Chemistry--climate feedback of atmospheric methane in a methane-emission-flux-driven chemistry--climate model

Kurzfassung

The chemical sink of atmospheric methane (CH4) depends on the temperature and on the chemical composition. Here, we assess the feedback on atmospheric CH4 induced by changes in the chemical sink in a warming climate using a CH4-emission-flux-driven set-up of the chemistry–climate model EMAC (ECHAM/MESSy Atmospheric Chemistry), in which the chemical feedback of CH4 mixing ratios can evolve explicitly. We perform idealized perturbation simulations driven either by increased carbon dioxide (CO2) mixing ratios or by increased CH4 emission fluxes. The CH4 emission flux perturbation leads to a large increase of CH4 mixing ratios. Remarkably, the factor by which the CH4 mixing ratio increases is larger than the increase factor of the emission flux, because the atmospheric lifetime of CH4 is extended. In contrast, the individual effect of the global surface air temperature (GSAT) increase is to shorten the CH4 lifetime, which results in a significant reduction of CH4 mixing ratios in our set-up. The corresponding radiative feedback is estimated at −0.041 and −0.089 W m−2 K−1 for the CO2 and CH4 perturbation, respectively. The explicit response of CH4 mixing ratios leads to secondary feedbacks on the hydroxyl radical (OH) and ozone (O3). Firstly, the OH response includes the CH4–OH feedback, which enhances the CH4 lifetime change, and, secondly, the formation of tropospheric O3 is reduced. Our CH4 perturbation induces the same response of GSAT per effective radiative forcing (ERF) as the CO2 perturbation, which supports the applicability of the ERF framework for CH4.