Estimating the impact of the radiative feedback from atmospheric methane on climate sensitivity

Kurzfassung

Methane (CH4), the second most important greenhouse gas directly emitted by human activity, is removed from the atmosphere via chemical degradation. In this study we assess the radiative feedback from atmospheric CH4 resulting from changes in its chemical sink, which is mainly the oxidation with the hydroxyl radical (OH) and, which is influenced by temperature and the chemical composition of the atmosphere. We present results from numerical simulations with the chemistry-climate model EMAC perturbed by either CO2 or CH4 increase. The essential innovation in the simulation set-up is the use of CH4 emission fluxes instead of prescribed CH4 concentrations at the lower boundary. This means that changes in the chemical sink can feed back on the atmospheric CH4 concentration without constraints. For both forcing agents, CO2 and CH4, we explore so called rapid radiative adjustments in simulations with prescribed sea surface temperatures, as well as slow radiative feedbacks and the climate sensitivity in respective simulations using an interactive oceanic mixed layer. To quantify individual physical and chemical radiative adjustments and feedbacks we use the partial radiative perturbation method in offline simulations with a radiative transfer model consistent with the one used in the online simulations. First results show a negative feedback of atmospheric CH4 in a warming and moistening troposphere. As water vapour is a precursor of OH, increased humidity leads to increasing OH mixing ratios. This leads in turn to a shortening of the CH4 lifetime and a reduction of the CH4 mixing ratios accordingly. This decrease in CH4 also affects the response of tropospheric ozone (O3) leading to a less pronounced increase of O3 in the tropical upper troposphere compared to previous studies of the O3 response following a CO2 perturbation (Dietmüller et al., 2014;Nowack et al., 2015;Marsh et al., 2016).