New evidence put forward by leading scientists will add pieces to a mosaic of evidence which suggests the climate crunch is heading our way faster, and with a harsher outcome, than previously thought.

The three-day conference, running from Tuesday to Thursday in the southwestern English city of Exeter, is bound to have a wide political impact.

It will add the objective weight of science to the political pressures on Washington to help curb carbon pollution.

It comes just before the UN's Kyoto Protocol -- the greenhouse-gas treaty fiercely opposed by President George W. Bush -- is due to take effect on February 16.

Bush's isolation on this issue is particularly poignant, for the Exeter meeting is a pet project of his closest ally, British Prime Minister Tony Blair, who is pushing for action on global warming in the Group of Eight (G8), chaired by Britain this year.

The scientists, hailing from 30 countries, will give a state-of-play about knowledge and try to define what is a dangerous level of global warming but not offer advice to policymakers on how to combat it, conference chairman Dennis Tirpak said.

"The next 25 years are quite critical as to what will happen over the next century," he said. "(...) The conference will try to collect the best evidence it can."

The last big scientific review of the evidence was in 2001, when the UN's Intergovernmental Panel on Climate Change (IPCC) gave birth to a magnum opus.

That landmark report -- due to be updated in 2007 -- crushed the last doubts among the majority of scientists that carbon gases, spewed by oil, gas and coal, are trapping solar heat and causing the Earth's surface to warm.

But the IPCC said there remained much uncertainty as to how fast this would happen, exactly how the climate system would be affected and which regions would be hit most.

It cautiously calculated that by 2100, temperatures would rise by between 1.4 C (2.5 F) and 5.8 C (10.4 F) compared to 1990 levels, according to whether carbon dioxide (CO2) levels in the atmosphere doubled or nearly quadrupled from pre-industrial levels.

The sea level would rise by between nine and 88 centimetres (four and 35 inches) according to the CO2 scenario cited by the IPCC.

In the past five years, the IPCC's margin of uncertainty has narrowed, eroded almost weekly by new studies that appear in Science, Nature and other prestigiouis journals.

"People are starting to realise that the higher end of climate-change scenarios are now possible," said Chris Jones, a researcher at the British Met Office's Hadley Centre, which is hosting the talks.

"Indeed, it's fairly accepted now, as every year goes by, that we are already seeing changes" in the climate system, he said.

Referring to a heatwave that scorched Europe in 2003, Jones said, "By the middle of this century, 2003 summers are going to be the norm, and by the end of this century, it's probably going to be considered a cold summer. It is frightening stuff."

The latest studies, some of which will be submitted in Exeter, suggest the clock is ticking faster than thought.

One piece of research will suggest that just 15 years are left to ensure that atmospheric CO2 pollution will be stabilised by the end of the century at 550 parts per million (ppm), twice that of the pre-industrial age.

At present, the level is 379ppm, which seems reassuringly far from 550ppm.

But it is rising quickly as China and India, which have huge populations, gobble up fossil fuels to power their fast-growing economies and America, the biggest single polluter, maintains its rivers of gas-guzzling cars.

Even if this 550ppm target is reached, the climate system may still be wrecked, according to the latest calculations.

A study of more than 2,000 computer models suggests that 550ppm will crank up temperatures by nearly two C (3.6 F) and more than 11 C (19.8 F).

In the latter part of this range, ice caps would shrink, glaciers melt and droughts, floods, El Ninos and hurricanes could become commonplace.

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