The Carbon Cycle Behind Net Zero

(whoosh)- So I'm Myles Allen, the Frank Jackson Professor of the Environment at Gresham College, and today we're talking about the carbon cycle behind net zero. So understanding where the carbon dioxide that is causing the problem that we've been talking about in the previous three lectures, where it all comes from, where eventually it will go, and what is going to happen next. So you'll notice, we've got a new toy with us today, which is its first starting, so we hope it's going to work. But I'm going to start off talking about this curve. It's high time in any series of lectures on climate and climate change, we have to look at, at some point at the Keeling Curve. Charles David Keeling started measuring, back in the late 1950s, he started measuring the concentration of carbon dioxide in the atmosphere on Mauna Loa in Hawaii, sort of well away from any local sources of pollution. And he noticed immediately that it was going up. So the first thing you notice about this curve is clearly that it's going up. The second thing you probably notice, regardless of where you are in the room, is that it's curving upwards. As our rates of emissions have been going up throughout this period, the rate at which carbon dioxide has been accumulating in the atmosphere is also going up. And the third thing you notice, perhaps, is this sort of zigzag, which is part of this, which is every year, carbon dioxide goes up rapidly and then goes down again and up again. And each year it goes up a little bit further. And in fact, the size of this zigzag is comparable to about five years or so of the overall trend. And this is the first hint you get that putting carbon dioxide in the atmosphere, it's not like any other pollutant. Well, it's not like most pollutants, which just come from human activity. When we're putting carbon dioxide in the atmosphere, we're adding something to what's already complicated, but also very predictable because these wiggles are much the same size every year, natural carbon cycle. And if we look at this animation from produced by NASA of carbon dioxide in the course of a year, transparent air is the concentrations at the beginning of the year. And notice how we see this pulsing of carbon dioxide coming out of the tropical rainforests. You're probably used to thinking of tropical rainforests as the lungs of the planet absorbing carbon dioxide. But they also, during the course of a day, release a lot of carbon dioxide. That's why you're seeing this orange air is high carbon dioxide air. Now look what's happening now, this is, look at the time of year, we've got to, it's November. So all the Northern Hemisphere forests are dying off and releasing loads of carbon dioxide into the atmosphere. So average concentrations in the atmosphere are rushing upwards in the space of a couple of months by more than about five years worth of human emissions. And you can see, it's accumulating in the Northern Hemisphere. This orange is higher carbon dioxide air. This is sort of getting into February now. But watch carefully what happens now, as surely as spring follows winter, watch the Northern Hemisphere carbon dioxide concentrations as we come through April. And just as the, as you watch carefully what's going on, we come into May and all the plant life in the Northern Hemisphere starts to grow again. And suddenly, it mops up in a very short space of time, mops up very rapidly that excess carbon dioxide in the Northern Hemisphere as the trees regrow and reabsorb it into the biosphere, the plant life of this planet. And then we're left come the same time the following year with just wisps of carbon dioxide in the tropics. But of course, you will notice, there's more orange there than there was to start with. That's just one year. And that's the accumulation from human emissions during that, from emissions due to human activity just during that period. So you can see how there is this natural carbon cycle we have to understand in order to understand what human activities are doing to carbon dioxide in the atmosphere. But it's also very predictable because it's so tied to the seasonal cycle to the growth and decay of forests. But it's amazing. It's amazing the amount of carbon dioxide that's passed every month between the surface and the atmosphere. And this also raises some very important practical questions about how we deal with carbon dioxide that we'll come to later on in this lecture. Particularly relating to trying to use trees to absorb the carbon dioxide that we're releasing into the atmosphere when trees are doing all this anyway. It makes it even more difficult to manipulate the biosphere to help us out with dealing with what we're doing to the rest of the climate of the carbon-cycle system. Just to emphasize how predictable this is, if we look back over the past, so Charles David Keeling's observations of the blue line here, which was started in the 1950s. If we want to look back further than that, we have to look at bubbles trapped in ice cores which preserve ancient air and allow us to see the concentration of carbon dioxide back over time. And if we go back to the year zero, so 2000 years ago, we can see that concentrations were very constant. I'm not quite sure what happened here and, but it may have been sort of specific to Antarctica. So you can see that and notice the sort of truncation of time scales here. But you can see they were very constant till we invented the steam engine and started burning coal in a serious way. And then immediately, concentrations started going up. If you look even further back in time, these are logarithmic scales. So as you move to the left on this, you are really going back in time scales as well as time. And you can see that it's really, you have to go back to a couple of million years before you see carbon dioxide concentrations as high as they are now. And you'd have to go back over 20 million years to see carbon dioxide concentrations as high as where they're going if we carry on emitting at the rate we're doing over the course of this century. So that's what carbon dioxide has done. And you know, the obvious next question is also why is it going up so fast at the moment? And for most people these days, the answer's