53.606 years for complete overturning there. Now, the whole system is driven by the fact 00:30:53.606 --> 00:31:00.985 that the water sinks down where it has the highest density, and that's in the 00:31:00.985 --> 00:31:07.376 northern Atlantic and around Antarctica, around the Antarctic continent. And it has 00:31:07.376 --> 00:31:12.360 the highest density there, not only because it's very cold, but also quite 00:31:12.360 --> 00:31:17.927 salty. This is why you don't have deep water formation in the North Pacific, in 00:31:17.927 --> 00:31:22.648 the Northern Hemisphere. You only have that in the North Atlantic. And that's 00:31:22.648 --> 00:31:28.784 because the North Atlantic waters are quite salty. And this is because this 00:31:28.784 --> 00:31:34.311 North Atlantic current exists and brings salty water from the subtropics up to the 00:31:34.311 --> 00:31:39.052 high latitudes, where normally it isn't very salty because it gets diluted by 00:31:39.052 --> 00:31:44.158 excess rainfall, whereas the subtropics have excess evaporation and that's why 00:31:44.158 --> 00:31:50.486 they're salty. And so it's like a chicken and an egg situation. The Northern 00:31:50.486 --> 00:31:55.049 Atlantic is salty because you have this overturning circulation and you have this 00:31:55.049 --> 00:32:00.055 overturning circulation because it's salty there. And so you can see the self 00:32:00.055 --> 00:32:06.419 amplifying feedback there again, which means it is a self stabilizing system up 00:32:06.419 --> 00:32:11.654 to a certain breaking point, a tipping point which can be reached if you add too 00:32:11.654 --> 00:32:19.338 much fresh water, diluting the northern Atlantic. And the stability diagram, 00:32:19.338 --> 00:32:24.564 again, looks like that second one. You've seen it for the Greenland ice sheet. As I 00:32:24.564 --> 00:32:29.618 said, this has been verified in a detailed model simulations with many different 00:32:29.618 --> 00:32:36.021 models that it really works like that in a complex 3D situation where you have 00:32:36.021 --> 00:32:40.630 depending on how much fresh water you add into the northern Atlantic, this is the 00:32:40.630 --> 00:32:46.846 control parameter here, you can move along that upper stable branch with the 00:32:46.846 --> 00:32:54.341 overturning circulation until that Stommel bifurcation point. And there this 00:32:54.341 --> 00:33:00.282 overturning breaks down and you fall down onto that lower branch without this 00:33:00.282 --> 00:33:06.698 overturning. It's labeled here NADW Flow that NADW stands for north Atlantic 00:33:06.698 --> 00:33:11.402 deepwater. It's a, I would say, one of the favorite water masses of the 00:33:11.402 --> 00:33:21.280 oceanographers. Now, let's look at the Gulf Stream, the surface circulation in a 00:33:21.280 --> 00:33:28.320 climate model. This is the CM 2.6 global coupled climate model ocean atmosphere by 00:33:28.320 --> 00:33:34.320 the Geophysical Fluid Dynamics Laboratory in Princeton. You can beautifully see the 00:33:34.320 --> 00:33:40.320 Gulf Stream and dark red here because it's warm leaving the coast of the United 00:33:40.320 --> 00:33:45.410 States at Cape Hatteras there, starting to meander, breaking up into these eddies, et 00:33:45.410 --> 00:33:52.880 cetera. And it actually meets the cold waters coming down inshore from the north, 00:33:52.880 --> 00:33:59.040 which are shown in blue here. And so this is what this the surface part of the 00:33:59.040 --> 00:34:07.360 circulation looks in a global climate model. And if you add carbon dioxide to 00:34:07.360 --> 00:34:13.520 that climate models atmosphere, the climate warms, of course, but it does show 00:34:13.520 --> 00:34:20.000 a peculiar pattern of sea surface temperature change, which you see here. 00:34:20.000 --> 00:34:24.800 And this actually shows the sea surface temperature change relative to the global 00:34:24.800 --> 00:34:29.520 mean. So everything that is blue has either warmed less than the global average 00:34:29.520 --> 00:34:34.640 or even cooled, which is actually the case south of Greenland. And everything that is 00:34:34.640 --> 00:34:42.720 orange or red has warmed substantially more than the global average sea surface. 