Ocean acidification is a long-term decline in ocean pH, driven mainly by the sea absorbing excess carbon dioxide from the atmosphere. It changes seawater chemistry and can make it harder for some organisms to build shells and skeletons. The ocean generally remains alkaline—not acidic in the everyday, pH-below-7 sense—but its chemistry is changing. Cutting carbon dioxide emissions addresses the main cause; monitoring and reducing local stressors help coastal communities respond.
What is ocean acidification?
Ocean acidification is a sustained decrease in ocean pH, primarily caused by the ocean taking up carbon dioxide (CO2) from the atmosphere. “Acidification” describes the direction of change, not a switch to acidic seawater: ordinary surface ocean water is usually near pH 8 and remains alkaline. NOAA’s Ocean Acidification Program describes it as a fundamental change in ocean chemistry.
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How does carbon dioxide change seawater chemistry?
When CO2 dissolves in seawater, it forms carbonic acid, which dissociates into hydrogen ions and bicarbonate. The added hydrogen ions lower pH and also react with carbonate ions. That leaves less carbonate available to combine with calcium to form calcium-carbonate shells and skeletons.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsThe resulting change is not simply “more acid in the water”: it is a shift in the carbonate system that affects several related chemical measures. NOAA’s overview of ocean acidification explains the process.
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How much has ocean acidity changed?
NOAA’s Ocean Acidification Program says the ocean has become about 26% more acidic on average globally over the past 250 years. Separately, NOAA’s education overview reports that surface-ocean pH has fallen 0.1 units since the start of the industrial era, corresponding to approximately 30% greater acidity. These are figures as each NOAA page states them, with different wording and time framing.
For context, NOAA reports a global average atmospheric CO2 partial pressure of 422.7 parts per million in 2024. That is an atmospheric measurement, not an ocean pH reading. NOAA’s education overview also says the ocean absorbs about 30% of carbon dioxide released into the atmosphere.
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What marine life is affected?
Organisms that build calcium-carbonate structures are among the groups of concern because declining carbonate availability can make it more difficult to build or maintain them. NOAA identifies oysters, clams, corals, sea urchins, and calcareous plankton as examples. Effects differ among species and depend on environmental conditions; it is not accurate to say that every marine organism responds in the same way.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallNOAA also describes observed or studied effects on some fish behaviors. Changes to individual species may affect food webs, but NOAA notes that ecosystem-wide cascades are difficult to predict. See NOAA’s ocean exploration explanation and its program overview.
What causes coastal acidification?
Rising atmospheric CO2 is the principal global driver, but local processes can shape coastal conditions. Upwelling can bring deeper, more acidic water toward the surface. Nutrient and organic-carbon runoff can promote algal blooms; when the algae decay, the process consumes oxygen and releases CO2. Circulation, wind, temperature, and salinity also influence local chemistry.
These local influences can compound conditions in a particular place, but they do not replace the global role of atmospheric CO2. NOAA summarizes these coastal processes in its ocean acidification overview.
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How do scientists measure ocean acidification?
NOAA calls four measurements the “Big four”: pH, partial pressure of CO2 (pCO2), total alkalinity, and dissolved inorganic carbon (DIC). Researchers generally measure two of these parameters and use them to calculate the others, allowing them to characterize carbonate chemistry rather than relying on pH alone. NOAA also highlights aragonite saturation state as an indicator.
Measurements can be collected with buoys, moorings, research cruises, autonomous vehicles, and other observing platforms. A consumer pH meter can show one parameter in a demonstration or limited measurement, but it does not characterize the full carbonate system. NOAA explains the indicators in OA Indicators Explained and describes platforms and methods on its monitoring page.
What can be done about ocean acidification?
Reduce the main driver
Reducing CO2 emissions addresses the central global cause: excess atmospheric carbon dioxide absorbed by the ocean. Local projects cannot reverse global acidification on their own.
Monitor and manage local conditions
Coastal monitoring and modeling help communities understand changing conditions and inform fisheries and ecosystem management. Reducing additional stressors—especially excess nutrient runoff—can support ecosystems facing multiple pressures, though it does not eliminate the global driver.
Support research and community response
NOAA describes community science, habitat restoration and protection, improved observing, and research into emerging marine carbon dioxide removal approaches. Carbon removal approaches remain an area of research, not an established substitute for emissions cuts. NOAA Fisheries discusses monitoring and adaptation in its ocean acidification overview; the NOAA program overview covers broader responses.
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