On the eve of COP28 in Dubai, is the goal of limiting global warming to 1.5°C still alive? What does this mean and is it even possible?
Given warming has reached 1.25°C, increasing at around ¼°C per decade, what happens if we miss our target? While every tenth of a degree matters, passing 1.5°C does not mean an inexorable slide into climate chaos, but every year’s delay increases the clean-up bill for future generations.
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This lecture was recorded by Myles Allen on 21 November 2023 at Barnard's Inn Hall, London.
The transcript and downloadable versions of the lecture are available from the Gresham College website:
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I wanna talk about what it actually means, um, to limit the temperature increase, uh, above, uh, to 1.5 degrees above pre-industrial levels. Um, I'll talk about some of the implications of allowing warming to go above 1.5 degrees that we considered in the, uh, IPCC reports, the intergovernmental panel on climate change reports on 1.5 degrees, uh, which really catapulted me into this, uh, debate, uh, when I, I was one of the, uh, coordinating lead authors of that report back in 2018. Um, and, you know, I'll come back to this narrative of deadlines and how deadlines are important. They're quite, they can be useful for people, uh, but they can also be quite dangerous. Um, and we need, so we need to understand what's happening and what it all means. Um, but first of all, you know, we live in interesting times. And, and thanks to John for help with getting the data read in last night for this plot, um, I gave the last Gresham lecture, the, the seventh in the series, uh, in September of this year. You probably heard September was a pretty, well, you probably noticed it was a pretty warm month in the uk. It was also a very warm month globally. Um, there it is at the end of that, um, series. It was the hottest month recorded, um, relative to the average for that month over the past, um, 150 years. And, um, but you can see, and there was a lot of talk at the time of, you know, does this mean climate has cost a threshold or something? But we have a big El Nino going on in the Pacific, uh, at the moment. And, um, the last time we had a big El Nino in the Pacific was in 2016. And you can see that we had a big spike in global temperatures back then, and that what happened in September was more or less what we'd have expected to be when the next big El Nino came along. Now, what was different was that in the last El Nino, the really high temperatures didn't happen until January, February. So we're watching what happens in this El Nino, um, and we may see higher temperatures yet. Um, but what is striking about it is that, you know, in many ways what's perhaps slightly disheartening about it is quite high predictable. What's going on is, I mean, we kind of knew the next big El Nino was gonna be if it happened, um, eight years after the last storm, which is what, what's happened, uh, was gonna be a couple of tenths of degree warmer, because that's the speed with which the world is warming at the moment. And that's exactly what we've seen. And of course, when we, uh, um, and, and, and what's happening in the world has not gone un gone unnoticed. And if we look back to 2019, I, I love this picture because it's such a inspiring picture. And I dunno if anybody, um, here in the room was out on these demonstrations in 2019, the, the, uh, the school strikes, um, where, and one of the big headlines of the school strike movement was we got 12 years left. And that came directly from the IPCC 1.5 Degrees report, which I was involved in. And I'll talk a little bit about where that narrative, you can see this, you know, the banner 12 years, um, was, was very prominent in that, uh, in, in those school strikes. And I spent a lot of time during 2019 that, that, that summer, um, going, going to the school strikes, actually, I was wearing, we, we, um, a colleague had some T-shirts printed up. I'm a climate scientist asked me a question. So we wore these T-shirts and went down to the school strikes. And one of the best interactions I had was I was, I was sitting on, on the tube on my way to the strike, uh, to the school strikes in Trafalgar Square. And a, uh, a a a sort of elderly gentleman was sort of looking at me slightly quizzically, sitting opposite. And I was wearing, I, I, I had had a jacket on. I thought it was quite hot. And I took my jacket off, and I thought, okay, here we go. What's gonna happen? And, uh, and he said, do you mind if I, I I'm gonna ask you a question in a very sort of like, and I thought, okay, here we go. But it turned out, actually he was a, um, he was, uh, heavily involved in sustainability himself, and this is what he did. And he actually asked me quite a technical question about, um, about climate change solutions. So you should never judge a book by its cover. Um, but, uh, anyway, back to, um, uh, 2019 and the 12 year, uh, deadline, uh, which was all over the place. Um, we had school strikes all over the world led by Greta Thunberg. And I love that picture of Greta Thunberg giving to death, stare to Donald Trump. Um, and you know, the origin of this was, um, just, you know, a few years earlier, we had, um, the Article two of the Paris Agreement, which introduced much to everybody's surprise, this target of holding the increase of green, the holding the, uh, increase of global mean temperature above pre-industrial to well below two degrees, and pursuing efforts to limit the warming to 1.5 degrees. That was the, the wording they agreed on in Paris. And I say, much to everybody surprised it, it definitely took me and most of the academic community by surprise.'cause up until that time, the, the target was basically two degrees. Two degrees was considered a good result. Um, and that was what they agreed back in. Well, they sort of started to agree it in Copenhagen in 2009, but then Copenhagen rather felt bits, and then they, they reconvened in Durban the following year, and they managed to agree on two degrees as the goal. And a lot of countries were very unhappy with two degrees being the goal. They, they saw, you know, that they were gonna suffer a lot from a two degree of warming, and were pursuing this, uh, and consistently pursuing an agenda, um, to make ratchet up the ambition of the UN process to 1.5 degrees. And they secured this in Paris. And, um, as I say, it caught the academic community vice price. Perhaps it, it shouldn't have done, because if we'd have been sort of more engaged with the negotiations, we'd have known what was going on. But, but we, we didn't. Um, and, uh, it also, um, you know, put us, uh, on, on our metal straight away. Um, because, uh, one of the first things that, um, happened, um, was that the, as, as a condition I believe, of agreeing to the 1.5 degree goal going into the Paris Agreement, the countries who were rather skeptical about 1.5 degrees insisted that they commission a report from the governmental pan, uh, governmental panel on climate change on 1.5 degrees to answer, to answer two questions. Um, what would, what was, what difference would it make limiting warming to 1.5 degrees