00:34:42.720 --> 00:34:50.080 And you see a very strong pattern in the northern Atlantic with this big cold blob, 00:34:50.080 --> 00:34:55.600 the blue blob south of Greenland and a very warm region inshore of the Gulf 00:34:55.600 --> 00:35:02.480 Stream along the coast of North America. And in the climate model, of course, we 00:35:02.480 --> 00:35:07.440 are a bit like gods in that sense that we have complete information about what's 00:35:07.440 --> 00:35:12.720 going on there. If we store all the data at every grid point, we know exactly everything 00:35:12.720 --> 00:35:18.160 that's happening and we can analyze the reasons. And the reason for this funny 00:35:18.160 --> 00:35:24.960 pattern in the northern Atlantic actually is a slowdown of the North Atlantic 00:35:24.960 --> 00:35:34.960 overturning circulation. That means that less heat is transported to the subpolar 00:35:34.960 --> 00:35:41.440 ocean south of Greenland there. That blue area, which makes it cool down and the 00:35:41.440 --> 00:35:46.960 Gulf Stream proper at the surface, moves inshore there is complicated dynamical 00:35:46.960 --> 00:35:52.880 reasons for that. But there is already long before this was shown in this model, 00:35:52.880 --> 00:35:59.360 a theoretical underpinning for this. It has to do with the vorticity dynamics on a 00:35:59.360 --> 00:36:04.880 rotating sphere too technical to go into in such a talk. But it's a well understood 00:36:04.880 --> 00:36:10.560 phenomenon. And so we know that this slowdown of the Gulf Stream system is the 00:36:10.560 --> 00:36:15.760 reason behind this peculiar temperature pattern. And this pattern is predicted by 00:36:15.760 --> 00:36:22.640 this climate model for a global warming situation. And my PhD student, Levke 00:36:22.640 --> 00:36:28.880 Caesar, who was the first author on this nature paper from 2018, she looked at all 00:36:28.880 --> 00:36:34.320 the available measurements of sea surface temperatures since the beginning of the 00:36:34.320 --> 00:36:39.760 20th century. And of course, because we have only limited ocean temperature 00:36:39.760 --> 00:36:43.840 measurements, we have only a fuzzy picture here, not a sharp one like in the climate 00:36:43.840 --> 00:36:50.560 model. But you can see a similar pattern in the North Atlantic in the observations 00:36:50.560 --> 00:36:55.680 compared to what the model predicts in response to a slowdown of the overturning 00:36:55.680 --> 00:37:01.040 circulation. And our conclusion here is that we are actually observing this 00:37:01.040 --> 00:37:07.040 slowdown of the circulation. Why do we take indirect evidence for this like this? 00:37:07.040 --> 00:37:13.680 Because we don't, of course, have measurements going back 100 years or more 00:37:13.680 --> 00:37:18.800 about the strength of that overturning circulation. We have actually only started 00:37:18.800 --> 00:37:24.720 to measure this regularly in 2004 with a so-called rapid array, At twenty six 00:37:24.720 --> 00:37:31.600 degrees north in the Atlantic, and what we reconstructed about the evolution of this 00:37:31.600 --> 00:37:37.520 current for the last period where we do have the direct measurements, agrees well 00:37:37.520 --> 00:37:46.480 with what the direct measurements show. We concluded that the overturning circulation 00:37:46.480 --> 00:37:56.202 has declined since at least the mid 20th century by about 15% so far. There are, of 00:37:56.202 --> 00:38:01.220 course, other indirect types of measurements. You can use sediment data of 00:38:01.220 --> 00:38:05.560 various kinds and with various methodologies to reconstruct the strength 00:38:05.560 --> 00:38:11.316 of this Atlantic overturning and a number of different studies compiled