Long-term sea level increases that could have a devastating effect on southern Florida and highly populated coastal areas may be even larger than once thought, a report suggests. Some studies have suggested that melting of ice in Antarctica and other areas could raise sea levels by 16 feet to 17 feet over the long run, a potential threat to coastal areas such as Washington, DC, New York City and California. But a report in Friday's edition of the journal Science warns that factors not previously considered could one day boost that increase to up to 21 feet in some areas. The study did not list a time frame for such a dramatic change. But co-author Peter Clark, a geoscientist at Oregon State University, stressed that they "aren't suggesting that a collapse of the West Antarctic Ice Sheet is imminent." The most recent International Panel on Climate Change report estimated sea level rise of up to 3 feet by the end of this century. "People have been trying prepare for sea level rise for some time, it's not a new issue," Clark said, noting that the US Fish and Wildlife Service and US Geological Survey are holding a meeting in San Francisco on the effects of coastal change. Earlier research has focused on melting ice adding water to the oceans and on thermal expansion of sea water in a warmer climate over long periods of time. In the new report geophysicist Jerry X. Mitrovica and physics graduate student Natalya Gomez of the University of Toronto, Canada, and Clark, say other factors need to be considered. - 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. - The melting of the West Antarctic ice sheet will cause the Earth's rotation axis to shift, potentially moving water northward. "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," Mitrovica said in a statement.