Ice cover on the Great Lakes has declined more than 30 percent since the 1970s, leaving the world's largest system of freshwater lakes open to evaporation and lower water levels, according to scientists associated with the National Oceanic and Atmospheric Administration.

They're concerned about how the milder winter freeze may affect the environment. But they're also trying to come to terms with a contradiction - the same climate factors that might keep lake ice from freezing might make freezing more likely if lake levels drop due to evaporation.

Scientists at the Great Lakes Environmental Research Laboratory in Ann Arbor, Michigan, say global climate change can be at odds with regional climate patterns. Accurately measuring ice cover across a lake system that spans 243,500 sq km in the United States and Canada is no small task, they say.

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," researcher Ray Assel said. "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 and erosion of coastal areas unprotected by shore ice.

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

However, there might be one short-term advantage to decreased ice: Shipping may someday be more possible in the winter months.

(China Daily via Agencies March 25, 2009)