The Earth's temperature could rise under the impact of global warming to levels far higher than previously predicted, according to the United Nations' team of climate experts.
A draft of the next influential Intergovernmental Panel on Climate Change (IPCC) report will tell politicians that scientists are now unable to place a reliable upper limit on how quickly the atmosphere will warm as carbon dioxide levels increase. The report draws together research over the past five years and will be presented to national governments in April and made public next year. It raises the possibility of the Earth's temperature rising well above the ceiling quoted in earlier accounts.
Such an outcome would have severe consequences, such as the collapse of the Greenland ice sheet and disruption of the Gulf Stream ocean current.
The shift in position comes as Tony Blair is expected to pledge today to work towards a date for stabilising international greenhouse gas emissions when he meets Stop Climate Chaos, the climate change equivalent of Make Poverty History. The group is campaigning for a target date of 2015 for stabilisation, saying a later date would endanger the planet.
The IPCC's removal of the upper temperature estimation is posited on new predictions about how the atmosphere would react to the carbon blanket wrapped around it. The three previous reports assumed that a doubling of carbon dioxide in the atmosphere would increase average global temperature by between 1.5 and 4.5C. Since then, computer models have foreseen increases as high as 11C, and some scientists wanted the naturally conservative IPCC to raise the upper end of the range. Others said such a move would be increase would be misleading and alarmist.
According to sources who have seen it, the draft now assumes a doubling of carbon dioxide would cause a likely temperature rise of between 2 and 4.5C, but says higher increases are possible.
The shift follows several high profile studies convincing some scientists the atmosphere may be much more sensitive to greenhouse gases than they had thought. Peter Cox, a leading climate expert at the Centre for Ecology and Hydrology in Winfrith, Dorset, said: "The scientific agenda has moved from improving the predictions to thinking about what are the chances of something awful happening."
Something awful has already happened and it's only going to get worse. I think we need to reconcile ourselves to the likelihood that humans will become extinct at some point during this century. I have certainly read that prediction before and all the news coming out about climate change indicates that it is a distinct possibility.