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How does carbon dioxide absorption cause ocean acidification?

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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. — Biosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

Ocean acidification occurs when the oceans absorb large quantities of carbon dioxide (CO₂) from the atmosphere, triggering a chemical process that increases the acidity of seawater.1

When CO₂ dissolves in seawater, it reacts with water (H₂O) to form carbonic acid (H₂CO₃). This acid subsequently dissociates, liberating hydrogen ions (H⁺). 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

Furthermore, these hydrogen ions react with carbonate ions (CO₃²⁻) present in the water to form bicarbonate (HCO₃⁻). This reaction 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. Consequently, the absorption of CO₂ not only lowers the pH of the ocean but also threatens the structural integrity of various marine life forms.1