Could you please help me welcome Professor Helen Chomsky? Thank you. Hello. Thank you for coming along in person and hello to everyone online. Right, carbon in our society gets a bit of a bad rap. It's a bit strange to think that carbon, this atom that we think about so much, actually makes up 0.02% of the entire mass of our planet. It's not very much. And the bit of the carbon that we are talking about almost all of the time is what's in the atmosphere, and that is 2% of the 0.02%. It's a very small amount. So I think a bit of context would be useful. So what this lecture is going to do is take a bit of a bigger picture at the global carbon cycle. I think that what we need to do in the world today is understand how the natural world works, our planetary life support system, and then learn to work with it rather than against it. And, you know, science fiction writers love to describe us as carbon-based life forms, but it's not just our anatomy, it's not just our bodies that are made of carbon that matters. We are carbon-dependent life forms because everything around our world is really, really, really everything that matters to us in the world is either based on carbon or very, very, very strongly influenced by carbon. So I would like to send you out of here thinking that carbon perhaps isn't automatically a terrible thing, but having the bits of it that are causing a problem, put in a bit more context. And so we're going to start going all the way back to 1773 in Paris. So what's going on? There's quite a lot happening in Paris at that time. The really big events are still to come. Marie Antoinette isn't quite Queen. The big harvest that will really start the rumblings for the French Revolution is still a few years away. America only became independent in 1776. That's not quite happened yet. But all of the sort of foment that led to all of that is just at the beginning. And in Paris, while all that's going on, this is happening. Now, this the key character in this wonderful diagram is this guy here. This is Antoine Lavoisier. He's an extremely famous scientist in his own time. He is the chemist who revolutionized chemistry. He turned it into a quantitative science instead of a qualitative science. So instead of alchemy and the, you know, these sort of ideas of air worth, air, earth, water, and fire being the elements, he starts to measure things. And really quantitative measurement was his huge gift to the scientific world. And he became interested in combustion. So he did a lot of measurements around combustion. And what this wonderful creation here is, is a solar furnace. And what he's doing with it is using this colossal lens at the top, which was owned by the Royal Palace. It weighed 70 kilometres. It was about 90, 80 or 90 centimetres in diameter. It's a huge piece of glass, a biconcave lens, and it's focusing the sun's length, the sun's light on this critical bit down here. You can just see some smoke and steam coming off at the bottom there. And the reason he's doing this is it's a way of heating things up without the fire itself influencing his chemical measurements. It allows him to make even more precise measurements. So lots of things went down here. The thing I like about this, by the way, is it's solar-powered science back in 1773. And what he what the the part of the story which is relevant here, he put lots of things down there. But the one that is relevant in 1773, and this is very perhaps typical, you might, in retrospect, of a scientist of his time, what he's putting down there is diamonds. And what he discovered about the diamonds was that if he put a little collection of diamonds on this plate, he'd focused the sun's the sun's rays on it, they would heat up and heat up, and then the diamond that he'd focus the light at would start to go black and then would evaporate, and the diamonds around it would start to look a little bit like soot. And he didn't really make the connection that the stuff that diamond is made of is the same stuff that soot is made of. But he was he was almost there and he was making the measurements that would make that story clear. And the reason I'm showing you this is that this is the beginning of the realization that carbon is a multifaceted thing. It can do many, many, many jobs. Back then, diamonds and soot seemed about as far apart in the chemical world as you can possibly imagine. And this was the experiment, the type of experiment that eventually showed them to be the same thing. And I mean, it's probably worth pointing out that Lavoisier had a very interesting history, which I'm not going to cover here. But while there was a political revolution going on outside, he was building a chemical revolution with this experiment as he showed that we could think about combustion differently. And actually, the law of conservation of mass in French is referred to quite often still as Lavoisier's law, because it was his quantitative measurements that allow people to understand that the number of atoms you've got is fixed, they hang around. Anyway, so while he's getting on with his chemical revolution here, this is the beginning of the idea that carbon can take many forms. And of course, we take that for granted today. The pure forms of carbon that we know of include the ones along the top. We've got diamond, graphite, buckyballs, graphene, and but these are relatively rare in the natural world. The ones along the bottom are much more common. The carbon is the backbone of biomass. There's plenty of types of it out in the modern world. And also not all organic carbon is actually currently living, although all of it has been, almost all of it has been produced by life. So this um on the bottom right here, we have some definitely organic matter that has come about as a side effect of life, if you like, a byproduct, even though it's not itself living. So we we we know today that carbon has these many, many, many faces it can present to the world. Um, so why is it then that when we hear about carbon, when it's in the news, it's all this kind of thing. Uh it's if you were if you just arrived on planet Earth and you just heard these headlines, you'd think carbon was a terrible thing. We want to we want carbon emissions to drop. Can we be carbon neutral? We have to pay for carbon dioxide. The message from all of these is that carbon is bad. And of course, in the wrong place in the natural system, carbon definitely causes some problems. But what happens if we take a step back and put all that in context? Um, so this is the carbon dioxide molecule. This is the baddie in the plot here. Uh, and you'll be for maybe familiar with this. It's got a carbon atom in the middle, it's got two oxygen atoms on either side, that's the the CO2, one C and two O's. So, where is it? If we look at all the carbon, there isn't just carbon dioxide, but it's all, you know, it's filling the system. That 0. not not just the 0.02%, but the carbon that's in places that are kind of accessible. Um, what is that? Well, if we imagine that the area of this oblong, this rectangle here, this is the accessible carbon in the world. We'll come to what that means later. How much of that is actually in the atmosphere? And the answer is it's about this much. Before humans got going with the Industrial Revolution, it was just the uh the small, the sort of two-thirds of that on the left, and then the the sort of spotted area is the amount of carbon that humans have moved from other places into the