Evaluation of a Computationally-Efficient CH4-CO-OH (ECCOH) Module with Climate-Sensitive Wetland CH4 Emissions Within the GEOS-5 Climate Model

We have developed a capability to simulate the methane-carbon monoxide-hydroxyl radical (CH₄-CO-OH) system within the NASA Global Modeling and Assimilation Office (GMAO) GEOS-5 atmospheric general circulation model. A parameterization of OH is used to capture nonlinear chemical feedbacks of the CH₄-CO-OH system in a computationally-efficient way, allowing for simulation of many decades of this system and many sensitivity simulations. Concentrations of OH in the troposphere are calculated as a function of a set of meteorological, solar irradiance, and chemical variables, including CH₄ and CO. The module includes a wetland CH₄ emission parameterization based on soil temperature and precipitation with sensitivities derived from a process-based wetland emission model that allows for further coupling of the methane and climate simulations. We run the model for a historical period (1988-present) as well as a future projection (through 2050) with various emissions scenarios, including an alternative interannually varying wetland CH₄ emissions distribution from the TransCom-CH₄ study in addition to our parameterized wetland emissions. We evaluate our simulated distributions of atmospheric CH₄ against ground-based observations and satellite data sets, including SCIAMACHY and GOSAT. The model captures much of the observed spatial and temporal variations in CH₄, with largely similar variability for the two different wetland emissions estimates though with differing trends over the historical period. The model CH₄ lifetime with respect to removal by OH is generally within the range estimated by previous model and observation-based studies. We discuss the contributions of model sources, sinks, and transport to the variability.