1 00:00:00,000 --> 00:00:13,884 rc3 prerol music 2 00:00:13,884 --> 00:00:18,068 Herald angel: Greeting creatures im Neuland. In 2015 governments from around 3 00:00:18,068 --> 00:00:22,814 the world met in Paris and agreed to attempt to limit anthropogenic climate 4 00:00:22,814 --> 00:00:28,336 change to well below two degrees. Unfortunately, it seems that since then we 5 00:00:28,336 --> 00:00:32,764 have not done enough and the climate crisis has only gotten more urgent. Our 6 00:00:32,764 --> 00:00:38,152 next speaker, Stefan Rahmstorf, has more accolades than I have time to tell. He's 7 00:00:38,152 --> 00:00:42,440 published more than 100 papers, including in the journals Nature and Science, co- 8 00:00:42,440 --> 00:00:46,875 authored four books and won the Climate Communication Prize from the American 9 00:00:46,875 --> 00:00:52,955 Geophysical Union, the first European to do so. Please welcome him. And heed his 10 00:00:52,955 --> 00:00:55,246 advice. Here's Stefan. 11 00:01:04,616 --> 00:01:10,560 Stefan Rahmstorf: Hi, everyone, my name is Stefan Rahmstorf, and I'm thrilled to be 12 00:01:10,560 --> 00:01:17,680 invited to give a talk at the Chaos Computer Club's remote chaos experience 13 00:01:17,680 --> 00:01:26,720 2020. I want to give you an overview of climate tipping points, a very exciting 14 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 15 00:01:34,400 --> 00:01:40,160 background on climate change. You probably know this image. It shows the global 16 00:01:40,160 --> 00:01:46,720 temperature evolution since the year 1880. Every line is one year. This is the more 17 00:01:46,720 --> 00:01:54,080 conventional way of viewing this time series. And the last seven years have been 18 00:01:54,080 --> 00:02:04,028 the hottest seven years since record keeping began in the 19th century. We know 19 00:02:04,028 --> 00:02:08,560 the reason for this warming: it's the increase of carbon dioxide, which you can 20 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 21 00:02:13,924 --> 00:02:21,602 curve, how the increase has accelerated in ever shorter time spans, we have seen an 22 00:02:21,602 --> 00:02:30,788 ever greater increase in the amount of carbon dioxide in our planet's atmosphere. 23 00:02:30,788 --> 00:02:37,822 This increase causes what we call a radiative forcing that is a kind of 24 00:02:37,822 --> 00:02:44,678 heating in terms of energy release per square meter of Earth's surface. And the 25 00:02:44,678 --> 00:02:51,295 increase in CO2 in the atmosphere until now is causing heating at a rate of two 26 00:02:51,295 --> 00:02:59,360 Watts per square meter surface. We understand the energy budget of our planet 27 00:02:59,360 --> 00:03:04,320 pretty well. On the left here in this diagram, you can see the incoming solar 28 00:03:04,320 --> 00:03:09,600 radiation in yellow. Part of that is reflected already in the atmosphere by the 29 00:03:09,600 --> 00:03:15,760 clouds, for example. Another part is reflected by the bright surfaces, that's 30 00:03:15,760 --> 00:03:22,960 the snow and ice surfaces primarily, and the rest is absorbed. On the right hand 31 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 32 00:03:28,640 --> 00:03:36,080 clearly distinct from the incoming short wave solar radiation by its wavelength and 33 00:03:36,080 --> 00:03:42,240 this thick arrow of long wave radiation leaving the Earth's surface basically to a 34 00:03:42,240 --> 00:03:47,600 large extent gets absorbed by the atmosphere. And the atmosphere itself 35 00:03:47,600 --> 00:03:53,840 emits like anything, any substance, any matter depending on its surface 36 00:03:53,840 --> 00:03:59,600 temperature, sorry, depending on its temperature, emits also infrared 37 00:03:59,600 --> 00:04:07,920 radiation. And one thing that few people realize is that the back radiation coming 38 00:04:07,920 --> 00:04:13,680 down from the atmosphere through the greenhouse effect, the greenhouse gases, 39 00:04:13,680 --> 00:04:20,240 is actually twice as large at the Earth's surface as the absorbed solar radiation. 40 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 41 00:04:26,080 --> 00:04:32,240 absorbed solar radiation at the Earth's surface. And so it's little wonder that if 42 00:04:32,240 --> 00:04:38,800 we are increasing this natural greenhouse effect, which actually makes our planet 43 00:04:38,800 --> 00:04:43,680 livable in the first place, if we are increasing this effect that it is going to 44 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 45 00:04:54,000 --> 00:04:58,640 increase, but other human caused greenhouse gases and also cooling effects 46 00:04:58,640 --> 00:05:05,040 caused by humans, then you see that the total human caused warming that we see in 47 00:05:05,040 --> 00:05:12,880 the orange bar is to, within uncertainty, as big as the observed global warming 48 00:05:12,880 --> 00:05:21,440 since the 1950s. And that means that about 100% of the observed global warming over 49 00:05:21,440 --> 00:05:28,320 the past 70 years is human caused, and the best estimates of the human caused warming 50 00:05:28,320 --> 00:05:34,560 is actually even slightly more than the observed warming, which has to do partly 51 00:05:34,560 --> 00:05:40,800 or is consistent with the fact that solar activity has gone down. So the decrease in 52 00:05:40,800 --> 00:05:49,840 solar activity has compensated a small part of the human caused global warming. 53 00:05:49,840 --> 00:05:55,840 It's also very interesting, and especially to me as a paleoclimatologist who studies 54 00:05:55,840 --> 00:06:01,840 natural climate variations in Earth's history and has done so for more than 25 55 00:06:01,840 --> 00:06:10,720 years, how the modern warming compares with the changes throughout the Holocene, 56 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 57 00:06:17,360 --> 00:06:23,360 decades of paleoclimate research, countless sediment cores taken at the sea 58 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 59 00:06:29,680 --> 00:06:35,440 meaningful global average temperatures. And you can see here the warming from the 60 00:06:35,440 --> 00:06:40,880 height of the last ice age into the Holocene, the Holocene optimum, the 61 00:06:40,880 --> 00:06:46,960 warmest period about until about five thousand years before present. And since 62 00:06:46,960 --> 00:06:55,200 then, we have seen a very slow cooling trend, which we have bent around due to 63 00:06:55,200 --> 00:07:02,720 human activities. And we have within 100 years more than undone 5000 years of 64 00:07:02,720 --> 00:07:08,640 natural cooling trend, which normally would have very slowly continued. These 65 00:07:08,640 --> 00:07:14,510 natural variations, by the way, are due to the Earth orbital cycles, these so-called 66 00:07:14,510 --> 00:07:24,320 Milankovitch cycles. You can easily read up on those, for example, at Wikipedia. 