pretty obvious. But it's worth digressing briefly on, you know, how we're confident about cause and effect here. Because there is a persistent myth out there that oh, scientists don't really know whether the carbon dioxide is going up because of fossil fuel burning. It might be being released by the oceans, for example. And the origin of this myth is this relationship you see between carbon dioxide concentrations and temperature over the past 800,000 years through the cycle of the Ice Ages. And for a while, people thought they could detect that carbon dioxide was falling shortly after temperatures started falling and rising shortly after temperatures started rising. In fact, the modern assessment is you can't really tell which is leading which, there's not, there's too much noise in the data. But this gave rise to the idea, oh, it's temperature's driving carbon dioxide, not the other way around. In fact, what's going on here is that as temperatures rise, the oceans warm, they release carbon to the atmosphere and that reinforces the rising temperature. So it is a positive feedback happening, but we wouldn't actually expect on these time scales to be able to detect any delay. What's going on at the moment, of course, that's what's happened over the past, that's where we are now and that's what's happened over the past century. That's a very, very different process. And we know it's a different process. We know it's being driven by something entirely different. Thanks to the work actually of Ralph Keeling, who was actually, who is Charles David Keeling's son, who took over the job of monitoring carbon dioxide in Hawaii and actually added a twist of his own. He started measuring oxygen concentrations in the atmosphere. And this is why everybody really panics because they think, oh, we're going to, you know, we're running out of oxygen, we're all going to die. Don't panic. The changes he was measuring. This is a tremendously impressive measurement, these blue line here, which is a measurement of the falling concentration of oxygen in the atmosphere. And you should expect he was, he expected to find this because when you make a carbon dioxide molecule, of course, by burning carbon, you need a molecule of oxygen. C plus O2 two makes CO2. So you should expect that oxygen's got to come from somewhere. It comes from the atmosphere. And, but the difficulty, the reason this is a such an impressive observation is that there wasn't that much carbon dioxide there to start with. So you could detect it going up, it's gone up by 30, 40% or so in the course of David Keeling's record. But oxygen, over 20% of the atmosphere is oxygen. So you are detecting a very, very small reduction in the concentration of oxygen, but it's exactly the amount you'd expect if the carbon dioxide is coming from burning something. And we know what we're burning because of this curve at the bottom here, which shows the isotopic signature, the fraction of Carbon-13, which is a radioactive form of carbon that we see in the atmosphere. And that fraction is going down. And the reason we understand it is going down is because fossil carbon has sat underground for millions of years, has very little radioactivity left in it, whereas the natural carbon in the, it's all natural carbon, but the carbon dioxide that occurs or that occurred already in the atmosphere is being irradiated all the time from the sun. So it's got a certain amount of radioactivity, of radioactive Carbon-13 in it. So as we add fossil carbon, which has no radioactive Carbon-13 in it to the atmosphere, we see this Carbon-13 fraction going down. So it's all kind of belt-and-braces stuff because it was pretty obvious to start with what was driving it. But this is just making it very clear. There's lots of lines of evidence. We know why the carbon dioxide's going up. It's going up because we're burning fossil fuels. Of course, it's not just confirming that. What Ralph's observations in particular did was they allowed us, they gave us an extra fix on how much of the increase in carbon dioxide was due to burning fossil fuels and how much was due to burning forests or land-use change or cutting down trees because of course, a big chunk of what we're putting into the atmosphere does come from land-use change. So this here from the Global Carbon Project, shows you how different sources of carbon have contributed to the increase we've seen in the atmosphere. And you can see, the usual suspects, coal, oil, gas, others that's primarily burning limestone or heating limestone to make lime for cement that releases carbon dioxide. So that's what that gray wedge is. It's mostly cement production, but this beige area here is emissions due to changing land use. So cutting down trees to clear land for agriculture or changing agricultural practices, releasing carbon from soil. If you over-cultivate a region, degrade the soil, that carbon ends up in the atmosphere. This of course is coming from by recent, it would've been taken out of the atmosphere fairly recently, this carbon. So that has a lower, that has a higher Carbon-13 signature. So what Ralph's observations and what the observations of Carbon-13 help us to see is to confirm exactly how much of the carbon we're putting in the atmosphere is coming from fossil fuels and how much is coming from changing land-use practices. And that's the ratio, it's overwhelmingly fossil these days. Of course, what we really need to think about, because we'll emphasize a lot in these lectures, how car carbon dioxide has a cumulative effect on climate. What matters is the total amount of carbon you dump in the atmosphere. So this is just taking these data, okay, so this is emissions every year and just adding them up over time to say where have the emissions come from? And we're not sort of differentiating, we're just lumping all the fossils together. And the land use, what's interesting here, notice that up until 1960 or so, cumulative land-use change emissions were bigger than fossil fuel emissions. So land use was the big driver up until the mid-20th century. And it's only quite recently that fossil fuels have taken over and of course now dominating the picture. It's also, not only do we know where this carbon dioxide has come from, we also interestingly, we