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updated 7/3/2007 4:38:49 AM ET 2007-07-03T08:38:49

Ancient ponds in the Arctic are drying up during the polar summer as warmer temperatures evaporate shallow bodies of water, Canadian researchers said on Tuesday.

They said the evaporation of these ponds — some of which have been around for thousands of years — illustrates the rapid effects of global warming, threatening bird habitats and breeding grounds and reducing drinking water for animals.

Falling water levels and changes in chemistry in the ponds first were noticed in the 1990s, and by last July some of the ponds that dot the landscape were dry, according to a report in Tuesday's Proceedings of the National Academy of Sciences.

John Smol of Queens University in Kingston, Ontario, and Marianne Douglas of the University of Alberta in Edmonton have been studying about 40 ponds on Ellesmere Island in northern Canada since 1983.

The ponds are habitat for algae and invertebrates such as insect larvae, and waterfowl use them.

Smol likens the warming conditions to a pot of soup on a stove.

"If you take the lid off, it is similar to what we are observing in these ponds. The soup will slowly decrease in volume and it will get saltier and saltier as the water evaporates, leaving the salts behind."

That same evaporation process is taking place with these ponds, he said.

Area free of ice earlier
Weather records show there has been no decline in rain and snowfall in the region and, while some arctic ponds have drained when the permafrost melted beneath them, these ponds sit above bedrock.

What was different was that ponds that had formerly remained frozen until mid-July were free of ice as early as late May.

"No small wonder that we are seeing evaporation occurring," Douglas said. "An extra month is tremendously long up there where the growing season is so short."

The changes will have significant impact on the birds and animals that rely on these sources of fresh water to survive and breed.

"The ecological ramifications of these changes ... will cascade throughout the Arctic ecosystem. ... Lower water levels will have many indirect environmental effects, such as further concentration of pollutants," the researchers wrote.

Douglas and Smol were able to use paleological techniques to trace the history of the ponds back thousands of years. "We basically followed them from cradle to the grave," Smol said.

Changes set in with more mosses growing and shorter periods of ice, followed by lowering water levels and increasing salinity until some dried up completely.

In addition to the ponds, the researchers also reported a drying of nearby wetlands.

'Tipping point' at ponds
In the 1980s they often needed to wear hip waders to make their way to the 6,000-year-old ponds, they noted, while by 2006 the same areas were dry enough to burn.

"We were surprised. We arrived in early to mid-July and the ponds we had been monitoring were dry. Some of them had dried up completely. Some were just about to lose the last remaining centimeters of water," said Douglas, director of the Canadian Circumpolar Institute at the University of Alberta.

"It's really interesting to see how quickly it is happening. We could see this trend had started a while ago but at no time did we expect it to accelerate," said Douglas.

Douglas said a study of the fossilized sediments in these pools of water — which are less than 7 feet deep — showed climate changes beginning as long as 150 years ago.

The researchers had thought these ponds were permanent. But change has come rapidly.

"It is a bit of a tipping point. We don't know how far this warming or drying will go," she said in a telephone interview.

Reuters and The Associated Press contributed to this report.

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