An abrupt change in the saltiness of surface water in the Greenland Sea has almost halted the formation of dense, cold water that helps create deep ocean currents, scientists have reported.
The change in deep-water formation, detected by repeated measurements of radioactive markers and chemicals in the sea, suggests that natural systems controlling global climate are more delicately balanced than had been thought and that they can change quickly.
The finding was published in Friday's edition of Science magazine.
"The results demonstrate that, on average, formation of Greenland Sea deep-water stopped almost completely during the 1980s," geochemist Peter Schlosser of Columbia University said in a telephone interview from Germany, where he was visiting. The amount of cold, dense water that forms and sinks to the sea floor in the Greenland Sea has decreased by 80 percent during the 1980s, apparently because of decreased salinity, he explained.
The formation of cold, dense, salty water "has significant impact on global climate, because by this process heat is exchanged between the deep oceans and the atmosphere," Mr. Schlosser said.
Scientists have yet to quantify the impact.
Co-authors were Gerhard Boenisch and Reinhold Bayer, at the University of Heidelberg, and Monica Rhein at the University of Kiel.
Researchers said that a worldwide system of deep ocean currents s driven by the formation of cold, dense water that sinks to the sea floor north of Iceland. These currents flow around the globe and play a critical role in local climates.
Northern Europe, for example, is relatively warm because of heat released from surface seawater flowing to the North Atlantic.
After giving up its heat, the dense water sinks again to form new deep water.
Oceanographer William Jenkins, at the Woods Hole Oceanographic Institution in Massachusetts, said that the abrupt change in deep-water formation "is compelling evidence" that the process "is sensitive to climate fluctuations."
"And if we do indeed begin to modify our climate, then the deep-water systems will begin to respond in fairly short order," he said.
Although it is not known what caused the decrease in salinity in Greenland Sea surface waters, Mr. Schlosser said that a measurable decrease was detected earlier in the North Atlantic Ocean.
"During the late 1960s and early 1970s an anomaly expressed as lower salinity moved into the Greenland and Norwegian Seas," Mr. Schlosser said.
"It was detected in the Greenland Sea area in about 1978."
The decline in salinity may have been caused by increased fresh-water runoff from land or, Mr. Jenkins said, by changes in the amount of ice formed at high latitudes.
The removal or addition of fresh water can alter sea surface salinity easily, since fresh water floats on denser brine.