Global warming could lead to sea level rises of more than 20 feet -- far higher than current estimates, and enough to swamp densely populated coastlines of north America and Europe, say scientists.
A team of experts has calculated that if the West Antarctic Ice Sheet collapses -- as many expect it to -- the outcome will be worse than has been forecast.
The sea level in the wake of the collapse is already projected to rise between 16 and 17 feet. The new research suggests the true figure could be almost 21 feet in the North Atlantic.
If this was to happen, many coastal areas in both north America and Europe would be devastated, and much of southern Florida would disappear.
Professor Peter Clark, one of the scientists, from Oregon State University in Corvallis, US, said: "We aren't suggesting that a collapse of the West Antarctic Ice Sheet is imminent, 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 (IPCC) estimates that losing the ice sheet would raise sea levels by 16.5 feet. But the scientists argue that the official forecast does not take account of several key factors, such as gravity or changes in the Earth's rotation.
The West Antarctic Ice Sheet towers more than 6,000 feet above sea level over a large section of Antarctica and has enough mass to exert substantial gravitational attraction, say the researchers.
They argue that water is pulled towards the ice sheet in the same way the tides are influenced by the Sun and moon. When the ice sheet goes, so will its grip on the oceans, allowing more water to move.
The study, reported in the journal Science, also suggests that when the ice is removed the land beneath it will "bounce up".
At the same time, the Earth's axis of rotation would shift by about a third of a mile.
All these forces would cause sea level around the vicinity of Antarctica to fall, it is claimed. Correspondingly, sea level in many other areas -- mostly in the northern hemisphere -- would go up.