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

Abstract

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 to 1987. This result is consistent with the estimates used in global carbon budget analyses.