here in this 00:38:11.316 --> 00:38:17.362 diagram. And even though, of course, they differ somewhat in the detail, they all 00:38:17.362 --> 00:38:23.006 tend to agree in this overall picture that the Atlantic overturning circulation has 00:38:23.006 --> 00:38:28.815 been quite stable for the previous thousand years or so before the 20th 00:38:28.815 --> 00:38:37.109 century, but then in the 20th century has showed a clear declining signature. And 00:38:37.109 --> 00:38:43.078 one example of the media coverage of this is that Washington Post article here, 00:38:43.078 --> 00:38:48.047 which if you can see the small print of the most read articles there on that, they 00:38:48.047 --> 00:38:53.195 actually made it to number three or the most read Washington Post articles. There 00:38:53.195 --> 00:38:59.650 is definitely an interest in science and climate change science by the readers in 00:38:59.650 --> 00:39:09.920 the newspapers. So far we've talked about a slow down and not so much about where 00:39:09.920 --> 00:39:13.920 this tipping point is. One reason is we don't know really. We know there is this 00:39:13.920 --> 00:39:18.000 tipping point, that is a robust result of many different studies and model 00:39:18.000 --> 00:39:23.760 experiments and theory, but we don't know how far away we are from this. That is 00:39:23.760 --> 00:39:28.000 very typical for these tipping points because they involve highly nonlinear 00:39:28.000 --> 00:39:34.320 dynamics. That means they can depend very sensitively on the exact conditions, for 00:39:34.320 --> 00:39:39.920 example, in this case, the exact salinity distribution in the Atlantic and the exact 00:39:39.920 --> 00:39:46.400 circulation pattern. And models get these things kind of approximately right, but 00:39:46.400 --> 00:39:52.560 not exactly right. And if you have a situation where the question of where the 00:39:52.560 --> 00:39:57.040 tipping point is is very sensitive to the exact conditions, then you have a large 00:39:57.040 --> 00:40:03.280 uncertainty about where the tipping point is. And so there is discussion in the 00:40:03.280 --> 00:40:08.480 literature. I just point out to one study here in science advances that try to 00:40:08.480 --> 00:40:16.960 correct for the inaccuracies in how we can reproduce the salinity in the Atlantic 00:40:16.960 --> 00:40:22.080 waters and found that if you correct for that, the circulation is actually a lot 00:40:22.080 --> 00:40:28.480 more sensitive than in other models. And maybe that model is more correct. And of 00:40:28.480 --> 00:40:32.400 course, it has other weaknesses as well. We don't know which of the models is 00:40:32.400 --> 00:40:39.840 correct, but should we cross this tipping point then the North Atlantic circulation 00:40:39.840 --> 00:40:44.160 system would break down and you get a temperature pattern like the one shown 00:40:44.160 --> 00:40:49.200 here, the cold blob in the Atlantic that is now only over the ocean. It exists, 00:40:49.200 --> 00:40:53.040 right? It's the only part of the world that has cooled since the beginning of the 00:40:53.040 --> 00:40:58.480 20th century, but it hasn't affected any land areas. But if the circulation would 00:40:58.480 --> 00:41:03.920 break down altogether and not only weakened by 15%, this cold would expand 00:41:03.920 --> 00:41:09.760 greatly and affect Great Britain, Scandinavia, Iceland, as you can see here, 00:41:09.760 --> 00:41:13.760 which would then get a much colder climate, whereas the rest of the globe 00:41:13.760 --> 00:41:19.600 continues to have a warmer climate. This is really distinct from an ice age. And so 00:41:19.600 --> 00:41:23.600 this is also really distinct from that Hollywood movie The Day After Tomorrow, 00:41:23.600 --> 00:41:29.200 where the earth goes into a huge ice age, an instant freeze. That, of course, is 00:41:29.200 --> 00:41:33.520 totally unrealistic. And the the screenwriter and the director, they knew 00:41:33.520 --> 00:41:40.720 this. They actually told me that if they were in the business of making a movie for 00:41:40.720 --> 00:41:45.520 a few million viewers, they would stick to the laws of physics. But since they make 00:41:45.520 --> 00:41:50.720 movies for a few hundred million viewers, they stick to the laws of Hollywood drama. 