The catastrophic increase in sea level, already projected to average between 16 and 17 feet around the world, would be almost 21 feet in such places as Washington, DC, scientists say, putting much of the U.S. capital city underwater.
Much of Southern Florida would disappear under the waves along with coastal areas of North America, the study predicts.
The report titled "The Sea Level Fingerprint of West Antarctic Collapse" was published Friday in the journal "Science," by researchers from Oregon State University and the University of Toronto.
"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. "But these findings do suggest that if you are planning for sea level rise, you had better plan a little higher."
NASA image of melting in West Antarctica obtained by satellite. January 2005.
"Scientists are particularly worried about the ice sheet because it is largely marine-based, which means that the bedrock underneath most of the ice sits under sea level," says co-author geophysicist Jerry Mitrovica, director of the Earth System Evolution Program at the Canadian Institute for Advanced Research.
"The West Antarctic is fringed by ice shelves which act to stabilize the ice sheet," Mitrovica explained. "These shelves are sensitive to global warming, and if they break up, the ice sheet will have a lot less impediment to collapse."
The edges of the vast ice sheet flow out into floating ice shelves, including the huge Ross Ice Shelf and Ronne Ice Shelf. This topography makes it "inherently unstable," Clark said.
"There is widespread concern that the West Antarctic Ice Sheet, which is characterized by extensive marine-based sectors, may be prone to collapse in a warming world," Clark and his team report.
This concern was reinforced by a recent study led by Eric Steig of the University of Washington that showed the entire region of the West Antarctic Ice Sheet is indeed warming.
A team of NASA and university scientists has found clear evidence that extensive areas of snow melted in west Antarctica in January 2005 in response to warm temperatures. This was the most significant melt observed using satellites during the past three decades.
It's still unclear, Clark said, when or if a breakup of the West Antarctic Ice Sheet might occur, or how fast it could happen. It may not happen for hundreds of years, he said, and even then it may not melt in its entirety. Research should continue to better understand the forces at work, he said.
The Intergovernmental Panel on Climate Change has estimated that a collapse of the West Antarctic Ice Sheet would raise sea levels around the world by about 16.5 feet, on average, and that figure is still widely used.
But Clark and his team found that the IPCC's theoretical average does not consider several key forces, such as gravity, changes in the Earth's rotation or a rebound of the land on which the massive glacier now rests.
An iceberg off Davis Base, Antarctica. Icebergs are formed when ice breaks off the Antarctic ice sheet. Every year about two million tonnes of ice break off the ice sheet to form icebergs. (Photo by Graham Blight courtesy CSIRO)
Right now, the West Antarctic Ice Sheet has a huge mass, towering more than 6,000 feet above sea level over a large section of Antarctica.
This mass exerts a substantial gravitational attraction, the researchers say, pulling water toward it - much as the gravitational forces of the Sun and Moon cause the constant movement of water on Earth - the rise and fall of tides.
"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 new study adds further factors to the calculation - the weight of the ice forcing down the land mass on which it sits and also affecting the orientation of the Earth's spin.
When the ice is removed, it appears the underlying land would rebound, and the Earth's axis of rotation defined by the North and South Pole would actually shift about one-third of a mile, also affecting the sea level at various points.
When these forces are all taken into account, the sea level anywhere near Antarctica would fall, the report concludes, while the sea level in the Northern Hemisphere would rise.
If the West Antarctic Ice Sheet completely melted, the east coast of North America would experience sea levels more than four feet higher than had been previously predicted - almost 21 feet - and the west coast, as well as Miami, Florida would be about a foot higher than that.
Most of Europe would have sea level rise of about 18 feet. Nations in the southern Indian Ocean would also see their coastal areas inundated.
"If this did happen, there would also be many other impacts that go far beyond sea level increase, including much higher rates of coastal erosion, greater damage from major storm events, problems with ground water salinization, and other issues," Clark said. "And there could be correlated impacts on other glaciers and ice sheets in coastal areas that could tend to destabilize them as well."
The research was funded with support from the Canadian Institute for Advanced Research, the Natural Sciences and Engineering Research Council of Canada, the John Simon Guggenheim Memorial Foundation and the U.S. National Science Foundation.
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