by Michael Reilly, Discovery News, August 1, 2008
It's one of the most dramatic examples of climate change in Earth's history, and scientists now say it happened almost entirely in one year's time.
Thirteen thousands years ago, Europe was much like it is today -- cool but temperate, with great forests carpeting the land. Ice sheets still nibbled at Finland and Sweden, but for much of the continent the last Ice Age was a distant memory.
Suddenly, the climate went haywire. Warm Gulf Stream currents that brought heat from the equator up toward the pole began to fail. Temperatures plummeted 34 °C, and stayed that way for a millennium.
Now scientists believe they've pinpointed the exact time the northern hemisphere was plunged back into a deep freeze. Examining sediments preserved at the bottom of a remote lake in western Germany, they found that what's known as the Younger Dryas cold period took just a year to sweep across the continent, starting in the autumn, 12,679 years ago.
Led by Achim Brauer of the German Research Center for Geosciences in Potsdam, Germany, the team believes such a quick, profound change in climate could only have been brought about by a shift in winds across the northern hemisphere.
Today prevailing winds in the northern hemisphere above the tropics tend to blow from the southwest to the northeast. Air that flows over Texas soon crosses the Atlantic and winds up over Norway.
As it travels the air passes over the Gulf Stream, a warm ribbon of water pouring northward from the tropics. The balmy air brings heat to Europe, which otherwise would be chilly.
The same was probably true just before the Yonger Dryas set in. But as the vast Ice Age glaciers retreated, their melt water flowed into the northern reaches of the Atlantic Ocean. The injection of fresh water made the sea easier to freeze, and a new skin of ice began advancing south.
The warm conveyor belt of Gulf Stream waters soon ebbed to a trickle. And as the sea ice advanced, the winds shifted into a west-east pattern. Within a year the breezes that warmed Europe had vanished.
"The Younger Dryas continues to surprise us in providing a message as to how quickly climate change can occur," said Daniel Sigman of Princeton University in N.J. Sigman is a co-author on the study, which appears today in the journal Nature Geoscience.
"The hypothesis on this paper is I think a very nice one," Richard Alley of Pennsylvania State University said. Winds tend to blow parallel to temperature gradients, and the gradient between sea ice and open water can be very sharp, up to 40 °C.
The sea ice border probably extended in a rough west to east direction, and the winds would've followed it, bringing cold air to much of Europe.
"You can think of it as a front pushing down across Germany," Alley said. "Winds go where something's pushing them hard. A steep temperature gradient along the edge of sea ice would push hard in an east-west direction."
http://dsc.discovery.com/news/2008/08/01/abrupt-climate-change.html
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