The chemistry of dissolved inorganic carbon in seawater is reviewed and used to predict the potential effect of rising levels of carbon dioxide in the atmosphere. In agreement with more detailed treatments, we find that calcium carbonate (aragonite) may become unsaturated in cold surface seawater by the year 2100 C.E., resulting in the destruction of calcifying organisms such as coral. Our analysis is intended for the environmental chemistry classroom and illustrates the importance of gas solubility, buffers, acid-base chemistry, and the effect of temperature on equilibrium. Calculations are adapted to spreadsheets and are intended to be accessible to upper-level chemistry students. (Contains 2 tables and 3 figures.)