A theory for the ocean-atmosphere partitioning of anthropogenic carbon dioxide on centennial timescales is presented. The partial pressure of atmospheric CO2 (P CO2) is related to the external CO2 input (ΔΣC) at air-sea equilibrium by: P CO2 = 280 ppm exp(ΔΣC/<I A + I O /R>), where I A , I O , and R are the pre-industrial values of the atmospheric CO2 inventory, the oceanic dissolved inorganic carbon inventory, and the Revelle buffer factor of seawater, respectively. This analytical expression is tested with two- and three-box ocean models, as well as for a version of the Massachusetts Institute of Technology general circulation model (MIT GCM) with a constant circulation field, and found to be valid by at least 10% accuracy for emissions lower than 4500 GtC. This relationship provides the stable level that P CO2 reaches for a given emission size, until atmospheric carbon is reduced on weathering timescales. On the basis of the MIT GCM, future carbon emissions must be restricted to a total of 700 GtC to achieve P CO2 stabilization at present-day transient levels.