How does ocean absorption of carbon dioxide lead to ocean acidification?

Ocean acidification is the ongoing reduction in the pH of seawater caused by the absorption of atmospheric carbon dioxide (CO₂) by the oceans.1

When CO₂ dissolves in seawater, it reacts with water to form carbonic acid (H₂CO₃). This compound subsequently dissociates, liberating hydrogen ions (H⁺) into the water. Because the acidity of a solution is determined by the concentration of these hydrogen ions—where a higher concentration results in a lower pH—the release of H⁺ ions directly increases the acidity of the ocean.12

These liberated hydrogen ions then react with carbonate ions naturally present in the water to form bicarbonate (HCO₃⁻). This chemical process reduces the overall concentration of carbonate ions, which are essential building blocks for marine organisms such as coral and shellfish to construct their shells and skeletons. The rapid decline in pH poses a significant threat to marine ecosystems and the stability of food chains.1