Even in the dead of winter, long stretches of subzero temperatures are pretty rare in London. It may come as a surprise, therefore, to learn that the capital of Britain lies nearly 400 miles farther north than Montreal--or that Paris is farther from the equator than Fargo, N.D. The relatively balmy climate of much of Western Europe suggests that many countries in that region should lie well south of where they actually are, and that's all thanks to the Gulf Stream, a gigantic river of tropical water that flows up and across the Atlantic, warming the waters that lap figuratively against Europe's western shores. Turn it off, and the region's temperatures could plunge disastrously.
That was precisely the specter raised last week when scientists from Britain's National Oceanography Center reported in Nature that a component of the oceanic current system that drives the Gulf Stream has slowed by 30% since 1992. The likely, paradoxical cause? Global warming. While climate experts around the world caution that the data are too preliminary to be definitive, "the result," writes University of Hamburg climatologist Detlef Quadfasel in a commentary on the study that also appears in Nature, "is alarming."
It's also not entirely unexpected. Back in the 1980s, Wallace Broecker, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory, was trying to understand why temperatures in Greenland had plunged dramatically several times over the past 70,000 years. His theory: fresh water, perhaps from melting glaciers, might have diluted the ocean's salinity, making it harder for cooling water to sink and return southward to pick up more heat. That could shut off the entire "conveyor belt" of water that keeps Europe temperate. It's hard to determine precisely what would have caused such a big thaw 70,000 years back, but we do know that today global warming is causing more meltwater to stream into the North Atlantic from glaciers and older sea ice, which is lower in salt. Could the conveyor belt stop again?
Climate experts are not sure--and some have serious reservations about the new paper--mostly because the observed change is happening too fast. Computer models predict that it should take at least 100 years to weaken the ocean conveyor belt. What's more, nobody was even measuring those currents before 1957. Says Broecker: "We don't know how much the flow bounces around normally."
Clearly, pieces are still missing from the equation, so even the scientists who wrote the study counsel against panic. Rather than be worried, says co-author Stuart Cunningham, "people should be more interested and concerned. The ocean seems to have changed in a large enough way to be detectable." It's something, in other words, to keep an eye on. [The following text appears in a complex diagram -- see PDF or hardcopy of magazine] GLOBAL WARMING Increased rainfall as well as the melting of sea ice, glaciers and the Greenland ice sheet could add enormous amounts of freshwater to the Atlantic currents, reducing their salinity enough to slow the sinking of cooler water and shut down the heat conveyor