Can water flip the planet’s climate? : paleoclimatologist Christo Buizert
Could shifts in the world’s water cycle tip the earth’s climate? In 2023, a paper was published that exploded into a huge scientific debate. It was about the circulation of our oceans, and how it might be slowing down and changing, which was a matter of huge concern. It is called the AMOC, the Atlantic Meridional Overturning Circulation. The paper raised the possibility that the AMOC might approach a major transition in the near future, which could have profound consequences for our climate. But why should ocean water moving around matter so much? To look at the history, let’s go back in time to the mid-20th century, when a Danish scientist, Willi Dansgaard, was looking at ice-core data that he had persisted through a military operation to gain access to. After much persistence, he was able to examine the data. He was expecting to see slow changes in the ice record, where temperature would change gradually. Instead, something very peculiar popped up. There seemed to be jumps in the record where the temperature would change very quickly. At first, he thought it might be some error in the collection or measurement. But after carefully checking, he realized it was real. The climate was making sudden shifts. Then another scientist, Hans Oeschger, went to check if these shifts were real, and using a totally different technique also found the same fluctuations. The temperature appeared to shift by several degrees Celsius over just a few decades. There were roughly 25 such events during the last 100,000 years. These events became known as Dansgaard-Oeschger events. And this created a rather uncomfortable question for the scientists who were expecting something else. What could make a climate system suddenly jump from one state to another? Oeschger and his colleagues began considering whether the ocean might be involved. The ocean could store enormous amounts of heat, and changes in the North Atlantic could potentially alter how that heat was distributed. Then Wally Broecker built on these ideas and proposed that the ocean was acting something like a giant conveyor belt, carrying heat from one part of the world to another. The circulation moves in a complex, three-dimensional pattern, with surface waters flowing in one direction and deep waters returning in another. Imagine heating a kettle of water. The water stores a lot of that heat. Now imagine a bathtub with hotter water coming in from the faucet, and cooler water further away from faucet. If you swish the bathtub, it changes where the distribution of heat. Now imagine the bathtub is the whole ocean. Its water now has different temperatures and different amounts of salt, and those differences determine how dense it is. Dense water tends to sink; lighter water tends to stay near the surface. That simple fact, repeated across an ocean thousands of meters deep, helps drive some of the largest flows on Earth. And now imagine changing that balance. Put more freshwater into the North Atlantic, and you’re not simply adding water. You’re changing the salinity, and therefore the density, of an enormous body of ocean. If enough of that happens in the right place, it can alter where water sinks. If you alter where water sinks, you can alter the circulation. And if you alter the circulation, you can alter where heat goes. And if you alter where heat goes, you can alter the climate. Its perhaps surprising that something as seemingly simple as water sinking or rising in the ocean could help determine the climate state of an entire planet. Which raises a question: How much of Earth’s climate is really being controlled by where the water goes? Broecker’s ideas helped lead toward today’s concern about the AMOC and whether anthropogenic climate change could weaken or reorganize this circulation. If the ocean transports different amounts of heat from one part of the world to another, the consequences need not be confined to the ocean. They can propagate through the atmosphere, sea ice, and the rest of the climate system. Why might AMOC slow down in the future? A warmer world changes the water going into the North Atlantic. As the planet warms, the atmosphere can hold more moisture, changing rainfall and freshwater input to the North Atlantic. At the same time, Greenland is losing ice, adding freshwater to the ocean. Both processes can make surface waters in the North Atlantic less salty and therefore less dense. If the water becomes less dense, it becomes harder for it to sink. And if less water sinks, the overturning circulation can weaken. There are other influences too. Changes in winds, aerosols, ocean heat, and the broader hydrological cycle can all affect the circulation. Climate models generally project a weakening of the AMOC as greenhouse gases rise, although there is substantial uncertainty about how large that weakening will be. (nature.com) If the AMOC weakens, it means that the distribution of heat across the planet can change. The North Atlantic could cool relative to the surrounding world. Rainfall patterns could shift. The position of major atmospheric circulation systems could move. European and North American climate could change, while tropical rainfall and monsoon systems could also be affected. And if the circulation has something like the multiple states suggested by the Daansgard-Oeschger record, there is another possibility: a gradual forcing could produce a sudden transition. We don’t know that the AMOC is about to collapse. In fact, the IPCC’s assessment has been that a complete collapse is unlikely during the 21st century, while more recent studies continue to disagree about how close the system may be to a critical transition. I was super glad for the opportunity to talk to Christo Buizert