Correction to This Article
The A-section article about the melting of the polar ice caps misspelled the name of Ted Scambos?, a researcher at the National Snow and Ice Data Center at the University of Colorado at Boulder.
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At the Poles, Melting Occurring at Alarming Rate

As the Arctic ice melts and ice shelves collapse in the Southern Ocean, vast areas of open water are exposed. The water absorbs heat from the sun that until now was reflected by the ice. As that heat warms the seas, the treadmill is expected to slow, the IPCC has reported. In the worst case, it could stop. The previous time that happened, 15,000 years ago, the Northern Hemisphere was plunged into a brief and brutal ice age, apparently within decades.

"It's like having a pool of warm water sitting in the middle of what is supposed to be the air conditioner of the north," Scanbos said. "And it will be within our lives, not our grandchildren's."

The northern Arctic is changing first, and most noticeably. But the changes there are likely to be followed by a "one-plus-two-plus-three punch" in the southern polar region, said Ellen Mosley-Thompson, at the Byrd Polar Research Center at Ohio State University. She has been drilling and measuring ice in polar regions and mountain glaciers for 25 years, and has lived in frigid tents and battled brutal weather for weeks at a time to get ice core samples.

The Antarctic has sent complicated signals, she said. The interior is "a monstrously large ice sheet that creates its own microclimate." Winds circling Antarctica have buffered it, so average temperatures on the continent have changed little in 50 years and the snowpack in the interior has been growing.

Furthermore, Antarctica's ice sits on land, and the warming of even the most gloomy predictions is unlikely to make a big dent in that huge ice pack very soon.

But western Antarctica and the peninsula that juts northward have a different weather scheme. The air over the Antarctic peninsula is one of three places on the globe with the fastest heating; Arctic western Canada and Alaska and the high mountains of China and Tibet are the others. The sea water also is heating, accelerating the breakup of ice shelves that stick out over the sea. When they go, the glaciers on land behind them begin to accelerate their slide to the sea, adding to global sea levels.

"We thought the Southern Hemisphere climate is inherently more stable," Scanbos said. But "all of the time scales seem to be shortened now. These things can happen fairly quickly. A decade or two decades of warming is all you need to really change the mass balance.

"Things are on more of a hair trigger than we thought."

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