Ocean Acidification, Explained: Why a More Acidic Sea Matters

In the mid-2000s, oyster hatcheries along the Pacific coast of the United States ran into a mystery. Billions of larvae were dying before they could grow shells, and nobody could say why. The water looked fine. Bacteria were ruled out. Then chemists tested the seawater itself, and the answer turned out to be invisible: the water had become corrosive. The baby oysters were not being poisoned. They were being dissolved.

That episode was many coastal communities’ first encounter with ocean acidification — a shift in seawater chemistry driven by the same carbon dioxide that warms the atmosphere. It is sometimes called climate change’s quieter twin, because it does not arrive with storms or heatwaves. It arrives as a number on a pH meter, moving slowly in the wrong direction, with consequences that are only now coming into focus.

The chemistry, without the fog

Seawater chemistry sounds forbidding, but the core reaction fits in a sentence. When carbon dioxide from the air dissolves in seawater, it reacts with water to form carbonic acid. That acid releases hydrogen ions, and those ions bind with carbonate ions already in the water, converting them into bicarbonate. Two things follow: the water’s pH falls, and the supply of free carbonate ions shrinks.

The pH scale runs from 0 to 14, with 7 neutral. The ocean’s surface water currently averages around 8.1 — still on the alkaline side of neutral, which is why scientists say the ocean is becoming more acidic rather than actually acid. The scale is logarithmic, like the Richter scale for earthquakes, so small numbers hide large changes: a drop of 0.1 pH units means roughly 30 percent more hydrogen ions in the water.

That 0.1 is not a projection. According to the US National Oceanic and Atmospheric Administration (NOAA), surface ocean pH has fallen by about 0.1 units since the start of the Industrial Revolution, a roughly 30 percent increase in acidity. The driver is the ocean’s appetite for carbon dioxide: it absorbs about 30 percent of the CO2 humans release into the atmosphere. That absorption has, in one sense, been a favour — without it, the atmosphere would have warmed faster. The price is paid in seawater chemistry.

How do scientists know what ocean pH was two centuries ago, before routine measurements? They read chemical proxies preserved in marine sediments and corals — most reliably, boron isotopes in the shells of tiny marine organisms, whose ratios vary with the acidity of the water in which they formed. On that evidence, the World Meteorological Organization reported that in 2021, open-ocean surface pH reached its lowest point in at least 26,000 years, and that the current rate of change is unprecedented over that span.

Why carbonate ions are the whole story

A falling pH number would matter little on its own. What makes it dangerous is the second half of the reaction: the disappearing carbonate.

A vast range of marine life builds shells and skeletons from calcium carbonate — corals, oysters, clams, mussels, crabs, and microscopic plankton. They do this by combining calcium with carbonate ions from the surrounding water. As hydrogen ions mop up carbonate, building becomes harder and more energy-expensive. In waters where carbonate minerals become undersaturated — where the water holds so little carbonate that existing shells begin to dissolve — the sea stops being a construction site and becomes a solvent.

The most vivid evidence comes from pteropods, tiny swimming sea snails nicknamed sea butterflies. They are food for salmon, herring, and other fish, and their thin shells are made of aragonite, the more soluble form of calcium carbonate. NOAA researchers examining pteropods collected off the US West Coast found that more than half had shells showing severe dissolution — ragged, pitted surfaces, like a tooth eroded by soda. In laboratory seawater adjusted to the pH and carbonate levels projected for 2100, pteropod shells dissolved measurably within 45 days.

Corals face the same arithmetic from a different angle. A reef is a city built by animals: coral polyps lay down calcium carbonate skeletons, generation upon generation, and the reef that results shelters a quarter of known marine species. Acidification slows calcification — the rate at which corals build — and weakens the framework that already exists. Combined with warming, which drives coral bleaching, it leaves reefs growing more slowly, eroding faster, and recovering less completely from damage. A 2025 NOAA-linked assessment found that acidification has already shrunk the suitable habitat of tropical and subtropical coral reefs by an estimated 43 percent, and of polar pteropods by up to 61 percent.

Scientists track this erosion with a measure called aragonite saturation state — essentially, how easy it is for aragonite to stay solid in a given patch of seawater. A recent global assessment by oceanographers, including researchers from NOAA’s Pacific Marine Environmental Laboratory, found that since the pre-industrial era, aragonite saturation has fallen by a fifth across 40 percent of the surface ocean and 60 percent of subsurface waters down to 200 metres. In plain terms, two-fifths of the ocean’s sunlit surface is now measurably worse at holding shells together than it was before industrial emissions began.

