A U.N. conference on global warming ended Saturday with a watershed agreement by more than 150 nations — an unwilling United States not among them — to open talks on mandatory post-2012 reductions in greenhouse gases.

The Bush administration, which rejects the emissions cutbacks of the current Kyoto Protocol, accepted a second, weaker conference decision, agreeing to join an exploratory global "dialogue" on future steps to combat climate change.

However, that agreement specifically ruled out "negotiations leading to new commitments."

The divergent tracks did little to close the climate gap between Washington and the Kyoto supporters, which include Europe and Japan. But environmentalists welcomed the plan to negotiate "second-phase" emissions cuts.

"The Kyoto Protocol is alive and kicking," said Jennifer Morgan of the World Wide Fund for Nature.

Before finally gaveling the two-week conference to a close early Saturday after working overtime in snowy Montreal, conference president Stephane Dion told delegates, "What we have achieved is no less than a map for the future, the Montreal Action Plan."

But Dion, Canada's environment minister, later acknowledged to reporters, "I would prefer to have the United States in Kyoto."

The Montreal meeting was the first of the annual climate conferences since the Kyoto Protocol took effect last February, mandating specific cutbacks in emissions of carbon dioxide and five other gases by 2012 in 35 industrialized countries.

A broad scientific consensus agrees that these gases accumulating in the atmosphere, byproducts of automobile engines, power plants and other fossil fuel-burning industries, contributed significantly to the past century's global temperature rise of 1 degree.

Continued warming is melting glaciers worldwide, shrinking the Arctic ice cap and heating up the oceans, raising sea levels, scientists say. They predict major climate disruptions in coming decades.

The United States is the world's biggest greenhouse-gas emitter, and the Clinton administration was instrumental in negotiating the treaty protocol initialed in 1997 in Kyoto, Japan — a pact the Senate subsequently refused to ratify.

When Bush rejected Kyoto outright after taking office in 2001, he said its mandatory energy cuts would harm the U.S. economy, and he complained that major developing countries were not covered.

The protocol's language required its 157 member nations by 2005 to begin talks on deeper emissions cuts for the next phase, which begins when Kyoto expires in 2012.

In days of tough negotiation, the Kyoto nations settled on a plan whereby a working group would begin developing post-2012 proposals. The agreement set no deadline for completing that work, except to say it should be done early enough to ensure that no gap develops after 2012.

That would guarantee an uninterrupted future for the burgeoning international "carbon market," in which carbon reductions achieved by one company can be sold to another to help it meet its target.

At the same time, the host Canadians tried to draw in the Americans on the parallel track, under the umbrella 1992 U.N. climate treaty, which does not mandate emissions cuts. As the days wore on, the language offered to the Americans, and finally accepted by them, weakened.

"It's clear the Bush administration isn't willing to accept its responsibility," climate expert Bill Hare of Greenpeace International said of the continued U.S. rejection of global negotiations and emissions controls.

Explaining that stand earlier in the week, U.S. delegation chief Paula Dobriansky said the Americans "believe firmly that negotiations will not reap progress, as indicated, because there are differing perspectives."

Instead, the U.S. delegation said it favors voluntary efforts and bilateral and regional arrangements to tackle climate change. It repeatedly pointed to $3 billion-a-year U.S. government spending on research and development of energy-saving technologies as a demonstration of U.S. efforts on climate.

Most of the conference was devoted to the nuts-and-bolts work of the climate pacts.

Environmentalists were pleased at agreements in such areas as how to quantify gas emissions and how to penalize nations that do not meet Kyoto targets.

Others expressed disappointment, meanwhile, that there wasn't more progress in such areas as helping finance developing countries' adaptation to damaging climate change.

British Prime Minister Tony Blair, who has made tackling climate change a key priority for his presidency of the G-8 group of the world's richest nations this year, welcomed the agreement.

"This agreement is the result of years of hard work and is a vital next step in tackling climate change, the biggest long-term challenge facing the world," Blair said in a statement. "Of course it is only a beginning but it is important and demonstrates why it is always worth engaging with America and the rest of the world."