versus allowing it to go to two? And was it even possible? Those are the sort of two key questions for this report. And as I say, I was one of the authors dragged in to do this, and we had to do this, the rules of the intergovernmental panel on climate change is you don't do research that the Intermental Panel on Climate Change doesn't do research. It assesses the academic literature, um, which was a challenge at the time in, in sort of early 2016 when we, it was announced they were going to require this report done in a couple of years, because there was no academic literature on 1.5 degrees. Nobody had ever bothered to consider the possibility that we might limit warming to 1.5 degrees. I mean, it makes us sound very bad and callous. But that was just what, that's what the academic community had done. And so we all worked pretty hard, actually, um, to, and it was a pretty frenetic couple of years. The Gresham College lecture that you're listening to right now is giving you knowledge and insight from one of the world's leading academic experts, making it takes a lot of time. But because we want to encourage a love of learning, we think it's well worth it. We never make you pay for lectures, although donations are needed. All we ask in return is this, send a link to this lecture to someone you think would benefit. And if you haven't already, click the follow or subscribe button from wherever you are listening right now. Now, let's get back to the lecture. Um, Oxford actually played quite a big role in this. We convened a conference. Um, it, it's so happened we had a conference booked, um, for September the following year. And we just sort of, as soon as the Paris result landed, we just, I ran into the, the, my unit director and said, look, we're just gonna rebrand the conference, um, and make it about 1.5 degrees. And it was actually, it was a very, it was a very successful conference. We got a lot of people together, and we got a lot of academic journals involved as well to help, uh, produce special issues to, to get peer reviewed, lit, uh, you know, literature, um, out there so that people can understand what the implications were of 1.5 degrees. And so anyway, we, we got the report done, um, in, in 2018. And, uh, the headline and The Guardian when it came out, we got 12 years to limit, uh, climate change catastrophe, warms un. And that was really the origin of the 12 year deadline. And the deadline narrative continues, you know, the Guardian loves it. Um, this was five, four years later. Um, world closed, irreversible climate breakdown. Uh, five years later, climate, this is this year. Um, climate crisis, carbon emissions budget is now tiny scientists say, um, the unip, the United Nations Environment Program releases an annual emission gap report where they look at how well or badly we're doing, um, at achieving, uh, climate goals. It landed this morning, and this was the cover. Um, I think it's, it's a pretty unfortunate, um, title, really, they've chosen for it this year.'cause, you know, broken record, yes, lots of weather records are being broken. That's the sort of, you know, but it's, it's, it's also the whole deadline narrative is starting to sound a bit like a broken record as well. I mean, we're keeping on saying the same thing and the same thing's happening. And, and, uh, um, I, I worry about whether this deadline narrative is really working for people. It doesn't seem to be, um, and, and is it potentially doing some harm? And I think maybe we'll come onto that in the questions. So, um, you know, how people reacted back in 2019 was very interesting. Um, we kicked off the school strikes, of course. Um, here's a tweet, uh, from a a, a millennial, um, saying, uh, uh, if you can't read it from the back, uh, uh, Tom Elliot, um, is saying, um, we're like, you know, millennials are saying, we're like, the world is gonna end in 12 years if we don't address climate change. And Alexandria Ocasio-Cortez, who's a, uh, uh, progressive Democrat, uh, in the US Congress, um, picked up on this and, you know, pointed out, you know, the UN says, we've got 12 years left to fix climate change and, and gave a link to the Guardian, um, article there. So that was the reaction. And of course, there was inevitably a backlash. Um, of course, I'm, you know, the backlash was strongest in the US because, well, everything's bigger in the us. Um, and, uh, um, so here's some energy website saying, uh, let's break down the alarmist talking points, fueling these kids' climate strikes. Um, more specifically, uh, a chap called Bjorn Lomborg, uh, who's quite prominent in these arguments popped up, um, to say, wait a minute. Um, the UN estimates, um, that climate change by the end of the century will be equivalent to an income loss of two to 4% of GDP. Um, he loves saying this and loves pointing out, well, that would only be like a couple of years growth. So we're sort of deferring the, the wealth will be at, um, in 2100 to 2102, you know, so what, um, and these are the estimates he quotes, um, for those, uh, in support of this, um, from various studies. And we'll come back to that. Um, because I think this, this whole question of, um, you know, what difference does it make allowing warming to go beyond 1.5 degrees is really the, the, the crux of this lecture. But I would like to spend a little bit time, 'cause it was my job on that 1.5 degrees report, to talk to you a little bit about what reaching 1.5 degrees actually means. Um, it, I, I should have pointed out that squiggly line that I plotted last night, but John's help, um, um, we've already had a month, well over 1.5 degrees. Uh, in fact, I believe yesterday or two days ago, there was a daily temperature, which was close up to two degrees. I mean, you know, we're getting, if you take it short enough timescale, um, we're seeing fluctuations that are well up there. But that's not what we meant, uh, or not what the Paris Agreement meant, either by temperatures reaching 1.5 degrees. Um, it's the warming, um, the, the underlying warming that we're talking about, which is shown in, in this figure by that orange line. The orange line here shows you the, um, human induced warming. We spent some time in the last lecture talking about how we estimate the human-induced warming. That's a human contribution to the warming. Um, and if we carry on that warming at the current rate, then uh, we would, uh, reach one point we estimated back in 2018 that we'd reach 1.5 degrees sometime around 2040 with a range of uncertainty, sometime between 2030 and early in the 2050s. Um, so the Guardian looked and others, of course, took that range, um, looked at, okay, we might get to 1.5 degrees as early as 2030, and that was the origin of the sort of 2030, um, uh, headline, uh, that was fed on into the 12 year narrative. Well, you can immediately see, you know, okay, um, there's, it's looking at that graph, you wouldn't say we were only 12 years away from, from, uh, climate disaster. Um, but, um, clearly the impacts are getting worse as the world warms. And we'll come back to that in a minute. But first of all, we've gotta talk a little bit about, you know, what it actually means to reach 1.5 degrees. What does it mean? What does