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The Great Lakes are poised to become a lot less great because global warming will cause a dramatic plunge in the amount of water they contain, a new report says.

Compiled by a team of scientists from Canada and the United States, the report says the rising temperatures accompanying climate change will cause the lakes to shrink, with water levels in Lake Huron and Lake Michigan likely falling about three metres by 2030.

If concentrations of greenhouse-gas emissions continue to grow unchecked and reach double the level that occurred naturally before industrial times, the drop could be as much as eight metres, or about the height of a two-storey house.

Such declines would cause major ecological changes as shorelines recede, and also harm shipping, cut the supply of electricity from hydro power stations and diminish the grandeur of global fixtures such as Niagara Falls.

The projections were in a report called Confronting Climate Change in the Great Lakes Region, which has been billed by its authors as the most authoritative study on the impact of climate change on the world's largest fresh-water ecosystem.

The water-level forecasts are based on computer models that simulate what would happen to the lakes as the temperature of the air above them warms. Rising temperatures will cause the water in the lakes to heat up and lose more of their moisture to evaporation.

At the same time, the higher temperatures will cause the land around the lakes to become drier, reducing the supply of groundwater feeding streams that flow into the lakes.

"It's the higher temperature which is really driving these future scenarios," said John Magnuson, a limnologist (the scientific study of fresh water) and emeritus professor from the University of Wisconsin. He said "almost every" computer model used by researchers is projecting future declines in the amount of water in the lakes.

The report was jointly released yesterday in major cities around the Great Lakes. It was produced by the Union of Concerned Scientists and the Ecological Society of America, both based in the United States, and the David Suzuki Foundation in Canada.

Lake Huron and Lake Michigan have huge surface areas, making them more vulnerable to evaporation and declining water levels than Lake Superior and Lake Ontario. The report estimated that Lake Superior, the deepest of the lakes, could drop only 0.6 metres by 2030.

The report also projects significant changes to Ontario's climate because of global warming. By the end of the century, if nothing is done to curb greenhouse-gas emissions, average summer temperatures will be four to eight degrees warmer than they are now.

That would mean Ontario would have the hot, humid temperatures that now prevail in the southern United States.

"By the end of the century, summers in southern Ontario will feel like those of Virginia today," said Katharine Hayhoe, a researcher who compiled the climate projections in the report. She said Ontario winters would resemble those currently experienced in New York.

The report says there are some potential gains from warmer temperatures, such as longer growing seasons for farmers. But this gain could be offset by lower soil-moisture levels and droughts, and the likelihood of new insect pests from southern regions.

The potential impacts of climate change could be diminished through reductions in the emissions of greenhouse gases, primarily carbon dioxide from the burning of fossil fuels for electricity and transportation.

Ms. Hayhoe said Canadians are global gluttons when it comes to energy use and have per capita annual carbon dioxide emissions of 23 tonnes, the second- highest total in the world after Australia. The other industrialized countries have an average of 13 tonnes a person. "We're a big part of the problem," she said.