We have estimated the in situ CaCO₃ dissolution rates for the global ocean from total alkalinity and chlorofluorocarbon data. Calcium carbonate dissolution rates, ranging from 0.003–1.2 µmol kg^-1 yr^-1, are observed beginning near the aragonite saturation horizon. The total water-column CaCO₃ dissolution rate for the global ocean is approximately 0.5 ± 0.2 Pg CaCO₃-C yr^-1, which is approximately 45–65% of the export production of CaCO₃. Rising atmospheric CO₂ concentrations over the past two centuries have changed the saturation state of the ocean with respect to calcium carbonate. The penetration of anthropogenic CO₂ into the ocean interior has caused an upward migration of the calcite and aragonite saturation horizons by about 40 - 200 m over large regions of the global ocean. Over the next several decades, these changes in the aragonite and calcite saturation state will have profound impacts on the health of coral reefs and other CaCO₃ shell-forming organisms in the ocean. The calcification rate of almost all calcifying organisms studied to date decreased in response to decreased CaCO₃ saturation state, even when the carbonate saturation level was >1.