of Oregon State University, who has made significant contributions to our understanding of the Daansgard-Oeschger (D-O) shifts, work for which helped win him the European Geosciences Union Early Career Scientist award. Christo Buizert and his colleagues looked at ice cores from both the Northern and Southern Hemispheres and found a striking relationship between the timing of Greenland and Antarctic climate changes. Greenland warming was followed by Antarctic cooling with a lag of roughly 200 years, while Greenland cooling was followed by Antarctic warming with a similar lag. This suggested a long-timescale seesaw in the distribution of heat between the hemispheres associated with changes in the ocean. At the same time, Buizert and colleagues found evidence that the atmosphere could respond much more quickly, meaning that the climate system was communicating these changes through more than one pathway. This is the strange thing about the climate system. A change that happens relatively quickly in one place can leave a signal that unfolds over centuries somewhere else. In addition to discussing his work on the Daansgard-Oeschger shifts, Buizert and I also discussed the nature of regime shifts in climate in general. When you push a complicated system, how do you know when it’s about to start behaving differently? Where are its tipping points? And what roles does the water cycle play in creating these shifts? We also discuss how the Sahara was once green, with lakes, rivers and grasslands; and how wobbles in the earth axis, and shifts in the water cycle, lead to Sahara becoming a desert. We discuss how there are two metastable states that the Sahara can be in. A desert can be the result of feedback loop, where less trees means less rain recycling, which then leads to even less trees. Here is some of our conversation (edited for clarity) Alpha: Today I’m super excited to have Christo Buizert with me. He’s going to talk about paleoclimate, the water cycle, and regime shifts. So welcome. Christo: Pleasure to be here. Paleoclimate is studying the history of the past. When it comes to our understanding of our climate system and our Earth, we really have quality observations going back maybe only 150 years for some of them, like temperatures, and for the really good observations, such as satellite records, only maybe 50 years. If we want to understand what happened before that period, before we had direct observations of how our Earth worked, we have to rely on other ways of reconstructing what the climate may have been like. My work focuses on using ice cores to reconstruct how climate has been changing over the last five million years Alpha: So water in a frozen state encapsulates some of our history. And also it’s interesting that the water cycle is actually a key player in how a lot of these climate regime shifts have happened. Christo: Yeah, absolutely. I think the reason we talk about regime shifts, the reason we talk about tipping points, actually stems a lot from this paleoclimate research. If we go further back in time, we see evidence for massive changes in the climate and sometimes extremely abrupt changes in the climate. And so that really has given the impetus to studying these kind of tipping points. So the reason we talk about these regime shifts and tipping points in the first place is because we’re looking into our past climate, which is a lot more variable, a lot more dynamic than the kind of changes we’ve seen over the last 150 years Alpha: So they might help us understand some of our climate change issues. Christo: Oh, absolutely. The hope is, or the goal is, that we can use these, first of all, as analogs. Can we look at these events in the past and use them as analogs to understand what might happen in the future? And second of all, we use it for testing the models that we have. When we look into the future, when we’re projecting future climate change, we do this using numerical models of how we think our climate works. But those models, of course, are imperfect, and we can improve those models, we can test those models by applying them to these paleoclimate problems and seeing how well they fare. How well do they actually compare to our real observations? So it’s a unique opportunity to test our models and see if they’re up to snuff. Alpha: Where did you grow up and how did you get into this? Christo: I was born and raised in the Netherlands, and I’ve always really had a fascination for physics through high school. So I ended up doing an undergraduate degree in physics and also a master’s degree in physics, and I ended up studying quantum physics, experimental quantum physics, quantum dots in particular. I did that for maybe a year and a half, and it was very fascinating research, but my heart was never really in it. And I had taken some classes in environmental physics, and I always found that a fascinating topic. So after I finished my master’s, I was a high school teacher for a year. And during that time I had to develop some curricula, some material for my students. And I ended up developing this whole module on climate physics, and through that I did a lot more research and reading, and I became even more fascinated by this topic. And then a PhD position opened up at the University of Copenhagen in Denmark to work on ice cores. So I just applied, and I’ve spent the last 20 years of my life studying ice cores and frozen ice and how they can help us to understand how the climate is changing. Alpha: What was it that appealed to you initially about ice core research? Christo: I guess it’s the sort of fascination of deep time, right? It’s sort of—you can almost think of it as a detective novel or something, right? Or CSI. We know that all these changes have happened. The Earth is a very dynamic place, but