it mean for the climate to reach 1.5 degrees?'cause when you're in a warming world, the traditional definition of climate that we used to use when I was a graduate student, um, in the, the nineties, it sort of worked back then. Um, it was, um, the, the World Meteorological Organization defines the climate as the average over a 30 year period. That's the sort of standard definition of weather, you know, stand of, of average weather. And they insist on a 30 year appearance so that you can average out all the fluctuations and get a handle on what the weather's normally like. But of course, if the world's warming, if you think about the, if the average over the most recent available 30 years is always going to be misleading, it'll always be the world's warming at 0.2 degrees per decade, then it would be 0.3 degrees two cold. Did I get that right? More or less? Um, I think so. Somebody will correct me at the end. Okay. So, um, oh, well, um, it's not warming at that over that period. It wasn't warming at quite, uh, 0.2 degrees per decade. So it's, um, uh, at the, at the rates, the world has been warming or had been warming up to, uh, 20, uh, 18, um, if you'd just taken the average over the past 30 years, um, we'd have said that the world was, you know, quarterly cooler than it actually was. And that's sort of fairly obvious from this graph. Um, but of course, this, you know, when you're talking about, um, something like temperatures reaching 1.5 degrees in the Paris Agreement, um, this all starts to get quite loaded because it's not just a, a scientific exercise to work it out. It actually has political and even legal, even financial ramifications. Lots of countries, um, argue that the trigger for what they refer to as loss and damage payments should be when temperatures cross 1.5 degrees. Other countries strongly disagree. You can imagine which countries are which. But the point is, this has become a, a legal question, um, when we reach 1.5 degrees. And so, actually, I spent a lot of my time with the inter intergovernmental panel on climate change report on, on this kind of rather ArcHa argument about what is the present day climate, do you think? What a completely stupid question. But it's not, it, it becomes quite complicated because the standard world meteorological organization definition would've always been 15 years out of date. And, um, as a physicist, uh, I was per, i, I would much prefer to say, well, the climate is the, uh, the moments of the attractor of the weather. Everybody knows that. And if you're a physicist, that statement means something. And if you're not, it probably doesn't mean a huge amount. And certainly I just go, if you're a lawyer, it means nothing at all. So, um, what I mean is, um, that if you, if you allowed the weather to evolve many, many times, uh, with the same drivers and then took, took the, uh, distribution over the possible states that you found and took the average, that would be the climate. Um, but that's too, that was regarded as too esoteric a, um, definition of present day climate for the UN purposes. So, um, we, we ended up with a definition of the temperature this year as the temperature we would find if we were able to take a 30 year average with the middle of the 30 years in this year, assuming whatever trend that's happening were to continue. Okay? So that's, that's where we landed eventually as a sort of bit of a compromise. And, and actually one of the reasons I was, I spent a lot of time worrying about this is, is illustrated by this next figure. Um, so we, we talked about these in the previous lecture. There's sort of multiple things driving climate on, um, inter-annual timescales. The trend is overwhelmingly driven by anthropogenic human induced, uh, increases in greenhouse gases. But every now and then, you can see here this blue lines, that's the contribution of natural variability to global temperatures. And those dips are associated. Those are, those are volcanoes. Um, they're the cooling effect of volcanoes. We'll, we'll come back to them in a minute, but, um, there was, during 2018, we, we had this very tight deadline for producing this, uh, IPCC report. We knew that, um, there was no slip. We, there was no opportunity of a slippage because it was sort of fitted into the whole U-N-F-C-C-C negotiation process. And all the meantime, there were rumors of a volcano rumbling on in Java, and everybody was saying, oh, it's gonna blow. It's gonna be another big one. You know, just like that one. And I was completely convinced that a matter of months before our final report landed, and by, by the way, these reports are reviewed in painstaking details. So once you've, once it's sort of rolling, you know, it, it really is set in stone several months before the final acceptance date. And so if this volcano had gone off and then cooled the whole planet down by, you know, a couple of tenths of a degree, it would've been a total nightmare if we hadn't landed the point that the present day temperature is not what we're talking about. It's this, you know, background 30 year, this background trend with the 30, 30 year average accounting for the underlying trend. So, um, that's what, um, we, we, we, we established, we managed to get that nailed down. Um, and, uh, we in fact use the apparatus that I described to you in the previous lecture of attributing temperature changes to different drivers to work out not just a trend, but how much of the trend is due to human influence. And that allows us to pull out that orange line, which is the expected response to anthropogenic, to human, uh, factors, um, which allows us to, and we with some uncertainty on that to say where warming had got to. So the formal definition of where the warming has got to is where human induced warming has got to, or where this average over a 30 year timescale has got to accounting for the human induced trend, and also removing the impact of any volcano if it had gone off. And as it happened, we were lucky and the volcano didn't go off. I was, in a sense, I was a little disappointed 'cause I was kind of, we'd done all this prep so that, so that if the volcano had gone off, um, it wouldn't have scuppered the report, but yeah. Anyway, it didn't, so, so, so it was all, you know, various colleagues thought I was being completely unnecessary pedantic throughout anyway. I, you know, they would've been grateful if it had gone off, you know, that was what I, anyway, so, so, um, so this is how we work out. Um, this is how we work out the current level of warming, and that's what we're sort of heading towards. Um, as we, um, and if you compare that to the, uh, those decal averages that I showed you earlier on, the sort of WMO definition of average is over 30 years. Um, you can see that since the 1960s or so, um, most of the warming we've seen is, is intel. Well, all of the warming you've seen really is due to human influence, which was the conclusion we talked about in the previous lecture about how it's extremely likely that, um, human influence has been the dominant cause of the observed warming since the mid 20th century. That's just to connect up this with the previous lecture. Um, and, you know, on this basis, we reached one degree