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 new IPCC report will underpin international talks on how to cut greenhouse gas emissions when the first phase of the Kyoto protocol expires in 2012.
Set up in 1988 by the UN, the IPCC brings together hundreds of experts to summarise the state of climate science for policymakers. It has produced three reports since 1990, each of which has been instrumental in establishing national and international strategies to address global warming. Government officials have until June to comment on the new draft, when scientists will gather in Bergen, Norway, to produce a final version.
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."
Dr Cox said the IPCC's move is significant because it will force governments to seriously consider extreme scenarios that are unlikely but potentially devastating. "The most probable thing is not the most important thing to worry about. The upper end is where the big problems are, because the impact rises as the temperature does."
If we continue to burn fossil fuels at current rates, levels of carbon dioxide in the atmosphere will reach 550 ppm (parts per million) - double pre-industrial levels - by around 2050. The most recent IPCC report, published in 2001, said this would increase global temperatures by between 1.4 and 5.8C by 2100, and that sea levels would rise by between 0.09 and 0.88 metres.
Climate scientists remain divided about the likelihood of the worst-case scenario being realised. James Annan, a British climate scientist who works on the Japanese Earth simulator supercomputer in Yokohama, says the risks of extreme climate sensitivity and catastrophic consequences have been overstated. He is about to publish a study showing that the chance of climate sensitivity exceeding 4.5C is less than 5%. He said: "It seems to me that some people seem to be talking up the possibility of disaster in order to scare people into doing something."
Dave Stainforth, a climate modeller at Oxford University, said: "This is something of a hot topic but it comes down to what you think is a small chance - even if there's just a half per cent chance of destruction of society, I would class that as a very big risk."
The IPCC findings mirror a British report on avoiding dangerous climate change published last month, in which Mr Blair admitted that the risks may be more serious than previously thought. It included a warning from Chris Rapley, head of the British Antarctic Survey, that the huge west Antarctic ice sheet may be starting to disintegrate, an event that would raise sea levels around the world by five metres. "The last IPCC report characterised Antarctica as a slumbering giant in terms of climate change," he said. "I would say it is now an awakened giant. There is real concern."
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