Reached in 1997 after two years of talks, the pact calls for the world's industrialized countries to reduce emissions of greenhouse gases linked to global warming. Proponents still need backing from Russia's parliament, both chambers of which are controlled by Russian President Vladimir Putin's loyalists, who are expected to fall in line with Thursday's decision.

With Washington's refusal to endorse the pact, Russia's ratification was needed for the accord to become effective.

The treaty needs to be approved by no fewer than 55 industrial countries that cumulatively represent at least 55 percent of greenhouse gas emissions. The U.S. represents 36 percent of the world's greenhouse gas emissions, and its rejection of the pact left Russia as the only other industrial polluter large enough to push the treaty over the 55 percent threshold.

''Today's decision was historic,'' said Georgy Safanov, director of the Russian Regional Environment Center and a longtime advocate of the pact. ''Human beings affect climate, and there is just one way to prevent this - a global initiative on reducing our impact on climate.''

Klaus Toepfer, chief of the U.N. Environment Program, called Russia''s decision a ''cause for celebration. . . . I hope other nations, some of whom like Russia have maybe been in the past reluctant to ratify, will now join us in this truly global endeavor.''

For more than a year, Russia had been sending mixed signals about its policy on the Kyoto treaty. In the summer of 2003, the Kremlin appeared ready to ratify the accord, but by December, a key Kremlin economic adviser had convinced Putin that the pact posed too great a threat to Russia's economy.

That adviser, Andrei Illarionov, became the treaty's most ardent Russian critic. He questioned the link between greenhouse gas emissions and climate change and warned that curbs on emissions that derive mostly from the burning of fossil fuels would unnecessarily restrain the country's economic growth.

At the time, the treaty appeared doomed. But in May, after Putin met with leaders of the European Union, a staunch advocate of the Kyoto treaty, the Russian leader expressed a new enthusiasm for the accord. He vowed to move Russia toward fast ratification of the pact, a decision that appeared to be tied to the EU's decision to back Russia's bid to join the World Trade Organization, a goal the Kremlin has eyed since 1993.

Analysts said Illarionov's pessimism about the pact gradually lost out to the advice of other Kremlin advisers who told Putin that the treaty's impact on the Russian economy would be minimal.

The pact calls for developed nations to cut greenhouse gas emissions 5.2 percent below 1990 levels by the year 2012. Greenhouse gases covered by the treaty include carbon dioxide, methane, hydrofluorocarbons, perfluorocarbons, nitrous oxide and sulfur dioxide. Once in effect, the pact requires member countries to abide by emissions quotas assigned to them for five years, from 2008 through 2012.

Bush opposes ratification

* The United States signed on to the treaty under President Clinton in 1998, but the Senate did not ratify the pact.

Since then, the Bush administration has said it opposes ratification, complaining that the treaty imposes unrealistic cutbacks in greenhouse gas emissions that ultimately would hinder economic growth.

President Bush's aides also argue that major polluters that are deemed by the pact to be developing nations, such as India and China, would not be bound by the same stringent cutbacks industrialized nations face, and therefore would have an unfair economic edge.