for National Geographic News
In the 2004 eco-disaster film The Day After Tomorrow, Europe and North America are gripped by a deep freeze after global warming halts the circulation of a North Atlantic ocean current. The film is pure Hollywood hyperbole.
But some scientists say the current is vulnerable to rising temperatures.
Acting like a conveyor belt, the current transports warm, surface waters toward the Poles and cold, deep waters toward the Equator.
In the Atlantic Ocean, these warm surface waters push northward, releasing heat into the atmosphere and becoming cooler and denser. As they do, the waters sink and flow southward in the deep ocean.
"The Atlantic circulation moves heat toward the Arctic, and this helps moderate wintertime temperatures in the high-latitude Northern Hemisphere," said Ruth Curry, a physical oceanography research specialist at the Woods Hole Oceanographic Institution on Cape Cod, Massachusetts.
Curry noted that excessive amounts of freshwater dumped into the North Atlantic could alter seawater density and, in time, affect the flow of the North Atlantic ocean current. (Global warming has boosted freshwater runoff in the form of glacier meltwater and additional precipitation, Curry said.)
Just how much extra freshwater it would take to alter the circulation system, known as the Atlantic Meridional Overturning Circulation, is a gray area of climate science.
Suffice it to say that the conveyor belt continues to work today. But freshwater runoff into the North Atlantic has increased in recent decades, and runoff is expected to increase further as global temperatures climb higher, Curry said.
Curry and research colleague Cecilie Mauritzen of the Norwegian Meteorological Institute in Oslo estimate that it will take about a century, at present rates, for the circulation pattern to be seriously affected by the increase in freshwater runoff.
The scientists conclude that it would take about two centuries for freshwater runoff to halt the North Atlantic conveyor belt entirely. Curry and Mauritzen published their findings in the June 17 issue of the research journal Science.
Stefan Rahmstorf, a professor of ocean physics at Potsdam University in Germany, said the researchers' calculations appear accurate.
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