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In the 2004 blockbuster "The Day After Tomorrow," the world faced a doomsday scenario: Global warming was causing the Earth's mighty ocean currents to shut down, bringing about a new ice age.
The apocalyptic plot was something cooked up by Hollywood, but its origins are steeped in reality — and it's happening now, scientists say.
A major system of currents in the Atlantic Ocean that carries millions of cubic meters of water north each day is slowing down, according to a new peer-reviewed study published in the journal Nature. That system is known as the Atlantic Meridional Overturning Circulation, or AMOC.
"We know somewhere out there is a tipping point where this current system is likely to break down," said study co-author Stefan Rahmstorf, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany. "We still don't know how far away or close to this tipping point we might be. ... This is uncharted territory."
Think of the movement as a large conveyor belt: The currents ferry warm water north from the Gulf toward the North Pole. There, the water gets colder and then is redistributed back south via the deep ocean.
But scientists say climate change is disrupting the belt by melting the ice in the Arctic, including from Greenland. That means freshwater, which is less dense than salt water, is draining into the ocean and slowing down the currents. That would spell trouble for parts of the world such as the East Coast of the United States, which is susceptible to rising sea levels, and Europe, which could experience more extreme winters and stormier weather.
A change in the ocean's circulation could also be disruptive to U.S. fisheries, which studies have shown are being devastated by warming waters.
The latest study suggests that the weakening current is at a record low that's unprecedented in the past 1,600 years, although it's unclear when exactly the decline began.
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Since the middle of the 20th century, the speed at which the ocean moves water in the AMOC has dropped 15 percent, the study found, using cold subpolar water temperatures as an indirect measurement. And it has plummeted in recent years, the study concluded.
Understanding how ocean currents move is important, said Alexey Fedorov, a climate scientist at the Yale Department of Geology and Geophysics who co-authored a study last year about the relationship between Arctic ice loss and the AMOC.
He said that climate models have predicted a slowdown of the currents by the end of the 21st century and a potential collapse in the 22nd century — affecting humans' not-so-distant generations.
"There is a growing concern that the models may be underestimating the risk of the AMOC collapse, and it can happen much sooner than anticipated," Fedorov said.
Reducing carbon emissions, however, could play a beneficial role, he added.
The Potsdam study noted that direct, continuous measurements of the AMOC have only been available for a little more than a decade and that longer-term evolution of the AMOC must be reconstructed.
Not all scientists agree entirely with the study's findings, however, indicating there's more research to be done.
MIT's Carl Wunsch told The Associated Press that the paper's "assertions of weakening are conceivable, but unsupported by any data."
Kevin Trenberth of the National Center for Atmospheric Research said his recent work faults regular cycles in the atmosphere more than the ocean. He said the Potsdam study doesn't explain year to year variability, while atmospheric cycles do.
Jeffrey Dukes, the director of the Climate Change Research Center at Purdue University, said these studies deserve attention and it's a "big deal" that the AMOC appears to be more feeble. Still, he cautioned that a series of cataclysmic events wouldn't just happen swiftly and suddenly.
"It's not like Europe is going to freeze over next week," he said.