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

Abstract. The CH 4 growth rate in the atmosphere showed large variations after the Pinatubo eruption in June 1991. A decrease of more than 10 ppb yr −1 in the growth rate over the course of 1992 was reported, and a partial recovery in the following year. Although several reasons have been proposed to explain the evolution of CH 4 after the eruption, their contributions to the observed variations are not yet resolved. CH 4 is removed from the atmosphere by the reaction with tropospheric OH, which in turn is produced by O 3 photolysis under UV radiation. The CH 4 removal after the Pinatubo eruption might have been affected by changes in tropospheric UV levels due to the presence of stratospheric SO 2 and sulfate aerosols, and due to enhanced ozone depletion on Pinatubo aerosols. The perturbed climate after the eruption also altered both sources and sinks of atmospheric CH 4 . Furthermore, CH 4 concentrations were influenced by other factors of natural variability in that period, such as El Nino–Southern Oscillation (ENSO) and biomass burning events. Emissions of CO, NO X and non-methane volatile organic compounds (NMVOCs) also affected CH 4 concentrations indirectly by influencing tropospheric OH levels. Potential drivers of CH 4 variability are investigated using the TM5 global chemistry model. The contribution that each driver had to the global CH 4 variability during the period 1990 to 1995 is quantified. We find that a decrease of 8–10 ppb yr −1 CH 4 is explained by a combination of the above processes. However, the timing of the minimum growth rate is found 6n9 months later than observed. The long-term decrease in CH 4 growth rate over the period 1990 to 1995 is well captured and can be attributed to an increase in OH concentrations over this time period. Potential uncertainties in our modelled CH 4 growth rate include emissions of CH 4 from wetlands, biomass burning emissions of CH 4 and other compounds, biogenic NMVOC and the sensitivity of OH to NMVOC emission changes. Two inventories are used for CH 4 emissions from wetlands, ORCHIDEE and LPJ, to investigate the role of uncertainties in these emissions. Although the higher climate sensitivity of ORCHIDEE improves the simulated CH 4 growth rate change after Pinatubo, none of the two inventories properly captures the observed CH 4 variability in this period.