The Life Scientific: Pierre Friedlingstein

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1:06Hello, I'm Jim Al-Khalili and today on The Life Scientific on Discovery, we meet Professor Pierre Friedlingstein. He's a prominent climate scientist and chair in mathematical modelling of the climate system at Exeter University. His models have transformed our understanding of climate change, revealing a complex dynamical system with carbon at its centre, cycling between the atmosphere, oceans and the land to directly influence the level of carbon dioxide in the atmosphere. He's director of the Global Carbon Budget, which estimates the remaining amount of carbon dioxide that can be emitted before we breach our global climate targets.

1:46Pierre Friedlingstein, welcome to The Life Scientific. Thank you. Now, Pierre, as I understand it, this remaining global carbon budget that I just mentioned is the additional amount of carbon dioxide that we can afford to emit into the atmosphere, from burning fossil fuels, from deforestation, while limiting global warming to a specific temperature target. The Paris Agreement's goal, of course, is keeping this below a 2 degree rise, preferably 1.5 degrees. So how are we doing with this? Let's take the 1.5 degree rise. How long before we reach that? Not so long anymore. The current level of warming is about 1.3 degrees.

2:22So if you look at the amount of carbon we can still emit, It's about five years of carbon emissions. And we breach the 1.5. And we breach the 1.5. So what about the two degrees then? So two degrees is more time than another half a degree, which if you translate this into an equivalent carbon dioxide emissions, just about the amount of emission we would emit for like 25 years or something, if we keep emitting at the same rate, which means we have to reduce. Right. So tropical rainforests take up a vast amount of carbon dioxide from the atmosphere, while deforestation releases it back into the atmosphere.

2:57How do you explain that process? What's going on? So if you have a forest, I mean, of course it accumulates a lot of carbon in the leaves, in the stems. This is just photosynthesis. It's just photosynthesis. And if you cut your forest, most of the carbon that was stored in the forest will either decay on the ground or they will burn the forest. And this carbon that was stored in the forest will end up in the atmosphere at some point. I mean, just how important are the rainforests in keeping carbon out of the atmosphere? So we emit about 40 billion tonnes of carbon dioxide every year from our activities.

3:29For burning fossil fuel, deforestation, we emit about 40 billion tonnes. About half of it, 20 billion tonnes, stays in the atmosphere. The other half goes into the ocean and into all land ecosystems. And again, it's about 50-50. So you've got 40 divided by 220, 10 goes into the ocean, 10 goes into the land. and when the fraction that goes into the land, it is mainly in forest because they store more carbon than grass or shrublands. So it's a big fraction of the cake that goes into the land. OK, we're going to come back to this shortly, but first let me take you back, Pierre Friedenstein. You grew up in Brussels in Belgium.

4:02What do you remember about that time, about your childhood? I think I had a relatively happy childhood, going to school and going in the countryside on the weekends with my parents because they had a house in the countryside and I loved being outside and playing like a little farmer with some of the people in the village. I mean, nothing dramatic happened to me. Nothing dramatic. I wasn't fishing for anything dramatic, but it's good that you had a nice, happy, settled childhood. It was during secondary school, though, that a career path was suggested to you. Kind of. It was a bit indirect, but I was good in mathematics.

4:34And my maths teacher had some connection at university as well. and he was trying to scout, I mean, the best math students and he would help us and prepare us to get in the exams and everything. And that's what you did. And that's what I did. So you got into this Ecole Polytechnique in Brussels, embarking on a five-year engineering degree. What was student life like? Student life was great. I mean, you have a lot of free time, so you can have a nice time in the evening. So that's all great. I mean, the thing that I didn't really like was, I mean, the study itself. I mean, engineering, I realised after like a couple of years that it wasn't really my thing.

5:10I gather you got interested in mining engineering. Why mining? There was the last option I could find. You ruled out everything else. Yeah, so you've got two years which are quite general and then you have to specialise. Mechanical engineering, robotic engineering, I mean chemical engineering, civil engineering of course obviously and I couldn't see myself doing any of this more important job afterwards. I mean working like on a big factory or something. Right. So there was this last option that was called mining engineering that involved a bit of earth science. Because there was some geology, you have to know where to mine, right?

