A published report says the amount of ice covering the Great Lakes has declined about 71 percent over the past 40 years, a drop that the lead author partly attributes to climate change.
The report published last month by the American Meteorological Society said only about 5 percent of the Great Lakes surface froze over this year.
"There was a significant downward trend in ice coverage from 1973 to the present for all of the lakes," according to the study, which appeared in the society's Journal of Climate.
Researchers determined ice coverage by scanning U.S. Coast Guard reports and satellite images taken from 1973 to 2010, the Duluth News Tribune reported. They found that ice coverage was down 88 percent on Lake Ontario and fell 79 percent on Lake Superior. However, the ice in Lake St. Clair, which is between Lakes Erie and Huron, diminished just 37 percent.
The study's lead researcher is Jia Wang of the National Oceanic and Atmospheric Administration lab in Ann Arbor, Mich. He attributed the decline to several factors, including broad climate change and smaller cyclical climate patterns like El Nino and La Nina.
He told WBEZ-FM in Chicago that diminished ice can accelerate wintertime evaporation, causing water levels to fall. The lack of ice could also lead to earlier and increased algae blooms that can damage water quality, and could speed up erosion by exposing more shoreline to waves.
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The study doesn't include the current winter, but satellite photos show that only about 5 percent of the Great Lakes surface froze over this winter. That's a steep drop from years such as 1979, when there was as much as 94 percent ice coverage. On average, about 40 percent of the surfaces freeze over.
The results are consistent with other studies that have found higher surface water temperatures on Lake Superior in recent years.
Even Lake Superior's protected Chequamegon Bay, just north of Ashland, Wis., has been remarkably free of ice. It usually freezes enough for trucks to drive on it, but the ice was never thick enough. That forced the local Madeline Island ferry to operate all season, which has only happened once before.