The dominant processes controlling the magnitude and spatial distribution of the preindustrial air-sea flux of CO2 are atmosphere-ocean heat exchange and the biological pump, coupled with the direct influence of ocean circulation resulting from the slow time-scale of air-sea CO2 gas exchange equilibration. The influence of the biological pump is greatest in surface outcrops of deep water, where the excess deep ocean carbon resulting from net remineralization can escape to the atmosphere. In a steady state other regions compensate for this loss by taking up CO2 to give a global net air-sea CO2 flux of zero. The predominant outcrop region is the Southern Ocean, where the loss to the atmosphere of biological pump CO2 is large enough to cancel the gain of CO2 due to cooling. The influence of the biological pump on uptake of anthropogenic CO2 is small: a model including biology takes up 4.9% less than a model without it. Our model does not predict the large southward interhemispheric transport of CO2 that has been suggested by atmospheric carbon transport constraints.