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

Abstract. The CH₄ 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 CH₄ after the eruption, their contributions to the observed variations are not yet resolved. CH₄ is removed from the atmosphere by the reaction with tropospheric OH, which in turn is produced by O₃ photolysis under UV radiation. The CH₄ removal after the Pinatubo eruption might have been affected by changes in tropospheric UV levels due to the presence of stratospheric SO₂ 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 CH₄. Furthermore, CH₄ concentrations were influenced by other factors of natural variability in that period, such as El Niño–Southern Oscillation (ENSO) and biomass burning events. Emissions of CO, NO_X and non-methane volatile organic compounds (NMVOCs) also affected CH₄ concentrations indirectly by influencing tropospheric OH levels.

Potential drivers of CH₄ variability are investigated using the TM5 global chemistry model. The contribution that each driver had to the global CH₄ variability during the period 1990 to 1995 is quantified. We find that a decrease of 8–10 ppb yr⁻¹ CH₄ is explained by a combination of the above processes. However, the timing of the minimum growth rate is found 6&nash;9 months later than observed. The long-term decrease in CH₄ 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 CH₄ growth rate include emissions of CH₄ from wetlands, biomass burning emissions of CH₄ and other compounds, biogenic NMVOC and the sensitivity of OH to NMVOC emission changes. Two inventories are used for CH₄ 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 CH₄ growth rate change after Pinatubo, none of the two inventories properly captures the observed CH₄ variability in this period.