Global warming could, quite literally, bring about a sea change.

Oceanographers have linked temperature changes in Greenland during the last ice age to abrupt changes in ocean salinity levels. Their research, published in the Oct. 5 issue of Nature, finds more evidence that global warming could affect ocean salinity levels in ways that could have far-reaching ramifications.

As part of their study, geologists Howard Spero, of the University of California, Davis, and Matthew Schmidt, of the Georgia Institute of Technology, recorded salinity levels in the North Atlantic Ocean between 45,000 and 60,000 years ago.

They constructed this record by examining ocean sediment, which is comprised of plankton fossil shells and contains varying amounts of oxygen, magnesium, and calcium. The levels at which these elements are present depend on the temperature and salinity of the water at the time of fossil formation. Changes in the ratios of these elements can then be used to measure changes in salinity.

When the researchers compared this salinity data to temperature changes recorded in Greenland’s ice cores, they found that the Atlantic got saltier during cold periods and fresher during warm intervals. The measurements revealed that temperature changes of 5° to 10° C were enough to cause three to five percent fluctuations in salinity.

The total changes took place over about 200 years, “but the measurable changes were on the order of decades,” Spero said. “That’s incredibly fast.”

The fluctuation likely occurred as a result of varying rainfall patterns. As the Earth cooled, it would have shifted the Intertropical Convergence Zone, a band of heavy rainfall that exists over the tropics. Normally, this rainy zone dilutes Caribbean ocean water with freshwater, and then ocean currents push this freshwater north into the Atlantic.

But during cold periods—such as during an ice age—the zone moves farther south. As a result, less rain falls into the Caribbean and less freshwater flows into the North Atlantic.

Global warming, scientists hypothesize, could have the opposite effect: increasing the amount of freshwater that accumulates at northern latitudes. Such a change could have serious implications for the world’s interconnected systems of circulating ocean currents. Because saltwater is denser than freshwater, and therefore sinks, currents are sensitive to salinity.

Freshening in the North Atlantic could slow deep-water currents in this region—or even one day stop them altogether.

“The salinity of the North Atlantic may be the canary in the coal mine,” Spero said. “Measurable changes in the salinity,” he continued, “could give us a clue as to whether ocean circulation is slowing down.”

Lloyd Keigwin, an oceanographer at the Woods Hole Oceanographic Institution, cautions that it is hard to predict exactly what will happen due to the many factors involved in shifting circulation.

“These things take thousands of years when the process is natural,” Keigwin said of possible freshening effects. “The troubling thing is that man’s influence on climate is decidedly unnatural. The climate system is naturally subject to abrupt change of very large magnitude, and we are messing with it in ways we don’t understand.”

Originally published October 25, 2006

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