Methane lasts longer in atmosphere.

Methane lasts longer in atmosphere

Laboratory experiments indicate that the greenhouse gas methane remains in the atmosphere about 25 percent longer than previously suspectd -- a finding that may clear up some nagging questions about this important player in the global warming drama.

The concentration of methane gas rises almost 1 percent per year, but scientists lack a good explanation for the increase. A problem arises when researchers tally the known sources for methane and compare those with the "sinks" that remove the gas from the atmosphere. The calculations suggest methane levels should remain constant or even decrease, despite what current measurements show.

The new research, conducted by scientists at the National Oceanic and Atmospheric Administration in Boulder, Colo., examined the speed of a chemical reaction between methane and the hydroxyl radical, the principal methane sink in the atmosphere. The experiments revealed hydroxyl reacts about 25 percent slower at atmospheric temperatures than suggested by previous, less accurate experiments. That means the average methane molecule remains in the atmosphere about 12.5 years rather than 10 years, the scientists report in the April 4 NATURE. Methane gas comes from natural wetlands as well as from such human activities as rice cultivation, raising domestic animals, biomass burning and landfilling wastes.

Because the findings suggest a weaker removal process for methane, they help resolve the discrepancy between the calculations and the observed methane increase. They also suggest tha methane, with its longer lifetime, contributes more to the greenhouse effect than previously thought.