Estimating the Impact of the Radiative Feedback from Atmospheric Methane on Climate Sensitivity

• Applicant: Dr. Franziska Winterstein
• Subject Area: Atmospheric Science
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429337746

Methane (CH4) is the greenhouse gas with the second most important contribution to anthropogenic radiative forcing (RF), next to carbon dioxide (CO2). The main topic of the present project proposal is to quantify, for the first time, the relevance of atmospheric CH4 in its role as a radiative feedback component for the climate sensitivity, which links the RF to the global temperature response. An approximation available from a preliminary model setup suggests a significant negative CH4 feedback that may considerably enhance the already well established negative ozone feedback in CO2-driven climate change simulations including interactive chemistry. However, such evidence requires confirmation from more advanced models. In this project we will make use of a newly established option in an approved and evaluated chemistry-climate model (CCM) that enables climate sensitivity simulations with predefined CH4 emission fluxes instead of the common fixed lower boundary mixing ratio conditions and thereby a direct calculation of the climate feedback of CH4. We will mainly rely on the effective radiative forcing (ERF) concept, where rapid radiative adjustments, including those via chemical species, are assigned to the forcing component and only radiative changes driven by the sea surface temperature are interpreted as radiative feedbacks modifying the climate sensitivity.The particular objectives of the project are, first, to determine the rapid radiative adjustment and the slow radiative feedback from CH4 changes in CO2-driven CCM simulations with and without CH4 flux boundary conditions. Then, an equivalent CH4-driven CCM simulation series will be performed with changed CH4 flux boundary conditions, where CH4 works as the forcing as well as a radiative feedback component. In either case a complete analysis of all physically and chemically driven radiative adjustments and feedbacks will be carried out for the simulations. This will yield further information about the sensitivity of the climate sensitivity parameter to the inclusion of interactive chemistry for CO2- as well as for CH4-driven climate change.In total, the project will advance the present, rather sparse, knowledge on radiative feedbacks controlled by atmospheric chemistry, will reveal the potential interactions between radiative adjustments and feedbacks of both physical and chemical origin, and will constrain the net contribution of methane and ozone feedbacks in CCMs on the climate sensitivity. Such knowledge will, in particular, allow more precise estimates of the net climate impact of future CH4 emissions, which is a prerequisite for potential mitigation concepts.

• DFG Programme: Research Grants
• International Connection: Switzerland, United Kingdom
• Cooperation Partners: Professor Dr. Keith Shine; Dr. Andrea Stenke
• Co-Investigator: Professor Dr. Martin Dameris