Arctic Circle’s Post

Transcript

Yes, hello everyone. Fantastic to be here with such a great audience. It's a great honor. What you see here, this is what is happening in the last decades in the oceans around surrounding us, and the one striking thing you immediately see is this so-called cold BLOB, an area which has been cooling over the past decades. But I want to also point out another feature and that is a region of excessive warming along the North American coast. And now these data show this for the time since we have satellite data since 1993, but we also have data going back longer in time. Here's a data set showing the trend since 1940 and we see exactly the same picture, the cold BLOB and the excess of warming in along the. American coast here, and also data starting in the 19th century show the same thing. And in fact, this cold BLOB is the only part of Earth that has been cooling since the 19th century, while the entire rest of the planet has warmed. So what's going on there? Now? This has to do with the AMOC, as you might have guessed, because it's in the title of my talk, and the AMOC is. And overturning circulation along the whole length of the Atlantic, with a warm surface flow starting from South Africa going right up to the high northern latitudes. There the water gives off its heat to the atmosphere, thereby gets colder and thereby dense enough to sink down to depth of two 2000 to 3000 meters, where it returns as a cold current shown in blue here to the South. And that works like a central heating system. As the minister has already referred to, it transports a huge amount of heat into the high northern latitudes, equivalent to about 50 times the energy use of humanity. And sometimes this is confused with the Gulfstream and that is because the AMOC flows along the Gulf Stream part of the way, but it contributes only 20% of the water flow of the Gulfstream. But it is the majority of the heat transport to the north because of the fact that the return flow is very cold. So that temperature difference of course makes the net heat transport. Now when this current slows down that region, that cold BLOB region will cool. So that is a something that has been well known, predicted by models. So that is an indicator of a slowing AMOC, but also this warming along the American coast is also an indicator of a slowing AMOG. It's a bit more complicated physics on a rotating sphere, but believe me, that's a well understood phenomenon. So taken together the cold BLOB and that warm part. Are considered the fingerprint of a slowdown in the AMOC and the cold blood. That's not just a surface phenomenon. If you plot the heat content anomaly in the ocean down to 2000 meters, that area stands out just the same. And so it indicates a slowing AMOC. Why would the AMOC slow down? It's because this region of the cold blood is not only cold, but it's also freshening. So the salinity is declining and that makes the water less dense and therefore harder to sink down and. Actually, in the cold BLOB region, the salinity now is the lowest since measurements began 120 years ago. And why is the salinity declining there? That's a result of global warming. We enhance the water cycle, more evaporation in the subtropics, more precipitation into the high latitudes. Then there's additional melting sea ice, melting glaciers, melting Greenland ice sheet, all contributing freshwater, diluting the ocean waters there and slowing down the AMOC. Now the AMOC has a very powerful effect on climate, and you can see that in a model. If you shut the AMOC down, you can do that by dumping a lot of fresh water in the North Atlantic. Then the overturning stops, the AMOC dies down, and then you get this kind of temperature change pattern. First of all, you see a really shocking cooling in the Nordic Seas, 20��C in. Annual mean temperatures, but you can also see the entire Northern hemisphere cooled. The entire Southern hemisphere warns. And that, of course, is not a future projection. This is just the effect of the AMOC alone. I will later come to the effect when combined with global warming. One thing we know from paleoclimate is that there have been some of the most abrupt and striking temperature changes. Climate changes in the paleoclimate record have been caused by instabilities in the AMOC. This shows in blue Greenland ice core data for 60,000 to 10,000 before present and in the green sediment data and I I have studied these abrupt glacial climate events. Since the 1990s. So I could give you an hour long talk on those, but here I don't have time for that. And let me just say that the concern about instability of the AMOC actually originated from paleoclimatologists. I'm thinking for example, of the famous American oceanographer Wally Broker, who warned that climate change would not necessarily proceed smoothly, but there could be disruptions of ocean circulation and so on. Now if we look into the future, this is what the high warming IPCC models predict for 2100. It's a straight from the IPCC report and you see again this cold BLOB here and in the models of course it is due to the global warming as I have explained and you can see here, the cold BLOB actually expands in these scenarios, cooling down Ireland, Scotland for example. So models, at least part of them predict this kind of thing happening, but generally the models underestimate the cold BLOB until now. So this is again also straight from the summary for policymakers from IPCC. And on the left you see the observed cold BLOB. And on the right hand side, what the models predict until now if you drive them with historic emissions data and you don't see the cold BLOB, you see just a somewhat reduced warming. And so this is 1 indication that. The climate models are underestimating this problem. They are. There's also a whole subsection of the scientific literature on AMOC. Arguing that the models are have an amount that is inherently too stable. And there's additional factors like the climate model still haven't got interactive Greenland ice sheet, so they don't include the growing meltwater influx from Greenland, for example. We can also turn to paleoclimate data. These are 7 different studies with proxy data from sediments, from corals and other things. So very different data sources, different scientific teams and of course different time resolutions. But all these reconstructions of amok for