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Since 1978 chemists at the University of California, Irvine, have been collecting air in 40 locations from northern Alaska to southern New Zealand. Using gas chromatography, the scientists have measured the levels of methane–CH₄–in the lowest layer of our atmosphere. Although not nearly as abundant as carbon dioxide–CO₂–methane remains the second most important greenhouse gas, both because each molecule of CH₄ in the atmosphere traps 23 times as much heat as carbon dioxide and it helps create more ozone–yet another greenhouse gas–in the atmosphere. During the two decades of measurements, methane underwent double-digit growth as a constituent of our atmosphere, rising from 1,520 parts per billion by volume (ppbv) in 1978 to 1,767 ppbv in 1998. But the most recent measurements have revealed that methane levels are barely rising anymore–and it is unclear why.

Chemist Isobel Simpson led the research examining samples from 1998 through 2005 and found that methane levels had practically stopped rising, reaching 1,772 ppbv in 2005. During this period some years did see rises while others actually saw slight decreases, according to the paper presenting the result in the November 23 issue of Geophysical Research Letters. By also measuring levels of ethane (C₂H₆) and perchloroethylene, or perc, (C₂Cl₄) the researchers determined that these pulses in methane levels during this period could be linked to major forest fires, such as the massive burn in Indonesia from late 1997 to early 1998. “All three of these molecules are removed by the same process–reaction with hydroxyl,” a radical formed from water in the atmosphere, explains Nobel Prize-winning chemist F. Sherwood Rowland, who participated in the research. “Both methane and ethane are produced in biomass burning, but perc is an industrial solvent. If biomass burning is the source, then perc [levels] should behave quite differently from the two hydrocarbons, and this is what we observed.”

But that does not solve the larger question of why methane in the atmosphere seems to have reached a plateau. “The scientific community agrees that the pause is source-driven rather than sink-driven, that is, caused by decreasing emissions of methane,” Simpson says. “I don’t believe we have reached a consensus on which sources have decreased and by how much.” Leading hypotheses include: the collapse of the Soviet Union, which resulted in a decline in energy use in Russia and the other former Soviet republics; repairs to oil and gas lines to prevent leaks; decreasing emissions from coal mining; widespread drought that led to decreased emissions from natural wetlands; and a decline in rice production. “The trends of major man-made sources such as rice fields and cattle have greatly slowed down over the last two decades,” notes physicist Aslam Khalil of Portland State University. “As these–rice and cattle–were once big sources, their lack of continued increase would then cause atmospheric methane to stop increasing as well.”

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