How does added carbon dioxide lower the pH of seawater?

When carbon dioxide (CO₂) dissolves in seawater, it reacts with water to form carbonic acid (H₂CO₃). This acid subsequently dissociates, liberating hydrogen ions (H⁺) into the solution.123

The pH of a solution is a measure of its acidity, which is determined by the concentration of these hydrogen ions; specifically, a higher concentration of H⁺ ions results in a lower pH value, indicating increased acidity. As more CO₂ is absorbed by the oceans, the resulting increase in H⁺ ion concentration drives this reduction in pH, a process known as ocean acidification.1

Furthermore, these liberated hydrogen ions react with carbonate ions (CO₃²⁻) present in the seawater to form bicarbonate (HCO₃⁻), which reduces the availability of carbonate ions necessary for marine organisms to build and maintain their shells and skeletons.1