To assess the role of the boreal and temperate forests and the tundra ecosystems in a future CO2-induced climate change, the Frankfurt biosphere model (FBM) was applied to the 3 × CO2 climate as calculated by the GCM of the MPI für Meteorologie in Hamburg. The FBM predicts on a 1° × 1° spatial grid the seasonal and perannual course of leaf biomass and feeder roots, woody biomass, soil carbon and soil water in response to the seasonal course of light, precipitation and temperature. The phenology is controlled by the flux balance of carbon gains and losses, thus being dependent on the driving climate and the state of vegetation. Two equilibrium runs based on the 3 × CO2 climate were performed: (1) Considering the pure climate effect (with no direct CO2 fertilization) we obtained a 22% decrease of the net primary production (NPP) due to enhanced autotrophic respiration and increased water limitation. Together with the effect on the soils this results in a 170 Gt carbon source. (2) Considering a CO2 -induced enhancement of the maximum photosynthesis the pure climate effect is more than compensated and we predict a NPP increase of 9% and a total carbon sink of 50 Gt C. This effect may even be an underestimate if one takes into consideration a shift in the optimum temperature for photosynthesis under enhanced levels of atmospheric CO2 as proposed by Long and Drake. DOI: 10.1034/j.1600-0889.47.issue1.16.x