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As arctic ice melts, South Pole ice grows

Scientists are puzzled, but the phenomenon seems to fit the latest global-warming models.

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Since 1978, the satellite record shows that Antarctica's sea ice has expanded by about half a percent a year. Declines in sea ice recorded between 2000 and 2002 have significantly moderated the overall rise.

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These long-term data have let scientists tease out relationships between Antarctic sea ice and natural climate variations, such as swings between El Niño and La Niña in the tropical Pacific. Recent modeling work has given scientists a sense that they are on the right track as they explore the processes affecting sea ice. Dr. Yuan, who uncovered Antarctica's coherent response to El Niño, has developed a seasonal sea-ice forecast model for key regions that scientists now use to plan expeditions.

The ice also plays a key ecological role in the region, some of which bears on the exchange of CO2 between the atmosphere, ocean, and ice, and on cloud formation.

Researchers have found that bacteria and algae that live in the ice trigger the production of huge amounts of dimethyl sulfide, a compound that, when exposed to oxygen, reacts to form aerosol particles around which moisture can condense as cloud droplets. In the ocean, algae and plankton produce the compound. But on the ice, researchers find concentrations some 3,000 times higher than in seawater. And where ice was once thought to keep the ocean from taking up CO2 or returning it to the atmosphere, the picture has grown more complex, Dr. Ackely says. Cold ice does seal in CO2. But slightly warm ice or ice under a little bit of snow begins to flush CO2 out of the ice and back into the air.

Antarctic ice may be melting from underneath

Given the complex role sea ice plays directly or indirectly in the biology and climate of the Southern Ocean region and beyond, its future under global-warming scenarios is of keen interest. Currently, models suggest that through the end of the century, Antarctic sea ice will begin an overall decline, although it isn't projected to be as dramatic as the Arctic's. There, some researchers predict summer sea ice will virtually vanish by 2013, 27 years earlier than previously projected.

A key measurement scientists are trying to make beginning this year involves the mass of Antarctica's sea ice. In the Arctic, ice began to melt from underneath before major shifts in its extent appeared. Thus, measurements of the sea ice's overall mass may uncover changes that aren't readily seen in satellite images.

One factor that could complicate this mass balance is snowfall. Researchers have long known that snow builds glaciers. Two years ago, a team of scientists combined snow-thickness measurements with modeling studies and found that, at least in Antarctica, snow also may build Antarctica's sea ice.

As the climate has warmed, more moisture has made its way to high latitudes. "In the Antarctic in particular, we expect more snowfall," says Achim Stoessel, a researcher at Texas A&M University in College Station, who took part in the study. Simulations showed that with increased snowfall, a sufficiently thick snow layer would push the ice underwater. The seawater in the snow-ice boundary would freeze, thickening the floe.

Some researchers suggest that this process may eventually arrest the decline of Arctic sea ice as well.