MONTREAL, Dec. 9 - Two weeks of treaty talks on global warming neared an end today with the world's current and projected leaders in emissions of greenhouse gases, the United States and China, still refusing to take any mandatory steps to avoid dangerous climate change.

The Bush administration was sharply criticized by other governments and by environmental groups for walking out of a round of informal discussions shortly after midnight that were aimed at finding new ways of curbing gases.

"This shows just how willing the U.S. administration is to walk away from a healthy planet and its responsibilities to its own people," said Jennifer Morgan of the World Wildlife Fund.

American officials declined to comment Thursday afternoon on their actions. They released a printed statement, but it referred only to the expected visit and speech later today by former President Bill Clinton.

Paula Dobriansky, the head of the American delegation, said that public events like Mr. Clinton's presentation were "useful opportunities to hear a wide range of views on global climate change."

The meeting is the latest in a 17-year string of sessions aimed at moving both industrial powers and fast-growing developing countries toward cutting emissions of the greenhouse gases, most notably carbon dioxide, which are an unavoidable byproduct of burning coal, oil and forests.

They have produced two agreements. The first, the 1992 Framework Convention on Climate Change, was accepted by nearly all the world's countries, including the United States, but includes no binding targets and never defines an unacceptably dangerous concentration of greenhouse gases.

The Kyoto Protocol, an addendum to the first treaty, took effect in February but only requires about three dozen industrial countries to make cuts in the gases. It was rejected in 2001 by President Bush.

At the Montreal meeting on Friday, countries bound by the Kyoto pact were close to agreeing on a plan to negotiate a new set of targets and timetables for cutting emissions after its terms expire in 2012.

But under pressure from some countries that were already having trouble meeting Kyoto targets, the language included no specific year for completing talks on next steps, instead indicating that parties would "aim to complete" work "as soon as possible."

In a news conference, environmental groups tried to cast that decision as a successful signal to emerging markets in credits earned by cutting greenhouse gases.

But even if those talks generate new targets, some scientists said today that they would be insufficient to stem harmful warming without much broader actions by the biggest and fastest-growing polluters.

In a statement from London, Lord Martin Rees, the new president of Britain's Royal Society, an independent national scientific academy, said that ongoing disputes among wealthy nations over how to cut the gas emissions were distracting them from actually carrying out steps to make the cuts.

Environmental campaigners insisted that the Kyoto process would eventually force other countries, particularly the United States, to act by building a market for credits achieved by making deep cuts in carbon dioxide and the other gases.

"As Kyoto deepens and broadens, U.S. business and industry will mount irresistible pressure on United States leadership to re-engage in the process rather than be shut out of markets of the future," said Alden Meyer of the Union of Concerned Scientists, a private group that supports binding cuts in heat-trapping gases.

But lobbyists and groups associated with businesses that oppose such restrictions scoffed at the prospect of a meaningful carbon market.

The National Center for Public Policy Research, one such group, worked the halls, distributing mock emissions credits.

These are the chits created under a "cap and trade" system for controlling pollution that allow those businesses that make cuts beyond requirements to sell the extra tons to others.

In this case, the mock credits were printed in five languages on rolls of toilet paper.

Environmental groups responded in kind.

The National Environmental Trust distributed custom-printed noise-making rubber Whoopee Cushions printed with a caricature of President Bush and the words "Emissions Accomplished."

Science Times Podcast
Andrew C. Revkin reports from the climate talks in Montreal, Dec. 5 - Dec 9, 2005.
Audio: Revkin Reports From Montreal (mp3)
Audio: Listen in on the "bed-in" (mp3)

Other climate talk coverage.

E-mail Andrew C. Revkin at revkin@nytimes.com 

- end -