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.
Scientists originally thought that maybe some mysterious pattern in deep-ocean currents, such as an invisible hand stirring a giant bathtub, created this temperature see-saw.
And that may be part of it. But there's a new idea: The cause isn't in the water; it's above it — a kind of air pollution called aerosols.
NASA Earth Observations
This NASA map shows the size of aerosol particles in the atmosphere. Green areas indicate larger, more naturally occurring particles like dust. Red areas indicate smaller aerosol particles, which can come from fossil fuels and fires. Yellow areas indicate a mix of large and small particles.
NASA Earth Observations
Click to see a high-resolution version of this image
Ben Booth, a climate scientist at Britain's Met Office Hadley Center, says that aerosols create clouds.
"The more aerosols you have, the more places there are for water vapor to condense," he says. "And so what aerosols do is they cool."
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 act like sunblock.
Volcanoes create aerosols, too, but air pollution appears to produce more, and then the aerosols sweep across the Atlantic sky.
Booth has calculated their effect on sea surface temperature swings.
"If you combine the role of volcanic activity and the human emissions of aerosols, we account for 76 percent of the total variation in sea surface temperature in our study," Booth says. That's a huge amount.
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.
"If they're right, human activity has a huge influence on just so many climate processes around the Atlantic Ocean," he says.
Surface temperatures around the Atlantic influence the amount and timing of rainfall in West Africa and the Amazon in South America, and whether there's drought there. They affect the number and strength of Atlantic hurricanes and even where hurricanes go.
That's if, as Evan says, Booth and his team are right.
Booth used computer models to analyze a very complicated process — the interaction of ocean and atmosphere over many decades. 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 he's observed a similar process going on in the Indian Ocean.
"The same type of release of pollution aerosols coming from the Indian subcontinent is actually changing the monsoon," he says, referring to the pattern of rainy and dry weather in the Indian Ocean.
The new research appears in the journal Nature. If it's confirmed, it could foretell a warmer Atlantic, because the aerosol pollution has apparently cooled the Atlantic some. But new pollution controls are reducing the amount of those aerosols — that's good for public health, but it also means the ocean loses its sunblock.