Science is making headway in predicting how the planet's climate will evolve, but it's anyone's guess what actions policymakers meeting in Bali will take -- or not -- to slow global warming.

With just over two weeks left before the crucial 11-day forum in Indonesia, decision-makers are under intensifying pressure to do something about greenhouse gases -- and do it quickly.

The starkest warning of all came on Saturday from UN's top scientists, gathered under the Nobel-winning Intergovernmental Panel on Climate Change (IPCC).

Summarising a massive review of the evidence, the IPCC declared it was more than 90-percent sure that humans were to blame for rising temperatures.

On current trends, it said, surging emissions of greenhouse gases will relentlessly warm Earth's atmosphere, damaging ice and snow cover and causing the oceans to expand and thus rise -- and the impacts could be "abrupt or irreversible."

In such a scenario, mankind would face wide-ranging misery in the form of crop failure, storm damage and ill health as drought, floods, cyclones, mosquito pests and water-borne disease become more frequent or intensive, it added.

The sober, neutrally-worded report by this prestigious group gives little "wriggle room" for politicians to deny or play down the global warming issue.

In political terms, it is the equivalent of a blast from a sawn-off shotgun as environment ministers prepare for Bali, where they will be asked to agree a two-year "roadmap" of negotiations for accelerating cuts in greenhouse gases.

"It makes it very difficult for politicians to say 'no' to a launch of negotiations," UNFCCC Executive Secretary Yvo de Boer told AFP.

Those clamouring for action at the December 3-14 Bali talks include the European Union (EU), which is leading a charge to halve global emissions by the middle of the century compared to 1990 levels.

Behind the ponderous scientific data, though, lurk the quicksilver shades of political reality.

The negotiation "roadmap" sketched at Bali must satisfy the EU, Japan and Canada and other countries that want deep, fast-track cuts in emissions.

But it also has to meet the positions of the United States and China, which together account for roughly half of all carbon emissions.

For different reasons, both countries are refusing to sign up to targeted pledges for curbing their dangerous pollution.

China, a voracious burner of coal, says it cannot afford any measures that imperil its long rise out of poverty, even though it is acutely aware of the global-warming crisis.

Beijing also argues that, even if it is poised to surpass the United States as the world's top emitter of CO2, rich countries are historically to blame for the global warming.

Asking developing nations in the throes of robust economic growth -- including India, Brazil and Mexico -- to simply end their use of fossil fuels is unfair, they argue.

The oil-addicted United States -- which emits twice as much CO2 per capita as Europe -- abandoned Kyoto under President George W. Bush in 2001 and is fiercely opposed to any mandatory approach for tackling emissions.

Washington is pushing for a voluntary, technology-led tack to reduce CO2, with particular focus on sectors such as energy, transport, steelmaking and the cement industry.

Accommodating all these different positions will be a massive undertaking, a juggling act complicated further by terrible pressures of time.

The IPCC says that to limit the average increase in global temperatures to 2.4 C (4.3 F) above pre-industrial levels -- the most optimistic of any of its scenarios -- the concentration of greenhouse gases would have to stabilise at around 450 parts per million (ppm) of carbon dioxide (CO2) or its equivalent in the atmosphere.

To achieve this goal, says the IPCC, CO2 emissions would have to peak by 2015 at the latest and then fall between 50 and 85 percent by 2050.

But the International Energy Agency (IEA) has poured cold water on this scenario.

To achieve the 450 ppm target would mean that CO2 from energy sources would have to peak by 2012, the IEA said earlier this month in an annual report on energy needs.

This would require "exceptionally quick and vigorous action by all countries and unprecedented technological advances, entailing substantial costs," the IEA said bluntly