Ocean acidification is the ongoing reduction in the average pH of seawater caused primarily by the ocean absorbing additional carbon dioxide from the atmosphere. It is not the same as the ocean becoming literally acidic in the everyday sense, because modern seawater remains slightly alkaline with a pH generally above 7. The term refers to a shift towards greater acidity, meaning a decrease in pH and an increase in hydrogen ion concentration.
When atmospheric carbon dioxide dissolves in seawater, it participates in a series of chemical equilibria involving dissolved carbon dioxide, carbonic acid, bicarbonate ions, carbonate ions and hydrogen ions. The resulting increase in hydrogen ion concentration lowers seawater pH and alters the relative abundance of carbonate species. Because carbonate ions are important for organisms that build shells and skeletons from calcium carbonate, these chemical changes can affect marine ecosystems, particularly where carbonate mineral formation or preservation is already difficult.
The basic chemistry begins when atmospheric carbon dioxide dissolves at the ocean surface. Dissolved carbon dioxide reacts reversibly with water to form carbonic acid, which can dissociate to release hydrogen ions and bicarbonate. A second dissociation can produce additional hydrogen ions and carbonate ions. The equilibria can be represented conceptually as carbon dioxide plus water forming carbonic acid, carbonic acid dissociating into hydrogen and bicarbonate, and bicarbonate dissociating into hydrogen and carbonate.
In seawater, most dissolved inorganic carbon exists as bicarbonate, with smaller proportions as carbonate and dissolved carbon dioxide. Adding carbon dioxide shifts these chemical equilibria towards greater bicarbonate and lower carbonate availability. The pH scale is logarithmic, so a seemingly small numerical change represents a meaningful proportional change in hydrogen ion concentration. Seawater chemistry is also affected by temperature, salinity, pressure, alkalinity and biological activity, meaning that acidification varies geographically and seasonally.
Ocean acidification matters particularly because many marine organisms use calcium carbonate to construct shells, tests or skeletons. Important groups include corals, some molluscs, echinoderms and certain planktonic organisms. Calcium carbonate exists in several mineral forms, including aragonite and calcite, which differ in stability under seawater conditions. The availability of carbonate ions influences the saturation state of these minerals.
When saturation state is high, precipitation of calcium carbonate is generally more favourable, whereas lower saturation states make calcification energetically more difficult and can increase the tendency of exposed calcium carbonate to dissolve. Organisms are not affected identically because they possess different physiological mechanisms, habitats and tolerances. Some can regulate their internal chemistry or compensate for external changes, while others show reduced growth, altered development or increased energetic costs under more acidic conditions.
Coral reefs are especially relevant because reef building corals rely on calcium carbonate skeletons and exist within complex biological communities. Ocean warming and acidification can act simultaneously, creating multiple stresses that interact rather than operating independently. Acidification can affect calcification, skeletal properties, reproduction and larval development in some coral species, while warming can increase the risk of coral bleaching. Other organisms may respond differently, and some species can acclimatise or adapt under certain conditions.
Acidification can also alter food webs through effects on plankton and shell forming organisms. The ecological outcome depends on species composition, environmental variability, nutrient availability, temperature, oxygen concentration and other factors. Consequently, acidification is best understood as a broad chemical change in marine carbonate chemistry rather than a single biological disease affecting every marine organism in the same way.
The ocean has absorbed a substantial fraction of the carbon dioxide emitted by human activities, which has slowed the increase of atmospheric carbon dioxide compared with a hypothetical Earth without ocean uptake. This absorption provides an important climate service but has a chemical consequence. The ocean's carbon uptake is mediated by physical and biological processes. Cold seawater can dissolve relatively large quantities of carbon dioxide, and ocean circulation transports dissolved carbon into the interior.
Marine organisms also incorporate carbon into organic matter and calcium carbonate, contributing to the biological carbon cycle. When carbon dioxide enters seawater, however, the resulting chemical reactions alter carbonate chemistry. Ocean carbon uptake therefore reduces the rate of atmospheric accumulation while simultaneously driving acidification. This illustrates that carbon storage and environmental impacts can occur together and should not be treated as mutually exclusive processes.
Ocean acidification is measured using variables including pH, dissolved inorganic carbon, total alkalinity, carbonate ion concentration and saturation state. Direct pH measurements require careful calibration because seawater pH depends on the particular scale and temperature at which it is reported. Long term observations from ships, fixed stations, autonomous instruments and research programmes reveal changes in marine carbonate chemistry over time. The magnitude of acidification is not identical everywhere.
Polar waters can be particularly sensitive because cold water can dissolve more carbon dioxide, while upwelling regions can naturally bring carbon dioxide rich deep water to the surface. Coastal areas can experience additional variability from freshwater inputs, nutrient cycling, respiration and local biological processes. These factors can cause short term pH fluctuations superimposed on the broader anthropogenic trend.
Ocean acidification has consequences for fisheries, aquaculture, ecosystem services and marine biodiversity because changes in the physiology or abundance of particular organisms can propagate through food webs. Shellfish hatcheries, for example, may experience difficulties when carbonate chemistry becomes unfavourable for early life stages, although management techniques can sometimes reduce exposure. Some marine organisms may benefit from increased carbon dioxide under particular circumstances, especially certain photosynthetic organisms, but benefits are constrained by nutrient availability and other environmental conditions.
Acidification also interacts with warming, deoxygenation and pollution, making future ecosystem responses difficult to predict using a single variable. Adaptation, acclimatisation, migration and changes in community composition can all occur. The overall trend is nevertheless directly connected to rising atmospheric carbon dioxide and can be reduced by limiting carbon dioxide emissions.
A common misconception is that ocean acidification means the entire ocean will soon become strongly acidic. The ocean remains generally alkaline, and the scientific concern is the progressive shift towards lower pH and reduced carbonate availability. Another misconception is that only shell forming organisms are affected. Changes in carbonate chemistry can influence metabolism, sensory processes, reproduction, behaviour and food web relationships in some organisms, although effects vary substantially among species.
Acidification is also distinct from ordinary local fluctuations in pH caused by biological respiration or freshwater input, because anthropogenic acidification represents a broad and persistent change superimposed on natural variability. The fundamental cause is increased carbon dioxide absorption, making atmospheric carbon dioxide concentration a central control on the long term trajectory of ocean carbonate chemistry.