it’s also a very complex place. And when we go further back in time, we have very limited records and very limited observations, and sort of puzzling together the history of our planet to understand how it works and to understand how it has changed in the past. I’ve always just found it really intriguing, like a mystery novel. You know, you find all these clues. Maybe there’s a hammer in the library, you know. We find in the ice cores these weird events that may have happened, and then we can look at other different parts of the planet, and we can combine it with models and different lines of evidence to try to form a full picture of what happened. Alpha: So when you were a graduate student, were you going off to these faraway places drilling ice? Christo: Yes, and I’m not gonna lie, that’s probably one of the reasons I applied in the first place. I’ve done two expeditions to Greenland, research expeditions to Greenland, and two expeditions to Antarctica to drill ice cores. Every year there’s a layer of snow that falls, but it’s so cold that that snow never melts. And it’s been piling up to a height of about a few miles of ice. If you go to the interior of Antarctica, the ice there is about three kilometers, or two miles, thick, and the same is true in the interior of Greenland. So you can think of these individual layers of snow, sort of like the pages in a book, perhaps, right? So every year there’s a page, and that’s where, drilling all the way to the bottom, we collect all those pages. We get a little bit of all those pages, and inside of those is a record of what the Earth was like. We can reconstruct what the atmosphere was like. Maybe there was a volcanic eruption and we see a layer of ash. Maybe it was very warm, and we can reconstruct that from the water molecules, and we can get into that later. In particular what my research is about are the bubbles of gas. So inside that ice, there are little bubbles of air that become trapped during the snowfall process, and those contain a pure sample of what the atmosphere looked like at the time that the snow fell, right? And we can use that. Our lab here specializes in extracting the air from those ice samples and using that to reconstruct the concentration of, for example, carbon dioxide or methane or all the gases in the atmosphere, or the greenhouse gases in the atmosphere. And that way we can really reconstruct how our atmosphere, how our climate, how our aerosols on Earth, how all these things have been changing through time. I mentioned the analogy of the book, right? So as you go down to the bottom, the pages get older and older and older. The oldest ice that we can find in Antarctica in the interior is about a million years if you want to look for pages that are in order. And there’s special places where the ice is even older. We found ice down at six million years old. That’s Antarctica. In Greenland, the oldest ice is maybe about 130,000 years old. So those are the time scales on which we can reconstruct how the Earth has been changing. And ice cores really revolutionized the way that we understand climate change over these time periods. Again, because we have a direct observation from these bubbles, from what the atmosphere was like. Alpha: That sounds crazy to me. I didn’t know the air was still trapped from millions of years or tens of thousands of years ago. Christo: It is kind of crazy. But then if you look at a picture of snow, look at a picture of a snowflake, it’s actually more air than ice, right? And so when all those snowflakes fall in Antarctica, they first sort of get roughed around a little bit until they form round grains. But if you go to a ball pit, you know, like go to your local McDonald’s or something, go in the kids’ ball pit, in between those balls, there’s a lot of little pockets of air, right? And so as that ice becomes compressed, those little pockets of air just sort of get stuck in there. And it’s about 10%. If I should have brought a piece of ice to share with you, but if you look at a piece of ice, you can see that about 10% of the volume of that piece of ice is air if it comes from an Antarctic ice sheet. So it’s quite a bit of air trapped in there, yeah. Alpha: And can you also see the quality of the water back like tens of thousands or hundreds—like whether it was cleaner or had more chemicals? Christo: Yeah, absolutely, right? So there are different labs. Again, our lab specializes in measuring the gases. There are other labs that specialize in the chemistry of the ice. So it all depends on the time period you’re interested in. And so when you talk about, you know, clean or purity, maybe you’re thinking about contamination. And so we can see a complete history of every contaminant that’s been in our atmosphere going back all the way through the Industrial Revolution. We can see the Romans, we can see the Greek civilizations. They started mining lead, you know, you can see peaks in lead, you can see empires rise and fall, all in the ice cores. Yeah, it’s all preserved. We can see refrigerants. We know when people first introduced CFCs for refrigerant, it’s preserved in the ice core. Bomb tests. So during the 1940s, 50s, and 60s, there were a lot of open-air nuclear tests. We can see all of them in the ice cores. Alpha: And you can also see how much it rained in the past, right? Christo: In a sense, yes, but we can see what—in two ways, I would say. So we can see how much snowfall there was at the location where we drill our ice core. And you can imagine we can reconstruct how thick those layers are. We can count those layers the same way you can count tree rings, for example, in a tree. And you can imagine if those layers are thicker, it means there was a year with a lot of rainfall, a lot of snowfall. If the layer is very