Flood Cooled Atmosphere in Ice Age

By CHRIS KAHN, AP

Sometimes global warming can result in cold weather.  Scientists say that as the glaciers melted at the end of the Ice Age, so much cold fresh water gushed into the North Atlantic 8,200 years ago that it cooled the atmosphere for hundreds of years.  The cold spell has been well known to researchers, but its cause was a mystery.

The flood scenario, outlined in Thursday's issue of the journal Nature, demonstrates how global warming can, paradoxically, trigger a global freeze, and do it in a matter of decades.

If a modern glacier such as the Greenland Ice Sheet melts as a result of rising temperatures in the next century, it could trigger a similar flood and climate fluctuation, researchers said.

``Ultimately, that's the interest here, said Richard Alley, a climate expert at Penn State University. ``We've been pretty lucky, the climate hasn't varied much in 8,000 years. But could the big changes come back?''

The study by University of Colorado and Canadian researchers examines evidence of a huge flood in the Hudson Bay region of Quebec and Ontario.

The Laurentide Ice Sheet covered parts of North America with ice up to 2 miles thick for more than a million years. As it retreated toward the poles, the ice sheet left in its wake at least two lakes containing more water than the Great Lakes combined.

In the Hudson Bay, ice held the water in place like a plug in a bathtub. But when the plug finally melted, trillions of gallons gushed into the Labrador Sea, shooting out at 100 times the rate water leaves the Mississippi.

Independent research showed the temperature dropped significantly within several hundred years of the flood. But nobody could pinpoint if these two events were connected, said the study's lead author, University of Colorado geologist Don Barber.   The evidence linking the temperature drop to the flood includes radiocarbon dating of clams in the flood sediment.

The Atlantic Gulf Stream normally acts like a conveyor belt to deliver warm tropical water to temperate regions. By adding so much fresh water in such a short time, the flood shut down the Gulf Stream, Alley said.

Temperatures in Greenland and Europe dropped by 6 to 15 degrees for at least 200 years, according to ice core data.

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