Russia formally ratified the Kyoto Protocol on global warming on Thursday, clearing the way for the environment pact to come into force in February 2005.

The move means that from Feb. 16, industrialized nations that are signatories to the pact will be legally bound to meet quantitative targets for reducing or limiting emissions of so-called greenhouse gases.

Scientists say carbon dioxide released from burning oil, coal and gas in power plants and cars is the main source of global warming.

U.N. climate experts have already detected many early signals of global warming, including the shrinking of mountain glaciers and Arctic and Antarctic sea-ice, reduced ice cover on lakes and rivers, longer summer growing seasons and the spread of many insects and plants toward the poles.

The U.N. accord, backed by more than 120 countries, will enter into force 90 days after Thursday's filing of the Russian ratification documents with the United Nations (news - web sites) -- meaning on Feb. 16, according to the accord's administrators.

Russia's documents were handed to U.N. Secretary-General Kofi Annan (news - web sites) by Moscow's U.N. ambassador, Andrei Denisov. Both are in Nairobi, the home of the U.N. Environment Program (UNEP), for an extraordinary U.N. Security Council meeting on Sudan.

"This is a historic step forward in the world's efforts to combat a truly global threat," Annan said. "Most important, it ends a long period of uncertainty.

The accord was ratified by both houses of Russia's parliament last month and then signed by President Vladimir Putin (news - web sites) on Nov. 5. Russia's support become crucial after the United States, the world's biggest polluter, rejected the pact in 2001.

"We discussed it for a long time," Denisov said. "We believe this step will be helpful in preventing emissions of greenhouse gases, which is very important for all mankind.

The protocol commits 55 industrialized nations to make big cuts in emissions of gases such as carbon dioxide by 2012.

The 1997 Kyoto Protocol obliges rich nations to cut overall emissions of heat-trapping carbon dioxide by 5.2 percent below 1990 levels by 2008-12 by curbing use of coal, oil and natural gas and shifting to cleaner energies like solar or wind power.

To come into force, the pact needed to be ratified by countries accounting for at least 55 percent of developed nations' greenhouse gas emissions.

Russia, which accounts for 17 percent of global emissions, became the key to Kyoto after Washington pulled out saying the pact was too costly and unfairly exempted large, rapidly industrializing countries such as China and India.

Russia signed the Kyoto Protocol in 1999. But it agreed to ratify the treaty only in exchange for European Union (news - web sites) agreement on Moscow's admission to the World Trade Organization (news - web sites).

Joke Waller-Hunter of the Bonn-based Climate Change Secretariat, which services the protocol, said only four industrialized countries have not yet ratified the Kyoto Protocol: Australia, Liechtenstein, Monaco and the United States.