Rising atmospheric methane: 2007–2014 growth and isotopic shift

From 2007 to 2013, the globally-averaged mole fraction of methane in the atmosphere increased by 5.7±1.2 ppb yr-1. Simultaneously, δ13CCH4 (a measure of the 13C/12C isotope ratio in methane) has shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5±0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics: for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies, or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13C-depleted values together with its significant interannual variability, and the tropical and Southern Hemisphere loci of post-2007 growth, both indicate fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.