Instant Genius - The surprising role of clouds in climate change

Episode Date: March 8, 2024

Clouds aren’t just the harbingers of bad weather. Turns out, they are crucial players in the climate – and so, too, in climate change. In this episode we speak to climate scientist Dr Paulo Ceppi, who contributed to Greta Thunberg’s The Climate Book, to learn about how clouds change our world. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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Clouds aren't just omens of bad weather. Turns out they're crucial players in the climate, and so too in climate change. To learn about how clouds change our world, we spoke to Dr. Paolo Chepi, the senior lecturer in climate science at Imperial College London, who contributed to Greta Thunberg's The Climate Book, which has just been re-released in soft cover. Talking to Paolo, we learned about the so-called seeds that form the centres of clouds, how some people want to use clouds to fight climate change, and what would happen to global temperatures if we reached net zero carbon emissions.

So, Paolo, what is the role of clouds in climate change? Well, that's a big question, and maybe it's easier to first say what the role of clouds in climate is before we even talk about climate change, right? So you might not realize, but actually clouds have a really profound effect on Earth's climate, and they do this in two main ways. The first way is perhaps the more intuitive one. We've all noticed that when it's cloudy, we don't see the sun, right? So clouds block the sun.

For example, on a summer day, if it's cloudy, it will typically be a bit cooler than on a clear cloud-free day where we get all the heat from the sun. So this I like to call the parasol effect, because it's exactly like that. It's a parasol shielding you from the heat from the sun. And so in that way, clouds actually keep the earth a little bit cooler than if they didn't exist than if they weren't there. That's the first effect. The second effect is perhaps less intuitive, but it's also important. And it's the fact that clouds have a greenhouse effect of their own.

So when you have a cloud, it's a bit like having extra greenhouse gas in the atmosphere. And so the cloud being there actually absorbs some of the heat that's trying to escape from your surface out of space. So it's the tiny droplets and ice crystals in the cloud that do that. Again, another example, some listeners may have noticed this. On a winter night, if it's clear, if there's no clouds, it can often get a lot colder than if it's cloudy. And that's because the clouds act a bit like a blanket.

They trap the heat and they keep the earth a little bit warmer. And so you have these two effects playing opposite roles. One is acting to cool the planet, the other is acting to warm. And one key question for climate change is to understand how each of these effects change as the world gets warmer through changes in clouds. And that's a really difficult question to answer, but it turns out to have actually a big impact on climate projections. And that's one of the key uncertainties that we climate scientists are working on reducing

currently. And this, by the way, is what we call a feedback process because it's something, you know, it's a change in the climate that has the potential to have a knock-on effect. on global temperature. So take, for example, an imaginary scenario. Actually, it's not so imaginary. We can come back to that in a second, but just as a thought experiment, imagine that as the Earth forms, the amount of cloud decreases a little bit, right? So you have slightly less cloud. And we're not thinking about a massive change, but maybe just something subtle of like one or two percent change in the amount of clouds globally. So if that decrease in cloud amount

happens, then it's like making your parasol a little bit smaller, right? So you have less shelter from the sun that allows more sunlight to come in and warm the planet, and this leads to further warming. So this is what we would call an amplifying feedback. It's a knock-on effect that leads to some further warming. And by the way, I mentioned these two effects, the parasol cooling effect and the blanket warming effect. The solar effect typically dominates. So if you have a change in the amount of clouds, it's mostly, you know, the change in absorbed sunlight that is the bigger effect compared to the change in the greenhouse effect of the cloud. Can we harness those qualities of clouds in any way? What are some of the ways that people are using clouds as a kind of weapon in the fight

against climate change? Right. So there are some ideas of, as you say, harnessing clouds to mitigate climate change, right? So this is completely different from the feedback that I was telling you about, which is a completely natural process. You know, if it warms, then clouds naturally respond to that warming. They change in a certain way. And as it happens, this slightly enhances global warming. But then there are some proposals out there to actually instead use clouds, perhaps to our advantage. We can think of ways to do this.