know exactly because the fossil fuel industry keeps good records. We know who owned it. And that wedge in there is just, and for example, carbon that is now in the atmosphere or has been released to the atmosphere that was once owned by a product sold by 90 odd companies in the world. So we know where this carbon's come from. Quite precisely, which is important for what we do about it, and we know where it's going. So if you look at this figure, this shows you the carbon that's accumulated in the atmosphere. That's just the Keeling Curve again, it's exactly the same shape as the curve I showed you earlier. It's now in units of tons of CO2 here, but that's just the same as parts per million. There's a simple conversion there. And then the green wedge is the carbon that's accumulated into the land that's been taken up by the land. And the blue is carbon. That's this extra carbon that we've put into the atmosphere that's been taken up by the ocean. And if you just, if I flick between this slide and this one, you can see that the dotted line here is the total amount we've released over the past couple of centuries and these three wedges pretty much exactly add up to the dashed line. And that small discrepancy is a lot of people still working on it. This is what the Global Carbon Projects and Corinne Le Quere and Pierre Friedlingstein founded this project. And it's again, it's a remarkable achievement that we understand so well what's happened to carbon in our climate system over the past century because a lot of this is, goes back a long way and well beyond many standard records. The fact that we understand what's going on now is much less impressive than the fact we can work out what was going on back in the mid-20th century when there were all sorts of things happening. And so, and, they're working to close this gap even further. In fact, Pierre was complaining that I still show this slide because in fact, it's a bit better now so that annoying discrepancy is getting smaller. So I should stress that point 'cause anyway, but it's a nice, tidy slide. So the important, so what we want to talk about now is this carbon that's not gone into the atmosphere, but has gone into the land or the ocean. The land sink and the ocean sink because they're, it's a source of much unnecessary confusion. And for example, here's one source of confusion that this extra carbon going into the land and the oceans gives rise to. I've shown you this slide before, I think in the opening lecture and it goes back to climate policy before we realized the need for net zero. And I'm just picking it up because it is representative of the way people thought about the problem that we would need, they looked at the emissions in 1990, they realized that the oceans were taking up about 50%, the oceans and the land biosphere between them were taking up about 50% of the carbon we were putting into the atmosphere in the 1990s. And so it seemed perfectly logical to say, well, if we reduce our emissions by 50%, then concentrations in the atmosphere will stop rising in the oceans. And so, back in 1990, emissions were, say maybe 30 billion tons of carbon dioxide per year. 15 billion tons were appearing in the atmosphere every year. The other 15 billion tons were being taken up by the oceans and the land biosphere. And so, the sort of logic went, okay, so if we reduce our emissions by 50%, Mother Nature will take care of it. Concentrations in the atmosphere will stop rising and we're good. And you can see this was the scenario. They had a bit of an increase here, but basically, they're talking about the target of climate policy being something like a 50% reduction. As I say, that was the goal before net zero. And it sounds like quite a reasonable argument. I mean, nature is taking up 15 billion tons of carbon dioxide per year. This was a situation in the 1990s. So provided we get our emissions below 15 billion tons, we'll be good. Nature will sort of take care of any further emissions. And understanding why this logic is wrong is one of the main points, probably the main point of this lecture. So this is the point where we need to go back to our climate models. But now, the Gresham Carbon-Cycle Model, and if you're thinking this looks a little bit familiar, yes, but there are differences. So do spot the differences here between this model and the model we were using to understand the flow of energy through the climate system. We're now going to introduce a very similar model to understand the flow of carbon through the climate system. So the first important difference is the fluid here is blue, not red. That's so because it represents carbon, and not thermal energy. But there are some other really important differences between this. I'm so tempted to ask the audience what's more. But let me just set this thing going so you can sort of see it at work. And so, we got a low level of emissions being pumped in at the bottom of the atmosphere, okay? So you can see as a result, concentrations are rising in the atmosphere, but you may notice straight away only half of what we're putting in the atmosphere is staying there. Half of it's going over here, but what's going to happen if I stop?(machine whirs) Well, it didn't take care of it or if I, so, and let me just do a different experiment. And I'll start it going again. So here we go. So, we're emitting into the atmosphere, only half of what we emit is staying in the atmosphere, but I'll just(machine whirs) halve the speed to see what happens.(machine whirs) And it slows down(machine whirs) only slightly because it won't receive that. Okay, sorry about this.(machine whirs) And. Sorry, I hadn't rehearsed that one. It didn't go quite as planned. Okay. Because there's a minimum speed and I was trying to drop below it. Anyway. And so in order to, so at this point, I'm going to do something that'd be great if we could do it in the real system, which is to pump it all out again just to sort of reset, reset the system.(machine whirs) Yeah. Wouldn't it be nice if we could do that for the atmosphere as well? We can, but it's a little bit harder than just typing a command.