67 00:07:24,320 --> 00:07:33,023 Now let's come to the famous, much feared tipping points in the climate system. What 68 00:07:33,023 --> 00:07:40,164 is a tipping point? That has been described in a seminal paper which I'm 69 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 70 00:07:50,336 --> 00:07:56,011 called tipping elements in the Earth's climate system. And it says that the term 71 00:07:56,011 --> 00:08:01,503 tipping point commonly refers to a critical threshold at which a tiny 72 00:08:01,503 --> 00:08:08,719 perturbation can qualitatively alter the state or development of a system and the 73 00:08:08,719 --> 00:08:13,702 different parts of the Earth's system, which can undergo such a transition, they 74 00:08:13,702 --> 00:08:21,120 are called the tipping elements. This whole concept is illustrated in the red 75 00:08:21,120 --> 00:08:26,366 line that's shown here: In the horizontal axis, we see a control parameter and that 76 00:08:26,366 --> 00:08:32,153 could be the greenhouse gas content of our atmosphere, it could be the temperature, 77 00:08:32,153 --> 00:08:37,565 it could be, if you talk about natural climate changes, for example, those 78 00:08:37,565 --> 00:08:43,692 orbital changes, the what we call the Milankovitch forcing, which drives 79 00:08:43,692 --> 00:08:48,886 changes. And on the vertical axis, you see the response. And if you imagine the 80 00:08:48,886 --> 00:08:54,840 control parameter changing from left to right in this diagram, you would march 81 00:08:54,840 --> 00:09:01,702 along that upper part of the red curve here, the branch, until you come close to 82 00:09:01,702 --> 00:09:07,278 a threshold. And at that threshold, the system will undergo a major change and 83 00:09:07,278 --> 00:09:17,070 reach then this lower part of the curve, a different kind of equilibrium state. So 84 00:09:17,070 --> 00:09:24,493 it's basically a small change in the driver causing a very big systemic 85 00:09:24,493 --> 00:09:31,336 response. That is what defines a tipping point. If we want to be very accurate 86 00:09:31,336 --> 00:09:36,422 here, we can distinguish two different types of tipping points. The first one is 87 00:09:36,422 --> 00:09:41,560 what I just showed you, is repeated here on the left side, and it is characterized 88 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 89 00:09:49,210 --> 00:09:55,003 axis. So every amount of forcing corresponds to one particular system 90 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 91 00:10:02,130 --> 00:10:12,310 driving parameter around this threshold. Now, a second, even more drastic or non- 92 00:10:12,310 --> 00:10:18,069 linear type of tipping point is shown in the right hand side, where the equilibrium 93 00:10:18,069 --> 00:10:24,040 states are somewhat more complex than the single red line on the left. You can see 94 00:10:24,040 --> 00:10:30,270 here that there is, again, an upper stable branch and there is also a lower stable 95 00:10:30,270 --> 00:10:37,415 branch, but they overlap. So there is a region that is shaded here where two 96 00:10:37,415 --> 00:10:44,955 stable equilibria exist. And it depends on the initial conditions on which of these 97 00:10:44,955 --> 00:10:55,474 branches you are. Now, there is what is called a bifurcation structure underlying 98 00:10:55,474 --> 00:11:03,403 this with a bifurcation point. There is an unstable branch which separates the basins 99 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 100 00:11:10,050 --> 00:11:15,390 you start kind of away from an equilibrium but above the dashed line, you will fall 101 00:11:15,390 --> 00:11:19,891 up onto that upper stable branch; if you start out below the dash line, you will 102 00:11:19,891 --> 00:11:25,056 fall down on the lower branch. That actually is pretty standard non-linear 103 00:11:25,056 --> 00:11:30,719 dynamics. It's a whole branch of physics which investigates exactly this type of 104 00:11:30,719 --> 00:11:37,996 behavior in many different physical systems. So the second type of tipping 105 00:11:37,996 --> 00:11:43,791 point, the right hand side one, is corresponding to multiple equilibrium 106 00:11:43,791 --> 00:11:48,302 states, in this case two stable equilibria. That's why this error range 107 00:11:48,302 --> 00:11:54,893 here is called bistability, two stable equilibria. It is coming with 108 00:11:54,893 --> 00:11:59,790 irreversibility, so basically, if you march to the right here on that upper 109 00:11:59,790 --> 00:12:06,150 stable branch at that bifurcation point, you fall off down onto the lower stable 110 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 111 00:12:11,543 --> 00:12:16,702 left to that second lower blue point there until you can go back onto that stable 112 00:12:16,702 --> 00:12:24,529 branch. The second type is actually as an everyday system that behaves like that it 113 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 114 00:12:29,577 --> 00:12:35,867 little bit to one side, then you experience a counterforce. So the kayak is 115 00:12:35,867 --> 00:12:41,720 trying to upright itself, it's resisting you tipping it. But if you move further 116 00:12:41,720 --> 00:12:47,241 and further and further, eventually you will reach a tipping point. This is the 117 00:12:47,241 --> 00:12:53,820 point where the kayak stops resisting your further leaning over and instead it starts 118 00:12:53,820 --> 00:12:59,991 tipping over further by itself and then it flips right over until it's upside down 119 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 120 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 121 00:13:11,262 --> 00:13:18,232 flip over. Now, this kayak also has a range of bistability, so once it's flipped 122 00:13:18,232 --> 00:13:23,326 over, it's also in a stable state and it takes considerable effort to turn it 123 00:13:23,326 --> 00:13:28,935 upright again into the other stable state when it's vertical, upright rather than 124 00:13:28,935 --> 00:13:37,168 upside down. Now, the whole point is that systems like this exist also in the 125 00:13:37,168 --> 00:13:45,120 climate system. The kind of first type on the left hand side corresponds, for 126 00:13:45,120 --> 00:13:51,920 example, to sea ice and on the right hand side this type of tipping element compares 127 00:13:51,920 --> 00:13:58,640 to refers to the Greenland ice sheet or continental ice sheets, also Antarctica or 128 00:13:58,640 --> 00:14:06,640 the Atlantic Ocean circulation. In terms of the trends in behavior, and that means 129 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 130 00:14:12,160 --> 00:14:19,440 right in these diagrams, then in that sense, they don't differ very much because 131 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, 132 00:14:26,000 --> 00:14:32,400 the green line more or less follows more or less closely the red equilibrium line 133 00:14:32,400 --> 00:14:37,280 with a certain delay, depending on how sluggish the system responds. So that's 134 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 135 00:14:44,880 --> 00:14:50,560 right hand side case, you have a similar thing. You are kind of, in theory, in 136 00:14:50,560 --> 00:14:56,320 equilibrium, you would fall off the cliff at this bifurcation point. But in praxis, 137 00:14:56,320 --> 00:15:01,440 the system has some inertia, it takes some time. So if you gradually move on the 138 00:15:01,440 --> 00:15:05,920 right towards the right there, you will also follow a green line, which is very 139 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 140 00:15:10,960 --> 00:15:16,160 go back, but you just going forward, progressive global warming, the difference 141 00:15:16,160 --> 00:15:20,960 isn't all that big. And the main difference comes from the intrinsic 142 00:15:20,960 --> 00:15:27,200 timescale of the system. Obviously, sea ice can respond much more quickly to being 143 00:15:27,200 --> 00:15:32,320 just a few meters thick compared to continental ice sheet like Greenland ice, 144 00:15:32,320 --> 00:15:36,720 which is about three thousand meters thick. And that just takes a very long 145 00:15:36,720 --> 00:15:45,466 time to melt. Now, here's an overview of different tipping elements in the climate 146 00:15:45,466 --> 00:15:55,840 system. A few examples you can see starting on the left here, the boreal 147 00:15:55,840 --> 00:16:02,080 forest, that are the kind of northern forests, which typically, like ecosystems, 148 00:16:02,080 --> 00:16:10,480 do have a tipping point, a point of collapse. The whole idea of these tipping 149 00:16:10,480 --> 00:16:15,360 points and system collapse is very strongly linked actually to ecosystem 150 00:16:15,360 --> 00:16:21,280 research and the boreal forests, They have a point where they get too dry, that fires 151 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 152 00:16:27,040 --> 00:16:34,240 Siberia, they go up in flames lit by lightning. Or the Amazon rain forest. This 153 00:16:34,240 --> 00:16:43,760 is also a tipping element, has been shown in many vegetation dynamics models, which 154 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 155 00:16:51,120 --> 00:16:56,480 by storing water in the soil, keeping it there and then bringing it up again 156 00:16:56,480 --> 00:17:02,640 through evapotranspiration, as we call it, the tree brings up water to the leaves 157 00:17:02,640 --> 00:17:08,560 then into the atmosphere again, and then it moves with the winds and maybe 50, 100 158 00:17:08,560 --> 00:17:14,720 kilometers downwind, it falls again as rain. So it's a kind of perpetual rain 159 00:17:14,720 --> 00:17:20,880 recycling system which keeps the whole forest nice and moist. But if you stress 160 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, 161 00:17:27,680 --> 00:17:34,880 and also you make it more drought prone by warming up the climate, which leads to 162 00:17:34,880 --> 00:17:40,160 faster loss of moisture, etc. greater moisture requirements by the trees. Then 163 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 164 00:17:45,000 --> 00:17:53,440 forest can go up in flames. Another example of how you see the top right is 165 00:17:53,440 --> 00:17:59,120 the permafrost thawing. This is when it gets too warm. There is a very simple 166 00:17:59,120 --> 00:18:02,960 threshold, namely the freezing point. Of course, that is a tipping point in the 167 00:18:02,960 --> 00:18:11,040 sense of freezing point of water. When the permafrost thaws, then there is methane 168 00:18:11,040 --> 00:18:19,360 gas escaping to the atmosphere, which then also can enhance the further warming, 169 00:18:19,360 --> 00:18:25,200 which then leads to more permafrost thawing and so on. Typically, these 170 00:18:25,200 --> 00:18:34,640 tipping points are associated with such amplifying feedbacks. I will discuss three 171 00:18:34,640 --> 00:18:41,040 of these in a little bit more detail. The Greenland ice sheet, which is undergoing 172 00:18:41,040 --> 00:18:47,840 accelerated ice loss, the Atlantic overturning circulation, often called Gulf 173 00:18:47,840 --> 00:18:54,480 Stream system. And the third one is the coral reefs, which are suffering from 174 00:18:54,480 --> 00:18:59,520 large scale die-off, which also as a typical ecosystem response, have a 175 00:18:59,520 --> 00:19:06,880 critical threshold. These examples are discussed in our paper 'Climate tipping 176 00:19:06,880 --> 00:19:12,720 points - too risky to bet against' which we published in Nature about one year ago. 177 00:19:12,720 --> 00:19:20,080 And they are also some of these tipping points interact, they are interlinked. And 178 00:19:20,080 --> 00:19:24,960 one of our quotes there is that the clearest emergency would be if we were 179 00:19:24,960 --> 00:19:30,080 approaching a global cascade of tipping points. That is a situation where one 180 00:19:30,080 --> 00:19:36,400 tipping element is triggering the next one in a kind of domino effect. This is what 181 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 182 00:19:44,960 --> 00:19:50,480 video showing based on GRACE satellite data where the ice sheet is losing mass. 183 00:19:50,480 --> 00:19:56,800 You can see increasing blue colors here that the Greenland ice sheet is indeed 184 00:19:56,800 --> 00:20:03,280 losing mass. You can look up at the NASA Vital Signs website, which has very good 185 00:20:03,280 --> 00:20:09,200 indicators of various vital signs of our planet, including the data on Greenland 186 00:20:09,200 --> 00:20:17,760 ice loss, constantly updated. Now, the point with the Greenland ice sheet is that 187 00:20:17,760 --> 00:20:24,000 it does have a stability diagram like the schematic one that I showed you earlier 188 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 189 00:20:29,920 --> 00:20:38,720 time by my colleagues, Calov and Ganopolski in 2005 in this article where 190 00:20:38,720 --> 00:20:43,440 they used the three dimensional ice sheet model coupled inside a global climate 191 00:20:43,440 --> 00:20:49,840 model with ocean atmosphere and so on and on the x axis is basically increasing 192 00:20:49,840 --> 00:20:54,240 amount of heating going on, in this case because they were interested in the 193 00:20:54,240 --> 00:21:01,360 paleoclimate question, it is this driving force by the orbital cycles and 194 00:21:01,360 --> 00:21:06,640 Milankovitch cycles. You don't need to understand the numbers, but on the 195 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 196 00:21:11,600 --> 00:21:17,840 million cubic kilometers. And you can see that upper branch in the blue line, we're 197 00:21:17,840 --> 00:21:24,080 actually moving towards the right here in this model simulation experiment. And you 198 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 199 00:21:28,720 --> 00:21:35,760 of around about five hundred. And this is where the tipping point is. There the ice 200 00:21:35,760 --> 00:21:41,280 mass declines, melts away, away very quickly. And you then end up at that lower 201 00:21:41,280 --> 00:21:47,120 branch with no ice on Greenland. And they played this game. They ran the simulation 202 00:21:47,120 --> 00:21:52,640 out to more than 550 watts per square meter. And the light blue line is what 203 00:21:52,640 --> 00:21:58,960 happens when they return, when they turn down the heat again. You move towards the 204 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 205 00:22:03,600 --> 00:22:09,360 line. You have to go to much lower radiation values until the ice sheet 206 00:22:09,360 --> 00:22:15,520 starts to grow again and comes back. The dots, by the way, are points where this 207 00:22:15,520 --> 00:22:21,600 has to has been run for many thousands of years really into an equilibrium just to 208 00:22:21,600 --> 00:22:27,520 show that there are really for the same value on the x axis, two very different 209 00:22:27,520 --> 00:22:34,960 equilibrium states with and without Greenland ice sheet. And the fact that we 210 00:22:34,960 --> 00:22:39,600 now and in the Holocene in the last ten thousand years have the Greenland ice 211 00:22:39,600 --> 00:22:45,280 sheet and it actually is stable in the Holocene climate is only because of the 212 00:22:45,280 --> 00:22:50,400 initial condition, because we came out of an ice age. If you took away the Greenland 213 00:22:50,400 --> 00:22:55,920 ice sheet now, then in the current climate or the Holocene or pre-industrial climate, 214 00:22:55,920 --> 00:23:01,680 it would never grow back. What is the positive feedback? The most positive? We 215 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 216 00:23:06,560 --> 00:23:12,000 and amplifying feedback and the key amplifying feedback here is what is called 217 00:23:12,000 --> 00:23:18,480 the ice elevation feedback. The Greenland ice sheet does not melt because it's very 218 00:23:18,480 --> 00:23:24,640 cold at the surface, mostly below freezing. And why is it so cold? Because 219 00:23:24,640 --> 00:23:28,800 it is very high up in the atmosphere, this ice sheet of three thousand meters thick 220 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 221 00:23:35,440 --> 00:23:41,040 away that ice sheet, though, the surface then would be down at sea level or even 222 00:23:41,040 --> 00:23:46,000 below if you did this quickly because the the bedrock is