in 2017 and, um, uh, on that, that rate, um, we also, um, reached, um, in, you know, on the basis of this record, we reached one degree in 20, uh, 2017. Um, also on the basis of this record, this was the record that was available to us at the time, back in, um, 2017. Um, the, the, the record was also consistent with the statement that at the time they were negotiating the Paris Agreement, the negotiators were told the world had reached 0.85 degrees of warming. So that was the, the information that was provided to the negotiators when they were, um, when, when they, when they were arguing over what they were going to agree in Paris, um, they had a very formal system called a structured Expert dialogue, where experts from the environmental panel on climate change gave presentations to governments. And this was the number that they, that they gave them. Um, and the current level of warming was deemed to be 0.85 degrees above pre-industrial levels. And that is indeed roughly the temperature of the decade or decade and a half or so prior to 2015, um, by coincidence, um, conveniently that happened to be a period where global temperatures were quite stable, that you probably can remember the Daily Telegraph would publish a headline saying Global warming stopped in 2000, you know, quite frequently. Um, they, they stopped doing that. In fact, there was a, there's a, there's a, there's an outfit called the Global Warming Policy Foundation, um, which are various, um, uh, it keeps being revealed, different conservative politicians are members of it. Um, but they, um, they, they used to have as their logo, global temperature, um, over the 20th century, over the 21st century.'cause it was impressively flat. Um, but they've, they've actually abandoned that logo and moved to a, uh, a much more neutral logo I noticed recently. Um, because obviously it's sort of, doesn't really work for them anymore. Anyway, interesting that, um, I be, I'll get some emails about that afterwards. There we are. Nevermind. Hello everybody. Um, but, uh, um, so, um, what that means, you know, with that record, at that rate, it implies we'll reach 1.5 degrees around 2040, um, however, um, and there's of course a range of uncertainty around that, but the record, of course, is uncertain. H how, you know, what has actually happened in the past. And here's a bunch of estimates, different estimates, you'll notice they are slightly different from each other, um, of global temperatures over the past century. And we haven't talked very much about observations in these lectures so far. So it's important to recognize that this, you know, I, if I show you a plot of global temperature, there's a lot, a lot has gone into it. If you stop and think about, you know, how how on earth do people know what's happened to climate over the past century? And, and the fact is that it's, it's, you know, there's a huge science in this, in, in gathering together observations from all over the world, and particularly when you go back to the 19th century to get that, that baseline that we're measuring against, um, a lot large parts of the world like the Pacific are, are badly observed. Um, because the only observing network they had back then was, um, merchant ships. Um, and the Royal Navy, in fact played a big role in this. They, they, I think it was part of ruling the waves, was you measured their temperature. Uh, it seemed to be a sort of fairly standard thing they did. Um, but they did it using, um, uh, and this is, this is a, there's an interesting sort of, sort of digression here, but, um, there, there's a remarkable role of this wooden bucket, um, in the history of climate science or these buckets like it because, um, Royal Navy ships used to use these standard wooden buckets, chuck 'em over the side, pull it up, dip a thermometer in, and then record the temperature. Um, and then they switched. Um, but that was very heavy wooden bucket. And so they switched, um, in the sort of nine, they modernized the system, um, to that canvas bucket. Okay? But the problem with the canvas bucket is that it's canvas, wet canvas being dragged up the side of a ship in a howlin gale is gonna cool off. And that's exactly what you see happening. Wooden buckets, suddenly they introduced canvas buckets, and it got a whole lot colder. The world got colder, apparently. Um, and then they, and then they realized that was the canvas buckets weren't very reliable. So they introduced these modern insulated rubber buckets, okay? And the world warmed by half a degree. Boom. Okay? So now fortunately they've kept all these gadgets and, you know, people like David Parker spent a admirable amount of time chucking 'em over the sides of ships in various different weather conditions to work out. So it's not, this is not this, fortunately because we've kept all the how, not just the records, but how the records were made. It's possible to make, to correct for these things and, um, to gather the stage together and, and, and, and establish, um, what actually happened. But as you might imagine, these corrections, and in particular, the method you use to fill in the gaps in a record going back over a century, do affect the outcome. They do. They do, they do, they do affect the, the estimate you end up with a little bit, not, not, not, you know, massively so, but enough. And, uh, as the record gets updated. So here was the record that we were using, it's now been updated in 2021. And you'll notice the impact of the update is to make it a little bit warmer today. But of course, if you stop and think about it, wait a minute. Now, today didn't change. What changed was the estimate of what temperatures were back in the 19th century, because we concluded that the global temperature in the 19th century was about a 10th of a degree cooler than we thought previously. Which of course means that, um, you know, 2017 was over 1.1 degree warmer than, uh, pre-industrial. Um, and, um, you know, the 2006 to 2015 period, which was 0.85 degrees warmer than pre-industrial, according to the people informing the Paris Agreement, was actually closer to one degree warmer. So what do we do with all this information? Because, you know, what that implies is that we'll reach 1.5 degrees, not in 2040, but in the early 2030s. Um, and that's where you get to with a certain range of uncertainty. And of course, cue more headlines from the Guardian about how close we are to, um, disaster. But what's actually changed here, we've changed what we think the temperature was in the 19th century. That doesn't make any difference whatsoever to what's happening today. Um, so, but of course it matters because the governments have agreed that they want to limit warming to 1.5 degrees above pre-industrial. And, you know, if we got it wrong, we got it wrong. And, you know, that's, that means that we're gonna get to 1.5 degrees earlier. But it doesn't. But what this should tell you is sometimes when you read the headlines, you might think that something very dramatic is going to happen exactly when we reach 1.5 degrees. And I'm telling you this little anecdote because it shows that, you know, it's, it's even what 1.5 degrees means is vague to within a tent or two of a degree, because, you