Ocean acidification is the long-term decline in seawater pH, caused mainly by the ocean absorbing excess carbon dioxide from the atmosphere. Surface seawater remains alkaline, but changing carbonate chemistry can make it harder for some organisms to build shells and skeletons. Cutting CO₂ emissions addresses the main cause; coastal monitoring and reducing local stressors help communities manage impacts.
What is ocean acidification?
Ocean acidification is a sustained decrease in ocean pH. The name describes a change in direction: ordinary surface seawater is still alkaline, generally near pH 8, rather than acidic in the everyday, pH-below-7 sense. NOAA summarizes it this way: “Ocean acidification occurs when the ocean absorbs carbon dioxide. This causes a fundamental and global change in the chemistry of the ocean.”
How does carbon dioxide change seawater chemistry?
Atmospheric carbon dioxide (CO₂) dissolves into seawater and reacts to form carbonic acid. That acid dissociates, adding hydrogen ions and bicarbonate. More hydrogen ions lower pH and also react with carbonate ions, reducing the carbonate available to organisms that make calcium-carbonate shells and skeletons.
The ocean absorbs about 30% of the carbon dioxide released into the atmosphere, according to NOAA’s education overview. This is a share of emissions, not a measure of ocean pH.
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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. NOAA’s education page reports a different measure and time framing: surface-ocean pH has fallen 0.1 units since the start of the industrial era, representing approximately a 30% increase in acidity. These figures are not identical measurements, so they should be read in the terms each NOAA page gives them.
As a separate atmospheric indicator, NOAA reports a 2024 global average atmospheric partial pressure of CO₂ (pCO₂) of 422.7 parts per million. That figure describes the atmosphere, not an ocean pH reading.
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Which marine life is affected?
Organisms that build calcium-carbonate structures are among the clearest groups of concern. Lower carbonate availability can make it more difficult for some species to form or maintain shells and skeletons. Effects vary with species and environmental conditions; acidification does not affect every organism in the same way.
- Shellfish: oysters and clams build shells from calcium carbonate.
- Corals: reef-building corals form calcium-carbonate skeletons.
- Other calcifiers: sea urchins and calcareous plankton also make calcium-carbonate structures.
- Some fish: NOAA describes observed or studied effects on some fish behaviors, but responses are not uniform.
Changes to individual species can affect food webs, but NOAA cautions that ecosystem-wide cascades are difficult to predict. A specific biological response should not be treated as a universal outcome for all marine life.
What causes coastal acidification?
Rising atmospheric CO₂ is the main global driver, but coastal waters can also be shaped by local processes. These influences can vary by location and time:
- Upwelling: winds and currents can bring deeper, more acidic water toward the surface.
- Nutrient and organic-carbon runoff: runoff can fuel algal blooms; when algae decay, the process consumes oxygen and releases CO₂.
- Local physical conditions: circulation, wind, temperature, and salinity can affect coastal chemistry.
How do scientists measure ocean acidification?
pH is important, but a single pH reading does not describe the full carbonate system. NOAA identifies four key measurements: pH, partial pressure of CO₂ (pCO₂), total alkalinity, and dissolved inorganic carbon (DIC). Researchers measure two of these parameters and use them to calculate the others. NOAA also highlights aragonite saturation state as an indicator.
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Measurements may come from buoys, moorings, research cruises, autonomous vehicles, and other observing platforms. A consumer pH meter can demonstrate one parameter in an educational setting, but it does not measure the complete carbonate system or substitute for a scientific monitoring program.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What can be done about ocean acidification?
Reduce the main driver
Reducing CO₂ emissions addresses the central global cause: the excess atmospheric carbon dioxide that the ocean absorbs. Local actions can help manage conditions and impacts, but they do not reverse the global driver.
Monitor and manage coastal waters
Monitoring and modeling help communities understand local conditions and support science-based ecosystem management, including decisions affecting fisheries. NOAA also describes improved observing, community science, and restoration or protection efforts as part of the response.
Reduce additional local stressors
Where nutrient runoff is contributing to coastal problems, reducing excess inputs can address one local stressor. Such steps support coastal resilience; they should not be presented as a substitute for cutting emissions.
Treat emerging approaches as research
NOAA notes research into marine carbon dioxide removal approaches. These are emerging areas of investigation, not established replacements for emissions reductions.
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