(machine whirs) And I'll just pull this down(machine whirs) so we can go back to where we started(machine whirs) and I'll talk you through the meaning of these different parts of(machine whirs) the carbon cycle. Okay, so It's always a bit more tense using this component, this demonstration than the other one because there's a crucial element missing here or I mean, it's accurate that it's missing, it's also missing in the real world. Remember the other Gresham climate model we had, which was the model of the flow of energy through the climate system. It had an outlet over here, energy was escaping back out in space. If you raised the level in the, raised the temperature of the Earth, it got rid of energy back out into space faster and it stopped rising. Well, if I do that here, there's no outlet, so it doesn't stop. That's the point. Carbon dioxide is completely stable, it doesn't go anywhere. If you put carbon dioxide into the atmosphere, into the active climate system, it stays there. It stays there until it's reformed into rock, which as you might imagine, takes a very, very long time. So putting blue fluid into this system is an irreversible process. Putting carbon dioxide into our atmosphere into the active carbon cycle is also an irreversible process. I can reverse it here because I can change the direction, but it's something I have to actively do. Whereas in the other model, Mother Nature would sort of get rid of a certain amount, would just get rid of energy faster for us. So that's an important first distinction. Let me tell you what the different things mean here. So this over here, this is the reservoir down here is labeled the geosphere, that's carbon underground, carbon in rocks, carbon in sediments, carbon of course, in fossil fuels. Here is carbon dioxide in the atmosphere. Over here, is carbon dioxide in the near-surface oceans and the biosphere, mostly the biosphere. And over here, is carbon dioxide in the deep oceans and the amount of carbon dioxide here we'll talk about in a minute. But again, if I just rerun the experiment, remembering, I hope to get the changing direction right, here we are. So we're pumping carbon dioxide into our atmosphere and you'll notice that concentrations are rising, they're only rising at half the speed that you would expect them to, they're only rising at half the speed as you expect 'em to rise if all of what you were pumping in was staying in the atmosphere. But if I were to stop emissions, They don't, it's not as if I was to halve the rate of emissions, it would just go up at half the speed. If I was to double the rate of emissions, it goes up at double the speed. Okay, in fact, let's do that and push it up to a higher level. But I'll do this carefully so I don't actually make a huge mess. So this is, we've cranked up our emissions now, okay? And you can see, it's a steady rate of emissions. Concentrations are rising and watch carefully now what happens. I'm going to have to time this right so that I make sure that I don't let it all overflow, but this is just, imagine constant emissions for a set period of time and then somehow, I just switch off the emissions, okay? Imagine being able to do that in the real world. I'll do that here.(machine whirs) Now look at what happens, okay? There's a rapid fall over a short period of time and then it stops falling or does it? I'll put my finger where it is. Okay. Notice it's still gradually falling, okay. Very slowly because through exactly an analogous, through exactly the same process actually, that we were talking about, about temperature being mixed down into the deep ocean, we're seeing carbon being slowly mixed down into the deep ocean. So if we stop emissions, first of all, we get a rapid equilibration between the atmosphere and the biosphere and near-surface oceans. And then we see a very gradual decline as the deep oceans absorb more carbon from the surface. But it's not going to go all the way back down to pre-industrial. So pre-industrial was down here, you can probably see by eye, we'll leave it at it and we'll see where it gets to. But you can see by eye where it's what's going to happen, we're going to end up with permanently elevated carbon dioxide concentrations in the atmosphere. So just remind me what we hear. This is part number one, additional carbon in the atmosphere and the level indicates the concentration in the atmosphere. Number two is additional carbon in the biosphere and the near-surface ocean. And the level indicates the concentration of carbon dioxide in the atmosphere that would be in equilibrium with that amount of carbon in the biosphere and near-surface ocean. As you think the concentration of carbon dioxide in a plant is a bit of a sort of odd, so we can't, we don't, we can't use that, but that's what I mean by the level in that. And finally, the additional carbon in the deep ocean. And likewise, the level indicates that sort of equivalent atmospheric CO2 concentration. So one question you might ask, just looking at this, if you're sort of a skeptical bent is why's he made the deep ocean so small, because you probably know the deep oceans, we emphasized that in the last lecture how enormous the deep oceans are. And yet, this pipe is, its area's what matters, of course. And the area of that pipe is double the combined area of those two pipes, but only double. See, I think why is it not massively larger? In fact, naturally, there are 40-odd trillion tons of natural carbon in the oceans, that's equivalent to 148 trillion tons of CO2, many, many times what's in the atmosphere or even what's in total fossil-fuel reserves. So why can't we rely on the deep oceans to just dilute away the carbon we put into the system indefinitely? And that's an important question because for many, many years people thought we could. Svante Arrhenius wasn't really worried about carbon dioxide concentrations going up very much or he reckoned it would take thousands of years, although, so therefore his theory about CO2 infecting climate was for him a completely academic idea because it wasn't, he didn't expect it to happen in any foreseeable timeframe because he knew that there was this amount and for many years, that was the view. And it wasn't until Roger Revelle came along in the 1950s to point out a very important aspect of ocean chemistry that was also going on as we raised carbon dioxide concentrations in the