5:45And also my two best mates came to the same conclusion as me, which is, I mean, this is probably the only thing we can do. So we decided to go as a team. So you spent the next three years of your degree exploring mining engineering. Did you manage to get down any mines? Yeah, so we went on several mines. To be honest, I didn't really like it. I enjoyed it. But after spending like a month underground, I kind of realised that I didn't want to be like a mole for the rest of my life and it didn't really appeal to me. Having realised then that mining engineering wasn't for you and of course needing a subject for your final year project in order to complete your degree, you, I gather, approached the physics department.

6:24And as luck would have it, there was a scientist there who needed a student to help her on some interesting research. Her name was Cathy Nicolis. She was working on physics of chaos theory. Which, of course, I guess eventually links to the weather and climate and the butterfly effects. Exactly, exactly. And she was working on environmental problems. So the project I worked on with her, if you look at deep sea sediments, scientists can use these sediments as an information of how the Earth works. It's a bit like the ice core sediment that you get from Antarctica or Greenland, right? the assumption is that when you go deeper you go further back in time which of course it's true but it's not that simple because at the surface of the seafloor it's not completely still you have like little animals going there and digging and so mixing this all creating a chaotic it's creating a chaotic system not like the ice cores in the arctic yeah exactly so it's a bit more complicated so she developed this mathematic model to try to reconstruct the time from the depth including some chaotic noise in the system.

7:26And this is what I was working with her. So you wanted to see if introducing noise or chaos into the system, for example, as you say, a sea creature mixing up the sediment, you would change what people assumed at the time. The deeper the sediment, the older it is. This project came to an end when you finished your degree, but a climate scientist across the corridor offered you a summer internship at the National Centre for Atmospheric Research in Boulder, Colorado. What did he want you to work on? He was working on ozone chemistry. It was back in the 90s and the ozone hole was a big issue still.

8:00And there were lots of scientists trying to understand and resolve. And he said, invite me to come to Boulder, Colorado with him on chemistry in the atmosphere and the link with the ozone hole. So this is computer modelling at this point? It was computer modelling. Okay. I mean, the hole in the ozone layer is, of course, an important reminder of how human activities have impacted the environment. other than just through carbon emissions. I remember very well talking about CFCs from fridges, aerosol sprays and so on. And also it's a reminder to us that, I mean, we can actually fix it because, I mean, the ozone hole was a big issue.

8:34We identified the problem. There was a protocol to ban an emission of this substance in the atmosphere and the ozone hole is slowly recovering. If only climate change was straightforward as that. It's a bit more complicated. Essentially it shows that, I mean, with action we can do it. We can do something. This was in 1990, and the supervisor who had given you this internship suggested you also apply for a PhD to study the global carbon cycle. cycle? So in 1990, there was a paper that's still one of the key milestone papers, at least to me, that came out in science and say, if you look at the concentration of carbon dioxide in the atmosphere, if you look at the emissions from fossil fuel burning, as we know, if you look at the behaviour of the ocean, there is too much carbon dioxide being removed from the atmosphere than the ocean can do.

9:25At the time, and thanks to that publication, they identified that it wasn't just going into the ocean, it has to go somewhere else. And the only place he could go was land, essentially forest. Okay, so just to clarify, we know that carbon is emitted by fossil fuels into the atmosphere in the form of carbon dioxide. But 35 years ago, when this paper first came out, the thinking was that only the oceans took some of that carbon out of the atmosphere. They weren't including the land in their calculations. Exactly. I mean, the ocean is huge. It takes like 75 % of the surface of the planet. The amount of carbon in the ocean is 100 times larger than the amount of carbon in the atmosphere.

9:59So it kind of makes sense to say, well, if there is an exchange of carbon dioxide between the atmosphere, well, it has to be with the ocean. But it's not just about the ocean. Well, in 1990, Pierre, you started your PhD at the University of Brussels, splitting your time between there and Colorado, where your supervisor was based. What was the focus of your PhD then? So the focus was specifically on trying to develop a land carbon cycle model. So a model that can explain the photosynthesis that goes from the atmosphere into the land, then the growth of biomass in forest and other ecosystems, the slow decay when a tree dies and falls down, and the decomposition from all this organic matter in the soil and the release of all of this into the atmosphere.

