Arctic ponds that have hosted diverse ecosystems for thousands of years are now disappearing because of global warming, according to a new study.

These ponds, which lie atop bedrock, freeze solid in the winter and then melt for a few months each summer, becoming hot spots of activity in the forbidding Arctic terrain.

"If you fly over, you see them everywhere," said study leader John Smol of Queen's University in Kingston, Canada.

The ponds brim with moss, algae, fairy shrimp, and other organisms that need liquid water to live during the summer.

The ponds vary in size, with some reaching three feet (a meter) deep and around several hundred feet across. But now these ponds have reached a tipping point, according to a study that appears in the journal Proceedings of the National Academy of Sciences this week.

Some of these ponds are going bone dry in the summer or shrinking to tiny puddles, while others are a fraction of their former size, because of global warming believed to be caused by humans.

The wetlands around some of the ponds are also disappearing, threatening the creatures that inhabit these areas, Smol added.

"Now they're so dry you can put a match to it and they'll burn," he said.

Drying Out, Dying Out

Smol and Marianne Douglas at the University of Alberta in Edmonton have spent more than two decades tracking about 45 ponds on Canada's Ellsmere Island, which is located just off the northwest coast of Greenland (see a map of Canada).

In earlier studies the researchers searched sediments in these ponds and dug out the shells of diatoms - microbes that grow protective mineral sheaths around themselves.

The record of diatoms revealed that the ponds had been relatively stable for several thousand years - until the 1800s. Then new kinds of diatoms and mosses took over the ponds, the first signs of warming's effects.

In recent years there has actually been more rain and snow in this region than a couple of decades ago.

But because of global warming, the summers are warmer, so the ponds are losing more water to evaporation - greater than the gain from the extra precipitation.

So like a pot of simmering stew, the ponds are becoming smaller and saltier.

Of the 24 or so ponds that Smol and Douglas were tracking, all were drying and shrinking, and two or three went completely dry in the summers of 2005 and 2006.

"We've seen, in our lifetime, in front of our eyes, some of these ponds dry up," Smol said. "It is quite striking."

In wetter years the now-dry ponds may again fill with water, Smol said.

But they won't be the same again, he says, since many of the organisms that once lived in the ponds have probably died.

More to Come

"The disappearance of lakes is occurring widely across the Canadian, Alaskan, and Siberian Arctic," said Larry Hinzman of the University of Alaska at Fairbanks.

A 2005 study by Hinzman and his colleagues showed lakes are also disappearing in Siberia, just south of the Arctic Circle.

Scientists speculate that there the permafrost beneath the lakes has been melting, draining the water like pulling the plug on a bathtub.

Another study last year used satellite photos to show that lakes in Alaska were also shrinking.

"These shrinking lakes will have a drastic ecological impact, which will reverberate through the northern life cycle," Hinzman said.

It will hurt migratory waterfowl and the people who depend on the lakes for subsistence hunting and fishing, he pointed out.

And there's more drying in store, he said.

"This will certainly continue, and probably accelerate for the foreseeable future."

The Arctic ponds in the new study "are like the miners' canaries of the planet," study leader Smol added, "showing the first signs of warming."