atmosphere that was likely to have an impact on how much carbon the oceans could absorb. And that's the fact that as we raise carbon dioxide concentrations in the atmosphere, we're also acidifying the oceans or perhaps more accurately, making the oceans slightly less alkaline. Dunno if you knew that the oceans are not actually neutral, they're slightly alkaline and as we add carbon dioxide to the system, we're making them slightly less alkaline. But fortunately for us, the oceans act as a giant buffer solution. So a bit like the antacid drink, this chap who's clearly had a bit of a heavy night is taking, that neutralizes a buffer solution, neutralizes the addition of acid or alkaline and the oceans act as a giant buffer solution keeping ocean pH, the acidity of the ocean's relatively constant. Okay, it is changing, but it's not changing nearly as much as it would do if the oceans didn't have this buffering capability. And that's a very good thing because if it wasn't for the buffering capabilities of the oceans, life could never have evolved on this planet because we need a very constant pH environment. Well, the microbes needed a very constant pH environment in order to evolve in the first place. So it's a great thing the ocean does this, but it does it at a cost in that the oceans that limits the amount of carbon dioxide the oceans can absorb'cause they're having to work so hard to buffer the change in pH that occurs as a result of dissolving carbon dioxide. So that means that actually when you take this into account, oceans have a limited ability to take up carbon from the atmosphere. So if we look at what we expect to see, do you remember when I switched off emissions? So this is a simulation, lots of simulations, in fact. The gray band is sort of the range of uncertainty from lots and lots of different models. Simulation of what happens when you put a whole lot of carbon dioxide into the atmosphere and then just see what happens to concentrations after that. And just like our Gresham Carbon-Cycle Model, it fell very, falls very rapidly to start with and then it starts to fall much more slowly. And as you can see, it's still falling in the atmosphere but very, very slowly indeed. Okay? And in fact, that's after a hundred years. If we look over the first thousand years and we add that sort of this is the first 100 years, I've just added it on here, you can see it goes down very rapidly to start with and then very, very slowly indeed. And in fact, we can even look over even longer time scales. And this was work done largely by David Archer in the 2000s about what, he asked the question of what actually happens to CO2 that we put into the atmosphere. And to start with, it goes down through reactions with calcium carbonates. And that's in David Archer's terms, pretty quick. He's a geologist. So 2000 years is lightening quick, But then he, beyond that, the only way of getting rid of carbon dioxide is reaction with igneous rocks. In other words, basically turning carbon dioxide back into the Earth's crust and that takes a long time, millions and millions of years. So that's the origin of this completely irreversible increase or irreversible over any imaginable time scale in carbon dioxide concentrations that results from its interaction with the geosphere. So that was sort of the first hint I guess, David Archer's work and others before him, like Siegenthaler and Oeschger in the late 1980s. They did point out that carbon dioxide would have an irreversible impact on climate and that therefore, there must have been a limit to how much we could put into the atmosphere. But it was, they were very much talking on geological time scales. And in the 2000s, this again, was seen as a bit of a sort of theoretical issue, but what was going to happen on much more sort of policy-relevant time scale, say the next sort of few decades to couple of centuries or so. And this shows you what this kind of system would do if we were to put carbon into it for a set period of time and then switch it off again, okay? And remember what it did, it fell rapidly to start with and then it levels off and it's going to carry on going down slowly, but never go all the way back down to zero for a very, very long time. So it's a, and that's the concentration of carbon dioxide in the atmosphere. And if we express this as the impact on the energy imbalance due to the excess carbon dioxide in the atmosphere, it's pretty much the same shape. There's a slight change in shape because of what you may remember from the second lecture. The fact that every doubling of carbon dioxide has the same impact as the last on the energy budget. So there's a slight change in shape between the impact on the energy budget and the impact on carbon uptake concentrations. But that's a detail for us now. So we can understand this with an expression, with an equation. The change in the height of the fluid in this column is equal to the amount of fluid I'm pumping in over a period of time minus a small term which corresponds to the fluid trickling out into the deep oceans. So that's a sensible expression for the behavior of the fluid in these pipes. And we can rearrange that and say emissions in is just the change plus this trickle out into the deep ocean. Why is this relevant? We can write down exactly the same expression for the carbon cycle. Emissions in is just the increase in the energy imbalance over a period plus the average, some constant times the average energy imbalance over that period. You might think why is he bamboozling us with this math? So at this point, well, I just want to remind you, if you're looking at that expression, does it remind you of anything? Well, I'm hoping it does. So basically, if you look, one of the things you'll notice about this is that if we shut that off, if I set this to zero, then this change, this has to be zero, which means that this change has to be, remember in the last lecture here, this change has to be negative because that is positive. They have to add up to zero so we get a decline, okay? A fractional decline once this initial adjustment is over of interestingly about 0.3% per year, that's a number you may remember from the ocean's lecture. And you've seen an expression rather like this before. This