Atmosphere Journal Entry

Methane Levels Rising Again (December 1, 2008)

Carbon dioxide isn't the only greenhouse gas we need to worry about. Methane in the atmosphere contributes to global warming as well. Now, new research shows methane levels are rising again after leveling off for nearly a decade.

Methane levels shot up in 2007 with several million more metric tons of the gas added to the atmosphere. Until recently, the addition of methane was balanced by the atmosphere's natural ability to get rid of it thanks to a kind of molecule called the hydroxyl radical (OH). This molecule, which has a single oxygen atom bonded to a single hydrogen atom, works as the atmosphere's methane cleanser. Now, more methane is added than can be removed naturally.

Natural sources of methane include wetlands, rice paddies, and cattle. The gas is also produced by industry. A big jump in the number of factories in Asia is one reason for the increase. Some scientists also link the methane rise in the Northern Hemisphere to warmer weather in Siberia throughout 2007. The extra warmth increased decomposition by bacteria in Siberian wetlands. It's less clear why methane levels are rising in the Southern Hemisphere as well.

Molecule for molecule, methane is about 25 times stronger as a contributor to the greenhouse effect than carbon dioxide. But carbon dioxide has a much greater effect on rising global temperatures because there's much more of it in the atmosphere.

Data from air bubbles trapped in ice sheets shows that methane is more abundant now in the atmosphere than at any time in the past 400,000 years. Since the middle of the 18th century, methane levels have more than doubled.