atmosphere. But it's a relatively small proportion of the whole. Um, so the way this fits into you heard from Christine that I'm giving a series of lectures over this academic year, and the point of the all of them together is that we're going to look at these big cycles, how these different atoms and molecules weave in and out of each other to form a global system. So, this is the context for today's lecture, and obviously we're here on the carbon lecture. So, so let's start out with saying where all the carbon is, that'd be a useful thing. Someone's got to have added it up. So, if we go back to our big area here, this represents the accessible carbon in the world. Now, most of the carbon on planet Earth is locked up in rocks, but it's so deeply locked away that it doesn't interact much with the world that we live in. So, this is all the other carbon, mostly, that is uh much more accessible. It's out and about where we can see it, it's not buried miles underground. So let's look at where all the carbon atoms are. Um, those are the ones from the atmosphere that we just saw. Here are the ones from the known reserves of fossil fuels. We'll come back to that. Vegetation has about that much, 450 gigatons. Um, quite a lot in the permafrost. If you care about where carbon is, permafrost is important. Turns out that soil is even more important. And then we get to the bits that are in the ocean. So it's split between the coastal ocean, ocean sediments, ocean dissolved organic matter. We'll come back to what all of that means. But there's still quite a lot of area here which doesn't fit into any of those categories, right? It's a lot. That's probably all the places you can think of where you might find some carbon. So where's the rest of it gone? And the rest of it, it turns out, is all in the ocean as an inorganic form, as dissolved inorganic carbon. And we'll come back to what that means. And there is a little bit, so I I the fossil fuels there in the top uh top left-hand corner, they are the known reserves of fossil fuels. Obviously, there is coal and oil and gas that's trapped either in tiny quantities or it's really inaccessible. It used to be out in the world, it used to be trees and organic matter. Um, and there's a lot of that. It's not clear exactly how much, but on the scale of this, approximately that much, somewhere around five to ten thousand gigatons. So, this is the carbon. If we're interested in understanding the carbon that runs our world, this is where it all is, and that's approximately how much of it is, how much of it there is. So, the reason carbon is interesting is because it's reactive, it's got a few properties that make it interesting. But so most of the time when we're talking about carbon in the natural world, we're not talking about carbon in its pure form, because that's actually really unusual. But just in case any of you really still believe that diamonds are forever, I'm about to pop that little balloon, sorry. Um so this is a sentence in a paper that I found when I was looking through this, and it's talking about graphite and diamonds. And basically, what it says is that if you have a diamond and you drop it into water at high enough temperature and pressure, not actually that high a temperature and pressure, it'll dissolve. So that's nice. So they said that you know that in science language, although graphite and diamonds are thermodynamically stable at high pressure and temperature conditions, they remain surprisingly soluble, especially in hot hydrous fluids, which means hot water, uh, because water is a remarkable organic solvent. So basically, if you drop your diamond ring into a volcano and it goes far enough down, it'll dissolve. So that's nice to know. Don't do it. If you value your diamonds. But one of the reasons we value these forms of carbon is that they are rare. Carbon is so reactive that it's always making friends with other atoms. And so it's the huge variety of other molecules that we're interested in here. Um, so those numbers on where carbon was found in our natural systems, they come from diagrams like this. And I've just outlined them all in red boxes here. It doesn't really matter uh exactly which ones are which for now, but they come, they they live in diagrams like this, and this is a diagram of the flows of carbon around our natural world. It's made by something called the Global Carbon, the Global Carbon Project, and that's been established for uh 20 or 25 years, and every year they make a budget. So it comes out in November or December, they take the latest knowledge, they add up what we know about where carbon is in the Earth system and where it's going, and they condense it into a set of very useful graphics. So this is um this year's Global Carbon Budget published in December 2025. And the most important thing about it is that there's carbon in lots of places, and there's some big arrows, and the arrows are the important bits because they're where the net flow of carbon is in or out of something. And of course, the most important arrow on here from the point of view of our civilization is this big grey one up here, where carbon is coming from the fossil fuels down there, it's being burned and it's it's ending up in the atmosphere. That's not actually the only carbon that matters. There's another little arrow here, which refers to land use change. So as we change our soils and we change how we farm, and we'll come to why a little bit later, we also cause them to emit carbon to the atmosphere. So these arrows here, these are some of the big changes. We are shifting flows of carbon. Things that carbon that was in one part of a system and our actions are moving it into another part. Um, so I didn't explain the units for a reason. It's that the units in which we count carbon, there's this rather awkward uh sort of ambiguity unless you're really paying attention. And the ambiguity is that sometimes people measure carbon as the number, the amount of the carbon atoms themselves, and sometimes they measure carbon as carbon dioxide. So we can imagine that you know, if you have a kilogram of just carbon atoms, by the time you've combined them with some oxygen to make carbon dioxide, you've got the same number of carbon atoms, but you've got some other atoms as well that are adding to the mass. And so you can measure in something like gigatons of carbon, or you can measure in gigatons of carbon dioxide. And so it's a little bit confusing, but just if you're looking at the numbers on some of these charts and you're thinking, oh, I thought we emitted 40 billion, 40 gigatons of carbon dioxide every year. We do, but that's only equivalent to 10 and a bit gigatons of carbon. Slightly unhelpful, um, but bear that in mind if you're thinking about these numbers. Either way, it's a lot. Our civilization emits around 40 gigatons of carbon dioxide up into the atmosphere. It's a colossal amount, right? It's a huge amount of stuff that we are moving around. So, what I want to do is take a step back from all the nitty-gritty of climate change, which I assume you're aware of the outline of, and I want to look down into what are the critical features of carbon in these different forms. The carbon that's in our world, including the carbon dioxide, but in other places as well, it's doing things. It's not passive, it's interacting with the system around it, it's shaping the world. So let's have a think about what is it, what are the features of carbon that allow