Ocean acidification is the ongoing decrease in the pH of seawater caused primarily by the ocean absorbing carbon dioxide (CO₂) from the atmosphere. When carbon dioxide dissolves in seawater, it undergoes chemical reactions that increase the concentration of hydrogen ions, making the water more acidic. Although the ocean remains slightly alkaline rather than acidic in the everyday sense, this change in its chemistry can make it harder for many marine organisms to build and maintain their shells and skeletons.
The process is closely linked to human activities, particularly the burning of coal, oil, and natural gas, as well as deforestation and other sources of carbon dioxide emissions. Because the ocean absorbs a substantial portion of the carbon dioxide released into the atmosphere, it helps slow the pace of climate change. However, that benefit comes with a chemical cost for marine ecosystems.
Ocean acidification affects organisms in different ways, from microscopic plankton to oysters, corals, sea urchins, and some fish. Its consequences can extend beyond individual species, influencing food webs, fisheries, coastal economies, and the health of marine habitats.
What causes ocean acidification?
The primary cause of modern ocean acidification is the rising concentration of atmospheric carbon dioxide produced by human activities. Carbon dioxide enters the atmosphere when people burn fossil fuels for electricity, transportation, heating, and industrial production. Deforestation also contributes by releasing stored carbon and reducing the amount of carbon dioxide that plants can absorb.
The ocean exchanges carbon dioxide with the atmosphere across its surface. When atmospheric concentrations rise, more carbon dioxide tends to dissolve into seawater. Ocean currents, temperature, winds, and biological activity influence how quickly and where this exchange occurs.
The ocean acts as an important carbon sink, meaning it absorbs more carbon from the atmosphere than it releases over a given period. This process limits the accumulation of carbon dioxide in the air, but it also changes seawater chemistry. The additional carbon does not simply remain dissolved as an unchanged gas. It reacts with water and other substances, shifting the balance among several forms of dissolved inorganic carbon.
Ocean acidification is distinct from ocean warming, even though both are driven in large part by rising atmospheric carbon dioxide. Warming occurs because greenhouse gases trap heat in Earth’s climate system. Acidification occurs because carbon dioxide changes the chemical balance of seawater. The two processes can interact, placing additional stress on marine organisms and ecosystems.
Natural processes also influence ocean acidity. Respiration by marine organisms, decomposition of organic matter, freshwater runoff, and volcanic activity can all affect local or regional water chemistry. These processes may produce substantial variations over short periods or in particular locations. However, the long-term global trend in ocean acidification is primarily driven by the ocean’s absorption of human-produced carbon dioxide.
The chemistry behind ocean acidification
To understand ocean acidification, it helps to follow what happens when carbon dioxide enters seawater.
First, carbon dioxide from the atmosphere dissolves in water. Some of it reacts with water to form carbonic acid, a weak acid. Carbonic acid can then release hydrogen ions, producing bicarbonate ions. Some bicarbonate can release another hydrogen ion, forming carbonate ions.
These reactions can be represented in simplified form:
CO₂ + H₂O ⇌ H₂CO₃
H₂CO₃ ⇌ H⁺ + HCO₃⁻
HCO₃⁻ ⇌ H⁺ + CO₃²⁻
Here, H₂CO₃ represents carbonic acid, H⁺ represents a hydrogen ion, HCO₃⁻ is a bicarbonate ion, and CO₃²⁻ is a carbonate ion. The double arrows indicate that these reactions can proceed in either direction, depending on the conditions in the water.
As more carbon dioxide enters the ocean, the chemical balance shifts. The concentration of hydrogen ions increases, while the availability of carbonate ions generally decreases. This change in carbonate chemistry is especially important for organisms that use calcium carbonate to construct shells, skeletons, or other hard structures.
The ocean’s chemistry naturally resists sudden changes in acidity through a process called buffering. Dissolved substances, especially bicarbonate and carbonate, help absorb or release hydrogen ions as conditions change. This buffering capacity moderates pH changes, but it does not prevent acidification. As additional carbon dioxide enters the ocean, the carbonate system shifts and the water’s pH declines.
What does pH tell us about seawater?
