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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

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And I'm Audie Cornish.

The Atlantic Ocean, especially the North Atlantic, is peculiar every few decades the average temperature of surface water there changes dramatically. Scientists want to know why that is, especially because these temperature shifts affect the weather. New research suggests that human activity is part of the cause.

NPR's Christopher Joyce reports on new research that suggests human activity is part of the cause.

CHRISTOPHER JOYCE, BYLINE: Scientists originally thought that maybe some mysterious pattern in deep ocean currents creates this temperature seesaw, like an invisible hand stirring a giant bathtub. And that may be part of it. But there's a new idea: The cause isn't in the water; it's above it. It's a kind of air pollution called aerosols.

Ben Booth, at Britain's Met Office Hadley Center, explains that aerosols create clouds.

DR. BEN BOOTH: The more aerosols you have, the more places there are for water vapor to condense.

JOYCE: They're like seething clouds.

BOOTH: Yes, and what aerosols do is they cool.

JOYCE: They cool the ocean because clouds reflect sunlight back into space before it can hit the ocean.

Aerosols are fine particles like soot or sulfur compounds, mostly from burning fuel. They seed a kind of cloud that's especially good at reflecting solar radiation back into space. Even on their own, without clouds, these aerosols are kind of like sun block.

Volcanoes create aerosols too, but air pollution appears to produce more and then they sweep across the Atlantic sky.

Booth has calculated their effect on sea surface temperature swings.

BOOTH: If you combine the role of the volcanic activity and the human emissions of aerosols, we account for 76 percent of the total variation in sea surface temperature in our study.

JOYCE: That's huge.

BOOTH: Yes.

JOYCE: Booth and his colleagues aren't the first to propose that aerosols influence sea surface temperatures. But climate scientist Amato Evan, at the University of Virginia, says they've done the most thorough job to date of tracking and confirming those changes.

AMATO EVAN: If they're right, human activity has a huge influence on just so many climate processes around the Atlantic Ocean.

JOYCE: Surface temperatures around the Atlantic influence the amount and timing of rainfall in West Africa and in the Amazon in South America, and whether there's drought there. They affect the number and strength of Atlantic hurricanes if, as Evan says, they're right.

Booth used computer models to analyze a very complicated process. The models' predictions didn't match all the changes people have actually observed in the Atlantic. Evan says scientists need more hard evidence to nail down exactly how aerosols affect oceans, but Evans says he's observed a similar process going on in the Indian Ocean.

EVAN: The same type of release of pollution aerosols coming from the Indian subcontinent is actually changing the monsoon.

JOYCE: The monsoon is a pattern of rainy and dry weather in the Indian Ocean.

The new research appears in the journal Nature. It could foretell a warmer Atlantic. Warmer because the aerosol pollution has apparently cooled the Atlantic, but new pollution controls are reducing the amount of those aerosols. And that's good for public health but it also means the ocean loses its sun block.

Christopher Joyce, NPR News.

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