Lake Superior has been warming even faster than the climate around it since the late 1970s due to reduced ice cover, according to a study by professors at the University of Minnesota Duluth.

Summer surface temperatures on the famously cold lake have increased about 4.5 degrees since 1979, compared with about a 2.7-degree increase in the region's annual average air temperature, the researchers found. The lake's "summer season" is now beginning about two weeks earlier than it did 27 years ago.

"It's a remarkably rapid rate of change," Jay Austin, an assistant professor with the university's Large Lakes Observatory and Department of Physics, told the Star Tribune newspaper. Austin co-authored the study with geology professor Steve Colman.

The study is based on data collected by National Oceanic and Atmospheric Administration buoys on the lake and on 102 years' worth of daily temperature readings at a hydroelectric plant near Sault Ste. Marie, Mich.

Austin said the surface temperature increase is not only "a symptom of climate change," but also could reinforce itself. A trend toward warmer winters would mean less winter ice cover, which would allow more solar radiation of the lake and continued warming, he said.

Lake Superior freezes over completely about once every 20 years, according to the Minnesota DNR's climatology office. If trends continue, it could be routinely ice-free by about 2040, the study found. This would cause water levels to continue to drop because the lake loses more water to evaporation in a winter without ice cover than it does during the summer. In recent months, the lake's level has been lower than at any equivalent time since 1926.

The study was first published by the American Geophysical Union on March 23.