For years, scientists have been trying to figure out why the atmospheric concentration of methane, a heat-trapping gas, stopped increasing in the early 1990’s after tripling during the preceding 200 years. Some scientists and environmental groups said the change could be a sign of success in efforts to stem emissions of methane, a gas that contributes to global warming. They included stanching pipeline leaks and capturing methane from landfills. Now a new study by an international research team supports the suspicions of some experts that the leveling off was probably temporary and caused by a downturn in emissions from industry and most likely related to the collapse of the Soviet Union and its economy. After 1999, emissions from industry and other human activities began rising again, particularly in China, according to the study, which will be published today in the journal Nature. But that increase in methane from manmade sources appears to have been masked by a reduction in methane from sources in nature. Tropical droughts reduced methane released by bacteria in muddy wetlands, the study said. Inez Y. Fung, an atmospheric scientist at the University of California, Berkeley, said the study was a convincing portrait […]