A collapse of the West Antarctic Ice Sheet would raise sea levels by about 10 feet, not the nearly 20 feet as earlier predicted, according to a new study.
Researchers led by Jonathan L. Bamber of the University of Bristol in Britain report their recalculation of the hazard in Friday's edition of the journal Science.
However, they add, the maximum increase is expected along the East and West Coasts of the United States, where sea levels could rise as much as 25 percent more than in other regions.
The West Antarctic Ice Sheet can be unstable and has long concerned researchers who fear it could collapse as a result of global warming.
Previous studies had estimated that failure of the ice sheet, causing it to slide into the ocean, would raise global seas levels by 5-to-6 meters, or 16-to-19 feet.
But Bamber's team calculated that the entire sheet would not collapse, with parts of it remaining grounded on the continent. Thus, they said, sea level rise would be only about 3 meters, or just under 10 feet.
Even 10 feet, however, would mean some coastlines and even islands would become submerged unless expensive seawalls were built as fortifications.
How fast this might happen was unclear, but an earlier study suggested the melting could take 500 years, which would mean a sea-level rise of about a quarter-inch a year.
The melting would also redistribute the balance of mass between water and land on the planet, potentially affecting Earth's rotation and causing water to build up along the North American coasts and in the Indian Ocean.
The research was supported by the United Kingdom Natural Environment Research Council and the Colorado University Cooperative Institute for Research in Environmental Science.
The study did not look at Greenland's ice sheet, which has shown signs of rapid melt.