American Scientist

Worry over the effects of fossil-fuel carbon dioxide in the air has become a familiar theme in public discourse about climate change. But news accounts (and movies by former Vice Presidents) that focus exclusively on CO₂ in discussing global warming neglect an inconvenient truth: Other gaseous emissions add substantially to the atmosphere's ability to trap heat. In particular, methane (CH₄) produces a climate forcing that is more than a third of that produced by carbon dioxide. The concentrations of methane and carbon dioxide in the atmosphere have both risen dramatically since the start of the industrial revolution, but unlike its more familiar greenhouse-gas cousin, atmospheric methane has recently stopped increasing in abundance.

This happy development wasn't entirely unanticipated, given that the rate of increase has been slowing for at least a quarter-century. Yet the United Nations Intergovernmental Panel on Climate Change has predicated many of its conclusions on scenarios in which methane concentrations would continue growing for decades to come. Thus the recent stabilization of methane levels is something that some scientists are trying very hard to explain.

Edward J. Dlugokencky, an atmospheric chemist with the National Oceanic and Atmospheric Administration (NOAA), has tracked atmospheric methane for many years. He says that "even as the reduction was happening, people doing emission scenarios weren't accounting for it." Dlugokencky maintains that the evolution of methane levels in the atmosphere mostly just reflects the attainment of a chemical equilibrium, such that methane production is balanced by its destruction. In sum, he says, atmospheric methane "looks like a system approaching steady state."

Methane has many sources. Some are natural; others are clearly the consequences of modern society. Natural sources include wetlands and also terrestrial plants, which earlier this year were discovered to give off methane. Sources tied to human activities include fossil-fuel production, landfills, ruminant animals, rice agriculture and wastewater treatment. Methane is destroyed principally by its reaction with the hydroxyl radical (OH) in the lower atmosphere.

Given that people have been extracting fossil fuels from the earth, dumping their garbage in landfills, cattle ranching, growing rice and treating sewage in ever-increasing amounts, it is indeed hard to understand why atmospheric methane levels are not going up and up. One hint might come from the recent discovery that land plants constitute a significant source of methane (though one that is poorly quantified at the moment). Frank Keppler of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and three colleagues reported this surprising result in Nature last January. In that paper, they note that "severe anthropogenic deforestation has considerably reduced tropical biomass over the past decades," suggesting that this "reduced biomass has probably contributed to the recent decrease in the atmospheric growth rate of CH₄ concentration." That is to say, cutting down rain forest might have reduced the atmospheric methane burden.

Another possible explanation comes from work published in June in Geophysical Research Letters. Arlene M. Fiore of NOAA's Geophysical Fluid Dynamics Laboratory, Dlugokencky and two colleagues report the results of their efforts to simulate in detail the evolution of methane concentration using a sophisticated numerical model of the atmosphere. That paper, titled "Impact of meteorology and emissions on methane trends, 1990–2004," suggests that changes in the weather may have played a key role in what Fiore regards as an anomalously abrupt flattening of the methane curve. In particular, an increase in the prevalence of tropical thunderstorms may have raised the amounts of the various nitrogen oxides (gases often referred to collectively as "NO_x ") high in the atmosphere. There NO_x has the side effect of boosting the production of OH, which in turn acts to destroy methane. Rising temperatures over this interval contribute to the elimination of methane as well, but to a lesser extent.

Fiore admits that "there's huge uncertainty" in her estimates of the size of these effects and that other explanations are certainly possible. She suggests, for example, that anthropogenic sources of NO_x may also have increased—or perhaps that the distribution of these pollutant gases has shifted toward the equator as low-latitude nations industrialize. (These gases are that much more likely to foster the destruction of methane there, because that process depends on the amount of incoming solar radiation, which is greatest at low latitudes.)

M. Aslam Khalil, a physicist at Portland State University in Oregon, helped establish a sampling network for methane as long ago as 1979. He, like Dlugokencky, believes that the recent stasis in methane levels fundamentally represents the system coming to equilibrium. Khalil suspects that there have been no significant changes in the overall magnitude of emissions, but he does recognize that some of the individual sources must have become larger over the past few decades. The explanation for the enigmatic stabilization of methane levels, in his view, is that at least one of the other sources—rice agriculture in particular—has simultaneously become much smaller.

In a paper soon to be published in Greenhouse Gases and Animal Agriculture: An Update (Proceedings of the 2nd International Conference on Greenhouse Gases and Animal Agriculture), Khalil and his Portland State colleague Martha J. Shearer point out that China has produced much of the world's rice for many decades, yet for the past 30 years, the area devoted to rice agriculture in that country has fallen from about 37 million hectares to a little more than 27 million. Khalil and Shearer further note that in these rice paddies nitrogen-based fertilizer has to a large extent replaced animal manure or "night soil" (human wastes). This change in how rice is grown in China reduces the amount of methane given off. What is more, these rice farmers are using less water than they did before—another change in agricultural practice that has the unintended side benefit of reducing methane emissions.

Clearly, it will be some time before atmospheric scientists are able to quantify with great certainty the changing sizes of the various sources of methane. But as Khahil says, it's important to get at least a crude handle on what is going on for the purpose of shaping policy: "You don't want to try to control something that's already going down."