depressed, but the surface 223 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 224 00:23:51,440 --> 00:23:57,120 meters altitude in the atmosphere. And there it is actually too warm to keep any 225 00:23:57,120 --> 00:24:02,640 snow on the ground year round, which would be required to regrow a new Greenland ice 226 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 227 00:24:06,800 --> 00:24:12,560 Holocene to get the Greenland ice sheet back once it were lost. This is a typical 228 00:24:12,560 --> 00:24:18,400 example of this amplifying feedback, which leads to a self stabilizing system. It can 229 00:24:18,400 --> 00:24:23,600 either self stabilize in the upper branch here when you start there or it self- 230 00:24:23,600 --> 00:24:28,720 stabilizes in the lower branch with no ice when you start there. This is what makes 231 00:24:28,720 --> 00:24:37,360 it a bi-stable system. To summarize, the Greenland ice sheet is melting as another 232 00:24:37,360 --> 00:24:43,200 data the great satellites show, but also other data sets. It has a tipping point 233 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 234 00:24:48,800 --> 00:24:55,600 with many climate models, simulation experiments going through more than two 235 00:24:55,600 --> 00:25:00,480 hundred thousand years of simulations from the past through the Eemian interglacial 236 00:25:00,480 --> 00:25:05,600 period where we know how much the ice sheets shrank back. And we could use those 237 00:25:05,600 --> 00:25:13,440 data from the past behavior of Greenland to calibrate the model. And so we know the 238 00:25:13,440 --> 00:25:17,920 tipping point for the complete loss of the Greenland ice sheet is somewhere between 239 00:25:17,920 --> 00:25:22,556 one degree and three degree global warming. We're already at one point two 240 00:25:22,556 --> 00:25:27,507 degrees global warming. So we have started to enter the danger zone where we crossed 241 00:25:27,507 --> 00:25:34,271 that tipping point. It doesn't mean that it suddenly starts to melt very fast also 242 00:25:34,271 --> 00:25:39,235 because it has its own intrinsic slow response time. But what that crossing, 243 00:25:39,235 --> 00:25:44,187 that tipping point means is that even without further warming, the Greenland ice 244 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 245 00:25:49,120 --> 00:25:56,171 seven meters of global sea level rise, drowning most of our big coastal cities 246 00:25:56,171 --> 00:26:04,047 and to many island nations. Here is a look at the future from models, simulations 247 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 248 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 249 00:26:15,088 --> 00:26:20,452 the ice flowing. You can see it retreating. So in purple, that's bedrock 250 00:26:20,452 --> 00:26:27,000 that is exposed where the ice sheet has withdrawn in this simulation. And so it's 251 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 252 00:26:32,686 --> 00:26:43,180 substantial fraction of the Greenland ice sheet. Now, let's look at another kind of 253 00:26:43,180 --> 00:26:46,850 tipping element, and that is the Gulf Stream system or the North Atlantic 254 00:26:46,850 --> 00:26:52,184 current. And I can't really introduce this topic is one of my favorite topics, which 255 00:26:52,184 --> 00:26:59,323 I have studied since the early 90s, without showing a clip from the famous 256 00:26:59,323 --> 00:27:05,217 Hollywood blockbuster The Day After Tomorrow. What about the North Atlantic 257 00:27:05,217 --> 00:27:10,680 current? What about it? The current depends upon a delicate balance of salt 258 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 259 00:27:16,100 --> 00:27:21,140 fresh water has been dumped into the ocean because of melting polar ice. I think 260 00:27:21,140 --> 00:27:32,100 we've hit a critical desalinization point. Yeah, now that statement about the 261 00:27:32,100 --> 00:27:37,027 critical desalination point is a completely correct description of the 262 00:27:37,027 --> 00:27:41,915 bifurcation point of the Atlantic circulation, I'll show it in a minute. And 263 00:27:41,915 --> 00:27:48,910 the statement that nobody has taken into account the meltwater from the Greenland 264 00:27:48,910 --> 00:27:55,922 ice sheet is also was completely correct when the movie appeared in 2004. Until 265 00:27:55,922 --> 00:28:01,670 then, the typical climate simulations that you could see in the IPCC reports, 266 00:28:01,670 --> 00:28:07,266 actually until quite a few years later, still had not taken account Greenland melt 267 00:28:07,266 --> 00:28:12,614 water because basically at that point in time, the models, almost all climate 268 00:28:12,614 --> 00:28:18,032 models were just ocean-atmosphere models plus land surface, but they didn't have 269 00:28:18,032 --> 00:28:24,821 continental ice sheet models coupled into them. And so in the meantime, of course, 270 00:28:24,821 --> 00:28:30,160 we have better models that include experiments either with artificially added 271 00:28:30,160 --> 00:28:36,838 Greenland meltwater from data estimates or fully coupled with ice sheet models. And 272 00:28:36,838 --> 00:28:41,168 from that, an example here being that nature article by Claus Boening and 273 00:28:41,168 --> 00:28:48,500 colleagues. We know that the meltwater input from Greenland has a non-negligible 274 00:28:48,500 --> 00:28:52,992 effect on the North Atlantic overturning. It's probably not the dominant effect, but 275 00:28:52,992 --> 00:29:00,900 it adds to various factors that weaken this North Atlantic current. And we also 276 00:29:00,900 --> 00:29:07,020 know that this system has a well-defined tipping point. Actually, I described that 277 00:29:07,020 --> 00:29:17,440 in a nature article in 1996 due to a salt transport feedback. The basic idea behind 278 00:29:17,440 --> 00:29:25,054 that has actually been known since the late 1950s or early 60s since work by the 279 00:29:25,054 --> 00:29:29,960 famous American oceanographer Henry Stommel. But what I showed in my Nature 280 00:29:29,960 --> 00:29:35,750 article in 96 is that it actually works that way in a complex, three dimensional 281 00:29:35,750 --> 00:29:43,045 global ocean circulation model, not just in very simplified models. And since then, 282 00:29:43,045 --> 00:29:49,775 this has been shown for a whole range of different climate models. The sole 283 00:29:49,775 --> 00:29:54,578 transportation feedback is also one of these amplifying feedbacks, and it's easy 284 00:29:54,578 --> 00:30:01,744 to explain. The overturning circulation of the Atlantic is called overturning because 285 00:30:01,744 --> 00:30:08,772 it's really a vertical overturning where water sinks down from the surface to great 286 00:30:08,772 --> 00:30:16,192 depth of two to three kilometers in the Atlantic because this water is heavy and 287 00:30:16,192 --> 00:30:21,177 it spreads thin in the deep ocean until it rises up in other parts, mainly around 288 00:30:21,177 --> 00:30:29,483 Antarctica in the Antarctic circumpolar current area and comes back at the 289 00:30:29,483 --> 00:30:35,733 surface. So basically the whole ocean is overturned with deep water being renewed 290 00:30:35,733 --> 00:30:45,008 and then coming back to the surface on very long timescale of about 1000 to 2000 291 00:30:45,008 --> 00:30:53,606 years for complete overturning there. Now, the whole system is driven by the fact 292 00:30:53,606 --> 00:31:00,985 that the water sinks down where it has the highest density, and that's in the 293 00:31:00,985 --> 00:31:07,376 northern Atlantic and around Antarctica, around the Antarctic continent. And it has 294 00:31:07,376 --> 00:31:12,360 the highest density there, not only