The pH scale measures how acidic or alkaline a solution is based on its hydrogen ion concentration. Lower pH values indicate greater acidity, while higher values indicate greater alkalinity. A pH of 7 is neutral under standard reference conditions, and seawater at the ocean surface is generally slightly alkaline, with a typical pH near 8.1 before accounting for local and seasonal variation.
Ocean acidification does not mean that the entire ocean has become acidic in the everyday sense. Instead, it means that seawater is becoming less alkaline than it was before.
The pH scale is logarithmic rather than linear. A decrease of one pH unit corresponds to a tenfold increase in hydrogen ion concentration. Even a seemingly small decrease can therefore represent a meaningful chemical change. For example, a drop of 0.1 pH unit corresponds to roughly a 26% increase in hydrogen ion concentration.
The magnitude of the change matters, but pH alone does not describe every aspect of the ocean’s chemical environment. Carbonate ion concentration, dissolved carbon dioxide, alkalinity, temperature, salinity, and pressure can all influence the conditions marine organisms experience.
The ocean’s pH also varies naturally. Photosynthesis removes carbon dioxide from seawater, often raising pH during periods of strong biological activity. Respiration and decomposition release carbon dioxide, often lowering pH. These changes occur over daily and seasonal cycles, and coastal waters may experience much larger fluctuations than the open ocean.
Human-driven acidification shifts the background conditions against which these natural variations occur. In some places, organisms that already experience periods of low pH may face more frequent or more intense exposure to unfavorable conditions.
Why carbonate chemistry matters to marine life
Many marine organisms build hard structures from calcium carbonate, a mineral found in shells, coral skeletons, and certain other biological materials. To form calcium carbonate, organisms need calcium ions and carbonate ions in seawater.
When seawater absorbs additional carbon dioxide, carbonate ions become less available because some are converted into bicarbonate. As a result, organisms may have to expend more energy to build or maintain calcium carbonate structures.
The relevant chemical balance is often described by the saturation state of calcium carbonate. Saturation state indicates whether seawater favors the formation and persistence of a particular calcium carbonate mineral or its dissolution.
When seawater is supersaturated with respect to a mineral, conditions generally favor its formation or preservation. When it is undersaturated, the mineral is more likely to dissolve. Two important forms of calcium carbonate in marine organisms are aragonite and calcite. Aragonite is more soluble than calcite under typical ocean conditions, making aragonite-forming organisms particularly sensitive to changes in carbonate chemistry.
Many surface ocean environments remain supersaturated with respect to these minerals, but their saturation states have declined as ocean acidification has progressed. In some naturally carbon dioxide-rich waters, including certain coastal habitats and deeper ocean environments, conditions may already be corrosive to particular calcium carbonate structures.
Organisms can sometimes regulate the chemistry around their shells or skeletons, protecting these structures even when surrounding water conditions become less favorable. However, that regulation requires energy and may become more difficult under sustained chemical stress.
Not all marine life depends on calcium carbonate, and not all organisms respond in the same way. Some species may tolerate changing conditions relatively well, while others experience slower growth, reduced survival, or difficulties reproducing. The effects depend on the organism’s biology, life stage, exposure, and ability to adjust to its environment.
How ocean acidification affects corals and coral reefs
Corals are among the best-known organisms affected by ocean acidification. Reef-building corals are animals that produce calcium carbonate skeletons, creating the complex structures that support coral reef ecosystems.
These skeletons provide the physical foundation of a reef. They create shelter, feeding areas, and breeding habitats for a wide variety of fish, invertebrates, and other marine organisms.
As seawater carbonate ion concentrations decline, many reef-building corals find it harder to produce their skeletons. Their calcification rates may decrease, depending on the species and environmental conditions. At the same time, chemical conditions can make existing calcium carbonate structures more vulnerable to dissolution, particularly when saturation states become low.
Over time, reduced skeletal growth can affect the ability of reefs to maintain their structure. If the combined effects of erosion, biological breakdown, and physical damage exceed new skeletal growth, a reef may lose structural complexity.
