The partitioning of excess CO2 in a five-reservoir atmosphere-ocean model

The two-box-diffusion model of the oceans developed by Oeschger et al. (1975) is extended to include a well-mixed, deep polar reservoir in contact with the atmosphere and a split diffusive deep ocean comprising two reservoirs with characteristics akin to the Atlantic and Pacific oceans. It is shown that the division of the deep ocean has little influence on the airborne fraction of fossil fuel CO2. The partial or complete replacement of the diffusive Atlantic reservoir with the well-mixed polar reservoir significantly reduces the airborne fraction, although it still remains above the observed value for the period 1959–78. The sensitivity of the airborne fraction to buffer factor and fossil fuel CO2 emission rate is seen to be large enough to limit the usefulness, as predictive tools, of linear models in which these parameters are held constant.