Role of OH variability in the stalling of the global atmospheric CH₄ growth rate from 1999 to 2006

The growth in atmospheric methane (CH₄) concentrations over the past 2 decades has shown large variability on a timescale of several years. Prior to 1999 the globally averaged CH₄ concentration was increasing at a rate of 6.0 ppb yr⁻¹, but during a stagnation period from 1999 to 2006 this growth rate slowed to 0.6 ppb yr⁻¹. From 2007 to 2009 the growth rate again increased to 4.9 ppb yr⁻¹. These changes in growth rate are usually ascribed to variations in CH₄ emissions. We have used a 3-D global chemical transport model, driven by meteorological reanalyses and variations in global mean hydroxyl (OH) concentrations derived from CH₃CCl₃ observations from two independent networks, to investigate these CH₄ growth variations. The model shows that between 1999 and 2006 changes in the CH₄ atmospheric loss contributed significantly to the suppression in global CH₄ concentrations relative to the pre-1999 trend. The largest factor in this is relatively small variations in global mean OH on a timescale of a few years, with minor contributions of atmospheric transport of CH₄ to its sink region and of atmospheric temperature. Although changes in emissions may be important during the stagnation period, these results imply a smaller variation is required to explain the observed CH₄ trends. The contribution of OH variations to the renewed CH₄ growth after 2007 cannot be determined with data currently available.