The average amount of ice covering the Great Lakes declined 71 percent over the past 40 winters, with Lake Superior ice down 79 percent, according to a report published by the American Meteorological Society.
"There was a significant downward trend in ice coverage from 1973 to the present for all of the lakes,'' states the study appearing in the society's Journal of Climate.
Researchers used Coast Guard reports and satellite photographs taken from 1973 to 2010 to determine the ice coverage of all the lakes, with Lake Ontario ice dropping 88 percent while ice in Lake St. Clair (between Lake Huron and Lake Erie) diminished just 37 percent.
The findings don't include the current winter, but 2011-12 will only speed up the decline. Only about 5 percent of the Great Lakes surface froze over this winter, the least since satellite photos first were taken from space. That compares to winters that saw as much as 94 percent ice coverage, such as in 1979. It's also way down from the average winter of about 40 percent coverage.
The results won't be a surprise to Northlanders who have gazed out all winter over open water on the western tip of Lake Superior, where almost no ice has formed. Even in protected Chequamegon Bay, which usually freezes enough for trucks to drive on, strong ice never formed this winter, forcing the Madeline Island ferry to operate all season. That's only the second time that's ever happened.
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The results echo other studies that show much higher surface water temperatures on Lake Superior in recent years and far fewer days of ice cover.
The study's lead researcher is Jia Wang, who works at the National Oceanic and Atmospheric Administration lab in Ann Arbor, Mich. He told Chicago's WBEZ-FM that diminished ice can speed up wintertime evaporation, reducing water levels. It also may lead to increased and earlier algae blooms, which can damage water quality, and may accelerate erosion by exposing more shoreline to waves.
Wang said the decline in ice cover probably is due to several factors, including cyclical climate patterns like El Niño and La Niña (unusually high temperatures and unusually low temperatures, respectively, in the Pacific), changes in the Arctic Oscillation and broader climate change.