know, what we thought 1.5 degrees meant back when we were negotiating the Paris Agreement is slightly different from what we think it means now. And of course, you know, what does pre-industrial mean even? And again, I I'm not trying to send the message Oh, we dunno, anything. No, but I'm just pointing out that at, at the sort of 10th of a degree level, you know, there is a certain amount of conventionality, should we say, at least about this. You gotta kind of agree what pre-industrial means, because you could argue as long as you liked about what is pre-industrial. And this is a, this is an estimate of global temperature going all the way back to, um, you know, the, uh, naught of the common era. Um, and you can see that, you know, there was a gentle decline. And again, it's important to stress, you know, if buckets were an issue for this part, when you go back beyond the, even the invention of the thermometer, you can imagine the uncertainties in constructing a record of global temperature all the way back there is, is, you know, there's all sorts of things get used, pollen, tree ring, ice cores, and so on. But it's, it's a, it's a, it's an art. It's detective work. It's, it's, it's not something you've gone and actually measured. So, but it is believed that there's been this very gradual decline of a couple of tenths of degree over the past thousand years, um, and indeed in this. So, so, you know, when you're choosing a period to regard as pre-industrial to measure 1.5 degrees against, you've gotta make a decision. Um, and they used this period in the late 19th century, um, 1850 to 1900. We actually had quite a lot of argument about that then, because, um, uh, the physical climate scientists, um, people like myself, uh, preferred a a 20 year period, 1860 to 80, um, because it excluded, um, something very dramatic that happened in, uh, uh, 1883, which was the eruption of Mount Pin, uh, mount, sorry, Mount was, was the recent one. Um, KRA tour was an even bigger eruption, which occurred, um, in, uh, 1883. Um, and an island in, in, uh, uh, uh, Indonesia blew up, um, and, um, it's, uh, put so much, um, uh, material into the stratosphere, cooled the planet significantly, and, uh, gave him lots of, um, material to Turner to paint his sunsets. These were the origins of the turner, those very dramatic sunsets, the turn us saw, because this material passed all the way around the world and, and, and produced these several years of very, very dramatic red sunsets, but also, you know, cool the planet. Um, and, um, uh, of course that, you know, should we include, um, uh, kra guitar in the period that defined pre-industrial? And we had a long, long arguments about this. I lost that argument. So it was included, um, and, uh, it, it didn't make that much difference. Um, but you, you can, you can see the sort of, you know, decisions that have to be made here, uh, in, in, in, in defining where, where we're starting from. But more importantly, you know, I promised, uh, I, I I promise we'd come back to this about how, how, how it matters, how much it, how it matters, not so much. Yes, it matters to lawyers when exactly we reach a 1.5 degrees, 'cause they're, you know, they love to be pedantic about these things. But for the rest of us, um, what matters is, um, you know, how much every incremental amount of warming we see is what, how much harm it's doing. And, and I, I talked about this, this backlash, um, that happened after the original, uh, 1.5 degrees report landed, and we, we had all these headlines about 12 year deadline. And, and Bjorn MBO indeed was tweeting this, um, with a, a figure showing, um, estimates of damage as a function of global temperature. And one thing you'll notice is that, you know, he claims that, you know, three degrees of warming is an income loss of about 3% of GDP, a couple of years of growth. Um, now those numbers are highly contestable, and I'm gonna contest them in a minute. Um, but at least we can agree on the overall shape. So Bjorn and I agree that the shape of this, of the damage function is, you know, starts off really flat and then gets steeper. And that sort of makes sense because, um, as we move away from the familiar climate that our ecosystems and our economy and so on have been adapted to, we expect the impact of every additional 10th of a degree of warming to be worse than the last one. Why do we expect that? Because just ask yourself what would happen if we went away in the other direction that would also cause harm? You know, cooling the planet would, would cause harm because we're adapted to that. Um, we're adapted to that, um, uh, point in the middle where, uh, where, uh, civilization, if you like, developed. Um, and so the simplest shape we could adopt, um, is what we call a quadratic. That's a sort of a, a, a temperature squared curve. Okay? And, and, um, again, uh, physicists love to do this kind of thing on a small enough scale. Everything's a quadratic if it's not a straight line. Yeah, I mean, uh, so, and, uh, uh, if you, okay, sorry, in, in, in jokes I suppose, but anyway, um, but the point is therefore, you know, for small enough warming, um, you are, you are sort of ex you should expect to see this kind of shape, which means that if, you know, according to lomborg, beyond Lomborg, um, a three degrees of warming costs two to 4% of GDP, then six degrees of warming would cause, uh, a loss of, you know, around 10% of GDP. And of course, you know, con con, conversely, as you go to the other side, um, uh, six degrees of cooling should cost sort of eight to 10% of GDP. Well, I don't know, but I think six degrees of cooling would probably cost a little bit more than 10% of gross domestic products. You know, we, we have experienced that in the past and, um, it sucked, um, for the humans that were around at the time. So in fact, we've done quite a lot of work. One of the things we did for the 1.5 degrees report was in fact, um, a, a study of the economic impact of, uh, 1.5 degrees of warming versus two degrees, uh, versus the present day and versus two degrees, um, using not the kind of back of the envelope on the estimates that Bjorn MBO was using, but actually using data of how growth, uh, observed growth in the economies of different countries responds to variations in global temperature. And Felix Preiss led this study. Um, it was a very elaborate, uh, statistical study because of course he had to take into account all the other things that were going on, like wars and, you know, global economic growth and so on. Um, and, um, but he, you know, he was able to tease out a temperature impact with a lot of uncertainty in it. It's important distress. So there's large uncertainty bars there, but he was able to identify not just, um, the global impact of, uh, a further, uh, half a degree of warming above where we'd reached, but also, um, the impact on different, how, how this affects different countries, uh, individually. Um, and as we go to two degrees, you can see all the purple countries here are the ones that are being adversely affected by two degrees of warming relative to the present. And you can see where they all are. Predictably enough. These are the big countries in the tro. These