it to either interact with the world or not? Because if what we care about is carbon doing something, we should ask when it can do it and how easily it can do it. So there's a couple of characteristics that matter. Um here's a question for you. Don't know if you've ever asked yourself this, but here you go. Why is it that we can't eat coal, right? We think of ourselves as creatures that can eat organic matter. You know, you probably all had breakfast and lunch and dinner today made out of things that either grew in fields or ate things that grew in fields. That's definitely all organic matter, it's all organic carbon. And coal comes from trees, which are definitely organic, and we can't eat trees, but other creatures can. And trees have definitely got energy in them because if you burn them, you release energy. So, why is it that we can't eat coal, right? Coal is just squashed trees that have been put under some pressure. What is it about coal that means we can't eat it? Um, there's two things. There's fundamentally two reasons we can't eat coal. One of them, and they illustrate the big picture here. One of them is the form that those molecules are in. When ancient trees fell into a swamp, were trapped in anoxic conditions, were compressed under a huge amount of pressure, and eventually became coal, a huge number of chemical reactions happened. And mostly what those chemical reactions did was they stripped out oxygen and they stripped out hydrogen. So they're all these organic molecules that something could eat. Uh, and then you strip away these other bits of the molecules. So these are little pie charts here showing you basically how much carbon in blue, how much oxygen in green, uh, the hydrogen is in orange, and the light blue is everything else. And you can see that initially the cellulose and the lignin that make up a tree are probably only actually half carbon itself. There's a lot of other stuff in there. But as you squish trees into them into different molecules, you strip out the hydrogen, you strip out the oxygen, what you're left with are molecules that are much, much, much richer in carbon. They have less of everything else, they're bonded together in different ways. And because those chemical bonds are different, nothing in nature has evolved to be able to break those bonds and release that energy. So that's why we can't eat coal. It's fundamentally that the carbon is definitely there, it's definitely got energy, it's definitely organic matter, but the chemical bonds that hold it together are a sort of bottleneck. We can't, there is no enzyme in nature that will get through that bottleneck. Actually, um, some people, there is a scientific paper where people have claimed that some of the microbes that live in the guts of termites might be able to digest coal. But no one seems to have repeated that experiment, so I'm not entirely sure whether to believe it or not. Um, but anyway, so the form that the carbon is in influences how it interacts with the world. And the other thing is uh the location. It sounds obvious, right? But you know, coal is mostly a long way underground. There's no reason for anything living up above to evolve the enzymes to eat it because it's just not around. So those two things, the form that a molecule is in and its location, dictate how it can influence the world. So let's think about the form first. And this is one of the things that makes carbon really unusual in the world is that it can do this. So let's consider Sertse. Now, you may have heard of Sertse, it's a lovely story, really. Um, off the bottom of Iceland, the south coast of Iceland, in 1963, nothing much was happening. No one really went there to look at anything. And one day in 1963, a earthquake, um, a sort of um geophysical station started measuring some weird little tremors. Thought that's a bit odd, don't know what's going on. And a few days later, a fishing trawler was off the coast of Surtsi and they saw these big plumes of black smoke, and they thought maybe a ship was on fire. They should go and help. So they went over there and they discovered that it wasn't a ship that was on fire, it was great plumes of ash that were coming out of the ocean. And that was how Sertse arrived in the world. It's a volcano, you know, Iceland sits on the uh the ridge, the mountain ridge that splits two great tectonic plates, and it was erupting, so it burst out of the world, there was burst out of the water, there's lots and lots of water and steam, so it was explosive, it was full of ash, and you know, lava built up and built up and built up. It erupted for four years, and then there was an island that became known as Surtie. And the thing that Iceland did that I think was brilliant with that island is they said, you know what, we're gonna leave it alone. We're not gonna go there, we're not gonna mess with it, we're not gonna change anything. Every year we're gonna send some scientists to, you know, go on there, measure the surface, but apart from that, no one is going to be allowed to go and visit Surtsi. We'll just we'll just let it sit there and do its thing. But as a result, this is a fabulous natural experiment, right? Because you've got bare rock emerging, and we know, because of places like Hawaii, for example, which have lush tropical greenery, that islands that start as bare lava can become fabulous places of biodiversity. But we didn't know a lot about exactly how it went from one to the other. So the thing that Certsey allowed the world to do was to watch and to see what happened. So every year they sent these scientists, they looked at how many animals there were, they looked at how many plants there were, uh, and they they plotted graphs like this. And it turns out that, you know, in the years since in the sort of 60 years that Certsi has existed, it's gone through four major stages. And so what's plotted here is the number of plants that are living on Certsi. And you can see in the first few years there's not very many. Maybe it gets up to 10. And what's happening at that point is that they're arriving from the ocean, you know, they sort of live on the sand, they're not really, they're kind of barely on the island itself, they're just kind of stuck in the edges. And then there's 10 years where that's all you've got. You've got this sort of tiny sprinkling of green around the edges of Certsey, but you haven't really got anything else. And then the seabirds discover Certsey. And what the Seabirds are doing the gulls, is they're feeding out in the ocean, in the rich seas around Iceland, full of fish. So they're feeding out there, they're coming back to the island to have a rest, and then they're pooping. So, of course, nutrients everywhere. Hooray! And you can see that then Sirtse goes through this really dramatic increase because basically, as the seagulls bring nutrients from the ocean, soil can grow and you start getting cycling. So you're growing soil, more and more plants can live. If they blow in, they might actually take root in the soil and grow. And so you go through this explosion of all the life on Certsey, and then you you kind of get to the top, and then everything that's happened is more or less it starts being much more stable. And uh for the last few years, everything's kind of stable, you know, it's stabilized a bit. And actually, Certsey is shrinking a bit, and the number of plants is going down ever so slightly. Um, what's happening here? We've got shore plants, we've