thin, there was a year with less. So yes, we can see directly how much snow falls in those locations, but also we can say something about the sort of the upstream terrestrial locations. So, for example, in Greenland ice cores, we see a lot of dust. And most of the dust all comes from the Chinese deserts, the Gobi Desert. If the Chinese deserts dry out, it becomes drier, they’ll produce more dust, and we see an increase in dust in our Greenland ice core. So in that way, we can definitely track what’s happening in the hydrological cycle. Now, if you are really interested in the hydrological cycle, there are other places you can go. So there are colleagues of mine who also work on paleoclimate, but, for example, work on cave deposits. They can tell you exactly how strong the monsoons were in Asia or how strong the monsoons were in South America. So that’s a different type of research, but it’s all in that broader field of paleoclimate. Alpha: And then you can also find regime shifts in paleoclimate. Christo: Absolutely, yeah. I think there’s broadly three places where we see really clear evidence for regime shifts or tipping points in the climate. One I would say is the ocean circulation in the North Atlantic, sometimes known as the AMOC. Maybe you’ve heard of that acronym. It means the Atlantic Meridional Overturning Circulation. It’s a very large ocean current. The Gulf Stream is a part of it. So it’s a very large current in the North Atlantic that’s extremely important for global climate, for regional climates, for the temperatures in Europe, etc. And we have really strong evidence that there were abrupt shifts in that current. The second example, I think, where we have fairly clear evidence for these regime shifts or tipping points, are ice sheets. We know that ice sheets, they’re very large. They contain a lot of fresh water. If they collapse or very quickly, they can raise sea level very quickly. And we have clear evidence that in the past there have been very relatively abrupt changes in the amount of ice on Earth. Relatively large means, for example, 100 meters of sea level. During the last ice age, we know that global sea level was about 130 meters, or 400 feet, lower than it is today. That means that these ice sheets must have been a lot bigger. And we can study these transitions. So as climate changes throughout the ice ages, these big ice sheets, they grow and so sometimes they grow, sometimes they collapse or decay or melt. And so we can track that, and through that, we combine that also with models, and we have quite strong evidence, I would say, that there are these tipping point transitions in these ice sheets. The third example, which I think you will find really interesting given your interest in the hydrological cycle, is that there’s a lot of evidence for changes in the hydrology of North Africa, the Sahara region, right? Maybe you’ve heard of the Green Sahara or the African Humid. So this happened about six, seven thousand years ago, where most of the Sahara was a lot greener than it is today. There’s lots of evidence for lakes, there was very different vegetation, very different kinds of animals. And this was all happening in the Sahara, which is today a desert, right? A very dry, arid region. And so we have very clear evidence, both again from paleoclimate evidence from models, but also in this case, uniquely from cave drawings that show that the climate has changed a lot. There’s all these very interesting cave drawings that have been found all over northern Africa. There’s one that’s called the Swimmer’s Cave. There’s all these images of people that are swimming, and there must have been lakes, right? And you go outside there today, and there’s a dry, arid desert. So there’s been really strong evidence for big changes, very abrupt changes in the hydroclimate of Northern Africa. So I think those would be the third example that we have in paleoclimate, where we are very certain that there likely are some of these abrupt transitions or tipping points involved. Alpha: Let’s turn to your research about how these ocean overturning events were relate to climate change. Christo: Let’s a little step back first. So these ice cores can record local temperatures, and they’re quite good at that. And we’ve known, I think, since about the ‘90s or so, we’ve had pretty reliable records from these ice cores in Greenland, and what they show is a sequence of really abrupt changes. And I’m talking about 15 degrees Celsius, or 20-some degrees Fahrenheit, within a few decades—mean annual temperature. So that’s extreme climate change, an extremely rapid climate change, right? We see a sequence of them. So during the last ice age, so say between about 110,000 years ago to about 10,000 years ago, during that time we see about 25 examples of extremely abrupt warming in Greenland. Again, you know, 10 to 15 degrees Celsius within a few decades, and then it stays in that warmer state for a while, and boom, all of a sudden it cools as abruptly as it warmed. It stays cold for a while, and very, very abruptly it warms again, right? So this sequence is known as the Dansgaard-Oeschger cycle. So Dansgaard was a famous Danish glaciologist who led some of those first ice-coring expeditions, and Oeschger was a Swiss scientist who worked on some of the greenhouse records from those ice cores. And so these oscillations have been named after the two of them. So the Dansgaard-Oeschger oscillations is what we call them. These are, I would say, the poster child for abrupt climate change. This is like repeated, very abrupt, very large-magnitude climate change happening in this region in the North Atlantic and Greenland. That’s the observation that was around, and as I was working on ice cores, you know, it’s sort of the biggest signal out there, right? And