Investigation of strongly enhanced methane Part I: Chemical feedbacks and rapid adjustments.

Methane (CH₄) is the second most important greenhouse gas, which atmospheric concentration is influenced by human activities and currently on a sharp rise. We present a study with numerical simulations using a Chemistry-Climate-Model (CCM), which are performed to assess possible consequences of strongly enhanced CH₄ concentrations in the Earth's atmosphere for the climate.

Our analysis includes experiments with 2xCH₄ and 5xCH₄ present day (2010) lower boundary mixing ratios using the CCM EMAC. The simulations are conducted with prescribed oceanic conditions, mimicking present day tropospheric temperatures as its changes are largely suppressed. By doing so we are able to investigate the quasi-instantaneous chemical impact on the atmosphere. We find that the massive increase in CH₄ strongly influences the tropospheric chemistry by reducing the OH abundance and thereby extending the tropospheric CH₄ lifetime as well as the residence time of other chemical pollutants. The region above the tropopause is impacted by a substantial rise in stratospheric water vapor (SWV). The stratospheric ozone (O₃) column increases overall, but SWV induced stratospheric cooling also leads to enhanced ozone depletion in the Antarctic lower stratosphere. Regional  patterns of ozone change are affected by modification of stratospheric dynamics, i.e. increased tropical up-welling and stronger meridional transport  towards the polar regions. We calculate the net radiative impact (RI) of the 2xCH₄ experiment to be 0.69 W m^-2 and for the 5xCH₄ experiment to be 1.79 W m^-2. A substantial part of the RI is contributed by chemically induced O₃ and SWV changes, in line with previous radiative forcing estimates and is for the first time splitted and spatially asigned to its chemical contributors.

This numerical study using a CCM with prescibed oceanic conditions shows the rapid responses to significantly enhanced CH₄ mixing ratios, which is the first step towards investigating the impact of possible strong future CH₄ emissions on atmospheric chemistry and its feedback on climate.