The oceanic uptake of anthropogenic CO2 changes the seawater chemistry and potentially can alter biological systems in the upper oceans. Estimates of future atmospheric and oceanic CO2 concentrations, based on the Intergovernmental Panel on Climate Change (IPCC) emission scenarios, indicate that atmospheric CO2 levels could approach 800 ppm by the end of the century. Corresponding models for the oceans indicate that surface water pH would decrease by approximately 0.4 pH units, and the carbonate ion concentration would decrease by as much as 48% by the end of the century. The surface ocean pH would be lower than it has been for more than 20 million years. Such changes would significantly lower the ocean;s buffering capacity, which would reduce its ability to accept more CO2 from the atmosphere. Recent field and laboratory studies reveal that the carbonate chemistry of seawater has a profound impact on the calcification rates of individual species and communities in both planktonic and benthic habitats. The calcification rates of nearly all calcifying organisms studied to date decrease in response to decreased carbonate ion concentration. In general, when pCO2 was increased to twice preindustrial levels, a decrease in the calcification rate ranging from about -5% to -60% was observed. Unless calcifying organisms can adapt to projected changes in seawater chemistry, there will likely be profound changes in the structure of pelagic and benthic marine ecosystems.