10:42And I was trying to do this at the global scale to estimate how much carbon is being taken by these ecosystems globally. So what did you discover? So it took like several years to build the model. So for the first couple of years, I didn't discover much. It had to be worth a PhD, for goodness sakes. But eventually we discovered that indeed, according to the model we developed, the land ecosystems are taking a large fraction of carbon dioxide from the atmosphere. However, if you look at how we expect this carbon optic to occur over time, over the course of the 20th century, it wasn't really matching the observation.

11:16So it was not just carbon dioxide. it was responsible for the uptake of carbon from the biosphere. There was something else. Climate, climate change. It was getting more complicated, as always. As always, as if it weren't already complicated. So if there's more carbon dioxide in the atmosphere, plants grow more, right? Because that's the food through photosynthesis. And that then increases the land's ability to take up more carbon dioxide. So that's the positive thing, right? Exactly. So that's the main positive things. and this is what's happening. The land, as you see it, I mean, it's used to its plant food.

11:52So if there is more carbon dioxide in absorption, they will grow faster. Right. And that explains what we call the carbon sink. What about the different types of vegetation? How do these different types vary and how much carbon dioxide they can take up? Forests are the number one. Trees are big, trees are big. There's more room to store and for longer. Right. If you store everything in grass, which dies at the end of the season, the carbon is going back to the atmosphere. In the forest, it goes in for 30, 50 years. And is this because the tree trunks that are the main volume? Yes, of course. Everything is happening in the leaf, but then the organic carbon which is generated within the leaf is transported into the roots, into the trunk, and it grows bigger.

12:34So the carbon in the leaf is not so big. It's just this carbon which is taken in the leaf and then exported and stored for a long time. It's like a bank. So for your PhD, you developed a computer model to map the different types of land and their carbon content on a global scale. What did this model reveal? So we applied the model to different timescales. We looked first at all the way back in time, what we call the last glacial maximum, which is like 20 ,000 years ago, when it was a glacial time, five, six, seven degrees colder than today. And the question was how much carbon was stored in the ocean, how much carbon was stored in the lands.

13:15We knew how much was stored in the atmosphere because you have the concentration, again, from this ice core when they drilled in Greenland Antarctica. They knew the concentration of carbon dioxide. So there was less land and there was less tropical forest because it was cold, but there were still pockets of forest and they were still storing enough carbon to explain the difference we observed with the atmosphere. Presumably, to include all the information in your model, you had to look at the availability of light, of water, of nutrients in the land. all of which can affect how much carbon can be taken up.

13:45Yes, well, I mean, I'm still working on these things. It's not over yet. So when I started, there was water. There was a limiting factor, of course, light as well, or maybe temperature, which is more like a proxy of light. But there was no nutrients, for example. It wasn't doing the nitrogen cycle or phosphorus. I mean, this is something that came after. Your PhD focused on the carbon in the land, but you soon became interested in how this links to climate change. You began working at Columbia University in New York, and your goal was to understand and model how changes in the climate affect the cycling of carbon between the atmosphere, land and the oceans, and vice versa, how this cycle of carbon affects the climate.

14:30Before we get into this, how was climate change modeled back then, 30 years ago? So 30 years ago, there were many, many groups across the world, including at Columbia. They were developing climate models. And the assumption is that you can simulate and project the future climate condition if you know the concentration of carbon dioxide. So you prescribe to your system a trajectory of carbon dioxide concentration. And the model calculates the warming that would be associated to this increase in carbon dioxide. So the more carbon dioxide in the atmosphere, the more of a greenhouse effect you have, the higher the temperature.

15:07Exactly. So what was missing from these climate change models, of course, at the time, was the movement of carbon between the atmosphere, the oceans and the land. And you began to create a model that put carbon at its centre. Yes. Essentially, what we do as humans, we emit carbon dioxide and the carbon dioxide we emit goes into the atmosphere. And as we discussed earlier, a fraction stays there and the fraction goes back into the carbon sinks. the ocean and to the land. As of today, it's about 50%. There is no guarantee it will be 50 % in the future. That will depend on how the ocean behaves in the future.

