How does carbon dioxide dissolved in seawater form carbonic acid?

When carbon dioxide (CO₂) dissolves in seawater, it reacts with water (H₂O) to form carbonic acid (H₂CO₃). This chemical process is represented by the following equilibrium reaction:123

CO₂ + H₂O ⇌ H₂CO₃13

In this reaction, the predominant species are simply loosely hydrated CO₂ molecules. Once formed, carbonic acid can dissociate and liberate hydrogen ions (H⁺), which increases the acidity of the seawater—a process known as ocean acidification. These hydrogen ions subsequently react with carbonate ions (CO₃²⁻) present in the water to form bicarbonate (HCO₃⁻).132

It is important to recognize that the increasing absorption of anthropogenic CO₂ by the oceans is causing a significant decline in seawater pH, which threatens marine ecosystems, particularly calcifying organisms like coral and shellfish that rely on carbonate ions to build their skeletons and shells.2