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.