is what we said temperatures did over decade to century time scales. Change in temperature is proportional to the change in energy imbalance plus the term which is proportional to the average energy imbalance over a period. So when we put these things together, temperature response to forcing looks like that and the forcing response to emissions looks like that. You'll notice these expressions look remarkably similar. Indeed, if these two constants are the same and we'll talk in the next lecture about whether they are, then we end up that the warming cause by carbon dioxide is very simply proportional to the total amount of carbon dioxide we put into the atmosphere over any multi-decade period. Now, I had to kind of speed through that and we're going to go back over that in the next lecture. So, if this was sort of slightly bamboozling, just contain your excitement and we'll come back to all this in the next legislature when we bring the ocean system and the carbon-cycle system together. So are we all done with the carbon cycle and net zero? Unfortunately, there's a problem and now we're kind of done with the science now. I'm getting on to what people are doing with it and what people are thinking about. So what do people, policy makers, companies, the world at the moment, actually mean by net carbon dioxide emissions? You may think that's perfectly obvious. In fact, we thought it was perfectly obvious when we were doing this work back in the 2000s. We never thought there was really, it never occurred to me personally, to worry about what people meant by carbon dioxide emissions. I thought it was obvious. You burn something, you put it in the atmosphere, that's an emission. Okay, well for us, for carbon-cycle scientists, it means net emissions means the emissions that you cause by human activities minus what you remove from the atmosphere also as an immediate consequence of human activities. So if I actively take carbon dioxide out of the atmosphere and get rid of it, that's a removal. That's what we called a removal. And these are emissions minus removals resulting directly from ongoing human activities. That's what we meant by an emission or net emissions. For emissions accountants, There isn't any accountants in the room, but we're now going to get a bit heavy on accountants here . For emissions accountants, they also include the uptake of carbon dioxide on so-called managed land. That's any land that somebody declares they own and they manage that results from the impact of past emissions. So if I take a, remember that amazing animation of the world and how the trees were absorbing all that carbon in Northern Hemisphere, now they're absorbing carbon, remember, there's wedges as well. They're absorbing carbon at a faster rate than they would be if we hadn't raised carbon dioxide in the atmosphere. In fact, we've got plenty of evidence of this. Let me just go onto the evidence. Here's some pictures of a undisturbed region of African savannah, 1925, 1993, 2011. Look how the vegetation is changing and we understand how, why the vegetation's changing. It's changing because there's more carbon dioxide in the atmosphere and trees like carbon dioxide, they're growing faster in certain conditions. If you're not limited by water, if you're not limited by lots of other things, trees will grow faster, plants will grow faster if you have more carbon dioxide available. So they're doing this, I would say perfectly naturally. And us naive scientists back in the 2000s regarded this as natural. But of course, it's not exactly natural because it's only happening because we humans have put more carbon dioxide into the atmosphere and this is why we were really naive, of course, it's also happening on somebody's land and these days, carbon has a value and if you can show that carbon is being absorbed on your land, you can sell that carbon on the offset markets for a healthy sum and suddenly, all land in the planet is managed apart possibly from Greenland and Antarctica that nobody really cares about'cause it doesn't really absorb carbon in the first place. So we have a situation that everybody is suddenly turning around and saying, oh great, there's all this carbon's being absorbed by the oceans and by the land biosphere. So that's a negative emission that we're going to take credit for that and sell it to somebody to offset their flight. You see where this is a problem? Okay, who owns these helpful forests? Well, Canada for example, Canada's forests are absorbing over 900 million tons of carbon dioxide every year, additional carbon dioxide because of all the carbon dioxide we've put into the atmosphere over the past century. Canada's emissions are about 730 million tons. So those lovely Canadians are not causing climate change at all or are they? Okay, now we can be confident. Canadians are really nice people. Any Canadians in the room? There's no way they would take advantage of such an obvious colossal accounting loophole to declare themselves net zero prematurely. Some other countries have large forests and are taking a rather different line in UNFCCC negotiations, which is why we have a problem here. Are these really equivalent activities, this, now we're back to Canada now. I'm not going to say anything controversial about other countries. Canadian forests absorbing CO2, Canadian tar sands releasing CO2. I mean, that's not net zero. Those activities do not net out. So this challenge we have is that the carbon accounting systems we use, the carbon accounting systems that will be used if you are tempted to tick the box to say, yes, I'd like to offset my flight. Next time you take a flight, rest assured the person behind, the company behind the box tick will be using a carbon accounting system that is fully endorsed by UK government, UNFCCC, everybody else that allows them to take credit for this carbon that's being taken up by managed land that is happening entirely natural. Well, not entirely naturally. It's happening because we've put carbon dioxide into the system, so trees are growing faster. And of course, if we look at, this is the part that's being taken up. There's all this in unmanaged land and there's all this going into the oceans that so far, nobody's taking credit for, but it's only a