Britannica Sources used in this answer

How does carbon dioxide absorbed by oceans contribute to ocean acidification?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

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

When CO₂ dissolves in the ocean, it reacts with water to form carbonic acid (H₂CO₃). This acid 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⁺ directly increases the acidity of the seawater.12

Furthermore, these hydrogen ions react with carbonate ions naturally present in the ocean to form bicarbonate (HCO₃⁻). This process reduces the availability of carbonate ions, which are essential building blocks for marine organisms such as coral and shellfish to create and maintain their calcium carbonate shells and skeletons.1