A complex Earth system model including atmosphere, ocean, ice sheets, marine carbon cycle and terrestrial vegetation was used to study the long-term response (100--1000 yrs) of the climate to different increased atmospheric CO2 concentrations. A 3.2 K global mean surface temperature increase is simulated for a 3xCO2 experiment. The freshwater input by melting of the Greenland Ice Sheet due to global warming is of minor importance compared to hydrological changes in the atmosphere. Increased equatorial upwelling enhances the tropical outgassing of CO2 from the oceans, lowering the total marine carbon uptake by 16--22%. On land, carbon release due to increase in soil temperature reduces the anthropogenic carbon uptake from CO2 fertilization up to 43%. Thus, we show that both marine and terrestrial carbon cycle have a positive feedback on climate, which has to be considered for future carbon emission scenarios.