Analysis of global methane changes after the 1991 Pinatubo volcanic eruption

Abstract. The global methane growth rate showed large variations after the eruption of Mount Pinatubo in June 1991. Both sources and sinks of tropospheric methane were altered following the eruption, by feedback processes between climate and photo-chemistry. Such processes include Ultra Violet (UV) radiative changes due to the presence of volcanic sulfur dioxide (SO2) and sulphate aerosols in the stratosphere, and due to stratospheric ozone depletion. Changes in temperature and water vapour in the following years caused changes in the tropospheric chemistry, as well as in natural emissions. We quantify the effects that these processes had on methane concentrations using a one-dimensional chemistry model representative for the global tropospheric column. To infer the changes in UV radiative fluxes, we couple the chemistry model to a radiative transfer model. We find that the overall effect of the eruption on the methane growth rate is dominated by the effect of stratospheric ozone depletion. However, all the other processes are found to have non-negligible effects, and should therefore be taken into account in order to obtain a good estimate of methane concentrations after the eruption. We find that the overall effect was a small initial increase in the methane growth rate after the eruption, then a decrease by about 8 ppb yr−1 by mid-1993. When changes in anthropogenic emissions are employed according to emission inventories, an additional decrease of about 5 ppb yr−1 in the methane growth rate is obtained between the years 1991 and 1993.