 It's simple. When ice gets above zero degrees, it melts. But is it really that simple? If we are not talking about a small ice cube, but a big sheet of ice covering an entire continent, is that really the only factor? And am I right with my assessment? I'm looking forward to be enlightened by Professor Dr. Ricardo Winckelmann. Ricardo Winckelmann is a professor of climate science at the University of Potsdam. And she's also a researcher for climate impact. She leads the ice dynamics working group and co-leads the future lab on Earth resilience in the Anthropocene. Her research focuses on tipping elements in the Earth system. And today she'll be talking about the Greenland and Antarctic ice dynamics and the future sea level rise that are impacted by them. It appears like she's truly an expert on all things registered to ice. So please give a warm hand of applause for Professor Dr. Ricardo Winckelmann with her talk, The Big Melt, Tipping Points in Greenland and Antarctica. Have fun. And welcome. Today we're going to take a little excursion to the far north and the far south to our polar ice sheets on Greenland and Antarctica. And as this year is coming to a close, I thought we'd take a brief moment to reflect back. 2020 has certainly been an exceptional year for all of us. It was supposed to be a super year for nature and the environment, as world leaders put it at the beginning of the year. It's five years after the Paris Climate Accord. It's five years after the sustainable development goals have been announced. However, 2020 turned out to be the year when we've had to face several global crises, including the ongoing COVID-19 pandemic and also the ongoing climate crisis. What almost got lost in the turmoil is that this year also saw several weather and climate extremes which span the globe from pole to pole, with temperatures reaching record highs in the Arctic and Antarctica, with plus 38 degrees of Celsius in the Arctic and in Siberia. That's the highest temperature that was ever recorded north of the Arctic Circle. And it's roughly 18 degrees warmer than the average maximum daily temperature in June when this was recorded. And we also saw plus 18 degrees at the Antarctic Peninsula, which is again the highest temperature ever recorded in Antarctica. And this was followed by widespread melting on nearby glaciers. Now, if we're kind of zooming out and taking a look at the bigger picture, we're also at a very significant point in Earth's history. Here you see the global mean temperature evolution since the last Glacier Maximum, so the last Ice Age, until today. And whenever I look at this graph, I see two things that still strike me to this day. One is that the Holocene, the interglacial or the warm age in which human civilizations have developed and thrived that has been characterized by very stable climate conditions, by a very stable gloom mean temperature. And the other thing is that the difference between an Ice Age, here 20,000 years ago, roughly, and a warm age, that's roughly three to four degrees of global average temperature change. And we're right now on the verge of achieving the same temperature difference, but at much, much faster rates. So here you see several future temperature projections from the IPCC. And what you can see is that in all of them, the temperature increase, even the lowest one, the temperature increase is much faster than has ever recorded before. So I think it's safe to say that we have truly entered the entropocene and that humans have become a geological force. So in the entropocene, humans have become the single most important driver of global change, affecting the entire Earth system, including our ice sheets. But it was kind of the opposite in the past. Like no other force on the planet, Ice Ages have actually shaped our surroundings and thereby determined our development as human civilizations. For instance, we owe our fertile soils to the last Ice Age that also carved our current landscape that we see all around us, leaving glaciers behind rivers and lakes. So even though the ice sheets on Greenland and Arktika might seem far away sometimes, they're actually crucial also for us here today. And today I want to leave you with an impression why they are so important. And one reason they're so important is because they're an amazing climate archive. Here you see an ice core taken from one of the deepest parts of an ice sheet. And this is basically like counting tree rings. You can go back to the past and you can see what the climate was like in the deep past, ranging several hundreds of thousands of years back. And you can see the conditions, for instance, in the CO2 change, the temperature change over these really long time scales. So that's one of the reasons why the ice sheets are so important. Another one is their so-called sea level potential. Greenland and Arktika are truly sleeping giants. And to give you an idea of the sheer size of these two ice sheets, one way of doing that is to compute their ice volume in the so-called sea level equivalent. What this means is if we were to melt down the Greenland ice sheet and distribute that meltwater around the entire globe, then this would lead to global sea level rise of roughly seven meters. For the Western Arctic ice sheet, it's about five meters. And for Eastern Arktika, the tenfold, so more than 65 meters in total of sea level potential that are stored in these two ice sheets. Now over the past decades, the ice sheets have both been losing mass, and they've been losing mass at an accelerating pace. In fact, we're currently