The outcome of the 12-day meeting, unfolding in Montreal from Monday and expected to draw as many as 10,000 participants from 180 countries, could reverberate for years.

It is the first meeting of the UN Framework Convention on Climate Change (UNFCCC) since its offspring, Kyoto, took its first faltering steps nine months ago.

Yet already the battle lines are being drawn about what should be done when the treaty's present commitment period expires in 2012.

Sources expect some early skirmishes between the United States and Europe, heralding a bitter negotiating process likely to last several years.

The biggest question is whether to stick with the present Kyoto format of setting a legally-binding cap that will encourage market mechanisms for reducing greenhouse-gas pollution -- or to yield to President George W. Bush's demand for a purely voluntary approach and a technological fix.

"What we are looking for is a broader and deeper regime, and it's clear that we can't do this with a technology-driven approach alone," said Lars Friberg of the environment group Climate Action Network Europe.

"We need a market driver, and that driver is a global price for carbon."

Kyoto is almost as sickly as Earth's climate system.

It was abandoned by the US, the world's No. 1 carbon polluter, in 2001, when Bush complained the treaty was too costly for the oil-dependent American economy.

And, he said, it was unfair because big developing countries were not required to make targeted emissions cuts.

Even if the remaining signatories meet their pledges under Kyoto's notoriously complex rule book, greenhouse gas levels by developed countries will decline overall by only one or two percent in 2012 over their 1990 benchmark.

In the meantime, India and China are becoming huge polluters in their own right.

"Given the accelerating pace of the climate change escalator we have set in motion, future generations will be asked to make even greater sacrifices if we do not act now to reduce our greenhouse gas emissions," warns Lord May, who heads the Royal Society, Britain's de-facto academy of sciences.

The challenge facing the UNFCCC conference, which climaxes in a three-day ministerial haggle from December 7-9, will be to set in motion a process that ensures the post-2012 Kyoto delivers dramatic results.

But in today's sour political atmosphere, it may be impossible to come up with a dynamic -- even a vague statement of intentions, for instance -- that simultaneously satisfies Bush, the European Union (EU), green groups and businesses.

Even as politicians haggle, the evidence from climatologists is ever gloomier.

Only a small minority of scientific diehards, along with the fossil-fuel lobbies, now contest that global warming exists and is a threat.

And more and more scientists are convinced that higher temperatures are already disrupting weather patterns.

Melting glaciers in Europe and the Himalayas, the shrinkage of the northern polar icecap, movement in the Greenland icesheet and the crumbling of iceshelves in Antarctica are among events that have been documented this year.

Amounts of carbon dioxide in the atmosphere are their highest levels for 650,000 years, according to research published last Thursday in the latest issue of the US journal Science. Meanwhile, 2005 is on course for being the hottest year ever.

By a yardstick adopted notably by the European Union, emissions of greenhouse gases will have to peak by 2020 and fall drastically thereafter to peg the planet's temperature rise to 2C (3.8 F) over pre-industrial times.

Even this modest increase will carry a cost in warmer oceans, higher sea levels and damaging changes to rainfall patterns that will disrupt agriculture and water supplies. Some regions will be hit far worse than others.

Beyond this threshold, the impacts will be worse -- more El Ninos, worsening desertification -- and at their extreme range apocalyptic, with drowned coastlines and the plunging of Western Europe into an Ice Age as the Gulf Stream closes down.

Climatologists say the world has to make cuts in the order of 50 or 60 percent over the next few decades, a goal that entails much greater fuel efficency and a switch to cleaner energy sources.

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.