Sensitivity of pelagic calcification to ocean acidification

Abstract. Ocean acidification might reduce the ability of calcifying plankton to produce and maintain their shells of calcite, or of aragonite, the more soluble form of CaCO₃. In addition to possibly large biological impacts, reduced CaCO₃ production corresponds to a negative feedback on atmospheric CO₂. In order to explore the sensitivity of the ocean carbon cycle to increasing concentrations of atmospheric CO₂, we use the new biogeochemical Bern3D/PISCES model. The model reproduces the large scale distributions of biogeochemical tracers. With a range of sensitivity studies, we explore the effect of (i) using different parameterizations of CaCO₃ production fitted to available laboratory and field experiments, of (ii) letting calcite and aragonite be produced by auto- and heterotrophic plankton groups, and of (iii) using carbon emissions from the range of the most recent IPCC Representative Concentration Pathways (RCP). Under a high-emission scenario, the CaCO₃ production of all the model versions decreases from ~1 Pg C yr⁻¹ to between 0.36 and 0.82 Pg C yr⁻¹ by the year 2100. The changes in CaCO₃ production and dissolution resulting from ocean acidification provide only a small feedback on atmospheric CO₂ of −1 to −11 ppm by the year 2100, despite the wide range of parameterizations, model versions and scenarios included in our study. A potential upper limit of the CO₂-calcification/dissolution feedback of −30 ppm by the year 2100 is computed by setting calcification to zero after 2000 in a high 21st century emission scenario. The similarity of feedback estimates yielded by the model version with calcite produced by nanophytoplankton and the one with calcite, respectively aragonite produced by mesozooplankton suggests that expending biogeochemical models to calcifying zooplankton might not be needed to simulate biogeochemical impacts on the marine carbonate cycle. The changes in saturation state confirm previous studies indicating that future anthropogenic CO₂ emissions may lead to irreversible changes in Ω_A for several centuries. Furthermore, due to the long-term changes in the deep ocean, the ratio of open water CaCO₃ dissolution to production stabilizes by the year 2500 at a value that is 30–50% higher than at pre-industrial times when carbon emissions are set to zero after 2100.