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 percent per year between 1979 and 2005. Because dark, open waters absorb more solar radiation than ice does, the date on which surface water reaches 4°C—the temperature at which it reaches maximum density and sinks, thereby mixing the lake’s waters—comes nearly 2 weeks earlier now than it did in 1979. Afterward, layers remain stable and surface waters begin to warm.
The second major factor has been warmer summertime air temperatures. The earlier onset of lake-water turnover combined with warmer air have brought about the surprisingly rapid surface warming in the lake, the researchers suggest.