00:41:50.720 --> 00:41:56.880 But you would get a substantial regional cooling with a major impact on ecosystems, 00:41:56.880 --> 00:42:03.920 on human society. Now, let me come to the third type of tipping point that I want to 00:42:03.920 --> 00:42:09.600 discuss today. This is the coral reefs. Coral reefs, like many ecosystems, do have 00:42:09.600 --> 00:42:17.040 critical thresholds. Coral reefs are very important, even though they only cover a 00:42:17.040 --> 00:42:23.539 very small percentage of the Earth's surface, they support a quarter of all 00:42:23.539 --> 00:42:31.597 marine life. 40% coral cover of the world has already been lost, 100 countries 00:42:31.597 --> 00:42:38.528 depend quite substantially on corals. There's 800 billion total global assets of 00:42:38.528 --> 00:42:45.557 coral reefs. So it does have a major impact on people. Now, corals, when they 00:42:45.557 --> 00:42:52.739 are about to die, they bleach. They are abandoned by their algae that provides 00:42:52.739 --> 00:43:00.092 them with nutrition and that's why they lose their color. And then after a while, 00:43:00.092 --> 00:43:07.167 they die. They get covered by other by seaweed, non symbiotic algae, and they 00:43:07.167 --> 00:43:13.351 die. And they do have a temperature threshold. It's a critical warming 00:43:13.351 --> 00:43:19.594 threshold where this bleaching happens. But an additional factor, not yet the most 00:43:19.594 --> 00:43:25.474 important factor, is the acidification of water. It's a direct chemical effect of 00:43:25.474 --> 00:43:32.287 adding carbon dioxide to the atmosphere, which then goes partly into the oceans and 00:43:32.287 --> 00:43:39.354 acidifies the ocean waters. But the main effect until now is the marine heatwaves, 00:43:39.354 --> 00:43:44.840 which cross more and more frequently the temperature tolerance threshold of coral 00:43:44.840 --> 00:43:50.093 reefs. And here you can see that for the Great Barrier Reef, a huge, fantastic 00:43:50.093 --> 00:43:55.973 world wonder that you can see from space. And you can see here the bleaching in the 00:43:55.973 --> 00:44:05.576 year 2016, 2017, 2020, three major bleaching events which affect it in each 00:44:05.576 --> 00:44:11.081 case, the red area here with the most severe bleaching, you can see that by now 00:44:11.081 --> 00:44:17.359 a very large part of the Great Barrier Reef has bleached in these three events. 00:44:17.359 --> 00:44:23.892 And it's very tragic. And you can see here, for example, the March, the 2016 00:44:23.892 --> 00:44:33.041 bleaching event in March, the coral was bleached. By May, it was already overgrown by seaweed. 00:44:33.041 --> 00:44:40.208 And just in 2015 and 2016, we actually had worldwide coral reef bleaching, not only 00:44:40.208 --> 00:44:45.620 at the Great Barrier Reef in Australia, only the blue ones out of these hundred 00:44:45.620 --> 00:44:53.935 reefs that were observed in this study, only the blue ones escaped bleaching. So 00:44:53.935 --> 00:45:02.252 we are actually in the midst of a great worldwide coral die off event, which is 00:45:02.252 --> 00:45:08.022 another prediction of climate science coming true. If you look at the latest 00:45:08.022 --> 00:45:13.925 IPCC report, it states that with two degrees warming, virtually all coral reefs 00:45:13.925 --> 00:45:19.826 will be lost, more than 99%. One point five degree warming. If we manage to limit 00:45:19.826 --> 00:45:26.215 the warming to one point five degrees, we can save between 10% and 30% of the 00:45:26.215 --> 00:45:36.758 corals. That is really depressing. Now, let me talk briefly about what can we do. 00:45:36.758 --> 00:45:42.311 A major