because it's very cold, but also quite 295 00:31:12,360 --> 00:31:17,927 salty. This is why you don't have deep water formation in the North Pacific, in 296 00:31:17,927 --> 00:31:22,648 the Northern Hemisphere. You only have that in the North Atlantic. And that's 297 00:31:22,648 --> 00:31:28,784 because the North Atlantic waters are quite salty. And this is because this 298 00:31:28,784 --> 00:31:34,311 North Atlantic current exists and brings salty water from the subtropics up to the 299 00:31:34,311 --> 00:31:39,052 high latitudes, where normally it isn't very salty because it gets diluted by 300 00:31:39,052 --> 00:31:44,158 excess rainfall, whereas the subtropics have excess evaporation and that's why 301 00:31:44,158 --> 00:31:50,486 they're salty. And so it's like a chicken and an egg situation. The Northern 302 00:31:50,486 --> 00:31:55,049 Atlantic is salty because you have this overturning circulation and you have this 303 00:31:55,049 --> 00:32:00,055 overturning circulation because it's salty there. And so you can see the self 304 00:32:00,055 --> 00:32:06,419 amplifying feedback there again, which means it is a self stabilizing system up 305 00:32:06,419 --> 00:32:11,654 to a certain breaking point, a tipping point which can be reached if you add too 306 00:32:11,654 --> 00:32:19,338 much fresh water, diluting the northern Atlantic. And the stability diagram, 307 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 308 00:32:24,564 --> 00:32:29,618 said, this has been verified in a detailed model simulations with many different 309 00:32:29,618 --> 00:32:36,021 models that it really works like that in a complex 3D situation where you have 310 00:32:36,021 --> 00:32:40,630 depending on how much fresh water you add into the northern Atlantic, this is the 311 00:32:40,630 --> 00:32:46,846 control parameter here, you can move along that upper stable branch with the 312 00:32:46,846 --> 00:32:54,341 overturning circulation until that Stommel bifurcation point. And there this 313 00:32:54,341 --> 00:33:00,282 overturning breaks down and you fall down onto that lower branch without this 314 00:33:00,282 --> 00:33:06,698 overturning. It's labeled here NADW Flow that NADW stands for north Atlantic 315 00:33:06,698 --> 00:33:11,402 deepwater. It's a, I would say, one of the favorite water masses of the 316 00:33:11,402 --> 00:33:21,280 oceanographers. Now, let's look at the Gulf Stream, the surface circulation in a 317 00:33:21,280 --> 00:33:28,320 climate model. This is the CM 2.6 global coupled climate model ocean atmosphere by 318 00:33:28,320 --> 00:33:34,320 the Geophysical Fluid Dynamics Laboratory in Princeton. You can beautifully see the 319 00:33:34,320 --> 00:33:40,320 Gulf Stream and dark red here because it's warm leaving the coast of the United 320 00:33:40,320 --> 00:33:45,410 States at Cape Hatteras there, starting to meander, breaking up into these eddies, et 321 00:33:45,410 --> 00:33:52,880 cetera. And it actually meets the cold waters coming down inshore from the north, 322 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 323 00:33:59,040 --> 00:34:07,360 circulation looks in a global climate model. And if you add carbon dioxide to 324 00:34:07,360 --> 00:34:13,520 that climate models atmosphere, the climate warms, of course, but it does show 325 00:34:13,520 --> 00:34:20,000 a peculiar pattern of sea surface temperature change, which you see here. 326 00:34:20,000 --> 00:34:24,800 And this actually shows the sea surface temperature change relative to the global 327 00:34:24,800 --> 00:34:29,520 mean. So everything that is blue has either warmed less than the global average 328 00:34:29,520 --> 00:34:34,640 or even cooled, which is actually the case south of Greenland. And everything that is 329 00:34:34,640 --> 00:34:42,720 orange or red has warmed substantially more than the global average sea surface. 330 00:34:42,720 --> 00:34:50,080 And you see a very strong pattern in the northern Atlantic with this big cold blob, 331 00:34:50,080 --> 00:34:55,600 the blue blob south of Greenland and a very warm region inshore of the Gulf 332 00:34:55,600 --> 00:35:02,480 Stream along the coast of North America. And in the climate model, of course, we 333 00:35:02,480 --> 00:35:07,440 are a bit like gods in that sense that we have complete information about what's 334 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 335 00:35:12,720 --> 00:35:18,160 that's happening and we can analyze the reasons. And the reason for this funny 336 00:35:18,160 --> 00:35:24,960 pattern in the northern Atlantic actually is a slowdown of the North Atlantic 337 00:35:24,960 --> 00:35:34,960 overturning circulation. That means that less heat is transported to the subpolar 338 00:35:34,960 --> 00:35:41,440 ocean south of Greenland there. That blue area, which makes it cool down and the 339 00:35:41,440 --> 00:35:46,960 Gulf Stream proper at the surface, moves inshore there is complicated dynamical 340 00:35:46,960 --> 00:35:52,880 reasons for that. But there is already long before this was shown in this model, 341 00:35:52,880 --> 00:35:59,360 a theoretical underpinning for this. It has to do with the vorticity dynamics on a 342 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 343 00:36:04,880 --> 00:36:10,560 phenomenon. And so we know that this slowdown of the Gulf Stream system is the 344 00:36:10,560 --> 00:36:15,760 reason behind this peculiar temperature pattern. And this pattern is predicted by 345 00:36:15,760 --> 00:36:22,640 this climate model for a global warming situation. And my PhD student, Levke 346 00:36:22,640 --> 00:36:28,880 Caesar, who was the first author on this nature paper from 2018, she looked at all 347 00:36:28,880 --> 00:36:34,320 the available measurements of sea surface temperatures since the beginning of the 348 00:36:34,320 --> 00:36:39,760 20th century. And of course, because we have only limited ocean temperature 349 00:36:39,760 --> 00:36:43,840 measurements, we have only a fuzzy picture here, not a sharp one like in the climate 350 00:36:43,840 --> 00:36:50,560 model. But you can see a similar pattern in the North Atlantic in the observations 351 00:36:50,560 --> 00:36:55,680 compared to what the model predicts in response to a slowdown of the overturning 352 00:36:55,680 --> 00:37:01,040 circulation. And our conclusion here is that we are actually observing this 353 00:37:01,040 --> 00:37:07,040 slowdown of the circulation. Why do we take indirect evidence for this like this? 354 00:37:07,040 --> 00:37:13,680 Because we don't, of course, have measurements going back 100 years or more 355 00:37:13,680 --> 00:37:18,800 about the strength of that overturning circulation. We have actually only started 356 00:37:18,800 --> 00:37:24,720 to measure this regularly in 2004 with a so-called rapid array, At twenty six 357 00:37:24,720 --> 00:37:31,600 degrees north in the Atlantic, and what we reconstructed about the evolution of this 358 00:37:31,600 --> 00:37:37,520 current for the last period where we do have the direct measurements, agrees well 359 00:37:37,520 --> 00:37:46,480 with what the direct measurements show. We concluded that the overturning circulation 360 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 361 00:37:56,202 --> 00:38:01,220 course, other indirect types of measurements. You can use sediment data of 362 00:38:01,220 --> 00:38:05,560 various kinds and with various methodologies to reconstruct the strength 363 00:38:05,560 --> 00:38:11,316 of this Atlantic overturning and a number of different studies compiled here in this 364 00:38:11,316 --> 00:38:17,362 diagram. And even though, of course, they differ somewhat in the detail, they all 365 00:38:17,362 --> 00:38:23,006 