The study used a programme that ran on PCs around the world (BBC)

The impact of global warming could be twice as severe as the worst scenario feared by United Nations scientists, the world's largest climate-modelling experiment has shown.

Average temperatures could rise by 11C (20F) to reach highs that would change the face of the globe, researchers who have run 60,000 computer simulations of climate change said yesterday.

The conclusions suggest that forecasts by the UN's Intergovernmental Panel on Climate Change (IPCC) may be much too conservative. In the worst case, the world would eventually heat up by almost double the maximum increase envisaged by the panel. The IPCC's latest report predicted that temperatures will rise by between 1.4C (2.5F) and 5.8C (10.4F) by 2100.

A world 11C warmer than it is today would be unrecognisable: while records show that the planet has been hotter than it is today for about 80 per cent of its history, there is no evidence that it has ever been more than about 7C warmer.

Although it would take hundreds of years for the full effects to be felt, the polar ice caps eventually would melt completely, causing sea levels to rise by 70m to 100m (230ft to 330ft). Coastal and low-lying cities such as London and New York would be submerged.

As the 11C figure is a global average, temperatures would be expected to climb even further in some regions.

David Stainforth, of the University of Oxford, the study's chief scientist, said: "When I start to look at these figures, I get very worried about them. An 11-degree warmed world would be a dramatically different world."

(By Ruth Gledhill, Times, January 27, 2005)