The study says rising temperatures could boost sea levels by as much as 1.4 meters (4.6 feet) by 2100, almost twice the rate previously forecast, increasing the flood risk in low-lying areas, and the threat of storm surges to cities such as New York and London.

Climatologists so far agree that sea levels will increase 9-88 centimeters (4-35 inches) over 1990 levels by the end of the century, but in the paper published in the journal Science, a German researcher suggests the range could be much higher, 50-140 centimeters (20-55 inches).

Stefan Rahmstorf, professor of ocean physics at Potsdam University, said current working assumptions are unreliable because the computer models that generated them significantly underestimate the rise in sea levels already seen.

"In the past 40 years, sea levels have increased about 50 percent more than the climate models predicted. That tells us that we haven't understood the problem of rising sea levels yet," said Rahmstorf.

Current computer models used by the Intergovernmental Panel on Climate Change, an UN-sponsored body that advises policymakers on the effects of climate change, considers a number of factors including thermal expansion and the effect of melting ice sheets.

For his analysis, Rahmstorf examined the relationship between rising sea levels and increases in global average near-surface air temperatures. He concluded that the rate of increase in sea levels was proportional to rising temperatures and accounted for the changes seen in the 20th century.

Rahmstorf said the higher range has serious implications, not just for low-lying areas which have already been identified as global warming-related flood risks, but also for some major cities in the Western hemisphere.

In a study published last year, Rahmstorf and his team at the Potsdam Institute for Climate Impact Research argued that global warming could increase sea levels in the North Atlantic by shutting down or weakening an ocean current called the overturning circulation or conveyor belt.

Under that scenario, regional sea levels could rise by up to a meter (yard), and once the greenhouse gas effect is factored in, the increase could be as much as two meters, which could expose London and New York to potentially "devastating storm surges," Rahmstorf said.

At the very least, Rahmstorf says his study shows just how wide the margin of error is when it comes to forecasting climate-related change in sea levels.

"The fact that we get such different estimates using different methods shows how uncertain our sea level forecasts still are."