Ocean acidification

The ocean continually exchanges carbon dioxide with the atmosphere. When additional carbon dioxide from human activities enters seawater, it reacts with water to form carbonic acid, which releases hydrogen ions. The increased hydrogen-ion concentration lowers pH and shifts the balance among dissolved carbon compounds; hydrogen ions also bind with carbonate ions, leaving fewer of those ions available. Ocean acidification therefore refers to a change in the ocean's acidity, not a claim that ordinary seawater has become acidic: average surface seawater remains alkaline, with a pH near 8.1. NOAA reports that surface-ocean pH has fallen by about 0.1 unit since the Industrial Revolution began, an increase of roughly 30 percent in acidity because the pH scale is logarithmic. The principal driver is the increase in atmospheric carbon dioxide from human activities, including fossil-fuel use and land-use change.

Many marine organisms use calcium carbonate to build shells or skeletons. Because acidification reduces the supply of carbonate ions and can lower the saturation of calcium-carbonate minerals, it may make forming or maintaining these structures more difficult for organisms such as corals, molluscs and some plankton. Changes to seawater chemistry can also affect organisms that do not build shells. The consequences are not identical across species or places: a meta-analysis of 228 studies found an overall negative response across the marine organisms studied, while also finding substantial variation among groups and life stages. Nutrition, local conditions, interactions among species and simultaneous warming can change how organisms respond. Researchers therefore distinguish a broad, well-supported risk from any claim that every species will be affected in the same way.

Ocean acidification is one of several climate-related changes affecting the sea, alongside warming and declining oxygen in some areas. These pressures can interact, influencing marine habitats and the communities that depend on them. Coral reefs and shellfish are among the systems of particular concern because organisms that make calcium-carbonate structures can be vulnerable; consequences may extend to food webs, fisheries and coastal livelihoods. The scale and timing of effects vary, so continued measurement is important. The Intergovernmental Oceanographic Commission of UNESCO supports coordination of the Global Ocean Acidification Observing Network, through which scientists and participating organizations work to improve observations from local to global scales. Reducing the human-driven carbon dioxide entering the atmosphere addresses the principal global cause, while monitoring helps communities and researchers assess changing conditions and local risks.