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Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms


Today's surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms—such as corals and some plankton—will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean–carbon cycle to assess calcium carbonate saturation under the IS92a ‘business-as-usual’ scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

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We thank M. Gehlen for discussions, and J.-M. Epitalon, P. Brockmann and the Ferret developers for help with analysis. All but the climate simulations were made as part of the OCMIP project, which was launched in 1995 by the Global Analysis, Integration and Modelling (GAIM) Task Force of the International Geosphere–Biosphere Programme (IGBP) with funding from NASA (National Aeronautics and Space Administration). OCMIP-2 was supported by the European Union Global Ocean Storage of Anthropogenic Carbon (EU GOSAC) project and the United States JGOFS Synthesis and Modeling Project funded through NASA. The interannual simulation was supported by the EU Northern Ocean Carbon Exchange Study (NOCES) project, which is part of OCMIP-3.

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Correspondence to James C. Orr.

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Supplementary Notes

Supplementary Methods, uncertainties, Supplementary Table, Supplementary Figures S1–S5 and additional references (PDF 2126 kb)

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Further reading

Figure 1: Increasing atmospheric CO2 and decreasing surface ocean pH and [CO32-].
Figure 2: The aragonite saturation state in the year 2100 as indicated by Δ[CO32-]A.
Figure 3: Climate-induced changes in surface [CO32-].
Figure 4: Key surface carbonate chemistry variables as a function of p CO 2 .
Figure 5: Average surface [CO32-] in the Southern Ocean under various scenarios.
Figure 6: Shell dissolution in a live pteropod.


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