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Saltier Atlantic may help decipher global warming

Cape Cod scientist gauges changes in salinity, evaporation

Researchers announced yesterday that the tropical Atlantic Ocean is much saltier than it was 50 years ago, a discovery that may help shed light on a poorly understood climate system that could have implications for global warming.

For years, scientists assumed that global warming would speed evaporation in parts of the world's oceans but had no direct way of measuring the change. Ocean evaporation is a fundamental part of earth's climate system, a giant hydrological cycle that sends water from tropical seas into the atmosphere to fall in high latitudes as rain or snow.

In the journal Nature, researchers led by Ruth Curry at the Woods Hole Oceanographic Institution compared the salinity of the Atlantic Ocean over several decades. If evaporation was increasing, they reasoned, then more fresh water would be removed from the ocean and salinity would increase. They found that evaporation was increasing and estimated a 10 percent rise in tropical evaporation rates during the last 15 years.

At the same time, they also found a corresponding freshening of water in far northern and southern Atlantic.

"They found a neat way of getting at it," said Peter Stone, professor of climate dynamics at the Massachusetts Institute of Technology. "They found what looks like the kind of acceleration of the hydrological cycle people have been expecting."

The report is a new piece of evidence in a global-warming puzzle that could help scientists make more accurate predictions about the earth's climate. Global temperatures are increasing, scientists say, in part because of human-related activities, such as the burning of fossil fuels that release greenhouse gases into the atmosphere. These gases trap heat and radiate it back to earth, slowly warming the planet.

Curry's article examined the salinity of the Atlantic from the tip of Greenland to the tip of South America, using data compiled since 1955. Aiding her were two researchers from the Centre for Environment, Fisheries and Aquaculture Science in England and the Bedford Institute of Oceanography in Nova Scotia.

Although global warming may be the reason ocean evaporation is increasing, Curry believes that increased evaporation may itself, in turn, be accelerating global warming. Water vapor is a potent greenhouse gas that can trap more heat in the atmosphere as evaporation rates increase.

"The system appears to be revving up," Curry said.

Research in the Mediterranean Sea and the Indian and Pacific oceans has yielded parallel salinity trends, but no one has been able to measure a change over time until this research, she said. "The climate system is slow to change, but we've definitely kicked it, and we are beginning to see the fingerprint of that change."

Scientists have long hypothesized that increasing evaporation rates could eventually disrupt a key ocean system, a "conveyor belt" that includes the Gulf Stream and transports warm tropical water up to the North Atlantic, where it loses heat and warms the air. That system explains why London is a temperate city, even though it is farther north than Toronto.

Driving the conveyor belt is cold, dense salty water in the North Atlantic that sinks, allowing warmer water to enter the region. Scientists have long recognized that if North Atlantic waters become too fresh, the cold water wouldn't be heavy enough to sink -- and the conveyor belt could stop or slow, causing more severe winters in New England and Europe.

Beth Daley can be reached at bdaley@globe.com.

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