When carbon dioxide mixes with seawater it has the effect of reducing the availability of carbonate ions, which many marine organisms—corals, marine plankton, and shellfish among them—need to build their shells.
There are several chemical reactions involving carbon dioxide (CO2), water (H2O), carbonic acid (H2CO3), bicarbonate ions (HCO3-), hydrogen ions (H+), and carbonate ions (CO3-2) that are important. The series of reactions below describes how hydrogen ions are produced when carbon dioxide reacts with seawater. Since pH is a measure of hydrogen ions in a solution—albeit, an inverse measure—this has the effect of lowering pH.
CO2 + H2O => H2CO3 => H+ + HCO3-
Critically for many shell-forming marine organisms, carbonate (CO3-2) is simultaneously consumed by reaction of the excess of hydrogen ions produced from the reaction above:
H+ + CO3- => HCO3-
Combining the two reactions, the net affect is approximately that one unit of carbonate ion is consumed for each unit of carbon dioxide added to seawater.
CO2 + H2O + CO3-2 ↔ 2HCO3-
Because the reactions run simultaneously, both pH and the availability of carbonate are reduced as the atmospheric concentration of carbon dioxide rises.