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The CH4 growth rate in the atmosphere showed large variations after thePinatubo eruption in June 1991. A decrease of more than 10 ppb yr−1 inthe growth rate over the course of 1992 was reported, and a partial recoveryin the following year. Although several reasons have been proposed to explainthe evolution of CH4 after the eruption, their contributions to theobserved variations are not yet resolved. CH4 is removed from theatmosphere by the reaction with tropospheric OH, which in turn is produced byO3 photolysis under UV radiation. The CH4 removal after the Pinatuboeruption might have been affected by changes in tropospheric UV levels due tothe presence of stratospheric SO2 and sulfate aerosols, and due toenhanced ozone depletion on Pinatubo aerosols. The perturbed climate afterthe eruption also altered both sources and sinks of atmospheric CH4.Furthermore, CH4 concentrations were influenced by other factors ofnatural variability in that period, such as El Niño–Southern Oscillation(ENSO) and biomass burning events. Emissions of CO, NOX and non-methanevolatile organic compounds (NMVOCs) also affected CH4 concentrationsindirectly by influencing tropospheric OH levels. Potential drivers of CH4 variability are investigated using the TM5 globalchemistry model. The contribution that each driver had to the global CH4variability during the period 1990 to 1995 is quantified. We find that adecrease of 8–10 ppb yr−1 CH4 is explained by a combination of theabove processes. However, the timing of the minimum growth rate is found 6&nash;9months later than observed. The long-term decrease in CH4 growth rate overthe period 1990 to 1995 is well captured and can be attributed to an increasein OH concentrations over this time period. Potential uncertainties in ourmodelled CH4 growth rate include emissions of CH4 from wetlands,biomass burning emissions of CH4 and other compounds, biogenic NMVOC andthe sensitivity of OH to NMVOC emission changes. Two inventories are used forCH4 emissions from wetlands, ORCHIDEE and LPJ, to investigate the role ofuncertainties in these emissions. Although the higher climate sensitivity ofORCHIDEE improves the simulated CH4 growth rate change after Pinatubo,none of the two inventories properly captures the observed CH4 variabilityin this period

Can we explain the observed methane variability after the Mount Pinatubo eruption?