Sea levels will rise by twice as much as previously predicted as a result of global warming, an important international study has concluded.
The Scientific Committee on Antarctic Research (SCAR) calculated that if temperatures continued to increase at the present rate, by 2100 the sea level would rise by up to 1.4 metres - twice that predicted two years ago.
Such a rise in sea levels would engulf island nations such as the Maldives in the Indian Ocean and Tuvalu in the Pacific, devastate coastal cities such as Calcutta and Dhaka and force London, New York and Shanghai to spend billions on flood defences.
Even if the average global temperature increases by only 2C - the target set for next week's Copenhagen summit - sea levels could still rise by 50cm, double previous forecasts, according to the report.
SCAR, a partnership of 35 of the world's leading climate research institutions, made the prediction in the report Antarctic Climate Change and Climate. It far exceeds the 0.59 metre rise by the end of the century quoted by the Intergovernmental Panel on Climate Change (IPCC) in 2007. This was based on a "business as usual" approach by governments that allowed temperatures to rise by 4 degrees. It will underpin the negotiations in Copenhagen.
SCAR scientists said that the IPCC underestimated grossly how much the melting of the Antarctic and Greenland ice sheets would contribute to total sea-level rises.
One of the world's leading experts on climate science has called for the world to intensify efforts to control global warming by actively removing carbon dioxide from the atmosphere.
In an interview with The Times, Dr Rajendra Pachauri, chairman of the IPCC, said that geo-engineering, where carbon is stripped from the atmosphere using specialist technologies, would be necessary to control runaway damage to the climate. "At some point we will have to cross over and start sucking some of those gases out of the atmosphere."
He added that world leaders meeting in Copenhagen should aim for a tighter target of no more than a 1.5C rise in global temperatures.
The IPCC report predicted that the melting of ice sheets would contribute about 20 per cent of the total rise in sea levels, with the majority coming from the melting of glaciers and the expansion of the water as it warms. It said that it was not able to predict the impact of melting ice sheets, but suggested this could add 10-20cm.
The SCAR report uses detailed climate observations over the past century linking temperature to sea levels to produce a more sophisticated estimate. It puts the likely contribution from ice sheets at more than 50 per cent.
The calculations were carried out by Stefan Rahmstorf, Professor of Physics of the Oceans at the Potsdam Institute for Climate Impact Research in Germany. Sceptics seized upon his figures as further evidence of the unreliability of climate change predictions.
"It's 50cm, 60cm, 100cm - 60m if you ask James Hansen from Nasa," said Benny Peiser, director of the Global Warming Policy Foundation . "The predictions come in thick and fast, but we take them all with a pinch of salt. We look out of the window and it's very cold, it doesn't seem to be warming. We're very concerned that 100-year policies are being made on the basis of these predictions"