Ocean acidification is a sustained reduction in the pH of the ocean, caused primarily by its uptake of carbon dioxide from the atmosphere. It involves changes in seawater chemistry, including increased hydrogen-ion concentrations and decreased carbonate availability. The term describes a movement toward more acidic conditions, not necessarily water becoming acidic: average surface seawater remains slightly alkaline. Although closely associated with climate change, acidification results directly from dissolved carbon dioxide rather than from warming itself. (oceanservice.noaa.gov)
Chemical mechanism
Ocean acidification forms part of the global carbon cycle. Since the Industrial Revolution, burning fossil fuels and changing land use have increased atmospheric carbon dioxide. The ocean absorbs a substantial share of these emissions—approximately 30 percent in widely cited estimates—thereby limiting atmospheric accumulation while altering seawater chemistry. (oceanservice.noaa.gov)
Dissolved carbon dioxide participates in a series of chemical equilibria. It reacts with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions. Hydrogen ions also react with carbonate ions, producing additional bicarbonate. Simplified reactions are:
CO₂ + H₂O ⇌ H₂CO₃
H₂CO₃ ⇌ H⁺ + HCO₃⁻
H⁺ + CO₃²⁻ ⇌ HCO₃⁻
The combined effect is an increase in dissolved carbon dioxide, bicarbonate, and hydrogen ions, accompanied by a decrease in carbonate ions. These acid–base reactions redistribute the forms of dissolved inorganic carbon rather than simply adding an unrelated acid to seawater. (pmel.noaa.gov)
Carbonate ions are used by many organisms to produce calcium carbonate shells and skeletons. Declining carbonate availability lowers the saturation state of minerals such as aragonite and calcite. Below saturation, dissolution is thermodynamically favored; biological difficulties can also occur before that threshold is crossed. (pmel.noaa.gov)
Observed changes and geographical variation
Historical reconstructions and instrumental observations indicate that average surface-ocean pH has fallen by approximately 0.1 units since preindustrial times, commonly expressed as a change from about 8.2 to 8.1. Because pH is logarithmic, this corresponds to roughly a 26 percent increase in hydrogen-ion concentration, often rounded to 30 percent. It does not mean that pH itself has decreased by 30 percent. (noaa.gov)
Long-term records show declining surface pH across ocean regions. The Intergovernmental Panel on Climate Change’s 2021 assessment concluded that human influence is the main driver of observed surface open-ocean acidification. Acidification also reaches deeper waters as currents and mixing transport anthropogenic carbon into the ocean interior. (ipcc.ch)
Conditions vary with circulation, biological activity, freshwater inputs, and local chemistry. Coastal upwelling can bring carbon-rich, low-pH subsurface water toward the surface. In estuaries, nutrient enrichment and eutrophication can intensify acidification through the production and subsequent breakdown of organic matter. These processes produce seasonal or episodic changes superimposed on the long-term anthropogenic trend. Consequently, a low-pH event alone does not establish the magnitude of human-driven acidification at a particular location. (ipcc.ch)
Biological and ecological effects
A major concern is biomineralization: the formation of mineral structures by living organisms. Reduced carbonate saturation can make shell and skeleton production more difficult for oysters, clams, sea urchins, corals, and calcifying plankton. Effects on growth and survival can be especially consequential in nutrient-rich, lower-salinity coastal environments. (oceanservice.noaa.gov)
Responses are not uniform across organisms or environments. Increased carbon dioxide can stimulate photosynthesis in some phytoplankton and macroalgae, while reduced mineral saturation can suppress net calcification in some shell-forming organisms. Nutrient availability and light conditions influence these responses. Ocean acidification therefore cannot be represented accurately as an identical physiological effect on every marine species. (ipcc.ch)
Changes in organisms can propagate through the food web and affect marine ecosystems. Acidification also occurs alongside warming and oxygen loss, making ecological outcomes dependent on interacting environmental pressures. Human consequences include risks to shellfish production and seafood-dependent communities, although local exposure and sensitivity differ. (oceanservice.noaa.gov)
Measurement and monitoring
Researchers characterize the seawater carbonate system using four principal quantities: pH, carbon dioxide partial pressure, total alkalinity, and dissolved inorganic carbon. Total alkalinity describes acid-neutralizing capacity; dissolved inorganic carbon comprises dissolved carbon dioxide, bicarbonate, and carbonate. Measurements of two of these quantities, together with temperature and salinity, allow calculation of other carbonate-system properties, including aragonite saturation. (oceanacidification.noaa.gov)
Ship-based sampling and instruments on moored buoys provide complementary observations. Repeated surveys describe spatial patterns, while continuous records reveal short-term variability and long-term trends. Monitoring links changes in chemistry to biological exposure and provides information relevant to coastal industries. (oceanacidification.noaa.gov)
Projections and responses
Future acidification depends strongly on carbon dioxide emissions. The Intergovernmental Panel on Climate Change projects a further average open-ocean surface pH decline of approximately 0.08 under SSP1-2.6 and 0.37 under SSP5-8.5 for 2081–2100 relative to 1995–2014. These are conditional scenario projections, not predictions of a single inevitable outcome. (ipcc.ch)
Reducing anthropogenic carbon dioxide emissions addresses the principal global driver. Local responses can reduce exposure without reversing ocean-wide change. Some shellfish hatcheries monitor incoming seawater, adjust intake timing, or add buffering substances to protect larvae from unfavorable chemistry. Such measures operate within managed production systems rather than restoring the carbonate chemistry of the surrounding ocean. (oceanacidification.noaa.gov)