IF THE world fails to slow global warming, it will overwhelm us. And it will do so more certainly, more quickly and more dangerously than we ever feared. That, in essence, is the stark message delivered to world leaders by climate scientists yesterday. The increase in storms, floods and droughts caused by pollution will threaten the lives and livelihoods of billions around the planet.

"Unmitigated climate change would, in the long term, be likely to exceed the capacity of natural, managed and human systems to adapt," concludes a report released by the UN Intergovernmental Panel on Climate Change (IPCC) at a meeting in Valencia, Spain.

The report will be the main evidence to be considered at a summit on climate change which begins in two weeks in Bali, Indonesia. There, international governments will have to decide how to replace the Kyoto Protocol, which limits pollution and is due to run out in 2012.

Climate change is "the defining challenge of our age" said UN secretary-general Ban Ki Moon yesterday. "Today the world's scientists have spoken clearly and with one voice. In Bali I expect the world's policymakers to do the same."

Fears that the US might succeed in diluting the report's findings appear to have been unfounded. It was greeted by a chorus of support from environmental groups across the globe, and in Scotland.

"The report is a wake-up call to the world," said Dr Dan Barlow, WWF Scotland's acting director.

"Given the Scottish government's commitment to an 80% cut in emissions by 2050 and a Scottish Climate Bill we are well-placed to demonstrate real leadership in tackling climate change."

The IPCC report, the summation of six years' work by 2500 scientists from 130 countries, warns that global warming "could lead to some impacts that are abrupt or irreversible". Melting of only some of the Polar ice sheets could cause sea levels to rise by "metres", flooding coastlines, river deltas and small islands.

Between 20% and 30% of all the world's plants and animals are "likely to be at increased risk of extinction" if average temperature increases exceed 1.5-2.4C. If temperatures rise by 3.5C, 40-70% of species could become extinct. The report predicts that before the end of the century average temperatures could increase by between 1.8 and 4C.

"There is now higher confidence in the projected increases in droughts, heatwaves and floods as well as their adverse impacts," it says.

The report summarises a frightening series of predicted impacts: mass water and food shortages in Africa, flooding in Asia and Europe, agricultural decline in Australia and heatwaves in North America (see panel). Many of these are expected to occur by 2020 or 2030, much sooner than previously predicted. Though no region can escape, the effects are likely to be worst in Africa, the Arctic and small islands.

Scotland will probably be spared one of the worst-case scenarios - a big freeze triggered by the loss of the Gulf Stream - at least for 100 years. Although the current which warms northern Europe is likely to slow in the 21st century, the report concludes it is "very unlikely to undergo a large abrupt transition".

Yesterday's report attempts to "synthesise" three reports and 3000 pages on the science, impacts and mitigation of climate change published by the IPCC earlier this year. Next month the IPCC will receive the Nobel Peace Prize, shared with the former US vice-president, Al Gore, for its work.

The report is the fourth major assessment from the IPCC since it was set up nearly 20 years ago, and is by far the most alarming. It lists possible ways of combating climate change, including increased energy efficiency, renewable energy, cleaner vehicles and better building insulation, although it doesn't recommend any particular course.

"We need a new ethic by which every human being realises the importance of the challenge we are facing and starts to take action through changes in lifestyle and attitude," said IPCC chairman Rajendra Pachauri.

Andrew Pendleton from Christian Aid said: "It's tough to welcome a report that carries such bad news. But we must heed its warning and get on with an emergency programme of cuts in emissions."

THE report also puts to rest any notion that climate change is a result of natural variation rather than human pollution. Human influences "very likely" contributed to sea level rises in the second half of the last century, and are "likely" to have contributed to changes in wind and storm patterns and increases in extreme hot nights.

The report was described as "a milestone" by the EU environment commissioner, Stavros Dimas. "The report's findings amount to a stark warning that the world must act fast if we are to prevent climate change from reaching devastating levels," he said. "The good news is that it also shows deep emission cuts are both feasible and affordable."

One group, however, sounded a note of caution. The Global Forest Coalition, a group of environment, development and indigenous people's organisations, called on governments to reject "false solutions" like biofuels and carbon trading.

Chairman Dr Miguel Lovera said biofuels could make climate change worse, and wreak social havoc by destroying forests, while carbon offsetting schemes, the coalition argued, had led to "devastating impacts" on local communities.

"Many indigenous peoples in the Pacific face the possibility of losing everything - their homes, territories and livelihoods because of rising sea levels," said Sandy Gauntlett, chair of the Pacific Indigenous Peoples Environment Coalition. "We insist governments do something now to stop this. They need to invest in forest conservation, sustainable transport and solar and wind energy."