Reduced methane growth rate explained by decreased Northern Hemisphere microbial sources

Elusive methane sources traced Methane is a greenhouse gas with a significant warming effect on climate — only water vapour and carbon dioxide are more important — yet the factors influencing its atmospheric concentration are poorly understood. In particular, a rapid rise in methane levels in the mid-twentieth century gradually (but temporarily) levelled off around the turn of the millennium; the reasons for this decline in growth rate are still being debated. Two new studies shed light on this conundrum — but reach conflicting conclusions. Fuu Ming Kai et al . measure differences in the concentration and isotopic signature of methane between the Northern and Southern Hemispheres and conclude that reduced microbial activity in wetlands was primarily responsible. Changing rice agricultural practices seem to explain about half of the Northern Hemispheric trend. By contrast, Murat Aydin et al . combine measurements of ethane trapped in Antarctic ice with a simple atmospheric model and conclude that the slow-down was caused by reduced methane emissions from fossil-fuel production. In News and Views, Martin Heimann discusses the differing findings of these two studies. Atmospheric methane (CH 4 ) increased through much of the twentieth century, but this trend gradually weakened until a stable state was temporarily reached around the turn of the millennium 1 , 2 , after which levels increased once more 3 . The reasons for the slowdown are incompletely understood, with past work identifying changes in fossil fuel, wetland and agricultural sources and hydroxyl (OH) sinks as important causal factors 1 , 4 , 5 , 6 , 7 , 8 . Here we show that the late-twentieth-century changes in the CH 4 growth rates are best explained by reduced microbial sources in the Northern Hemisphere. Our results, based on synchronous time series of atmospheric CH 4 mixing and 13 C/ 12 C ratios and a two-box atmospheric model, indicate that the evolution of the mixing ratio requires no significant change in Southern Hemisphere sources between 1984 and 2005. Observed changes in the interhemispheric difference of 13 C effectively exclude reduced fossil fuel emissions as the primary cause of the slowdown. The 13 C observations are consistent with long-term reductions in agricultural emissions or another microbial source within the Northern Hemisphere. Approximately half (51 ± 18%) of the decrease in Northern Hemisphere CH 4 emissions can be explained by reduced emissions from rice agriculture in