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

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).