How Ocean Acidification Affects Marine Life: A Deep Dive

Deep dive: This is the expert companion to our plain-language guide. For the short version — what ocean acidification is, in everyday terms — see: Ocean Acidification Explained: Why a More Acidic Sea Matters.

Our Complete guide gave the one-sentence version: carbon dioxide dissolving in seawater forms carbonic acid, the acid eats away at carbonate ions, and the animals that build shells from carbonate pay the price. That answer is correct, but it leaves the interesting questions unasked. How do we know the ocean is changing, when the change is two thousandths of a pH unit a year? How fast is this happening compared with the last time the ocean acidified, 56 million years ago? Where does the water turn corrosive first, and why there? And is there any way to put the chemistry back? This Deep dive answers those questions, with the numbers researchers actually use.

A boundary crossed in 2025

The newest reason to take the numbers seriously arrived on 24 September 2025, when the Potsdam Institute for Climate Impact Research published its Planetary Health Check 2025. Ocean acidification became the seventh of nine planetary boundaries to be assessed as breached — the first new breach since the framework’s 2023 update. The boundary is defined on a single control variable, the global mean aragonite saturation state of the surface ocean, with the safe limit set at 80 percent of the pre-industrial value. Revised model estimates put the pre-industrial value at 3.57, which sets the boundary at 2.86. The observed value in 2024 was 2.84 — just below the line. The margin is thin, and that is itself the point: the assessment rests on a high-quality global observational dataset, not on a model projection, and it says the ocean has already left the safe operating space for this measure. Only stratospheric ozone and aerosol loading remain inside their boundaries.

A saturation state of 2.84 still sounds comfortable — the water is still supersaturated, shells can still form. The boundary is a warning line, not a cliff edge. But it is a line drawn from the chemistry, and the chemistry is moving in one direction. To understand why scientists are confident about that direction, start with how the change is measured.

How we know: measuring a 0.002-a-year change

Ocean pH is falling by about 0.002 units a year. That is a tiny annual change, smaller than the seasonal swing at any one place, and far too small to taste or see. Detecting it required building the longest-running chemistry experiment in oceanography: fixed stations where the same water is sampled the same way, month after month, for decades.

The two longest records are the Hawaii Ocean Time-series, which has sampled Station ALOHA north of Oahu since 1988, and the Bermuda Atlantic Time-series Study, running since 1983. A third, the European Station for Time-series in the Ocean near the Canary Islands, adds the eastern Atlantic. Together with a station in the Drake Passage, these time series show surface pH falling at a mean rate of about 0.02 units per decade, while the partial pressure of carbon dioxide in the surface water rises by about 19 micro-atmospheres per decade — tracking the rise in the atmosphere almost exactly. At Station ALOHA, the measured pH fell by 0.035 units over twenty years, and the decline matched, almost exactly, the rate expected from chemical equilibration with the atmosphere. That match is the core of the case: the ocean is acidifying because the air above it holds more carbon dioxide, and the numbers agree quantitatively.

Measuring pH at sea is itself a technical achievement. Seawater pH is reported on the total hydrogen ion scale, and the most precise measurements use a spectrophotometric method: a dye is added to a seawater sample and the colour change reveals the pH to within a few thousandths of a unit. The method is precise enough that a single station can resolve the trend — but only after years of data, because seasonal cycles and year-to-year variability are larger than the annual change. A 2019 analysis of 40 autonomous moored buoys found that 8 to 15 years of continuous observation are needed at an open-ocean site before the human-driven trend emerges from the noise; at coastal sites, where rivers and biology stir the chemistry, it takes 16 to 41 years.

The ship-based records are now backed by a global observing system: repeat hydrographic surveys that re-measure the same ocean sections every decade, a surface carbon dioxide database built from millions of ship measurements, and a growing fleet of biogeochemical Argo floats that profile pH and oxygen through the upper two kilometres. For the deeper past, before any instrument existed, scientists read the chemistry from fossils: the ratio of boron isotopes in the shells of tiny plankton varies with the acidity of the water they grew in, which is how the pH of oceans millions of years old can be reconstructed at all.

The chemistry, one level deeper

The reaction chain is short. Carbon dioxide dissolves in seawater and reacts with water to form carbonic acid, which releases hydrogen ions. Those ions bind with carbonate ions to make bicarbonate. Two things follow: the water grows more acidic, and the supply of free carbonate ions shrinks. It is the second half that matters most to marine life, because carbonate is the raw material for shells and skeletons.

Whether a shell can form or must dissolve is captured in one number, the saturation state, written as the Greek letter omega (Ω). It is the product of the calcium and carbonate concentrations in the water, divided by the solubility of the mineral in question. When Ω is above 1, the mineral can stay solid; below 1, it dissolves. Aragonite, the form of calcium carbonate used by corals, pteropods and many shellfish larvae, is more soluble than calcite, the form used by coccolithophores and foraminifera — so aragonite is the first to cross the line.