Ocean acidification does not act alone. Coral reefs are also threatened by marine heat waves, coral bleaching, pollution, sediment runoff, overfishing, and habitat destruction. Elevated temperatures can cause corals to lose the symbiotic algae that supply much of their energy, a process known as coral bleaching. Acidification and warming can occur together, compounding the challenges faced by some reef organisms.
The response of coral reefs varies across regions and species. Some corals can maintain growth under moderately changing conditions, while others are more sensitive. Local water chemistry, food availability, temperature, and ecological relationships all influence the outcome. Nevertheless, sustained ocean acidification creates an additional obstacle to reef growth and recovery.
Effects on shellfish and other shell-forming animals
Oysters, mussels, clams, and many other mollusks use calcium carbonate to build their shells. Ocean acidification can make shell formation more difficult, particularly during early development when larvae must construct their first protective structures.
In some species, exposure to unfavorable carbonate chemistry is associated with slower shell growth, thinner shells, abnormal development, or lower survival. The severity of these effects depends on the species, the duration and intensity of exposure, and other environmental conditions.
Young shellfish are often especially vulnerable because they have small energy reserves and must build essential structures rapidly. If the surrounding water has low carbonate availability, larvae may need to devote more energy to shell formation and maintenance, leaving less energy for growth and other biological functions.
These effects have practical consequences for aquaculture, the farming of aquatic organisms for food. Shellfish hatcheries depend on suitable water chemistry to produce healthy larvae. When coastal waters contain elevated levels of carbon dioxide or experience naturally low pH, hatchery production can become more difficult or unpredictable.
Some hatcheries respond by monitoring seawater chemistry and adjusting the water used for larval rearing. Such measures can reduce immediate exposure, but they do not eliminate the broader environmental causes of acidification.
Other shell-forming organisms, including some sea urchins and marine snails, can also be affected. The details differ among species, and some animals can compensate for changing conditions through physiological adjustments. Still, reduced growth, increased energy demands, and greater vulnerability during early development can create disadvantages that accumulate over time.
Effects on plankton and the marine food web
Ocean acidification can affect marine organisms that are difficult to see without a microscope. Plankton includes a wide range of small organisms that drift in the water, including photosynthetic organisms called phytoplankton and animal plankton called zooplankton.
Some planktonic species produce calcium carbonate structures. Pteropods, small swimming sea snails sometimes called sea butterflies, are one example. Their shells can be vulnerable to dissolution when carbonate saturation is low. Because pteropods serve as food for fish and other predators, changes in their abundance or condition can affect organisms higher in the food web.
Other plankton species do not make calcium carbonate shells. Their responses to acidification vary, with some showing changes in growth, reproduction, metabolism, or other biological processes and others exhibiting relatively limited effects under particular conditions.
Phytoplankton also respond to changes in carbon dioxide availability, but the effects are not uniformly harmful or beneficial. Carbon dioxide is essential for photosynthesis, and some photosynthetic organisms may benefit from increased availability under certain conditions. However, their responses depend on factors such as nutrient supply, light, temperature, and interactions with other species.
The overall effects on food webs are therefore complex. If acidification reduces the survival or abundance of particular plankton species, predators that rely on them may face changes in food availability. Shifts in plankton communities can also influence competition among species and alter the movement of carbon through marine ecosystems.
These changes matter because plankton form the foundation of many ocean food webs. They support commercially important fish, marine mammals, seabirds, and other animals, while also contributing to the ocean’s role in the global carbon cycle.
How fish and other marine animals respond
Fish do not generally build calcium carbonate shells, so their responses to ocean acidification differ from those of corals and shellfish. Nevertheless, changing seawater chemistry can affect fish physiology, development, and behavior.
Fish regulate the balance of acids and bases within their bodies, a process known as acid-base regulation. As carbon dioxide levels rise in seawater, some fish must adjust internal chemical conditions to maintain normal cellular function. These adjustments can affect other physiological processes, depending on the species and the level of exposure.
Research has found that elevated carbon dioxide can influence sensory systems, behavior, growth, and development in some marine animals. However, responses vary substantially among species, life stages, and experimental conditions. Some early studies reported dramatic behavioral changes in certain fish exposed to high carbon dioxide levels, but subsequent research has produced mixed results, and the extent to which some effects occur under realistic ocean conditions remains an area of scientific investigation.