we didn’t understand it. So it seemed very intriguing for me to work on. Alpha: So you saw this oscillation of the ocean signal, and you were like—and you also knew it was connected to climate change, and so you’re like, you wanted to work on that problem more. Christo: Absolutely, yeah. So the link to the ocean is, you know, so we talked about the ice core now, right? So that’s where these were first discovered. The ice cores were the first place where people saw this abrupt sequence of really large climate change events. And then more research happened. People, you know, other people studying the marine sediments of the North Atlantic, and they reconstructed the ocean circulation and the different ocean masses that are in the North Atlantic Ocean, and they saw the same changes. If you look at the surface temperatures, if you look at ocean circulation, for every time we see Greenland warm, we also see a change in the circulation of the North Atlantic. And now we also have models that can do it. Models, again, if you tune all the knobs, right? If you put the wobble of the Earth in the right place, you put the CO2 concentration in the right place, you make the size of the ice sheet just right, in those models also the current starts to oscillate, right? And so we have a lot of evidence now that these really abrupt Dansgaard-Oeschger events are linked to very abrupt switches in this current in the North Atlantic. Alpha: Ocean currents are important for global temperatures because the ocean holds so much of the heat, right? Christo: Absolutely, yeah. And so it depends on how they move, they can absorb some of the heat in the atmosphere and so shift our global temperatures. The ocean currents move around heat, they move around nutrients, they move around CO2. So all the gases and all these properties of the climate are being regulated by the oceans to some degree. And so if you perturb the ocean circulation, you’re gonna perturb everything in the climate. Alpha: There’s something called AMOC, the Atlantic Meridional Overturning Circulation which people are worried about. It is the changing of the circulation of the ocean, which will lead to a change in global temperatures. Christo: Yeah. So when it comes to these tipping points, the underlying dynamics, even though the systems are very different, right? You’re talking in one case about rainfall in the Sahara, now we’re talking about ocean current, right? Very different system, but the dynamics that lead to these tipping points are actually somewhat similar in that they have these what we know as positive feedbacks, right? And so the same thing happens when this current is going strong. It has mechanisms built in that ensure that it can stay strong. The AMOC is a current, it consists of some surface waters that are flowing north, and the Gulf Stream is a very famous example of this, right? These are very warm waters. The Atlantic happens to be saltier than the other ocean basins, for example, it’s saltier than the Pacific. It’s important in this case because to get this circulation to keep going, what you need to do is get that water to sink. So when that water comes out of, you know, it goes through the Caribbean, to the Florida Strait. So these are warm waters. They go north, they’re warm, salty waters. They go north and they start to cool down. Particularly in the winter, right? You can imagine in the winter you get that really cold air blowing off of Alaska, blowing off of Greenland, sort of sucking heat out of that ocean water. So that water was already very salty, and now it’s also cooling down, particularly in the winter, and those two things make it very dense and heavy, right? So water can become dense because it’s saltier or colder, and this water is both. So it becomes really dense and it sinks to the bottom of the ocean and then it flows back south. That’s what the AMOC is. So it’s a surface current that is warm, that moves north. That water sinks because it got really cold and it’s still very salty, and then it flows back at the bottom of the ocean. What keeps that current going is the fact that you can make really dense waters in the North Atlantic. We call that water North Atlantic Deep Water. So as long as you can form North Atlantic Deep Water, you’ll keep this current going strong. As long as this current is going strong, it transports the salt it needs to make dense waters. Also, this ocean current actually keeps the ocean ice-free. If you go to the Nordic seas, the Nordic seas are between Greenland and Norway. If you go there in the middle of winter, there’s no sea ice there almost. Kind of remarkable, because you’re really far north. And the reason is that this warm water goes all the way north, and so it keeps the ocean ice-free. As long as it’s ice-free, it means it can contact that really cold air, right? That really cold air is blowing directly on the water, so it can extract all that heat from the ocean, make that really dense water, that North Atlantic Deep Water that sinks to the bottom. So those are the two ways the current keeps itself going. It has its own momentum. Now imagine we were to weaken that current, right? Some people talk about shutting down, that’s not quite accurate, it never really goes away, but it gets to be a lot weaker. And so once it’s in the weak state, the weak current is no longer transporting all the salt water north, right? When it’s weak, now we have sea ice covering the whole North Atlantic, and that sea ice is like an insulation layer, like a big layer of insulation. So that water in the winter is no longer touching the very cold air, so the water cannot cool anymore, right? So we’re not—we don’t have the salty water anymore, we don’t have the strong cooling anymore, so the water is not sinking. So you