Ocean Acidification and India’s Coasts Explained

Ocean acidification is the gradual fall in the pH of seawater caused by the ocean absorbing carbon dioxide from the atmosphere, and it is a slow but serious threat to shell-forming animals, coral reefs and fisheries along India’s long coastline in the Arabian Sea, the Bay of Bengal and the Indian Ocean. The ocean is not turning into acid; it remains slightly alkaline. But even a small drop in pH changes the chemistry that corals, oysters, mussels, clams and tiny plankton depend on to build their shells and skeletons.

This explainer (last updated 4 November 2026) explains the chemistry in plain language, describes what is known about acidification in the seas around India, and looks at risks for reefs, fisheries and mollusc and pearl culture. The numbers are given in rounded form because ocean chemistry varies by place, depth and season, and Indian monitoring is still developing.

Key Takeaways

• Ocean acidification happens because seawater absorbs roughly a quarter of the carbon dioxide people emit, forming carbonic acid that lowers pH and removes carbonate ions that shell-builders need.
• Average ocean surface pH has dropped by about 0.1 units since pre-industrial times, which is roughly a 30 percent rise in acidity because the pH scale is logarithmic.
• Corals, oysters, mussels, clams, pearl oysters and calcifying plankton find it harder to build and keep shells and skeletons in more acidic water, and young larvae are often the most vulnerable.
• India’s seas have special features, including monsoon upwelling of carbon-rich deep water in the Arabian Sea and large freshwater inflow into the Bay of Bengal, which may make some coastal waters more vulnerable.
• Acidification interacts with warming and low oxygen, so reefs and fisheries face several stresses together rather than one at a time.
• Only deep cuts in global carbon dioxide emissions can stop acidification, but better monitoring, pollution control and protection of mangroves, seagrass and reefs can help Indian coasts cope.

What is ocean acidification?

Ocean acidification is the ongoing decrease in the pH of the world’s oceans, caused mainly by the uptake of carbon dioxide from the air, which makes seawater less alkaline and changes its carbonate chemistry. It is sometimes called the “other carbon dioxide problem”, because it arises from the same gas that warms the climate but works through chemistry rather than heat.

Seawater is naturally slightly alkaline, with a pH of around 8.1 to 8.2 at the surface today. A pH below 7 would count as acidic, and the ocean will not reach that point in any realistic scenario. The term “acidification” describes the direction of change, towards lower pH, rather than a crossing into acidity. Even so, the shift is fast by geological standards, and the last time the oceans changed this quickly was probably tens of millions of years ago, according to studies of ancient sediments.

The effect is easy to underestimate because the numbers look small. The pH scale is logarithmic, so a fall of one-tenth of a unit means about a 30 percent rise in hydrogen ions, which is a big change for organisms whose body chemistry has been tuned to stable conditions over millions of years.

How does carbon dioxide make the ocean more acidic?

Carbon dioxide makes the ocean more acidic because when it dissolves in seawater it reacts with water to form carbonic acid, which releases hydrogen ions and lowers the pH. This is the same basic reaction that gives fizzy drinks their slight sourness.

The chemistry has several steps. Dissolved carbon dioxide combines with water to make carbonic acid. The carbonic acid splits into hydrogen ions and bicarbonate. The extra hydrogen ions then grab carbonate ions, which are the building blocks for calcium carbonate, and convert them into more bicarbonate. The net result is that pH falls and the supply of carbonate ions shrinks, which lowers what chemists call the saturation state of the minerals aragonite and calcite that organisms use to make shells and skeletons.

The ocean has a natural buffering system that softens the change, and it is the reason the sea has absorbed so much carbon dioxide without turning sour more quickly. That buffer, however, is gradually used up as more carbon dioxide dissolves, and colder and fresher waters have less capacity. The ocean also takes up carbon dioxide over decades and centuries, so the pH will keep falling for some time even if emissions were halted.

Which marine animals are most affected?

