What Is Actually Happening to the WaterWhen carbon dioxide dissolves into seawater, it reacts with water to form carbonic acid.
That acid releases hydrogen ions, gradually changing the ocean’s chemistry and making it less alkaline.
It is one of the fastest shifts in ocean chemistry known from the geological record, unfolding over just two centuries rather than thousands or millions of years.
The cost has been borne by the ocean’s chemistry and the marine life that depends on it remaining stable.
A more acidic ocean is a less efficient carbon sink.
Climate change discussions usually focus on what is happening above the water: rising temperatures, heatwaves, melting glaciers, sea-level rise. But roughly a quarter to a third of all the carbon dioxide humanity emits never reaches the atmosphere or remains there. It dissolves into the ocean. For decades, this has slowed the pace of global warming considerably — without the ocean acting as a vast carbon sink, temperatures would already be higher than they are. But nothing comes free. Every tonne of carbon the ocean absorbs changes its chemistry. That change is what ocean acidification measures, and in 2025, it became the seventh planetary boundary confirmed as crossed.
What Is Actually Happening to the Water
When carbon dioxide dissolves into seawater, it reacts with water to form carbonic acid. That acid releases hydrogen ions, gradually changing the ocean’s chemistry and making it less alkaline. The ocean is not becoming acidic in the everyday sense—it is still alkaline overall. But marine organisms evolved over millions of years in waters whose chemistry remained remarkably stable. Even relatively small changes can disrupt biological processes that depend on that stability, particularly for species that build shells and skeletons from calcium carbonate.
Since the Industrial Revolution, the average pH of the ocean’s surface has fallen by about 0.1 units. That may sound insignificant, but the pH scale is logarithmic, much like the Richter scale used to measure earthquakes. A decline of 0.1 means the ocean has become roughly 30 per cent more acidic. It is one of the fastest shifts in ocean chemistry known from the geological record, unfolding over just two centuries rather than thousands or millions of years.
Why Small Changes Have Large Effects
The first creatures to feel the effects are those that build shells and skeletons from calcium carbonate—corals, oysters, mussels, clams, sea urchins, and tiny free-swimming sea snails known as pteropods. They all rely on seawater that contains sufficient carbonate minerals to build and repair these protective structures. As the ocean absorbs more carbon dioxide, those minerals become less available. Shells grow more slowly, become thinner and weaker, and under more acidic conditions can even begin to dissolve.
Scientists track this using a measure called the aragonite saturation state, because aragonite is the form of calcium carbonate many marine organisms use to build their shells and skeletons. When the planetary boundaries framework was first proposed in 2009, researchers estimated that the ocean should not lose more than 20 per cent of its pre-industrial aragonite levels. New research published by the Plymouth Marine Laboratory in 2025 found that the safe limit had been reached sooner than previously thought. The latest Planetary Health Check confirmed that the ocean has now crossed this boundary, as updated evidence shows pre-industrial oceans contained more carbonate than earlier estimates had suggested.
The consequences do not stop with corals or shellfish. Pteropods, despite being only a few millimetres long, are an essential food source for salmon, mackerel, herring and even whales. Coral reefs, though they cover less than one per cent of the ocean floor, support around a quarter of all marine species at some stage of their lives. They also act as natural breakwaters, shielding coastlines from storms while supporting fisheries and tourism. As reefs decline, fish populations fall, coastal communities become more exposed to extreme weather, and millions of people who depend on healthy oceans for food and livelihoods are affected.
What This Means for India
India has four major coral reef systems: the Andaman and Nicobar Islands, Lakshadweep, the Gulf of Mannar, and the Gulf of Kutch. All four face a double stress that makes ocean acidification harder to absorb: warming sea temperatures — which cause bleaching by breaking the relationship between corals and the algae living in their tissue — combine with acidification, which weakens coral skeletons and reduces their ability to recover from bleaching events. A coral that might have recovered from a single temperature spike is less resilient when its calcium carbonate skeleton is simultaneously dissolving more easily.
Live coral cover in the Gulf of Mannar fell from 37 per cent in 2005 to 27.3 per cent in 2021. A Nature Conservation Foundation study found a 50 per cent decline in coral cover in Lakshadweep between 1998 and 2022. The 2023-2025 period has been confirmed as the fourth global mass coral bleaching event, affecting 83.9 per cent of the world’s reefs.
The human cost is concentrated and specific. A peer-reviewed study published in iScience in October 2025 found that 67.3 per cent of India’s fishing households live at or below the poverty line, making them highly vulnerable to any decline in marine productivity. India has approximately 1.4 million marine fishers. As reef fish populations fall and shellfish aquaculture becomes harder to sustain in more acidic water, these communities have limited alternatives to fall back on. Ocean acidification is, for them, not an abstract Earth-system process. It is a direct threat to the fish in the net and the income from selling it.
A Buffer That Cannot Absorb Forever
The ocean has, for the past two centuries, quietly protected humanity from the full consequences of its carbon emissions. By absorbing roughly a quarter of what we have put into the atmosphere, it has bought time that the climate system itself would not otherwise have provided. The cost has been borne by the ocean’s chemistry and the marine life that depends on it remaining stable.
There is also a feedback that makes this boundary more urgent than it might appear: as the ocean becomes more acidic, its ability to absorb further carbon dioxide decreases. A more acidic ocean is a less efficient carbon sink. This means more of what we emit stays in the atmosphere, accelerating the climate boundary this series will examine later. The boundaries are not independent problems. They are interconnected pressures on the same system, and weakening one makes the others harder to hold.
The ocean is not infinite. It is responding. That response is now, for the first time, measured in the language of planetary boundaries — and it is outside the safe zone.
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Utkarsh Mishra is a journalist based in Ranchi writing on law, labour rights, and the environment. His work has appeared in Feminism in India, The India Forum, Down to Earth, The Policy Circle, Verdicto News and Zee News.
