Earth Systems Science

If planetary diplomacy is to work, we can't represent the planet's interests until we understand how the planet actually works. From various different disciplines we have come to understand that Earth is more a single, interconnected system rather than a collection of separate parts. This understanding has given rise to a new field, earth systems science.

In this episode, we sat down with Sonia Seneviratne, Professor of Land-Climate Dynamics at ETH Zurich and a member of the IPCC , to explore the science of Earth systems. Her research focuses on climate extremes, land-atmosphere interactions, and the feedback loops that amplify heatwaves, droughts, and other climate impacts.

In the conversation, we discuss how vegetation and soils influence the climate, the role of positive feedback loops and tipping points, the risks facing ecosystems like the Amazon rainforest, and why, despite remaining uncertainties, we already know enough to act. We also explore what a more balanced Earth system could look like—and the pathways that could help us get there.

Transcript

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Welcome back to the Tectonic podcast, presented by Swissnex in Boston and New York. I'm your host, Brendan Karch.

We’re continuing our series on Planetary Diplomacy — the idea that the interests of other species, of ecosystems, of the living systems we depend on, all deserve a seat at the table where decisions get made. But before we can represent the planet's interests, we have to understand how the planet actually works. And it ends up the study of the earth is far more advanced, and interconnected, than most of us were taught in school.

Traditionally, we studied the Earth in pieces. Physics here, biology there; oceans, forests and atmosphere each their own domain. But the last half century has yielded a critical mass of new discoveries pointing to the same direction: that these systems don't operate in isolation. They talk to each other. A rainforest drought can amplify a heat wave. A dying forest can flip from absorbing carbon to releasing it. The Earth, we’ve learned, behaves a lot like a single, breathing system.

We have a name for this new interconnected field: Earth systems science. And my guest today is one of its leading voices in Switzerland.

Sonia Seneviratne is a professor of land–climate dynamics at ETH Zurich, and a globally leading expert on climate extremes: heat waves, droughts, and the feedback loops that make them worse. She's also a member of IPCC, the UN's climate science body, which puts her work at the intersection of science and policy.

In this conversation, she walks us through how the land itself is an active agent in our climate. We talk about attribution science, and how researchers can now say, with real confidence, that a given heat wave was made far more likely by human activity. We also get into tipping points — the Amazon, the permafrost, the great ocean currents — and why our models may be underestimating them. And we end, perhaps surprisingly, on a note of optimism about our climate future.

Here's my conversation with Sonia Seneviratne.

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Brendan Karch: 
So, thanks for joining us today Professor Sonia Seneviratne from ETH Zurich. Maybe you can start off by telling us who you are and what you do and what you research?

Sonia Seneviratne:
Yeah, so I'm a professor at ETH Zurich for land climate dynamics. I'm a climate research professor. The main topic I'm investigating are climate extreme events such as heat waves and droughts. I also investigate specifically processes on continents so the role of vegetation for the climate system and also how change in the climate system affect condition on continents. So beside droughts also impact on vegetation.

Brendan Karch: 
Yeah, very timely, I think, on both sides of the Atlantic. You had the hottest June on record in Central Europe. And then as we record this much of North America is under smoke warnings with bad air because of wildfires. What got you interested in this topic going back in time?

Sonia Seneviratne: 
Well I would say first I was always interested at the same time in physics and biology. So, I actually started by studying biology for two years, but then I realized I also have a strong interest in physics. And I then came for basically one year to Zurich in the Department for Environmental Science. So, I was an exchange student and then I realized I'm interested in environmental physics. So, I finished my master degree at ETH Zurich in environmental physics but always with this joint interest both in the biology aspects and the physics aspect. And specifically when I was a student I also did an internship in South America in the Amazon Rainforest. I was in a forest that's called cloud forest where they have like a permanent cloud around the forest. And there you see basically that much of the cloud is sustained by the evaporation of the trees. And I found this fascinating and I think that's a moment when I thought I would like to understand these processes better.

