Global warming is destined to have a far more destructive and earlier impact than previously estimated, the most authoritative report yet produced on climate change will warn next week. A draft copy of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, obtained by The Observer, shows the frequency of devastating storms - like the ones that battered Britain last week - will increase dramatically. Sea levels will rise over the century by around half a metre; snow will disappear from all but the highest mountains; deserts will spread; oceans become acidic, leading to the destruction of coral reefs and atolls; and deadly heatwaves will become more prevalent.

The impact will be catastrophic, forcing hundreds of millions of people to flee their devastated homelands, particularly in tropical, low-lying areas, while creating waves of immigrants whose movements will strain the economies of even the most affluent countries.

'The really chilling thing about the IPCC report is that it is the work of several thousand climate experts who have widely differing views about how greenhouse gases will have their effect. Some think they will have a major impact, others a lesser role. Each paragraph of this report was therefore argued over and scrutinised intensely. Only points that were considered indisputable survived this process. This is a very conservative document - that's what makes it so scary,' said one senior UK climate expert.

Climate concerns are likely to dominate international politics next month. President Bush is to make the issue a part of his state of the union address on Wednesday while the IPCC report's final version is set for release on 2 February in a set of global news conferences.

Although the final wording of the report is still being worked on, the draft indicates that scientists now have their clearest idea so far about future climate changes, as well as about recent events. It points out that:

• 12 of the past 13 years were the warmest since records began;
• ocean temperatures have risen at least three kilometres beneath the surface;
• glaciers, snow cover and permafrost have decreased in both hemispheres;
• sea levels are rising at the rate of almost 2mm a year;
• cold days, nights and frost have become rarer while hot days, hot nights and heatwaves have become more frequent.

And the cause is clear, say the authors: 'It is very likely that [man-made] greenhouse gas increases caused most of the average temperature increases since the mid-20th century,' says the report.

To date, these changes have caused global temperatures to rise by 0.6C. The most likely outcome of continuing rises in greenhouses gases will be to make the planet a further 3C hotter by 2100, although the report acknowledges that rises of 4.5C to 5C could be experienced. Ice-cap melting, rises in sea levels, flooding, cyclones and storms will be an inevitable consequence.

Past assessments by the IPCC have suggested such scenarios are 'likely' to occur this century. Its latest report, based on sophisticated computer models and more detailed observations of snow cover loss, sea level rises and the spread of deserts, is far more robust and confident. Now the panel writes of changes as 'extremely likely' and 'almost certain'.

And in a specific rebuff to sceptics who still argue natural variation in the Sun's output is the real cause of climate change, the panel says mankind's industrial emissions have had five times more effect on the climate than any fluctuations in solar radiation. We are the masters of our own destruction, in short.

There is some comfort, however. The panel believes the Gulf Stream will go on bathing Britain with its warm waters for the next 100 years. Some researchers have said it could be disrupted by cold waters pouring off Greenland's melting ice sheets, plunging western Europe into a mini Ice Age, as depicted in the disaster film The Day After Tomorrow.

The report reflects climate scientists' growing fears that Earth is nearing the stage when carbon dioxide rises will bring irreversible change to the planet. 'We are seeing vast sections of Antarctic ice disappearing at an alarming rate,' said climate expert Chris Rapley, in a phone call to The Observer from the Antarctic Peninsula last week. 'That means we can expect to see sea levels rise at about a metre a century from now on - and that will have devastating consequences.'

However, there is still hope, said Peter Cox of Exeter University. 'We are like alcoholics who have got as far as admitting there is a problem. It is a start. Now we have got to start drying out - which means reducing our carbon output.'

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http://observer.guardian.co.uk/uk_news/story/0,,1995348,00.html