So maybe just to provide some context, clouds are formed of tiny water droplets or also tiny ice crystals when it's cold enough. And to form these droplets and ice crystals, often you need a little seed that's known as a condensation nucleus. So basically a little seed that water can deposit onto, to form a droplet or an ice crystal onto. And these seeds are commonly known as aerosols.

They're basically tiny particles of stuff. It can be natural things like organic molecules or compounds from trees and things like that. It can be sea salt crystals, tiny ones, or also it can be pollution particles from human activities. Anyway, all of these things that are suspended in the air can serve as little seeds for clouds to form onto. And one interesting thing that we've observed

in recent decades over the course of the 20th century and into the 21st century, is that as humans emit pollution, that changes the clouds. So we have already changed clouds, not just by changing global temperature, but also by emitting these little cloud seeds. And so in some regions, we've seen more clouds. So the more seeds you have, the more aerosols you have, the more pollution you have, the more easily clouds form. But we see this particularly in certain regions

where we have, for example, shipping corridors over the ocean. So where heavy ships circulate, they tend to follow preferred paths. And so often you will see a kind of a trail of clouds over the ocean, and these are easy to observe from space, from satellites. These are known as ship trails. And so these are exactly because of the aerosol pollution emitted from these ships. And so the idea then that some people have proposed is to deliberately emit more of these aerosols over vast areas of the oceans,

so as to extensively form more clouds over the ocean. oceans that would then act like a giant parasol and keep the earth a little bit cooler. Now, this is all very controversial because, of course, there will be a huge economic cost of coming up with all that infrastructure to constantly emit aerosols around the globe, around big areas of the ocean, but also undoubtedly there will be side effects. So you would make a lot of the ocean darker, and it's really hard to tell how this would affect marine life, you know, marine organisms, photosynthesizing organisms in the ocean, and so on. So I would say this is all very experimental, but there's absolutely no doubt that technically,

from a sort of technical scientific standpoint, we can, if we want, we can deliberately modify clouds and we are already doing so in fact. That's really fascinating. And this is called cloud seeding, am I right? So when you do it deliberately, yes, this would be known as cloud seeding exactly because you're emitting these seeds that will form more clouds. Yeah, you're kind of creating that initial kernel. But how does that actually work?

I'm guessing that you couldn't just go outside and kind of spray something from inside your house and form a cloud. So I know that there are kind of planes involved and sometimes hot air balloons. What are we actually talking about when we're talking about creating clouds? Right. So yes, it will be, as you said, it would be hard to do it yourself with a spray can. You would need vast amounts of aerosols and also you want them to go high enough into the layers of the atmosphere where clouds can form. Over the ocean, it turns out that over vast ocean areas, clouds form actually had quite low, We're talking about, say, 1,000, you know, a few hundred meters up to, say, 2,000 meters above the ocean surface.

And these clouds turn out to have a powerful cooling effect. It's actually coming back to what I said earlier about the parasol cooling and the blanket warming effects of clouds. These low ocean clouds have a very strong parasol cooling effect, and they don't have much of a warming blanket effect. So they're actually the clouds you want to have if you want to cool the planet, if that makes sense. And then another consideration is the type of seeds because not all seeds or not all aerosols are equally effective. And also, of course, some aerosols are just bad for us, right? I mean,

we don't want pollution in principle. And so the main proposals are revolving around the use of sea salt aerosols, which are, of course, present naturally. They're completely natural. In case you're curious, the reason why we have sea salt aerosols is because over the ocean, you know, you have winds, you have waves, and when the waves crash, you have like sea spray coming out, and the tiny droplets evaporate, and when they evaporate, they leave behind a little salt crystal, so basically a sea salt aerosol. And so it is known that the sea salt aerosols can facilitate or promote the formation of clouds. So then the idea would be to have something like ships covering large areas of the ocean

and sort of like sailing around and constantly spraying large amounts of seawater into the air and as the spray evaporates leaves behind these aerosols. Now, I don't think, I'm not aware that in that form, this proposal has actually been tested to scale. We know that ships and ship pollution causes clouds, these ship tracks that I was mentioning before, but that's a different type of aerosol, because obviously ships don't emit sea salts.