tend to agree in this overall picture that the Atlantic overturning circulation has 366 00:38:23,006 --> 00:38:28,815 been quite stable for the previous thousand years or so before the 20th 367 00:38:28,815 --> 00:38:37,109 century, but then in the 20th century has showed a clear declining signature. And 368 00:38:37,109 --> 00:38:43,078 one example of the media coverage of this is that Washington Post article here, 369 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 370 00:38:48,047 --> 00:38:53,195 actually made it to number three or the most read Washington Post articles. There 371 00:38:53,195 --> 00:38:59,650 is definitely an interest in science and climate change science by the readers in 372 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 373 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 374 00:39:13,920 --> 00:39:18,000 tipping point, that is a robust result of many different studies and model 375 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 376 00:39:23,760 --> 00:39:28,000 very typical for these tipping points because they involve highly nonlinear 377 00:39:28,000 --> 00:39:34,320 dynamics. That means they can depend very sensitively on the exact conditions, for 378 00:39:34,320 --> 00:39:39,920 example, in this case, the exact salinity distribution in the Atlantic and the exact 379 00:39:39,920 --> 00:39:46,400 circulation pattern. And models get these things kind of approximately right, but 380 00:39:46,400 --> 00:39:52,560 not exactly right. And if you have a situation where the question of where the 381 00:39:52,560 --> 00:39:57,040 tipping point is is very sensitive to the exact conditions, then you have a large 382 00:39:57,040 --> 00:40:03,280 uncertainty about where the tipping point is. And so there is discussion in the 383 00:40:03,280 --> 00:40:08,480 literature. I just point out to one study here in science advances that try to 384 00:40:08,480 --> 00:40:16,960 correct for the inaccuracies in how we can reproduce the salinity in the Atlantic 385 00:40:16,960 --> 00:40:22,080 waters and found that if you correct for that, the circulation is actually a lot 386 00:40:22,080 --> 00:40:28,480 more sensitive than in other models. And maybe that model is more correct. And of 387 00:40:28,480 --> 00:40:32,400 course, it has other weaknesses as well. We don't know which of the models is 388 00:40:32,400 --> 00:40:39,840 correct, but should we cross this tipping point then the North Atlantic circulation 389 00:40:39,840 --> 00:40:44,160 system would break down and you get a temperature pattern like the one shown 390 00:40:44,160 --> 00:40:49,200 here, the cold blob in the Atlantic that is now only over the ocean. It exists, 391 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 392 00:40:53,040 --> 00:40:58,480 20th century, but it hasn't affected any land areas. But if the circulation would 393 00:40:58,480 --> 00:41:03,920 break down altogether and not only weakened by 15%, this cold would expand 394 00:41:03,920 --> 00:41:09,760 greatly and affect Great Britain, Scandinavia, Iceland, as you can see here, 395 00:41:09,760 --> 00:41:13,760 which would then get a much colder climate, whereas the rest of the globe 396 00:41:13,760 --> 00:41:19,600 continues to have a warmer climate. This is really distinct from an ice age. And so 397 00:41:19,600 --> 00:41:23,600 this is also really distinct from that Hollywood movie The Day After Tomorrow, 398 00:41:23,600 --> 00:41:29,200 where the earth goes into a huge ice age, an instant freeze. That, of course, is 399 00:41:29,200 --> 00:41:33,520 totally unrealistic. And the the screenwriter and the director, they knew 400 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 401 00:41:40,720 --> 00:41:45,520 a few million viewers, they would stick to the laws of physics. But since they make 402 00:41:45,520 --> 00:41:50,720 movies for a few hundred million viewers, they stick to the laws of Hollywood drama. 403 00:41:50,720 --> 00:41:56,880 But you would get a substantial regional cooling with a major impact on ecosystems, 404 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 405 00:42:03,920 --> 00:42:09,600 discuss today. This is the coral reefs. Coral reefs, like many ecosystems, do have 406 00:42:09,600 --> 00:42:17,040 critical thresholds. Coral reefs are very important, even though they only cover a 407 00:42:17,040 --> 00:42:23,539 very small percentage of the Earth's surface, they support a quarter of all 408 00:42:23,539 --> 00:42:31,597 marine life. 40% coral cover of the world has already been lost, 100 countries 409 00:42:31,597 --> 00:42:38,528 depend quite substantially on corals. There's 800 billion total global assets of 410 00:42:38,528 --> 00:42:45,557 coral reefs. So it does have a major impact on people. Now, corals, when they 411 00:42:45,557 --> 00:42:52,739 are about to die, they bleach. They are abandoned by their algae that provides 412 00:42:52,739 --> 00:43:00,092 them with nutrition and that's why they lose their color. And then after a while, 413 00:43:00,092 --> 00:43:07,167 they die. They get covered by other by seaweed, non symbiotic algae, and they 414 00:43:07,167 --> 00:43:13,351 die. And they do have a temperature threshold. It's a critical warming 415 00:43:13,351 --> 00:43:19,594 threshold where this bleaching happens. But an additional factor, not yet the most 416 00:43:19,594 --> 00:43:25,474 important factor, is the acidification of water. It's a direct chemical effect of 417 00:43:25,474 --> 00:43:32,287 adding carbon dioxide to the atmosphere, which then goes partly into the oceans and 418 00:43:32,287 --> 00:43:39,354 acidifies the ocean waters. But the main effect until now is the marine heatwaves, 419 00:43:39,354 --> 00:43:44,840 which cross more and more frequently the temperature tolerance threshold of coral 420 00:43:44,840 --> 00:43:50,093 reefs. And here you can see that for the Great Barrier Reef, a huge, fantastic 421 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 422 00:43:55,973 --> 00:44:05,576 year 2016, 2017, 2020, three major bleaching events which affect it in each 423 00:44:05,576 --> 00:44:11,081 case, the red area here with the most severe bleaching, you can see that by now 424 00:44:11,081 --> 00:44:17,359 a very large part of the Great Barrier Reef has bleached in these three events. 425 00:44:17,359 --> 00:44:23,892 And it's very tragic. And you can see here, for example, the March, the 2016 426 00:44:23,892 --> 00:44:33,041 bleaching event in March, the coral was bleached. By May, it was already overgrown by seaweed. 427 00:44:33,041 --> 00:44:40,208 And just in 2015 and 2016, we actually had worldwide coral reef bleaching, not only 428 00:44:40,208 --> 00:44:45,620 at the Great Barrier Reef in Australia, only the blue ones out of these hundred 429 00:44:45,620 --> 00:44:53,935 reefs that were observed in this study, only the blue ones escaped bleaching. So 430 00:44:53,935 --> 00:45:02,252 we are actually in the midst of a great worldwide coral die off event, which is 431 00:45:02,252 --> 00:45:08,022 another prediction of climate science coming true. If you look at the latest 432 00:45:08,022 --> 00:45:13,925 IPCC report, it states that with two degrees warming, virtually all coral reefs 433 00:45:13,925 --> 00:45:19,826 will be lost, more than 99%. One point five degree warming. If we manage to limit 434 00:45:19,826 --> 00:45:26,215 the warming to one point five degrees, we can save between 10% and 30% of the 435 00:45:26,215 --> 00:45:36,758 corals. That is really depressing. Now, let me talk briefly about what can we do. 436 00:45:36,758 --> 00:45:42,311 A major success is, of course, the Paris accord, the biggest failure of which is 437 00:45:42,311 --> 