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Ocean acidification is a sustained decrease in seawater pH, commonly caused by absorption of atmospheric carbon dioxide. Dissolved carbon dioxide shifts chemical equilibrium toward bicarbonate and hydrogen ions. Reactions involving carbonate buffer part of the perturbation while reducing carbonate availability.

The change can affect the conditions for forming or dissolving calcium carbonate, even while the water remains alkaline with a pH above seven. Characterizing the carbonate system requires measurements such as pH, dissolved inorganic carbon, carbon-dioxide partial pressure or alkalinity, together with temperature and salinity. Two suitable independent carbonate-system measurements constrain the others through an equilibrium model.^[ref]