# Ocean acidification is changing the chemistry that builds shells

> NOAA's Ocean Acidification Program tracks a quiet chemical shift across the sea. As the ocean absorbs more carbon dioxide from the air, seawater holds more hydrogen ions and fewer carbonate ions. That matters because carbonate helps many marine animals build shells and...

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Byline: ARGO.net Editorial Team
Published: 2026-07-22T13:00:21+00:00
Categories: Explainer, Oceans

![Pteropod shell affected by ocean acidification](https://www.argo.net/wp-content/uploads/2026/07/reviewed_ocean_acidification_thumbnail.jpg)

[NOAA's Ocean Acidification Program](https://oceanacidification.noaa.gov/what-is-ocean-acidification/) tracks a quiet chemical shift across the sea. As the ocean absorbs more **carbon dioxide** from the air, seawater holds more hydrogen ions and fewer carbonate ions. That matters because carbonate helps many marine animals build shells and skeletons. The change reaches from coral reefs to tiny drifting snails and it can also affect the fisheries and coastal communities that depend on them.

## Carbon dioxide shifts seawater chemistry

Each day, the ocean takes in carbon dioxide from the atmosphere. That service slows the rise of carbon dioxide in the air, yet it changes the water's chemistry. Dissolved carbon dioxide reacts with seawater and forms carbonic acid. The reactions release **hydrogen ions**, which lower pH. Scientists use the term **ocean acidification** for this long-term change in ocean chemistry.

The phrase can sound confusing because seawater remains slightly basic on the pH scale. The important point is the direction and speed of the shift. A lower pH means a higher concentration of hydrogen ions. The pH scale is logarithmic, so a small-looking change represents a meaningful chemical change. NOAA reports that the global ocean has become about 26 percent more acidic on average over the past 250 years.

Measurements from ocean stations and repeated surveys show the trend alongside rising atmospheric carbon dioxide. The change is linked to emissions from burning coal, oil and gas, as well as land-use changes that add carbon dioxide to the atmosphere. Ocean acidification therefore belongs to the larger carbon cycle. It also has its own direct effects on seawater and marine life.

## Carbonate is a building block for shells

The chemical shift also changes the supply of materials that shells need. The extra hydrogen ions combine with **carbonate ions** in seawater and turn some of them into bicarbonate. That leaves fewer carbonate ions available for animals that make hard parts from **calcium carbonate**. Corals, oysters, clams, sea urchins, some crabs and several kinds of plankton all rely on this material in different ways.

For a shell-building animal, finding enough carbonate can become an energy problem. It may need to spend more energy building and maintaining a shell or skeleton. Young animals can be especially sensitive because early life stages must form structures quickly. NOAA's [shellfish research](https://oceanacidification.noaa.gov/ocean-acidification-research/ocean-acidification-biological-response/shellfish/) describes how changing carbonate chemistry can affect growth, survival and physiology.

Conditions vary from place to place. Coastal waters can change quickly when deep water rises to the surface, when rivers bring in freshwater, or when local organisms alter carbon dioxide levels. Those local patterns help explain why monitoring matters. A global average shows the broad trend, while local observations can warn hatcheries and resource managers about conditions that may arrive within days.

## Cold seas face an early squeeze

Cold water can hold more dissolved carbon dioxide than warm water. That gives polar and subpolar seas an early exposure to changing carbonate chemistry. The Arctic, the Southern Ocean and parts of the North Pacific are closely watched because many organisms there are adapted to waters where calcium carbonate is already less available than it is in warm tropical seas.

One familiar example is the **pteropod**, a small swimming sea snail sometimes called a sea butterfly. Pteropods form delicate calcium carbonate shells and are food for animals ranging from fish to whales. NOAA's [Ocean Exploration program](https://oceanexplorer.noaa.gov/ocean-fact/acidification/) notes that reduced carbonate can slow skeletal growth in pteropods and other calcium-secreting organisms.

Scientists often describe shell conditions with a measure called saturation state. Higher values favor the formation of calcium carbonate. Lower values make it harder to build or maintain those structures. This measure helps connect chemistry with biology. It also reminds researchers that the response of a species depends on temperature, food, life stage and the other stresses it experiences.

## Food webs and coastal fisheries feel the change

The ocean is a connected food web. When plankton or shellfish struggle, effects can travel upward to predators and people. Some responses are direct, such as slower shell growth. Others involve behavior, development, or the quality of habitat. NOAA's [plankton studies](https://oceanacidification.noaa.gov/ocean-acidification-research/ocean-acidification-biological-response/plankton/) emphasize that many animals, from salmon to whales, depend on these small drifting organisms for food.

**Coral reefs** face a particularly difficult mix of pressures. Reef-building corals use carbonate to form their skeletons, while warming water can trigger bleaching. Pollution, disease and destructive fishing can add still more stress. Acidification works alongside these pressures. It changes the background chemistry while other threats shape how well reefs and their communities can recover.

Shellfish farms have shown why timely data can be valuable. In the Pacific Northwest, managers use observations of carbon dioxide and seawater chemistry to prepare for difficult water conditions. The [U.S. Integrated Ocean Observing System](https://ioos.noaa.gov/project/ocean-acidification/) describes how early warnings can help hatcheries adjust water intake or treatment. Such steps can reduce risk at a facility, though they cannot change chemistry across the open ocean.

## Cutting carbon dioxide limits the damage

Ocean acidification follows the amount of carbon dioxide entering the atmosphere. That gives the problem a clear main lever. Lower emissions slow the chemical changes that marine ecosystems must handle. Protecting coastal habitats such as seagrass beds, kelp forests and mangroves can help support local resilience, especially when paired with cleaner water and careful fisheries management.

Observation networks are also expanding what scientists can see. They measure pH, carbon dioxide, oxygen, temperature and other conditions in the same places over time. Those records help separate long-term change from short-lived weather and seasonal swings. NOAA's [National Ocean Service](https://oceanservice.noaa.gov/facts/acidification.html) explains that the process has been unfolding for more than two centuries as human activities raised atmospheric carbon dioxide.

Scientists still study why species respond differently. Some organisms can adjust parts of their internal chemistry, while others have less flexibility during sensitive stages of life. Field observations and laboratory experiments each add useful evidence. Together, they help researchers identify which waters, seasons and species need the closest attention. Those comparisons also help avoid treating every organism or every coast as though it will respond in exactly the same way. That knowledge can guide monitoring and local decisions while emissions reduction addresses the global driver.

The sea has absorbed a large share of humanity's carbon dioxide emissions. That has moderated climate warming in the air while placing a chemical burden on ocean life. Understanding **seawater chemistry** makes the stakes easier to see: less available carbonate can make shell-building harder and the consequences can move through ecosystems. Reducing carbon dioxide emissions offers the broadest way to limit the change.