Understanding Controls on the Atmospheric Methane Sink: Towards Synergistic Climate-Air Quality Solutions (Invited)

The dominant sink for methane, reaction with the hydroxyl radical (OH), is highly sensitive to temperature, restricting a major portion of the atmospheric methane loss by this pathway to the lower tropical troposphere. This sink of methane to chemical oxidation is tightly coupled to both production and loss pathways of tropospheric ozone, another greenhouse gas and a harmful air pollutant, and we will briefly review these complex chemical connections. With multiple configurations of the Community Earth System Model version 2 (CESM2), we will assess the robustness of regional distributions of the tropospheric methane-OH reaction to climate variability and emission changes. We will then introduce a highly simplified configuration of CESM2 designed to elucidate fundamental processes controlling how the methane sink (and atmospheric chemistry in general) will respond to climate change. This “AquaChem” configuration consists of the CESM2 Community Atmosphere Model version 6 (CAM6) aquaplanet with zonally symmetric sea surface temperatures (SSTs), perpetual equinox conditions, and fully coupled atmospheric chemistry. This simplified climate model enables us to assess the sensitivity of methane loss rates to a much larger number of processes than is possible with a standard chemistry-climate model where long simulations are needed to detect signals from anthropogenic change amidst climate variability. By increasing the SSTs in AquaChem, we demonstrate that the influence of a warming atmosphere on methane loss rates operates both through the reaction kinetics and OH (via water vapor), such that the spatial response of the methane loss rate does not simply mirror that of OH. We also highlight key linkages with tropospheric ozone, including the sensitivity of methane loss rates to U.S. and global air pollutant emissions.