matter of time before everybody wants to start taking credit for carbon uptake on managed oceans. Watch this space, it's bound to happen and 30% of the world's oceans are in somebody's economic exclusion zone. So that country will say, right, we want to take credit for this sink. And if that's then used offset, ongoing fossil-fuel emissions, we end up with net zero, not really meaning what we meant by it at all. That's the natural uptake, quotes natural, because obviously, it's not entirely natural uptake due to past CO2 emissions. And of course, that's what would happen if we allowed people to carry on burning fossil fuels saying it's good as long as you've offset it with this stuff that's being taken up by the natural sinks because then you wouldn't ever get any decline at all because you'd just be allowing the fossil fuels to be carried on being burnt to balance it. So that's where we're at. We need to get back, I think to what we originally meant by net CO2 emissions, i.e. the net consequences of our actual activities, what we did last year, not just the consequences of what we've done over the past 150 years. Recent paper by Felipe Grassi drawing attention to this. They've done a great job of documenting this problem, but they say don't worry about it because it's all going to go away soon anyway,'cause as the world warms, trees won't be able to absorb so much carbon anyway. So the loophole will close. I don't like this logic because it's a little bit like saying we've got this great loophole that we can take advantage of now, but the next generation won't be able to. So we're good. So let's exploit it as hard as we can. I think that's the certain sort of problem. It's problematic, it seems to me, that logic and plus anyway, if we count on this loophole all the time and we use all these natural uptakes to offset ongoing fossil-fuel emissions, we're not going to get to, we're not going to stabilize temperatures anyway because if all we do is make concentrations constant in the atmosphere, temperatures will continue to rise. So that's the problem. There is a solution which is to stop pretending that carbon in the biosphere is somehow exchangeable, fungible with carbon coming out of the geosphere. Taking carbon out of the geosphere, rocks, fossil fuels, and putting it into the active carbon cycle is an irreversible process. The only way to get these concentrations back down again is to pump it back out. So if I've put it in, I've got to pump it back out, back into the geosphere taking credits for carbon that's going into other parts of the active carbon cycle doesn't cut it, but we're going to have to work on that because it is the way everybody regards carbon at the moment. And it's important to stress promoting reabsorption of carbon by the biosphere through particularly, nature-based solutions has many wonderful benefits. And so I'm not saying we shouldn't do that, but what I am saying is we shouldn't kid ourselves that that is somehow compensating for the impact of us taking carbon out of the geosphere and putting it into the active carbon cycle. So here's a question. We have an energy bill before Parliaments at the moment. The government has a golden opportunity to embrace the concept of geological net zero as proposed in Chris Skidmore's review of the UK's net-zero target that was released only a couple of months ago, in which he expressly said,"Government should recognize the importance of geological net zero." That's a balance between what's being taken out of the geosphere and what's being actively put back in the geosphere."And work to align international ambitions towards geo zero," that's what he called it, "by 2050, in line with net zero." So watch this space because our government has an opportunity to do something really historic here. We could be the first country in the world to actually declare a goal of geological net zero and that's what it'll actually take to stop global warming. So if we were the first to declare it, eventually, all countries will have to declare for geological net zero and Britain could be the first, which would be fitting 'cause after all, we were kind of the first to start the problem in the first place.(audience laughs) So that sort of concludes this lecture. Just to remind you what we've covered in this lecture, how is CO2 emissions are distributed in the atmosphere and biosphere in the oceans? It's relatively simple, relatively predictable, but important and important to understand it and how fossil the release of fossil carbon dioxide has a permanent impact. I've stressed how accounting for carbon dioxide in the biosphere is a bit of a mess, but I don't want just to take away with you the message that, oh, it's a bit of a mess and it's complicated and I don't want to think about it. There's something very simple that we do need to do, which is getting the accounting right of carbon coming in and out of the geosphere. We take carbon out of the geosphere in only four forms, coal, oil, natural gas, and limestone for cement. We have right now we put it back in only one form, which is liquid CO2. If we want to stop global warming those have to balance as I emphasized right back at the beginning of these lectures. And our government has an opportunity right now to embrace that idea and pioneer the roots towards a stable climate future. Let's see what they do. Thank you.(audience claps)- Thank you so much Professor Allen for such a fascinating lecture. I've got a question for you online and then perhaps we can go to the room. The question is, what impact do you think this rise in CO2 emissions will have on the lives of the next generation and for example, in terms of lifespan, et cetera?- Well, the important, I mean, there'll be many impacts because every ton of CO2 we put into the atmosphere drives up global temperatures. And we'll talk about the amount by which it drives it up in the next lecture. But so clearly, the next generation will be stuck with the impacts of that elevated global temperature unless and until we work out how to reverse the pump and literally take that carbon dioxide back out of the atmosphere. So that's the important thing for everybody to understand is that what we're doing now is causing irreversible harm or harm that can only be reversed with a lot of work. I mean, getting carbon dioxide back out of the atmosphere, we will talk about this in the last lecture, is hard and it can be done but bequeathing that task to the next generation or the next generation, but one is rough on them because it's a hard one.