Engineering the Climate: Can Injecting Chemicals in the Sky Help Save the Planet? (Professor David Keith)

22:03Professor David Keith:That's an incredible paradox to think that we are causing global warming by pumping CO2 into the atmosphere, but we're lessening the effect of that. We're mitigating the effect of that by polluting the atmosphere. There's this incredible detail about a volcanic eruption in the Philippines in, I think it was the early 90s, that provides some indication of what the effects of putting sulfur into the atmosphere might have on global temperatures. Can you talk about that a bit? Yes. So volcanoes do put sulfur in a stratosphere and cool the planet down. But I think it's much more relevant to keep talking about the pollution.

22:41So if you look at the global temperature record, you'll see that from like the 1940s till the 1980s, temperatures didn't really go up very much, despite the fact we were putting all that CO2 in the air. And then in the 1980s, they began to go up quickly. Why was that? That's because many things, but the dominant thing is CO2, as we put it in, is this long-lived, long-term incremental warming. These sulfur gases we put in are very short-term. The lifetime of sulfur gas in the lower atmosphere is only a week or so. And so we were fooling ourselves. We were masking ourselves. So during, say, the 1970s, we were warming the earth with CO2, but at the same time, we were pumping tons of sulfur in a lower atmosphere that was having a short-term cooling effect.

23:26And scientists knew that at the time. And in the short term, that cooling effect was winning. That's why the earth wasn't warming up until about 1980. Then after that, partly because we started to reduce the amount of sulfur, but also because the CO2 had built up, then warming really started. And now we're accelerating warming partly because we're doing such a good job in pollution.

23:48Professor David Keith:There was another extraordinary fact that I learned from watching your lecture that I certainly didn't know. Maybe a lot of people did know, but that it's better to fly during the day than at night. Can you explain that? I will, but I first want to get my one crucial number in. that I think should reassure you. I mean, I think it's absolutely natural that you and most listeners should say it's completely bonkers for somebody to talk about protecting the environment by spraying sulfuric acid in the stratosphere, but the numbers matter. So if we did it carefully, we could cool the world half a degree centigrade, which is this giant benefit to humans in the environment, with something like one and a half million tons a year of sulfur.

24:28So maybe we would be doing that in 2070 or 2080. One and a half million tons. Compare that to the fact that now we're putting 30 million tons in the lower atmosphere that's having roughly the same cooling effect. So that ratio is really important. So I am not saying, and nobody who's sensible is saying, that this has no risks. There absolutely are risks of putting a million tons or a million and a half tons of sulfur in the stratosphere. But those risks are very much known and knowable because we have so much experience with sulfur. And they're comparatively tiny compared to the risks of what we're now doing with the sulfur in the lower room.

25:03Professor David Keith:So I want to get onto the specifics of how it would work, how you would do it, what the benefits could be, what the dangers might be. But can you just tell me why it's better to fly during the day? Sure. Many people will have seen that airplanes make contrails. That's a word for condensation trails. That's because when you burn the fuels in the airplane, you partly make water vapor and that water vapor in the cold atmosphere will freeze and make little cloud crystals. And in some places where the atmosphere is already pretty moist, it can trigger what's called persistent contrails, where it kind of makes what's effectively an artificial high cirrus cloud.

25:39And low clouds are always reflecting sunlight and cooling the planet. But a high cloud both has two functions. A high cloud reflects some sunlight, which just cools the planet, but it also traps infrared heat just like carbon dioxide does. So it's got a warming component and a cooling component. But if you think about it, the cooling component obviously doesn't work at night. So if you have a cirrus cloud at night, it's warming the earth, but it's not doing anything to cool the earth because there's no sunlight to reflect. But that same cirrus cloud during the day or contrail, that same contrail during the day is reflecting some sunlight as well as heating.

26:16So if you think about it, an aircraft contrail, or if you choose to book a flight today, an aircraft that flies during the day is adding carbon dioxide, which is warming the Earth, but it's also adding some contrails, which are warming the Earth, but it's also adding contrails that are cooling the Earth by reflecting sunlight. But at night, no sunlight to reflect. So that means the net effect is a little bit less for flying.