Acidification may also affect fish indirectly. Changes in plankton communities can alter food supplies, while damage to coral reefs can reduce the availability of shelter and feeding habitats. A fish species may therefore experience consequences even if it is not especially sensitive to the direct chemical effects of acidification.
Other marine animals, including crustaceans such as crabs and shrimp, also show varied responses. Some can regulate internal chemistry or compensate for changes in shell formation, while others may face greater energetic demands or reduced performance. Their vulnerability depends on their biology, environmental exposure, and access to food and other resources.
Why some regions are more vulnerable than others
Ocean acidification does not proceed at the same rate everywhere. The chemistry of seawater is shaped by water temperature, circulation, biological activity, freshwater inputs, and the movement of carbon between the ocean and atmosphere.
Cold water can generally dissolve more carbon dioxide than warm water. As a result, some high-latitude regions are particularly vulnerable to acidification because their waters can absorb substantial amounts of carbon dioxide and may have relatively low carbonate saturation states.
Deep ocean waters can also contain elevated concentrations of dissolved carbon dioxide. As organic matter sinks and decomposes, respiration releases carbon dioxide into the surrounding water. Because deep water may remain isolated from the atmosphere for long periods, it can accumulate dissolved carbon and develop lower pH and carbonate saturation.
Coastal waters present a different set of challenges. Rivers can carry nutrients and organic matter into estuaries and coastal ecosystems. When excess nutrients stimulate biological growth, the subsequent decomposition of that organic matter can release carbon dioxide and lower pH. This process, often associated with nutrient pollution and eutrophication, can intensify the effects of ocean acidification.
Upwelling also influences regional vulnerability. In upwelling systems, winds and currents bring deeper water toward the surface. This water may naturally contain more dissolved carbon dioxide and fewer carbonate ions than surface water that has recently exchanged gases with the atmosphere. When upwelling brings this water into coastal habitats, it can expose marine organisms to challenging conditions.
These local and regional processes can combine with global acidification, producing large variations in exposure. For that reason, understanding the risks to a particular ecosystem requires more than knowing the global average pH. Scientists also examine seasonal fluctuations, carbonate saturation, temperature, oxygen levels, and the duration of exposure to unfavorable conditions.
The difference between ocean acidification and ocean warming
Ocean acidification and ocean warming are closely related consequences of rising atmospheric carbon dioxide, but they operate through different physical and chemical mechanisms.
Ocean warming occurs because greenhouse gases reduce the rate at which Earth loses heat to space. The resulting increase in ocean temperature can alter species distributions, increase the risk of coral bleaching, reduce oxygen solubility, and change ocean circulation and stratification.
Ocean acidification occurs because carbon dioxide dissolves in seawater and changes the balance of dissolved carbon compounds. Its effects are particularly pronounced for organisms that depend on calcium carbonate, although other marine animals can also be affected.
The two processes can reinforce one another. Higher temperatures increase the metabolic demands of many organisms, potentially raising their need for energy at the same time that acidification makes some biological functions more costly. Warming can also change the chemical balance of seawater, while shifts in ocean circulation and stratification influence the distribution of carbon and oxygen.
However, the interactions are not identical for every species or ecosystem. Some organisms may be more strongly affected by warming, others by acidification, and still others by the combination of multiple stressors. Pollution, habitat destruction, and overfishing can add further pressure.
Distinguishing between these processes helps scientists identify the mechanisms behind observed ecological changes and develop more effective conservation and management strategies.
How scientists measure and monitor ocean acidification
Scientists monitor ocean acidification by measuring seawater properties that describe its chemical condition. These include pH, dissolved inorganic carbon, total alkalinity, and the partial pressure of carbon dioxide in seawater. Together, these measurements provide a more complete picture than any single value can offer.
pH measurements reveal changes in hydrogen ion concentration, while dissolved inorganic carbon describes the total amount of carbon present in dissolved forms such as carbon dioxide, bicarbonate, and carbonate. Total alkalinity measures seawater’s capacity to neutralize added acid, providing important information about its buffering chemistry.