on track with the worst case climate change scenario. Here you see the observations in gray, and you also see several of the projections from the past for the ice sheets. And as you can see, we're tracking this upper branch here. So we're really on track with the worst case climate change scenario for the ice sheets. And what this means is even if we were to stop global warming today, the ice sheets would still keep losing mass because of the inertia in the system. So sea levels would keep rising for decades or even centuries to come. Why is that? Well, there are several processes that need to understand in order to keep track of sea level change and also to understand the ice sheets evolution in the past and in the future. Here you see sort of an exemplary cut through an ice sheet shelf system, where the ice sheet is in contact with the atmosphere. You have the grounded part. And then in many places, you also have these extensions, these floating extensions, the so-called ice shelves that surround particularly Antarctica. The separation between the two is the so-called grounding line. Now generally, ice sheets gain mass through snowfall just on top of the ice sheet, which then is compressed into ice. And over time, due to the sheer gravity and the sheer size of the ice sheets, it's basically pushing its own mass towards the ocean. And that's one of the reasons why there's a constant flow of ice. So ice is really not only solid, it's also fluid. The ice sheets can also lose mass through surface melting, but also through melting at the undersides of the floating ice shelves where they're in contact with warmer ocean waters. And then there can of course also be ice shelf carvings, so icebergs that break off at the margins of the ice sheet. Now what we see here, this left-hand side, that's a typical situation for the Greenland Ice Sheet. The Greenland Ice Sheet is generally grounded above sea level in most parts, and it's not only much smaller than Antarctica, but it's also located further south, so further away from the pole. And that means it's generally warmer in Greenland, leading to more surface melt for the Greenland Ice Sheet. Whereas in Antarctica, it's not only much colder there, but also the ice sheet is covered and surrounded by floating ice shelves almost all around the coastline. And that means that one of the most important driving processes for mass loss in Antarctica is this melting underneath the ice shelves, so the sub-shelf melting in contact with the warmer ocean waters. Just to give you an impression of the sheer ice thickness, I brought this picture here. This is my very first impression of the Antarctic coastline, the ice shelf margin. This is close to the German research station, Neumeyer 3, and I will never forget the moment that I first saw the ice shelf edge. It was in the middle of the night, but we were there in summer, so we had 24 hours of daylight, and I woke up because it suddenly got dark in our cabin, so I went up to the bridge to see what was going on, and I saw myself in front of a wall, like really a cliff of ice. And knowing that these ice shelves behave like the ice cubes in the water glass, so only roughly 10% are visible above the sea level. This means that in this case, we had an ice shelf edge that was more than 100 meters thick, and that really impressed me. I immediately had to think of this German expression, das ewige Eis, the eternal ice, and I really wondered if this is maybe yeah, the right expression, because it seemed like it was so static and nothing was moving. However, that's not true, because even in equilibrium, the ice is constantly moving, so you just visualize by these little snowflakes, and you can see how the ice is moving from the interior towards the coastlines, and we have a wide range of velocities at the surface, ranging from almost zero in the interior of the ice sheet to several kilometers per year in the larger ice shelves, and also in the so-called ice streams, the faster flowing ice. If I were able to take a dive underneath the ice shelves, and I could actually take a look at the grounding line, this would probably be what I could see. This is the triple point, basically, where solid earth, the ice, and water all come together, and this grounding line plays a very important role for Antarctic ice dynamics and also for the future fate of Antarctica. So what makes the dynamics of the ice sheets and shelves so particularly difficult to understand and also to project the future evolution is that both ice sheets are subject to several so-called positive so self-reinforcing feedback mechanisms. Here are just some examples with some of the major ones we know very well. One is the ice albedo feedback, and another one is the so-called melts elevation feedback. As I said, in Greenland, we observe a lot of surface melting. If you've ever flown across the Greenland ice sheet in summer, you can really see these rivers forming and then even lakes forming at the ice sheet surface. Over the recent decade, Greenland has been subject to several extreme melt events, including particularly the years 2010, 2012, and also last year. The reason there is this extreme melting at the surface is due to a combination of factors. It has to do with the duration of the summer, but also even here in Europe we observed very warm and dry summers, and that's also something that was observed for Greenland. So that, for instance, in the year 2019 in August, almost the entire ice sheet surface was covered with melt water. Now, why is this surface melting so important? The reason is that