There is a second number that governs the future: the Revelle factor, which measures how much the ocean’s carbon dioxide content changes for a given change in the air above it. In today’s surface ocean it is around 10, and it rises as more carbon dioxide dissolves. A rising Revelle factor means the ocean’s buffering capacity is falling: each additional tonne of carbon dioxide causes a larger pH change than the tonne before it. The ocean absorbed roughly a quarter of the carbon dioxide humans have emitted, a service that slowed atmospheric warming. That service is not guaranteed to continue at the same rate, because the chemistry that provides it is itself being used up.

Ω also varies with depth. Cold, deep water holds more carbon dioxide and less carbonate, so Ω falls as you go down, reaching 1 at a depth called the saturation horizon. Below that horizon, aragonite dissolves. As the surface ocean acidifies, the horizon rises — the corrosive zone expands upward. In the Southern Ocean, models project the aragonite saturation horizon rising from about 730 metres to about 60 metres by 2100 under a high-emissions pathway. That is not a distant abstraction: cold-water corals live on seamounts and slopes in exactly that depth range.

The rate is the story: 56 million years ago, ten times slower

The closest geological parallel to today is the Paleocene-Eocene Thermal Maximum, or PETM, about 56 million years ago, when a massive release of carbon warmed the planet and acidified the ocean. Researchers reconstructed the event from boron isotopes in the shells of plankton preserved in seafloor sediments drilled off Japan. They found that surface ocean acidity roughly doubled — a pH fall of about 0.3 units, comparable to what is projected for the end of this century — but that the change took a few thousand years, and the acidified conditions then lasted for 70,000 years or more. As one of the study’s authors put it, the PETM “still happened about 10 times more slowly than what we are doing today.”

The magnitude is similar; the rate is not. And rate is what decides whether life can keep up. Over thousands of years, species can migrate, shift their ranges, and evolve. Over a century and a half, the options narrow to tolerating the change or dying where they are. Even at the PETM’s slower pace, the event drove a mass extinction of deep-sea benthic foraminifera — single-celled organisms that build calcite shells on the ocean floor. Today’s change is faster than the slowest catastrophe in the geological record, and it is happening to ecosystems that have no deeper, cooler water to retreat into, because the change is fastest where the water is coldest.

The PETM also sets the timescale for recovery. Acidified conditions persisted for 70,000 years after the carbon release, because the only processes that restore ocean chemistry — the slow weathering of rocks on land and the burial of carbonate sediments — work on hundred-thousand-year timescales. Carbon dioxide absorbed today will go on shaping seawater chemistry for centuries, and the deep ocean, which has barely begun to feel the change, will keep acidifying long after emissions fall. The chemistry of the sea is set by the composition of the sky, and the sky’s composition changes far faster than the sea can forget it.

Where the water turns corrosive first

Acidification is not uniform, and its geography matters as much as its average. Cold water absorbs more carbon dioxide than warm water, so the polar oceans acidify fastest. Upwelling regions, where deep, carbon-rich water is brought to the surface, are next. The places where the change arrives first are also the places where the food webs are most exposed.

The Southern Ocean is the sharpest case. A 2016 study in Nature Climate Change, using an ensemble of ten Earth system models, found that seasonal aragonite undersaturation — months when surface waters cross below Ω = 1 — will begin around 2030, spread to about 30 percent of Southern Ocean surface waters by 2060, and cover more than 70 percent by 2100. Once these events begin, their duration lengthens abruptly, from about one month a year to six months a year within two decades, across more than three quarters of the affected area. An earlier study put the tipping point at 450 parts per million of atmospheric carbon dioxide — a concentration the atmosphere is on course to reach in the 2030s. Pteropods, the swimming sea snails that are a staple food for fish, birds and whales in the Southern Ocean, live exactly where this is happening.

In the Arctic, undersaturated waters have already been observed seasonally in parts of the Canada Basin, where sea-ice melt adds fresh water that dilutes alkalinity — the ocean’s buffering capacity — on top of the carbon dioxide effect. And on the Pacific coast of North America, the upwelling of deep water is why the oyster hatcheries were hit first: the water drawn into the hatcheries in the 2000s was already corrosive, and it arrives earlier each year as the deep reservoir acidifies. What organisms experience is not the annual global mean but the worst season in their own patch of water — which is why researchers increasingly argue that winter minima, not annual averages, are the number that matters for survival.

Beyond shells: the effects we are less sure about

Shell-builders are the clearest victims, but they are not the only ones. Fish do not build carbonate shells, yet laboratory experiments have reported that acidified water alters behaviour in some reef fish larvae — how they sense predators, how they choose habitat. This is the least settled part of the science, and honesty requires saying so: a large replication study published in Nature in 2020 found no such impairment in the species it tested, and the field is still resolving which results hold. The behavioural literature is genuinely contested, and any guide that presents it as settled is overstating the evidence.

Other effects are on firmer ground. Squid metabolisms slow in more acidic water. Sea urchin larvae change shape. The byssal threads that mussels use to anchor themselves to rocks weaken. None of these is as clean or as certain as shell dissolution, but the direction is consistent: a chemistry shift at the base of the food web rarely stops there. The organisms most at risk are the small calcifiers — pteropods, coccolithophores, foraminifera — that sit at the bottom of marine food webs and also, in the case of coccolithophores and foraminifera, help carry carbon to the deep ocean when they die and sink. If their shells thin, both the food web and the carbon cycle feel it.

