Ocean acidification is the ongoing decrease in the pH of seawater caused primarily by the ocean absorbing carbon dioxide from the atmosphere. As carbon dioxide dissolves in seawater, it triggers chemical reactions that increase the concentration of hydrogen ions and reduce the availability of carbonate, a substance many marine organisms need to build shells and skeletons.
The process does not mean that the ocean is becoming acidic in the everyday sense of turning into something like vinegar. Seawater remains slightly alkaline under normal open-ocean conditions. Instead, ocean acidification describes a shift toward lower pH that changes the chemical environment in which marine life grows, feeds, reproduces, and survives.
The consequences extend from microscopic plankton to coral reefs, shellfish, and the larger food webs that support fisheries and coastal communities. Understanding ocean acidification requires looking at how carbon moves between the atmosphere and ocean, how seawater chemistry responds, and why even a relatively small change in pH can matter to marine ecosystems.
What causes ocean acidification?
The main cause of modern ocean acidification is the increase in atmospheric carbon dioxide produced by human activities. Burning coal, oil, and natural gas releases carbon that was stored underground for millions of years. Deforestation and other land-use changes also contribute by releasing carbon dioxide and reducing the amount of carbon that vegetation can absorb.
The ocean acts as a major reservoir for this excess carbon. Carbon dioxide continually moves between the atmosphere and the ocean surface, with the direction and rate of exchange influenced by factors such as temperature, wind, and the difference in carbon dioxide concentrations between air and water. As atmospheric carbon dioxide rises, more of it enters the ocean.
This absorption helps slow the accumulation of carbon dioxide in the atmosphere, limiting some of the warming that would otherwise occur. However, it also changes seawater chemistry. The ocean cannot absorb additional carbon dioxide without affecting the balance among dissolved carbon compounds and the minerals that marine organisms use to construct shells and skeletons.
Natural processes also influence ocean acidity. Respiration by marine organisms releases carbon dioxide into seawater, while the decomposition of dead organisms adds more. Upwelling, the movement of deep water toward the surface, can bring carbon-rich water into coastal regions. Freshwater runoff, changes in nutrient availability, and biological activity can further alter local conditions.
These natural influences can cause substantial differences in ocean chemistry from one location to another. Nevertheless, the widespread, long-term decline in surface-ocean pH is primarily the result of the ocean absorbing human-produced carbon dioxide.
How does ocean acidification work chemically?
The chemistry begins when carbon dioxide from the atmosphere dissolves in seawater. Once in the water, some of the dissolved carbon dioxide reacts with water molecules to form carbonic acid. That weak acid can release hydrogen ions, which lower pH.
The process also changes how dissolved inorganic carbon is distributed among three main forms: dissolved carbon dioxide, bicarbonate ions, and carbonate ions. These forms exist in a dynamic chemical balance, and changes in one influence the others.
The overall process can be summarized in three steps:
- Atmospheric carbon dioxide dissolves in seawater.
- Dissolved carbon dioxide participates in reactions that increase hydrogen ion concentration.
- The resulting chemical shifts increase bicarbonate relative to carbonate and make carbonate less available for shell and skeleton formation.
Carbonic acid is only partly responsible for the chemistry. The key point is that the reactions redistribute carbon among different chemical forms while increasing hydrogen ion concentration.
Seawater contains many dissolved substances, including salts and minerals, that help buffer changes in pH. A buffer is a chemical system that resists sudden changes in acidity when acids or bases are added. The ocean’s carbonate system provides much of this buffering capacity, but it is not unlimited. As more carbon dioxide enters seawater, the system shifts toward a new balance rather than preventing the change altogether.
Why pH matters
The pH scale describes the concentration of hydrogen ions in a solution. Lower pH means a higher hydrogen ion concentration, while higher pH means a lower concentration. Because the scale is logarithmic, a change of one pH unit represents a tenfold change in hydrogen ion concentration.
This relationship makes even seemingly small pH changes important. For example, a decrease of 0.1 pH unit corresponds to an increase of roughly 26 percent in hydrogen ion concentration, assuming the comparison is made on the same pH scale and under comparable conditions.
