# What is ocean acidification?

> Ocean acidification is the long-term decline in seawater pH caused mainly by the ocean absorbing carbon dioxide from the atmosphere. The ocean remains alkaline, yet its chemistry is moving toward the acidic end of the pH scale. That shift changes the forms...

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Published: 2026-09-04T12:52:48+00:00
Categories: Explainer, Oceans

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**Ocean acidification** is the long-term decline in seawater **pH** caused mainly by the ocean absorbing carbon dioxide from the atmosphere. The ocean remains alkaline, yet its chemistry is moving toward the acidic end of the pH scale. That shift changes the forms of carbon dissolved in seawater and reduces the **carbonate ions** many marine organisms use to build shells and skeletons.

The process is global because air and surface water constantly exchange gases. According to the [NOAA definition](https://oceanservice.noaa.gov/facts/acidification.html), the ocean absorbs about 30 percent of the carbon dioxide released into the atmosphere. Local conditions can strengthen or soften the change. Near coasts, river runoff and pollution alter water chemistry. Biological activity and upwelling add separate influences.

## How carbon dioxide changes seawater

Carbon dioxide does more than dissolve and remain as free gas. Some of it reacts with water to form carbonic acid, which can separate into bicarbonate and hydrogen ions. A higher concentration of hydrogen ions lowers pH. The [NOAA Ocean Acidification Program](https://oceanacidification.noaa.gov/what-is-ocean-acidification/) describes pH, partial pressure of carbon dioxide, total alkalinity and dissolved inorganic carbon as four central measurements used to track the process.

Extra hydrogen ions also react with carbonate ions and form more bicarbonate. Carbonate is therefore less available even before seawater becomes corrosive to a particular mineral. Marine chemists often measure the saturation state of aragonite or calcite, two forms of **calcium carbonate**, to estimate how favorable the water is for building and maintaining hard structures.

The pH scale is logarithmic, so a small numerical decline represents a meaningful chemical change. NOAA reports that average global ocean acidity has risen by about 26 percent over roughly the past 250 years. Temperature and salinity complicate comparisons between places. Pressure and biological activity add further variation, which is why scientists rely on **sustained observations** rather than a single water sample.

The word acidification sometimes causes confusion because ordinary seawater has a pH above 7. Scientists use the term to describe movement toward lower pH, just as a warm afternoon can cool without becoming cold. In chemical terms, the direction and rate of change affect carbonate availability even while the water remains alkaline.

## Shells and coral skeletons face a chemical squeeze

Oysters, mussels, sea urchins, corals and some plankton draw calcium and carbonate from seawater to make calcium carbonate. When carbonate becomes scarcer, an organism may need more energy to build the same structure. Very low saturation can also favor dissolution of exposed mineral surfaces. Sensitivity differs among species and life stages, so acidification does not produce one universal biological response.

Young shellfish can be especially vulnerable because larvae must form their first shells quickly. The [NOAA Pacific Marine Environmental Laboratory](https://www.pmel.noaa.gov/co2/story/Ocean+Acidification) explains how changes in carbonate chemistry can weaken conditions for **calcifying organisms**. Hatcheries in affected regions have learned to monitor incoming water and adjust operations when corrosive water reaches their intakes.

Coral reefs face a broader set of pressures. Acidification can slow reef building while warming raises the risk of bleaching. Disease adds further strain, as do pollution and physical damage. A reef's future depends on the balance between new calcium carbonate produced by living organisms and material lost through erosion or dissolution.

Some organisms can regulate chemistry around their tissues or find enough food to offset added energetic costs. Others may acclimate across a lifetime, while populations could adapt over generations. Those possibilities are active research areas, but they do not guarantee that entire ecosystems will keep their present structure as conditions change.

## Effects can travel through food webs

Marine ecosystems are networks, so a chemical effect on one group can reach its predators and alter fisheries. Habitat can change as well. Pteropods, sometimes called sea butterflies, are small swimming snails eaten by fish and other animals. Their thin aragonite shells make them useful indicators of corrosive conditions in cold waters where carbon dioxide dissolves readily.

