Martin Heimann (email@example.com)
The Carbon-Cycle Model Linkage Project (CCMLP) has brought together several research groups to study the role of the terrestrial biosphere in the Earth system using TBMs. The focus of phase 1, recently completed, was to evaluate model responses to CO2, climate and land use for the terrestrial carbon cycle. Four models (including LPJ) were run in parallel for the period 1920-1992. Results indicated that the dominant influences on the terrestrial carbon cycle are the large and opposing effects of historical land-use changes and CO2 fertilization. In comparison, the effects of climatic changes are (still) small and the direction of the response, whether source or sink of CO2 to the atmosphere, differs among models. Nevertheless, the net fluxes over the 1980s for all four models were in agreement with inferred estimates (based on observed atmospheric CO2 concentrations, fossil-fuel emissions and ocean fluxes as computed from a state-of-the-art ocean model). The seasonal cycle of atmospheric CO2 is almost entirely due to the dynamics of the terrestrial biosphere, with a net flux to the atmosphere during the winter months associated with heterotrophic respiration and a net flux of carbon from the atmosphere to the biosphere during the summer as photosynthesis exceeds respiration. In the last decades an increase has been observed in the amplitude of the atmospheric CO2 seasonal cycle at monitoring stations, indicating an increase in terrestrial biosphere activity. LPJ captured the observed trend in amplitude of the seasonal cycle of atmospheric CO2 at the Mauna Loa monitoring station between 1960-1992.