success is, of course, the Paris accord, the biggest failure of which is 00:45:42.311 --> 00:45:47.544 that it hasn't come 20 years earlier. After all, the world community already in 00:45:47.544 --> 00:45:54.652 1992 decided to stop global warming at the Rio Earth Summit. The nations signed the 00:45:54.652 --> 00:46:01.818 United Nations Framework Convention on Climate Change, and it took a full 25 00:46:01.818 --> 00:46:12.237 years of further negotiations to finally reach the Paris accord. Now, you can see 00:46:12.237 --> 00:46:17.190 here that the goal of this is to hold the increase in the global average temperature 00:46:17.190 --> 00:46:21.683 to well below two degrees above pre- industrial level. So it's not two degrees, 00:46:21.683 --> 00:46:26.467 it's well below two degrees. That's a very important point. Many countries would not 00:46:26.467 --> 00:46:32.915 have signed up if it simply had said two degrees, which was an older goal, but it 00:46:32.915 --> 00:46:45.520 has shown to be insufficient and. And to sorry and to pursue efforts to limit the 00:46:45.520 --> 00:46:50.640 temperature increase to one point five degrees above pre-industrial levels. So 00:46:50.640 --> 00:46:56.080 that is a more stringent Paris goal, but at least the nations have committed to 00:46:56.080 --> 00:47:03.200 pursue efforts. So my view is that every person should ask their own government 00:47:03.200 --> 00:47:08.400 what you are doing here. Is this a credible effort to try and limit warming 00:47:08.400 --> 00:47:13.840 to one point five degrees? We might not make it, but at least we should try to 00:47:13.840 --> 00:47:18.560 limit the warming to one point five to avoid the risk of destabilization of the 00:47:18.560 --> 00:47:26.080 Greenland ice sheet, almost complete coral die off and many further risks. So what 00:47:26.080 --> 00:47:32.960 does this entail? That is an important point. If you want to limit global warming 00:47:32.960 --> 00:47:38.880 to some value, whatever it is, one point five, two, three, whatever you choose, it 00:47:38.880 --> 00:47:45.040 means you can only emit a limited amount of carbon dioxide. That is because the 00:47:45.040 --> 00:47:53.120 amount of global warming is to a good extent proportional to the total amount of 00:47:53.120 --> 00:47:59.200 CO2 that we have ever emitted. So to the cumulative emissions, it's like filling a 00:47:59.200 --> 00:48:05.040 bathtub with water. If you want to draw the line at any level and say no further 00:48:05.040 --> 00:48:10.720 than here, you can only add a limited amount of water. And if you want to limit 00:48:10.720 --> 00:48:15.840 global warming to some value, you can only add a limited amount of CO2 to the 00:48:15.840 --> 00:48:23.360 atmosphere. And this is shown here for two different examples, two different amounts. 00:48:23.360 --> 00:48:30.560 This is actually, the numbers here are emissions from the year 2016. So it's 00:48:30.560 --> 00:48:39.440 don't take these numbers from now. We have already had four more years of emissions. 00:48:39.440 --> 00:48:50.480 The solid lines throw show three scenarios with six hundred billion tons of CO2 and 00:48:50.480 --> 00:48:56.160 they all have the same amount of emission. So they're all three solid lines, get the 00:48:56.160 --> 00:49:01.280 same amount of warming. This is about actually these lines correspond to about a 00:49:01.280 --> 00:49:08.160 50 percent chance of ending up at one point five degrees. And so they will get 00:49:08.160 --> 00:49:16.160 you the same amount of warming, but with different times of when the peak emissions 00:49:16.160 --> 00:49:23.440 are reached. So 2016 went past without us getting over the peak of the emissions. 