fter a 200-year rise driven mainly by human activities, atmospheric levels of methane, the second most important greenhouse gas, have stopped growing, scientists are reporting. Climate experts said the stabilization of methane, though probably temporary, is important evidence that steps to curb emissions could slow global warming even as disputes persist over what to do about carbon dioxide, the dominant greenhouse gas.

"This is a big deal," said Dr. James A. Hansen, a climate scientist at NASA's Goddard Institute for Space Studies who has highlighted the importance of methane as a heat-trapping gas for years.

It is clearer than ever that "methane presents an opportunity for a global warming success story," he said. "We could get it to stop increasing and even decrease somewhat, mostly with actions that make sense for other reasons." Such actions could include stanching leaks in pipelines or capturing gas released during mining or oil drilling.

The side benefits would include improved air quality, he and other experts said. Methane not only warms the atmosphere but also contributes to the formation of ozone, an ingredient of smog.

Actions to cut methane emissions would also produce far quicker results than measures to curb carbon dioxide. Once released, methane, the main component in natural gas, remains in the atmosphere for only 8 to 10 years before it breaks down. Carbon dioxide, which is released every time a fossil fuel or forest is burned, can last a century and has been accumulating steadily in the air.

The American and Dutch researchers who measured the change in methane levels said they found evidence that human actions, while not aimed at stemming climate change, appeared to be the cause — specifically the near shutdown of oil and gas extraction after the disintegration of the Soviet Union.

Old production methods released vast streams of the gas from leaking pipelines, uncapped wells and the like. Newer, less leaky methods are slowly being adopted now.

Some climate experts had already noted that emissions of methane were more variable, and perhaps more controllable, than those of carbon dioxide. But this is the first time that scientists have found a sustained plateau in methane concentrations, from 1999 to 2002. A global analysis has not been completed for 2003.

The new analysis, described in the current issue of Geophysical Research Letters, is by the Commerce Department's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., and the National Institute for Space Research of the Netherlands.

Dr. Hansen said it was premature to point to a particular reason for the change because methane has so many sources. About 70 percent comes from human activities — with most from fossil-fuel extraction. But methane also comes from sources as varied as the digestive tract of cattle and termites, wetlands, rice paddies and garbage dumps.

The scientists collected measurements of methane in the air taken at 43 monitoring stations around the world and compared them with a European database of records kept on emissions from various sources.

By examining regional difference in methane concentrations, they found the plateau appeared to result mainly from a sharp drop in emissions in 1991 and 1992 in latitudes north of 50 degrees — a region dominated by Russia and Canada.

Other evidence, they said, pointed more precisely to Russia, including measurements taken at Canada's Alert military base — the northernmost inhabited spot on Earth — that tracked air masses drifting directly from Siberia.

The drop in methane levels there appears to have more than compensated for a rise in emissions from Asia, said Dr. Edward J. Dlugokencky, the lead author of the study and a scientist at the Boulder laboratory.

"If we hadn't had decreases in the former Soviet Union, we wouldn't have seen methane flat for the last four years," he said.

Some scientists concurred with that conclusion, but other experts on methane, though agreeing a plateau had been reached, said they were not yet convinced that the Russian downturn in emissions was the reason.

"Methane is an incredibly messy problem," said Dr. Inez Y. Fung, the director of the atmospheric sciences center at the University of California at Berkeley. She said its level varies not only as sources shrink or grow, but also because it is destroyed at changing rates — depending on other substances, including other pollution, in the air.

And, she added, the estimates of emissions from various sources, particularly nonindustrial ones like rice cultivation, are extremely rough.

She said an untapped source of evidence that could show the link to Russian gas fields could be satellite images of the world at night. Flares on oil rigs destroy only a portion of the methane and could reveal overall activity, and leakage, in such areas, she said.

The biggest question now is whether methane will resume the climb that has more than doubled its concentration since the start of the industrial revolution.

Dr. Dlugokencky said that all depends on how assertively countries and companies work to stem emissions — noting that human behavior is the most uncertain factor of all.

Prof. Jesse H. Ausubel, the director of Program for the Human Environment at Rockefeller University in Manhattan, said that particularly now that natural gas has become a valuable commodity there are strong economic incentives to stop leaks.

"This is better than a no regrets action," he said. "This should be a case where, with little outlay, people can actually win an economic benefit."