Sensitivity of the atmospheric CH4 growth rate to global temperature changes observed from 1980 to 1992

Abstract

The response of the atmospheric CH4 growth rate to observed global temperature fluctuations recorded from 1980 to 1992 is explored using a 2-dimensional (2-D) chemistry/transport model. Two competing mechanisms are considered in this study: the temperature dependency of CH4 emissions from wetlands and the temperature dependency of the CH4 photochemical sink, principally via the positive temperature dependencies in the the reaction rate between CH4 and OH and in the tropospheric content of water vapour, the source of OH. The results provide some bounds on these temperature effects and the regions where they are most effective at perturbing the CH4 growth rate. The OH-induced effect could have caused interannual variations in the CH4 growth rate of the order of few ppbv yr-1; their pattern is consistent with the variations observed during the 1980s, but not in 1991/1992, when the Mount Pinatubo eruption led to global perturbations of not only the temperature but also of stratospheric ozone, stratosphere-tropospheric exchanges and the general circulation. Using the current range of temperature sensitivity of wetland emissions, the wetland effect could explain the entire CH4 anomaly observed in 1991-1992 in the northern hemisphere. However, this mechanism cannot account for any of the CH4 anomalies observed during the 1980s. When these temperature mechanisms are combined, we find that wetland effects dominate in the northern hemisphere and the OH effect being more important in the tropics. Any future model calculations addressing the impact of temperature fluctuations on CH4 concentrations should be compared to the entire observational record instead of focusing on a particular CH4 anomaly.