00:49:23.440 --> 00:49:29.520 2020, maybe we still have a chance. Emissions have dropped a bit in 2020, but 00:49:29.520 --> 00:49:34.240 not for structural change and mostly, but due to Corona. But we still we have a 00:49:34.240 --> 00:49:41.410 chance that maybe next year they are lower still. And what this shows is that the 00:49:41.410 --> 00:49:46.160 longer you wait, the steeper your reductions have to be, not only because 00:49:46.160 --> 00:49:51.186 you're starting later, but also because you have to reach zero earlier at the end. 00:49:51.186 --> 00:49:57.147 Notice how all these three lines, the later you start with reducing, the earlier 00:49:57.147 --> 00:50:01.837 you have to reach zero emissions, because the surface area under these curves is 00:50:01.837 --> 00:50:07.317 what counts for the climate goal. The dashed lines a more generous goal, which 00:50:07.317 --> 00:50:15.727 would end at about 1.75 degrees or so, best estimate. this is kind of the weaker 00:50:15.727 --> 00:50:21.452 Paris goal of well below two degrees, which would allow us to gradually reduce 00:50:21.452 --> 00:50:28.054 emissions to zero by 2050. This is not counting in any negative emissions 00:50:28.054 --> 00:50:34.010 afterwards, by the way. This is the net emissions, if you like. So we have to 00:50:34.010 --> 00:50:40.315 reach net zero emissions in 2050. But of course, if we wait five more years until 00:50:40.315 --> 00:50:44.454 the emissions start to decline, then they'll have to be at zero five years 00:50:44.454 --> 00:50:50.680 earlier. So this is why it's so important to start now. This, by the way, so from an 00:50:50.680 --> 00:50:58.397 article by Christiana Figueres et al. in Nature, published 2017, where I was a 00:50:58.397 --> 00:51:06.380 coauthor as well. Now, a final point. Can tipping points maybe help us? And I'm 00:51:06.380 --> 00:51:10.330 talking here about societal tipping points. And there are also some 00:51:10.330 --> 00:51:17.474 interesting studies on that. The basic idea is that shown in the top right here, 00:51:17.474 --> 00:51:24.544 we are in a kind of stable equilibrium where the red ball is now and we are stuck 00:51:24.544 --> 00:51:31.191 there. It's hard to get out of this, but there is a better equilibrium, a more 00:51:31.191 --> 00:51:36.207 stable one further off to the right. And the question is, how do we get over the 00:51:36.207 --> 00:51:43.070 hill into that beneficial equilibrium of a sustainable global economy, a sustainable 00:51:43.070 --> 00:51:49.037 energy system, a stable climate and so on? Complete decarbonization, that means no 00:51:49.037 --> 00:51:55.720 more fossil fuel use. And these this green addition there that is added there, this 00:51:55.720 --> 00:52:02.299 is just some examples of how we can make this transition earlier, easier and the 00:52:02.299 --> 00:52:08.165 hill that we have to get over smaller, so we can make this current status quo that 00:52:08.165 --> 00:52:15.160 we're in a little bit less comfortable by putting a price on carbon. We can make the 00:52:15.160 --> 00:52:23.275 transition easier by subsidizing renewable energies. There are there is a greening of 00:52:23.275 --> 00:52:28.905 values. There is a tipping point in thinking, in society. There are many co 00:52:28.905 --> 00:52:34.080 benefits of this transformation in terms of avoided air pollution. For example, 00:52:34.080 --> 00:52:39.528 millions of people die every year from outdoor air pollution, which would which 00:52:39.528 --> 00:52:45.801 to a large extent go away if we stop fossil fuel use. And we have seen a 00:52:45.801 --> 00:52:51.940 massive movement by the young people Fridays for future. He is Greta Thunberg 00:52:51.940 --> 00:52:56.892 talking to me at our institute. She came last year to visit us here, here is a 00:52:56.892 --> 00:53:02.596 Fridays demonstration in Berlin where I took this photo. This is really changing 00:53:02.596 --> 00:53:08.554 the societies values and it's changing election results and it could be a tipping 00:53:08.554 --> 00:53:16.400 point towards a sustainable global society. And with that hopeful message, I 00:53:16.400 --> 00:53:21.993 want to end and I thank you very much for your attention. If you want