Study: Global warming slowing ocean currents


Published: Thursday, December 1, 2005 at 6:01 a.m.
Last Modified: Thursday, December 1, 2005 at 12:00 a.m.
The powerful ocean currents that transport heat around the globe and keep northern Europe's weather relatively mild appear to be weakening - a likely and problematic consequence of global warming - a new study by British scientists has concluded.
The currents, like mighty rivers flowing at different depths of the ocean, act like radiator pipes to carry warmth from the tropics to northern latitudes. The best known is the Gulf Stream, which carries warm water from the Gulf of Mexico to the coasts of Britain and France. Other currents return colder water from the poles.
With the prevailing winds, the currents warm the climate of much of Europe by several degrees. Rome, for instance, is at a latitude similar to Boston and would be much cooler if not for the warmth coming from the sea.
In the new study, published today in the journal Nature, a group of British oceanographers surveyed a section of the Atlantic Ocean stretching from Africa to the Bahamas that has been studied periodically since 1957. They found the overall movement of water had slowed 30 percent in the past five decades, particularly in the flow of cold water back to the south. The report is the first evidence of such a slowdown.
"The result is alarming," Detlef Quadfasel, a climate expert at the University of Munich, wrote in a commentary accompanying the research. The findings provide "worrying support for computer models" that have predicted that global warming could disrupt the way the planet regulates heat, he said.
Computer models have long predicted that the warming of the oceans and the "freshening" of the seas with water from melting glaciers and increased precipitation - all linked to warming of the Earth by greenhouse gases - could slow the currents. But scientists did not expect to see such changes so soon.
Scientists differ on the potential effect. Some say weaker currents would cool Europe by several degrees, causing problems for agriculture and ecosystems and ushering in far more severe winters. Others say the cooling would likely balance out the effect of global warming in Europe, which is expected to raise temperatures globally by several degrees over the next century.
"My personal guess is there would be no overall cooling, just a slowdown of the warming," Quadfasel said in an interview.
In the past, the currents have stopped completely - most recently 8,200 years ago, climate records show. When that happened, temperatures in parts of Europe dropped by 10 and sometimes 20 degrees within a decade, said Richard Alley, an expert on abrupt climate change at Penn State University.
The episode happened when a large ice-dammed lake in North America suddenly drained and freshened the Arctic, he said.
A complete shutdown of the currents is considered a "low probability, high impact" scenario that could cause up to 10 degrees of cooling in Europe. Most computer models, however, show the currents would slow and not stop completely, even when much larger amounts of greenhouse gases occur in the atmosphere.
Terrence Joyce, a physical oceanographer at the Woods Hole Oceanographic Institution, is among scientists concerned about the effect of global warming on ocean currents and the abrupt changes to temperature and precipitation that could follow. But he remains unconvinced by the new report. "I think the case still remains open," he said. The reason for the doubt is that gauging the flow of currents is extremely difficult, and, like all measurements, subject to error.
The oceanographers are trying to gauge changes in water in massive currents that in some cases move volumes of water equivalent to 150 Amazon rivers.
The new measurements, taken by Harry L. Bryden and two colleagues at England's National Oceanography Centre, show that while the northward flow of warm water in the Gulf Stream has not changed, the return flow of cold water south has slowed by 50 percent, and more water is now recirculating back toward the Bahamas instead of flowing north to make a full circuit of the current system.
The authors acknowledge that because of the measurement difficulties, it is possible that the current has not slowed as much as they report, and that some of the apparent change is caused by errors due to eddies and other perturbations that can affect ocean currents.
But they say they are convinced they are seeing real slowing because of changes in temperature and density of the water that would lead to slowing and because the changes appears to be occurring at a depth where computer models predicted they would be found.
Joyce agrees that the system appears to be more sluggish, but said it looks like most of the slowing occurred since 1992, not over the past five decades as the authors of the report suggest. The changes, he said, could be due to some shorter-term natural variation in the ocean and not climate change.
"We need 10 more years of data to get compelling evidence of a long term trend," he said.
Those who are persuaded by the new data say evidence of a slowing ocean circulation system is buttressed by other observations, including the fact that seawater in the North Atlantic has been growing less salty in past decades.
The salinity of sea water is key. Salty water is denser and sinks. Currently, cold, salty water in the northern Atlantic sinks near the poles, moves toward the bottom of the ocean and begins its journey back to the south. But as the ocean water becomes diluted with fresh water, it will sink less, potentially blocking the recirculation.

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