Numerical simulation of the impact of atmospheric OH variability on the global mean δ13C(CH4) trend.

The hydroxyl radical (OH) serves as a primary sink for CH₄ in the atmosphere and plays an important role in interpreting the global CH₄ budget. Changes in the OH trend have recently been proposed as a potential explanation for the renewed increase of CH₄ and the simultaneous decrease in δ¹³C(CH₄) since 2007. In this work, we introduce comprehensive numerical sensitivity simulations to explore the impact of temporal OH variations on the globally averaged CH₄ mixing ratio and δ¹³C(CH₄) signature. We apply the state-of-the-art global chemistry-climate model EMAC and use a simplified approach to simulate methane loss. Our simulations apply different OH fields, including climatologically described and transient OH fields, and assume moderate changes in the CH₄ tropospheric lifetime. We also consider methane isotopologues and the kinetic isotope effects in physical and chemical processes. The setup uses recent CH₄ emission inventories and accounts for regional differences in the isotopic signatures of individual emission source categories. Our results suggest that the influence of an OH reduction on the global δ¹³C(CH₄) is rather small and does not explain the observed trend in CH₄. Additionally, we examine the impact of the latitudinal OH distribution on the relative contribution of different emission source categories to the global CH₄ rise and the global mean surface δ¹³C(CH₄).