| Summary: | The surface ocean is everywhere saturated with calcium carbonate (CaCO[SUB]3[/SUB]). Yet increasing atmospheric CO[SUB]2[/SUB] reduces ocean pH and carbonate ion concentration and thus the level of saturation. Here we show with ocean data and models that due to this anthropogenic acidification, some surface waters will become undersaturated within decades. When atmospheric CO[SUB]2[/SUB] reaches 550 ppmv, in year 2050 under the IS92a business-as-usual scenario, Southern Ocean surface waters begin to become undersaturated with respect to aragonite, a metastable form of CaCO[SUB]3[/SUB]. By 2100 as atmospheric CO[SUB]2[/SUB] reaches 788 ppmv, undersaturation extends throughout the entire Southern Ocean (<60[SUP]o[/SUP]S) and into the surbarctic Pacific. Meanwhile, Weddell Sea surface waters also become undersaturated with respect to calcite, the stable form of CaCO[SUB]3[/SUB]. These transient changes are much larger than seasonal, interannual, and decadal variability. They threaten high-latitude aragonite secreting organisms including cold-water corals, which provide essential fish habitat, and shelled pteropods, i.e., zooplankton that serve as an abundant food source for marine predators.
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