DULUTH, Minn. – For Jay Austin, who has made a career of studying the Great Lakes, the warming climate around Lake Superior is no mystery.
But he was surprised to find the waters of the lake itself warming even more rapidly.
Austin, a Duluth professor and a researcher with the University of Minnesota-Duluth's Large Lakes Observatory, has studied decades of data.
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What he found was water temperatures rising almost twice as fast as air temperatures — more than 4 degrees for the average surface temperature.
The increase is having dramatic effects.
"The date of what we call the spring overturn has been getting earlier in the year," Austin said. "It's basically the start of the summer season in the lake. It's when you start to develop strong positive stratification: warm water sitting on top of cool water."
In two decades, the spring turnover has moved up two weeks from early July to mid-June.
Part of that likely is due to a loss of ice cover. Since ice is reflective, when it's not there it makes it easier for the lake to absorb heat.
In another 35 to 40 years, Austin said, Lake Superior will have very little ice cover.
While that may sound good to people who swim or sail on the lake, it's not so good for plants and animals, including the lake's native whitefish.
"If there's less ice over time, and there appears to be, there's a chance for greater storminess in the sort of shallow water [bays] that the whitefish spawn in," said Steve Coleman, who directs the Large Lakes Observatory.
Bob Sterner, a University of Minnesota biologist, said warming usually speeds the growth of fish and the plants they feed on. But when it's too fast, it can create big problems.
"Paradoxically, you may well see the lake essentially becoming even more desert-like in the sense that you've reduced the flow of nutrients into the system across that temperature gradient," Sterner said.
The research will be published soon by the American Geophysical Union. The Duluth scientists' next project is trying to prove their suspicion that diminishing ice is contributing to falling lake levels.