15:42That will depend on how the forest behaves in the future. And we had some indication already that they probably wouldn't behave well with climate change. So even if you knew that half of the carbon dioxide we emit gets absorbed into the oceans and the forest, that doesn't mean if it gets warmer that 50 % is going to remain the same. Exactly. And it was the implicit assumption. So it was becoming clear to you then, Pierre, that the way that carbon cycles between land and the atmosphere has a subtle relationship with the climate. And this subtlety is to do with the temperature. So if the climate is getting warmer, either because of future climate change or El Niño's, which happen every three or four years and you have like slightly warmer condition.

16:26We know that the land carbon cycle is not functioning as well. and the main reasons is that when it's warm and dry you have less photosynthesis so you reduce the amount of carbon that goes into the biomass into the trees and also you have a huge amount of carbon which is sitting on the ground which is decomposed by microorganisms i mean worms and bacteria so there's just decaying there's the decaying part and the decaying part increase with temperature so not only you have less carbon going in because photosynthesis is stressed you have more carbon going out. And so we see this on every timescale we can look at.

17:04So we know that, I mean, warming leads to less carbon stored on land and more carbon ending up in the atmosphere. And it's the same for the ocean. The warming reduces the amount of carbon it goes in. So it's still going in, but not as fast. Okay, let's bring the ocean back into the story now then. In 1998, you joined the National Centre for Scientific Research in France to continue working on these models. What did you discover during this time about the relationship between the land, the atmosphere and the oceans and this rise in temperature. This is the first time we put all these pieces together.

17:34So I came back in France with my land carbon cycle model. There was a team where I landed working on ocean carbon cycle model and it was part of a larger institute developing the traditional climate models. Right. And so we got together and said actually we've got to do it right. So we've merged all of these different models into one single model. Right. It does everything. And so every single year, you can tell the model how much we emit, and the model will calculate how much CO2 stays there because it will calculate how much goes into the land and goes into the ocean. So what did you discover would happen with temperature rise?

18:06So what we discovered, so when you couple the whole system with simulation with climate change, without climate change, and looking at the difference, and we identified what we call the climate carbon cycle feedback, which is climate change reduce the land carbon sink, reduce the ocean carbon sinks, which means there is more carbon dioxide that stays in the atmosphere. So this means there is more warming. We know why the land gets less efficient at taking carbon dioxide as the temperature goes up. For the oceans, what is the reason for that drop in efficiency when the temperature rises? For the oceans, it's more the solubility of carbon dioxide into the seawater, decline with increasing temperature.

18:46And also, the export of carbon from the surface of the ocean to the deep ocean is due to the large-scale circulation of the ocean, and this also reduces with warming. It doesn't mix. It doesn't mix. Take the carbon dioxide down to the bottom. So carbon dioxide levels in the atmosphere don't directly predict climate change. As the temperature rises because of climate change, the land and the oceans get less efficient at locking in carbon dioxide. Yeah, and so the land and the ocean are controlling the level of carbon dioxide into the atmosphere, and therefore the warming. Your computer model, Pierre, which simulates the Earth's climate and other components, including the atmosphere, land and the oceans now, was published in 2001.

19:31It was known as the Earth System Model. It was one of the first, actually, to link the carbon cycle to future climate change. How was it received around the world when it was published? At the same time, there was a little group in the UK, actually, from the Met Office. And what was really striking is that they came to the same conclusion as us, which is future climate change does reduce the efficiency of the land in the ocean but their projection was much more dramatic than ours so there is a warming because of co2 there would be less uptake more co2 and therefore more warming so we said the more is about 10 percent they said the more is about 30 percent i mean agreeing on the sign of the response yes but not on the magnitude and then of course to generate a lot of research because in science it's always the same if you have two results in contradiction we need to understand which one is right so everyone say okay it's a hot topic now and we need to understand because it's really important because this is what will control the future climate system and then five years later there were 10 or 11 models doing the same thing trying to understand what's going on and so we generate this activity across many modelling groups across the world.

