Modelling the impact of atmospheric sink variability and CH4 biomass burning emissions on the global mean δ13C(CH4) trend with the comprehensive chemistry-climate model EMAC.

Changes in atmospheric sinks could explain the renewed increase of CH₄ and the simultaneous decrease in δ¹³C(CH₄) since 2007. In this work, we present comprehensive numerical sensitivity simulations to explore how atmospheric methane oxidation (both by OH and Cl), uptake by soil, and uncertainties in the kinetic isotope effect (KIE) influence the simulated global atmospheric δ¹³C(CH₄) trend. Furthermore, we examine the sensitivity of the latter to reduced emissions from biomass burning, which have relatively high isotopic source signatures. We use the state-of-the-art global chemistry-climate model EMAC with a simplified approach to simulate CH₄ loss. The model considers all four CH₄ isotopologues and the (partly temperature-dependent) isotope fractionation during  physical and chemical loss of CH₄. Our setup uses recent CH₄ emission inventories and accounts for regional differences in the corresponding isotopic signatures depending on source material and type of formation. Our results emphasize the importance of atmospheric sinks when interpreting the global CH₄ budget with respect to δ¹³C(CH₄).