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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Antarctic sea ice has gotten little respect, especially compared with its top-of-the-world cousin, or with the enormous ice sheets on Greenland and the Antarctic continent. The sea ice is hard to reach. It has little direct effect on people. And the Southern Ocean was not a cold-war playground for US and Soviet submarines, which amassed a wealth of information on changes in Arctic sea ice before the era of long-term satellite observations.
But as a research target, southern sea ice's stock appears to be rising.
Over the past 20 years, southern sea ice has expanded, in contrast to the Arctic's decline, and researchers want to understand why. Many climate-model experiments show the Arctic responding more rapidly than Antarctica as global warming kicks in. But after looking at the latest projections from the Intergovernmental Panel on Climate Change, "Arctic sea ice is well ahead of the models, and Antarctic sea ice is well behind what the models project," says Stephen Ackley, a polar scientist at the University of Texas, San Antonio.
Moreover, recent studies have shown that in key regions off the Antarctic coast, sea ice shows a strong, coherent response to El Niño-La Niña cycles, decade-scale climate swings in the tropical Pacific whose length, strength, and timing may be affected in uncertain ways by global warming. Indeed, outside the tropics, Antarctica boasts the strongest climate response to El Niño of any region on the planet. This suggests strong climate connections and feedbacks among sea, ice, and air in the Southern Ocean that are poorly understood.
Some scientists say trends in sea ice in key spots around the continent may be bellwethers for worrisome changes that could accelerate the melt of nearby land ice, most notably the West Antarctic Ice Sheet.
The overall growth in Antarctica's sea ice over the past two decades masks significant regional declines in the Bellingshausen and Amundsen Seas – the destination for glaciers flowing from the West Antarctic Ice Sheet. Researchers say these glaciers are losing ice to the sea faster than snow is replenishing the ice. Thus, the large regional drops in sea ice could also signal the presence of "a very big threat to glacier ice" on the continent, says Xiaojun Yuan, a polar scientist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y. The leading suspect: relatively warm water upwelling near the coast as a result of global warming's effect on wind patterns in the region.
To address some of these issues during the International Polar Year, which ends in March 2009, scientists are installing a network of buoys off Antarctica's coast. The buoys will track changes in sea ice and measure the factors in air and atmosphere that trigger those changes. Last August, 10 international science groups joined forces on a project dubbed SOPHOCLES, which aims to use the latest information on the Southern Ocean and Antarctica's land and sea ice to improve climate models.
'Antarctic sea ice is such a different animal'
For some commentators, the out-of-sync trends in sea ice at the two poles is evidence that warming isn't global and doesn't deserve the international angst it triggers.
Not so fast, many researchers respond. Northern and southern sea ice shouldn't necessarily act in lock-step. "Antarctic sea ice is such a different animal," says Douglas Martinson, another polar-ice specialist at Lamont-Doherty. Geographic and oceanographic differences – a virtually landlocked ocean in the north versus an open ocean in the south – encourage the buildup of thick, long-lasting, multiyear ice in the Arctic Ocean. Antarctica's sea ice, by contrast, is largely thin and seasonal. In winter, Antarctic sea ice covers an area nearly twice the size of Europe. By the end of summer, it shrinks to one-sixth of its winter extent. These wide swings make it difficult to tease out long-term trends in ice cover there.