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.
But he emphasized that their findings do not provide a forecast; they only give an impression of the amount of freshwater required to alter or halt the North Atlantic ocean current.
"It is of course very unlikely that the freshening [freshwater inflow] will simply continue at the same rate it has for the past few decades," he wrote in an e-mail to National Geographic News.
Rahmstorf said the freshening could be part of a natural fluctuation in Earth's climate system that will stop and reverse.
He added, however, that if the phenomenon is due to global warming, which he said is likely, then the freshening will probably accelerate as glaciers melt and more rain falls at high latitudes in response to rising temperatures.
According to Curry, scientists are uncertain as to the exact course global warming will take and how it will affect the amount of freshwater flowing in the North Atlantic. A particular wild card, she noted, is Greenland.
"As it does melt, it will release freshwater into the Nordic seas"—water bodies found between Iceland, Greenland, and Norway—"and that probably represents the biggest source of freshwater that could have an impact on the conveyor belt," she said.
There are a number of mechanisms that could inject large amounts of freshwater into the Nordic seas at the precise region that is critical to the conveyor belt. They include
• pooling and release of glacial meltwater,
• collapse of an ice shelf followed by a surge in glacier movement, or
• lubrication of a glacier's base through increased melting.
According to an unpublished survey by Potsdam University researchers Kirsten Zickfeld and Anders Levermann, expert scientific opinion varies widely on the likelihood that excess freshwater runoff from the Arctic will alter the North Atlantic conveyor belt in this century.
Some scientists consulted for the survey said there is no chance that the current will break down. Others estimated that the chance of a complete shutdown exceeds 50 percent if global warming climbs by 7.2° to 9° Fahrenheit (4° to 5° Celsius) by 2100.
Rahmstorf believes the chance of a circulation shutdown is as high as 30 percent. He said any possibility of such a scenario, even if slight, is cause for concern.
"Nobody would accept expanding nuclear power if there was a 5 percent risk of a major accident," he said. "Why would we accept expanding oil and coal power if there is a 5 percent risk of a major climate accident?"