Documentation of a geographically explicit dynamic carbon cycle model

The terrestrial biosphere is a crucial part of the C cycle, because of its large C exchange capacity and the sensitivity of its fluxes for changes in environmental conditions. The terrestrial C cycle model, described in this report, is a component of the Integrated Model to Assess the Greenhouse Effect (IMAGE 2) but can also be used in a stand-alone mode. The model simulates major C fluxes between the terrestrial biosphere and the atmosphere, and determines C storage in different compartments of the terrestrial biosphere on a 0.5 x 0.5 degree grid.

The driving factor is the net primary production (NPP) of ecosystems. The actual value of NPP depends on the land cover characteristics in each grid cell and on different local environmental conditions. Actual NPP is altered by CO₂ fertilization (again dependent on climate and several local conditions), the effects of a changing temperature and moisture availability on net plant photosynthesis and soil decomposition, and an increase of the water use efficiency of plants under increased atmospheric CO₂ concentration.

Shifting vegetation zones as a response to climatic change can also influence the actual land cover and therefore the global C cycle. Applications of the model are presented and the importance of the terrestrial C cycle for the global C cycle is evaluated. A major conclusion is that this role cannot be neglected and that large regional differences in response patterns exist. The model has the possibility to create a better regional understanding of the terrestrial C cycle and has the capability to evaluate the consequences of different impact and mitigation scenarios.