The previously increasing atmospheric methane concentration has inexplicably stalled over the past three decades. This may be due to a fall in fossil-fuel emissions or to farming practices that are curtailing microbial sources. See Letters p.194 & p.198
There has been a recent unexpected variation in the atmospheric concentration of methane (CH4) — the second most important anthropogenically modified greenhouse gas after carbon dioxide. Its growth rate has declined almost to a standstill in the past three decades, despite more than doubling since pre-industrial times; signs of renewed growth are only now starting to appear. What has caused the puzzling behaviour of this atmospheric trace gas? In potentially contradictory studies on pages 194 and 198 of this issue, Kai et al.1 and Aydin et al.2 attempt to answer this question.
The growth rate of the atmospheric methane concentration effectively reflects the global methane budget (Fig. 1), which at present delicately balances large sinks and sources. Global sinks arise primarily from the activity of the hydroxyl radical (OH•), which is involved in photochemical oxidation reactions in the atmosphere. Although sinks are quite well understood, the relative contributions of the various processes that produce methane are uncertain.
About 60% of global methane stems from human activities. This includes emissions associated with energy production and usage — such as coalmining, incomplete combustion of fossil fuels and gas leaks. These fossil-fuel-derived emissions, together with methane from landfills, organic waste, cattle raising, rice agriculture and biomass burning, were probably the main cause of the increasing concentrations after pre-industrial times. Methane also forms naturally, mostly from anaerobic decomposition of organic carbon in wetlands, with lesser contributions from the ocean, termites, wild animals, wildfires and various geological sources.
How can the relative contributions of the various sources to the global methane budget be determined? One technique depends on the fact that most of the source processes have a distinct geographical signature, which is reflected in the atmospheric methane-concentration distribution. For example, fossil-fuel methane emissions that predominantly occur in the Northern Hemisphere cause a north–south concentration gradient that is visible in atmospheric measurements, including those made at surface stations3 and from space-based observations4. This method identified a fall in fossil-fuel-derived methane emissions in the 1990s, reinforced by a coincident decrease in wetland emissions since 2000, as the main drivers of the global decline in methane growth rate over the past 30 years5, albeit with substantial uncertainties. Other methane sources also have distinct geographical patterns, but many of these overlap, and so the attribution and quantification of the different sources from atmospheric measurements are not unique.
Aydin and colleagues2 offer new information on the history of methane emissions from fossil fuels in the Northern and Southern Hemispheres over the past 60 years. They achieve their reconstruction by measuring the ethane (C2H6) in air trapped in porous snow-ice (firn) in Greenland and Antarctica. Like methane, ethane is released during the mining, transport and incomplete burning of fossil fuels, but also from incomplete biomass burning. With the help of a simple atmospheric mixing model, the authors resolve the ethane-concentration record into changes in ethane-source strength and, given typical hemispheric patterns of fossil-fuel and biomass-burning sources, into changes over time of these two sources. They then assume fixed methane-to-ethane emission ratios in estimating the history of fossil-fuel methane emissions.
Their results are surprising. Whereas the emissions deduced for biomass burning are consistent with independent bottom-up estimates, the inferred history of fossil-fuel-derived methane emissions before 1980 is strikingly different — double the estimates from standard databases based on the statistics of fossil-fuel production. During 1980–2000, the record of fossil-fuel methane emissions shows an almost 30% decline, which would go a long way towards explaining the observed decrease in the global methane growth rate.
Kai and colleagues1 offer an alternative explanation. They make use of the fact that methane from fossil fuels is enriched in its stable 13C/12C carbon-isotope ratio, whereas methane from microbial sources (mainly wetlands and rice paddies) is depleted in 13C/12C with respect to the atmospheric background. Furthermore, variations in the 2H/1H hydrogen-isotope ratio in methane are primarily affected by changes in the photochemical sink. Observations of the atmospheric methane concentration, in conjunction with measurements of its stable-isotope ratios, thus provide an alternative means to distinguish between different source categories. In combination with a simple model of atmospheric mixing, the authors compare the simulated atmospheric signatures of various methane-source scenarios with observations from the past two decades in each hemisphere.
Surprisingly, Kai et al. find that a reduction in the fossil-fuel methane source is not compatible with these measurements, and that the isotope record can be explained only by a reduction in the microbial sources in the Northern Hemisphere. A drying trend in northern wetlands could have contributed to this finding6, but the authors convincingly show that methane emissions from rice agriculture, particularly in China, must also have decreased. Their conclusion is based on changes in agricultural practices: new high-yield rice species, together with greater application of fertilizer, require shorter inundation periods, making substantial water savings and reducing methane emissions.
Can these conflicting inferences on the recent slow-down of global methane growth be reconciled? Because of the limited data coverage and simplistic analysis assumptions of the two studies1,2, there are considerable uncertainties in the deduced methane-source variations, but the different scenarios are plausible and compatible with their respective observations. The challenge now is to bring the different lines of evidence together, perhaps by using a more advanced modelling framework and improved bottom-up inventory information on the various methane-source categories. More extended observations will help too — particularly the mapping of atmospheric methane concentration by current and upcoming satellite missions.
These studies1,2 illustrate the importance of high-precision, long-term observations of methane concentration and isotope composition, and of auxiliary trace gases such as ethane, in distinguishing between the contributions from different sources. But more insight is needed to solve the enigma of the recent methane budget if the evolution of this important greenhouse gas over the twenty-first century is to be predicted.
Kai, F. M., Tyler, S. C., Randerson, J. T. & Blake, D. R. Nature 476, 194–197 (2011).
Aydin, M. et al. Nature 476, 198–201 (2011).
Dlugokencky, E. J. et al. Geophys. Res. Lett. 36, L18803; http://dx.doi.org/10.1029/2009GL039780 (2009).
Schneising, O. et al. Atmos. Chem. Phys. 9, 443–465 (2009).
Bousquet, P. et al. Nature 443, 439–443 (2006).
Jung, M. et al. Nature 467, 951–954 (2010).