Numerical simulation of the atmospheric CH4 increase and the corresponding decrease of δ13C(CH4) after 2007

The global atmospheric CH₄ growth rate stagnated between 2000 and 2007, and has continued to grow since 2007. This renewed CH₄ rise has been analysed with respect to a 2007 onward decline in  δ¹³C(CH₄), indicating changes in the relative contribution of CH₄ sources. However, this is still subject to debate and a variety of hypotheses have been put forward. In our work, we present numerical sensitivity simulations that investigate the impact of different inventories of methane emission fluxes on the globally averaged δ¹³C(CH₄) signature. We apply the state-of-the-art global chemistry-climate model EMAC and use a simplified approach to simulate methane loss. We include methane isotopologues and take the kinetic isotope effects in physical and chemical processes into account. We further consider regional differences in the isotopic signatures of individual emission source categories, such as, for example, the differences between signatures of tropical and boreal wetlands emissions. Based on recent emission inventories and isotopic source signatures from the literature, our chemistry climate model reproduces the actual atmospheric methane and δ¹³C(CH₄) distribution adequately. We show that our setup is suitable to constrain the individual influence of different CH₄ sources on the global average δ¹³C(CH₄). We further present an approach to optimize the global methane level with respect to station measurements probing for a strategy to include the isotopic information into such an optimization process.