Scientists Rule Out One Threat Of Antarctic Collapse

They found that one threat from the roster of suspects that could cause the collapse of the West Antarctic Ice Sheet, the most unstable part of the region, seems less of a threat. A collapse would raise the world's sea level and threaten to flood coastal areas.

John Anderson, professor of geology and geophysics, and graduate student Stephanie Shipp have been studying how the West Antarctic Ice Sheet slides across the bed of rock or sediment beneath it, a mechanism believed to be one that could lead to breakdown of the ice sheet. The sliding allows it to accelerate, become thinner and eventually float off the sea floor. This phenomenon is known as ice sheet collapse.

To test the plausibility of this mechanism, Rice geologists have been examining the geological record of the ice sheet's behavior during the past 20,000 years.

"The evidence we have today shows that this was not an important mechanism in the past," Anderson says. "The evidence suggests that the recent retreat of the ice sheet has been slower and more continuous than previously thought. So this portion of the ice sheet appears to be more stable than some previous models predicted."

Using sophisticated instruments such as side-scan sonar, the Rice team of scientists imaged the sea floor on which the ice sheet formerly rested. These records show a variety of glacial landforms that formed beneath the ice sheet, such as giant lineations or ridges that extend for miles across the sea floor and mark the former route of the advancing ice sheet. Other features were formed in the wake of the retreating ice sheet and indicate that it never actually floated off the bed or collapsed.

Anderson's and Shipp's results were presented at a September meeting of glaciologists, oceanographers, meteorologists and geophysicists interested in the fate of the ice sheet. The results will be published in a spring issue of the Geological Society of America Bulletin.

Anderson has made 20 visits to Antarctica over the past 28 years to study the structure, movement and erosion of this icy continent.

There is a large range of uncertainty in trying to predict how much the ice sheets will contribute to sea level rise over the next century as Earth's climate warms. Reports published by an international government panel estimate a rise in sea level by 2100 that varies in range from 20 centimeters, which is minimal and the present rate, to 96 centimeters. The West Antarctic Ice Sheet is believed to be the most likely source of this rise, but there is still debate concerning the ice sheet's stability and those mechanisms that could cause the ice sheet to melt.

If this extreme prediction comes true, Anderson says, Texas's Galveston Island would not survive, South Louisiana would be flooded, and Bangladesh would be under water. To date, research has focused on the Ross Sea, where 50 percent of the ice sheet drainage into the ocean occurs.

To get a more accurate prediction of future behavior of the ice sheet, scientists look at a variety of complex factors, including temperature, precipitation, the ice sheet system, and climate change and its direct impact on surface oceans. The most predictable impact of global warming is that of expansion of the oceans as surface waters become warmer. Scientists also want to understand those complexities over a 100-200 year scenario.

Other mechanisms that have been tied to a rapid retreat of the ice include melting under the ice from warm ocean currents.

"The undermelting now seems to take on greater importance," Anderson says, "now that the ice sliding across a bed of deforming sediment is played down by our results."

In visits to Antarctica this year, Anderson and his team of graduate and undergraduate students will focus on other factors that could lead to collapse. They will head to Pine Island Bay in the Antarctic Peninsula region, an area of the ice sheet that currently is retreating rapidly. The area has been dubbed the "weak underbelly of the ice sheet" by some glaciologists. The Rice team will try to determine the recent retreat history of the ice sheet from the bay to see if evidence exists there for a collapse.

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Editors: For more information about Antarctica visit http://www.glacier.rice.edu/.