got slower colonization, tough times, the gulls arrive, they bring nutrients, they establish soil. So here's the thing is that on the day Sertse erupted, the carbon in the atmosphere and you know, puffing out of the volcano, it was almost all carbon dioxide, and there was plenty of carbon, but it was stuck as carbon dioxide. And it's only when life comes along, and all these cells that life is made of, a cell is just a biochemical factory, it's building things out of the world around it, and it can build carbon atoms into a different type of molecule. So life is this facilitator for these for turning carbon from very, very simple molecules, inorganic molecules, into these very complex forms where you've got chains of carbon atoms. And as soon as you have life, you you create the conditions for all those little biochemical factories to get going. And that's when the world of Certsey started to change. So, oh yeah, that's so this is actually a seabird colony on Certsey now. So you can see it looks, I mean, there are other bits of the island that look quite bare, but wherever the seabirds were nesting, lots of soil, loads of life, loads going on. So what is it, right? What is it that life is doing that takes all this carbon from the atmosphere and puts it into, it turns it into something different? What's the key at the root of all of this? And so, chemical equation coming up, sorry. Um this will be familiar to many of you. We've got carbon dioxide and water and some energy on the left-hand side, and those turn into this collection that's in brackets with some carbon and hydrogen and oxygen, plus extra oxygen. And any of you who remember your GCSE science may identify this as photosynthesis. We've got carbon dioxide and water that are turning into some stuff and oxygen, and this stuff, this is where the carbon atoms have joined together to form big long chains. And the thing about chains, and especially chains that can branch, is that you have this enormous richness of molecules, and that's what gives you access to all of the life that we see on Earth. And there are a couple of words for what's on either side of this. Uh, everything that's on this side, where the where you've got carbon atoms in long chains, is an organic molecule. That's the definition of an organic molecule, where you've got multiple carbons locked together in these big long chains so you can have huge complexity. And then by default, everything on the other side is called inorganic. And this is the big shift, right? This is what life enables. It enables the world to take really simple molecules and build them up into something where so you have building blocks, and once you have building blocks, you can build lots of things. So, how do we do it? So we've said photosynthesis takes you that way. Plants take energy from the sun and they take these raw materials, they build up sugars. Um, and of course, we usually associate photosynthesis with storing up energy, and it does, but it also stores up materials. So that pushes every pushes things this way. What pushes things the other way? Respiration is a thing that pushes it the other way, and you are all doing this right now. As you sit here, all of you are breathing, whatever you had for breakfast or lunch or dinner, those organic molecules that came to you as organic carbon, every time you know your body is busily respiring, every cell in your body will be respiring, you take burning those sugars to turn to take the energy, turn them back into carbon dioxide and water, and you have moved um carbon from the organic carbon category into the inorganic category. So just the process of living is shifting things back this way, of course, and you get energy as a result of that. And then we also know, and especially this summer, where we've had a lot of wildfires, combustion sends you back that way. You release a lot of energy. And then there's a few, you know, things like fermentation. It's only in brackets because it's a not such an important process. If you're really into whiskey, I'm really sorry. But you know, in general, not so important in the or bread actually, not so important in the grand scheme of things uh when it comes to moving carbon about. But the point I want to make is that you've got these two big pools, and life is what is tipping carbon from one form to the other. It's like a seesaw. So let's just imagine ourselves a little food chain here. Uh we've got a leaf there, and the leaf photosynthesizes, it takes energy from the sun, so it builds up a store of sugars, of building blocks, organic carbon. But then the leaf itself has to live. It hasn't made those sugars, you know, out of the goodness of its heart. It's made those molecules because it needs them in order to survive. So it's gonna itself, leaves respire at during the daytime, they are taking up oxygen, taking up carbon. At night, they're respiring and they're giving it back. So some of it immediately goes back to the organic pool. But then maybe a caterpillar comes along. But the caterpillar only has access to what's left after the leaf has had its bit. And the caterpillar, you know, will eat some, but it's also got to burn some energy to live. So chunks, another chunk goes back to the inorganic side, and then maybe a bird eats the caterpillar, and there's even less left on this side, and then another chunk has respide out, the bird's warm, it's got to, you know, it's got to generate heat energy to stay alive. And you sort of follow on down the food chain, fox might eat the bird, even less left, tiny bit goes over there, and then eventually, if the fox you know dies, it might get eaten by a microbe, and the microbe will finish the process off, and everything will end up back as organic carbon. And I should say here that I've drawn these, the length of the bars here are drawn for illustrative purposes. They are longer than they would be, right? The efficiency of what gets passed on from stage to stage is sort of between around 5 and 30 percent, so it's not a lot. So actually, these bars be much, much, much smaller as you get down the bottom, which is why, incidentally, it's more energy efficient to be a vegetarian because you're accessing the the levels of the food chain where there's much more available before a lot of it has been turned by respiration back into inorganic matter. So, this is how life is shifting carbon from organic to inorganic all the time. Um, but the one thing, so that could happen all by itself, and the world would not be an interesting place. Because if it becomes a potentially interesting molecule, and then straight away it's respired back into our inorganic carbon, which is less interesting. Sorry, any inorganic chemists are out there, um then nothing very you can't, you know, it's very quick, isn't it? You you you turn it from photosynthesis into organic carbon, you breathe it out, you've got inorganic carbon, you just you know, you're limited in what can happen. The reason our world is an interesting place is that that seesaw goes in the photosynthesis direction at a relatively consistent rate, but a very it's very variable in how quickly it goes back the other way. So have you ever wondered why it is that if you have an apple in a fruit bowl and you leave it for several days, in my case, sometimes a couple of weeks, and then you come, you arrive and you realize that instead of having an apple, what you've got is a bag of mold in the shape of an apple. So, why is it that that can