Estimates of anthropogenic carbon uptake from four three‐dimensional global ocean models

We have compared simulations of anthropogenic CO 2 in the four three‐dimensional ocean models that participated in the first phase of the Ocean Carbon‐Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ±19%, giving a range of 1.85±0.35 Pg C yr −1 for the 1980–1989 average. Regionally, the Southern Ocean dominates the present‐day air‐sea flux of anthropogenic CO 2 in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data‐based estimates of anthropogenic CO 2 suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real ocean's present uptake, based on comparison of regional data‐based estimates of anthropogenic CO 2 and bomb 14 C. Column inventories of bomb 14 C have become more similar to those for anthropogenic CO 2 with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO 2 would improve if systematic errors associated with the data‐based estimates could be provided regionally.