Sea levels could rise by up to one-and-a-half metres by the end of this century, according to a new scientific analysis.
This is substantially more than the Intergovernmental Panel on Climate Change (IPCC) forecast in last year's landmark assessment of climate science.
Sea level rise of this magnitude would have major impacts on low-lying countries such as Bangladesh.
The findings were presented at a major science conference in Vienna.
The research group is not the first to suggest that the IPCC's forecast of an average rise in global sea levels of 28-43cm by 2100 is too conservative.
The IPCC was unable to include the contribution from "accelerated" melting of polar ice sheets as water temperatures warm because the processes involved were not yet understood.
The new analysis comes from a UK/Finnish team which has built a computer model linking temperatures to sea levels for the last two millennia.
Glaciers 'flowing faster'
"For the past 2,000 years, the [global average] sea level was very stable, it only varied by about 20cm," said Svetlana Jevrejeva from the Proudman Oceanographic Laboratory (POL), near Liverpool, UK.
"But by the end of the century, we predict it will rise by between 0.8m and 1.5m.
"The rapid rise in the coming years is associated with the rapid melting of ice sheets."
Where will all these people go? - There are 10 million victims threatened by the annual floods that ravage Bangladesh. Millions of these find temporary shelter in the rivers, on islands that emerge when water levels drop during the summer. Experts say a third of Bangladesh's coastline could be flooded if the sea rises one metre in the next 50 years, creating an additional 20 million Bangladeshis displaced from their homes and farms. This is about the same as Australia's population. "Bangladesh is already facing consequences of a sea level rise, including salinity and UNUSUAL height of tidal water." It is unclear how the government could feed, house or find enough clean water for vast numbers of climate refugees in a country of 140 million people crammed into an area of 55,500 sq miles. In a taste of what the future might look like, Bangladesh suffered two massive floods and a cyclone last year that together killed about 4,500 people, made at least two million homeless and destroyed 1.8 million tons of rice.