the last thousand or more years agree on one thing that it's been relatively stable until in the last 100 years or so it's in steep decline. Now that in itself, of course. That's causing this cold BLOB, and that already is affecting our weather and is in itself a problem. But the even bigger problem comes through the fact that the AMOC has a tipping point. And what is this tipping point? It's demonstrated here. The green curves show the stable equilibrium strength of the AMOC as it depends on how much freshwater is flowing into the northern Atlantic. So when you start with the present climate there on the top branch where you have the AMOC flowing as we know it, and you add freshwater, you're moving along on the right and you see this decline of the AMOC gets steeper and steeper and then the curve bends back on itself. And that is a tipping point. It's a mathematically it's a bifurcation. Point and it results from an amplifying feedback which is well understood. This is understood since Henry Stommel and other very famous American oceanographer described this feedback system in 1961. And the feedback is such that if you add some fresh water, the amore gets weaker. But that means the AMOC itself transports less salty water from the subtropics where it's salty because of evaporation there to those high latitudes that. Makes it even fresher, the water weakening the AMOC more. So that's an amplifying feedback system. And these are always behind the physical tipping points in the climate system because these amplifying feedbacks at a certain point become so self reinforcing that you don't have to push the system anymore. It will basically go over the Cliff there where there is no more stable equilibrium with an AMOC and then it basically collapses. So that is the tipping point and. Also, the ICC has issued quite a stern warning about the tipping points in general. Namely saying that the risk associated with large scale singular events or tipping points transitioned to high risk between 1 1/2 and 2 1/2 degrees warming. Below 1.5, it's moderate risk, not not low. So that is a real problem, and it's one of the reasons why we should really try to stay as close to 1.5�� as possible. Another a new study that is still a preprint that hasn't been peer reviewed yet shows that what happened with those climate models that have continued beyond 2100, that's not been shown in the IPCC report. There's around about 30 of these models and 9 have since the IPCC continued their runs further into the future and for high emissions, that is the red curves. All nine of these collapse the AMOC. And you can see that here it happened already by 2100, they're well on the way to collapse. But the the last bits happen after 2100 and even some of the moderate emissions or low emission scenarios, some of those models also get a collapse in AMOC, despite the fact that probably these models have a too stable AMOC. And what is of particular concern is that the tipping point for a full collapse is most likely already passed much earlier in the in this century, probably even in the first half of this century. The consequences once the AMOC is collapsed in a globally warmed world looks something like this. In the top panel you see then the annual mean surface temperature changes and you know, the planet warms, the northern Atlantic region cools. The exact location of that cold patch depends of course on how much warming there is compared to the AMOC collapse. How far along are we already with warming? How early does the AMOC collapse? And so on. But the bottom panel shows changes in rainfall patterns, and we also know those from paleoclimate. There have been drastic changes every time the Amor collapsed during the last Ice Age along the tropics because the tropical rainfall belts. They shift S when the Northern hemisphere cools and the Southern hemisphere warmth. But of course, here in this part of the world, we are mostly concerned about this. And I sometimes get asked by journalists, well, isn't that great, say in northern? How many of the two effects just compensate? But that's an illusion of course, because that's only in annual mean. But we we don't have ever annual mean conditions out there. We have weather and it varies depending on whether we have cold air from the north or warm air from the South. And the contrast will be much greater. And these contrasts Dr. storms their drive extreme events. So if you have Southern Europe warming and Northern Europe cooling, that's really bad news for extreme events. And there are also other consequences. For CO2 uptake by the ocean, which normally with the AMOC gets drawn down into the deep sea by the deep water formation. The oxygen supply for the deep sea comes with this deep water formation. There are big sea level changes in response to an AMOC collapse, so there are many very serious problems. And to finish off, I want to point to that big report on global tipping points published last year by more than 200 researchers from over 90 organizations in 26 countries, which concluded that harmful tipping points in the natural world pose some of the gravest threats faced by humanity. They're triggering will severely damage our planets life support systems and threaten the stability of our society. So my conclusions are the AMOC is weakening, I think very strong evidence for that. The AMOG has a tipping point, well understood since the 1960s and confirmed again and again with more and more complex models. But the billion dollar question is how close are we? To this tipping point and will we maybe push it over the edge already in the next few decades and recent studies have really. Given us, given certainly me, the impression that this risk is much greater than we thought just five years ago, and This is why a group of among experts has come together to write an open letter to the Nordic Council of Ministers. Which points out to that risk and says please take this very seriously. We need more risk analysis and most importantly, we hope that the Council of Nordic Ministers uses its its diplomatic cloud that it certainly has in the world to push for more deep and stringent and rapid emissions reductions. And I'm very grateful that I have the opportunity here today to hand this letter to the Climate Minister, thought her son. And thank you very much for receiving this and passing it on to your Nordic Minister colleagues.