They emit things like sulfur, stuff that is much more nasty for us. And so, yeah, so then it's really a different idea. So in principle, it's doable, but I think whether it's cost-effective and just technically, well, it's technically feasible, but really whether it's cost-effective and whether we really want to do that instead of treating the root cause of the problem, which is really the carbon emissions, which we want to reduce, I personally think that we should focus more on reducing our carbon emissions before going into complex experiments, causing even further changes that might have unwanted side effects and might be very costly.

And by the way, what you described as cloud seeding is something known as geoengineering. So geoengineering is kind of the broader family of deliberate interventions to offset global warming. So cloud seeding is one, but there's other ways of doing this. And some other ways will potentially be more feasible from a cost standpoint. So one other example that's often cited, it's probably the sort of leading idea of geoengineering, is something known as stratospheric sulfate injection. So it sounds a bit complex, but it's actually not that difficult. So I already mentioned sulfur emissions from ships, for example,

but there are also natural sources of sulfur, for example, from volcanic eruptions. And it turns out when you have a large volcanic eruption, this can sort of catapult large amounts of sulfur aerosols into the upper layers of the atmosphere known as the stratosphere. And when they're there, once they're there, they can stay there for a few years, and they form some kind of haze. So it's actually not a cloud. It's just the aerosols themselves.

They're actually little droplets, and they can create some kind of haze that reflects a bit of sunlight and cause some cooling. So in the case of a volcanic corruption, this will only last for, say, two or three years until the aerosols have dissipated and the effect goes away. So then the proposal is to do this deliberately. So instead of relying on volcanoes,

to have some balloons up in the stratosphere that are constantly releasing a certain amount of sulfur and then causing this haze and keeping the climate cooler. And this is really something that would be cheap enough for even individuals to do if they have the resources. We're talking perhaps about some hundreds of millions or perhaps billions, but some people actually have those resources.

And there have already been experiments, and this is very controversial, because then it becomes a question of who decides, and if I want to cool the climate, who's going to stop me from doing that? But then what are the impacts on other people? there's again going to be side effects. It's a question that goes way beyond the issue of just physical climate science. It's really also a political and social science problem to decide

who can decide to do that and when. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're building. Fit for your ambition. First Citizens Bank. Hey, honey, it's mom. Did you know if we switched to Verizon, we can get four, phones for $0.00 plus four lines for $25 a line. Call me back. Me again. That's just $100 a month for four lines on Unlimited Welcome plus four phones no trade in needed. Call me. It's Mom. America's Best Network, Verizon. That's the one we're talking about. I'll send you text. America's Best Network based on Root Metrics, Best Overall Mobile Network Performance, US second half

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name audio creates systems that deliver exceptional sound, and unforgettable listening experiences at home. Try it for yourself at a focal powered by name boutique. Visit focal powered by name.com for more information. From when we're children, we know that clouds make rain. That's where rain comes from. So if we're talking about a world where we can make clouds, does that mean that we can effectively then make rain in areas of drought or in desert?

Would we be able to create more water effectively by creating clouds? Right. So these are actually different problems. So it's much easier to create a cloud than to create rain. Because to create a cloud, of course you need some moisture, you need some water vapor in the first place to create a cloud. So if the air is very dry, no matter how much aerosol you pump into the atmosphere, it's never going to form a cloud. However, over the ocean, there's plenty of water available

to evaporate from the ocean. So usually moisture supply is not the problem. You have enough moisture there. So it's more of a problem of having enough seeds. But then once you form the cloud, it's a really different question as to whether it's going to rain out or not, right? So most clouds actually don't precipitate. They don't produce any rain or snow. And so then if you have a cloud and you wanted to rain or snow, then that's a different problem. And there are some proposals around that as well. There are some experiments, so some ideas of injecting certain types of seeds into the cloud.

