For perhaps 25 million years, surface waters throughout the ocean have remained saturated with respect to calcium carbonate (CaCO₃).  Yet increasing atmospheric CO₂ reduces ocean pH and carbonate ion concentration [CO₃^2-] and thus the level of saturation.  Despite this acidification, it has been estimated that all surface waters will remain saturated for centuries. However, marine calcifiers are still expected to suffer reductions in the rate at which they form their exoskeletons out of CaCO₃. Here we show with ocean data and models that the anthropogenic acidification will actually cause some surface waters to become undersaturated within decades, thus exacerbating the problem for marine calcifiers [Orr et al., 2005]. For instance, by 2050 when atmospheric CO₂ reaches 550 ppmv under the IS92a business-as-usual scenario, Southern Ocean surface waters begin to become undersaturated with respect to aragonite, a metastable form of CaCO₃. By 2100 as atmospheric CO₂ reaches 788 ppmv under the same scenario, undersaturation extends throughout the entire Southern Ocean (all ocean south of 60°S) and into the surbarctic Pacific.