In situ measurements of the seasonal cycle of δ13C(CO2) provide complementary information on the seasonality of the global carbon cycle, but are currently not exploited in the context of process-based carbon cycle models. We use isotope-enabled simulations of the Bern3D-LPX Earth System Model of Intermediate Complexity and fossil fuel emission estimates together with a model of atmospheric transport to simulate local atmospheric δ13C(CO2). We find good agreement between the measured and simulated seasonal cycle of atmospheric δ13C(CO2) (mean seasonal amplitude mismatch of 0.02 ‰ across 19 sites), particularly at high northern latitude sites. Factorial simulations reveal that the seasonal cycle of δ13C(CO2) is primarily driven by land biosphere carbon exchange. Spatial and temporal fluxes of CO2 and their signatures are analyzed to quantify the terrestrial drivers. The influence of external forcings (climate and land use change) on seasonal amplitude is found to be small. Unlike the growth of seasonal amplitude of CO2, no consistent change in seasonal amplitude of δ13C(CO2) is simulated over the historical period, nor evident in the available observations. We conclude that the seasonal cycle of δ13C(CO2) is influenced by different carbon cycle processes, and its potential as a novel atmospheric constraint should be further explored.