So again, the idea is to sort of manipulate the little droplets and ice crystals present in the cloud. And for example, if we somehow were able to force them to grow or to coalesce, you know, to collide with one another and grow into larger droplets, such that eventually they become large enough to fall out of the cloud as rain, then that would promote rain. But to my knowledge, this is unripe. You know, the science is not there yet.

It's still very experimental. And I personally don't think that in most cases we have the power to make clouds rain or not. I think if there's any effect, it's going to be quite small. So we're not going to be able to suddenly turn the desert into a garden or vice versa to stop the rain from happening when we don't want it to happen. When it comes to renewable energy and clouds, there's a bit of a catch-22. Ironically, some of the temperature rise over the last decade has actually been caused by

switching to renewables. There was a news item in the journal Science in January, which pointed out that the decrease in those sulfur particles that you were talking about before in clouds that's causing warming because those particles usually reflect the sun's light away from the parasol effect. Exactly. I'm right. So if we're losing some of those sulfur particles, because of that renewable energy transition, what should we do?

Is that still something that we should be aiming for and how are clouds kind of complicating that choice of clean or unclean fuel? That's a really interesting question, and you're absolutely right. So earlier I said aerosol pollution from human activities cause more clouds because aerosols act as seeds, and we've seen, you know, especially over the second half of the 20th century, an increase in aerosol concentrations and a concomitory. increase in the amount of clouds.

And this has slightly offset the effect of greenhouse gases. So of course, greenhouse gas concentrations have been rising. This has been enhancing the greenhouse effect and causing global warming, but it has been partly offset by the aerosol pollution and its cooling effect, particularly by clouds. Now, what's been happening since, I would say, the 70s or 80s or so, is that increasingly we have put clean air regulations in place. Of course, aerosol pollution is bad for us just from

a pure health standpoint, right? Aerosols, like the pollution goes into our lungs, causes all kinds of health problems, so we don't want them. And we have successfully reduced aerosol emissions, for example, from ships, but also from cars and from all kinds of industries. And as a result, the concentration of aerosol particles in the air globally has been going down, and therefore this has reduced slightly the amount of clouds. And as you just said, this has promoted further warming because it has reduced that parasol cooling. effect of the clouds. Now, I don't think that this is a reason for not switching to renewables. There are many reasons for wanting to move away from fossil fuels. A key one is what I said a

moment ago about health concerns. Aerosol pollution is plain bad for us, and so we want the least possible amount of aerosols for that reason. So let's move on to the kind of other part of your research that's related to all this about net zero emissions. And obviously we've talked about renewable energy there. There's this big question when it comes to net zero about what? the impact will be if we get there, if and when, I should say, because we are kind of slowly moving that way. But it is a huge, incredible effort to do that because of the system that we've created for ourselves that's based on carbon emitting fuels, basically. You have actually made a prediction about what will happen if we are able to get to that point. So could you summarize that for us?

I mean, in essence, the question is, is there any point getting to net zero? What will the impact on global temperatures be? Yeah, I think maybe the first thing to say is why do we even need net zero? Why are we interested in net zero? And the key thing here is that we know human emissions, which are mostly from fossil fuel burning, enhance the greenhouse effect, so increased greenhouse gas concentrations, cause global warming. What we know is the more we emit, the more it warms. And in fact, we know that the amount of global warming is roughly proportional to the cumulative emissions, so meaning the accumulated emissions since the Industrial Revolution. So that means that every gram of extra CO2 we emit leads to further warming.

But the flip side of this is that once we stop emitting, so if we go to net zero emissions, then we can expect global warming to stop getting worse. So it's not a story of suddenly things going back to as they were before. We're not going to go back to the original pre-industrial revolution climate that was colder than today, but at least it's going to stop getting worse. So currently it's getting worse year by year. We're emitting more every year. Global temperature is rising year by year.

