Evaluation of the isotopic disequilibrium in the terrestrial biosphere by a global carbon isotope model

Assessing the global budget for atmospheric CO₂ through analyses of observed atmospheric and oceanic data sets, some approaches rely partially upon estimates on the magnitude of the isotopic disequilibrium between the terrestrial biosphere and the perturbed atmosphere. To test if these estimates agree with the results of grid-based global carbon cycle models, we developed a module that considers the carbon cycle of the stable isotope ¹³C, besides ¹²C, and coupled it with the high-resolution biosphere model. The module describes the fluxes of carbon in the system compartments phytomass, litter, soil, and atmosphere, separately for ¹³C and ¹²C. It enables us to predict the changes of the isotope ratios in historical times. Based on a transient model run since preindustrial times forced by observed atmospheric CO₂ concentrations and its ¹³C/¹²C ratios we find a global mean terrestrial biosphere ¹³C disequilibrium of 0.41‰ (18 Pg C‰ yr^-1) for the period 1970-1987. This result is consistent with the estimates used in global carbon budget analyses.