As the ocean absorbs carbon dioxide from the atmosphere, chemical reactions increase the concentration of hydrogen ions and lower the pH of seawater. This process is ocean acidification. Surface pH has already declined by about 0.1 units since pre-industrial times, representing a roughly 30 percent increase in acidity.

Lower pH reduces the availability of carbonate ions that many organisms need to build shells and skeletons of calcium carbonate. Corals, pteropods, oysters, and some plankton are particularly sensitive. Acidification can also affect physiology, behavior, and sensory abilities in fish and invertebrates. The impacts vary by species, life stage, and local conditions such as temperature and nutrient status.

Because CO2 is well mixed in the atmosphere, acidification is a global phenomenon, though coastal waters can experience additional pH variability from local processes. Reducing CO2 emissions is the only fundamental solution. Some aquaculture operations already buffer water chemistry to protect vulnerable larvae. Long-term monitoring of carbonate chemistry is now part of the global ocean observing system.

A pteropod’s thin shell can begin to dissolve in water that is undersaturated with respect to aragonite.