The oyster lesson, retold

Return to those Pacific hatcheries, because they show how an abstract chemical shift becomes a human problem. In the mid-to-late 2000s, hatcheries in Washington and Oregon watched larvae die in enormous numbers. The Whiskey Creek hatchery in Tillamook, Oregon, became the emblem of the crisis: water drawn from the ocean was so corrosive that larvae could not form their first shells, and without a shell, a larva dies within days. Once the cause was identified, hatcheries adapted — monitoring water chemistry, treating incoming seawater with sodium carbonate, and timing production around corrosive upwellings of deep water. The industry survived, but at a cost, and only because it could afford chemistry labs and treatment systems. Wild shellfish, which cannot be buffered, have no such option.

The pattern repeats wherever shell-building meets cold, deep, naturally carbon-rich water: the Pacific Northwest, the Arctic, parts of the Southern Ocean. These are also regions where fisheries and coastal livelihoods are most exposed. The ocean’s chemistry is changing fastest precisely where its food webs are least able to absorb the shock.

What about the fish?

Shell-builders are the headline victims, but they are not the only ones. Fish do not build carbonate shells, yet experiments show that acidified water can alter fish behaviour and development — in some species, affecting how larvae sense predators or find habitat. Squid metabolisms slow. Mussels’ threads, the fibres that anchor them to rocks, weaken. Sea urchin larvae change shape. None of these effects is as clean or as certain as shell dissolution, and scientists are careful to say so: the behavioural research, in particular, has been contested and refined over the past decade. But the direction of travel is consistent — a chemistry shift that touches the base of the food web rarely stops there.

It is worth being precise about what acidification is not. It is not the same as warming, though both come from the same emissions. A cooler ocean still acidifies if it absorbs CO2. It is not pollution in the familiar sense: no factory is pouring acid into the sea. It is the unavoidable chemistry of a gas dissolving in water, which is why it cannot be cleaned up locally. A single bay can be protected from sewage or overfishing. It cannot be fenced off from the atmosphere.

Why this matters

The stakes are often framed as environmental, and they are, but they are also economic and nutritional. Hundreds of millions of people depend on seafood for protein, and shellfish farming is a significant industry in many coastal countries. Coral reefs protect shorelines from storm surges, support tourism, and shelter the fish that coastal communities eat. When the chemistry that builds reefs and shells shifts, the losses compound: fewer fish, weaker storm defences, and livelihoods that become harder to sustain — with the poorest coastal communities, which contributed least to the emissions, facing the earliest consequences.

There is also a timescale problem that makes acidification unusually unforgiving. Carbon dioxide already absorbed will keep working its way through the ocean for centuries, and the deep ocean, which has barely begun to feel the change, will continue acidifying long after emissions fall. Decisions made this decade about emissions will still be shaping seawater chemistry in the year 2300. Few environmental problems lock in their consequences so far ahead.

What happens next

The future of ocean chemistry depends almost entirely on the future of emissions. Under high-emission scenarios, the IPCC has projected that surface ocean pH could fall by a further 0.3 to 0.5 units by 2100 — a change several times larger than what has already happened, taking surface waters to acidity levels not seen for more than 20 million years. Under lower-emission pathways, the decline slows and eventually stabilises, though it does not reverse quickly: the ocean gives back its chemistry far more slowly than it absorbs change.

Adaptation has a role, and the oyster hatcheries proved it: monitoring, buffering, selective breeding of more resilient larvae, and protecting the habitats — seagrass beds and kelp forests — that locally draw down CO2. But adaptation has limits, and it works best for farmed species in wealthy countries. For wild reefs, polar food webs, and the open ocean, there is no treatment plant. The chemistry of the sea is set by the composition of the sky, and that is where the decisive action lies. The baby oysters of the Pacific Northwest were an early warning, written small. The question now is whether it gets read in time.

Sources

• National Oceanic and Atmospheric Administration, Ocean acidification: https://oceanservice.noaa.gov/facts/acidification.html
• National Oceanic and Atmospheric Administration, Study finds ocean acidification is more pervasive than previously thought: https://research.noaa.gov/study-finds-ocean-acidification-is-more-pervasive-than-previously-thought/
• Intergovernmental Panel on Climate Change, Sixth Assessment Report, Working Group I: https://www.ipcc.ch/report/ar6/wg1/