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 as 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. Paris, for instance, is at a latitude similar to northern Maine 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% in the past five decades, particularly in the flow of cold water back to the south.
The findings are the first evidence of such a slowdown.
“The result is alarming,” Detlef Quadfasel, a climate expert at the University of Hamburg, wrote in a commentary accompanying the research. The findings provide “worrying support for computer models” predicting that global warming could disrupt the way the planet regulates heat, he said.
Computer models have long predicted that warming of the oceans and “freshening” of the seas with water from melting glaciers and increased precipitation -- all linked to warming of the Earth by greenhouse gases -- could slow down 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 probably 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 current has stopped completely -- most recently 8,200 years ago, climate records show, when a large ice-dammed lake in North America suddenly drained and freshened the Arctic, said Richard Alley, an expert at Penn State University on abrupt climate change. When that happened, temperatures in parts of Europe dropped by 10 and sometimes 20 degrees within a decade, Alley said.
Worldwide catastrophe brought on by a stopped current was portrayed in the 2004 movie “The Day After Tomorrow.” No scientists expect such a far-fetched scenario, even if the ocean currents do stop.
A complete shutdown of the current is considered a “low probability, high impact” scenario that could cause up to 10 degrees of cooling in Europe within a decade.
Most computer models, however, show the current would slow but not stop, 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 the 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 that the new changes are caused by global warming. “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 although the northward flow of warm water in the Gulf Stream has not changed, the return flow of cold water south has slowed by 50%, and more water is 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 appear 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 report’s authors 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 support 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 less salinity than in past decades.
The salinity of seawater is a key factor. Salt increases water density, causing salty water to sink. 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 freshened from melting ice and increased runoff, it will sink more slowly, potentially blocking the recirculation.
In June, Ruth Curry, a physical oceanographer at Woods Hole, published a paper showing that an additional 19,000 cubic kilometers of fresh water -- nearly four Amazon Rivers’ worth -- had entered the North Atlantic since 1960. Historically, 5,000 cubic kilometers entered the region each year.
The excess water is coming from additional precipitation, higher river runoff and melting ice from glaciers and ice sheets, she said.
Curry expects that the freshening she has measured in the North Atlantic, and the warming of the waters there will slow ocean circulation in the future. But she’s not convinced that the slowing has already occurred.
“This is not a certainty, but it could be a prelude,” Curry said of the new measurements. “Only time will tell us. We have no crystal ball here.”