Brendan Karch: 
That’s very fascinating that you went and studied biology, but then you went and studied physics, and these are two of the classic disciplines that everybody recognizes. But you and many others now might classify yourselves as Earth system scientists. A field, as far as I understand, that didn't really exist 50 years ago. There have been the classic fields, biology, physics. There's also been things like ecology, oceanography, perhaps atmospheric physics. But for a long time they were separate disciplines. So your journey seems to, in some ways, mimic the journey of Earth system science, of understanding that these things have to be considered together and have to be considered in interaction. So, obviously for you personally, that moment in the forest is a moment when you personally saw these connections. For the field as a whole, do you think there were key moments when there were realizations that they have to study these systems together?

Sonia Seneviratne: 
The studies I basically did in Zurich, it was a master in environmental science and it was conceived as really multidisciplinary. So the students in the first two years have a kind of interdisciplinary background in natural science and then they specialize in two different aspects: one is discipline so physics, biology, chemistry. At the time there was also microbiology or health science. And then there was a sphere so it was atmosphere or hydrosphere or the soils and also what they call anthroposphere so like the human dimension. So, these studies here at ETH Zurich were very much considered in this way and when I studied, in the 90s, I think there were only two places where you could more or less study this type of interdisciplinary environmental science. It was ETH Zurich and in Bayreuth in Germany they had a program in geoecology which was a bit similar. And I came here from the Frenchspeaking part of Switzerland so it was a bit of a challenge to come to the German-speaking part. But then once I saw the research and the teaching program I realized this is really fascinating. But as mentioned at the time there were only about two places in all of Europe where it was possible to study this. 

Brendan Karch: 
What do you think were some of the discoveries and realizations that were coming out of this department that were made possible by this new interdisciplinary constellation?

Sonia Seneviratne: 
I think there would be many. It's difficult to summarize this just in a few statements. But for me, I guess with my perspective, the research I've been doing at the interface between biology and physics is really important because the plants, I mean how much water is available to plants, is determined by physical constraints. The way you have precipitation, this would be constrained by physical relationship in the atmosphere. But then how much water the plants physically evaporate, this is very much a biological process. So, I would say this applies to my research but research of earth scientists looking at this interface is understanding how droughts affect possibly the carbon cycle or how plants will affect the climate system. But, of course, there are many other fields where this is relevant, if you're looking at the role of also biosphere and the ocean. 

Brendan Karch: 
Yeah, very interesting. So, let's talk maybe a little bit about your own research in this context. As I understand it, you work a lot on the interactions between land and atmosphere, sort of the interface there. How does the land turn out to be an active agent in the climate system?

Sonia Seneviratne: 
I mean the first point is the land processes, if you think of the interaction with the atmosphere the main process of relevance there is evapotranspiration. So, it's basically how much water is evaporated by plants but also at the surface from lakes and different sources of water. So, this is a direct flux of water to the atmosphere and it's a very large flux. On average you have sixty percent of all precipitation that is flowing back to the atmosphere in the form of evapotranspiration. So, it's a huge amount. I think sometimes it's underestimated because we don't see it. We do see the precipitation. So, everybody, of course, realizes “oh we have a huge downpour of water from the atmosphere”. Nobody sees this as a flux of water which is from the surface to the atmosphere even though it's very large. I think that's that's a main point. That's a main interaction there and they are then connected at the same time to the water cycle obviously because this is this flux of water. But you have also a connection to the energy balance because the energy required to evaporate water is very large. It's between fifty and sixty percent of all the available net radiation on the land surface is used for this flux of evapotranspiration, so a huge amount of energy is used for this. And then there is a connection to the carbon cycle because much of this flux happens through plants and the plants evaporate water through photosynthesis. So, it's in the process where they take up CO2 of the atmospheres and evaporate water and so it means those two fluxes are directly connected. It also means that if you have more droughts, the plants are going to evaporate less water to save water, but they're also going to take up less CO2. And in the worst case I mean if you get to a massive drought and maybe plants are dying then obviously not only they are not going to take up CO2 but maybe you're going to have fires which release more CO2 to the atmosphere.

Brendan Karch: 
You already hinted at how this process of evapotranspiration can lead to extreme weather events. Are there any other ties that you would make here between the work that you undertake, the research, and sort of the more extreme weather that people have been experiencing?