to read more, 00:53:21.993 --> 00:53:27.477 there's a couple of books of mine that have also come out in English. You can 00:53:27.477 --> 00:53:32.949 follow me on the blogs and of course, in social media, preferably Twitter, but also 00:53:32.949 --> 00:53:38.200 the scientist for future logo there, because many thousands of scientists are 00:53:38.200 --> 00:53:44.626 engaged there to try and stop the climate crisis. This is really a matter of 00:53:44.626 --> 00:53:50.695 survival of civilization. Thank you very much for listening. Stick to science and 00:53:50.695 --> 00:53:54.401 leave policy to us. Well, we tried that approach. You didn't want to hear about 00:53:54.401 --> 00:53:59.847 the science when it could have made a difference. 00:53:59.847 --> 00:54:03.791 Herald: Thank you so much Stefan for your talk. Now we have some questions from the 00:54:03.791 --> 00:54:10.114 Internets. Let's see the first question Question: Which additional tipping points 00:54:10.114 --> 00:54:17.696 will be triggered at two degrees, three degrees and so on? 00:54:17.696 --> 00:54:22.783 Stefan: That is actually a difficult question to answer because of the 00:54:22.783 --> 00:54:29.264 uncertainty that I mentioned in my talk about where these tipping points are. 00:54:29.264 --> 00:54:33.742 There is one in Antarctica, the Wilkes basin, that is a part of the Antarctic ice 00:54:33.742 --> 00:54:40.571 sheet that that could be triggered, say, below three degrees. There are others like 00:54:40.571 --> 00:54:46.444 the ocean circulation where you probably at least we hope you have to go beyond 00:54:46.444 --> 00:54:51.881 three degrees to really trigger a collapse of the Gulf Stream system. But the truth 00:54:51.881 --> 00:54:57.943 is that they are very large uncertainty ranges. And the main fact is that with 00:54:57.943 --> 00:55:06.502 every bit of extra warming, we increase the risk of crossing more tipping points. 00:55:06.502 --> 00:55:10.330 Herald: And are there some of these tipping points that are interrelated or 00:55:10.330 --> 00:55:16.008 correlated? For instance, could we save some tipping points if we are able to save 00:55:16.008 --> 00:55:19.714 others, for instance, the collapse of the Gulf Stream? 00:55:19.714 --> 00:55:24.880 S: Yes, there are these interconnections. For example, if the Gulf Stream system 00:55:24.880 --> 00:55:32.160 collapses, it will affect the atmospheric circulation. The monsoon systems then can 00:55:32.160 --> 00:55:37.120 shift the tropical rainfall balance. This is not just theoretical. We see that in 00:55:37.120 --> 00:55:41.440 paleoclimate where we have seen these collapses of the North Atlantic 00:55:41.440 --> 00:55:48.480 circulation and the paleo climatic proxy data show that it comes with shifts in the 00:55:48.480 --> 00:55:54.080 tropical rainfall belts that could then in this way trigger a major drought in the 00:55:54.080 --> 00:56:00.480 Amazon region if the Gulf Stream system collapses. And so it would be very wise to 00:56:00.480 --> 00:56:05.040 prevent these tipping points, especially when it comes to the ocean circulation or 00:56:05.040 --> 00:56:10.240 atmospheric circulation, because it's really going to mess up the weather 00:56:10.240 --> 00:56:17.200 patterns in a major way. Herald: How long have we known about 00:56:17.200 --> 00:56:23.416 human caused climate change? S: Well, in principle, in the 19th 00:56:23.416 --> 00:56:30.070 century, Alexander von Humboldt, actually, wrote in 1843, if I remember correctly, 00:56:30.070 --> 00:56:36.647 that humans are changing the climate by cutting down forests and emitting large 00:56:36.647 --> 00:56:40.705 amounts of gases at the centers of industry. That's almost a little literal 00:56:40.705 --> 00:56:46.485 quote by Alexander von Humboldt. We've known about how sensitive the climate is 00:56:46.485 --> 00:56:52.288 to a change in CO2 since the Swedish Nobel laureate Svante Arrhenius, remotely 00:56:52.288 --> 00:56:59.912 related to Greta Thunberg by the way, in India studied the effect of CO2 doubling. 