00:45:47,544 that it hasn't come 20 years earlier. After all, the world community already in 438 00:45:47,544 --> 00:45:54,652 1992 decided to stop global warming at the Rio Earth Summit. The nations signed the 439 00:45:54,652 --> 00:46:01,818 United Nations Framework Convention on Climate Change, and it took a full 25 440 00:46:01,818 --> 00:46:12,237 years of further negotiations to finally reach the Paris accord. Now, you can see 441 00:46:12,237 --> 00:46:17,190 here that the goal of this is to hold the increase in the global average temperature 442 00:46:17,190 --> 00:46:21,683 to well below two degrees above pre- industrial level. So it's not two degrees, 443 00:46:21,683 --> 00:46:26,467 it's well below two degrees. That's a very important point. Many countries would not 444 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 445 00:46:32,915 --> 00:46:45,520 has shown to be insufficient and. And to sorry and to pursue efforts to limit the 446 00:46:45,520 --> 00:46:50,640 temperature increase to one point five degrees above pre-industrial levels. So 447 00:46:50,640 --> 00:46:56,080 that is a more stringent Paris goal, but at least the nations have committed to 448 00:46:56,080 --> 00:47:03,200 pursue efforts. So my view is that every person should ask their own government 449 00:47:03,200 --> 00:47:08,400 what you are doing here. Is this a credible effort to try and limit warming 450 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 451 00:47:13,840 --> 00:47:18,560 limit the warming to one point five to avoid the risk of destabilization of the 452 00:47:18,560 --> 00:47:26,080 Greenland ice sheet, almost complete coral die off and many further risks. So what 453 00:47:26,080 --> 00:47:32,960 does this entail? That is an important point. If you want to limit global warming 454 00:47:32,960 --> 00:47:38,880 to some value, whatever it is, one point five, two, three, whatever you choose, it 455 00:47:38,880 --> 00:47:45,040 means you can only emit a limited amount of carbon dioxide. That is because the 456 00:47:45,040 --> 00:47:53,120 amount of global warming is to a good extent proportional to the total amount of 457 00:47:53,120 --> 00:47:59,200 CO2 that we have ever emitted. So to the cumulative emissions, it's like filling a 458 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 459 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 460 00:48:10,720 --> 00:48:15,840 global warming to some value, you can only add a limited amount of CO2 to the 461 00:48:15,840 --> 00:48:23,360 atmosphere. And this is shown here for two different examples, two different amounts. 462 00:48:23,360 --> 00:48:30,560 This is actually, the numbers here are emissions from the year 2016. So it's 463 00:48:30,560 --> 00:48:39,440 don't take these numbers from now. We have already had four more years of emissions. 464 00:48:39,440 --> 00:48:50,480 The solid lines throw show three scenarios with six hundred billion tons of CO2 and 465 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 466 00:48:56,160 --> 00:49:01,280 same amount of warming. This is about actually these lines correspond to about a 467 00:49:01,280 --> 00:49:08,160 50 percent chance of ending up at one point five degrees. And so they will get 468 00:49:08,160 --> 00:49:16,160 you the same amount of warming, but with different times of when the peak emissions 469 00:49:16,160 --> 00:49:23,440 are reached. So 2016 went past without us getting over the peak of the emissions. 470 00:49:23,440 --> 00:49:29,520 2020, maybe we still have a chance. Emissions have dropped a bit in 2020, but 471 00:49:29,520 --> 00:49:34,240 not for structural change and mostly, but due to Corona. But we still we have a 472 00:49:34,240 --> 00:49:41,410 chance that maybe next year they are lower still. And what this shows is that the 473 00:49:41,410 --> 00:49:46,160 longer you wait, the steeper your reductions have to be, not only because 474 00:49:46,160 --> 00:49:51,186 you're starting later, but also because you have to reach zero earlier at the end. 475 00:49:51,186 --> 00:49:57,147 Notice how all these three lines, the later you start with reducing, the earlier 476 00:49:57,147 --> 00:50:01,837 you have to reach zero emissions, because the surface area under these curves is 477 00:50:01,837 --> 00:50:07,317 what counts for the climate goal. The dashed lines a more generous goal, which 478 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 479 00:50:15,727 --> 00:50:21,452 Paris goal of well below two degrees, which would allow us to gradually reduce 480 00:50:21,452 --> 00:50:28,054 emissions to zero by 2050. This is not counting in any negative emissions 481 00:50:28,054 --> 00:50:34,010 afterwards, by the way. This is the net emissions, if you like. So we have to 482 00:50:34,010 --> 00:50:40,315 reach net zero emissions in 2050. But of course, if we wait five more years until 483 00:50:40,315 --> 00:50:44,454 the emissions start to decline, then they'll have to be at zero five years 484 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 485 00:50:50,680 --> 00:50:58,397 article by Christiana Figueres et al. in Nature, published 2017, where I was a 486 00:50:58,397 --> 00:51:06,380 coauthor as well. Now, a final point. Can tipping points maybe help us? And I'm 487 00:51:06,380 --> 00:51:10,330 talking here about societal tipping points. And there are also some 488 00:51:10,330 --> 00:51:17,474 interesting studies on that. The basic idea is that shown in the top right here, 489 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 490 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 491 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 492 00:51:36,207 --> 00:51:43,070 hill into that beneficial equilibrium of a sustainable global economy, a sustainable 493 00:51:43,070 --> 00:51:49,037 energy system, a stable climate and so on? Complete decarbonization, that means no 494 00:51:49,037 --> 00:51:55,720 more fossil fuel use. And these this green addition there that is added there, this 495 00:51:55,720 --> 00:52:02,299 is just some examples of how we can make this transition earlier, easier and the 496 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 497 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 498 00:52:15,160 --> 00:52:23,275 transition easier by subsidizing renewable energies. There are there is a greening of 499 00:52:23,275 --> 00:52:28,905 values. There is a tipping point in thinking, in society. There are many co 500 00:52:28,905 --> 00:52:34,080 benefits of this transformation in terms of avoided air pollution. For example, 501 00:52:34,080 --> 00:52:39,528 millions of people die every year from outdoor air pollution, which would which 502 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 503 00:52:45,801 --> 00:52:51,940 massive movement by the young people Fridays for future. He is Greta Thunberg 504 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 505 00:52:56,892 --> 00:53:02,596 Fridays demonstration in Berlin where I took this photo. This is really changing 506 00:53:02,596 --> 00:53:08,554 the societies values and it's changing election results and it could be a tipping 507 00:53:08,554 --> 00:53:16,400 point towards a sustainable global society. And with that hopeful message, I 508 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, 509 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 510 00:53:27,477 --> 00:53:32,949 follow me on the blogs and of course, in social media, preferably Twitter, but also 511 00:53:32,949 --> 00:53:38,200 the scientist for future logo there, because many thousands of scientists are 512 00:53:38,200 --> 00:53:44,626 engaged there to try and stop the climate crisis. This is really a matter of 513 00:53:44,626 --> 00:53:50,695 survival of civilization. Thank you very much for listening. Stick to science and 514 00:53:50,695 --> 00:53:54,401 leave policy to us. Well, we tried that approach. You didn't want to hear about 515 00:53:54,401 --> 00:53:59,847 the science when it could have made a difference. 