Model studies on the response of the terrestrial carbon cycle to climate change and variability

The first part of this thesis describes the further development of a dynamic global vegetation model, LPJ, and its application to selected scientific questions. LPJ has been extended to include isotopic fractionation of 13C at the leaf level during assimilation and includes a full isotopic terrestrial carbon cycle. Hence, it simulates the isotopic signature of the heterotrophic respiration fluxes. The model thus allows a quantitative analysis of the net biosphere exchange of CO2 and CO2 with the atmosphere as a function of changes in climate, land cover, atmospheric CO2, and the isotope ratio of CO2. The extended version of LPJ has been used to study the response of the global vegetation distribution to an abrupt climate change event (Younger Dryas) and the thereby incurred changes in the terrestrial carbon pools and fluxes and their isotopic 13C/12C ratio. Climate data from a 850-year-long coupled ocean-atmosphere general circulation model (ECHAM3/LSG) is used for these simulations. The comparison of the modelled vegetation distribution and shifts during this idealized Younger Dryas event with reconstructed vegetation maps for North America and Europe based on pollen records shows a reasonable agreement. The impact of the terrestrial carbon release during the Younger Dryas on the atmospheric CO2 and δ13C is analyzed using a simplified ocean model and compared to ice core measurements. In the standard case the simulation exhibits a significant change in global total terrestrial carbon stocks of about 180 Pg C leading to an atmospheric CO2 increase of ≈28 ppmv as a consequence of the climate change event. The robustness of the terrestrial signal during the Younger Dryas is studied by several sensitivity experiments concerning the initial values of the carbon pool sizes as well as the CO2 fertilization effect and the temperature dependency of the carbon decomposition rates. The resulting increase of atmospheric CO2 concentrations for the cold event varies between 16 to 33 ppmv among the different experiments. The simulated atmospheric δ13C values which are about 0.4 lower during the cold phase reflect major findings from ice core measurements and are fairly robust against the sensitivity experiments. The isotope version of LPJ has also been used to study the effects of climate variability, fire, and changes in land use on the exchange fluxes of CO2 and CO2 between the terrestrial biosphere and atmosphere for the last 100 years in greater detail. A transient, spatially explicit dataset of C4 crops and tropical C4 pastures has been compiled which, in combination with a land use scheme, allows the analysis of the impact of land use and C4 cultivation on the terrestrial stable isotope composition. LPJ simulates a global mean isotopic fractionation of 17.7 at the leaf level with interannual variations of ca. 0.3 in the case without land use for the years 1950 to 1998. In this case, interannual variability in the net CO2 flux between atmosphere and terrestrial biosphere is of the order of 15 Pg C yr−1. It is reduced to 4 Pg C yr−1 if the leaf-level fractionation factor is held constant at the long term mean. Depending on the chosen