happen within a week? But if you see a tree stump, it can last not just for years or for decades, but sometimes for centuries, right? It's all made of the same stuff, right? It's made of plants. So why is it that sometimes you get you turn it into organic matter and you turn it straight back really quickly, and sometimes you turn it into organic matter and it just hangs around. And the reason is that what you've got in an apple is basically sugar, it's really easy to digest. There's lots of enzymes that will do that job, and it's immediate fuel straight back. You into the organic pools, straight back, breathed out. But in the case of trees made of cellulose and lignin, there are relatively small numbers of organisms in nature that have the enzymes to digest these. And because they're hard to digest, they tend to hang around for a very long time, and that makes them really, really, really good at building things. So the reason our world is an interesting place, one of the many reasons, but a very big reason, is that carbon, once it's become organic, doesn't very easily go back to being inorganic, some forms of carbon, and that's what gives us structure in the world, right? That's why everything doesn't just get respired back immediately. And so you've got this time scale, quick quick turnover and slow turnover, and it can be days at that end and centuries at that end. So now our balance of what's shifting where depends on speed. But it doesn't just depend on what enzymes are available, it also depends on the physical environment. And I love this experiment, I love this idea. This is a it's called the Harvard Forest, it's in Massachusetts in America, and in 1991, somebody, for reasons I'd love to have been a fly on the wall on this day, they got scientific funding to heat up the soil in a patch of forest in certain places by five degrees above whatever the ambient temperature was. So basically they split this forest into lots of grid squares, and some of those grid squares were left alone, some of them had extra nitrogen added, some of them they heated up the soil, which is a lot of energy. And part of me, the one that thinks about the climate and energy use and all of that, is horrified that they're basically heating up the outdoors. And the other part of me thinks it's a really clever experiment because what you've got is a patch of the same forest in exactly the same conditions, exactly the same in every other way, but you heat the soil up a bit and you can just see what it changes. And of course, they were measuring regularly as time went on to see what happened. And what happened is that even though the same microbes were present, the same nutrients were present, everything else was the same. Changing the temperature changed the speed at which carbon went from the soil to the atmosphere. Now, the reason soil matters here is that forest soils, you saw that soils were a big chunk on that graph back near the beginning. Um, soils are a huge store of carbon. And you can see this is a quote from that paper here that's storing up to almost half of the total carbon in temperate forests. So it's a massive chunk of carbon in soil. And what's in soil is a huge range, there's an organic matrix, it's really beautifully complicated. I talked about some of this in my lecture about the ground last year, so go back on that and have a look at that if you're interested in soil. But the point is, there's a colossal carbon store in there, and it's stored in the kind of molecules that aren't digested very easily. They're just sitting in the soil, they're providing structure. And what they found was that this is a diagram from that paper, you can see a little thermometer, is that as they warmed up the soil, enzymes and microbes digested things faster. So, exactly the same soil, exactly the same microbes, exactly the same enzymes, you heat it up, everything moves faster, and what that means is you are giving off carbon back to the atmosphere. So you're taking it from that soil store and you're emitting it to the atmosphere. And so they saw that warming sped up decomposition. Um, it moved more carbon back into the atmosphere. It actually reduced the number of microbes in the soil because you know they didn't have as much to eat, because there were microbes will eat some of this stuff very slowly. But most importantly, it was destabilizing the pool of organic matter that made up the soil just by heating it up, not by changing anything. So the physical conditions also make a big difference to how carbon moves between these big pools. Okay, so that's a little bit about all the horizontal stripes that were on that map of where all the carbon is at the start. We're going to take a little dive into the ocean, not just because the ocean is what I study, but because, as you saw, the ocean is this colossal store of carbon, and there's a really interesting reason for that, and it matters for how our world works. So let's find out where that big store of carbon comes from. So here's it's this is the same information as that uh sort of sideways bar chart, same colours, but it's just presented differently. So here you are, I think 84% or something of all the carbon that's sort of around in the world we can see is in the ocean. So the question is what's it doing there? Or what's the ocean doing? That means that's where all the carbon is. And this is a case where the form of the chemistry, the form that matters, the form of the molecule, isn't just controlled by biology. Here the form is controlled by chemistry. If you don't like chemistry, I promise this bit will be quick, it'll be over. But the next equation is going to look a bit frightening, right? Uh sorry about that. So here's the thing: carbon can exist in the ocean in different forms that are all inorganic. Um, and this is the sort of equation you might have seen in school. So you can see you could dissolve carbon, carbon dioxide, you can dissolve that in the ocean, and quite a lot of carbon dioxide dissolves in the ocean because there's quite a bit in the atmosphere. So some of it finds its way into the ocean. So if we dissolve carbon dioxide into the ocean, it combines with the water to form something called carbonic acid, which is not very stable. But what the carbonic acid then does is dissociate, it splits into two ions, a positive and a negative one. So this is bicarbonate in the middle here, but even that is not completely stable, so that might dissociate again into carbonate. The names don't matter too much, but the point is here are our three stages. So when you dissolve carbon dioxide in the ocean, it doesn't stay as dissolved carbon dioxide. Quite a lot of it shunts itself down this chain and forms either bicarbonate or carbonate. And in the ocean today, the proportions are relatively fixed, they look a bit like this. About 90% of it is stored as bicarbonate, 10% of it is this end and it's carbonate, and 1% is that end, uh carbonic acid, acid and dissolved carbon dioxide. So the and the way the act the exact proportion depends on the pH and the temperature. You know, you can play about with the chemistry of that for a long time. But the reason that this matters is that when you dissolve carbon dioxide in the ocean, it doesn't stay as carbon dioxide. It moves along the chain. So actually, the ocean is a dilute solution of bicarbonate, generally. There's a lot of carbon here. So why does it matter? Why does it matter if when we dissolve carbon dioxide in