are countries in the tropics that are vulnerable to a little bit more additional warming.'cause a lot of, you know, they're, they're, they're, they're sensitive to, um, uh, uh, uh, uh, the, the heat stress, the, the, the and, and the additional, um, in burden on their economies that will result from, from any warming. You'll notice a couple of green countries are appearing here. These are countries who, according to the data, this was just a purely data driven study. And, you know, there's a bit of squeamishness about this. We said, no, no, we published the results. And these are countries where, um, according to the data, there would be a small benefit, um, from a, a, uh, a, uh, a further, um, uh, degree of warming above the present. Um, and we could also show who benefited and who lost out from limiting warming to 1.5 degrees versus allowing it to go to two. And the key point here was that nobody benefited significantly from allowing temperatures to go above 1.5, uh, to two, but several countries significantly lost out from that. And we could, you know, able to sort of identify these countries. And there's a big table of them. And the ones at the bottom of the table, just to sort of illustrate the countries that are most vulnerable to this additional warming are, you know, Saudi Arabia, Mauritania, Djibouti, Bangladesh, Saudi, you may be slightly surprised to see Saudi Arabia on that list, but of course, this is about the impacts of the warming, not the impacts of the process that causes the warming. So, you know, um, and, uh, and up at the top, you know, cold countries like Russia, Canada, Mongolia, and so on. So, so the, one of the key things which comes outta this of course, is that the impacts of warming are very, very different depending on, um, which, uh, uh, which, uh, country you're in or sort of what your, what your position is, you know, and how much warming costs you depends very much on who you are and where you live. Um, which is why in 2018, we said very firmly that one of the questions that we were being asked by the governments, we refused to answer it. They were basically asking us, is it worth it limiting warming to 1.5 degrees? And we said, any comparison between one and a half and higher levels of warming implies a risk assessment and value judgements that we can't reduce to a cost benefit analysis. This is the kind of kind of thing that the treasury loves to do, comparing cost of a policy with benefits of it and working out if it's worth it. Um, interestingly, in 2023, the intergovernmental panel on climate change really stuck its neck out on this, um, and actually said the global economic and social benefits of limiting global warming to two degrees exceed the cost of mitigation in most of the assessed literature. Um, but um, they, uh, they, so they, they, um, and the, the reason I, I was very surprised to see that statement, um, in the, the, um, sixth assessment report, which came out a couple of years ago, was they don't make the same statement about 1.5 degrees, um, because they, and, and so, so they were implicitly acknowledging, they actually have a footnote that explicitly acknowledges that they can't make the same claim about 1.5 degrees. So, so does that mean, and it sort of, someone could read this, oh, well, it's not even worth it. It's, it's economically, it's, it's not worth even trying to limit warming to 1.5 degrees. So, but I mean, the, the, the, the crucial issue that, that you, that, that I, you know, that that emerges here, is that how much globally you regard a warming of two degrees versus 1.5 degrees to cost depends critically on how much you weight impacts on Congo to impacts on Canada. And if you just add up dollar impacts, you often find the impacts on Canada are much more valuable than the impacts on Congo.'cause Congo didn't have much money in the first place. So, you know, you take 10% off their economy while it was wasn't great in the first place. So it's not very much money in the end. Whereas, you know, a couple of percent benefit to Canada is, is a big impact on global GDP, but clearly thank you very much. That's great. Um, and, uh, um, one of the, one of the crucial things which comes out in this is that climate change costs lives. And if you, um, if you want to add up the cost of climate change you need, you may, you may not be explicit about it, but you need implicitly to assign a statistical value to a life. And back in 1995, the intergovernmental plan on plan on climate change actually did this exercise, the sort of cost benefit analysis. And they did what was standard at the time, which was you assign a value to a life according to what somebody is willing to pay to prolong their life. And you look at the statistics of different countries and work it out. But of course, that's not a function of what the life's worth. It's what it's, it's a function of what the person's able to pay. And as a result, it sort of emerged in the course of producing the 1995 report, that they were effectively waiting the life of a Bangladeshi at some tiny fraction of the life of a resident of Florida. And there was a huge row, and that entire section of the report got ditched, um, I think rightly so. Um, and the IPCC, the Intermental Panel on Climate Change has been, you know, very squeamish about coming back into that arena ever since. And I think, I think correctly, so, so I was very surprised when they ventured back into this in the, in the last report, you'll notice that their, their, their comment is not about a judgment of the IPCC, it's just a comment about the literature. And so it's, it's it, you know, it's appropriately cautious, but I, you know, it's, it's, um, it's a, it's a problem, um, trying to assign value to different levels of warming in the way that people like Gil Mbo and the sort of a lot of the community saying, you know, it's not really worth it trying to do anything about climate change.'cause let's face it, it's not gonna do very much harm anyway. The way in order to say how much harm climate change is doing, they have to add up impacts in this way, and they have to make decisions about things like the statistical value of life. Are poor lives worth less than rich ones. Um, you know, social impacts a mass movement of people from a part of the world made uninhabitable by rising temperatures, which might be a poor part of the world to a rich part of the world would probably increase global GDP. Yeah, once everybody moved, they then sort of get much closer to automatically get much closer to the income levels of the country of their destination. Um, and o on average, the world would be richer. But you can just imagine the social disruption that might accompany that. Um, anybody listening who's worried about small boats? There you go. I mean, it's, it's, it's, these are not cost-free disruptions. Um, and also there's, you know, for for, for, for the Stratham College of Stratham School kids, there's a, a very important part of this, of course, is how do we wait impacts on the present day generation to impact on future generations? Um, and again, that's crucial element in, in, in deciding what the value is of different levels of warming. So what I think we can't do