Scott Rutherford only reluctantly brings up the far-fetched disaster movie “The Day After Tomorrow” to explain a recent research paper he co-authored that sheds new light on one effect of climate change.

Of course, the decade-old film about an apocalyptic cooling of the planet was a huge exaggeration, the associate professor of environmental science at Roger Williams University says, but “the premise of that is what we’re talking about here.”

What he and the other members of a team of researchers from the United States, Denmark and Germany are talking about is a significant slowdown in the overturning of the Atlantic Ocean, the natural system of currents, including the Gulf Stream, that pumps warm water north and cold water south.

According to their peer-reviewed article published in the journal Nature Climate Change, there was a rapid decrease in circulation between 1975 and 1995, the likes of which is unprecedented in at least a millennium.

And that slowdown could explain why the North Atlantic is one of the only regions in the world that has shown signs of cooling as temperatures elsewhere go up. As the study’s lead author, Stefan Rahmstorf, of the Potsdam Institute for Climate Impact Research in Germany, points out in a recent blog post, last winter was globally the warmest on record, yet for the North Atlantic, it was the coldest on record.

Differences in water density power the Atlantic overturning. Warm, lighter water flows north, pushing cold, heavier water down as it flows south in a regular circulation. If currents slow down enough, there could be widespread effects.

“Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas,” Rahmstorf said in a statement. “A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in the region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe.”

The cause of the recent slowdown appears to be the continued melting of the Greenland ice sheet. Freshwater from the melting ice is not as dense as saltwater, so it’s harder to sink and, as a result, harder to generate the necessary overturning to keep water moving in the Atlantic.

Other research had suggested a slowdown of the Atlantic overturning, but no previous studies had put the recent changes in a historical context.

Rutherford reconstructed temperature data as far back as reliably possible — about 1,000 years — to compare the recent slowdown with past changes in the circulation of the Atlantic Ocean. Some slowing and speeding up of the currents is normal.

Although there were changes in the past, such as a slowdown in the early 1600s, they could be accounted for through natural forces. But the model that Rutherford, Rahmstorf and researchers at Pennsylvania State University and the Geological Survey of Denmark and Greenland created could not explain the recent slowdown through natural variability. That suggests that man-made carbon emissions were to blame.

The circulation recovered in part after 1995, but in the last few years it has slowed down again, Rutherford says. And there’s no reason to think it won’t continue to decrease.

But if global warming is responsible for the slowdown, and temperatures still rose in the period after 1995, how do the researchers explain the partial recovery?

Rutherford says that although global mean temperatures are increasing, the effects of climate change have to be considered on a local basis. Quirks in weather patterns determine those local effects, and can help account for the temporary pickup in circulation. But overall, the data suggests that over the longer term the currents are slowing down.

So is it possible that in the real world the overturning could stop altogether like it did in the fictional world of “The Day After Tomorrow” and its depiction of a modern Ice Age?

Rutherford says the circulation did shut down 12,000 years ago during the Younger Dryas period — colloquially known as the “Big Freeze” — when temperatures in the Northern Hemisphere plummeted abruptly.

“Is that a possibility? Yes,” says Rutherford. “Is it likely? No.”

Still, he points to weather effects that may not be as dramatic or devastating but are still altering the way we live. There’s the drought in California, for one. And this past winter in New England, the seemingly never-ending snowfall is another example of extreme weather that may be due to climate change.

“That unusual winter may not be so unusual,” Rutherford says. “It’s consistent with what we would expect to see." 


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