HERE’S THE THING about the stratosphere, the region between six and 31 miles up in the sky: If you really wanted to, you could turn it pink. Or green. Or what have you. If you sprayed some colorant up there, stratospheric winds would blow the material until it wrapped around the globe. After a year or two, it would fade, and the sky would go back to being blue. Neat little prank.

This is the idea behind a solar geoengineering technique known as stratospheric aerosol injection, only instead of a pigment, engineers would spray a sulfate that bounces some of the sun’s radiation back into space, an attempt at cooling the planet. It’s the same principle behind a supervolcano loading the stratosphere with aerosols and blocking out the sun. And it, too, would rely on those winds distributing the material evenly. “If you do it in one place, it’s going to affect the whole planet,” says climate scientist Kate Ricke, who studies the intersection of geoengineering, human behavior, and economics at Scripps Institution of Oceanography. “Not just because you’ve cooled down and changed the global energy balance, but because the particles spread out.”

While it’s not likely that someone will colorize the atmosphere anytime soon, it’s getting increasingly likely that someone will decide it’s time for stratospheric aerosol injection. Emissions are not declining at anywhere near the rate needed to keep global temperatures from rising 1.5 degrees Celsius above preindustrial levels, and the climate crisis is worsening. 

But the science isn’t ready. This anthropogenic geoengineering might trigger unintended effects, like droughts in certain regions and massive storms in others. Plus, if engineers abruptly stopped spraying aerosols in the atmosphere, temperatures would swing back to where they started, potentially imperiling crops and species. 

Still, stratospheric aerosol injection would be fairly cheap. And there’s nothing stopping countries from unilaterally deciding to spray their airspace, even though those materials would ultimately spread around the globe. “I just have a hard time seeing with the economics of it how it doesn’t happen,” says Ricke. “To me, that means that it’s really urgent to do more research.” 

WIRED sat down with Ricke to talk about the allure and potential pitfalls of geoengineering, what makes it so politically perilous, and how scientists can make sense of it—for the good of humanity and the planet. The conversation has been condensed and edited for clarity.

Can you give me an idea of the scale that we’d be talking about with solar geoengineering—both spatial scales and timescales?

Kate Ricke: Let’s say you want to start geoengineering today to stabilize global temperatures where we are, or maybe bring it down a little bit. You basically need a fleet of airplanes that can reach the stratosphere. We’re talking on the scale of maybe tens to hundreds of airplanes, and the capability to spray aerosol precursors. 

But the way that the stratosphere works is that once you get up there, stratospheric winds take things around the planet relatively quickly in bands of latitude. And then slowly over time, on the timescale of months, things sort of migrate in general from equatorial regions up toward the poles, and then particles fall out near the poles. So you wouldn’t need to be flying through the whole stratosphere spraying stuff. The stratosphere does a lot of work to spread it out. And that’s part of the reason why you can’t do stratospheric geoengineering over just one area.

Would we notice this? Visually, would we see anything? 

KR: Yes, on an absolute scale. It changes the ratio of direct and diffuse radiation. So the idea is the sky would on average become a little bit whiter, and, for example, sunsets would become a bit more vivid. It’s definitely much smaller than the difference between going from the desert in California to the city. The white skies thing is also not, in my opinion, probably the biggest problem.

What about any concerns about toxicology? Is this stuff benign to living creatures on Earth? 

KR: It’s not benign—it’s the same stuff that comes out of power plants. Large concentrations of it in one area makes people and crops sick. But, in terms of the scale, the amount you need in the stratosphere is way, way smaller than what we emit from power plants, and it’s spread out over the planet. 

People have done some studies on this, too, and it seems like probably the biggest risk from the particles would be to sort of sensitive high-latitude ecosystems—so polar ecosystems that don’t get very much exposure to urban pollution right now, but would get more from this. Especially because the particles move towards the poles, generally, before they precipitate out of the stratosphere. 

Say a country unilaterally says, ‘We’re going to do this.’ They want to cool down their own country by spraying the stratosphere, and it doesn’t matter if it’s going to wrap around the planet.

