Their studies show that although the amount of ice cover can vary substantially from year to year, the overall coverage on the world’s largest system of freshwater lakes is diminishing, especially in the deepest, middle portions of Erie, Huron, Michigan, Ontario and Superior.

“The deeper the water, the greater the heat storage from summer, and it freezes later than the shallow areas,” research Ray Assel told The Plain Dealer. “Now, increase the air temperature and the lake takes in more heat and stores it longer, to the point that many of the midlake areas are freezing over less.”

Assel’s records indicate that ice formation at nearshore areas has decreased less than on the deepest parts.

Evaporation from open water can cause heavy lake-effect snow inland.

Researcher Jia Wang said ice loss can cause other problems, including the destruction of the eggs of fall-spawning fish by winter waves from an open lake, erosion of coastal areas unprotected by shore ice and less winter recreation on the lakes such as snowmobiling or ice fishing.

The Coast Guard has estimated it cost more than $245,000 to rescue 134 fishermen from a huge ice floe off Ohio last month. The fishermen became stranded Feb. 7 when a miles-wide chunk of ice broke away in Lake Erie.

There might be one short-term advantage to decreased ice: Shipping may someday be more possible in the winter months. The locks at Sault Ste. Marie now close each year in mid-January and reopen in late March. But shipping companies might haul less cargo to pass through low-water areas.