This is ALL THINGS CONSIDERED from NPR News. I'm Robert Siegel.


And I'm Guy Raz. When we burn fossil fuels, we add carbon dioxide to the atmosphere and carbon dioxide contributes to global warming. But the world's smokestacks also emit pollutants that offset some of that warming. A new study shows that cleaning up air pollution could actually heat the planet more than scientists had previously thought.

NPR's Richard Harris has this report.

RICHARD HARRIS, BYLINE: Power plants not only produce carbon dioxide, they spew soot, as well as compounds of nitrogen and sulfur and other stuff into the air. It's a complicated soup of material known as aerosols. And Natalie Mahowald at Cornell University says, so far, scientists have mostly tried to understand what those aerosols do while they're actually in the air.

NATALIE MAHOWALD: There are so many different kinds of aerosols and they have many different sources, and some warm and some cool, but in the net, humans are emitting a lot of extra aerosols and it seems to be that they tend to cool for the most part.

HARRIS: The aerosols reflect sunlight back into space or they stimulate clouds that keep us cool, but it turns out that's not all they do. Mahowald says these aerosols also influence how much carbon dioxide gets drawn out of the air by plants, on land and in the sea.

MAHOWALD: They can add nutrients, for example, to the oceans or to the land, but also while they were in the atmosphere, they changed the climate and so that also can impact how much carbon the land or the ocean can take up. So there's quite a few different ways that aerosols can interact.

HARRIS: In an article published in Science magazine, she concludes that those effects add up to quite a bit, which means, at the moment, aerosols are not only helping reduce global warming by cooling the atmosphere, but they're helping reduce the amount of carbon dioxide that stays in the air once we emit it.

That's good news for now, it means the planet isn't heating up quite as fast as it could be, but it's bad news looking down the road a little bit. That's because many aerosols make people sick, heart and lung disease, in particular. So some nations are now in the process of trying to rein them in.

MAHOWALD: As we clean up the aerosols, which we really want to do for public health reasons, we're going to be, perhaps, causing ourselves more trouble in terms of the climate situation.

HARRIS: This is not a brand new idea. For example, other research has found that, if we switch from coal to much cleaner natural gas, that might not do much to help with global warming because we'd also be reducing the pollutants in coal smoke that help offset warming.

Mahowald's results suggest that reducing those pollutants could be an even bigger problem than we'd realized when you consider that aerosols help remove carbon dioxide from the air by encouraging plant growth. Hard numbers on this effect are highly uncertain at the moment, but this could turn out to be quite significant.

MAHOWALD: This is something that is really poorly studied. I think that's the main point of the paper is we're really been ignoring this potentially important topic.

HARRIS: And studying it is not easy because the effects aren't well understood. For example, nitrogen can be a fertilizer, but it can stunt plant growth when nitrogen comes out of the air in acid form.

Lisa Emberson at the Stockholm Environment Institute studies those biological cycles and she says there are so many subtle effects, it's hard to be sure which ones will prove to be the most important.

LISA EMBERSON: I think a message of this paper is that we need to understand those interactions far better and we probably need to take action much more quickly than we are doing at the moment.

HARRIS: Right now, it seems we're much more likely to clean up aerosol pollution while increasing the amount of carbon dioxide in the air. So scientists, unfortunately, may have a chance to see how this inadvertent experiment on our planet starts to play out.

Richard Harris, NPR News.

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