there is also a self-reinforcing feedback that could be driven by surface melting, and we all know this mechanism from mountain climbing. If you climb down from the peak of a mountain towards the valley, it gets warmer around you. And the same is true also for the ice sheets. So if there's enough melting, it could actually lower the surface to a region where the temperatures are higher, the surface temperatures are higher, leading to more melting, which again lowers the surface elevation, leading to higher temperatures, leading to more melting, and so on and so on, so that this can trigger these self-reinforcing dynamics. And whenever we have such a positive or self-reinforcing feedback mechanism, we can also have a tipping point. And here's the depiction of a very simple way of computing where the tipping point might be for the Greenland ice sheet, where we've really done this with just analytical work, so pen and paper, trying to understand where we go from a stable Greenland ice sheet into an unstable regime, which would then lead to meltdown of the entire ice sheet until basically no ice is left at the surface. So this is something that we can understand in theory, but also something that we find in more complex numerical ice sheet models. And they find that this warming threshold that leads to basically a decay of the entire ice sheet lies somewhere between 0.8 and 3.2 degrees of warming above pre-industrial levels. And you can see that between these temperatures somewhere there's almost a step change from this is now computed in sea level rise. So up here this means that Greenland is ice-free, so we're going from an intact Greenland ice sheet to an ice-free Greenland ice sheet somewhere between these temperatures. What this looks like can be visualized with numerical ice sheet models, and here you see that once this threshold is exceeded, basically the eigen dynamics lead to a complete meltdown of the ice sheet, until there's almost no ice left except for in the highest regions here in the east, where there are some small ice caps remaining. Now something similar but also different that's going on in Antarctica, because as I said earlier Antarctica in Antarctica is much colder, so we have very little surface melt at the moment, but at the same time it's surrounded by the floating ice shelves, and they play the major role in driving the changes in Antarctica. Antarctic mass loss has tripled over the recent years, especially in the so-called Amundsen and Bellingshausen Sea regions, so these regions here where you see all these red parts, so this is all ice loss that's been detected here, and the reason for this is due to the ice shelf ocean interactions, so here you now see the ocean temperatures surrounding Antarctic ice shelves, and you can see a stark difference between the temperatures here around the Amundsen and Bellingshausen regions, and the temperatures for instance here in the Waddle Sea or in the Ross Sea, the temperature difference being roughly two degrees, so there's really been a switch from a colder to a warmer cavity for instance here in the Amundsen Sea region, and that drives more sub-shelf melting, which in turn leads to a decrease of the so-called buttressing effect. What this means is, well first of all, the ice shelves do contribute to sea level rise directly, at least not significantly, the reason being that they are like ice cubes in a water glass, and if that melts down it also doesn't raise the water level in the glass, so it's similar with the ice shelves, but at the same time they are still attached to the grounded part of the sheet, so if the ice shelves melt, or there are larger calving events in the ice shelves, that means that the flow behind them from the interior of the ice sheet into the ocean accelerates, it's almost like pulling a plug, and this is what is called the so-called buttressing effect, so the back stress at the grounding line, so if we have enhanced ice shelf melting, that means that this buttressing effect, this buffering effect, is reduced and therefore we have accelerated outflow into the ocean. Now the question is how does this impact the ice sheet dynamics overall, and in particular the stability of the western east and arctic ice sheets? You may have come across some of these headlines in recent years, my favorite one is still this one appeared from 2014, where the holy shit moment of global warming was declared, and the reason for this were these observations from the Amundsen region in western arctica, so we're now taking sort of a flight into the Amundsen Sea region, and what was observed over the recent decades is not only that the glaciers here have accelerated, so everything that's shown in red is accelerated ice flow, but at the same time the glaciers have also retreated and the deeper valley is behind, so you see this browning at the surface now, so all of these are changes where the the glaciers have basically retreated, and with this comes another self-reinforcing feedback, the so-called marine ice sheet instability. For the marine ice sheet instability to occur, we need two conditions to hold. One as depicted here is that the ice sheet is grounded below sea level, which is true for many parts of west and arctica, but also some parts of east and arctica, and also we need to generally have a retrograde sloping bed, so that means that the bedrock elevation decreases towards the interior of the ice sheet, and with these two conditions hold, then we can show in two dimensions mathematically, even prove mathematically, that an instability occurs in