Global warming is shrinking the Great Lakes

Tom Mackay reckons his backyard in Duluth shows what is happening in Lake Superior as well as any place. In November 2005, the metre-tall wooden "Bay Ness Monster" statue he installed in the water just off his home dock was submerged up to its gaping mouth. Today, his would-be water serpent is high and dry.

For residents of this lakeside Minnesota city, located more than 3000 kilometres by boat from the open Atlantic, the transformation is disturbing. Lake Superior, the largest body of fresh water in the world by surface area, is experiencing its lowest water levels since the record set in 1926. The lake is down by 34 centimetres from a year ago, and more than half a metre below its long-term mean. At least part of the drop can be attributed to a multi-year drought that has been particularly severe since 2006. More troubling, however, is evidence that global warming is driving a long-term shrinkage of this massive natural reservoir.

A rapidly warming lake is the key to understanding the change, says Jay Austin, a limnologist at the University of Minnesota Duluth's Large Lakes Observatory. Earlier this year he reported that Superior's surface waters had warmed by about 2.5 °C since 1979 - far more than average air temperatures in the region during the same period (Geophysical Research Letters, vol 34, p L06604). Austin's findings link the warming to a reduction in winter ice cover on the lake. The less ice is present to reflect sunlight, the more solar energy the lake can absorb. On average, the onset of summer warming of the lake is happening half a day earlier each year. The reduced ice cover also contributes to shrinkage by allowing more evaporation. "Most of the evaporation goes on in winter," Austin says, as cold, dry air

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