Responses without shells also deserve attention. Experiments have found changes in growth and metabolism in some species. Reproduction or behavior can also respond, although results vary with experimental design and population. The [U.S. Environmental Protection Agency](https://www.epa.gov/ocean-acidification) emphasizes that acidification can combine with warming and low oxygen, producing conditions that no single-stressor experiment fully represents.

Food supply can change an experimental outcome because well-fed animals may have more energy for acid-base regulation or shell repair. Exposure time also counts. A brief laboratory trial may reveal an immediate physiological response, while a multigenerational study can test acclimation and inherited differences. Careful reporting identifies the tested pH and exposure duration. It also names the life stage and other environmental conditions.

## Why coastal waters can change faster

Open-ocean acidification is driven chiefly by atmospheric carbon dioxide, while coastal chemistry has more moving parts. Rivers can bring fresh water with low alkalinity. Wastewater and fertilizer runoff can stimulate algal growth, followed by decomposition that releases carbon dioxide and consumes oxygen. Upwelling may deliver naturally carbon-rich deep water onto continental shelves.

Coastal chemistry can change within hours and follow a seasonal cycle, even as a decades-long trend continues. A sensor therefore records large swings around the longer change. Organisms in variable habitats may tolerate fluctuation better than species from stable waters, but they can also encounter sharper extremes. Researchers therefore examine both the average condition and the duration of stressful episodes.

Cold regions often reach low carbonate saturation sooner because cold water can hold more dissolved carbon dioxide. High-latitude seas also receive freshwater from ice melt and rivers, which may reduce buffering in some locations. Polar food webs include calcifiers that support larger animals, giving the chemistry ecological importance far beyond a shell's surface.

Estuaries present another challenge because salinity changes over short distances. A pH reading near a river mouth cannot be interpreted exactly like one offshore. Monitoring programs pair **pH with alkalinity**, dissolved inorganic carbon, temperature and salinity so scientists can identify the processes behind an observed change.

Natural carbon dioxide seeps offer glimpses of long exposure, while controlled laboratory studies isolate mechanisms. Neither setting perfectly predicts a future ocean. Field observations become more informative when experiments support them. Models can then test whether the agreement holds across different scales.

## How scientists monitor acidification

Research ships collect samples through the water column and repeat survey lines years apart. Moorings and autonomous floats record changes between cruises. Laboratories analyze dissolved inorganic carbon and alkalinity with high precision, then calculate related values in the carbonate system. The [**Global Ocean Acidification Observing Network**](https://goa-on.org/) helps coordinate measurements and data practices across regions.

Long records are essential because ocean chemistry varies naturally with seasons and climate cycles. Biological production removes carbon dioxide near the surface during periods of strong photosynthesis, while respiration returns it. Mixing and currents carry water masses with distinct histories. A trend becomes clearer when observations cover enough time to separate persistent change from recurring variation.

Reducing carbon dioxide emissions addresses the main global driver. Local actions can still improve the conditions organisms face by limiting nutrient pollution and protecting habitat. Identifying vulnerable fisheries guides targeted adaptation. Monitoring gives communities time to adjust hatchery operations or restoration plans while showing whether larger changes in atmospheric carbon are altering the chemistry as expected.

Scientists also use models to connect the measurements. Ocean circulation carries absorbed carbon away from the surface, while upwelling can return older carbon-rich water to coastal ecosystems. Models estimate where corrosive conditions may develop between observing stations and help design future surveys. Their projections are checked against new field data, since biological feedbacks and regional circulation can alter the pace of change.

Public reporting usually expresses the trend through pH and carbonate saturation together. Reading both prevents a simple pH number from hiding the chemical conditions faced by shell-building organisms.

**Related reading:** [how nutrient pollution changes coastal water](https://www.argo.net/what-is-nutrient-pollution/) and [what coral reefs are made of](https://www.argo.net/what-is-a-coral-reef-made-of/).

 **Related reading:** [how nutrient pollution changes coastal water](https://www.argo.net/what-is-nutrient-pollution/) and [what coral reefs are made of](https://www.argo.net/what-is-a-coral-reef-made-of/). **Explore this topic:** [How Are Ocean Currents Measured?](https://www.argo.net/how-are-ocean-currents-measured/) and [What is ecoforecasting?](https://www.argo.net/what-is-ecoforecasting/).