this kind of case. The reason is that we have a feedback between the grounding line retreat and the ice locks across the grounding line. If the grounding line retreats in a case where we have a retrograde sloping bed and the ice is grounded below sea level, that means that the ice thickness towards the interior is larger, and this generally also means that the ice flux across the grounding line is larger, leading to further retreat of the grounding line and so on and so on. So again, we have a positive feedback mechanism that could drive self-sustained ice loss from parts of the western eastern arctic ice sheet. And the concern is now that this marine ice sheet instability is potentially underway in the Amazon basin here in western arctica. Now what's unclear is how fast this change would actually occur. So if we have actually triggered the marine ice sheet instability in this region, and that means we have committed ice loss of roughly one meter sea level equivalent, then the question is still how fast does this occur, and for this it really matters how much further global warming continues and at which rate the temperature will change in the future. So this is what's happening in part of the western arctic ice sheet. We were also asking ourselves what could something like this also happen for eastern arctica and how stable are each of the different ice basins in arctica. So we did something of a stability check on the Antarctic ice sheet to assess the risk of long-term sea level rise from these different regions. What you will see next is an animation where we're increasing the global mean temperature, but we're increasing it very very slowly at a much slower rate than the typical rate of change in the ice sheet to test for the stability of these different parts. And what we see is that at roughly two degrees we are losing a large part of the western arctic ice sheet, so there's a first tipping point around two degrees and then as the temperature increases also the surface elevation is lowered and that leads to potentially then also triggering these surface elevation and melt elevation feedbacks in eastern arctica. So around 60 to 9 degrees there's another another major threshold and after this large parts of the eastern arctic ice sheet could also be committed to long-term sea level rise. At about 10 degrees the Antarctic ice sheet could potentially become ice-free on the long-term and this is really important. What we're seeing here are not projections, but what we're seeing here is a stability check. So we're not looking at something that's happening within the next century or so, but rather we're interested in understanding at which temperatures the Antarctic ice sheet could still survive on the long term. We also wanted to see if some of these changes are reversible and what we find is so-called hysteresis behavior of the Antarctic ice sheet. That means as we're losing the ice and we're then cool the temperatures back down the ice sheet does not regrow back to its initial state but it takes much much colder temperatures to regrow the same ice sheet volume that we are currently having at present day temperature levels. So there's a significant difference between this retreat and the regrowth path and this can be up to 20 meters of sea level equivalent in the difference between these two paths. What this looks like originally you can see here, so again we have the retreat and the regrowth paths at two degrees of global warming and four degrees of global warming. So these are the long-term effects at these temperature levels and you can see that for instance for for four degrees large parts of eastern Antarctica and also of the western Arctic ice sheet do not regrow at the same temperature level. So we clearly observe this hysteresis behavior that's another sign that the Antarctic ice sheet is a tipping element in the climate system. So both Greenland and Antarctica are tipping elements in the climate system. There are a number of more candidates for tipping elements including some of the larger biosphere components, for instance the Amazon rainforest, the tropical coral reefs and also the boreal forests, as well as some of the large-scale circulations. So for instance the Atlantic thermohaline circulation or what we often term the Gulf Stream and the Indians in Monsoon are tipping candidates in the climate system. Now if we go back to our temperature evolution since last geisha maximum and we now insert what we know about the tipping thresholds of these different components in the Earth system then this is what we get and we see that there are basically three clusters of tipping elements in comparison to the global mean temperature here and you see in these burning amber diagrams that some of these tipping elements are at risk of switching into a different state even within the Paris range of 1.5 to 2 degrees of warming and among these most vulnerable tipping elements are the western Arctic ice sheet and the greenland ice sheet and in general the cryosphere elements which seem to react to global warming and climate change much faster and therefore belong to the most vulnerable parts of the Earth system. So if there's one thing that I would like you to take away from this talk it is that ice matters. I've presented you with three reasons why. First of all polar ice acts as a climate archive it also acts as an early warning system. Secondly glaciers and ice sheets are important contributors already to currency level rise but they will become even more important in the future as the global mean temperature keeps