His study, based on air temperatures and past sea level changes rather than computer models, suggested seas could rise by 50-140 cms by 2100, well above the 9-88 cms projected by the scientific panel that advises the United Nations.

A rise of one metre might swamp low-lying Pacific islands such as Tuvalu, flood large areas of Bangladesh or Florida and threaten cities from New York to Buenos Aires.

"The computer models underestimate the sea level rise that has already occurred," Rahmstorf told Reuters of a rise of about 20 cms since 1900. "There are aspects of the physics we don't understand very well."

Sea level changes hinge on poorly understood factors such as the pace of the melt of glaciers and of ice sheets in Greenland and Antarctica. Water also expands as it gets warmer but the rate of penetration of heat to the depths is uncertain.

"My main conclusion is not that my forecast is better but that the uncertainty is much larger because of the different results you get with reasonable methods," he said.

Almost all climate scientists reckon the world is warming because of emissions of greenhouse gases from human use of fossil fuels in factories, power plants and cars. Rising temperatures could bring more droughts, floods and heatwaves.

TIDES

Rahmstorf likened his approach to predicting the height of tides along a coast, largely based on past observations.

He said seas were 120 metres below present levels during the last Ice Age 20,000 years ago and 25-35 metres higher than the present in the Pliocene epoch 3 million years ago.

In the Ice Age temperatures were 4-7 Celsius (7.2-12.6 Fahrenheit) cooler than today and 2-3 C (3.6-5.4F) warmer in the Pliocene. That suggested sea levels change 10-30 metres per rise or fall per degree Celsius (1.8F), over thousands of years.

The UN climate panel has projected temperatures will rise by 1.4-5.8C (2.5-10.4F) by 2100, mainly because of human influences.

"Sea level is a very slow component of the climate system so what we see by the year 2100 is just a small percentage of the total we are causing," Rahmstorf said.

There was still time for the world to cut greenhouse gas emissions but he said the slow pace of UN talks on extending the UN's Kyoto Protocol beyond 2012 "gives you the impression that governments are not very well aware of how urgent the whole problem has become". Coastal cities in the North Atlantic -- from New York to London -- could be especially vulnerable because a possible slowdown of ocean currents could also raise sea levels in the North Atlantic and lower them in the southern hemisphere.