the ocean it doesn't stay as carbon dioxide? It matters because of what I study, which is air sea gas transfer, and that means how the ocean breathes. There's a physical, there are physical processes that help uh gases cross the surface, but all of those physical processes will only do anything if there's a concentration gradient. So if you have more in the atmosphere than you have in the ocean, carbon dioxide will tend to go down into the ocean. If you have more in the ocean than you have in the atmosphere at that point, carbon dioxide will tend to come up out of the ocean. So you have a concentration gradient. So if there's no concentration gradient, on average, nothing moves. But if you have a big concentration gradient, then the world is going to try and keep up fast, quickly or slowly, it depends. So here's why this matters, right? Say we dissolve some carbon dioxide in the ocean, and it's an amount of carbon dioxide that would bring the atmosphere in the ocean into equilibrium. But it doesn't reach equilibrium because most of that, 99% of all that carbon, carbon dioxide, disappears off down this chain and becomes carbonate or bicarbonate. So our concentration gradient is maintained, even though the carbon is still there, it's just been shunted off into a form that doesn't matter. So what that means is that you can dissolve a lot more carbon dioxide in the ocean than you should be able to if this wasn't operating. Because you're shunting it down the chain into these pools that don't interact with that physical exchange process. And that's why the ocean holds so much carbon. And it's why it's doing us such an enormous favor. It's taking up around uh 25 or 30 percent of all the extra carbon we're putting in the atmosphere. That's ending up in the ocean. And it's ending up, so much of it ends up there because this process is squirreling away the uh carbon dioxide as something else, making it effectively invisible to that exchange process at the surface. So, this is the fundamental reason why there's this enormous pool of carbon in the ocean, far more than it looks like there should be. Um, and of course, once it's in the ocean, and that this bit will whiz through relatively quickly, um, it doesn't necessarily it's not distributed equally, right? If it's distributed through biological and physical processes as huge sort of there's a buff, this is a carbonate buffer system. Um so so what that looks like, that exchange process across the surface, red is where the carbon dioxide is coming out of the ocean, blue is where it's going in. This is a model, so it's based on a weather model and a model of what the concentrations are like, but you can see that this the ocean is breathing in close to the poles and it's breathing out close to the equator. And the net amount of carbon dioxide that ends up in the ocean is the balance of those two things. So it's an incredibly dynamic process, and this is just a model. We don't know exactly what it looks like yet, that's why I've still got a job studying bubbles. Um, so uh, and then there's this complicated set of processes that move carbon around in the ocean itself. There's a biological way of doing it, things that take up carbon if if a tiny it's only a tiny proportion, but if they fall down, then that will move carbon down to the deep ocean. There's also physical processes that can take up carbon and move it downwards. So even once it's in the ocean, it's still complicated, is one where I'm going. Um so just the then the the sort of point I want to get to at the end of all of that is that I want to just draw your attention to these arrows on this diagram from the start. These are the enormous natural flows before humans started messing about with the flows. So the numbers of our unnatural flows, if you like, are relatively tiny compared to these huge natural flows. But it means that if we change one of these huge natural flows by a tiny amount, it actually adds up to quite a big number. So as we're thinking about how carbon flows in and out of the ocean and in and out of life, um, everything we do is on top of this big natural pattern of these enormous flows that are constantly coming and going all the time. Like you breathing out, you know, you might grow something in your garden, eat your homegrown tomato, and then breathe it back out. And you've managed that process all by yourself. That's all in these gigantic natural flows. Um now the important thing, it's very easy to get deep into the carbon budget and think that the thing that matters about carbon is all where it the accounting, right, where the carbon is. But of course, carbon is doing far, far more than just existing as a pool of carbon. Because of those structural properties, because of the types of molecules that form the type of organic carbon you get that then don't turn into anything else, that they don't degrade very quickly. Carbon's actually really important in making structures, right? Gives the world shape. Uh, you know, trees and forests and grasses and things, they all have shape because carbon gives them shape. So it's um, and of course, we get biodiversity and a huge range of other things that you know, this enormous richness of planet Earth comes from carbon being in these forms. So it's not just that they're just accounting things. But this is really important. This was a paper that was published relatively recently, and it asked how stable ecosystems are. And it turns out that the stability of an ecosystem and therefore a carbon pool is really related to the biodiversity in it. Now, this plot is quite complicated. What it shows as you go that way to the left from the black line, the ecosystem is getting less and less rich, it's getting less biodiverse. And as you go up the side, it's uh measures in stability. And the important thing is that you can mess around with you know, water stress and fire and nitrogen, and these they do a bit when it comes to making the ecosystem less stable. But the thing that consistently makes an ecosystem less stable is lower biodiversity. And so the reason I'm showing you this and the way they say it in the paper is that regardless of the chords of biodiversity. Loss. It is biodiversity loss that makes that ecosystem less stable and therefore makes the carbon pool less stable. So biodiversity is not a nice to have. It's not, oh, look at the fluffy tigers, don't ever think a tiger is fluffy, they're horrible, you know, vicious predators doing their bit in the ecosystem, but I don't want anyone near me, anywhere near me. Rather than seeing biodiversity as a nice to have, it's actually essential to keeping these carbon pools stable so that the world continues to function as it does. Okay, so um influence, right? Just a very quick run through the types of influence that carbon atoms have. We can see that they're distributed, they have different forms, they can be organic or inorganic, they can be at different locations, they're moving around between all these pools. Where are the major influences that they have on the rest of the system, on all the other atoms out there that aren't in that 0.02%? So, I mean, you will be familiar with this carbon and dioxide, when it's in the atmosphere, it is a greenhouse gas. And so it contributes to us collecting energy on planet Earth, to the planet accumulating energy, and that's what climate change is. I'm not going to say any more about that because I gave a whole lecture on it last year. Go and watch that