The ponderous churning of the North Atlantic Ocean that carries warm water northward and returns deep, cold water to the south appears to have slowed in the past decade or two. That would mean this oceanic radiator is bringing less heat to warm Europe and, if global warming is behind the slowdown, will carry less and less heat to high latitudes in the future.
Oceanographers only last year put down a string of instrumented moorings spanning the Atlantic from West Africa to the Bahamas. So for a long conveyor record, physical oceanographer Harry Bryden of the National Oceanography Centre (NOC) in Southampton, U.K. and colleagues had to draw on five oceanographic surveys across that stretch of the Atlantic between 1957 and 2004. During ship crossings of a month or two, researchers measured seawater temperature and salinity from the surface to near the bottom. The NOC group used seawater densities calculated from those observations, plus current measurements of the Gulf Stream passing by Florida and a few standard assumptions, to estimate the currents heading north and south through the depth of the Atlantic.
The Gulf Stream remained steady through the 47-year period, and Atlantic flows remained much the same through the 1992 survey. But the conveyor appears to have slowed dramatically in 1998 and 2004, the NOC team reports 1 December in Nature. Fifty percent more Gulf Stream near-surface waters were turning back southward before reaching very far to the north, whereas part of the deep southward flow of cold water had decreased by 50%. All in all, the conveyor had slowed by 30%. The slowing, although sizable, is comparable to the estimated uncertainty of the observations, Bryden notes. Still, "it's real variability," he says.
"The pattern is reasonably convincing," says physical oceanographer Peter Rhines of the University of Washington, Seattle. "It's a pretty nice picture." Some think the picture is still fuzzy, however. Ocean and climate modeler Richard Wood of the Hadley Centre for Climate Prediction and Research in Exeter, U.K., says that, like any part of the complex climate system, the conveyor is bound to slow down at times and speed up at others. The two latest surveys may have happened to catch the Atlantic as the conveyor slowed temporarily, he says, giving the impression that a permanent change had taken place. It may take a decade or two more of watching and waiting to know for sure.