rising. And thirdly both Greenland and Antarctica are tipping elements in the Earth system and one of the next things we need to understand is how these tipping elements interact with one another because we have a very good understanding by now of the different mechanisms behind these tipping elements and of the individual temperature thresholds but one of the I think most important questions we need to ask ourselves is how the interaction of the tipping elements that changes the stability of the Earth system as a whole and if there could be something like domino effects in the Earth system. And with this thank you so much for your attention and I'm very much looking forward to questions. We are now going to have a Q&A and if you have any questions regarding this awesome talk then please post them to the Signal Angels. They are following on Twitter and the Fediverse if you're using the hashtag, hashtag RC31 because this is RC1 Hall and you can also post your questions to the IRC and I already have a first question. I don't know Riccardo if you can hear me but is there anything that this specific as a community of nerds and hackers can do more than anyone else to help with this issue? What what do you think that we can do to help this? Yeah thank you so much. Great question. Let me start by saying I'm a nerd and hacker myself. I'm a developer or co-developer of the parallel ice sheet model that's one of the the ice sheet models for Greenland and Antarctica that's being used around the globe with many different applications so yeah as a fellow nerd and hacker I can say there's lots we can do in particular towards understanding even better the different dynamics of the Greenland and the Antarctic ice sheet but also beyond that for the Earth system as a whole. I think we're now at a point where we understand the individual components of the Earth system better and better. We also have better and better observations, satellite observations but also observations at the ground to further understand the different processes but what we need now is to combine this with our knowledge in the modeling community and also with some of the approaches from big data machine learning and so on to really put this together all the different puzzle pieces to understand what this means for the Earth system as a whole and what I mean by that is we now understand that there are several individual tipping points in the Earth system and we also know that as global warming continues we're at higher risks of transgressing individual tipping points but what we still need to understand is what does this mean for the overall stability of our planet Earth. Thank you for this extended answer to this question. I have another one and I would like to know you showed a slide where you showed the browning of the eye surface and then explained that this speeds up the process of melting as well like done by deer but can't we just paint it white or with a reflective paint on it has this been simulated is this of interest to you scientists? Yeah very good question so basically what you are addressing here is the question of the so-called ice albedo feedback we all know this that as we're wearing black clothes and summer it's warmer than when we're wearing white clothes and the same is basically true for our planet as well so the ice sheets and also the sea ice and the Arctic and Antarctica they contribute considerably to a net cooling still off the planet so if we didn't have these ice landscapes that would mean that the planet would warm even faster and even further than it already is today so currently the ice albedo feedback is still helping us with keeping the temperatures at lower levels and they would be without the ice landscapes and yeah therefore it is definitely of interest to further understand what would this mean for for instance the global mean temperature but also regional changes if we were to lose our ice cover completely and also the reverse question of course if we were to widen parts of the planet then how would this affect the temperature one thing that we found out is that if we were to lose the ice sheets and the sea ice in terms of the ice albedo feedback alone entirely then this could already lead to additional global warming of roughly 0.2 degrees celsius now that may not seem very much but it certainly is important in the grand scheme of things as we're thinking of for instance the Paris range of 1.25 to 2 degrees of warming every tenth of a degree matters so yeah very interesting question and this is something that has been done with numerical ice models just to understand what kind of an effect these kind of what if scenarios would have also in terms of the albedo very interesting so should we now start to develop drones who can spray paint that's a good question i don't think that's the solution i think we have a much better solution and that is we know that we need to to mitigate climate change and reduce greenhouse gas emissions and that is one that would work for sure whereas these questions of well should we spray paint all of our buildings at the at the top white that is something that cannot be done at such a large scale as we would need it in order to reverse global warming and another thing to keep in mind is that even if we were able to reduce the global signal this still doesn't mean that we could also reverse the the regional scale changes we're already experiencing a large increase in extreme weather and climate events and and that is certainly something that i haven't seen so far that this could also be reversed just