26:41Professor David Keith:I absolutely did not know that. And that is amazing. I might change my flying patterns. It's actually built into lots of estimates of the CO2 impacts of your flight. So I'm not a promoter of Google, but if you go on Google Flights, it estimates how much CO2 a flight will emit based on the aircraft you're flying and the route and so on. But it also provides some estimate of this contrail warming potential. So if I understand your proposal correctly, we're focusing on reflecting more sunlight away from the Earth in order to mitigate the effects of global warming. We could do that by e.g. sticking mirrors in space, making things on the Earth's surface more reflective or clouds brighter.

27:25Professor David Keith:But you are focused on what's called stratospheric aerosol injection, which is spraying sulfur into the stratosphere. Can you just take us through, walk us through physically, how would you do it? I'll do that in a second, but a caveat. For a first time, this is very much not my idea. This idea was first proposed by Mikhail Badiko, that at least the first English translation was in 74. And he specifically talked about using aircraft as artificial volcanoes to cool the world, to reduce the risks of accumulating. And this is a Soviet scientist in the 1970s? Yes, maybe the leading climatologist, an amazing, amazing man.

28:08So these ideas actually date to the 1960s. That's the first English translation of Badeco's really specific idea of doing it from aircraft. So most definitely not my idea. And yes, it's true that I'm focused on, of all these different sunlight reflection methods, it's true that I'm focused on the stratospheric aerosols. But that's not because it's my idea or because I just kind of woke up liking it. It's simply because of the pragmatic reality that when you look at the engineering feasibility, it's the only thing we actually know how to do with current tools. So, yeah, we could build some giant space shield that's thousands of kilometers in diameter, but we have no idea how to do it with current tools.

28:47It's just -

28:47Professor David Keith:Well, it's the kind of thing Elon Musk might turn his mind to. Well, he might babble about, but the point is things aren't done just because somebody says they might be done. Things are done when actual engineering could go do them in the real world. And there's a whole host of things that any number of people, including Elon Musk, talk about that might in principle be possible within the laws of physics, but are completely implausible now. The point is that if you wanted to put a small amount of sulfur in the stratosphere now, there is no technology to do it. You don't need to invent any new technology.

29:22You could go to commercial companies and they could figure out how to do it. So I can tell you about how that works. But the crucial idea here is that it's something that is doable with today's. So how would you do it? So you'd use high-altitude aircraft. You could start with existing relatively high-altitude aircraft like business jets. So if you wanted to start putting 50 ,000 tons a year of sulfur into, say, the northern hemisphere, You could do that with one airfield and something like 10 of these conventional business jets flying from one airfield, and that would be enough to put that much sulfur in the stratosphere, which would be kind of doubling the background natural flow and beginning this process.

30:02And then if you want to do it more efficiently, you want to fly a little bit higher. So to give you altitudes, a conventional aircraft that many of your listeners will have flown in flies at about 10 kilometers. Business jets can get a little bit higher, 14 or something. you really want to put this material in at more like 20 kilometers, twice as high as a conventional aircraft flies on a typical route. And that's absolutely doable, but there are not commercial aircraft today that can put large payloads in at that altitude. But there are aircraft designs that could do that with, with existing engines and existing aircraft fabrication technology.

30:38So several different people who are leading aircraft designers have said, this is not hard to do. So the answer is if some coalition of countries wants to do this, they would procure a limited number, we're talking 30 or something, aircraft that were specifically designed to efficiently bring payloads up to 20 kilometers or so. But you could start with existing aircraft and then transition to these higher altitude aircraft.

31:01Professor David Keith:So you'd stick these aircraft up in the sky and you'd have a spraying mechanism that would spray out sulfur? Well, what you actually do is you release a precursor gas. So you're not spraying sulfuric acid droplets. Sulfuric acid droplets are what you ultimately want to make, but that's almost certainly not what you do. What you do is release a precursor gas like sulfur dioxide or hydrogen sulfide, probably hydrogen sulfide, and that's a gas that itself doesn't reflect any sunlight at all. It's just a gas, but it reacts in the stratosphere quite quickly to oxidize, to make sulfuric acid that then forms little droplets in the stratosphere.

