Ocean acidification is the long term reduction in the pH of seawater caused primarily by the ocean absorbing increasing amounts of carbon dioxide from the atmosphere. The phrase does not mean that the entire ocean has become chemically acidic in the ordinary sense, because normal seawater remains slightly alkaline, with a pH generally above 7. Instead, acidification describes a shift towards greater acidity, meaning a lower pH and a greater concentration of hydrogen ions. It is a change in ocean chemistry rather than simply a change in temperature. The process is occurring throughout the global ocean, although its magnitude varies between regions because ocean circulation, biological activity, temperature, alkalinity, freshwater inputs and local carbon dioxide sources differ geographically.
The fundamental chemical process begins when atmospheric carbon dioxide dissolves into seawater. Dissolved carbon dioxide reacts with water and forms carbonic acid, which can dissociate into bicarbonate ions and hydrogen ions. Some of the additional hydrogen ions react with carbonate ions, converting them into bicarbonate and thereby reducing the availability of carbonate ions. This matters because carbonate ions are a key component required by many organisms to construct calcium carbonate shells and skeletons.
Ocean acidification therefore changes several related chemical properties simultaneously, including pH, dissolved carbon dioxide, hydrogen ion concentration, carbonate ion concentration and the saturation state of calcium carbonate minerals such as aragonite and calcite. These interconnected changes are collectively more informative than pH alone when scientists evaluate ocean acidification.
One of the most important consequences of ocean acidification is its effect on organisms that build structures from calcium carbonate. Oysters, clams, mussels, sea urchins, corals, pteropods, some plankton and many other calcifying organisms can face greater difficulty forming or maintaining shells and skeletons when carbonate availability declines. If seawater becomes undersaturated with respect to a particular calcium carbonate mineral, dissolution becomes thermodynamically more favourable, although biological responses depend on species, life stage, habitat and other environmental conditions.
Acidification can therefore affect organisms during shell formation, growth, reproduction and development. Coral reefs are especially important because reef building depends on the balance between calcium carbonate production and dissolution. A chemical change that makes calcification more energetically demanding can consequently influence entire ecosystems rather than only individual organisms.
Ocean acidification can also affect organisms that do not make calcium carbonate shells or skeletons. Changes in seawater chemistry can influence physiological processes, energy allocation, sensory systems, metabolism, behaviour and interactions among species. NOAA notes evidence that more acidic conditions can affect predator detection in some fish and can influence the ability of certain larval fish to locate appropriate habitat. Responses are not uniform, however, and some species may be relatively tolerant while others are highly sensitive.
The effect on an ecosystem consequently depends on the composition of its communities and on simultaneous pressures such as warming, oxygen loss, pollution, disease, habitat destruction and overfishing. Ocean acidification is therefore best understood as one component of a broader set of environmental changes affecting marine ecosystems.
The scale of modern ocean acidification is closely connected with the human alteration of the atmospheric carbon cycle. Since the industrial era, large quantities of carbon dioxide have been released through fossil fuel combustion, cement production and land use change. The ocean has absorbed a substantial fraction of anthropogenic carbon dioxide, which has reduced the amount remaining in the atmosphere compared with a world in which the ocean did not absorb that carbon.
This uptake provides an important climate regulating service but simultaneously changes seawater chemistry. NOAA estimates that the average global surface ocean has become about 26 percent more acidic over roughly the past 250 years, illustrating why even a modest numerical change in pH can represent a substantial chemical change because the pH scale is logarithmic.
Ocean acidification is related to climate change but is not identical to global warming or climate change as a whole. Both processes are strongly connected to rising atmospheric carbon dioxide, but they operate through different immediate physical mechanisms. Global warming is primarily associated with changes in the Earth's energy balance caused by greenhouse gases, whereas ocean acidification results directly from carbon dioxide entering seawater and altering carbonate chemistry.
Ocean warming can nevertheless modify acidification patterns because temperature affects gas solubility, circulation, biological processes and stratification. Coastal areas can experience additional acidification from local processes, including nutrient pollution, decomposition of organic matter, freshwater inputs and upwelling of naturally carbon dioxide rich deep water. Consequently, local coastal acidification can be substantially different from the global open ocean trend.
Scientists monitor ocean acidification using several complementary chemical measurements rather than relying exclusively on pH. Important measurements include seawater pH, partial pressure of carbon dioxide, total alkalinity and dissolved inorganic carbon. Together these variables allow researchers to determine how carbon dioxide is entering or leaving seawater, how strongly the carbonate system is buffered and how conditions affecting calcium carbonate formation are changing.
Long term observations are important because seawater chemistry naturally varies with season, latitude, depth, biological productivity, temperature and circulation. A short term measurement may therefore reflect ordinary variability rather than a long term acidification trend. Researchers combine ship observations, autonomous instruments, laboratory measurements, satellites, numerical models and historical records to establish regional and global patterns and to determine how rapidly chemical conditions are changing.
The significance of ocean acidification extends beyond marine chemistry because marine ecosystems support food production, employment, coastal protection, cultural practices and economic activity. Fisheries and aquaculture can be affected when sensitive organisms experience impaired growth, survival or reproduction, while coral reef degradation can reduce habitat complexity and the ecological services associated with reefs. The severity of impacts varies considerably because species differ in tolerance and because organisms experience multiple environmental stresses simultaneously.
Reducing atmospheric carbon dioxide emissions is therefore central to limiting the underlying global driver, while monitoring, reducing local pollution and improving ecosystem management can help reduce additional pressures in vulnerable coastal regions. Ocean acidification is fundamentally a carbon cycle problem expressed through ocean chemistry, and its consequences connect atmospheric composition with biological, ecological and socioeconomic systems.
