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How does carbon dioxide cause ocean acidification?

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Illustration of various geoengineering techniques to address climate change, including carbon capture and storage methods like artificial trees, industrial scrubbers, and deep ocean CO2 storage, as well as solar reflectance strategies such as sulfur aerosols, reflective crops, cloud whitening, and orbital mirrors. The scene shows these interventions applied to land, sea, and atmosphere, with labeled features explaining each process. — Geoengineering Various geoengineering proposals designed to increase solar reflectance or capture and store carbon.

Ocean acidification occurs when the oceans absorb large quantities of carbon dioxide (CO₂) from the atmosphere.1

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

Furthermore, these liberated hydrogen ions react with carbonate ions in the water to form bicarbonate (HCO₃⁻). This process reduces the overall concentration of carbonate ions, which are essential building blocks for marine organisms that create shells and skeletons, such as coral and shellfish.1