31:40Professor David Keith:And so to be clear, this is not happening yet, but you're saying we have enough research that can tell us how it could happen and what it would likely do. That's correct. Nobody's doing this deliberately now. I mean, it's happening in the sense that the Earth is being cooled by human sulfur emissions today, as I said. And there even are human sulfur emissions in the stratosphere from transatlantic flights. But nobody's doing this deliberately to cool the Earth today. And yes, I think it's important to say with sulfur, there's an immense amount of science. So I'm not making these numbers up. There's more than 15 ,000 published papers on climate and aerosols in the lower atmosphere.

32:20It's really one of the topics we've studied the most in all of Earth sciences. And there's 2 ,500 papers on stratospheric aerosols. and this may sound like kind of obscure history, but the first review paper that told you kind of how stratospheric sulfur aerosols work was published in 1961. And the reason I say that is it gives you confidence that this science is really stable. So I think it's easy to picture climate engineering, the sunlight reflection is some wacky thing that somebody like me cooked up in a laboratory. And if you're a regular listener, you should be suspicious if somebody cooked up a thing in the laboratory and it hasn't been tested and it hasn't been tried.

32:59And in some deep way, that's not the right way to think about this. So while the idea of doing this deliberately is, you feel free to call it radical and people disagree about it, physics or nature doesn't care about intention. And the point is the science of what sulfur aerosols do in the atmosphere has been studied intensively for more than a century.

33:21Professor David Keith:So can we get down to like practical propositions? What is your proposition that should happen now? How much sulfur should be sprayed into the atmosphere? And crucially, what effect would that have on global temperatures? So there's no right answer. So very important thing to say, I'll tell you what I would kind of vote for, but I'm not claiming that there's some objective right answer. These are things that inherently are evaluated decisions. And so it's just a idea. So I would advocate for starting quite soon, within years, gradually increasing the amount of sulfur that was put into the stratosphere in a way that added a cooling trend of a tenth of a degree per decade.

34:06So to give you some context, the world is now warming up at about two-tenths of a degree per decade. So every decade it gets about 0.2 Celsius warmer. I would advocate at least starting, starting with zero cooling from sulfur aerosols in the stratosphere, but increasing the amount each year where the rate of that increase would be enough to cool the earth by one-tenth of the degree per decade. So that would be basically cutting the rate of warming in half. And that to me feels like a reasonable place to start. And after a decade or so of that, you'd be talking about a few hundred thousand tons of sulfur into the lower atmosphere to give you kind of a sense of scale into the upper atmosphere to give you a sense of scale.

34:49And I think an important thing about starting slowly is you could watch what's happening. They're sort of double-checking what we know about how sulfur moves around in the upper atmosphere. And if one wanted to stop after a decade, you could just stop. Some of your listeners or you may have heard about this concerns about termination or termination shock, which is the idea that if we did a huge amount of climate engineering, a huge amount of sunlight reflection maybe late this century, if you stop suddenly, there'd be a big risk, which is absolutely true. But that risk only grows if you're doing a whole lot.

35:22But if you start with a slow ramp, if after a decade one wants to stop, there's no giant risk of stopping.

35:27Professor David Keith:Could we have a sort of honest appraisal of the potential downsides or the unknowns and the risk for unintended consequences? What is it about this that might keep you awake? Well, I think mostly I'm kept awake by known consequences. So we know that sulfur aerosols are the most dangerous air pollutant that we have. So to me, that's number one. We know for sure that doing this would actually cause some additional air pollution harm. We can quantify it because we have so much knowledge about it and it seems pretty small, but that's where I start as a concern. We know there's some added ozone layer damage.

36:07Again, we know a lot about it. We're confident it's pretty small, but that's one that we know about for sure. There's concerns about how it alters the aerosols in the upper atmosphere, kind of complicated set of concerns. There's kind of fundamental concerns about the fact that doing this will reduce most climate changes in most places, not just temperatures, but also other climate changes. But it won't do it perfectly everywhere. This isn't anti-CO2. And so it will inevitably in some places make climate change worse, exacerbate.

