What is the term for the decrease in ocean pH due to CO2 absorption?

The decrease in ocean pH due to carbon dioxide absorption is called ocean acidification. When atmospheric CO2 dissolves in seawater, it forms carbonic acid, which releases hydrogen ions and lowers pH. Seawater remains slightly alkaline, but it becomes more acidic relative to its previous condition.

The oceans absorb roughly a quarter of human-produced CO2, making them an important climate buffer. However, this uptake changes carbonate chemistry and reduces carbonate-ion availability. Corals, oysters, mussels, and other calcifying organisms use carbonate compounds to build shells or skeletons, so acidification can make that process more difficult.

A common mix-up is confusing ocean acidification with global warming or thermal expansion. Warming describes increasing ocean temperature, while thermal expansion is the rise in water volume as it heats. Ocean acidification specifically describes a chemical change in seawater pH. Since the pH scale is logarithmic, even a seemingly small decline represents a substantial increase in hydrogen-ion concentration.