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Glaciers surge after ice shelf collapses. (Slippin' Slide).

Five of the six large glaciers that once fed into Antarctica's Larsen A ice shelf have sped up significantly since that floating ice mass collapsed and drifted away in January 1995, scientists report.

Analyses of satellite images and aerial surveys reveal the glaciers' acceleration, says glaciologist Hernan De Angelis of the Argentine Antarctic Institute in Buenos Aires. For instance, photos from orbit show that between February 2000 and September 2001, portions of the Sjogren Glacier moved at an average speed of about 2 meters per day. That's about double the speed measured in 1999 and four times the rate clocked in 1995 just after Larsen A's disintegration. De Angelis and his institute colleague Pedro Skvarca report their analyses in the March 7 Science.

Although most of the areas large glaciers now flow seaward more quickly, ice upstream at higher elevations has sped up only modestly, if at all. These flow-rate differences affect the glaciers' surface topography. Among the most telling features is a series of 20-to-40-m-tall ice terraces, says De Angelis. Those icy cliffs, first observed in aerial surveys about a year ago, formed as the ice in the glaciers' channels surged seaward and thinned while peripheral ice lagged behind.

Although the breakup of an already floating ice shelf doesn't affect global sea levels, any subsequent flux of new ice from land-based glaciers into the ocean could lead to sea level rises. Because scientists haven't measured the thickness of glaciers that are now freely spilling across the 100-kilometer coastline once fringed by Larsen A, they don't yet know how much global sea levels might be affected, says Skvarca.

Scientists have long debated whether ice shelves retard the flow of the glaciers that feed them. De Angelis and Skvarca's new finding strongly suggests that the presence of an ice shelf can matter, says Eric J. Rignot of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

A glacier's flow rate can also be influenced by climate and other local conditions, Rignot adds. De Angelis and Skvarca agree that many factors contribute to a glacier's flow rate, including the firmness of rocks or sediments that the ice glides across and the presence of subglacial melt water.
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Title Annotation:Antarctica's Larsen A ice shelf
Author:Perkins, S.
Publication:Science News
Geographic Code:8ANTA
Date:Mar 8, 2003
Words:362
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