Sonia Seneviratne: 
Yeah, so there are many links between evapotranspiration and extreme events. One typical example is also during heat waves. So, when you have very dry soils you are going to have much less water available for evapotranspiration. So, what is normally happening is a lot of energy would be used to evaporate the water but if the soils are dry this energy is then used instead to warm up the air and the soil. And it's very similar to what happens in the human body. So, we know actually sweating is also a mechanism to cool down the body. When we sweat basically this means that it's going to take energy and it cools us down. But we know also that if we are not drinking enough water we run the risk of having a heat stroke. And that's similar to what happens on the Earth. So basically you could say when the soils are dry you almost have like a heat stroke at the surface with this huge increase in temperature. And so this is a key mechanism for heat waves in midlatitudes. So, in regions like in Central Europe also in the central United States, basically regions which have strong variations in water availability where you can have droughts. And when you have those droughts you have this amplification of heat wave and you see big jumps in the temperature. You have also other effects, for instance, also on heavy precipitation. So, if you have a very wet soil it can contribute to the input of moisture to the atmosphere leading for instance to to heavy downpours

Brendan Karch: 
Yeah, these all sound like examples of something that I think earth system science talks a lot about, which is feedback loops. So the ways in which systems feed into each other, and droughts can precipitate heat waves, and heat waves can precipitate droughts, and then forest fires and all these things kind of run together and can I think make each other worse or make each other more likely. Is it fair to say that a lot of what you study are these sort of feedback loops that exist?

Sonia Seneviratne: 
Yeah definitely. So, I'm looking at a lot of the feedback loops that take place at the interface between continents and the atmosphere and this includes mechanisms which are called positive feedback loops, which lead to an amplification of the responses. So, for instance, in terms of temperature extreme, what we find is in projections this effect of the soil moisture, this amplifying effect, when you have a drying of the soil, it's about half of the projected change in the heat waves or of the temperature change. So, this means half of it is a typical mean increase in temperatures that we see everywhere, but in addition to this we'll have like a doubling of the temperature anomaly due to soil moisture feedback. So, yes it basically belongs to those processes that can amplify very much a response and lead to even stronger responses than what you would have without those feedbacks.

Brendan Karch: Yeah, so I’d like to talk a little bit about the concept of tipping points as well. This is something that I've read in the more popular scientific literature that we might be reaching tipping points with our Earth's climate. You hear about a runaway hothouse Earth scenario as well. You hear about specific tipping points, for example, in coral reefs with coral bleaching, that coral reefs might be reaching a tipping point of water temperatures where they cannot recover. Do you find this concept of tipping points useful for your own work?

Sonia Seneviratne: 
Yeah, I think the general concept makes sense. It's a concept that people understand and the illustration of tipping points is if you push something maybe it will stay stable and at some point if you push it too much it's going to fall down. Another way to refer to this could be critical threshold. So what is a critical threshold? After which you have a massive change in the system and often you have some type of residency. The system can go back to its past equilibrium but at some point it will come to a different equilibrium. So the concept itself is also a scientific concept and, for instance, there is a new IPCC report that will be released in 2028. The one on the physical science basis of climate change, we do have a chapter that is specifically also mentioning tipping points in the title. So it's recognized as certainly a scientific area. Indeed the concept can be used in a whole range of different areas. So you can refer to regional tipping points or as you mentioned possible impact on coral reefs or it can be a global tipping point on the climate system where suddenly you would have a reorganization of the whole climate system in a different way. But, I mean, we do have evidence of such tipping points. We know the climate system in the past has sometimes been through massive change and also quite rapid changes. So, it's not just a theory. There is evidence for this also from the paleoclimate records.

Brendan Karch: 
And in your own research, do you see us more likely reaching a new equilibrium? Are we already perhaps in a new equilibrium with the extreme events that you look at? Or do you see an acceleration and runaway, uh, events with floods and droughts and these things?