The ocean’s average surface pH has fallen by about 0.1 unit since the beginning of the Industrial Revolution. Although that change may appear modest, it represents a meaningful shift in seawater chemistry across an enormous volume of water.
The decline is not identical everywhere. Ocean temperature, salinity, water circulation, and biological activity affect local pH, and coastal waters can experience much larger short-term fluctuations than the open ocean. Still, the long-term global trend reflects the accumulation of human-produced carbon dioxide.
Why carbonate availability is so important
Many marine organisms build shells, plates, or skeletons from calcium carbonate, a mineral formed from calcium and carbonate ions. Corals, oysters, mussels, clams, some plankton, and certain other marine organisms rely on this material to create protective or structural parts of their bodies.
As ocean acidification progresses, hydrogen ions react with carbonate ions to form bicarbonate. This reduces the concentration of carbonate available in seawater. The total amount of dissolved carbon can increase even as the amount of carbonate decreases.
That distinction is central to understanding ocean acidification. More carbon in the ocean does not necessarily mean more of the carbonate needed by shell-building organisms.
The difficulty of forming calcium carbonate depends partly on a chemical property called the saturation state. This describes how favorable the surrounding water is for the formation or maintenance of a particular calcium carbonate mineral.
When seawater is highly saturated with respect to a mineral, organisms generally have more favorable conditions for building structures from it. As saturation decreases, calcification can become more difficult or energetically costly. If water becomes undersaturated, existing calcium carbonate structures can dissolve more readily, although biological protection and other environmental factors influence the actual outcome.
Two common forms of calcium carbonate are aragonite and calcite. Aragonite is generally more soluble in seawater than calcite, making it particularly sensitive to changes in carbonate chemistry. Many corals and some plankton produce aragonite, so declining aragonite saturation can be especially important for them.
Ocean acidification does not automatically prevent every organism from making a shell. Many species regulate the chemistry inside the spaces where their shells or skeletons form. However, that regulation can require additional energy, and its effectiveness varies among species and environmental conditions.
How ocean acidification affects marine life
The effects of ocean acidification differ among organisms. Some are sensitive to changes in carbonate availability, while others experience changes in growth, metabolism, behavior, or reproduction. The consequences also depend on life stage, exposure duration, food supply, temperature, and other environmental conditions.
Corals and coral reefs
Corals build calcium carbonate skeletons that provide the foundation of coral reefs. These structures create habitat for a wide variety of fish and invertebrates, support complex food webs, and help reduce wave energy along some coastlines.
As carbonate availability declines, many reef-building corals face less favorable conditions for calcification. Some may build their skeletons more slowly, while others may experience changes in skeletal density or structure. The response varies among coral species and locations, and some corals can partly compensate for changing conditions.
Ocean acidification can also affect the balance between reef construction and erosion. Reefs grow when organisms produce calcium carbonate faster than physical, chemical, and biological processes remove it. If acidification reduces calcification or makes reef structures more vulnerable to dissolution, the balance can shift toward erosion.
Acidification is not the only major threat to coral reefs. Marine heat waves can cause coral bleaching, a condition in which corals lose or expel many of the microscopic algae that supply them with energy. Warming, pollution, overfishing, and habitat damage can compound the effects of acidification. These pressures interact, making the future of a reef depend on more than seawater pH alone.
Oysters, mussels, and other shellfish
Shell-forming animals can be particularly vulnerable during their earliest life stages. Oyster and mussel larvae must build shells as they develop, and unfavorable carbonate chemistry can make this process more difficult.
Some young shellfish exposed to low-pH conditions have shown slower shell formation, reduced growth, or lower survival. The severity of these effects varies by species, developmental stage, and local conditions.
Adult shellfish may also face challenges if maintaining or repairing their shells requires more energy. When organisms devote additional resources to basic physiological functions, fewer resources may remain for growth, reproduction, or defense against disease.
These changes matter beyond individual animals. Shellfish support commercial fisheries and aquaculture, provide food for predators, and help filter particles from the water. In regions where shellfish farming is economically important, acidification can create practical challenges for hatcheries and growers.
