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Abstract:
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
