Possible Instability In Larsen C, Amundsen Sea Ice Shelves - AP

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PUNTA ARENAS, Chile — "Scientists looking south from the tip of South America, over steel-gray waters toward icy Antarctica, see only questions on the horizon about the fate of the planet. Now that one mammoth Antarctic ice shelf has collapsed into the ocean, when might another, bigger one crumble and slip into a warming sea? In 1,000 years? In 100 years? Sooner? Never?

"People don't have the answer to the question yet — what the probability is of that collapse, if any," said scientist Gino Casassa. "But there's some indication of instability." Casassa and fellow Chilean researchers had just flown back from the icy continent to this expedition staging point, and brought with them some potentially unsettling news. On a two-month round-trip trek by snow tractor to the South Pole, they pointed their sophisticated radar at the ground and found that the West Antarctic Ice Sheet may be thicker than thought, many hundreds of feet thicker in parts. Glaciologists like Casassa worry most about that western ice sheet, half a continent of frozen water believed enough, if gradually melted, to raise ocean levels worldwide by about 15 feet.

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Skvarca and American researchers, collating aerial reconnaissance with ICESat images, reported last September that land-based glaciers backed up behind Larsen B have accelerated their flow since its break-up — moving ice into the sea up to eight times faster than before. Now scientists are warily watching the Larsen C ice shelf, farther south and 20 times larger. Like the B sector before it collapsed, "Larsen C also appears to be thinning," said Robert Thomas, a researcher at NASA's Goddard Space Flight Center facility in Virginia. "It's quite possible that the thinning is the precursor to the breakup." Scientists expect Larsen C to disintegrate sometime in this century, Skvarca said in a telephone interview from Buenos Aires. "We should bear in mind what is happening to the Larsen ice shelves, because if it also happens to a big shelf, we are going to be in trouble," he said.

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No one has spotted signs of instability yet in the two giant shelves (ed. - Ross & Ronne). But scientists also worry about what Thomas calls the "back door" — a stretch of Antarctic coast whose glaciers feed off the western ice sheet directly into the Amundsen Sea, or into small ice shelves on that sea. "There's some indication of instability in the Amundsen Sea," Casassa said. Casassa, Thomas and colleagues have watched the "back door" since the early 1990s, most recently via satellite tracking. They reported in the journal Science last September that a half-dozen glaciers there are now thinning and accelerating. The big Pine Island glacier is carrying ice faster toward the sea as far as 186 miles inland, deep within the western ice sheet. "The instability and changes are migrating inland," Casassa said. "This could affect the whole of the West Antarctic Ice Sheet, because the ice sheet 'feels' what is happening at the coast."

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http://www.dailysouthtown.com/southtown/dsindex/07-ds6.htm

[Wilms]

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