if you're interested. Um, but the if you put carbon dioxide into the atmosphere, it does have this effect on energy flows out of Earth. Um, people often ask the question well, do plant if we're putting more carbon dioxide into the atmosphere, do plants grow better? And the answer is globally complicated, not really clear. Uh, and the reason for that is that if you, in a lab, put plants in the environment with more carbon dioxide, they do grow faster. But of course, in the real world, you've also made the world warmer. And those two things play off against each other. So this was an experiment published a couple of years ago, and basically what it's showing is that when carbon dioxide increases yields of these crop species, wheat and soy and rice, go up, but if you increase the temperature yields go down. So you're kind of trading off more carbon against lower temperatures, higher temperatures being a bad thing. So basically, that's a question that gets asked. The answer is it's complicated. So we're going to leave that there. And then, as I said before, carbon gives the world shape, right? The fact that there are carbon atoms making structures, making trees, making uh bodies, moving things, you know, existing as bodies, the fact that complex molecules can build all these structures gives us the shape which makes our world an interesting place. So even if carbon isn't, doesn't look like it's doing anything serious, even if it's just in the shape of an elephant, that elephant is a very important component in the world. It's changing the way the world functions around it just by existing in that shape, as well as making gigantic piles of poo. I didn't put one of those pictures here, but that big pile of poo at the start was from an elephant. Um okay, so I want to just very I mean you can tell talking about the carbon cycle could go on for a long time, right? I'm not going to subject you to that. But I did want to add a word at the end about what humans are doing to these carbon flows. So these big natural flows, the planet pre-industrial revolutions, approximately imbalanced. So we're clearly quite good at diverting carbon. And we have been for a long time. Um this was this is a this is a I don't quite know the word for it, it's not exactly a pamphlet or a book. It's definitely an angry thing, right? John Evelyn, who wrote this, who was um very uh he was a friend of the monarch, he was very concerned about how things were in society, he's a very big fan of trees, wrote this in uh the 1600s, and he if he sort of published it, but it was addressed to the king, to his majesty. And basically he was complaining about the amount of smoke in London that was coming from seacole. So sea coal had started coming from Newcastle, from the coast, that's why it was called seacole. We burnt it in London, and it produced a colossal amount of black smoke. This is way before the Industrial Revolution. And John Evelyn pretty much nailed the problem way back then. And um he said he said lots of things about it. He said, uh, our inhabitants breathe nothing but an impure and thick mist, accompanied by a folligymus and filthy vapour corrupting their lungs, disordering the habits of their bodies, um, making consumptions and rage. And then this wonderful sentence, I mean, he's very cross, this wonderful sentence here, the city of London resembles the face rather of Mount Etna, the court of Vulcan, Stromboli, or the suburbs of hell, than an assembly of rational creatures. So he wrote this about the pollution caused by sea coal. He didn't know what carbon was, but he did know that this colossal amount of smoke was not any good for anybody. Um, and actually, I mentioned this because this is a Gresham lecture. Uh, John Evelyn was writing at an at around the time when Gresham College had its own property and was established as a college, and there is a very famous poem called The Ballad of Gresham College in praise of all the wits and philosophers who meet on Wednesday. It's not Wednesday, is it? Chris Lintott was here yesterday. He was the he was he must be the wits and philosophers. Um, weekly at Gresham College. But this the this pamphlet written by John Evelyn was actually in the famous ballad of Gresham College, uh, to guess by everyone's merit. A book called Fumi Fungium Read. Its author hath a public spirit and doubtless to a subtle head. So it goes on about the sea coal smoke. Uh, and I like the last line left none at fumi fungi be scoffing who heard at church our Sunday's coughing. Um, so anyway, Gresham College itself is in the history of the early recognition that you know burning stuff's not great for everybody. So now we've got a more modern way of making it essentially the same argument. This is also from the global carbon budget, and I like these plots. It doesn't start at zero on the y-axis, uh, so bear that in mind. But what we've got here, all the ones that are going up are um where things were pre-industrial revolution. So we started off with 285 parts per million carbon dioxide in the atmosphere. We burned a load of coal, so that went into the atmosphere, we burned a load of oil, we burned some gas, we've made a lot of cement. Um, we changed our land use. So we've been putting lots and lots and lots of carbon into the atmosphere. But it didn't stay there because all of the work the Earth's carbon cycle is dynamic. So the land has taken up a lot of carbon. Um, the ocean has taken up a huge amount of carbon, which is very convenient from all of us, but it wasn't enough, and so a lot here you can see 422 parts per million in 2024 was left in the atmosphere. So we we've burned all this stuff and it has to go somewhere. It's a closed system. If it's not in coal, it must be somewhere else. So we're still burning a lot of stuff, causing ourselves problems because as that as this light blue on the right goes up, uh, all that's happening is it's making the Earth trap heat more and more and more and more. Um so we're clearly very good at putting carbon into the atmosphere. I assume you know quite a lot about carbon emissions. But the question is, can we take it out? Because that's the next question, right? We've manipulated our carbon cycle to put all this extra carbon into the atmosphere, to take it from the fossil fuel pool and push it up into the air where it's causing trouble. Can we take it out? And this goes by the name of carbon dioxide removal, if you're taking it from the atmosphere, and sometimes confused with uh CCS carbon capture and storage, carbon capture and storage happens at a chimney. It's where there's a concentrated source of carbon dioxide and you catch it there. Carbon dioxide removal is when you take the atmosphere and you say you're going to take carbon dioxide out of it. Is it gonna work? Um well, the problem we face is that things don't look very good if it doesn't work. So just recently, a couple of months ago, a set of scenarios was published for our future. And what you're looking at here are the scenarios that they're ideas for how we might, how the future might be organized. There's a very low emission one, there's a very high emission one, which is not as high as the high one in the previous thing, because you know, we have actually got a bit better, not better enough, but you know, a bit better. And the coloured um sort of bars here show as you run forward in time what the temperature change would be on the uh left hand side. So the important thing about all of this is that all