Substantial warming well below ocean’s surface will pose risk in this century

By Janet Raloff

Web edition: July 6, 2011

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DEEP TROUBLE

Subsurface warming of ocean waters threatens to undercut and accelerate calving of Greenland’s ice sheets, which serve as vast water reservoirs. Shown is West Greenland’s Store Glacier, which is losing mass rapidly.

Credit: E. Rignot/NASA JPL

Scientists have uncovered a potentially potent risk to Greenland’s ice sheets during the next century and beyond: rapidly warming deep water. The subsurface ocean off Greenland is now expected to warm at roughly double the rate that is projected for such waters globally, including off the coast of Antarctica.

Calling the ocean “the 900-pound gorilla of global warming and climate change,” oceanographer Josh Willis of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., says he’s excited to finally see long-term climate projections homing in on the ocean’s role. “How the oceans affect the great ice sheets in Greenland and Antarctica could mean the difference between getting two feet and six feet of sea level rise in the coming century,” observes Willis, who had no role in the new analysis. “So this is a big deal.”

Until now, “no one had noticed that the ocean surrounding Greenland and Antarctica will warm very differently during this century and the next,” says climate modeler Jianjun Yin of the University of Arizona in Tucson, who led the new study, published online July 3 in Nature Geoscience. Although his group’s conclusions come from computer analyses, Yin notes that the study’s projections “are quite consistent with recent observations of subsurface ocean temperatures around Greenland.”

Yin’s team ran 19 computer programs to project the effects of global climate change on the world’s great bodies of water, many for periods through 2200. These analyses consistently turned up surprises, Jin says, including signs of substantial warming during the next 90 years and beyond in waters off Greenland at depths of 200 to 500 meters.

The increase, projected to total about 2° Celsius, could erode the base of glaciers in fjords, where the ice can reach as deep as 1,000 meters below the ocean surface. And unlike the glacial surface, which remains frozen in winter, the deeply submerged portions of tidal glaciers can melt year-round, Yin says, greatly accelerating their underwater calving.

In contrast to projections for Greenland’s waters, deep-ocean warming globally will probably increase by only about 0.5° C, the researchers reported.

Antarctica’s ice sheets are expected remain relatively insulated throughout the next century because of a circumpolar current that effectively keeps warm waters from penetrating beyond 60° S latitude. “Multiple projections show that this current will become stronger during this century and to the end of the next,” adds Yin, based on his study. That’s good news, he says, because although Greenland’s ice sheets in total contain enough water to increase global sea level by 7 meters, or about 23 feet, Antarctica’s ice sheets contain roughly 10 times that much water.

Willis cautions, however, that computer analyses of climate lack the resolution to evaluate the impact of eddies that might weaken the insulating Antarctic current. Recent studies have suggested such eddies could dampen that current enough to let warm water breach it and approach the continent’s ice sheets. “So,” Willis says, “it might be that the scenario painted by Yin and company for Antarctica is a bit too rosy.”