Lake Superior has been warming even faster than the climate around it since the late 1970s. Its ice cover could completely vanish by the winter of 2040, according to a study by professors at the University of Minnesota Duluth.
A popular tourist spot,it is the largest freshwater lake in the world by surface area.
Summer surface temperatures on this 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 Minnesota Duluth.university's Large Lakes Observatory and Department of Physics, said.
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, Michigan.
Austin said a trend toward warmer winters would mean less winter ice cover. That would in turn lead to more solar radiation of the lake and continued warming.
The lake loses more water to evaporation in a winter without ice cover than it does during the summer. As a result water levels could drop sharply. In recent months, the lake's level has been lower than at any equivalent time since 1926.
Lake Superior freezes over completely about once every 20 years. If the current trends continue, it could be routinely ice-free by about 2040, the study found.
Presently it is the world's fourth-largest freshwater lake by volume.