Methane levels stabilize

Since 1998, levels of atmospheric methane, an important greenhouse gas, have been largely constant, reports F. Sherwood Rowland, professor of chemistry and earth system science at the University of California, Irvine. This leveling follows a methane rise that spanned at least two decades, says Rowland, whose research was published in the Nov. 23 online edition of Geophysical Research Letters . Since the beginning of the Industrial Revolution, atmospheric methane levels have nearly doubled, and two-thirds of what is currently in the atmosphere can be traced to human activities, he says. Man-made sources include leaking natural gas pipelines, coal mining, rice production, biomass burning, and landfills. Methane constitutes 10 to 15% of the gases linked to global warming. The new finding indicates that methane may no longer be as large a global-warming threat as previously thought. "If one really tightens emissions, the amount of methane in the atmosphere 10 years from now could be less than it is today," Rowland says.