36:39Professor David Keith:Do we know, sorry to interrupt, do we know where is it possible to pinpoint where it'll get worse over where it'll get better? No. That seems problematic. Well, maybe, but I've come up through working in public policy schools. I was a professor of public policy at the Harvard Kennedy School. And the fact is, everyday real public policy that politicians implement always has winners and losers and uncertainty. It's just the way the world works. So if you or your listeners are looking for something that will work perfectly with no risks and not make anybody worse off, give up. There is no such thing.

37:19But in the real world that we actually live in, we have to make hard choices between different risks. And climate change is today literally killing people from heat and slowing economic growth in the poorest parts of

37:32Professor David Keith:So this makes the question of who decides how and when to deploy this technology potentially extremely important. For sure. I mean, I think if you think about it unethically, there's two sets of issues at least. There's the distributional ethical question of whoever decides who actually is harmed and benefit. What's the distribution of harms and benefits? And then there's the procedural ethical question of who gets to make the decision. Both of those things matter. Because I was just looking at the UN Scientific Advisory Board of Independent Scientists who came up with some recommendations about a year ago, May 2025.

38:15Professor David Keith:And they talk about this uneven cooling that you mentioned. And they say that current modeling suggests there could be dramatic cooling in some regions, heating in others, including in the Antarctic. It could cause changes in monsoons, hurricanes, shifts in El Nino, etc., etc. It seems like the dangers here are that you start fiddling with a highly complex machine when you don't even really, we don't even really understand in full the way this machine works. We don't understand how it works, but we're already fiddling with it. So we've already added all the CO2 for the atmosphere, which is the way that climate science talks about increasing the radiative forcing, the amount humans are pushing the climate.

39:01We've done that because of the CO2 in the atmosphere. If we had a magic way to remove all the CO2, we wouldn't waste our time talking about this idea. We just remove the CO2, but we don't. We are already affecting the climate in a patchy way by the sulfur we've now put in the atmosphere, which is offsetting some of that CO2 radiative forcing. So what we're proposing here is a way to reduce the net radio forcing. Nobody disagrees that this reduces the net radio forcing, which is the primary thing that drives climate change. So I think it's important to bear in mind this risk-risk framing. So absolutely, there would be some side effects.

39:37I think that statement that you read was politically motivated and overstated. I could find you another UN report called the One Atmosphere Report that I think is much more balanced. The reason that report was able to say that is they were choosing really extreme scenarios with much more climate engineering than I'm talking about here. So I think that's not – those words don't apply to the kind of slow ramp I was talking about. But even the slow ramp I was talking about absolutely will have some uncertainties and risks. That's just the game. But it is incorrect to say that we're just kind of messing with the system by doing this.

40:15That's the wrong thing.

40:15Professor David Keith:I wasn't suggesting we are already. And I take that point. And I wasn't suggesting that you were just fiddling with the system for the hell of it. And I should say that, you know, the UN's scientific advisory board obviously isn't in the room and they might disagree with your characterization of it as political. But I think the point is that partly this is a scientific question. Should we or shouldn't we do this? Could it or could it not work from a scientific engineering standpoint? But partly, of course, it is a political question because of all the vital questions of who gets to decide how to do it, when to do it, and which people, frankly, get to get thrown under the bus.

40:54Absolutely. Just like all the other pieces of climate. So, yeah, let's not forget the basic politics of climate. Most of the CO2 in the atmosphere came from relatively richer people pumping CO2 in the atmosphere, and most of the harm now goes to poorer people in hotter countries who are suffering from heat. That's the current inequality that we live in. You got to look at the difference between what's actually written in peer-reviewed papers and what people say. People will say all sorts of stuff about solid reflection. Because of this underlying concern about moral hazard, people will say it's completely crazy, it has these huge damages.

41:28What they don't do is publish papers that show big damages from moderate hemispherically balanced scenarios like unproposent.

41:38Professor David Keith:So you published a paper all the way back in 1992 called A Serious Look at Geoengineering, in which you asked some of these questions about who authorizes these kinds of interventions, and crucially, who is liable if it causes harm. Let's imagine a scenario in which let's say the United States and China decide to go together to spray sulfur in the atmosphere to reduce warming in the way that you suggest. And it causes a terrible drought in the Sahel, or it disrupts the monsoons in another part of the globe. And there is a crop failure. Who is then liable for that? There's got to be some kind of global mechanism, hasn't there?

