Why does ocean acidification reduce the number of carbonate ions in seawater?

Ocean acidification reduces the concentration of carbonate ions in seawater through a chemical reaction triggered by the absorption of atmospheric carbon dioxide (CO₂).

When CO₂ dissolves in seawater, it reacts with water to form carbonic acid, which subsequently releases hydrogen ions (H⁺) into the water. This increase in hydrogen ion concentration raises the acidity of the seawater, which is measured as a decrease in pH.1

These newly liberated hydrogen ions then react with existing carbonate ions (CO₃²⁻) to form bicarbonate ions (HCO₃⁻). Because the carbonate ions are consumed in this process to create bicarbonate, their overall concentration in the seawater declines. This reduction is significant because marine organisms, such as coral and shellfish, rely on these carbonate ions to build and maintain their calcium carbonate shells and skeletons.1