Fossil fuel emissions required to achieve atmospheric CO₂ stabilisation using ANU-BACE: A box-diffusion carbon cycle model

Modelling the global flux of CO₂ 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 CO₂ concentrations. A box-diffusion global carbon cycle model (ANU-BACE) incorporating a biosphere/atmosphere CO₂ exchange model has been developed which accurately accounts for the observed atmospheric CO₂ record. The biosphere is clearly a significant sink for CO₂ taking up nearly 2 Gt C in 1990. A major component of this sink is occurring in tropical forests. While the atmospheric CO₂ concentration continues to increase, the biosphere will remain an important sink for CO₂ significantly reducing the rate of increase in atmospheric CO₂. However, a steady or declining CO₂ concentration will result in a rapidly diminishing biospheric CO₂ sink. This implies that in order to achieve CO₂ stabilisation fossil fuel CO₂ fluxes will need to reach much lower levels than are indicated by the present rates of removal of CO₂ from the atmosphere. While the biosphere will act to reduce the rate of increase of atmospheric CO₂, it will also respond to slow the rate of decrease in the atmospheric CO₂ concentration. This occurs as the biosphere outgases through the respiration of the carbon stored during a period of increasing atmospheric CO₂ concentration. The rapid stabilisation of atmospheric CO₂ concentrations may not be achievable without dramatic reductions in CO₂ emissions and substantial geophysical intervention thus making early action to reduce CO₂ emissions imperative.