It delivered a jolt to politicians about the perils of fossil-fuel pollution and reduced the powerful lobby of climate-change "deniers" to a shrill, if well-funded, rump.

Sources familiar with the drafting of this year's report -- the first of three mammoth IPCC tomes to be issued this year -- say it will not offer any good news.

"It will be a confirmation of what has been said for a long time, but point to additional risks," said French climatologist Herve Le Treut.

Over the past few years, many published studies suggest that climate change, which many had expected to kick in several decades from now, is already underway.

In alpine areas, glaciers are melting and snow cover is shrinking. The North Pole's summer icefield is a mere fraction of what it once was. Permafrost in high northerly latitudes is retreating. The oceans are becoming more acidic through absorption of carbon dioxide (CO2).

In 2001, the IPCC said that carbon pollution from burning oil, gas and coal had helped drive atmospheric levels of CO2 to their highest in 420,000 years. CO2 is the principal "greenhouse gas," a term that applies to half a dozen substances that linger invisibly in the atmosphere, trapping the Sun's heat instead of letting solar radiation bounce back into space.

Over the previous 50 years, temperatures climbed by around 0.1 C (0.2 F) per decade and most of the warming could be attributed to Man, the 2001 report said.

It predicted that by 2100, the global atmospheric temperature will have risen between 1.4 and 5.8 C (2.52-10.4 F) and sea levels by 0.09 to 0.88 metres (3.5-35 inches), depending on how much greenhouse gas is emitted.

The biggest interest in this year's report will be whether the nearly 500 scientists who make up this working group of the IPCC will amend these figures.

They are likely to attribute "probability" to specific forecasts of temperature rise in order to help policymakers, said Serge Planton, in charge of climate research at Meteo France.

Emissions of greenhouse gases determine the level of warming -- but these emissions, in turn, depend on two factors.

One is whether governments take action to rein in the pollution, a process that so far, under the Kyoto Protocol, has proven nightmarishly difficult. The other is equally hazy: whether stored carbon in Earth's surface, such as in the permafrost, may be released if temperatures rise beyond a certain point.

Such triggers could release hundreds of millions of tonnes of CO2 into the air. Like petrol thrown on a fire, it would dramatically amplify the warming.

Two other volumes will be issued in April in what will be the fourth assessment report on climate change by the IPCC since it was established in 1988.

The two others will focus on the impacts of climate change and on the social-economic costs of reducing greenhouse gases.

Fossil fuels provide the backbone of today's energy needs, so reducing carbon emissions implies an economic cost in shifting to a cleaner source.

Weighed against that, though, is what experts say is the longer-term cost of failing to tackle the pollution. The potential risks include worse droughts, floods, rising sea levels and more vicious storms, which thus poses a threat to agriculture, water supplies and even human settlement itself.

A report last year by former World Bank economist Sir Nicholas Stern said climate change could cost up to 20 percent of global gross domestic product if nothing is done.

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