For atmospheric CO₂ concentration stabilization, we projected allowable carbon emission with an Earth system model based on a general circulation model. Our calculations on centennial timescale in various scenarios reveal how saturation with respect to CO₂ and climate-carbon cycle feedback reduce natural carbon uptake, and hence allowable emission. In 450 ppm stabilization scenario, for example, climate-carbon cycle feedback reduces the accumulative allowable carbon emission until year 2300 from 1248 to 980 PgC. The Emission at the year 2050 is about the half of the year 2000 level for the SP450 scenario. Terrestrial carbon cycle is especially susceptible to climate-carbon cycle feedback, and is a significant source of projection uncertainty. Our model responds nonlinearly to CO₂ and climate, suggesting process-based models are indispensable tool for future climate-carbon cycle projections.