by reversing the global mean temperature change as a whole answer i have another question i think that's quite interesting how old is the oldest ice in Antarctica are you aware of that and how long would it take a minimum to lose that entirely yeah very good question um so the oldest ice there's actually an ongoing search for the oldest ice in Antarctica so to say we know that Antarctica was ice free for the last time roughly 34 million years ago so when we're talking about these scenarios that um eventually and artica could become ice free with of course very strong global warming scenarios of about 10 degrees of global warming then we need to keep in mind that this was the case for the last time about 34 million years ago now as we're speaking there is an ongoing project an international collaboration to find and and also drill for the oldest ice so that we can really understand our earth's history better and better and so this is a very exciting project because as i said the ice cores are kind of like tree rings and we can count back in time and really understand what our global climate was like several hundreds of thousands of years ago so yeah with with that being said i think it's important to keep in mind that this is something that humans certainly have never experienced and that's therefore unprecedented in our world for this very elaborate answer to this question i know it is not the core of your research but someone from the internet asked if it's possible for old viruses and old bacteria from back when Antarctica was like beginning to freeze over or from like millions of years ago is it possible for them to thaw out again and is that a danger for us oh that's also a very interesting question so i'm not no expert on this but i could imagine that um at the temperatures that we have in in arctica so especially at the core ice body there um we have temperatures that go down to well i think the coldest temperature was something like minus 90 degrees celsius that was recorded there but in any case it's very cold there so um there might be some bacteria that can survive these conditions and i've read about um bacteria like that but um i wouldn't know that there are many um bacterial species or specimen that that could survive uh these kinds of conditions so um to be honest i i would have to read up on that that's a very interesting question so yeah thank you for this answer um i remember that you uh watched that you showed an animation in the graph for a simulated ice decline to find the tipping points in arctica and on the x axis of that i couldn't see a time scale and now someone asked on the internet what are the timescales between reaching a tipping point and most of the ice being melted is the years the decades centuries millennia what's kind of the scale there yeah very important point um so it's important to note that um we're here um showing this over the global mean temperature change and the reason for this is that um the way these kinds of hysteresis experiments are run is that you have a very slow temperature increase um so slow in fact that um it's much slower than the sort of internal um timescales of the ice itself and in in this case for instance um we had a temperature increase of um 10 to the minus four degrees per year um and the reason for this is because this is the way you're approaching um the actual hysteresis curve that we were interested in so this should not be mistaken for um for sea level projections um of any sort so what we find here are the actual so to say tipping points the actual critical thresholds um that parts of the Antarctic ice you cannot survive nonetheless of course we're also working towards sea level projections and trying to understand what kind of sea level change we can expect from the ice sheets um over the next decades to centuries to millennia and one important thing there is that most of the ice loss that could be triggered now would actually happen after the end of this century so very often when we see these sea level curves we're looking until the year 2100 so for the next decades how does the sea level respond to changes in temperature because but because we have so much inertia in the system that means that even if the temperatures were the global warming signal was stopped right now we would still see continued sea level rise for several decades to centuries and that is something important to keep in mind so I think we really need to start thinking of sea level rise in terms of commitment rather than these short-term projections that being said another important question and factor is the rate of sea level change because this is actually what we need to adapt to as civilizations when we think of building dams there are two questions we need to answer one is the magnitude of sea level rise and also in an upper scale an upper limit to that and the other question is the rate at which this changes and what we find is that on the long term there is something like 2.3 meters per degree of sea level change so this is sort of a number to keep in mind when we think of the sea level projections and yeah I think it's really important to consider longer time scales than the one till the year 2100 when we talk about sea level rise thank you for this answer is very interesting and we are out of time now so thanks for all the questions and thank you Riccata for this amazing talk the next talk on this stage will be about the related topic and measuring co2 indoors but also in the atmosphere in general but before that we have a Harald News Show for you prepared so enjoy