BALI, Indonesia - World powers meeting at a U.N. climate-change conference in Indonesia this week won't be able to craft a meaningful plan to address global warming without cooperation from the United States, the top emitter of greenhouse gases, the United Nations' climate chief said yesterday.

Delegates from 190 nations will gather on the resort island of Bali today for one of the largest global-warming conferences ever, bringing together about 10,000 people, including Hollywood luminaries, former Vice President Al Gore, fishermen, and drought-stricken farmers for two weeks of marathon discussions.

The United States refused to sign the last major international treaty on reducing greenhouse gases, undermining its effectiveness.

Yvo de Boer, general secretary of the U.N. Framework Convention on Climate Change, said the role of the United States "would be critical" in the discussions and that delegates must come up with a road map that's embraced by Washington.

"To design a long-term response to climate change that does not include the world's largest emitter and the world's largest economy just would not make any sense," he told reporters.

World leaders will attempt to launch negotiations that could lead to a replacement for the Kyoto Protocol, which expires in 2012.

Among the most contentious issues will be whether emission cuts should be mandatory or voluntary and how to help the world's poorest countries adapt to a warmer climate.

The United States, which along with Australia refused to sign the Kyoto Protocol, said ahead of the Bali talks that it was eager to launch negotiations, but has been among industrialized nations leading a campaign against mandatory emission cuts.

But now the United States finds itself isolated at the conference, given that Australian Prime Minister-elect Kevin Rudd, whose party swept to power in general elections just one week ago, immediately put signing the Kyoto pact at the top of his international agenda.

The meeting on Bali comes after a Nobel Prize-winning U.N. network of scientists issued a report concluding the level of carbon and other heat-trapping greenhouse-gas emissions must be stabilized by 2015 and decline from there to stave off the worst effects of climate change.

The 1997 Kyoto pact required 36 industrial nations to reduce carbon dioxide and other heat-trapping gasses but it set relatively low emissions reduction targets: about a 5 percent required drop in the levels recorded in 1990 by 2012.

A new agreement must be concluded within two years to ensure a smooth, uninterrupted transition, proponents say.