The marine animals most affected are those that build shells or skeletons from calcium carbonate, including corals, oysters, mussels, clams, sea snails, sea urchins and many tiny plankton. For these animals, lower carbonate availability means they must spend more energy to build and maintain their hard parts, and in extreme cases the shells begin to dissolve.

Young stages are often more vulnerable than adults. Oyster and mussel larvae, which form their first shells within a day or two of hatching, have shown high mortality and deformed shells in experiments with acidified water. Pteropods, small free-swimming sea snails sometimes called sea butterflies, are an important food for fish, and their thin shells show damage in acidified waters. Coccolithophores and foraminifera, microscopic plankton with chalk plates, are another group at risk, although some species appear to tolerate more.

The effects are not only about shells. Laboratory studies suggest that acidified water can disturb the behaviour of some fish, including their sense of smell and ability to detect predators, and can raise the energy cost of regulating body chemistry. Scientists debate how widely these findings apply in the wild, and results vary from species to species, so the long-term effect on whole food webs remains uncertain.

What do we know about acidification in India’s seas?

Studies of the seas around India indicate that acidification is under way there too, but monitoring is thinner than in many other regions, and the Arabian Sea, Bay of Bengal and coral lagoons each behave differently. Indian ocean scientists at institutions such as the CSIR-National Institute of Oceanography in Goa, the National Centre for Polar and Ocean Research, INCOIS and various universities have been measuring carbon chemistry along the coast and in the open sea.

The Arabian Sea has strong seasonal swings. During the south-west monsoon, winds push surface waters offshore and draw up deep water that is rich in nutrients and carbon dioxide but poor in oxygen, particularly along the south-western coast. Such upwelled water is naturally lower in pH and carbonate saturation, so organisms there already live near the edge of their comfort zone in some seasons. Added to the human carbon dioxide signal, upwelling episodes can push local conditions into stressful territory.

The Bay of Bengal gets large amounts of freshwater from the Ganga, Brahmaputra, Irrawaddy and peninsular rivers, which lowers the surface water’s alkalinity and makes it less able to buffer added carbon dioxide. Research suggests that this layer of fresh, lightly buffered water can make parts of the Bay more sensitive to acidification than saltier seas, although local studies are still limited. River water carrying acids from soils, nutrients and organic matter can add further complications near the coast and in estuaries.

How does acidification threaten India’s coral reefs?

Acidification threatens India’s coral reefs by reducing the carbonate that corals need to build their skeletons, which slows reef growth and makes reef structures weaker and easier to erode. Reefs grow only when the rate at which corals and other organisms build limestone exceeds the rate at which it is eroded or dissolved.

In more acidic water, corals use more energy to build the same amount of skeleton, and the calcifying algae that cement the reef together also struggle. Studies elsewhere suggest that coral calcification has already slowed in some regions, and models indicate that a number of tropical reefs could start to see net erosion later this century under high-emission scenarios. In India, the atoll reefs of Lakshadweep, the fringing reefs of the Andaman and Nicobar Islands, and the Gulf of Mannar and Gulf of Kutch reefs are all calcifying communities that could be affected, though measurements on them are still limited.

The danger is compounded by warming. A coral stressed by heat bleaches more readily if it is also coping with more acidic water, and a bleached reef recovers more slowly if its skeleton-building ability is already reduced. This is why scientists see ocean warming and acidification as a double hit rather than two separate problems, and why the explainer on coral bleaching and this one belong together.

Will acidification harm Indian fisheries, mollusc farming and pearl culture?

Acidification could harm Indian fisheries and aquaculture, particularly mollusc farming and pearl culture, because the animals involved are shell-builders, though the real-world impact depends on local water chemistry and is still being studied. India has a large marine fishery that supports millions of coastal people, and a growing mariculture sector.

