Ocean salinity called sign of global warming

50-year change charted, with recent acceleration

December 18, 2003|By Robert Cooke | Robert Cooke,NEWSDAY

Signs that global warming is changing patterns of rain, snow and ocean currents that drive the climate system were reported yesterday by scientists monitoring ocean saltiness.

According to oceanographer Ruth Curry, sea surface waters in tropical regions have become significantly saltier in the past 50 years, while surface waters at high latitudes in Arctic regions have become much fresher. These changes in salinity seem to have accelerated in the 1990s.

"This is the signature of increasing evaporation and precipitation" caused by warming, Curry said, "and a sign of melting ice at the poles. These are consequences of global warming, either natural, human-caused or, more likely, both."

These changes in saltiness reflect increased seawater evaporation in the warm tropical regions, leaving the surface water saltier. The increased evaporation leads to increased rainfall and snowfall - plus more ice melting - dumping fresher water at the poles.

Curry, a research scientist at the Woods Hole Oceanographic Institution in Massachusetts, published the result with two colleagues yesterday in the journal Nature. Her co-workers were Bob Dickson of the Center for Environment, Fisheries and Aquaculture in Lowestoft, England, and Igor Yashayaev, at the Bedford Institute of Oceanography in Nova Scotia.

Their data came from more than 40 years of salinity measurements taken in the Atlantic, between Iceland in the north and the tip of South America.

She explained that changes in these patterns, called the hydrologic cycle, "can be very important because they can be a powerful amplifier of global climate change."

There has been much concern in the past two decades that human activities, especially burning of fossil fuels, are adding so much carbon dioxide to the air that the climate system is being tipped out of balance. Carbon dioxide is a so-called greenhouse gas because it also tends to trap and hold the heat that comes from he sun.

As the air gets warmer, global climate patterns are likely to change, bringing excess rainfall to some areas and drought to others.

Geochemist Wallace Broecker, at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., said "it's an extremely interesting report. There's no doubt that the data are solid. But the question is whether this is a cyclic thing," whether the changes will soon turn around and the salinity will revert to "normal."

Without a far longer record of salinity measurements, Broecker said, it's difficult to guess how long the trend might last or what the consequences might be.

Broecker proposed decades ago that a change in global climate might trigger an abrupt and major shift in the global ocean circulation system.

At present, a constant flow of warm water northeast along the U.S. East Coast, known as the Gulf Stream, delivers welcome warmth to Northern Europe. If that flow of warmth ceases or slows too much, Europe could be plunged into a deep freeze.

Along with global warming comes the danger of rising sea levels and coastal flooding, which would affect the American Northeast.

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