00:56:59.912 --> 00:57:04.535 He wasn't worried by that because he thought global warming would be great. 00:57:04.535 --> 00:57:13.566 Bring it on. It just died, now it's back. You can see my picture so? 00:57:13.566 --> 00:57:20.085 Herald: yeah A: and so he suggested, you know, burning 00:57:20.085 --> 00:57:25.766 a lot of coal to enhance global warming. I guess he came from Sweden and thought cold 00:57:25.766 --> 00:57:31.028 is bad without thinking it through properly. But the first real expert 00:57:31.028 --> 00:57:37.609 reports warning the US government, Lyndon B. Johnson, of the coming global warming 00:57:37.609 --> 00:57:44.289 due to fossil fuel use was a rebel report in nineteen sixty five, exactly 50 years, 00:57:44.289 --> 00:57:50.105 half a century before finally the Paris agreement was reached. 00:57:50.105 --> 00:57:53.384 Herald: Will you be publishing your slides from the talk? 00:57:53.384 --> 00:58:00.200 S: Yes, I will. Uploading the slides. Herald: What is or what should be the 00:58:00.200 --> 00:58:04.190 ultimate goal of the climate change mitigation? For instance, is it saving 00:58:04.190 --> 00:58:10.240 lives, saving other species? S: Well, I think the the ultimate goal is, 00:58:10.240 --> 00:58:15.920 of course, preserving human civilization, as we know it, but because I think if we 00:58:15.920 --> 00:58:24.640 let this run, we will not only destroy a lot of ecosystems and biodiversity, but we 00:58:24.640 --> 00:58:32.000 will probably cause major hunger crisis, which with big droughts like the one in 00:58:32.000 --> 00:58:39.440 Syria before the unrest in Syria started in 2011, the country went through the 00:58:39.440 --> 00:58:44.080 biggest drought in history. And according to settlement data from the eastern 00:58:44.080 --> 00:58:51.440 Mediterranean, it was the worst drought in at least nine hundred years. And then I 00:58:51.440 --> 00:58:58.320 think especially in some unstable, conflicted countries, this can really turn 00:58:58.320 --> 00:59:04.640 them into failed states. That is what happened in Syria. And it's what a German 00:59:04.640 --> 00:59:11.520 report for the German government actually warned in 2009. It was called climate 00:59:11.520 --> 00:59:15.520 change as a security risk, I was actually one of the coauthors of that report 00:59:15.520 --> 00:59:19.760 because I was in the German government's advisory panel on global change at the 00:59:19.760 --> 00:59:26.640 time. And I think we will see increasing hunger crisis, failed states and all the 00:59:26.640 --> 00:59:32.560 effects that that has on international politics if we cannot keep global warming 00:59:32.560 --> 00:59:39.760 below two degrees. Herald: And finally, is there a specific 00:59:39.760 --> 00:59:44.160 call to action for the chaos community? Is there anything that we can do with our 00:59:44.160 --> 00:59:50.640 mindset and our skills? S: That's a good question that I haven't 00:59:50.640 --> 00:59:58.080 thought about, but maybe you can know yourself the best thing, what you can do, 00:59:58.080 --> 01:00:03.840 I think the key is really to keep up the pressure on the political world, like 01:00:03.840 --> 01:00:11.440 Fridays for future has been doing: Go on the streets, protest, vote with climate as 01:00:11.440 --> 01:00:16.080 a priority. I think these are the key things that everyone should be doing and 01:00:16.080 --> 01:00:21.200 specifically in whatever profession they are. They will see some ways of how you 01:00:21.200 --> 01:00:26.640 can help to reduce emissions in your company, put sustainability at the top of 01:00:26.640 --> 01:00:31.330 the agenda and so on. Herald: Stefan, thanks so much for taking 01:00:31.330 --> 01:00:36.254 the time to join us today. Stefan: It's a great pleasure and honor. 01:00:36.254 --> 01:00:42.716 Herald: Always welcome. 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