Oxidation by tropospheric hydroxyl radicals (OH) is the main sink of atmospheric methane. Increasing OH would lead to lower methane. OH is sensitive to anthropogenic emissions of nitrogen oxides (NO_x), carbon monoxide (CO), volatile organic compounds (VOCs), and hydrogen (H₂), as well as to changes in halogens, stratospheric ozone, and climate variables. The chemistry is complicated and highly nonlinear. Global atmospheric chemistry models include hundreds of coupled species and reactions to simulate OH. But they typically overestimate the global mean OH concentration inferred from proxy species (1), they show long-term trends of OH at odds with these proxies (2), and they differ in projections of future OH including in sign (3). These model deficiencies hinder our ability to project future changes in methane lifetime and to develop effective strategies to decrease that lifetime.

The core problem to be addressed is the inability of global atmospheric chemistry models to accurately represent the loss of methane to OH and its response to perturbations. The key goal is to improve the models so that they can be used reliably for future projections of methane lifetime and for what-if scenarios aimed at shortening that lifetime. The project is timely because of (a) recent advances in the GEOS-Chem model simulation of relevant tropospheric chemistry (4,5), and (b) availability of global data on OH concentrations and OH reactivity (OHR) from aircraft campaigns surveying continental and oceanic environments with extensive chemical payloads (6,7).

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Gaining confidence in the processes controlling OH concentrations and their representation in global models will have a transformative impact for understanding the lifetime of methane and its response to perturbations. It will allow evaluation of geoengineering proposals aimed at decreasing methane lifetime by boosting the production of Cl atoms over the oceans, which could have large and currently very uncertain effects on OH. It will enable quantitative analysis of win-win scenarios to decrease anthropogenic emissions of CO and VOCs with joint benefits for air quality and methane lifetime.