"The momentum has built to an absolute peak," Yvo de Boer, the UN's top climate change official, said of Bali's goal of launching talks on a deal by 2009 to succeed the Kyoto Protocol, which binds 36 rich nations to cut greenhouse gas emissions.

"It's almost inconceivable to me there is no political response," in Bali to the scientific warnings, de Boer told Reuters. The Bali talks will be attended by about 130 environment ministers and thousands of other delegates.

Backers of Kyoto say a decision by Australia's Labour Prime Minister-elect Kevin Rudd to ratify Kyoto will add pressure for wider global action.

Australian ratification will make the United States the only big industrial nation outside Kyoto, which obliges cuts of 5 percent in emissions below 1990 by 2008-12.

"I hope that the Americans will follow the Australians' example," European Environment Commissioner Stavros Dimas said.

2009 OPTIMISTIC

But many experts say it may be too optimistic to agree a new pact by the end of 2009 as intended, especially since President George W. Bush's successor will take over only in January 2009.

"The goal of a deal in 2009 is extraordinarily ambitious," said Elliot Diringer, of the Pew Center on Global Climate Change, a Washington think-tank. Many governments are likely to want to wait and see what Washington is willing to do.

Still, many experts say a pact has to be in place by 2009 because it will take years to ratify. Investors, in coal-fired power plants or wind farms, want to know the rules after Kyoto's first period ends in 2012 as soon as possible.

"The conference must also fix the end of 2009 as the deadline for completing the negotiations," Dimas said.

Bush opposes Kyoto, saying it unfairly omitted 2012 emissions goals for developing nations and would damage the US economy. He has instead stressed big investments in cleaner technologies such as hydrogen or "clean" coal.

In a shift, Bush agreed with G8 allies in June on a need for "substantial cuts" in emissions and a goal of a UN deal in 2009.

So the problem will be partly to persuade developing nations, led by China and India, to join in a pact aimed at curing a problem caused largely by rich nations. Developing nations fear any curbs could hinder an escape from poverty.

Indonesian President Susilo Bambang Yudhoyono said Bali should be "a bold, global decision addressing climate change without significantly jeopardizing development efforts".

They reject the ideas of caps on emissions but might agree to other targets -- such as raising the share of renewable energy in their electricity generation or to cut the amount of carbon dioxide emitted per unit of gross domestic product.