The importance of ocean transport in the fate of anthropogenic CO 2

Abstract. We assess uncertainties in projected oceanic uptake of anthropogenic CO2 associated with uncertainties in model ocean transport using a suite of climate/carbon-cycle models. In response to a CO2 pulse emission of 590 Pg C (corresponding to an instantaneous doubling of atmospheric CO2 from 278 to 556 ppm), the fraction of CO2 emitted absorbed by the ocean (model mean ±2σ) is 37±8%, 56±10%, and 81±4% in year 30, 100, and 1000 after the emission pulse, respectively. Modeled oceanic uptake of excess CO2 on timescales from decades to about a century is strongly correlated with simulated present-day uptake of chlorofluorocarbons (CFCs) and anthropogenic CO2, while the amount of excess CO2 absorbed by the ocean from a century to a millennium is strongly correlated with modeled radiocarbon in the deep Southern and Pacific Ocean. The rates of surface-to-deep ocean transport are determined for individual models from the instantaneous doubling CO2 experiments, and they are used to calculate oceanic uptake of CO2 in response to emission pulses of 1000 and 5000 Pg C. These results are compared with simulated oceanic uptake of CO2 from a number of model simulations with the coupling of climate-ocean carbon cycle and without it. This comparison demonstrates that the impact of different ocean transport rate across models on the oceanic uptake of anthropogenic CO2 is of similar magnitude as that of climate-carbon cycle feedbacks in a single model associated with changes in temperature, circulation, and marine biology, emphasizing the importance of ocean transport in the fate of anthropogenic CO2.