see, there’s these two stable states again, right? You have the one state where the ocean is just going strong, happily chugging along, and by doing so, it creates the condition that it needs to keep going. Or the other state is the ocean is shut down. And by being shut down, it’s sort of happy to be in that state also. It doesn’t have the salt transport, it doesn’t lose its heat, and so it’s happy to stay in that weak mode also, right? So it has these two quasi-stable modes, and it can switch between them. Based on the evidence that we have from the ice core and other records, when it switches, it switches quickly. It can just go, it switches from the weak to the strong mode within a few decades, maybe up to a hundred years or so. Alpha: And we’re worried right now, right, that with global warming, we’re melting the ice and that sinks into the ocean, and then that fresh water somehow shifts the AMOC, the circulation of the ocean, leading to temperature rise. Christo: Yeah, that’s a real concern. And if you were to look at projections, again, we talked about climate models earlier. If you look at, say, 20 climate models and you look at what’s going to happen in the future, going maybe to 2100 or even 2300 or something, a couple hundred years in the future, this becomes very clear that the majority of those models will predict or project that the AMOC is going to get weaker in the future. And the reasons are clear, right? I mentioned earlier, what keeps this current going is the fact that you can make really dense waters at the surface that can sink to the bottom. Again, for water to be dense, it needs to be salty and it needs to be cold. What’s happening now with global warming is, first of all, we are warming, we’re adding CO2 to the atmosphere, that’s trapping some of that heat. So that surface ocean, the surface waters in the North Atlantic, are not getting as cold as they used to. Also, like you’re saying, we’re having melt and runoff from the Greenland ice sheet that makes the water fresher, that also suppresses—that makes the water lighter, less dense. So that also suppresses that deep convection. So we have two mechanisms. We’re making the water fresher and we’re making it warmer, and both of those make it harder for that water to sink to the bottom. So there’s a real concern that going forward into the future, this AMOC current is going to weaken again, just like it did during the last ice age. Alpha: What would you say is the cause of the jump in the temperature? Is it the ocean causing the temperature or the temperature causing the ocean, or is it like a coupled feedback? Christo: Yeah, good question. So what happens, you can think of it as—again, it’s related to that sea ice question. So when the ocean current is strong, it’s keeping much of the North Atlantic ice-free. That makes the climate a lot milder, right? So if you compare two cities, compare Moscow to London, right? They’re pretty much on the same latitude. But London is a lot milder than Moscow in the winter. And the reason is that London is very close to the ocean, and the ocean during the winter provides a lot of heat. It’s nice and relatively warm. And so during the winter the ocean warms up the atmosphere and it keeps the climate in London very moderate. Whereas in Moscow, you’re landlocked, you’re very far away from the ocean. So you don’t have that source of heat nearby. So you are much colder in the winters. So the way I would think of these Dansgaard-Oeschger switches, it’s basically if the current weakens, the whole North Atlantic is covered in sea ice, which basically acts as a lid on that heat from the ocean. So basically you’re turning London into Moscow. That’s basically what happens, right? So when I say it cooled a lot when the current shut down, it’s basically because the winters become extremely cold because you shut down that warm ocean source of heat, right? You put a lid on it, you put a little bit of insulation on there. So basically it’s as if London turned into Moscow within a few decades. That’s what happened during those past abrupt switches. Going into the future, that sea ice effect is probably gonna be a lot less big, but it’s still gonna drive massive cooling, right? It’s gonna drive cooling in Europe, which is gonna be important and have big implications. But one of the things that we learn from the ice cores is that what happens in the North Atlantic doesn’t stay in the North Atlantic. The impact is really global. So maybe we should talk—you want to talk about that a little bit? A couple of things that are important here. I think one of the most robust ones that we see throughout the tropics is that we get these big changes in, again, the hydroclimate in the monsoons. The monsoons in Asia, the monsoons in South America, the monsoon in Africa, are all really closely coupled to what happened to that ocean current. What we see in the evidence is that when the current is strong, all the monsoons in the Northern Hemisphere are also very strong. The Asian monsoon is strong, the Indian monsoon is strong. And at the same time, all the monsoons in the Southern Hemisphere are relatively weak. Then when the current shuts down, we cool the north, and now all of a sudden at the same time, we see weakening of all the monsoons in the north. And we see strengthening of the monsoons in the Southern Hemisphere, such as in South America. The way we normally think about this is through what’s called the ITCZ, or the Intertropical Convergence Zone. That’s sort of the warmest point on Earth, right? And so it moves around. And that’s where all the deep convection happens. That’s sort of the heat engine for the whole atmosphere. That’s where all the atmospheric motion is. So that’s the warmest part, it’s near the equator, and it moves around, right? So