Mussel and oyster farming is practised in Kerala, Karnataka, Maharashtra and along parts of the east coast, with green mussel farming in the backwaters and estuaries of north Kerala well known. Clam fisheries in lagoons such as Ashtamudi and Vembanad support small communities, and edible oyster culture has expanded as an income source for coastal women’s groups in several states. The Gulf of Mannar has a long history of pearl oyster fisheries, and modern pearl culture has been developed there using the pearl oyster Pinctada. All of these rely on animals that must form calcium carbonate shells, and early-life stages in hatcheries could be sensitive to low-pH water, particularly during monsoon upwelling or heavy river discharge.

Fish are affected more indirectly. Many commercial species, including oil sardine and mackerel in the Arabian Sea, depend on plankton food webs that include shelled plankton, and changes in the base of the food chain could ripple upward. Acidification, warming and shrinking oxygen levels combine in the sea, and fishing communities may feel the effects as shifts in where and when fish are found. Present evidence on fish stocks is still limited, so scientists mostly describe these as risks rather than established losses.

How is ocean acidification monitored in India?

Ocean acidification in India is monitored through research cruises, coastal time-series stations, moored sensors and laboratory analysis, mostly led by institutions under the Ministry of Earth Sciences and by universities. The goal is to build a record long enough to separate human-driven change from natural seasonal and year-to-year variation.

Scientists measure a set of linked variables, including pH, dissolved inorganic carbon, total alkalinity and the partial pressure of carbon dioxide in seawater. Ship-based surveys cover the Arabian Sea, Bay of Bengal and Indian Ocean basin, while time-series stations near the coast, such as one off Goa, track seasonal cycles. India also takes part in international efforts such as the Global Ocean Acidification Observing Network, and Sustainable Development Goal 14 includes a target to minimise and address ocean acidification, supported by shared methods and data.

The national Deep Ocean Mission and other ocean-science programmes have expanded observation capacity. Gaps remain, notably along the shallow coast, in estuaries, lagoons and reef flats where aquaculture and reefs are located and where chemistry swings daily because of tides, rain and biological activity. More sensors, standard methods and open data would help fisheries and aquaculture planners understand local risk.

How is acidification linked to warming and low oxygen?

Acidification is linked to warming and low oxygen because all three stem from the same excess of greenhouse gases and the same changes in ocean circulation, and together they form what scientists call the ocean’s “triple threat”. Warm water holds less dissolved oxygen and also less carbon dioxide, which speeds up carbon build-up in the air.

In the Arabian Sea and Bay of Bengal, natural oxygen-poor layers lie at mid-depth. Warming increases stratification, which can reduce the mixing of oxygen into deeper water, while more respiration of organic matter releases extra carbon dioxide, lowering pH in the same layers. The same water that is low in oxygen is often also high in carbon dioxide, so upwelling events in the Arabian Sea can bring both stresses to the coast at once.

These overlapping stresses mean that planning for any single factor can underestimate the danger. A mussel farm, for example, could face heat, low oxygen and low pH during the same few weeks. Researchers therefore call for integrated monitoring that records temperature, oxygen, pH and carbonate chemistry together.

What can be done about ocean acidification?

The only lasting solution to ocean acidification is to cut carbon dioxide emissions sharply, since the problem is caused by the gas dissolving in the sea, but local steps can reduce the harm and help coasts adapt. Nothing that is done at sea can match the effect of reducing the source.

At the global scale, India’s shift toward solar, wind, hydro and nuclear power, along with efficiency and clean transport, reduces the carbon dioxide reaching the ocean. At the local scale, controlling nutrient pollution and sewage reduces the extra acidity produced when algal blooms decay near the coast. Protecting mangroves, seagrass meadows and salt marshes helps, because these plants absorb carbon dioxide and can locally raise the pH of nearby water during the day, although their effect is limited and varies with the season.

For aquaculture, hatcheries can monitor water and time larval production to avoid the lowest-pH periods, buffer incoming water, and select resilient strains. Researchers are also studying techniques to add alkalinity to seawater, but these remain experimental and carry uncertain side effects. Awareness among fishers, farmers and officials is another important piece, and the link between a smoky power station and a shrinking oyster shell is not one most coastal communities have yet been told about.

How do ocean acidification, warming and pollution compare?

