In a dark assessment to policymakers, experts said that the latest evidence confirmed and amplified cautious warnings made four years ago by the UN's paramount scientific panel on climate change.

"In many cases, the risks are more serious than previously thought... (and) a number of new impacts were identified that are potentially disturbing," it declared.

The conference, staged in the southwestern English city of Exeter, was called by British Prime Minister Tony Blair, who has vowed to make global warming a priority of his presidency of the Group of Eight (G8) countries this year.

More than 100 scientists from 30 countries were invited to the meeting, entitled "Avoiding Dangerous Climate Change," with the task of an update of expert knowledge to help guide policymakers.

The conference looked at the latest studies and compared them to the 2001 benchmark, an assessment by the UN's top expert group, the Intergovernmental Panel on Climate Change (IPCC).

That report -- next to be updated in 2007 -- calculated that, by 2100, temperatures would rise by between 1.4 C (2.5 F) and 5.8 C (10.4) compared to

Mean global sea levels would rise by between nine and 88 centimetres (four and 35 inches).

The predicted consequences ranged from additional stress on water and food supplies in hot tropical countries to prolonged droughts and floods, violent storms, shrinkage of the polar ice caps and the meltdown of alpine glaciers.

But the Exeter conference went far further than the IPCC in saying that man-made carbon pollution -- "greenhouse" gases disgorged by the burning of fossil fuels -- was already starting to affect Earth's delicate climate sysstem.

"The impacts of climate change are already being observed," the conference's draft summary concluded.

"Ecosystems are already showing the effects of climate change. Changes to polar ice and glaciers and rainfall regimes have already occurred."

The brutal heatwave that killed some 30,000 Europeans in 2003, while a single event that could not by itself be held up as an example of climate change, was a "prime example" of the kind of extreme weather events that lay ahead.

The IPCC's estimates depend on how much CO2 will be in the atmosphere by the end of the century.

The lower and upper ends of the IPCC's estimates are based on 540 parts of CO2 per million (ppm) and 970 ppm. In 1750, the benchmark for pre-industrialisation, concentrations were 280ppm.

Today, levels are about a third over the 1750 level and rising quickly, disgorged in particular by China and India and the United States, which accounts by itself for almost a quarter of greenhouse-gas pollution.

The world's average temperature rose by 0.6 C (1.08 F) from 1900-1990 alone.

The conference, while not setting any recommendation for action and cautioning about complexities, said urgent work would be needed if the world wanted to keep the overall temperature increase to 2.0 C (3.6 F), a figure set by the European Union (EU) in 1996.

To achieve this, "global emissions would need to peak in 2020," the summary draft warned. At present, emissions stand at around 6 gigatonnes of CO2 per year. This would have to fall to 3.1 gigatonnes per year by 2095.

How would this be achieved, in a world so dependent on oil, gas and coal?

The conference's grim answer: "There are no magic bullets."

The good news, though, was that there are already many technology options for slashing emissions, "and the costs are likely to be smaller than previously considered."

The conference said that some light had been shed on several existing unknowns and new problems had been discovered elsewhere:

-- Beyond a regional temperature rise of 2.7 C (4.8 F), the Greenland icesheet is likely to start to melt. Its complete loss would drive up global sea levels by seven metres (24.5 feet), drowning vulnerable coastal cities.

-- Man-made CO2, if emitted at such unbridled rates, could eventually make the seas too acidic to sustain life, according to one study.

-- Is there a risk of runaway climate change? The worry is whether warming might at some point unleash the decay of fallen vegetation stored over millions of years in cold, northern soils. If so, billions of tonnes of CO2 could be released within a few years.

-- Concern was expressed for Antarctica, where the world's biggest source of freshwater is locked up in an icesheet.

"Until now, Antarctica has been called a slumbering giant. Now, the question has to be asked whether it is a wakening giant," said British polar scientist Chris Rapley, presenting evidence of accelerating ice flows into the Southern Ocean from three big glaciers.

The conference's findings will now enter the political arena.

In a statement, British Environment Secretary Margaret Beckett said the outcome "underlines the need for the international community to take urgent action... the conference will be seen as a turning point in the perception of climate change."

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