Methane (CH4), the second most important greenhouse gas directly emitted by human activity, is removed from the atmosphere via chemical degradation. The chemical sink of CH4 is influenced by temperature and the chemical composition of the atmosphere. It is further an important source of water vapour in the stratosphere and affects the ozone concentration in the troposphere and the stratosphere via secondary feedbacks.
The talk will focus on the role of these chemistry-climate interactions in numerical simulations with the chemistry-climate model EMAC perturbed by either CO2 or CH4 increase. For both forcing agents, CO2 and CH4, so called rapid radiative adjustments are assessed in simulations with prescribed sea surface temperatures, as well as climate feedbacks in respective simulations using an interactive oceanic mixed layer.
The simulation set-up uses CH4 emission fluxes instead of prescribed CH4 concentrations at the lower boundary so that changes of the chemical sink can feed back on the atmospheric CH4 concentration without constraints.
The results show a shortening of the CH4 lifetime and, accordingly, a reduction of the CH4 mixing ratios in a warming and moistening troposphere. This decrease in CH4 also affects the response of tropospheric ozone.
Furthermore, recently an additional radiation scheme was implemented into the EMAC model, which represents the direct radiative effect of CH4 better and also accounts for the absorption by CH4 in the solar shortwave spectrum. With the new radiation scheme the effective radiative forcing for the same perturbation of CH4 emissions is larger, and individual rapid radiative adjustments, e.g. of clouds, are changed.