Methane packs a big punch in the atmosphere — having 21 times the effect of carbon dioxide on global climate. Yet, many of the processes determining the balance between how much of the gas goes into the atmosphere and how much is stored are still uncertain. A team of climate scientists now says, however, that it has better determined the primary controls over the methane budget over the past two decades, and the team offers a warning for the future: methane emissions will likely rise.

Like all atmospheric gases, methane has sources and sinks, and the balance of the two comprises its budget. Sources of the gas include everything from bacterial activity in wetlands and rice paddies, to wildfires, plants and fossil fuels, says Arlene Fiore, a researcher at the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory in Princeton, N.J. The sink is the removal of methane from the atmosphere by a free radical chemical called hydroxyl, she says.

Atmospheric methane has been increasing for the past couple of decades. The annual growth rate of methane in the atmosphere, however, decreased a bit in the early 1990s, and methane abundances in the atmosphere have remained relatively constant since 1999. Whether this atmospheric balance, which implies that methane sources equal sinks, is due to a decline in methane emissions, or to an increase in the destruction of methane, has remained up for debate.

To better understand these patterns, a team led by Philippe Bousquet of the Institut National des Sciences de l’Univers du CNRS in Paris, France, compiled observations of atmospheric methane abundance in surface air from around the world from 1984 through 2003. They then plugged those numbers into a model to determine the likely sources and volumes of methane emissions, using isotopic signatures that vary for the different sources, as the team reported in the Sept. 28 Nature.

The researchers found that on the monthly or yearly scale, wetlands emissions seemed to dominate the variability in observed methane in the atmosphere, says Ed Dlugokencky, a researcher at NOAA’s Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., and a co-author on the paper. Wetlands emissions of methane depend largely on weather conditions in a particular season or year, he says; if it’s particularly warm or wet, emissions will increase and vice versa if it’s cold or dry.

Over longer time scales, however, the observed decline in atmospheric methane was likely due to an overall decline in human-induced methane emissions, Dlugokencky says. The reason for the anthropogenic decline in the 1990s, he says, was two-fold. First, the Soviet Union, which had accounted for much of the fossil fuel-based emissions, collapsed, causing energy usage and emissions there to fall significantly. Second, the United States and other industrial nations began using cleaner burning technologies, thus reducing emissions.

This research by Bousquet and colleagues supports what Ronald Prinn and Yu-Han Chen of MIT in Cambridge, Mass., reported in a paper in the May 31 Journal of Geophysical Research, although the two teams used different models and somewhat different data, Prinn says. “The role of wetlands as a significant, but variable methane source is becoming clear,” he says. And wetlands will likely continue to be the dominant factor year to year in the future, he says, as models project wetlands to release even more methane as global, and especially Arctic, temperatures rise.

“We also expect to see a significant increase in anthropogenic methane emissions in the future,” Prinn says, with increases in natural gas and other fossil fuel usage, especially in developing countries. “So unfortunately, this happy circumstance that we see right now” of an approximately balanced methane budget “is in fact temporary,” he says.

How big of a player methane will be in Earth’s temperatures, however, will also depend on the levels of hydroxyl in the atmosphere, Fiore says. She and her colleagues have found that the hydroxyl radical may have increased slightly in recent years, causing atmospheric methane concentrations to level off. Models the team ran, as reported in the June 24 Geophysical Research Letters, suggested that increased lightning may be increasing the hydroxyl radicals in the atmosphere. But the link is still tenuous, she says.

Methane is a very significant greenhouse gas, Fiore says, “and if we’re going to control it, we need to know how it behaves in the atmosphere.” Determining if changes in the current methane budget are driven more by its sources or sinks “is definitely important,” she says, and the paper by Bousquet and colleagues certainly helps.

Megan Sever

Links:
Geophysical Fluid Dynamics Laboratory
Institut National des Sciences de l'Univers du CNRS (in French)
Climate Monitoring and Diagnostics Laboratory

"Methane climate menagerie," Geotimes, December 2006

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