The perceived budget imbalance in the global carbon cycle has been suggested to result from, among other processes, CO2fertilization of the terrestrial biosphere and/or enhanced regrowth of previously felled temperate forest. These two processes are incorporated into a box diffusion model of the ocean‐atmosphere system coupled to a five‐box terrestrial biosphere. The extent to which historical fossil fuel and land use change emission data can be reconciled with the observed atmospheric CO2concentration record is examined. Furthermore, the sensitivity of future CO2projections to the nature of the budget imbalance is investigated. It is found that the CO2record can accommodate a carbon budget balanced by CO2fertilization but that the balance with forest regrowth is more difficult. Future CO2projections are found to be sensitive to how the carbon budget is balanced, even relative to uncertainties in future emissi