The system of circulating water currents that moderates northern Europe's weather is 30 percent slower than it was nearly 50 years ago, according to a study by British scientists.

The finding, being published in the journal Nature, is significant because it could signal a major climate shift in coming decades. Some scientists worry that a disruption in the "conveyor belt" of currents -- which brings warm surface water north toward England, Ireland and other northern countries, and returns cold, dense, deep-ocean water south -- could sharply decrease temperatures in northern Europe.

The warm water Gulf Stream that flows on the surface from the Gulf of Mexico to northern Europe is part of the larger system, but the researchers found no change in the stream itself.

Harry Bryden, an oceanography professor at England's University of Southampton and the paper's lead author, cautioned in an interview that he and other researchers are not predicting a drastic climate shift in a matter of years because of the slowing of this "overturning" circulation.

"We're not 'Day After Tomorrow' fanatics," Bryden said, referring to the 2004 movie that portrayed drastic climate changes occurring within days. He added, however, "I'm convinced we've measured a real change. . . . We've established there's variability in the overturning circulation."

Bryden and two colleagues compared the circulation rate and water temperature data in one section of the Atlantic stretching from Africa's coast to Miami during five years between 1957 and 2004. They determined that the deep water southward-flowing section of the conveyor is now moving 30 percent slower than it did in 1957 because it does not contain as much dense, cool water.

Bryden attributed the slowdown in part to the water's declining salinity caused by the addition of less-dense freshwater from melting Arctic sea ice and glaciers. He said the slowing is in line with computer models that suggest that Earth's warming climate could weaken and eventually halt the conveyor belt circulation altogether, causing northern Europe to become as much as 11 degrees Fahrenheit cooler in a matter of decades.

Terrence M. Joyce, a senior scientist at Woods Hole Oceanographic Institution who studies Atlantic Ocean currents, said the paper has produced "an interesting result" but should not be overinterpreted.

"I don't think there's a trend going back to 1957," Joyce said. "They can clearly say it's different now."

And James J. O'Brien, an oceans current expert and director of the Center for Ocean-Atmospheric Prediction Studies at Florida State University, said researchers have yet to determine how much the change reflects a natural climate cycle.

"There's so much climate variability in the system, we can't draw too many conclusions from two snapshots of ocean currents," he said.

Bryden said researchers should know much more in a year because they have set up 22 moored buoys in the section of the Atlantic current they have been studying.