- [Audience] What extent does the carbon cycle also extend to other potent greenhouse gases like methane or other hydrocarbons which have a higher global warming potential?- Yep, so the methane cycle is totally different from the carbon cycle in that when you put methane into the climate system, it's kind of as if there's an outlet as well because methane is oxidized naturally to carbon dioxide, which has a much lower global warming potential. So it almost couldn't be more different. And that's a rather short answer to your question and there will be a whole lecture on, when we come to year three of these lectures where we talk specifically about solutions and the different aspects of solutions to the climate issue, we will devote an entire lecture to the whole, the challenge of methane and the opportunities provided by methane reductions. But the big picture, and not to discourage anybody from coming to that lecture,'cause it's a very interesting one, but the really big picture is carbon dioxide. We have to appreciate that the most we can do on methane might cut a couple of tenths of a degree of warming of mid-century temperatures. That's a few years of fossil-fueled warming at the moment. So it can help, but this is the big picture.- [Audience] Thank you for the lecture. I was in London all through last July and to me, that was the standout event of the entire, in fact, probably the entire decade, it was terrifying. I dunno if you were here, but it was very warm. It was over 40 degrees heat, very dry for a very long time. And on, I think it was the July the 19th or something that the height of the temperature, London Fire Brigade had its busiest day since The Blitz and that was when the Germans were actively dropping incendiary bombs on the capitol. So that was, nevermind the Queen dying and nevermind Liz Truss and everything, that seems to me the most terrifying. That was one of the most terrifying times to be in a big city. And to answer your question about will this government act on this bill, I'm still amazed that the whole government machinery isn't reeling from last July. Are you at all optimistic that this, forgive me, this current conservative government will look back on last July just pre-Truss and say, wait a minute, we hit 40 degrees centigrade right across Britain?- Well, there's a lot in that question. So I'll just focus in on what, you have to be an optimistic in this game. You have to be an optimist to go to work. I mean, I spent my entire life worrying about global warming. I mean, imagine what that's like. So yes, there are, the important thing is there are solutions out there and there are people who get it. So Liz Truss commissioned Chris Skidmore, one of her first actions when she came into power was to commission Chris Skidmore to undertake a review of the UK's net-zero target. And I have to confess, a lot of us working on climate change, our hearts rather sank at this point.'Cause we thought, oh, is this just going to be a sort of a route to sort of diluting the whole ambition entirely and perhaps that was her plan. If it was, then I guess it didn't work out, bit like some other plans as well. But because he actually came up with a really strong review, which essentially, he was saying that we need to go much further, much faster and crucially identifying issues like the need for geological net zero, which are not, which are nowhere in legislation at the moment, but it would be a two-line amendment for them to acknowledge, yeah, we need to get to this and it would completely transform the global outlook in the run up to COP 28. So, they've got an opportunity there to make history.- [Audience] Would you discredit the carbon credit system then?- I'm not the first to discredit the carbon credit system. There's articles appearing pretty much all the time saying all the many problems with it. I guess what I would emphasize is even if it worked perfectly, even if everything was doing what it said it was doing, unless we fix this accounting problem, which is a really fundamental one that was sort of allowing people to take credit for carbon uptake that actually is happening quasi-naturally anyway, without them doing anything, then it's never going to deliver what we meant by net zero. Now of course, what somebody means by net zero is up to them. So, I can't tell the world's governments no, you should redefine net zero. But I can say, well, this is what we meant and this is what it'll take to stop global warming, which is actually to let nature do her thing and stop dumping carbon into the atmosphere from fossil fuels and not pretend that nature's uptakes are somehow compensating for our continued fossil fuel use. That I can do, but whether the governments will listen, I don't know. So there are simple things we can do and the really simple fundamental thing is to separate geological from biological carbon. Unfortunately, very few governments seem very interested in this idea. Very few companies seem very interested in this idea because of course, if you are a company with executives flying around all over the world, the last thing you want to do is say, okay, there's no way we can get to net zero by planting trees, we're actually going to have to get rid of that CO2. It's a lot more expensive to put CO2 back underground. It's perfectly possible, but it's a lot more expensive than just planting a tree somewhere. It's certainly much more expensive than taking credit for a tree that's growing anyway. I mean, that's really cheap. Okay, so, it's a big ask of the world to fix this problem, but unless we fix it, we're not going to stop global warming.- Before we thank Professor Allen in the usual way, can I just draw your attention to the Carbon Cycle Behind, sorry, The Trillionth Ton of Carbon, Why it Matters for Carbon Change, which is on Tuesday, the 18th of April. Tuesday, the 18th of April for your diaries, please. Now can you join me in wishing professor Allen many thanks.(audience claps)