Abstract. Plankton manipulation experiments exhibit a wide range of sensitivities of biogenic calcification to simulated anthropogenic acidification of the ocean, with the "lab rat" of planktic calcifiers, Emiliania huxleyi apparently not representative of calcification generally. We assess the implications of this observational uncertainty by creating an ensemble of realizations of an Earth system model that encapsulates a comparable range of uncertainty in calcification response to ocean acidification. We predict that a substantial reduction in marine carbonate production is possible in the future, with enhanced ocean CO₂ sequestration across the model ensemble driving a 4–13% reduction in the year 3000 atmospheric fossil fuel CO₂ burden. Concurrent changes in ocean circulation and surface temperatures in the model contribute about one third to the increase in CO₂ uptake. We find that uncertainty in the predicted strength of CO₂-calcification feedback seems to be dominated by the assumption as to which species of calcifier contribute most to carbonate production in the open ocean.