of these, all of these scenarios can only happen if there's some carbon dioxide removal, not sort of natural systems just clearing up our mess, but some kind of active human-driven carbon dioxide removal. And so that looks alright, except that this is something we can't do. We haven't got any demonstrated ways of doing it that will scale up. Anything that does scale up is probably expensive and intrusive and difficult, and it really is gonna cause a lot of legal arguments. Um, but people are putting a lot of work into thinking up different ways that we could manipulate the Earth's system to take some of this carbon out of the atmosphere. Now, this next diagram I'm gonna show you is terrifying, and I'm only showing it to you to give you a sense of how what's out there. Uh oh no, this is so this is the problem. This is the unmixing. Uh, so it's basically a needle in a haystack. The reason that this is hard is because to mix carbon dioxide into the atmosphere is really, really easy. To take it out again, you've got to find it, you've got to separate it out of all the other molecules that are in there and then put it somewhere else durably, that means for a long period of time, and in addition to what would have happened naturally. So you've got to basically extract a lot of needles from a massive haystack, and by the way, the more dilute it is, and you know, we're adding a lot to the atmosphere, but it's still quite dilute, the more energy it will take. So there's an energy cost to doing this, as well as the disruption, the ecosystem disruption, all of that other stuff. Um, this is the diagram with all the methods, and these are there is research being done into whether or not they would work. The problem at the moment is that often we think of carbon dioxide removal as the thing that might let us continue to emit carbon. That it's a sort of trade-off that, oh, if we just sort out the carbon removal, we can kind of buy our way to emitting a bit more. Maybe that's okay. And that fundamentally is the wrong way to think about it, I think. And it's for this reason, um, and this is an idea that was sort of made, you know, was made visible to the world in this uh nature article here, and it uses the analogy of a time machine. So you might remember back at the start, I said that every year we emit 40 gigatons of carbon dioxide into the atmosphere. So if you go forward a year, you emit 40 gigatons. And so if you have some carbon dioxide removal method, one of those things on the previous slides, what you need to do is you'll take some carbon out of the atmosphere and it kind of takes you back in time a little bit. So you have your one year of emitting all the carbon dioxide, and then you're going to do something which might pull you back in time a little bit, so it'll cancel out some of those emissions. So the question is that if we do some of the things that we could do, how much do they take you back in time every year? And um so there's our 40 billion forward. So let's look at something that um made a lot of press. There was a proposed, I actually don't know what's happened to them now, but a uh proposed um prototype plant to do direct air capture, which means to take carbon dioxide out of the air in America. It was funded under the Biden administration, and they said it's gonna it's gonna remove from the atmosphere a million tons of carbon dioxide every year. That sounds good. So let's remove a million tons of the carbon dioxide every year and see how far back it takes us in our time machine. And the answer is 13 minutes. So let's try something else, right? People like trees. Let's plant some more trees. What if everybody on earth planted a tree? Uh now they all trees have to stay alive and not burn down in any wildfires and all stay healthy and all that sort of not get chopped around to do other things, all that kind of thing. But let's say we planted 8 billion trees, one for every person on earth, and they all grew to maturity. That's a lot of trees, that's got to do some good. How far back does that take us? And the answer is 43 hours. And just imagine how colossally invasive it would be to plant 8 billion trees, as well as getting the right tree, keeping them all alive, not having them burned down in wildfires, not disrupting all the ecosystems you're planting them in and all of that kind of stuff. So basically, what this says is that the problem is not these numbers down the bottom, the problem is that number up the top. The problem is the 40 billion. But if in 20 years' time we have reduced our emissions to 4 billion, then instead of 43 hours, it would be 430 hours, right? So basically, at the moment, the big problem facing us is to reduce our emissions, to stop taking carbon from all these other pools and pushing it into the atmosphere. We are not going to get out of that problem by taking carbon out of the atmosphere and putting it back somewhere else. We just can't do it at scale, we can't sort out the legal things, we're not even sure which ones of those actually work in the big picture. And so trying to think of it as an offset isn't really the answer. We have to reduce, and if by the time we've made massive emissions, we have done enough science to show that some of those methods work, then we might be in a position to make that trade-off. But it doesn't make sense right now. So we are very good at manipulating the carbon cycle to send carbon one way, and at the moment we are very bad at doing the opposite. Okay, so we've had a gallop through the carbon cycle. You can feel, I'm sure, that there is much, much more in that topic. But hopefully you've got a picture of what's going on, a slightly different way of looking at the world. Hopefully, you're going to walk out into the world, and every time you eat something, or every time you grow something, or every time you pick up a cotton t-shirt, you will think of the carbon in it and think of where it's come from, where else in the Earth's system it's come from, and perhaps where it's going. So, um, as a quick sort of summary through here, carbon atoms are a tiny fraction of the weight of Earth, but they are disproportionately important. We are dependent on carbon, not just because of what it's of us being largely made of it, but because of what it makes possible in the world and the richness that it makes possible in the world. Um, there are lots of carbon reservoirs, and none of them really are just sitting there doing anything. They're all doing other stuff. Carbon is a dynamic thing, it's flowing around these big reservoirs all by itself quite naturally. Um so there are these huge carbon fluxes already. And even if we're making tiny changes, we if we just change one of those big natural flows by a bit, we can unbalance it without actually emitting carbon. That's where the land use thing comes from, right? So carbon actually is not just about our emissions, it's about how we're changing our biodiversity and changing our soils and changing the environment that they're in to move carbon around the system. Um, and so our challenge is to, this is a part of the bigger question, which is I think we need to understand how our planetary system works and then learn to work with it rather than against it. And that is absolutely, definitely true for carbon. So, carbon-based carbon-based life forms, thank you for joining me this evening, and thank you, thank you. Carry on as the world, enjoy your carbon, and you can make it a lot of the same.