42:24No, there doesn't. It'd be great if there was, but it doesn't. Who's liable for the harms from the CO2? People are dying now from high temperatures from CO2.

42:33Professor David Keith:Well, there's a lot of people talking about who should be liable for the CO2 releases. I understand. I've even been supporting some of those lawsuits. But in the real world, the answer is we don't have a global liability system. So I think it's just important to be... But should we build one? It would be great if there was some universal global government that had some just liability system, but that's not the world we actually live in. So the answer is if some set of countries, I think it's unlikely to be China and the US, started implementing, there would not be a clean universal system for liability.

43:09I wish there was, but that's just not the way the world actually works.

43:12Professor David Keith:And you're not saying we should build one before we start doing this? I don't think that's possible. I don't know how to do it. So if you don't think it's going to be China and the US... But also, hold on. I want to push back. The politics here are rough. So let me give you some facts about droughts. Because the gap between what people rhetoric says... Step back to the big politics. A lot of well-intentioned people in the environmental left, especially in the rich European environmental left, believe this idea really is dangerous because of its distraction from cutting fossil fuels. So they make all sorts of claims.

43:49But you do have to go back to what the literature says.

43:52Professor David Keith:I mean, the real answer must be that we simply we don't know everything, right? We don't know exactly what will happen if we did do this. But you could take an assessment of the risk. I would like to just drill down into like who gets to do this. Is it governments or is it private companies? Like who is going to end up, if this happens, who is going to end up spraying this sulfur into the stratosphere and physically doing this? And is it going to be a governmental thing or is it going to be a for-profit thing? How will this work? I think it is almost certainly controlled by governments, by a small coalition of governments.

44:30I think that's overwhelmingly likely. So there are now some private companies proposing different versions of this. None of them have, I think, any credible business plan or credible technologies. But supposing one had a credible technology, because of issues of private liability, because of kind of the way state power works, I just don't see a version of this that works privately. And to be clear, I strongly oppose private action on this. I think there's a certain kind of category of actions that just automatically become government actions. So even if some rich person starts doing it, other governments will push back and it becomes de facto the decision of a government or a coalition.

45:10Professor David Keith:So there is already a few private companies. There's one in particular, Stardust, that has seen quite a bit of coverage in the media. It's founded by some former Israeli nuclear program officials. They've raised$75 million from venture capital firms. And a lot of people are very worried about this idea of for-profit interventions in the Earth's climate. Do you think this is a useful model or piece of the puzzle, as it were? No, I think that Stardust is pursuing the wrong technical solution with the wrong structure. So they're not, I mean, again, they're not in the room, but they're not proposing sulfur, right?

45:48Professor David Keith:They're proposing some other particle to spray into the atmosphere. I know them pretty well. We've talked lots. You can look at videos of us together. I think it's the wrong solution because the central challenge for sunlight reflection is trust. And the one thing that capitalism like that doesn't do well is manufacture. Neither does governments these days. Well, but they have more. So I think the point is that you're right, governments don't have as much trust as they did, but capitalism can be great when you want to drive down the cost of something. But for this, it's already cheap enough. We don't need to make it cheaper.

46:23And what we centrally do is trust. So I've been giving you a bunch of arguments about why I think the science says X, and other people have said the opposite. And what you or listeners should want is high-quality independent assessments that provide a clean answer insofar as is possible in between those different views. But a for-profit company claiming their thing is great and the other thing is terrible is very unlikely to generate.

46:49Professor David Keith:Obviously suspicious. Can I just, I mean, you yourself set up a geoengineering company, Carbon Engineering, which you then sold to a fossil fuel company for a large amount of money. Do you think that the profit motive is problematic in this sphere in geoengineering? So you've chosen to lump those two things together because it's the most negative thing you can say. It's perfectly reasonable for you to say that, but I need to push back in several regards. Please do it. First of all, to claim that I owned it and sold it is false. I apologize. You own a little part of it. I was. Let me speak. I was the founder.

