That Other Greenhouse Gas
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
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."