CLIMATE change is real and set to cause dramatic temperature rises in the coming century, according to a leaked draft of a major United Nations report.
The study, by a panel of 2,500 scientists who advise the UN, is the most complete overview of climate-change science and will be published next month by the Intergovernmental Panel on Climate Change (IPCC) after a final review.
The IPCC's reports are regarded by many environmentalists as cautious, because the findings have to be agreed by member states including oil-producing countries such as Saudi Arabia.
The draft, which was leaked by scientific sources, says it is "very likely" - more than a 90 per cent chance - that human activities, led by burning fossil fuels, are to blame for warming since 1950. The previous report in 2001 said the link was "likely".
It projects temperatures will rise by 2-4.5C (3.6-8.1F) above pre-industrial levels, with a "best estimate" of a 3C (5.4F) rise, assuming carbon-dioxide levels are stabilising at about 45 per cent above current levels. The European Union says any temperature rise above 2C will cause "dangerous" changes.
Leading environmentalists said the report was the final nail in the coffin for "climate-change deniers" and also presented a challenge to government to impose tougher restrictions on greenhouse gases in order to prevent a temperature rise of 2C or more.
In New Delhi, Rajendra Pachauri, the IPCC chairman, said he hoped the report would act as a wake-up call to the world.
"I hope this report will shock people, governments, into taking more serious action, as you really can't get a more authentic and a more credible piece of scientific work."
The draft also predicts more droughts, rains and shrinking Arctic ice and glaciers, and rising sea levels to a foreseeable 2100, but cautions that the effects of a build-up of greenhouse gases will last far longer.
"Twenty-first century anthropogenic (human-caused) carbon-dioxide emissions will contribute to warming and sea-level rise for more than a millennium, due to the timescales required for removal of this gas," sources quoted the report as saying.
However, the report had some good news, quoting six models with central projections of sea-level rises this century of between 11in and 16.9in - compared with a far wider band of 3.5-34.6in in the 2001 report. Rising seas would threaten low-lying Pacific islands, coasts from Bangladesh to Florida and cities from Shanghai to Buenos Aires.
It also said there were uncertainties about whether higher temperatures would bring more cooling clouds - their white tops bounce heat back into space - and added that dust from volcanic eruptions and air pollution seems to have braked warming in recent decades by similarly reflecting sunlight.
Dr Richard Dixon, the director of WWF Scotland, said the report emphasised the need for governments to take action to restrict concentrations, currently at 380 parts per million, to 450ppm, rather than softer targets of 500ppm or 550ppm.
The draft says it was "very unlikely" that the temperature rise could be restricted to less than 1.5C above pre-industrial levels, but Dr Dixon said this suggested that the rise could still be held below 2C. "They are saying by implication that staying below 2C is possible, but requires massive effort."
Dr Dixon said the report should finally end the debate about whether climate change is happening.
However, while he still has hope, environmentalist Professor James Lovelock, who came up with the Gaia theory of the Earth as a living entity, said the world should prepare for the worst.
"Most people don't realise what a 3-5C rise would be like. It's as big a difference as between the last Ice Age and now. It's huge," he said.
He said Britain and particularly Scotland would be less seriously affected, because the ocean would help to keep the country cool. "Our main problem will be dealing with refugees. Every European has the right to come here - and there will be 500 million."
The report predicts:
- Temperatures are likely to rise by between 2C and 4.5C above pre-industrial levels if concentrations are kept at 550 parts per million in the atmosphere, as against about 380 now. The "best estimate" for the rise is about 3C.
- The warming is unlikely to be less than 1.5C.
- It is "very likely" that extremes such as heatwaves and heavy rains will become more frequent. Arctic sea ice could disappear in summer by the latter part of the 21st century in some projections. Warming is expected to be greatest over land and at high northern latitudes and least over the Southern Ocean and North Atlantic.
- Antarctica is likely to stay too cold for wide surface melting and is expected to gain in mass due to a rise in snowfall.
- A system of Atlantic currents including the Gulf Stream, bringing warm waters northwards, are likely to slow by 2100, but an overall warming will more than offset any cooling effect.