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 (OCMP), as means to identify their major differences. Simulated global uptake agrees to within ±19‰, giving a range of 1.85±0.35 Pg Cyr 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 occuring 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 ¹⁴C. Column inventories of bomb ¹⁴C 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.