The modern budget of atmospheric methane determined from measurements (Invited)

Reducing radiative forcing by atmospheric CH₄ is often considered a means to slow the rate of increase in climate forcing by long-lived greenhouse gases, because there are cost-effective ways to reduce its anthropogenic emissions. For this to be effective, we have to be sure that increased emissions from natural sources responding to climate change do not cancel reductions in anthropogenic emissions. Much of what we know about the global CH₄ budget is based on nearly 3 decades of high-precision, well-calibrated atmospheric observations of its spatial and temporal distribution. The observations indicate a decreasing growth rate of atmospheric CH₄ from 1984 to 1998, stabilization in the CH₄ burden from 1999 to 2006, and increasing CH₄ at a near-constant rate since 2007. Superimposed on this long-term behavior is significant interannual variability in growth rate. What can these observations tell us about emissions? At the global scale, total emissions of CH₄ are well constrained by measurements of CH₄ atmospheric abundance and rate of increase with an estimate of its lifetime (~9 yr). Assuming that the lifetime has remained constant, annual emissions from 1984 through 2012 have averaged 548 Tg CH₄, but emissions from 2007-2012 are 14 Tg CH₄ yr^-1 greater than the long-term average. While total global emissions are known reasonably well, quantification of emissions from individual sources remains difficult, in part because sources are spread over enormous areas, and emission factors vary over relatively small spatial scales and change over time. A common approach to estimating CH₄ emissions from specific sources is to assimilate the spatial and temporal variations of observed CH₄ abundance in a chemical transport model. Robust conclusions of these studies are that most of the interannual variability in CH₄ growth rate is caused by temperature- and precipitation-driven changes in wetland emissions, with smaller contributions from its major loss process (atmospheric oxidation, initiated by hydroxyl radical (OH)) and extreme biomass burning events. For the renewed increase in CH₄ that began in 2007, the largest contributions are from increased emissions from tropical wetlands and anthropogenic sources, driven by increased production of coal and unconventional natural gas, but the atmospheric data are not consistent with the rates of increase in anthropogenic emissions reported in inventories. Finally, while increased emissions from natural sources at high northern latitudes contributed to the increase in CH₄ starting in 2007, there is, so far, no detectable trend in Arctic emissions from melting permafrost and subsea hydrates.