during the Northern Hemisphere summer, the ITCZ moves to the north, and the monsoons are part of that ITCZ. They follow that. When we warm the north, that ITCZ band shifts to the north, and all the monsoons in the Northern Hemisphere become strong. When we cool the North Atlantic, because we shut down the current, now that ITCZ, the warmest point on the planet, shifts to the south. So that means that all these monsoons in the Northern Hemisphere weaken quite abruptly, and all the ones in the south are now inside this ITCZ shifting belt, and so they get stronger. This, I think, is the most important lesson, for when we think about the impacts on society. Because in Asia, we have billions of people whose livelihoods depend on the reliability and the strength of these monsoon systems. What the paleo evidence shows us: if we shut down that current in the North Atlantic, at the same time, we’re expecting to see a weakening of the monsoons in Asia and the monsoons of India and East Asia, for example. So that’s a profound impact. Alpha: So the world is actually very coupled. People don’t always realize that your ocean here on this part of the world is actually coupled to the rain in a continent on the other side of the world. In climate it’s called teleconnection. Christo: Yeah, teleconnection, yeah, exactly. That’s the word we use for that. So there’s a very strong teleconnection between the North Atlantic and the monsoon systems in Asia. There’s even a teleconnection that’s further away, and that’s what I’ve worked a lot, spent a lot of my time working on, which is Antarctica, right? Antarctica is all the way at the opposite end of the world. When we’re talking about the North Atlantic, you couldn’t almost be further away. And we see them in Antarctica too. So in Antarctica, we can see the impact of what happens in the North Atlantic. It’s pretty wild. But every time the current shuts down, we can see it in the ice cores in Antarctica very clearly. Alpha: Do you want to say a bit about how you do your research? Like how do you choose your hypothesis? How do you work? Christo: That’s a mysterious way, right? That people do research, and there’s not, I don’t think, a single way in which it goes. There have been times that I’ve been banging my head against the problem for months or years and not made any progress. And other times you get a new data set, or in the example of the postcards and the text messages, right? I just had the data set synchronized, I plotted them up, and within a minute I knew what it meant. Within a minute, it was like, “Ah, that’s what’s going on. So sometimes it’s really fast. You just get lucky, and you just plot the right things, you do the right analysis, and all of a sudden it just all clicks into place. Alpha: Why choose the particular experiment you looked at? Christo: Well, I guess again, because we knew that this phasing was probably gonna be important, right? So much of this research was really driven by questions that come from just modeling or theoretical experiments that told us that there was probably going to be atmospheric readjustment, right? We knew this ocean connection had been known actually for a longer time. I was not the one—I’m not the one who can claim to have discovered that. It was other people who discovered this already, you know, like maybe 15 years ago or so. But I was able to determine the timing better, right? So we knew already that there was a way that Antarctica and Greenland were communicating, but we also knew that the limitation was really the fact that these ice cores weren’t synchronized very well. So it was very obvious that you can learn a lot about how the synchronization works by synchronizing those ice cores better. And so in that sense, it’s one of those things that it was obvious that it needed to happen. And I was frankly lucky. Luck always plays a big role too, right?We like to think that people who discover big things just do it because of their genius. But you know, luck—you don’t want to underestimate the importance of luck. So I was a graduate student, I was a postdoc. I came to where I’m now, Corvallis, Oregon State University, as a postdoc, and at that time the US ice core community had just drilled a brand new ice core. They drilled this ice core in West Antarctica, a place where it snows a lot, which means you get really thick layers, right? The annual layers are really thick, which means that you can reconstruct climate at really high resolution. And so that community had been working for 20 years to drill that ice core, and I showed up, and the core was just done. They just had the records, and I happened to be the one who had the skills and the interest to work on that synchronization problem. I was really standing on the shoulders of giants, as they say. I mean, I was able to do that work only because my mentors and all the other senior researchers and younger researchers and engineers had spent decades working really hard to drill that particular ice core. And so I think luck and also just support from a community and long-term, healthy research environment are really all really critical to doing this kind of work. This is never an individual discovery. This was definitely a team effort. And so I end up writing the paper, and it had over a hundred authors on it, to give you an idea of how many people were involved in that project. So, yeah, in that particular case, that question was sort of an obvious one. It’s just somebody had to do it, right? And then when we did it, for the first time we could really see that signal. When I had done all the analysis and I plotted it up, it was immediate. You can see, “Oh, there’s the atmospheric signal, there’s the ocean signal.” It became very clear the moment I’d done it. But more often, you