Sonia Seneviratne, ETH Zurich: 
Yeah, so I would say in any case we do have massive change in climate extreme events and maybe the concept of tipping points could also apply to human society at which point you reach a tipping point where the impacts are so large that we can no longer face them in ways that doesn't mean massive suffering. And I think maybe that's for me the most relevant aspect because we see that there are more and more impacts and there could be also cascading impacts in the society in the economic system if you see how dependent we are, for instance, on supply chains. You can just imagine the number of cascading impacts and also that it would be a tipping point if the food prices basically are increased very much and some people don't have enough to eat. I think it's a very concrete aspect. So, for instance, in my research we do analysis of this probability of concurrent extremes, like these impacts on the breadbasket. We have also worked a lot on the impacts of droughts on the carbon cycle which is relevant for the Amazon rainforest tipping point. So, I would say that's one specific kind of global climate tipping point I've done research on. I mean my research was not specific on tipping point but it's related to it because we find in observation a very clear relationship between water availability on land and the change in CO2 in the atmosphere on a global scale. So, we find evidence in observation of a direct connection between the two and what we found was that climate models actually underestimate this relationship. 

Brendan Karch: 
Maybe we can dive more into the Amazon. So it's often been described as the lungs of the Earth's atmosphere, right? A huge CO2 carbon sink. I think one of the tipping points that's been discussed is just the size of the forest itself and the massive deforestation that goes on. Is it land use that is the potential biggest tipping point, or is it just the warming of the Amazon, or is it some other factor that might create a tipping point for the Amazon, or do we not know yet?

Sonia Seneviratne: 
Yeah, the literature on the Amazon dieback tipping point is not necessarily taking into account deforestation. It's just saying that with increasing climate change we know that there is more drought risk actually in the region. This is also already in the last IPCC report, So, we'll see a two degree of global warming. So, whole Amazon region is affected by much more drought. Now with increasing drought occurrence there's risks that the vegetation could not survive. Now, if it doesn't survive, this would mean that this region, which actually is a large sink for the CO2 emissions we have right now, would basically disappear as a land sink. So, we would have suddenly like a jump in CO2 concentration in the atmosphere which would be associated with a much stronger warming. So, basically at the moment we are rather actually having a negative feedback that some of the excess CO2 we are emitting is actually taken up by vegetation, by the ocean, which is moderating a little bit the response, but if this disappear we would have suddenly a huge increase and this could then lead to extremely fast increase of extreme events globally. 

Brendan Karch: 
Interesting. So you've mentioned IPCC a little bit in our discussion. For those who might not know the IPCC, can you tell us about what they are and what the work is that they do?

Sonia Seneviratne: 
Yeah, so IPCC is an intergovernmental panel on climate change. It's an organization under the United Nations that was founded by, on the one end, the World Meteorological Organization and then UNEP, it’s UN agency for environment protection. And IPCC since about the 90s has been basically preparing reports on the state of climate change. So, its main mandate is to provide an evaluation of how much climate change there is and also what would be dangerous climate change. It's basically informing the decisions that are taken under the UNFCCCs. The reports themselves are different working groups. So Working Group I is on the physical science basis, Working Group II is on impacts and adaptation, Working Group III is on mitigation. But what is unique in the IPCC report is that summary for policymakers, that's a short summary at the beginning of the report, it's about 10 pages long, this summary is approved by the government representative, line by line. So, it's a process, it lasts about one week and really we go through each sentence of that summary and any country can make a comment and propose suggestions to revise the text. On the other hand, the scientists keep basically the responsibility of the text. So, the scientists have to explain why they wrote what they wrote. They can also take into account some suggestion, but in the end it is really a very special type of report which is written by scientists but  is also then approved by the governments which means that those reports have a legal basis because they have been read and then approved by the governments.

Brendan Karch: 
Interesting. Now it seems that a lot of our discussions around climate science here have focused on the impacts on humans, and I think that's often how climate science is discussed in some ways, or climate change in particular. “Oh, humans will suffer in ways X and Y from this. But, of course, other species suffer, ecosystems suffer. The planet as a whole obviously suffers, but the specific parts of the planet that are the non-human parts, most of the planet, will suffer biodiversity, species loss. When you're doing your science, do you ever think about or frame it in terms of the loss of these other parts of the planet that are not just affecting humans directly?