47:27I'm very proud of what Carbon Engineering did. By the time Carbon Engineering was sold, I had a few percent of the shares and certainly didn't have control. My big concern as a Canadian was actually selling to the Americans. And I also had controls about selling to an oil company. But the idea that it was at that point my company and I sold it is a false read of the facts. And also, you've chosen to suddenly switch to calling that all geoengineering, which you're free to do, but I need to have the right to push back. So I think these things are quite different in the way they need to be regulated and run.

48:02So carbon removal is inherently very expensive. The problem with carbon removal right now is mostly that it's too expensive. And lots of people in the environmental movement agree carbon removal will be great if we can find cheap ways to do it that have low environmental impacts. And from my perspective, that's what carbon sharing was trying to do, along with lots of other companies. I'm not saying carbon sharing was so special. And for carbon removal, where it costs a lot of money to remove significant amounts of carbon, we'd be talking about kind of a trillion dollars a year, which is what I hope humans would spend late this century to remediate the CO2 in the atmosphere.

48:38That's something that almost certainly would involve large private industry in some form, probably more run like trash removal is run. But in any case, it requires industry. But it also doesn't provide any big risks. There's local environmental risks to any of these carbon removal things, including what carbon sharing did. But there's no global environmental risk because you're just taking CO2 out of the atmosphere. That's very different from some reflection where it's cheap and there are – well, I'm going to keep going. You can cut me off if you like. But you've made an implication that I think is unfair, and I'm going to push back.

49:11Professor David Keith:I just wanted to say to you that this is not a kind of gotcha podcast. I'm trying to drill down into the— But you just— No, I'm— You did the gotcha move, which was totally fair. People will read about it. But you guys control it, and you can cut this out. But I'm not going to stop. You deliberately, after keeping them separate, lumped the two together and said, I sold an oil company. No, no. That's what you did. So what I'm hearing from you is... That was what you did. Okay. We can end this now. No, no, I would like to continue talking. I'd just like to make sure that I've understood you clearly, which is that carbon removal is a different order of geoengineering from solar reflection.

49:54Professor David Keith:And that in your view, if I understand you correctly, private companies have a role to play in carbon removal, but they don't in the area of solar reflection. So first of all, few people now lump the two together as geoengineering. In general, both the IPCC, the environmental movement, other people just talk about these as different things. So the way I would talk about it is there's four basic things humans can do to manage the climate risk. One, and the most important, is decarbonization, which breaks the link between industrial activity and CO2 going in the atmosphere. The second is carbon removal, which breaks the link between historical emissions and the current concentrations.

50:38There's a whole set of issues with carbon removal. The third is sunlight reflection, which doesn't break the link, but weakens the link between climate change and the amount of CO2 in the atmosphere. And the fourth is adaptation, which doesn't break the link, but weakens the link between the amount of climate change there is and the harms to humans. So I think increasingly, most people talk about those as four pretty distinct things. And I'm not opposed to the word geoengineering, but I think it's not particularly useful to lump those middle two into one thing. And people often do that when they kind of want to make that critical bite and go for it.

51:15But I think they are legitimately quite different things, not just in the science, but in the public policy implications. and I'll try and say it in a shorter way, any carbon removal activity will have some local environmental harms. Any industrial activity does. But if you remove carbon from the atmosphere, that's a clean global environmental benefit. So it's a regulatory thing. The challenge is paying for it and managing those local environmental harms. But there's not a challenge of, you know, worry that people will take too much CO2 out of the atmosphere. Sunlight reflection is utterly different.

51:53It appears to be ridiculously cheap. The whole issue for sunlight reflection is who makes the decision, how the risks of doing it are balanced against the risks of not doing it. And that, I think, is just fundamentally different politics from carbon removal.

52:08Professor David Keith:That's clear. So it should be governments. And how do you guard against, If there isn't really space for a sort of framework agreement on how this is deployed, how do you guard against private companies, rogue actors, state actors deploying this as they might see fit in a way that might be, in your view, harmful? So I don't think in the end that private companies will be a big risk here because there just really isn't a business model. And very quickly, other states would force it to be state liability. So I just don't, it's not that I think the private actors are well-intentioned, but I just don't see a plausible scenario.

