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Vol 437 29 September 2005 doi 10 1038 nature04095 ARTICLES Anthropogenic ocean acidification over the twenty first century and its impact on calcifying organisms James C Orr1 Victoria J Fabry2 Olivier Aumont3 Laurent Bopp1 Scott C Doney4 Richard A Feely5 Anand Gnanadesikan6 Nicolas Gruber7 Akio Ishida8 Fortunat Joos9 Robert M Key10 Keith Lindsay11 Ernst Maier Reimer12 Richard Matear13 Patrick Monfray1 Anne Mouchet14 Raymond G Najjar15 Gian Kasper Plattner7 9 Keith B Rodgers1 16 Christopher L Sabine5 Jorge L Sarmiento10 Reiner Schlitzer17 Richard D Slater10 Ian J Totterdell18 Marie France Weirig17 Yasuhiro Yamanaka8 Andrew Yool18 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 Ocean uptake of CO2 will help moderate future climate change but the associated chemistry namely hydrolysis of CO2 in seawater increases the hydrogen ion

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