this, um, assigning an an absolute value, um, to global climate change. Um, what we can do is say that it's roughly that shape. I think that's pretty safe 'cause everybody agrees on that. And in a sense it sort of almost has to be if we just accept that that was the climate we were adjusted to, we expect climate damage to get worse and get worse faster as we move away from that, um, from that pre-industrial, uh, condition. So whatever damage climate change is causing today, we could expect two degrees of warming. We're at one and a quarter degree now. So we, the damage we've experienced over the past decade is around sort of like one degree damage. Okay? So we could expect, um, at two degrees, four times as much damage just square the temperature. Yeah, one and a one and a half degrees, two and a quarter times the most damage, slightly harder. Some, you can probably still do it in your head. Um, one and a quarter degrees where we're at now, 1.6 times, uh, the, the, the damage. So what this tells you is, you know, you don't have to agree with anybody about the absolute value of the damage, but the difference between 1.5 degrees and two degrees, that additional half a degree of warming is going to do as least as much damage as all the climate changes has happened to date. Because we expect the damage to get steeper, the damage curve to get steeper as we move ahead. So that allows you to put it into context. Um, and, you know, whatever you care about climate and what's happened already, you should care just that much and more about avoiding that additional half a degree of warming beyond one and a half degrees. So I just wanna wrap up by talking about briefly about how not to think about warming, um, by 2100 because you're gonna hear lots of headlines about this during cop. Um, this thermometer is a very, um, um, uh, beloved, uh, graphic that is flashed around a lot at cop. Um, it's massively misleading, um, because it implies that, um, you know, it basically what it's about is warming to 2100 and it tells you that, you know, if we, uh, if we manage our 2030 targets, we'll limit warming to 2.4 degrees. Um, and if we go, you know, much beyond that and include all the long-term pledges, we'll limit warming to two degrees. So you might think, well, that's a lot of work, work for 0.4 of a degree, okay? Um, and um, what this is doing is actually le leaving out the crucial points that what the numbers here they're talking about are only warming by 2100. They're not actually differentiating between, you know, conditions where we've actually stopped the warming in 2100 or situations where it's still carrying on warming. And this, even this fixation on 2100 even started to creep into the negotiations in, in Glasgow where, um, we, we, they, they started talking about limiting warming to 1.5 degrees by 2100 as if that was the goal. Um, and of course, if you, if that were the goal, and it certainly wasn't the goal agreed in Paris, um, you would never, you then would never know whether you'd met it or not until into 2020 second century as if the goal was limiting warming to 1.5 degrees by 2100. You wouldn't know until 2010 whether you'd succeeded or not, 2110. And, and, and, and whether you, whether you succeeded or not. So, you know, this obviously would have huge ramifications for things like loss and damage because then people would say, well, you know, we don't dunno if we made it or not, you know, even though temperatures are at, might be at two and a half degrees, but, you know, we may be, you know, on our way back down to one and a half degrees by two nine or two nos. Um, so, so that's, you know, fortunately in, in that was got rid of very quickly, um, from the draft decision text. Um, and, uh, um, but it, this, this way of thinking is, um, is hasn't gone away. Um, in the press press release that accompanied the UNAP gap report, um, that landed this morning, we see this, um, current targets basically would lead to temperatures not exceeding two and a half degrees above pre-industrial levels. Now, current targets would return the current rate of warming from roughly quarter of a degree per decade, which is at the moment to about one, uh, one point, uh, 0.17 degrees per decade. So reduce it by about a third or so. Um, and then if you work it out, that gets you to two and a half degrees by 2100. So it lowers the rate of warming slightly, but it's still warming. So it doesn't limit warming to two and a half degrees at all. It just means, you know, instead of getting 2.7 degrees by carrying on the current rate, you at 2.5 degrees. Um, so it's a, and, and, and, you know, they didn't, they in the press release, they didn't even mention 2100 at all. So this is very dangerous rhetoric, okay? Back to what 1.5 degrees actually means. Um, you know, the right way to think about it is there's a lot of sort of bogus precision banded around at these cops, you know, 2.7 degrees versus 2.4 degrees, really? Yeah, we, we don't know these. So you mustn't let yourself be distracted by this bogus precision. 1.5 degrees is a milestone. Um, it's important when we pass a milestone to note that we, if we do pass the milestone, we hope we won't pass it. Um, but if we do pass the milestone, you shouldn't expect anything dramatic to happen just because you passed the milestone. Um, but it does tell you the direction of travel. Um, and indeed, you know, it's, it's actually quite a good analogy, the sort of 24 miles to London, what does it mean 24 miles to London? I mean, London's, you know, however many miles across, so you have to choose rather specifically one bit of London as you know, the place you're gonna measure stuff from. It's chaing cross or something, isn't it? I I, I can't remember Anyway, but, but the point is, it's a convention. Um, and, and the same goes really for global temperature. We have to, we had to sort of agree what pre-industrial was, and so therefore, you know, there's no sort of nothing absolute that's gonna happen, but just to, you know, remember you, you know, as, as we approach, uh, this cop, remember this, uh, analogy that I gave you early on in these lectures, um, if we hit the brakes now, um, and managed to stop warming by mid-century, we would limit future warming, all future warming to less than what's already happened this century. I think, you know, if you remember that, that's, you know, that's not, doesn't mean it's not a big deal. Adding on the same amount of warming again that we've already had since 2000 will cause a lot of harm. But if we manage to do that, we'll avoid a lot of harm as well, and that would be a huge result. And, you know, I think we, um, need that, which is why this coming cop in Dubai is so important because we are finally starting to talk about the kind of things we need to do to actually stop fossil fuels from causing global warming by mid-century. So things to look for in the outcome of Cop 28. Um, will we agree to phase out all unabated use of fossil fuels at this cop? What, you know, that's what, that's the phrase to look for in the findings. Then of course, we've gotta agree on what abated use means, and we better agree that it means something that's compatible with the Paris Agreement, meaning permanent storage