Title: Recent decreases in fossil-fuel emissions of ethane and methane derived from firn air
Abstract: 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. Methane and ethane are the most abundant hydrocarbons in the atmosphere and they affect both atmospheric chemistry and climate. Both gases are emitted from fossil fuels and biomass burning, whereas methane (CH4) alone has large sources from wetlands, agriculture, landfills and waste water. Here we use measurements in firn (perennial snowpack) air from Greenland and Antarctica to reconstruct the atmospheric variability of ethane (C2H6) during the twentieth century. Ethane levels rose from early in the century until the 1980s, when the trend reversed, with a period of decline over the next 20 years. We find that this variability was primarily driven by changes in ethane emissions from fossil fuels; these emissions peaked in the 1960s and 1970s at 14–16 teragrams per year (1 Tg = 1012 g) and dropped to 8–10 Tg yr−1 by the turn of the century. The reduction in fossil-fuel sources is probably related to changes in light hydrocarbon emissions associated with petroleum production and use. The ethane-based fossil-fuel emission history is strikingly different from bottom-up estimates of methane emissions from fossil-fuel use1,2, and implies that the fossil-fuel source of methane started to decline in the 1980s and probably caused the late twentieth century slow-down in the growth rate of atmospheric methane3,4.