Effects of acidification and warming on long-term ocean carbon cycling constrained by observations

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The chemical changes associated with ocean acidification will make it more difficult for important marine species (such as coccolithophores, foraminifera, and pteropods) to build their calcium carbonate (CaCO3) body parts, and existing CaCO3 will dissolve more easily. Thus acidification will likely decrease the production of CaCO3 in the future ocean. Warmer temperatures, on the other hand, lead to faster metabolic rates, which will likely increase primary and CaCO3 production. Faster warming of surface waters than deeper waters leads to increased stratification and less nutrient input into the sunlit surface ocean (photic zone). This may cause shifts in plankton species composition favoring coccolithophores and thus increasing CaCO3 production. Any change in CaCO3 production may also affect organic carbon fluxes from the surface to the deep ocean due to the aggregation and association of CaCO3 with organic particles. This research examines the relative importance of these effects on future global CaCO3 production and carbon cycling on long time scales (hundreds to thousands of years) using an improved model calibrated with existing observations. CaCO3 production increases atmospheric CO2, thus its future evolution may be an important feedback on climate. The sign and uncertainty of this feedback will be evaluated.An existing global model of ocean biogeochemical cycles suitable for millennial time scale simulations will be improved by adding a process based formulation of particle aggregation and sinking. The model will consider two mineral forms of CaCO3, calcite and aragonite, as well as opal, terrigenous, and organic matter as components of the aggregates. A global dataset of particulate organic carbon (POC) will be created by analyzing and calibrating large volume filtration measurements, bottle, transmissometer and satellite data including error estimates. This dataset, together with a large array of existing other global-scale biogeochemical observations will be used to calibrate the model and estimate uncertain parameters as well as different structural formulations of (a) the effect of ocean acidification on the production of CaCO3 and (b) particle aggregation. A Bayesian data assimilation scheme, designed to quantify three hypothetic mechanisms regarding the control of the rain ratio (CaCO3 over POC export from the euphotic zone), will be applied. Probabilistic projections will be carried out to quantify the effect of each mechanism on long term ocean carbon cycling and its feedback on atmospheric CO2 concentrations. The ability of the existing observations to constrain the projections will be evaluated. The project aims to lead to a better, more quantitative understanding of multi-variable processes that control global cycling of CaCO3 and carbon in the ocean and their coupling. Improving ocean biogeochemical cycling in a widely used Earth System Model of intermediate complexity will enhance infrastructure for research and education. The improved model will be made publicly available and benefit future research of long-term carbon cycle processes such as studies of anthropogenic effects or paleoclimate. The global POC dataset will also be made publicly available. A 3-day workshop for K-12 educators will be organized with climate change and ocean acidification as its main topics. Society may benefit from the outcome of this project through an improved assessment (including uncertainties) of the effects of anthropogenic carbon emissions on ocean biogeochemical cycles.