20:44Well, so far, we've been talking about how you use your models to quantify the total global amount of carbon in order to see how much of it we're responsible for emitting through fossil fuels and deforestation, and then understand how all this drives climate change. The challenge, of course, is to convey the science in a way that's both accessible to the public and, of course, actionable by governments. With climate change, that means quantifying the impact of human emissions. And you found a neat way of doing this. Can you explain? So we discovered that there's a near-linear relationship between the total carbon dioxide emission that human activities put or will put in the atmosphere and the level of warming.

21:26This linear relationship is between the carbon emission into the atmosphere and the rise in temperature. It's a straight line. Exactly. So you double one, you double the other. Yeah. So for example, since the beginning of the Industrial Revolution, we emitted, it's a staggering number, 2.5 trillion tonnes of carbon dioxide. And this led to warming of about 1.3 degrees. So for every trillion tonnes of carbon dioxide we emit in the future, there's about half a degree of warming. This relationship between carbon dioxide emissions and global temperature increase has been crucial in getting the message across about how much carbon each country can emit before we breach some sets, increase in temperature.

22:06Now, we mentioned the idea of this remaining carbon budget at the start, but can you just explain where we're at now? So the concept of the remaining carbon budget comes from that linear relationship. It is really the amount of carbon dioxide we can still put in the atmosphere, total, all countries together, to limit warming to a given level. Every additional trillion tons of carbon dioxide will generate about half a degree. So if you want to limit warming below 1.5, given where we are today, you can translate this into a carbon number. And likewise, you can calculate how much carbon dioxide we can still limit before we reach, let's say, 2 degrees.

22:46And for 2 degrees, it's about the equivalent of 25 years of current emissions. So we have to reduce the emissions in order to reach zero, having emitted less altogether than this budget. Your work has shaped the assessment of climate change globally and the role that humans play in it. For the past 30 years, you've been actively involved with the Intergovernmental Panel on Climate Change. And now the remaining carbon budget has been incorporated into their assessments. Not only that, your Earth system models that we talked about earlier underpin these assessments, putting carbon at the very heart of how we think about climate change.

23:22That's quite an achievement. How do you personally feel about that? It's not just a personal achievement, right? I mean, there's a large community of scientists that have been working on this. That's what all my guests say. Yeah, but they're all right. If you ever get one guess who says it's just me, I'm not sure. But my wife always makes fun of me because she says, you discover a catastrophe and you are proud of it. And yes, it's complicated. I mean, it's like the dichotomy in the brain of a scientist, which is, I mean, of course, you discover something, you publish in literature, you're super excited, but you're discovering something bad.

23:58And we've done the same with like permafrost, for example. on permafrost, I mean, this frozen soil in high latitude, they will release carbon because of warming, but you publish a great paper. So it's complicated. It's bad news. And we try to portray the information, which is we have to act faster because things will get worse and we have to limit the warming as much as possible. Right. So the future is still in our hands, simply depending on us acting and reducing emissions. How do you feel then about all this, given that you've dedicated your life to showing the world what we need to do to combat climate change?

24:36When I started, like in the 90s, climate change was the concept of the future. So it wasn't like science fiction, but it was not far from science fiction. I mean, 30 years later, we are living climate change as we speak. How do you feel about this? Yeah, I mean, it's really frustrating, given that at least 30 years ago we knew already the cause, emissions from burning fossil fuel. There was enough information for responsible governments to act. I mean, this is COP30. 30 means 30 years. So we've been discussing and sitting around the table for like 30 years with no action. You're meant to be at COP30, of course, but I understand you decided not to go.

25:16So we are trying to share our carbon footprint and also to share our guilt about flying around the world. So with the carbon budget that I lead in Exeter, of course it's important and we know that we have to communicate on where we are in terms of emissions. So we have to attend this kind of event. So there's a team with me and we split the carbon footprint and we split the personal guilt of flying around. So I'm not at COP. So reducing carbon footprint by reducing overseas flying. What about this popular idea of carbon offsetting that we hear about? Flying emits CO2. So you need to remove the same amount if you want to stop warming.