In recent decades, the waters of Lake Superior have warmed significantly faster than have air temperatures at nearby sites onshore—a trend caused at least in part by a long-term decrease in the lake's winter ice cover, scientists say.
Between 1979 and 2006, the average summertime air temperature at 31 sites within 500 kilometers of the center of Lake Superior rose about 1.5°C, says Jay A. Austin, a geophysicist at the University of Minnesota in Duluth. During the same period, however, the average summertime temperature of the lake's surface water, as measured by instruments on three buoys, jumped about 2.5°C. "It is a remarkably rapid rate of change, and it is surprising," Austin notes.
At least two factors have conspired to boost the lake's temperature, he and his colleagues report in the March 28 Geophysical Research Letters. First, the area of Lake Superior that was covered by ice between Dec. 1 and May 31 dropped, on average, about 0.42 percen