of all the CO2 generated, and then of course, we've gotta get on with it and phase up the fraction of carbon dioxide that is safely and permanently disposed of. Um, at the same time as we phase down total fossil fuel use. And this is one sort of good bit of news, um, coming out of this cop because it's really important to emphasize the positives, uh, in the climate issue. Um, there is a little reported, um, development led by, um, uh, motley crew, including the United States, Saudi Arabia, Canada, the United Kingdom even, um, which is committing to store a million, a billion tons of carbon dioxide back into the Earth's crust by 2030. They're calling it the Carbon Management Challenge. And if you work out what this means, if they can maintain that rate of acceleration in driving up the fraction of carbon dioxide generated by the fossil fuels we use, um, that's re-injected back underground. If they can maintain that over the next 30 years, then we'll stop fossil fuels from causing global warming by the 2050s, which would be a huge result. Um, and let's, so let's watch, watch for that language and watch for the Carbon Management Challenge and watch for the countries that are joining in with it, where they're actually putting action where their words are. Anyway, to sum up, y 1.5 degree map, every 10th of degree causes more harm than the last one. Um, warming, remember this, you know, when people are saying warming from 1.5 degrees is, is is not that harmful? Well, warming from 1.5 degrees to two degrees is at least as harmful as the warming we've already experienced to date. Um, but it is a milestone, not a cliff edge. Um, we, there's no reason to to believe that anything dramatic will happen when we get to 1.5 degrees, but it is the point at which we have agreed collectively that we're gonna try and limit warming. So let's hope for a good outcome at Cop, uh, 28, um, and get us on the road to, um, achieving that m uh, to, to stopping before we get to that milestone. Thank you. Sorry, I've got a bit close to time, but hopefully we've got enough time for a couple of questions. Thank you, Professor Allen. Thank you. That was a fascinating lecture. And yes, I think we will squeeze in a couple of questions if we, if we can. Um, so the first one, uh, there's quite a few, um, 'cause a lot of the lecture covered elements of methods like how you come to these, these decisions. And so there's a few, I'm gonna sort of bunch them into one another. I'll have a go. Um, so when you sort of, as you talked about with the economics, it was striking how you use a different model, a different baseline for calculation. You get quite wildly different models, which then produces very different results. But thinking about the science, when you said, you know, you were the first sort of group of people that sat down and taught about 1.5, how do you start that? How do you go about getting, because science by its nature, it's all committed to truthfulness. We, we hope, um, we know. But, um, but so many different disparate sources, how do you bring them together into any sort of a, a unity to, to, to calculate from? Yeah, There's a, there's a, there's a, there's a lot to it, a lot of discussion and a lot of, and it's very important that the process is as inclusive as possible. And one of the things, so, um, you know, just to big up physics on this, yes, we have to make these decisions in the climate physics community, and they make differences of like a 10th of a degree. The economists make decisions and they make differences of like factors of two or three, you know, so, so, or, or more so, so, you know, let's be clear where the uncertainties are. And it, you know, it's not the economist's fault, it's just that, you know, that's where, um, that, that, that's where a lot of the uncertainties lie. I think it is very important in doing our science to be clear what is sort of dictated by the world out there and what we're bringing to the problem in terms of our own preconceptions. So, you know, when, and that, that for, um, for in, in, for the physical science part, the argument about whether krato should count as representative of pre-industrial. Okay. Fortunately, it didn't make that much difference, you know, and, you know, as I said, I lost that argument. It didn't, it didn't really matter. Um, and, uh, but the argument about whether we should assign equal weights to a dollar lost to a Congolese, to a dollar lost to a Canadian, that makes a huge amount of difference to the outcome of how much you, how you weight different levels of warming or, or a dollar loss to, you know, a, a school child today, um, in, you know, when they're in their eighties compared to a dollar loss to a pensioner today who's already in their eighties, for example. I mean, you know, so, so, um, these are, sorry. It's, it's a feature of economics that everything's in dollars. I don't, uh, sorry about that. Um, but, um, uh, and so, so, so I, I do think one of the things we have to do is be very clear what's our preconceptions versus what's come from outside. The other thing we have to really work on is, and something that the whole climate science community is, is working on, is being more inclusive of different perspectives in the research process itself. Um, because, you know, for a long time, climate science was very heavily dominated by people who looked like me, to be honest with you. Um, and, and that's, you know, and that's a problem, uh, because it's not really up to us to, you know, pontificate to the world how they should behave, Just, uh, just on the costs. You, you described them in a sort of quadratic as though there is a smooth increment. And I'm just wondering, taking Krater as a example on the other side, whether, whether you see these costs as mainly incremental, or are there some big costs that come at various tipping points? Yes, that need to be factored in. We have a whole lecture on tipping points later this year, so come back for our lecture on tipping points. If that's, if that's a, if that's a reasonable answer. I mean, it's, um, but, but yes, but for these, at the moment in our models for these small levels of warming, things seem to happen relatively smoothly. Um, that doesn't mean that's true of the real world. Um, and indeed in the real world, things in, when it comes down to impacts, particularly impacts on extreme weather, which we will be talking about in the next lecture coming up in January. Um, things do seem to be happening rather by fits and starts. Um, and perhaps not, not as smoothly as we would've liked. Um, so, so yeah, no, this is, this is a, this is a, a big, a big issue, but, but the difficulty you, we have is that if you can't predict when a jump will occur, it's almost like it, it's smooths the uncertainty smooths it out. If you, yeah, I mean, it's still there, but it's sort of, it's smoothed by uncertainty, if you see what I mean. Yeah. In, in, in the way, in our thinking, if not in our, in, not in reality and not in the experience of it. Okay. We'll get onto this in the tipping points lecture, if that sounded very mysterious to you. Okay, thank you very much. And in the meantime, please join me in thanking our guest speaker tonight, professor.