That's going to stop happen is our prediction if we get to net zero. So you probably have heard, or many listeners will have heard, I think, of the Paris Agreement. That's a big, you know, landmark in global international climate policy. And one of the things in this international agreement is that we've been, want to strive to 1.5 degrees of global warming, so try and not exceed 1.5 degrees. For context, we're currently at about 1.2 degrees, give or take. So we've got very little headroom. And by the way, these are differences from the pre-industrial climates from, say, 1850 or so. And so what that means is if we want to stay within 1.5 degrees, we have to go to net zero very quickly. And this is

the reason why many countries around the world, including the UK and the EU, for example, have put a net zero policies in place with an aim of achieving this by 2050, because what the science tells us is that given how much we've emitted up to now, we really need to get down to net zero by, say, middle of the century, so around 2050, in order to have a reasonable chance of staying within 1.5 degrees of the pre-industrial climate. Yeah, so that's the context we're in, and that's why net zero is so important. And by the way, when I say net zero by the middle of the century, it doesn't mean that we carry on as normal until 2049, and then suddenly in year 2050 we stop emitting and go to zero. It's rather that we need to decrease our emissions now, starting now, really now,

and quite rapidly year after year, and then reaching net zero around 2050, if we want to have a good chance of staying below 1.5 degrees. All of this research is something that you talk about in your chapter in the new Greta Thunberg book on climate change, your section is specifically on clouds and global temperatures and renewables. So what was it like to be part of such a massive project to bring expert voices like yours together? I think it's very exciting. First of all, obviously, I was very honored to be asked to contribute to the book, and I was very happy to do so. People may have different opinions about Greta Thunberg and her advocacy, but one thing she says, which I fully agree with, is listen to the scientists. And I think my impression,

anyway, is that that was at the heart of the project of the book to give a platform for scientists to say what the science is saying. I think it's really key for citizens and for politicians to have the information to make the right decisions, right? So one thing is to have the scientific facts on the table, and then another thing is to use those facts to make difficult political decisions. And I'm not saying it's an easy thing to transition to net zero. I'm not claiming that, on the contrary. So I think this book hopefully helps to inform citizens, you know, the general public, to know what the scientific challenges are, what the science is telling us. For example, this sort of question that we were discussing, why do we even need net zero and why by 2050, what does it mean? And yeah, so I'm very

excited that this book is hopefully any way contributing to educating the public in that way. I'm throwing this one at you. So I apologize if you haven't had time to think about this. But I wanted to end with a question about clouds again. So we need to. know that there are, or some of us may know that there are actually tons of different types of clouds, and I actually wouldn't be able to throw some names at you, and hopefully you can do so. But I was hoping to hear what your favorite type of cloud is. Oh, I see. Well, I have to be honest with you. My favorite type of cloud is a so-called cumulonimbus cloud. So in case you haven't heard of this, it's a typical thunderstorm cloud. I don't know

if you've noticed. Thunderstorms are not that common in the UK, but they're much more frequent in other parts of the world like the Midwestern US and also in continental Europe a bit more. But maybe on a hot summer day, you may have noticed that sometimes you have these large cauliflower shaped clouds forming. And then when they grow enough, they become deep enough so they reach high enough into the atmosphere that they hit some kind of ceiling. That's a layer known as the tropopause. And for reasons that I don't have time to go into, that's like lit. So the clouds can grow past that. And so what happens instead is they start spreading sideways. And they form something that's shaped like an anvil. So we sometimes call it.

these anvil clouds. And when you look at them from a distance, they look very spectacular. You know, they have this very impressive vertical structure, flat top, spreading sideways. And of course, when you're under them, it's maybe not as much fun. But seen from a distance, I think they just make me marvel at the beauty of nature. That was Dr. Paolo Cheppe on the role of clouds in climate change and live after net zero. Thank you for listening to this episode of Instant genius brought to you by the team behind bbc science focus magazine by the latest issue of science focus in your favorite shop or visit us online at sciencefocus.com this podcast is sponsored by name audio and focal the texture and emotional depth of music can be lost through digital sources or poor signal

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