Tropospheric Oxidation Chemistry and Methane Growth: A Process-Based Attribution of OH Variability (2003–2022)

Abstract. Earth system models that include interactive chemistry provide a mechanistic understanding of the hydroxyl radical (OH) sources and sinks. However, their use is often limited in the estimation of the global methane (CH₄) budget. We conduct 20-year emission-driven methane simulations (2003–2022) with the Community Earth System Model Version 2.2 (CESM2.2) nudged to different meteorological datasets and input various chemical emissions, including the Seventh Coupled Model Intercomparison Project (CMIP7). Our evaluation includes in-situ observations from the NOAA Marine Boundary Layer Reference and airborne field studies, as well as ground-based and satellite remote sensing observations of carbon monoxide (CO) and CH₄. We find that the model configurations with a lower OH bias have improved skill in reproducing both CO and CH₄ spatial and temporal variations, including the CH₄ annual growth rate. We quantify the OH impacts on the CH₄ growth rate and derive a comprehensive attribution of OH changes to Earth System chemical processes. Despite an interannual variability lower than 4 %, changes in OH explains around 30 % of the interannual variability of the CH₄ growth throughout the 2003–2022 period. The OH changes arise from both internal variability —via biogenic and lightning emission responses— and prescribed surface emissions, with remaining uncertainties. We find that OH is well buffered against chemistry perturbations, as increased chemical complexity raises the OH recycling probability. Consequently, studies relying on simplified and prescribed OH representations may neglect these complex chemical feedbacks and overestimate the role of OH changes in driving the CH₄ growth rate.