Simulating the carbon flux between the terrestrial environment and the atmosphere

A Terrestrial C Cycle model that is incorporated in the Integrated Model to Assess the Greenhouse Effect (IMAGE 2.0) is described. The model is a geographically explicit implementation of a model that simulates the major C fluxes in different compartments of the terrestrial biosphere and between the biosphere and the atmosphere. Climatic parameters, land cover and atmospheric C concentrations determine the result of the dynamic C simulations. The impact of changing land cover patterns, caused by anthropogenic activities (shifting agriculture, de- and afforestation) and climatic change are modeled implicitly. Feedback processes such as CO₂ fertilization and temperature effects on photosynthesis, respiration and decomposition are modeled explicitly. The major innovation of this approach is that the consequences of climate change are taken into account instantly and that their results can be quantified on a global medium-resolution grid. The objectives of this paper are to describe the C cycle model in detail, present the linkages with other parts of the IMAGE 2.0 framework, and give an array of different simulations to validate and test the robustness of this modeling approach. The computed global net primary production (NPP) for the terrestrial biosphere in 1990 was 60.6 Gt C a^?1, with a global net ecosystem production (NEP) of 2.4 Gt C a^?1. The simulated C flux as result from land cover changes was 1.1 Gt C a^?1, so that the terrestrial biosphere in 1990 acted as a C sink of 1.3 Gt C a^?1. Global phytomass amounted 567.5 Gt C and the dead biomass pool was 1517.7 Gt C. IMAGE 2.0 simulated for the period 1970–2050 a global average temperature increase of 1.6 °C and a global average precipitation increase of 0.1 mm/day. The CO₂ concentration in 2050 was 522.2 ppm. The computed NPP for the year 2050 is 82.5 Gt C a^?1, with a NEP of 8.1 Gt C a^?1. Projected land cover changes result in a C flux of 0.9 Gt C a^?1, so that the terrestrial biosphere will be a strong sink of 7.2 Gt C a^?1. The amount of phytomass hardly changed (600.7 Gt C) but the distribution over the different regions had. Dead biomass increased significantly to 1667.2 Gt C.