However, not every shellfish population responds in the same way. Some species or populations tolerate changing chemistry better than others, and food availability, temperature, and local water conditions can influence their ability to cope.
Plankton and the base of the food web
Plankton includes a diverse collection of organisms that drift in the ocean, from tiny photosynthetic cells to small animals. Many planktonic organisms form the base of marine food webs, supporting fish, seabirds, and marine mammals either directly or indirectly.
Some plankton species produce calcium carbonate structures. These include certain microscopic algae and small drifting animals whose shells or plates contribute to the ocean’s movement and storage of carbon. Acidification can interfere with shell formation in some of these organisms, potentially changing their growth, survival, and distribution.
Other plankton species do not make calcium carbonate shells and may respond differently. Some organisms could experience little direct harm under particular conditions, while others could benefit from changes in carbon availability. Even among related species, responses may differ.
These differences make the broader effects difficult to predict. If acidification changes the abundance or seasonal timing of key plankton species, the effects could spread through the food web. Predators may find less food, encounter prey at different times, or shift to other species.
The ecological consequences depend on how communities respond as a whole, not simply on whether an individual plankton species can survive in lower-pH water.
Fish and other marine animals
Fish do not generally build external calcium carbonate shells, but they can still be affected by changing ocean chemistry. Like other animals, fish must regulate the chemistry of their internal fluids to maintain normal physiological function.
Many fish can adjust to some changes in environmental conditions, including changes in carbon dioxide. However, the degree of adjustment varies among species and life stages. In some circumstances, elevated carbon dioxide has been associated with changes in growth, development, sensory function, or behavior.
Research on fish responses has produced variable findings, and some reported behavioral effects have been difficult to reproduce consistently. It would be inaccurate to assume that all fish experience the same effects or that a single behavioral response applies across marine species.
Other animals, including some crustaceans and echinoderms such as sea urchins, may experience changes in development, calcification, or energy use. Their responses depend on their biology and the conditions they encounter. Some species can compensate for changes in seawater chemistry, while others are less able to do so.
The wider concern is that even modest effects on individual organisms can become important when they alter reproduction, predator-prey relationships, or the structure of an ecosystem over time.
Why some ocean regions are more vulnerable than others
Ocean acidification is a global process, but its effects are not uniform. Differences in circulation, temperature, biological activity, and water chemistry influence how quickly local conditions change and how organisms experience those changes.
Cold water generally holds more dissolved carbon dioxide than warm water. As a result, cold regions can absorb substantial amounts of atmospheric carbon dioxide, and some high-latitude waters are particularly vulnerable to declines in carbonate saturation.
Coastal waters can face additional pressures. Rivers may deliver nutrients and organic matter that stimulate biological activity. When organisms respire or dead material decomposes, carbon dioxide enters the water. In areas with limited circulation, this carbon dioxide can accumulate, lowering pH and oxygen levels.
Upwelling also plays an important role in some coastal regions. Deep ocean water naturally contains elevated concentrations of dissolved carbon dioxide because organic matter sinks and decomposes below the surface. When that water rises toward the coast, it can bring naturally low-pH, carbonate-poor conditions into shallow habitats. The addition of human-produced carbon dioxide makes the underlying chemistry more vulnerable to further change.
Estuaries, where freshwater mixes with seawater, can be especially variable. River discharge, rainfall, tides, and seasonal biological activity can cause large changes in salinity and carbonate chemistry. Organisms living in these environments may experience rapid fluctuations rather than a steady decline in pH.
Local conditions can therefore amplify or temporarily mask the effects of the global trend. Understanding vulnerability requires examining not only average pH but also how often organisms encounter unfavorable conditions, how long those conditions last, and whether they have time and resources to recover.
How ocean acidification differs from ocean warming
Ocean acidification and ocean warming are closely connected because both are driven largely by rising atmospheric carbon dioxide, but they are distinct processes.
Ocean warming occurs when the buildup of greenhouse gases traps additional heat in Earth’s climate system. The ocean absorbs much of that excess heat, increasing water temperatures and changing circulation, oxygen levels, and the distribution of marine species.
