Richard Gray, Science Correspondent, Sunday Telegraph
Last Updated: 1:37am GMT 11/12/2006
Mankind has had less effect on global warming than previously supposed, a United Nations report on climate change will claim next year.
The UN Intergovernmental Panel on Climate Change says there can be little doubt that humans are responsible for warming the planet, but the organisation has reduced its overall estimate of this effect by 25 per cent.
In a final draft of its fourth assessment report, to be published in February, the panel reports that the level of carbon dioxide in the atmosphere has accelerated in the past five years. It also predicts that temperatures will rise by up to 4.5 C during the next 100 years, bringing more frequent heat waves and storms.
The panel, however, has lowered predictions of how much sea levels will rise in comparison with its last report in 2001.
Climate change sceptics are expected to seize on the revised figures as evidence that action to combat global warming is less urgent.
Scientists insist that the lower estimates for sea levels and the human impact on global warming are simply a refinement due to better data on how climate works rather than a reduction in the risk posed by global warming.
One leading UK climate scientist, who asked not to be named due to the sensitivity surrounding the report before it is published, said: "The bottom line is that the climate is still warming while our greenhouse gas emissions have accelerated, so we are storing up problems for ourselves in the future."
The IPCC report, seen by The Sunday Telegraph, has been handed to the Government for review before publication.
It warns that carbon dioxide emissions have risen during the past five years by three per cent, well above the 0.4 per cent a year average of the previous two decades. The authors also state that the climate is almost certain to warm by at least 1.5 C during the next 100 years.
Such a rise would be enough to take average summer temperatures in Britain to those seen during the 2003 heatwave, when August temperatures reached a record-breaking 38 C. Unseasonable warmth this year has left many Alpine resorts without snow by the time the ski season started.
Britain can expect more storms of similar ferocity to those that wreaked havoc across the country last week, even bringing a tornado to north-west London.
The IPCC has been forced to halve its predictions for sea-level rise by 2100, one of the key threats from climate change. It says improved data have reduced the upper estimate from 34 in to 17 in.
It also says that the overall human effect on global warming since the industrial revolution is less than had been thought, due to the unexpected levels of cooling caused by aerosol sprays, which reflect heat from the sun.
Large amounts of heat have been absorbed by the oceans, masking the warming effect.
Prof Rick Battarbee, the director of the Environmental Change Research Centre at University College London, warned these masking effects had helped to delay global warming but would lead to larger changes in the future.
He said: "The oceans have been acting like giant storage heaters by trapping heat and carbon dioxide. They might be bit of a time-bomb as they have been masking the real effects of the carbon dioxide we have been releasing into the atmosphere.
"People are very worried about what will happen in 2030 to 2050, as we think that at that point the oceans will no longer be able to absorb the carbon dioxide being emitted. It will be a tipping point and that is why it is now critical to act to counter any acceleration that will occur when this happens."
The report paints a bleak picture for future generations unless greenhouse gas emissions are reduced. It predicts that the climate will warm by 0.2 C a decade for the next two decades if emissions continue at current levels.
The report states that snow cover in mountainous regions will contract and permafrost in polar regions will decline.
However, Julian Morris, executive director of the International Policy Network, urged governments to be cautious. "There needs to be better data before billions of pounds are spent on policy measures that may have little impact," he said.