52:49But I absolutely do see that some government would want to do it to protect its citizens from climate change. And I think it's unlikely to do it purely alone. If you imagine that you're in the leadership of some government, maybe a government probably in the global south that's got some power that has lots of citizens suffering from heat or crop failures, in a government like that, you're going to know that some other governments will oppose. And so you're going to look for building a coalition. I think that's likely what would happen.

53:20Professor David Keith:Do you worry? I mean, I'm aware that I'm coming at you with all the negatives and I'm trying to stress test it and trying to ask the questions that I think our listeners will be asking. I mean, let's say India wants to do it to reduce heat, but let's say Russia doesn't want to do it. Do you worry about the potential for interstate conflict? Yes, for sure. I mean, way back in that article in 92, I worried about that. But I also think there's a way in which sometimes climate people take themselves too seriously. States obviously do go to war. We have more wars than we did in a lot of my lifetime right now.

53:59It's very sad. But I think climate is a pretty slow-moving thing. And I think it's unlikely to be an acute cause of war the way that, you know, I think disruptions from AI might be and other geopolitical disruptions we're seeing are. So I worry about it. But I don't worry about it.

54:19Professor David Keith:This idea, it feels a lot like AI to me and certainly the debate around it, because on the one side, just like with AI, you've got people a bit like yourselves. You think this is the solution. This could say or part of the solution could help save the world. But I absolutely don't think this is. But it's part of the solution. Yeah, but that's a very different statement. And that's what I said. So it's part of the solution that it could fundamentally create a better future. And on the other side, you've got people who think this could be the end of the world. I think that's true. But I think there really is a difference when it comes to kind of rationality in scenarios.

54:58So obviously other people aren't here to defend themselves, but let me say it. There is nothing that suggests end of the world scenarios from the kind of limited, hemispherically balanced sunlight reflection that I suggested. There's no scenario that I am aware of that's been proposed. because there's so much knowledge about sulfur in the stratosphere, because we've watched what Pinatubo did with 8 million tons a year in a single pulse. That's the volcano. The volcano in 1991. Because we've watched the atmosphere respond to, at the peak, about 70 million tons a year of sulfur from pollution, there's just no physically plausible way that, say, putting half a million tons or 1 million tons a year in the stratosphere in a careful way is going to have some terrible effect.

55:45That's not to say it's all predictable. There will undoubtedly be some harmful side effects or undoubtedly be some surprises. But I just don't believe there's a physically plausible scenario for how this is a disaster. Whereas for AI, there are tons of people, including some of the best AI scientists on the planet, who have published and thought about ways in which AI takes power away from humans and wipes them out. So I just think it's a very different thing.

56:12Professor David Keith:About 25 years ago, when I was a very young journalist, I got to interview Mikhail Gorbachev about his stewardship of the Soviet Union in its final years. And this is an imperfect analogy, but this conversation we're having makes me think of it. He said, I was in charge of this sort of sclerotic piece of machinery that was the Soviet Union, and I could see it wasn't doing well. And I thought I could fix it by tightening a few screws here, loosening a couple of screws over there. And then what started to happen is the whole thing started to wobble and it fell apart. I just wonder, you know, you seem like somebody who's fundamentally positive, who feels like we can we can do something here to help mitigate what is obviously a real and pressing problem.

56:54Professor David Keith:I just wonder how much doubt you have in your kind of darker moments. I have lots of doubt about the political future for the world. You mentioned that I was lucky enough to make money from carbon engineering being sold. And so now my wife and I have to give this money away. And I'm actually giving a bunch of money now, starting to look for ways to reduce risk of nuclear war. And I think that people really don't worry about it enough, especially young people. The fact that we have this enormous number of nuclear weapons in a more unstable world than we did before. And I think that's an example of a real risk that people are paying insufficient attention to in the current geopolitical moment.

