As CO2 reacts with seawater it forms carbonic acid, causing a reduction in seawater pH. Seawater is naturally 'buffered' against these pH changes, but the buffering process consumes carbonate ions. Carbonate ion is an essential ingredient in the creation of calcium carbonate shells and skeletons produced by a large number of marine organisms (e.g., corals, marine plankton, coralline algae, and shellfish).
Carbonic acid is formed as a result of dissolving CO2 in water. In seawater, some of the acid (H+) is consumed by reacting with carbonate ion (CO32-) to form bicarbonate ion (HCO3-). Thus, ocean acidification not only reduces surface ocean pH, but also reduces the availability of carbonate ion.
The term Ocean Acidification is new, but the concept has been recognized for more than three decades. Trends toward lower pH can now be clearly seen in sustained observations recorded at ocean time series stations and during repeated geochemical surveys. In coming decades, ocean acidification could affect some of the most fundamental biological and geochemical processes of the sea.