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https://doi.org/10.5194/acp-2020-519
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-519
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  26 Jun 2020

26 Jun 2020

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This preprint is currently under review for the journal ACP.

Effects of Strongly Enhanced Atmospheric Methane Concentrations in a Fully Coupled Chemistry-Climate Model

Laura Stecher1, Franziska Winterstein1, Martin Dameris1, Patrick Jöckel1, Michael Ponater1, and Markus Kunze2 Laura Stecher et al.
  • 1Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
  • 2Freie Universität Berlin, Berlin, Germany

Abstract. In a previous study the quasi-instantaneous chemical impacts (rapid adjustments) of strongly enhanced methane (CH4) mixing ratios have been analyzed. However, to quantify the influence of the respective slow climate feedbacks on the chemical composition it is necessary to include the radiation driven temperature feedback. Therefore, we perform sensitivity simulations with doubled and fivefold present-day (year 2010) CH4 mixing ratios with the chemistry-climate model EMAC and include in a novel set-up a mixed layer ocean model to account for tropospheric warming.

We find that the slow climate feedbacks counteract the reduction of the hydroxyl radical in the troposphere, which is caused by the strongly enhanced CH4 mixing ratios. Thereby also the resulting prolongation of the tropospheric CH4 lifetime is weakened compared to the quasi-instantaneous response considered previously.

Changes in the stratospheric circulation evolve clearly with the warming of the troposphere. The Brewer-Dobson circulation strengthens, affecting the response of trace gases, such as ozone, water vapour and CH4 in the stratosphere, and also causing stratospheric temperature changes. In the middle and upper stratosphere, the increase of stratospheric water vapour is reduced with respect to the quasi-instantaneous response. Weaker increases of the hydroxyl radical cause the chemical depletion of CH4 to be less strongly enhanced and thus the in situ source of stratospheric water vapour as well. However, in the lower stratosphere water vapour increases more strongly when tropospheric warming is accounted for enlarging its overall radiative impact. The response of the stratospheric adjusted temperatures driven by slow climate feedbacks is dominated by these increases of stratospheric water vapour, as well as strongly decreased ozone mixing ratios above the tropical tropopause, which result from enhanced tropical upwelling.

While rapid radiative adjustments from ozone and stratospheric water vapour make an essential contribution to the effective CH4 radiative forcing, the radiative impact of the respective slow feedbacks is rather moderate. In line with this, the climate sensitivity from CH4 changes in this chemistry-climate model setup is not significantly different from the climate sensitivity in carbon dioxide-driven simulations, provided that the CH4 effective radiative forcing includes the rapid adjustments from ozone and stratospheric water vapour changes.

Laura Stecher et al.

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Laura Stecher et al.

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Short summary
This study investigates the impact of strongly increased atmospheric methane mixing ratios on the Earth's climate. An interactive model system including atmospheric dynamics, chemistry, and a mixed layer ocean model is used to analyse the effect of doubled and fivefold methane mixing ratios. We assess feedbacks on atmospheric chemistry and changes in the stratospheric circulation, focusing on the impact of tropospheric warming, and their relevance for the model's climate sensitivity.
This study investigates the impact of strongly increased atmospheric methane mixing ratios on...
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