Modeling Terrestrial Ecosystems in the Global Carbon Cycle With Shifts in Carbon Storage Capacity by Land‐Use Change

Abstract

A five—compartment model for carbon cycling in the world's terrestrial ecosystems, which includes a concise treatment of the releases of carbon and shifts in carbon storage due to forest clearing, is presented. The dynamics of the less abundant isotopes, 13C and 14C, are included in the model to allow interpretation of available isotopic time series. The sensitivity of the net carbon flux between the terrestrial component of the model and the atmosphere to 10% variability in terrestrial rate coefficients and grown parameters is examined; for the particular case considered here, the variability in model response is °10%. Response of the model agrees reasonably well with observations of historical changes in the specific activity of 14C in the atmosphere. The model—calculated Seuss effect in 1952 is 2%, and the time constant of the expotential decrease in atmospheric 14C following the weapons test ban is 14 yr. By adjusting the releases of carbon due to forest clearing, a fit of model response to 13C/12C tree—ring time series is derived. The resulting forest—clearing carbon release function rises to 2.5 Pg/yr by 1910 and remains constant to the present. Due to establishment of ground vegetation following clearing, the net carbon flux from the terrestrial biotic system to the atmosphere is less than the release due to clearing in some instances. To accommodate this net input to the atmosphere in addition to that due to fossil fuel combustion, the pre—industrial CO2 concentration must be assumed to have been lower than is implied by extrapolating the modern instrument records backward in time.