know, you talked about sort of what it’s like to do research. I think it’s sort of like walking into a dark room, right? You don’t see much, and at first you’re maybe stumbling around, maybe you bump into the couch, you bump into a table, you’re feeling around, and slowly you start to get the feel of it, right? Like, “Oh, there’s something over here,” and that’s maybe how big it is. And as you’re stumbling around, at some point, maybe by luck, you find that light switch and you turn it on, and all of a sudden, all those pieces that you first had a hint of where they were, all of a sudden it sort of falls into place. That’s the satisfying moment. Yeah, Richard Feynman called it “the pleasure of finding things out,” right? Like all of a sudden you’re like, “Ha, that’s how it works,” and that’s a really good feeling. Alpha: Yeah, so you basically stumble, you’re finding lots of correlations, something interesting here, but it doesn’t fit into a bigger picture. Right. And suddenly there’s this aha. How would you say your research style compares to some other people? Christo: What I like to do in my personal work is to really combine data-driven approaches with climate models. So I think the integration of those two things is really, really valuable. I run a lab together with a colleague, we run a lab, and so some of my time is really just on instruments and cutting ice samples and making sure we have all the right samples. And some of it is really working with climate modelers. I work with really talented climate modelers. We integrate those insights you get from the climate models with what you can see in the observations. That’s where I really—that’s sort of the interface where I like to work, where I think we can make the most progress. Alpha: And what happens when you get stuck? Like what are some of the strategies in general? Christo: Yeah, that’s a good question. I mean, you get stuck a lot, right? That’s one of the features of research. I mean, if it was—you know, it’s sometimes easy, but more often you’re just stumped for a while, and you just have to try different approaches and keep going and checking all your assumptions. And then some things also just fizzle, right? The things that we publish and the things that we—the big ideas that come out of it are the stories that were successful. But for every successful paper and every successful discovery, there’s a whole bunch of things that just didn’t work out. Like stuff you tried, hypotheses you had, methods you developed, you know, and it just sometimes doesn’t go anywhere. Alpha: Do you keep a notebook, a journal, or all your research stuff? Christo: Yeah, I mean, on my computer, there’s a whole bunch of files, you know. So sometimes I’ll go through some old files or some old projects. So for every paper that’s published, there’s probably also a half-written manuscript somewhere with some half ideas in it. And sometimes I’ll go back to it and be like, “Oh, that was a good idea. I should pick that up again. Sometimes there’s also data sets that I know are interesting, but you just sometimes don’t have the time to because you’re focusing, deciding to focus on other things, right? So there’s also data sets that just sort of—they’re published, they’re online, like you could download them, you know, but we’ve never really had the bandwidth or the time to really figure out what all the details meant. Alpha: How do you write your papers? Do you write them as you’re doing your research, or do you kind of synthesize everything until it’s all neat and then start writing? Christo: So I’m a bit of a weirdo in that I really love writing papers. Many people don’t like it, but for me it’s the favorite part of the job. And so when I procrastinate, like maybe I have to submit a report or something, or I have to do some grading and I’m procrastinating, I’ll go write a paper. I start by making the same way I make my presentations, I start by making the figures, right? I’m starting to think of how I present the data visually, like how am I gonna plot things, how am I gonna show what data am I gonna show? There’s always many—you know, there’s—you make choices, right? Like what aspects do you want to highlight, what data do you want to feature more prominently, which ones tell the story, which are the most important parts of the most important pieces of evidence that link everything together. By making the figures, I try to come up with a narrative, like a red line, like a thread that runs through it all, like a narrative that you can sort of walk through and take the reader and explain all the different steps of your reasoning, your hypotheses, what your data show you, what your models show you, and then you sort of wrap it all up together. You make your first draft, you share it with all your colleagues and your co-authors, and they have comments and usually really good feedback, and then you go back and forth a couple of times as you refine it. References Brook, Edward J., and Christo Buizert. "Antarctic and global climate history viewed from ice cores." Nature 558, no. 7709 (2018): 200-208 Buizert, C., Gkinis, V., Severinghaus, J.P., He, F., Lecavalier, B.S., Kindler, P., Leuenberger, M., Carlson, A.E., Vinther, B., Masson-Delmotte, V. and White, J.W., 2014. Greenland temperature response to climate forcing during the last deglaciation. Science, 345(6201), pp.1177-1180 Buizert, Christo, Michael Sigl, Mirko Severi, Bradley R. Markle, Justin J. Wettstein, Joseph R. McConnell, Joel B. Pedro et al. "Abrupt ice-age shifts in southern westerly winds and Antarctic climate forced from the north." Nature 563, no. 7733 (2018): 681-685. This is a public episode. If you'd like to discuss this with other subscribers or get access to bonus episodes, visit climatewaterproject.substack.com/subscribe
