Implication of strongly increased atmospheric methane concentrations for chemistry–climate connections

• Main Authors: F. Winterstein, F. Tanalski, P. Jöckel, M. Dameris, M. Ponater
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-05-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/7151/2019/acp-19-7151-2019.pdf
• ISSN: 1680-7316; 1680-7324

<p>Methane (<span class="inline-formula">CH₄</span>) is the second-most important directly emitted greenhouse gas, the atmospheric concentration of which is influenced by human activities. In this study, numerical simulations with the chemistry–climate model (CCM) EMAC are performed, aiming to assess possible consequences of significantly enhanced <span class="inline-formula">CH₄</span> concentrations in the Earth's atmosphere for the climate.</p> <p>We analyse experiments with <span class="inline-formula">2×CH₄</span> and <span class="inline-formula">5×CH₄</span> present-day (2010) mixing ratio and its quasi-instantaneous chemical impact on the atmosphere. The massive increase in <span class="inline-formula">CH₄</span> strongly influences the tropospheric chemistry by reducing the OH abundance and thereby extending the <span class="inline-formula">CH₄</span> lifetime as well as the residence time of other chemical substances. The region above the tropopause is impacted by a substantial rise in stratospheric water vapour (SWV). The stratospheric ozone (<span class="inline-formula">O₃</span>) column increases overall, but SWV-induced stratospheric cooling also leads to a enhanced ozone depletion in the Antarctic lower stratosphere. Regional patterns of ozone change are affected by modification of stratospheric dynamics, i.e. increased tropical upwelling and stronger meridional transport towards the polar regions. We calculate the net radiative impact (RI) of the <span class="inline-formula">2×CH₄</span> experiment to be 0.69&thinsp;W&thinsp;m<span class="inline-formula">⁻²</span>, and for the <span class="inline-formula">5×CH₄</span> experiment to be 1.79&thinsp;W&thinsp;m<span class="inline-formula">⁻²</span>. A substantial part of the RH is contributed by chemically induced <span class="inline-formula">O₃</span> and SWV changes, in line with previous radiative forcing estimates.</p> <p>To our knowledge this is the first numerical study using a CCM with respect to 2- and 5-fold <span class="inline-formula">CH₄</span> concentrations and it is therefore an overdue analysis as it emphasizes the impact of possible strong future <span class="inline-formula">CH₄</span> emissions on atmospheric chemistry and its feedback on climate.</p>