Ocean acidification occurs when the ocean absorbs carbon dioxide and its chemistry changes. It reduces pH and carbonate availability, creating challenges for organisms that build calcium carbonate structures and potentially affecting other biological processes.
The two processes can reinforce one another in their effects on marine life. For example, corals exposed to unusually warm water may bleach, while acidification makes the formation and maintenance of their calcium carbonate skeletons more difficult. An organism already stressed by one condition may have fewer resources to cope with another.
Other stressors, including deoxygenation, pollution, habitat destruction, and overfishing, can add to these pressures. Their combined effects are not always simply additive; one stressor can intensify or sometimes alter an organism’s response to another.
Distinguishing the processes matters because their mechanisms differ. Reducing carbon dioxide emissions addresses both the main driver of ocean warming and the principal cause of modern ocean acidification, but protecting marine ecosystems also requires attention to local environmental pressures.
What ocean acidification means for people
Changes in marine ecosystems can affect people through food production, coastal economies, and the natural benefits that healthy ocean habitats provide.
Commercial fisheries and aquaculture depend on marine organisms completing their life cycles successfully. If acidification reduces the survival or growth of vulnerable shellfish, it can affect harvests, hatchery operations, and the communities that depend on them. Changes in plankton communities can also influence the food available to commercially important fish, although the consequences differ by region and species.
Coral reefs provide habitat for fisheries and tourism, and they can reduce wave energy along coastlines. If reefs lose their ability to maintain their structures, the benefits they provide may weaken over time. The extent of that risk depends on the combined effects of acidification, warming, reef erosion, and other local conditions.
Ocean acidification also illustrates how the ocean’s role as a carbon sink comes with a chemical cost. Absorbing carbon dioxide helps limit its buildup in the atmosphere, but the resulting changes in seawater can disrupt biological processes that have developed under different conditions.
These impacts are not distributed evenly. Communities that depend heavily on coral reefs, shellfish harvesting, or coastal fisheries may face greater economic and food-security risks than communities with more diverse sources of income and food.
Can ocean acidification be slowed or reduced?
The most effective way to slow human-driven ocean acidification is to reduce the amount of carbon dioxide entering the atmosphere. Because atmospheric carbon dioxide is the main driver of the long-term change, cutting emissions from fossil fuels and reducing carbon releases associated with land-use change can limit the additional carbon absorbed by the ocean.
The response will not be immediate. Carbon moves between the atmosphere, ocean surface, and deeper waters over different timescales, and some of the carbon already absorbed will continue to influence ocean chemistry for a long time. Even so, limiting future emissions can reduce the extent of further acidification.
Local measures can also improve conditions for marine ecosystems. Reducing nutrient pollution, managing wastewater, protecting habitats, and limiting destructive fishing practices can decrease other sources of stress. In some coastal waters, these measures can reduce locally intensified acidification or help organisms better withstand unfavorable conditions.
Such actions cannot fully counteract the global chemical changes caused by excess atmospheric carbon dioxide. They can, however, improve the resilience of ecosystems while broader emissions reductions address the underlying cause.
Scientists also monitor seawater pH, dissolved carbon, carbonate saturation, and biological responses to understand how conditions are changing. Long-term observations help distinguish natural fluctuations from persistent trends and identify regions or species that are especially vulnerable. Laboratory experiments and field studies provide additional evidence about how organisms respond to different combinations of temperature, acidity, food supply, and other environmental conditions.
There is no single response that will protect every marine species. Some organisms may adapt through changes in physiology or behavior, and populations can evolve over generations. Others may struggle to keep pace with rapidly changing conditions. The extent of these responses remains an important area of research, but adaptation does not eliminate the underlying chemical change.
Ocean acidification is ultimately a consequence of the ocean’s interaction with the atmosphere. As excess carbon dioxide enters seawater, it shifts a natural chemical balance, lowers pH, and reduces the availability of carbonate needed by many marine organisms. The biological consequences vary, but the underlying mechanism is well established. Reducing carbon dioxide emissions is essential to limiting further change, while protecting marine habitats and reducing local stressors can help ecosystems cope with the changes already underway.