516 00:53:59,847 --> 00:54:03,791 Herald: Thank you so much Stefan for your talk. Now we have some questions from the 517 00:54:03,791 --> 00:54:10,114 Internets. Let's see the first question Question: Which additional tipping points 518 00:54:10,114 --> 00:54:17,696 will be triggered at two degrees, three degrees and so on? 519 00:54:17,696 --> 00:54:22,783 Stefan: That is actually a difficult question to answer because of the 520 00:54:22,783 --> 00:54:29,264 uncertainty that I mentioned in my talk about where these tipping points are. 521 00:54:29,264 --> 00:54:33,742 There is one in Antarctica, the Wilkes basin, that is a part of the Antarctic ice 522 00:54:33,742 --> 00:54:40,571 sheet that that could be triggered, say, below three degrees. There are others like 523 00:54:40,571 --> 00:54:46,444 the ocean circulation where you probably at least we hope you have to go beyond 524 00:54:46,444 --> 00:54:51,881 three degrees to really trigger a collapse of the Gulf Stream system. But the truth 525 00:54:51,881 --> 00:54:57,943 is that they are very large uncertainty ranges. And the main fact is that with 526 00:54:57,943 --> 00:55:06,502 every bit of extra warming, we increase the risk of crossing more tipping points. 527 00:55:06,502 --> 00:55:10,330 Herald: And are there some of these tipping points that are interrelated or 528 00:55:10,330 --> 00:55:16,008 correlated? For instance, could we save some tipping points if we are able to save 529 00:55:16,008 --> 00:55:19,714 others, for instance, the collapse of the Gulf Stream? 530 00:55:19,714 --> 00:55:24,880 S: Yes, there are these interconnections. For example, if the Gulf Stream system 531 00:55:24,880 --> 00:55:32,160 collapses, it will affect the atmospheric circulation. The monsoon systems then can 532 00:55:32,160 --> 00:55:37,120 shift the tropical rainfall balance. This is not just theoretical. We see that in 533 00:55:37,120 --> 00:55:41,440 paleoclimate where we have seen these collapses of the North Atlantic 534 00:55:41,440 --> 00:55:48,480 circulation and the paleo climatic proxy data show that it comes with shifts in the 535 00:55:48,480 --> 00:55:54,080 tropical rainfall belts that could then in this way trigger a major drought in the 536 00:55:54,080 --> 00:56:00,480 Amazon region if the Gulf Stream system collapses. And so it would be very wise to 537 00:56:00,480 --> 00:56:05,040 prevent these tipping points, especially when it comes to the ocean circulation or 538 00:56:05,040 --> 00:56:10,240 atmospheric circulation, because it's really going to mess up the weather 539 00:56:10,240 --> 00:56:17,200 patterns in a major way. Herald: How long have we known about 540 00:56:17,200 --> 00:56:23,416 human caused climate change? S: Well, in principle, in the 19th 541 00:56:23,416 --> 00:56:30,070 century, Alexander von Humboldt, actually, wrote in 1843, if I remember correctly, 542 00:56:30,070 --> 00:56:36,647 that humans are changing the climate by cutting down forests and emitting large 543 00:56:36,647 --> 00:56:40,705 amounts of gases at the centers of industry. That's almost a little literal 544 00:56:40,705 --> 00:56:46,485 quote by Alexander von Humboldt. We've known about how sensitive the climate is 545 00:56:46,485 --> 00:56:52,288 to a change in CO2 since the Swedish Nobel laureate Svante Arrhenius, remotely 546 00:56:52,288 --> 00:56:59,912 related to Greta Thunberg by the way, in India studied the effect of CO2 doubling. 547 00:56:59,912 --> 00:57:04,535 He wasn't worried by that because he thought global warming would be great. 548 00:57:04,535 --> 00:57:13,566 Bring it on. It just died, now it's back. You can see my picture so? 549 00:57:13,566 --> 00:57:20,085 Herald: yeah A: and so he suggested, you know, burning 550 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 551 00:57:25,766 --> 00:57:31,028 is bad without thinking it through properly. But the first real expert 552 00:57:31,028 --> 00:57:37,609 reports warning the US government, Lyndon B. Johnson, of the coming global warming 553 00:57:37,609 --> 00:57:44,289 due to fossil fuel use was a rebel report in nineteen sixty five, exactly 50 years, 554 00:57:44,289 --> 00:57:50,105 half a century before finally the Paris agreement was reached. 555 00:57:50,105 --> 00:57:53,384 Herald: Will you be publishing your slides from the talk? 556 00:57:53,384 --> 00:58:00,200 S: Yes, I will. Uploading the slides. Herald: What is or what should be the 557 00:58:00,200 --> 00:58:04,190 ultimate goal of the climate change mitigation? For instance, is it saving 558 00:58:04,190 --> 00:58:10,240 lives, saving other species? S: Well, I think the the ultimate goal is, 559 00:58:10,240 --> 00:58:15,920 of course, preserving human civilization, as we know it, but because I think if we 560 00:58:15,920 --> 00:58:24,640 let this run, we will not only destroy a lot of ecosystems and biodiversity, but we 561 00:58:24,640 --> 00:58:32,000 will probably cause major hunger crisis, which with big droughts like the one in 562 00:58:32,000 --> 00:58:39,440 Syria before the unrest in Syria started in 2011, the country went through the 563 00:58:39,440 --> 00:58:44,080 biggest drought in history. And according to settlement data from the eastern 564 00:58:44,080 --> 00:58:51,440 Mediterranean, it was the worst drought in at least nine hundred years. And then I 565 00:58:51,440 --> 00:58:58,320 think especially in some unstable, conflicted countries, this can really turn 566 00:58:58,320 --> 00:59:04,640 them into failed states. That is what happened in Syria. And it's what a German 567 00:59:04,640 --> 00:59:11,520 report for the German government actually warned in 2009. It was called climate 568 00:59:11,520 --> 00:59:15,520 change as a security risk, I was actually one of the coauthors of that report 569 00:59:15,520 --> 00:59:19,760 because I was in the German government's advisory panel on global change at the 570 00:59:19,760 --> 00:59:26,640 time. And I think we will see increasing hunger crisis, failed states and all the 571 00:59:26,640 --> 00:59:32,560 effects that that has on international politics if we cannot keep global warming 572 00:59:32,560 --> 00:59:39,760 below two degrees. Herald: And finally, is there a specific 573 00:59:39,760 --> 00:59:44,160 call to action for the chaos community? Is there anything that we can do with our 574 00:59:44,160 --> 00:59:50,640 mindset and our skills? S: That's a good question that I haven't 575 00:59:50,640 --> 00:59:58,080 thought about, but maybe you can know yourself the best thing, what you can do, 576 00:59:58,080 --> 01:00:03,840 I think the key is really to keep up the pressure on the political world, like 577 01:00:03,840 --> 01:00:11,440 Fridays for future has been doing: Go on the streets, protest, vote with climate as 578 01:00:11,440 --> 01:00:16,080 a priority. I think these are the key things that everyone should be doing and 579 01:00:16,080 --> 01:00:21,200 specifically in whatever profession they are. They will see some ways of how you 580 01:00:21,200 --> 01:00:26,640 can help to reduce emissions in your company, put sustainability at the top of 581 01:00:26,640 --> 01:00:31,330 the agenda and so on. Herald: Stefan, thanks so much for taking 582 01:00:31,330 --> 01:00:36,254 the time to join us today. Stefan: It's a great pleasure and honor. 583 01:00:36,254 --> 01:00:42,716 Herald: Always welcome. 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