Ocean acidification, warming and local pollution are different kinds of stress on marine life that often act together, and the table below compares their cause, mechanism and main Indian impact.

Bottom Line

Ocean acidification is a quiet, chemical consequence of the same carbon dioxide that is warming the planet. For India, whose coasts support enormous fisheries, island communities and reefs, it adds another pressure on top of heat and pollution. The science of how it will play out in the Arabian Sea, Bay of Bengal and reef lagoons is still developing, which is a reason to expand monitoring now, protect coastal ecosystems, and cut emissions that drive the change.

Frequently Asked Questions

Is the ocean actually becoming acidic?

Not in the strict sense. Seawater is still alkaline, with a pH above 7, but it is moving towards lower pH. The term acidification describes this shift, which is already large enough to affect shell-building life.

How much has ocean pH changed?

Average surface ocean pH has fallen by about 0.1 units, from roughly 8.2 to about 8.1, since the start of the industrial era. Because the pH scale is logarithmic, that corresponds to roughly a 30 percent increase in acidity.

Why are shell-forming animals so sensitive to acidification?

Their shells are made of calcium carbonate, which needs carbonate ions from seawater. Acidification reduces the available carbonate and can make shells thinner, slower to form or prone to dissolving, especially in young larvae.

Does acidification affect pearl farming in India?

It could. Pearl oysters build shells and pearls from calcium carbonate, so low-pH water may stress larvae and growing oysters, particularly in hatcheries. Monitoring and careful timing of hatchery production can reduce the risk.

Is the Bay of Bengal or the Arabian Sea more at risk?

Both face risks for different reasons. The Arabian Sea has seasonal upwelling of carbon-rich water, while the Bay of Bengal has a freshwater layer that buffers poorly. Research is still ongoing, so neither can be called safe.

Can anything be done locally to slow acidification?

Local measures cannot stop global acidification, but reducing nutrient and sewage pollution, protecting mangroves and seagrass, and managing hatcheries can lessen the harm. The lasting fix is to cut carbon dioxide emissions.

{“@context”:”https://schema.org”,”@graph”:[{“@type”:”FAQPage”,”mainEntity”:[{“@type”:”Question”,”name”:”Is the ocean actually becoming acidic?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”Not in the strict sense. Seawater is still alkaline, with a pH above 7, but it is moving towards lower pH. The term acidification describes this shift, which is already large enough to affect shell-building life.”}},{“@type”:”Question”,”name”:”How much has ocean pH changed?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”Average surface ocean pH has fallen by about 0.1 units, from roughly 8.2 to about 8.1, since the start of the industrial era. Because the pH scale is logarithmic, that corresponds to roughly a 30 percent increase in acidity.”}},{“@type”:”Question”,”name”:”Why are shell-forming animals so sensitive to acidification?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”Their shells are made of calcium carbonate, which needs carbonate ions from seawater. Acidification reduces the available carbonate and can make shells thinner, slower to form or prone to dissolving, especially in young larvae.”}},{“@type”:”Question”,”name”:”Does acidification affect pearl farming in India?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”It could. Pearl oysters build shells and pearls from calcium carbonate, so low-pH water may stress larvae and growing oysters, particularly in hatcheries. Monitoring and careful timing of hatchery production can reduce the risk.”}},{“@type”:”Question”,”name”:”Is the Bay of Bengal or the Arabian Sea more at risk?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”Both face risks for different reasons. The Arabian Sea has seasonal upwelling of carbon-rich water, while the Bay of Bengal has a freshwater layer that buffers poorly. Research is still ongoing, so neither can be called safe.”}},{“@type”:”Question”,”name”:”Can anything be done locally to slow acidification?”,”acceptedAnswer”:{“@type”:”Answer”,”text”:”Local measures cannot stop global acidification, but reducing nutrient and sewage pollution, protecting mangroves and seagrass, and managing hatcheries can lessen the harm. The lasting fix is to cut carbon dioxide emissions.”}}]}]}