KR: Legally, it’s complicated, because countries own their airspace up to space, basically. It’s a little ambiguous. So people could spray stuff over their country, and it would go everywhere. And then [the particles] stay in the atmosphere for on average about a year and a half. They spread out and the radiative effects take effect immediately. That’s why after a large volcanic eruption, you see a dip in the global temperature immediately that persists for about a year to two years and then drops off again. So you wouldn’t need to be spraying stuff every day, necessarily. If you stopped doing it for two years, the effect would go away.

I’m having a hard time seeing how we’re not going to do it at this point, actually, because it’s so inexpensive. Already the impacts of climate change are looking to be so disruptive that I don’t see in this world how such a low-expense solution doesn’t get implemented by someone. There’s just nothing else in the world that can cool the planet as quickly. Even if we started rapidly decarbonizing and taking CO2 out of the atmosphere, it’s still a decade timescale for consequences. Whereas blocking sunlight, the climate response starts right away. 

I’ve seen some modeling that if you were to suddenly stop solar geoengineering, you’d have a problem with temperatures dramatically climbing and imperiling species.

KR: If the program got disrupted, and we were blocking a lot of warming with stratospheric geoengineering, you would get this really rapid warming if someone stopped doing it. I mean, it would be catastrophic if we stopped treating our drinking water too, right? There’s things that humans do that we need to keep doing, or it’s catastrophic. 

The technology’s not so complicated that we would need just the person who developed the technology to be the one to keep doing it. And so I’m a little skeptical about that argument being the biggest issue, because we already basically know how to do it. It’s within the grasp of a medium-sized country or something. The resources are substantial enough that it would be hard for a single individual, or a very small country to do it. But it’s not like nuclear weapons or something like that. 

Are we getting to the point where the science is sound enough that we can begin to make these decisions? And is that even going to be possible, given the general lack of cooperation on an international scale?

KR: There might be some technical experts, like me or other people who have worked on this, that would say: ‘Yes, I’ve seen enough to believe it.’ But in order to have collective decision-making at the global scale, you need science that’s viewed as legitimate by everyone. Not everyone, but a lot of people. And we’re not there, by a longshot, with geoengineering.

But that’s why we need more research. And we need more diversity of who’s doing research and where, because the results are going to need to be viewed as legitimate by a much broader group of people. They’re not right now. That’s definitely not true. 

Why not?

KR: Because it’s been a small group of mostly elite university white dudes in North America and Europe who’ve done all the research. And people just don’t automatically trust a small group of elites like that. It’s actually important that the ministry of the environment in Bangladesh has someone who’s Bangladeshi talking to them about geoengineering science. So that, I think, is the biggest problem with the science right now. You can look at certain areas of climate science and you see we’re saying the same thing over and over and over again. But there has been some value to that, too—replication and repetition. It builds consensus, and it builds trust in the science.

Country-scale commitments to reducing emissions are one thing, but this involves everybody simultaneously because we share one atmosphere. Is there going to be agreement on that?

KR: We’re not there where we can have global consensus about geoengineering, not by a longshot. But I would guess it’s more likely that this will happen not with a global consensus. Certainly, there are some actors that, if they did it, would be constrained by more powerful actors. But there are definitely other major actors in the world that already exist that could do geoengineering and get away with it. Because the alternative is: Is it bad enough for you that you’re willing to go to war over it?

What about the moral hazard? Wouldn’t geoengineering make it less urgent to reduce emissions?

KR: The moral hazard is a totally valid concern, and it’s a big one. In terms of the existing empirical research, the results are very mixed. It doesn’t seem like [for] individual humans, when you put them in behavioral experiments, that a moral hazard around geoengineering exists. Telling people about geoengineering in a controlled way tends to make people want to mitigate greenhouse gases more, because people think geoengineering is kind of nuts and scary. They see it as an indicator that climate change is a big problem.

This is me editorializing about my fellow climate scientists, but I think most climate scientists don’t like the idea of geoengineering. And the reason they still don’t like it the most is because of the moral hazard. They think we’ve got to tell people ‘This is a bad idea’ as long as possible because of that. And they’re probably right. But the risk is that if things get bad enough with climate change, people are going to do geoengineering anyway, and we’re not going to be ready to do it.