The rise in atmospheric CO2 has caused significant decrease in sea surface pH and carbonate ion (CO 22 3 ) concentration. This decrease has a negative effect on calcification in hermatypic corals and other calcifying organisms. We report the results of three laboratory experiments designed specifically to separate the effects of the different carbonate chemistry parameters (pH, CO 22 3 ,C O2 [aq], total alkalinity [AT], and total inorganic carbon [CT]) on the calcification, photosynthesis, and respiration of the hermatypic coral Acropora eurystoma. The carbonate system was varied to change pH (7.9–8.5), without changing CT ;C T was changed keeping the pH constant, and CT was changed keeping the pCO2 constant. In all of these experiments, calcification (both light and dark) was positively correlated with CO 22 3 concentration, suggesting that the corals are not sensitive to pH or CT but to the CO 22 3 concentration. A decrease of ,30% in the CO 22 3 concentration (which is equivalent to a decrease of about 0.2 pH units in seawater) caused a calcification decrease of about 50%. These results suggest that calcification in today’s ocean (pCO2 5 370 ppm) is lower by ,20% compared with preindustrial time (pCO2 5 280 ppm). An additional decrease of ,35% is expected if atmospheric CO2 concentration doubles (pCO2 5 560 ppm). In all of these experiments, photosynthesis and respiration did not show any significant response to changes in the carbonate chemistry of seawater. Based on this observation, we propose a mechanism by which the photosynthesis of symbionts is enhanced by coral calcification at high pH when CO2(aq) is low. Overall it seems that photosynthesis and calcification support each other mainly through internal pH regulation, which provides CO 22 3 ions for calcification and CO2(aq) for photosynthesis. The increase in atmospheric CO2 is associated with global warming, rising sea level, and surface ocean acidification (Brewer 1997; Feely et al. 2004). Atmospheric CO2 is expected to double relative to its preindustrial level sometime between 2050 and 2100 according to different IPCC scenarios (Houghton et al. 2001). Under such conditions the calculated surface ocean pH will be 7.9 by the year 2060 compared with a value of ,8.2 during the preindustrial time (Brewer 1997). This decrease will cause a significant decline in the carbonate ion concentration (CO 22