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A potential climate solution in the works: reflecting sunlight back into space

AYESHA RASCOE, HOST:

With leaders from around the world gathered in Dubai for the U.N. climate summit, we wanted to discuss an idea that might sound more like science fiction than science. Could humans cool the planet by reflecting sunlight away from Earth? It's called solar geoengineering, and it is something that scientists are working on, even if we don't hear much about it. Peter Irvine is a climate scientist and teaches at University College London. Welcome to the program.

PETER IRVINE: Thanks for having me.

RASCOE: So can you give us a better sense of what solar geoengineering is?

IRVINE: Solar geoengineering, yeah, as you said, describes a set of ideas to increase the amount of light that the Earth reflects back to space. And that could counter the warming effect of the greenhouse gases we've been adding ourselves. There's a couple of different ideas. The most promising one would aim to mimic the effect of volcanic eruptions.

So some of the biggest eruptions in history, like Tambora in 1815, Pinatubo in 1991 - the big eruption goes off, and the very powerful ones penetrate up into the stratosphere, which is a high layer in the atmosphere. And there the particles that they release scatter around the entire planet and then reflect light away for a couple of years. And in the years after these big eruptions, we see temperatures dip and then slowly recover. And the idea is that we could create that effect, but persistently, to help offset future global warming.

RASCOE: So how would this work?

IRVINE: Yeah. So volcanoes put up a few million tons - the big ones that have this cooling effect put up a few million tons of this gas, sulfur dioxide, into the stratosphere. You could get an aircraft to fly that high, dump this gas and come back down. And with enough of them, you could get millions of tons up. We're talking tens to hundreds of aircraft, so it's not a huge amount.

And every engineering assessment today comes to the same conclusion. This is feasible and relatively cheap. We could offset basically the difference between where we're headed, which seems to be something like 2 1/2 Celsius of global warming, to where we want to be, which is 1 1/2, or to limit it to that much, at about $20 billion per year, which is a lot of money, but not a lot of money compared to the costs of climate change and the costs of dealing with it.

RASCOE: How would sulfur dioxide cool the planet? Like, how does that work?

IRVINE: Just like the volcanic eruption, we would be adding this sulfur dioxide gas, which goes on to react in the atmosphere to form these really tiny droplets. And they're so tiny that they effectively stay suspended in the air. And now, this happens in the lower atmosphere. These little particles are responsible for the haze you see on many days. And they stay suspended until they get caught up in rain clouds and rained out.

But up in the upper atmosphere, it's very dry. There's no clouds. And so these particles persist for several years until they gradually, slowly fall down. And so yeah, this relatively small amount of material that we could put into the upper atmosphere would have this disproportionately large cooling effect.

RASCOE: What is the risk from, you know, releasing the sulfur dioxide?

IRVINE: Well, I think the first thing to note is this is not an alternative to cutting greenhouse gas emissions. The CO2 would still be there. It'd still be having this warming effect on the planet. And this would only be a temporary cooling effect to sort of offset some of the consequences.

It would also have some side effects similar - well, the same side effects that volcanoes do. The sulfur dioxide adds to acid rain a little. It would delay the recovery of the ozone hole by some time, and it would make the sky a little hazier. But yeah, it would have these side effects, and they're not good. But the question is, it seems that they're a lot less bad than climate change would be.

RASCOE: So, you know, bottom line - like, might this become a reality someday? And, like, would all the countries have to get together - major countries have to get together and agree to it? Because it seems like it would affect the entire planet.

IRVINE: Yeah, absolutely. You couldn't just create this patch of haze above your own country. It would affect the entire planet - this stratospheric aerosol engineering idea. And so, yeah, the whole world is at stake. The whole world should be involved, I think, in decisions around this.

At the minute, we're a long, long way from that. We don't have the planes that could do this. And we haven't done enough research to be confident that it can work in the ways I'm suggesting it could. So I think we're - yeah, we're - there's a lot of scientific work to do to sort of establish whether this is a good idea and then a lot of careful discussion and debate to figure out whether and how we can cooperate to pull it off.

RASCOE: That's Peter Irvine of University College London. Thank you so much for joining us.

IRVINE: No problem. Thank you. Transcript provided by NPR, Copyright NPR.

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Ayesha Rascoe is a White House correspondent for NPR. She is currently covering her third presidential administration. Rascoe's White House coverage has included a number of high profile foreign trips, including President Trump's 2019 summit with North Korean leader Kim Jong Un in Hanoi, Vietnam, and President Obama's final NATO summit in Warsaw, Poland in 2016. As a part of the White House team, she's also a regular on the NPR Politics Podcast.