Sonia Seneviratne: 
Yeah, it's a bit less of my research but I do think this is relevant. I mean the special report of the IPCC on 1.5 degrees, one of the big messages was also about impacts to coral reefs and impact to biodiversity, that at 2 degrees you would have much more impacts on biodiversity. For instance, for coral reefs already at 1.5 degrees it was absolutely critical but at 2 degrees most of the coral reefs would disappear. So, I would say for me it's definitely relevant. It's not an area in which I'm doing a lot of research myself but I did collaborate also with scientists working on biodiversity, for instance, sharing information on changing climate extremes so that they could integrate this in their models.

Brendan Karch: 
Yeah, so maybe a couple of questions to wrap it up. One is, as somebody who studies Earth systems, where do you think our biggest gaps in knowledge still are? Are there specific fields that you feel still really need to be developed and filled out to understand Earth systems better?

Sonia Seneviratne: 
Yeah, maybe the first point is we know definitely enough what we need to do if we want again to limit global warming, for instance within what has been set in the Paris Agreement, So, we know all what we need to know in this case. I would say we don't need more science to say what would be the better course. Actually the IPCC is also clear about that, that there are more negative impacts if we don't limit global warming. So, the net effect of climate change is negative if we have more global warming. So, all of this is known and again if you want to avoid those negative impacts then we know what needs to be done. We know that the main reason why we have global warming and climate change is because of the burning of fossil fuels, so it's the burning of oil, the burning of coal and gas. And this means that the main solution is also obvious, it's basically to have other sources of energy like renewables, so solar energy or wind energy or hydropower, and, of course, also have better energy efficiency use energy in the form of electricity rather than fossil fuels or in case of mobility moving from petrol cars to electric cars or for the heating system having a heat pump instead of gas or oil heating. So, I think for me it's really important to emphasize, we know a lot and we know enough to basically control this crisis and stop it and stabilize the climate system. Now I would say the areas in which we have remaining uncertainties is about this low probability risk, which could be extremely impactful like the global tipping points. Definitely these areas which are more uncertain we don't exactly know when those tipping points could happen, but we know they could happen. We know that it is plausible. We know such tipping points have been reached in the past. We know also this would basically lead to massive change in the climate system, probably to changes that would be very hard to manage in any way because the change will be so fast and so big. So, I would say we know that there are these super risky possible changes in the climate system. I would say I'm not in favor of doing an experiment to find out exactly when the Amazon tipping point is going to be reached or the permafrost tipping point, I think there I would stop the experiment and say “Yes we know it can happen but let's not go there.” So again, these are some of the areas that are uncertain but it's not a reason not to do anything about it because we know enough now to know that the course we are following right now doesn't make sense. It's just leading us to more suffering.

Brendan Karch: 
So maybe we can finish then with an optimistic vision. I don't know whether you personally consider yourself an optimist or a pessimist, but if you were to paint a positive vision for an earth system that you consider more in balance, say, in the next 25 years, um, what, what does that vision look like, and how do we get there?

Sonia Seneviratne: 
I do think I'm more of an optimist person but also in the context of climate change I would say I'm actually almost more optimistic now than I was 20 years ago, which might seem surprising. But the point is, of course, climate change is much worse than it was 20 years ago, but the difference is the solutions exist. So we knew already 20 years ago the main problem was fossil fuel. We knew also that ideally we would want to develop renewables, but there were very small amounts of them and it was very expensive. In the meantime renewables have been expanding very fast. They are also quite cheap. So the solution would be to first stop burning fossil fuels, using energy mostly in the form of renewables, so solar, wind, hydropower, geothermal. Of course, also improving energy efficiency maybe, this would also make sense. In terms of mobility, we would not have any petrol cars anymore. One issue that needs to be solved is flying, so there would need to be some type of alternative for flying, maybe solar planes if that's a possibility or really fast trains. So, again there would be a lot of very practical changes that could happen right now, I mean it wouldn't take new technologies actually for going away from fossil fuels.

Brendan Karch: 
Great. Well, thank you for that vision and thanks for joining us today

Sonia Seneviratne: 
Thank you very much

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Tectonic is created by Swissnex in Boston and New York, a Swiss science consulate promoting exchange in education, research, and innovation. Production and editing of the podcast is done by Frederick Atwood and Hayley Bartley. I'm your host, Brendan Karch.