Can we put the chemistry back?

If the problem is too little alkalinity, the obvious fix is to add some. Ocean alkalinity enhancement is the idea of adding alkaline material — ground minerals, lime, or electrochemically treated seawater — to shift the carbonate chemistry back toward its pre-industrial state and draw additional carbon dioxide out of the air. Modelling studies put the potential at 3 to 30 gigatonnes of carbon dioxide a year, a range so wide it is itself a measure of how little is known. Field trials are only just beginning, including a planned trial in a coastal channel in Tasmania, and the practical questions are large: how much rock must be mined, ground and shipped, and what happens to the ecosystems that receive it?

The chemistry also sets a hard limit that was only quantified in 2024. Experiments in Norway showed that if alkalinity is added too fast, calcium carbonate precipitates out of the water on its own — a runaway process that strips alkalinity back out and can leave the water worse off than before. The threshold is an aragonite saturation state of about 5, far above today’s surface values but easily reached near an injection point. Any large-scale deployment would have to add alkalinity slowly and dilute it quickly, which limits how fast the ocean can be treated and raises the cost.

Local adaptation works better at small scales. The Pacific oyster hatcheries proved it: they monitor water chemistry, treat incoming seawater with sodium carbonate, and time production around corrosive upwellings. Selective breeding is producing oyster larvae that tolerate lower saturation states. Seagrass beds and kelp forests draw down carbon dioxide locally and can raise pH in their immediate surroundings, though only during the day and only within the meadow. All of these help farmed species and sheltered bays. None of them treats the open ocean, the polar seas, or a wild reef. For those, there is no treatment plant. The chemistry is set by the sky.

What remains uncertain

Three uncertainties deserve to be stated plainly. First, coastal waters are messier than the open ocean: rivers, runoff, nutrient pollution and local biology can dominate the chemistry in a bay or estuary, so global averages hide large local differences — in both directions. Second, models disagree on the exact timing of regional thresholds, because the Southern Ocean and the Arctic are the hardest places to observe and the hardest to model. Third, biology may surprise us in both directions: some populations show real resilience, and selective breeding is already finding it, but evolutionary rescue is slow, and the organisms with the shortest generation times — the plankton — are also the ones whose shells are thinnest.

What is not uncertain is the direction, the cause, or the rate. The measurement record is now long enough that ocean acidification is observed, not modelled: three decades of time-series stations, forty moored buoys, and a satellite-era observing system all tell the same story. The geological record adds the context that makes the story alarming: the last time the ocean acidified this much, it took thousands of years, and it still drove extinctions and lasted 70,000 years. This time the same magnitude of change is being compressed into a century and a half. That compression — not the chemistry itself — is what makes this episode different from anything in the last 56 million years.

Sources

1. Potsdam Institute for Climate Impact Research (2025). Seven of nine planetary boundaries now breached — ocean acidification joins the danger zone. Planetary Health Check 2025. pik-potsdam.de
2. Penman, D. E., Hönisch, B., Zeebe, R. E., Thomas, E., & Zachos, J. C. (2014). Rapid and sustained surface ocean acidification during the Paleocene-Eocene Thermal Maximum. Paleoceanography, 29, 357–369. doi.org
3. Hauri, C., Friedrich, T., & Timmermann, A. (2016). Abrupt onset and prolongation of aragonite undersaturation events in the Southern Ocean. Nature Climate Change, 6, 172–176. doi.org
4. McNeil, B. I., & Matear, R. J. (2008). Southern Ocean acidification: A tipping point at 450-ppm atmospheric CO₂. Proceedings of the National Academy of Sciences, 105, 18860–18864. doi.org
5. Orr, J. C., et al. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437, 681–686. doi.org
6. Dore, J. E., Lukas, R., Sadler, D. W., Church, M. J., & Karl, D. M. (2009). Physical and biogeochemical modulation of ocean acidification in the central North Pacific. Proceedings of the National Academy of Sciences, 106, 12235–12240. doi.org
7. Takahashi, T., et al. (2014). Climatological distributions of pH, pCO₂, total CO₂, alkalinity, and CaCO₃ saturation in the global surface ocean. Marine Chemistry, 164, 95–125. sciencedirect.com
8. Sutton, A. J., et al. (2019). Autonomous seawater pCO₂ and pH time series from 40 surface buoys and the emergence of anthropogenic trends. Earth System Science Data, 11, 421–439. doi.org
9. Suitner, N., et al. (2024). Ocean alkalinity enhancement approaches and the predictability of runaway precipitation processes. Biogeosciences, 21, 4587–4604. doi.org
10. Intergovernmental Panel on Climate Change (2021). Climate Change 2021: The Physical Science Basis, Working Group I, Sixth Assessment Report. ipcc.ch
11. National Oceanic and Atmospheric Administration. Ocean Acidification Program. oceanacidification.noaa.gov