Positive feedback mechanism between biogenic volatile organic compounds and the methane lifetime in future climates

A multitude of biogeochemical feedback mechanisms govern Earth’s climate sensitivity to radiative perturbations. A feedback largely missing from most Earth System Models used in IPCC AR6 is the effect of warming on biogenic volatile organic compound (BVOC) emissions and the subsequent impact on hydroxyl radical (OH) concentrations. OH is the primary sink for many gases including methane (CH₄), the second most important anthropogenic greenhouse gas by radiative forcing. Using a one-dimensional chemistry–transport model (SOSAA) and a global chemistry–transport model (TM5), we quantify this feedback. In a +6 K scenario, the BVOC–OH–CH₄ feedback increases the local methane lifetime by 11.4% over the boreal region when the temperature rise affects only chemical reaction rates, not BVOC emissions. This implies a local increase in methane aerosol radiative forcing (ARFCH₄) of ~0.013 W m⁻² per year, about 2.1% of current ARFCH₄. Across the Northern Hemisphere, simulations indicate a CH₄ concentration increase of 0.024% per year when comparing temperature increase in chemistry alone vs. in both chemistry and BVOC emissions. This equals ~7% of the 2008–2017 annual CH₄ growth rate (6.6 ± 0.3 ppb yr⁻¹) and yields an ARFCH₄ of 1.9 mW m⁻² per year.