Icy sign of polar warming?

Icy sign of polar warming?

During the winter of 1987, ice covering the Arctic Sea north of Greenland was significantly thinner than during the winter of 1976, according to submarine measurements reported in the June 28 NATURE. From a single comparison like this, it's impossible to tell whether global warming is actually thinning Arctic ice. But scientists say the finiding underscores the importance of monitoring ice thickness for early signs of the predicted climate change.

Peter Wadhams of the University of Cambridge in England examined data from upward-looking sonar on two British submarines that took similar routes across the Arctic Sea. One sailed in May 1987, the other in October 1976. From the sonar data, Wadhams estimates that the ice over a region about the size of Nevada had a mean depth of 5.3 meters in 1976 and 4.5 meters in 1987.

Some scientists have suggested that thinning ice in the Arctic would emerge as one of the first signs of global warming. However, Wadhams says the difference in thickness between 1976 and 1987 does not represent a general thinning due to warmer temperatures, but instead stems from a radical change in the pattern of ice drift. During the earlier period, he suggests, wind had blown ice toward Greenland, building up the ice cover in that region. But winds during the later observation period didn't blow the same way, and the ice cover did not thicken to the same extent.

Oceanographer Alfred McLaren from the University of Colorado in Boulder calls the new report intriguing, but mentions that the two submarines traveled during different seasons and did not follow exactly the same track. In general, he says, "we still don't know enough about what's going on up there to make sense out of it."

McLaren captained the U.S. nuclear submarine Queenfish in August 1970 when it measured ice extent across the Canada basin, following the course made by the U.S.S. Nautilus in August 1958. Last year, he reported that the mean ice cover was thinner in 1970 by 0.69 meters, though it remains unclear whether the difference reflects a long-term trend or just the normal variability between two years. McLaren thinks an answer to that question lies buried somewhere in the reams of submarine data taken over the last 32 years.