Ocean‐atmosphere partitioning of anthropogenic carbon dioxide on multimillennial timescales

Ocean‐sediment and weathering interactions exert the primary control on how much anthropogenic‐emitted CO 2 remains in the atmosphere on timescales longer than about 1 kyr. Analytical theory is presented which predicts, from initial conditions, the remaining atmospheric fraction of emitted CO 2 after equilibrium with CaCO 3 burial in deep‐sea sediments but before silicate weathering removes all excess CO 2 on a >100 kyr timescale. The theoretical predictions of final atmospheric CO 2 partial pressure are tested against independent integrations of the GENIE‐1 Earth system model and are found to agree to within 10% for total emissions up to about 4000 PgC. The predicted theoretical relationship is linear and is based on the assumptions that ocean carbonate ion concentration is restored when CaCO 3 burial reaches a new steady state, and that the steady state change in global ocean CO 2 * is proportional to the change in atmospheric CO 2 ; where CO 2 * is the combined concentration of aqueous CO 2 and carbonic acid. We find that the residual fraction of anthropogenic CO 2 in the atmosphere can be determined without explicit use of ocean [CO 3 2− ], even though this concentration is known to be important in controlling the depth interval over which CaCO 3 ‐rich sediments accumulate. The simple theory developed here is particularly suited for efficient assessment of events recorded in the geological record as well as anthropogenic CO 2 influences on the long‐term stability of ice sheets.