Scientists collect measurements from research vessels, coastal monitoring stations, autonomous instruments, and other ocean-observing systems. Long-term records are especially important because natural variations can be substantial. Measurements taken over days or weeks may capture seasonal cycles without clearly revealing the underlying long-term trend.
Laboratory experiments help researchers determine how individual species respond to different levels of carbon dioxide, pH, and carbonate saturation. By comparing organisms exposed to different conditions, scientists can investigate effects on shell formation, growth, reproduction, metabolism, and survival.
Field observations complement these experiments by revealing how organisms respond in real ecosystems, where temperature, food availability, predators, competition, and other environmental factors interact. Neither approach is sufficient on its own: laboratory studies help isolate mechanisms, while field studies show how those mechanisms operate in complex natural settings.
Scientists also use models to project how ocean chemistry may change under different future carbon dioxide emissions pathways. Such projections depend on assumptions about future emissions, ocean circulation, biological processes, and other factors. Although the precise ecological consequences remain uncertain in many cases, the underlying chemical relationship between carbon dioxide absorption and declining carbonate availability is well established.
Can ocean acidification be reversed or reduced?
The most effective way to limit human-driven ocean acidification is to reduce carbon dioxide emissions. Because fossil fuel combustion is a major source of atmospheric carbon dioxide, shifting toward low-carbon energy, improving energy efficiency, and reducing emissions from transportation, industry, and electricity generation can help slow the accumulation of carbon in the ocean.
Protecting and restoring forests, wetlands, and other carbon-storing ecosystems can also contribute to climate mitigation. However, these measures cannot substitute for substantial reductions in fossil fuel emissions if the goal is to limit long-term changes in ocean chemistry.
The ocean responds to changes in atmospheric carbon dioxide over different timescales. Reducing emissions would slow the rate of acidification, but it would not immediately restore seawater to its earlier chemical state. Much of the carbon already absorbed by the ocean remains in the climate system, and its redistribution and removal occur over long periods.
Local measures can reduce additional stress on marine ecosystems. Improving wastewater treatment, limiting nutrient runoff, managing fisheries sustainably, protecting habitats, and reducing sediment and chemical pollution can strengthen ecosystem resilience. These actions do not directly eliminate the global carbon dioxide burden, but they can help marine organisms cope with changing conditions.
Some proposed approaches seek to alter seawater chemistry directly, including adding alkaline materials to increase the ocean’s capacity to neutralize acidity. Such approaches are being investigated, but their effectiveness, environmental risks, scalability, energy requirements, and potential consequences for marine ecosystems need careful evaluation. They are not a substitute for reducing the underlying source of excess carbon dioxide.
Marine conservation and climate mitigation therefore address different parts of the problem. Reducing emissions limits the chemical disturbance, while local environmental protections can reduce other pressures and help ecosystems retain their ability to adapt.
What ocean acidification means for the future of marine ecosystems
Ocean acidification is a gradual but consequential change in the chemistry of the ocean. It occurs as seawater absorbs human-produced carbon dioxide, increasing hydrogen ion concentrations and reducing the availability of carbonate ions. These changes can make it more difficult for many organisms to build calcium carbonate shells and skeletons, while also affecting other biological processes in species that do not produce hard structures.
The consequences extend beyond individual animals. Coral reefs may struggle to build and maintain their physical structures, shellfish populations may face challenges during early development, and changes in plankton communities can affect food webs that support marine life and human fisheries.
The extent of future damage is not predetermined. Species differ in their sensitivity, ecosystems vary in their exposure, and some organisms can acclimate or adapt to changing conditions. Yet these differences do not remove the underlying chemical pressure created by rising carbon dioxide concentrations.
Understanding ocean acidification requires recognizing both its global cause and its local complexity. Carbon dioxide emissions drive the broad trend, while regional conditions determine how that trend interacts with marine organisms and ecosystems. Reducing emissions, monitoring ocean chemistry, and limiting additional environmental stresses offer complementary ways to reduce risks.
The central scientific lesson is that the ocean’s role in absorbing carbon dioxide comes with consequences. Protecting marine ecosystems in a world with rising atmospheric carbon dioxide requires not only responding to warmer water, but also addressing the less visible chemical changes taking place throughout the ocean.