57:38And there are incremental ways we could reduce that risk. AI is another one. I think that I'm not actually that optimistic. I think there's a good chance that we have a big war or a real social breakdown. I simply think that for this incremental idea of sunlight reflection, I think for a bunch of reasons which intellectual historians could dig up, people have systematically exaggerated the risk. So we get to the point where, you know, I'm talking to a really responsible, amazing journalist at BBC who says that people think it could end the world. And I guess you can find somebody who thinks that, but there's no consistent story.

58:17There is a consistent story where one hour from now, people could have discharged thousands of nuclear weapons and killed a billion people. That is physically possible. The hardware is all there. It actually really could happen. There just isn't a physically plausible story that I've ever heard about how doing small amounts of sunlight reflection produced some awful answer like this. It wouldn't remove all climate risks. It wouldn't be perfect, but it would incrementally reduce risks. And I think there's a way in which it just – I get why it sounds like something like AI, but it just isn't. It's a slow-moving, incremental thing.

58:52Professor David Keith:I suppose fundamentally this is a question about the limits of human behavior, whether we need to accept limits that the earth places on us or whether we think that our human ingenuity can really overcome almost any challenge. I don't see that. It's an either or. I'm absolutely on the side of accepting limits. Okay. So, I mean, my whole life has been about environmental protection. I think that we must accept the limit that we need to stop filling the atmosphere with CO2 and using it as a waste dump. But do you fundamentally believe that climate change is a solvable problem and it is solvable with technology as part of the puzzle?

59:37It's solvable in the sense that we could stop making it much worse. We can't avoid the fact that damages have already happened. So ecosystems have already been harmed, people have already died, and you can't undo that. But we could reduce the overall climate risks and we could restore the climate towards pre-industrial. That is within human ability for sure. Whether we do or not, I can't say. And I don't think anybody thinks this can be done without technology. I don't think there's any way to deal with managing the world and providing food and shelter for people in a way that reduces CO2 emissions that doesn't involve technology.

1:00:18Professor David Keith:I wonder, maybe finally, as a scientist, obviously science is driven by argument and testing people's theses, but it's also a community. And I think you are, it's fair to say, quite far out on this subject of solar reflection. Does it ever feel lonely as a scientist to be David Keith? Well, much less than it did. I think that the world has changed very fast on this topic. So I'm very excited in the last couple of years, two of the largest and most historically kind of storied and powerful environmental groups, both U.S. Center in Environmental Defense and Natural Resources Defense Council, that have both played this huge role in practical environmental protection on toxic chemicals and on climate, now both publicly and strongly support research on sunlight reflection.

1:01:15And that really feels very different than it did a decade ago. And I'd say, while I'm more willing to publicly talk about deployment, behind the scenes in environmental groups and in governments, there's much more talk about this than it was even a year or two ago. So it doesn't feel very lonely.

1:01:36Professor David Keith:Okay. Okay. Professor David Keith, thank you very much for sharing your radical idea with us. Thank you very much.

1:01:49Professor David Keith:Well, what an interesting conversation. I think my first takeaway from Professor David Keith is that, I mean, he is quite out there. He is a bit of a lonely voice in this space, arguing for solar geoengineering to be practically deployed, to actually start dimming the sun. But he does feel like somebody who senses that he is gaining ground, that his arguments are gaining ground and that this idea is less on the fringes than it once was. One of the other things that I thought stood out was that he was quite open about the areas where we don't really know, where we don't really know the unpredictable consequences that could flow from his idea.

1:02:37Professor David Keith:and being open about the fact that deploying this idea, solar geoengineering, will have winners and losers, that some people will lose out, and that that is just a kind of inevitable part of the way the world works. And I think that is going to be quite hard for some people to hear. It comes out of this milieu, I think, or it feels like he's part of this milieu of tech builders. I mean, this is mostly Silicon Valley, right? And David Keith is a scientist, so he's slightly separate from that. But it's this idea that technology is going to shape the future, whether you like it or you don't, and you just need to go ahead and build it.

1:03:23Professor David Keith:And whether that scares us or makes us feel uncomfortable or not, that is currently quite a powerful force in politics and in society.

1:03:34Professor David Keith:And that's the end of this episode. To make sure you don't miss any future conversations, you can subscribe to Radical on BBC Sounds. But that's it from me. There'll be another episode of Radical coming your way next week.