Scientists have detected a substantial drop in the last 50 years in the flow of cold deep sea water leaving the Arctic and pouring into the Atlantic between Iceland and Scotland.
Climate experts say the obscure current, flowing south 2,000 feet beneath the surface, is one of the engines that drive the worldwide oceanic conveyer belt that also carries sun-warmed surface water north toward the pole. Because the outflow of cold deep water has diminished, the influx of warm surface water that usually replaces it also has to have declined. That decrease could explain a recent cooling of some coastal regions near the Norwegian Sea, said the authors of the study, which is described in today's issue of Nature.
The study was conducted by scientists in Norway, Scotland and the Faroe Islands, 400 miles east of Iceland, and centered on the flow of water over a submerged ridge east of those islands. The changes there mesh with observations of major shifts in temperature, sea ice, currents and winds above the Arctic Circle and match some computer simulations of global warming. But the scientists noted that the natural cycles in the area between the Arctic and the Atlantic remained poorly understood. They said it was too soon to say climate change caused by human activity had changed the flow.
Other climate experts cautioned that it was premature to predict whether the change could have broader effects on Europe, because other influences besides sea temperature contribute to the generally mild conditions there.
''If I lived in the Faroes, I might be worried,'' said Dr. Richard Seager, a senior research scientist at the Lamont-Doherty Earth Observatory of Columbia University. ''But in Paris I wouldn't be worried.''
Dr. Knut Aagaard, an oceanographer at the Polar Research Center of the University of Washington in Seattle, said the research benefited from a focus on a remote, but vital junction in the ocean's circulatory system, the fairly narrow passages between Greenland and Europe that link Arctic waters with the North Atlantic.
Enormous amounts of water affected by conditions far to the north flow through the gaps, Dr. Aagaard said, adding: ''These constrictions give you a wonderful way to monitor what's going on over larger areas. If you want to know changes in a big building, stand by the front door and you'll get a feel for it.''
One author of the study had the benefit of living and working in the Faroes. Dr. Bogi Hansen, an oceanographer at the Faroese Fisheries Laboratory, said instruments that tracked currents, salinity and temperature were placed on the ridge east of the islands.
The water flowing over that sill, he said, constitutes a submarine river 10 miles wide and more than 600 feet deep, with a flow twice that of all the world's freshwater rivers combined.
Detailed readings from the anchored instruments over five years were matched up with separate measurements taken nearby since 1948 from weather ships, providing a much longer-term estimate of shifts in the deep currents.
The amount of cold deep water in that time, Dr. Hansen said, has fallen 20 percent and is accelerating. ''If you look at the graph,'' he said, ''you see the decrease is much faster in the last five years than it was over the 50-year period.''
But Dr. Hansen said the record was still not long enough to determine whether the change was linked to warming of the atmosphere from rising levels of greenhouse gases.