Estimating the Impact of the Radiative Feedback from Atmospheric Methane on Climate Sensitivity
Final Report Abstract
The potent greenhouse methane (CH4 ) is a chemically active gas and induces a variety of environmental impacts in the atmosphere. Motivated from the complexity of its effects, the project IRFAM-ClimS was designed to address the interaction between atmospheric CH4 and climate changes. Recent model developments enable the application of flux boundary conditions for CH4 so that feedbacks of the atmospheric sink of CH4 can explicitly be simulated. For this project, two sets of simulations with the chemistry climate model EMAC are carried out with, either increased carbon dioxide mixing ratios (x1.35), or increased CH4 surface emissions (x2.75). Our first main result is that climate change induces an enhancement of the tropospheric sink of CH4 , which results in a shortening of CH4 lifetime. The associated decrease of CH4 mixing ratios (by approx. 7%) leads to reduced formation of ozone (O3 ). This effect is not included when surface CH4 mixing ratios are prescribed and might lead to a misestimation of the tropospheric O3 response. Secondly, we attributed individual contribution of rapid adjustments and climate feedbacks. Our results suggest that chemical climate feedbacks play a minor role (< 5% of the total climate sensitivity). However, chemical rapid radiative adjustments, which represent the fast/direct response to the perturbation, play an important role for the CH4 perturbation. For instance, rapid radiative adjustments of O3 represent 47% of the total effective radiative forcing, which confirms the necessity of chemistry-climate models and the differentiating of rapid adjustments and climate feedbacks for perturbations of chemically active trace gases. Finally, we calculated the climate sensitivity parameter λ, which links the radiative forcing of a perturbation to the global mean surface temperature (∆Tsurf = λ · RF). We applied the concept of effective radiative forcing, which yields a statistically similar value for the climate sensitivity parameter (i.e. 0.68±0.08 K / (W m−2 )) for both perturbations, confirming this to be invariant to the applied perturbation. Other concepts, without the separation of rapid adjustments and the climate response, show substantially different climate sensitivities for the two perturbations. Additionally, our results indicate different climate responses, if flux boundary conditions for CH4 are applied. These differences are not significant in the current analysis but are expected to be more pronounced for an updated radiation scheme representing the radiative forcing of CH4 more realistically. Significant differences would make flux boundary conditions literally indispensable for accurate climate projections.
Publications
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Effects of strongly enhanced atmospheric methane concentrations in a chemistry-climate model: Rapid adjustments and slow feedbacks. In American Geophysical Union Fall Meeting 2020, online, 2020.
Stecher, L.; Winterstein, F.; Dameris, M. & et al.
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Investigation of strongly enhanced methane part ii: Slow climate feedbacks. In EGU General Assembly 2020, online, 2020.
Stecher, L.; Winterstein, F.; Dameris, M.; Jöckel, P. & Ponater, M.
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Slow feedbacks resulting from strongly enhanced atmospheric methane mixing ratios in a chemistry–climate model with mixed-layer ocean. Atmospheric Chemistry and Physics, 21(2), 731-754.
Stecher, Laura; Winterstein, Franziska; Dameris, Martin; Jöckel, Patrick; Ponater, Michael & Kunze, Markus
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Estimating the impact of the radiative feedback from atmospheric methane on climate sensitivity. In EGU General Assembly 2023, Wien, Österreich, 2023.
Stecher, L.; Winterstein, F.; Dameris, M.; Jöckel, P. & Ponater, M.
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Updating the radiation infrastructure in MESSy (based on MESSy version 2.55). Copernicus GmbH.
Nützel, Matthias; Stecher, Laura; Jöckel, Patrick; Winterstein, Franziska; Dameris, Martin; Ponater, Michael; Graf, Phoebe & Kunze, Markus
