Modelling the global flux of CO 2 between the atmosphere and the biosphere has represented a major uncertainty in the development of global carbon cycle models despite long recognition of the significance of the biosphere in determining atmospheric CO 2 concentrations. A box-diffusion global carbon cycle model (ANU-BACE) incorporating a biosphere/atmosphere CO 2 exchange model has been developed which accurately accounts for the observed atmospheric CO 2 record. The biosphere is clearly a significant sink for CO 2 taking up nearly 2 Gt C in 1990. A major component of this sink is occurring in tropical forests. While the atmospheric CO 2 concentration continues to increase, the biosphere will remain an important sink for CO 2 significantly reducing the rate of increase in atmospheric CO 2 . However, a steady or declining CO 2 concentration will result in a rapidly diminishing biospheric CO 2 sink. This implies that in order to achieve CO 2 stabilisation fossil fuel CO 2 fluxes will need to reach much lower levels than are indicated by the present rates of removal of CO 2 from the atmosphere. While the biosphere will act to reduce the rate of increase of atmospheric CO 2 , it will also respond to slow the rate of decrease in the atmospheric CO 2 concentration. This occurs as the biosphere outgases through the respiration of the carbon stored during a period of increasing atmospheric CO 2 concentration. The rapid stabilisation of atmospheric CO 2 concentrations may not be achievable without dramatic reductions in CO 2 emissions and substantial geophysical intervention thus making early action to reduce CO 2 emissions imperative.