A puzzling decline in the growth rate of atmospheric methane finds conflicting explanations in this week’s Nature. Two independent studies, based on different data sets and analytical approaches, reach differing conclusions, but both point to human activities as the main cause of the slowdown.
Methane (CH4) is the most abundant hydrocarbon in the atmosphere, and is second in importance only to carbon dioxide among the greenhouse gases that are directly affected by humans. Atmospheric methane concentrations increased through much of the twentieth century, then levelled off during the past three decades, with signs of renewed growth appearing only very recently. Previous investigations of the late-twentieth-century slowdown have identified declining methane emissions from fossil fuels and wetlands as important causes, but large uncertainties remain.
Murat Aydin and colleagues at the University of California address this question by investigating the history of fossil-fuel emissions, based on measurements of another hydrocarbon, ethane (C2H6), trapped in air bubbles in perennial snowpack from Greenland and Antarctica. Combining their ethane measurements with a simple atmospheric model, the authors derive a fossil-fuel emission history that suggests a need to reassess existing estimates based on reported production. As ethane and methane are both released by fossil-fuel usage and biomass burning, the authors are able to conclude that the late-twentieth-century atmospheric methane slowdown was caused by a larger decrease in fossil-fuel sources from the 1980s than had previously been shown.
Meanwhile, Fuu Ming Kai and colleagues in Singapore take advantage of the different isotopic signatures of methane from fossil-fuel and microbial sources (primarily wetlands and rice paddies) to constrain the contribution of these sources to the atmospheric methane budget. Using a simple atmospheric model to simulate observed inter-hemispheric differences in methane concentration and isotopic composition, these authors exclude reduced fossil-fuel emissions as the main cause of the slowdown. Instead, they suggest that long-term reductions in Northern Hemisphere microbial emissions are primarily responsible, with reduced emissions from rice agriculture in Asia able to provide about half of the required decrease.