February 6, 2009

Unconsidered Factors Could Result In Higher Than Expected Sea Levels

Sea levels could rise up to 21 feet in certain regions, according to a new report that takes into account new previously unconsidered factors.

Geophysicists from the University of Toronto have previously reported that coastlines of North America and of nations in the southern Indian Ocean could face the greatest threats from a West Antarctic Ice Sheet collapse.

But in a new report, scientists found that some estimates failed to take certain factors into account.

"We aren't suggesting that a collapse of the West Antarctic Ice Sheet is imminent," said Peter Clark, a professor of geosciences at Oregon State University, and co-author of a new study to be published in the February 6 issue of Science magazine. "But these findings do suggest that if you are planning for sea level rise, you had better plan a little higher."

The Intergovernmental Panel on Climate Change has estimated that a collapse of this ice sheet would result in a sea level rise of between 16 and 17 feet worldwide, but scientists said it could rise to almost 21 feet, leaving places such as Washington, D.C. underwater.

The most recent International Panel on Climate Change report estimated sea level rise of up to 3 feet by the end of this century.

"There is widespread concern that the West Antarctic Ice Sheet may be prone to collapse, resulting in a rise in global sea levels," said geophysicist Jerry X. Mitrovica, who co-wrote the report along with physics graduate student Natalya Gomez.

The AP cited certain crucial factors that remain to be considered, including:

  • When an ice sheet melts, its gravitational pull on the ocean is reduced and water moves away from it. That means sea levels could fall near Antarctica and rise more than expected in the northern hemisphere.
  • Antarctic bedrock that currently sits under the weight of the ice sheet will rebound from the weight, pushing some water out into the ocean. "A study was done more than 30 years ago pointing out this gravitational effect, but for some reason it became virtually ignored," Clark said. "People forgot about it when developing their sea level projections for the future."
  • The melting of the West Antarctic ice sheet will cause the Earth's rotation axis to shift, potentially moving water northward.

The result of these factors would cause the sea level anywhere near Antarctica to decrease, while the levels in many other areas, mostly in the Northern Hemisphere, would go up, they wrote.

"The net effect of all of these processes is that if the West Antarctic Ice Sheet collapses, the rise in sea levels around many coastal regions will be as much as 25 per cent more than expected, for a total of between six and seven meters if the whole ice sheet melts," said Mitrovica.

Clark and Mitrovica acknowledged that it is still unknown whether the West Antarctic Ice Sheet will melt, and if so, how soon.

"There is still some important debate as to how much ice would actually disappear if the West Antarctic Ice sheet collapses "“ some fraction of the ice sheet may remain quite stable," said Mitrovica.  "But, whatever happens, our work shows that the sea-level rise that would occur at many populated coastal sites would be much larger than one would estimate by simply distributing the meltwater evenly."

"However," Clark added, "these same effects apply to any amount of melting that may occur from West Antarctica. So many coastal areas need to plan for greater sea level rise than they may have expected."


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