Environment ministers from around the globe will attend the final three days in Montreal. Some predict the negotiations they launch may last 5 years.

"It will be very complex," said Elliot Diringer, a director of the Washington-based Pew Center on Global Climate Change. "Any agreement has to be more flexible than Kyoto but at the same time has to deliver real cuts in emissions."

"And the Bush administration is adamantly opposed to any process aimed at widening Kyoto," he said.

Many scientists say that a buildup of greenhouse gases from burning fossil fuels could have catastrophic effects on the climate by spurring more hurricanes, spreading deserts, driving thousands of species to extinction and raising sea levels.

Under Kyoto, which entered into force in February after years of dispute between Washington and its main allies, about 40 rich nations have to cut emissions of heat-trapping carbon dioxide by 5.2 percent below 1990 levels by 2008-12.

One trick will be to extend a UN scheme to poor countries, which reject Kyoto-style caps because higher energy use -- like bringing electricity to slum dwellers or building roads to help trade -- is a key to ending poverty.

WRONG, COSTLY

Adding to the tangle, the world's biggest polluter has rejected Kyoto. President George W. Bush pulled the United States out in 2001, saying Kyoto was too costly and wrongly excluded developing nations before 2012.

Paula Dobriansky, US Under Secretary for Global Affairs, who leads US climate policy, will lead the US delegation in Montreal. Businesses planning long-term investments in new technologies, and investors in carbon markets set up to squeeze industrial emissions, want to know what to expect after 2012.

A report for the European Commission said the climate talks were "a Gordian knot that will need much creativity to unravel."

"Developed countries should continue after 2012 with Kyoto-type commitments with ever deeper cuts," said Jennifer Morgan, climate policy director at the WWF environmental group. "But developing countries should start with less strict goals."

Goals for poor nations could include brakes on the rise of emissions, promises to clean up heavy-polluting industries like coal-fired power plants or targets for higher use of non-polluting solar or wind power.

Some experts favour goals for industrial sectors, such as a target for how much carbon dioxide is emitted per tonne of steel or cement, for instance, or global auto emission standards.

Some nations favour a revival of nuclear power, which produces no greenhouse gases. The United States is expected to push at Montreal for a scheme to bury carbon dioxide underground.

"There are, let's admit it, very wide differences of opinion amongst governments," said Richard Kinley, acting head of the UN Framework Convention on Climate Change.

"It will be over the course of the next say 2-3 years that the process of coming to grips about what needs to be done about climate change could be addressed," he said. Many nations indicated agreement could be reached around 2008-10, he said.