Ocean acidification is the decrease in ocean pH caused by the absorption of CO₂ from the atmosphere. The oceans have absorbed roughly a quarter of all human carbon dioxide emissions since industrialisation, dropping pH from approximately 8.2 to 8.1 — a 26 per cent increase in acidity that threatens marine ecosystems globally.
What is the chemistry of ocean acidification?
To understand ocean acidification, you need to understand what pH means and what happens when CO₂ dissolves in seawater.
pH scale: pH measures acidity on a scale from 0 (most acidic) to 14 (most alkaline), with 7 as neutral. The scale is logarithmic — a change of 0.1 represents a roughly 26 per cent change in acidity, not a 0.1 per cent change. Pre-industrial ocean pH was approximately 8.2 (slightly alkaline). Current pH is approximately 8.1, and models project a further fall to 7.8–7.95 by 2100 if emissions continue.
The chemical reaction:
- CO₂ dissolves in seawater: CO₂ + H₂O → H₂CO₃ (carbonic acid)
- Carbonic acid dissociates: H₂CO₃ → H⁺ + HCO₃⁻ (bicarbonate)
- The increase in H⁺ ions is what lowers pH (increases acidity)
- H⁺ ions also react with carbonate ions: H⁺ + CO₃²⁻ → HCO₃⁻
Step 4 is critical for marine life: it reduces the availability of carbonate ions (CO₃²⁻) in seawater. Many marine organisms — corals, shellfish, sea urchins, pteropods — use carbonate to build calcium carbonate (CaCO₃) shells and skeletons. When carbonate availability falls, these organisms struggle to build and maintain their shells.
What evidence tells us that ocean acidification is happening?
| Evidence type | What it shows | Source |
|---|---|---|
| Direct pH measurement (Mauna Loa / ALOHA Station, Hawaii, since 1988) | Continuous pH decline matching atmospheric CO₂ rise | Long-term ocean monitoring |
| Coral core samples | Declining calcification rates since the 1970s | Palaeoclimatology |
| Boron isotope analysis of ancient shells | Ocean pH over hundreds of thousands of years | Geological record |
| Laboratory experiments | Reduced shell growth in shellfish at lower pH | Controlled research |
| Ocean model projections | pH trajectories under different emissions scenarios | Climate science |
The Mauna Loa oceanic station in Hawaii has provided the most continuous long-term record. The direct correlation between atmospheric CO₂ concentration (which has risen from 280 ppm pre-industrial to over 420 ppm today) and ocean pH decline is one of the strongest pieces of evidence connecting human emissions to ocean change.
Which marine species are most threatened by acidification?
The most vulnerable species are those that build calcium carbonate structures:
Corals: Coral reefs are built from the calcium carbonate skeletons of tiny animals called polyps. Acidification slows coral growth and weakens skeletons; combined with warming temperatures (which cause bleaching), acidification makes coral reefs the most severely threatened marine ecosystems. The Great Barrier Reef — the world's largest reef system — experienced mass bleaching events in 2016, 2017, 2020, 2022 and 2024.
Pteropods (sea butterflies): These tiny free-swimming molluscs are a crucial part of polar food webs — they are a primary food source for salmon, herring, whales and seabirds. Studies in the Southern Ocean have found pteropods with visibly dissolving shells in waters that are already undersaturated in aragonite (a form of calcium carbonate). This is happening now, not in the future.
Oysters, mussels and clams: Commercial shellfish face problems in the early stages of shell formation. The US Pacific Northwest oyster farming industry has already suffered significant losses linked to acidification of upwelling coastal waters.
Sea urchins: Important for reef ecosystems because they graze algae that would otherwise smother corals. Their larvae are highly sensitive to pH changes.
What are the SEEP consequences of ocean acidification?
Using the SEEP framework helps organise the wide-ranging impacts:
Social: Loss of coral reefs and shellfish fisheries threatens the food security of coastal communities in the tropics and Pacific Islands, where reef fish are a primary protein source. An estimated 500 million people globally depend on reefs for food, income or coastal protection.
Economic: Coral reefs generate an estimated US$375 billion per year through fisheries, tourism and coastal protection. Shellfish and aquaculture industries in the UK, USA and Asia face rising losses. Insurance costs for coral-adjacent coastlines are increasing as protective reef structures weaken.
Environmental: Ocean acidification is driving a shift in marine species composition — acid-tolerant species (certain bacteria, jellyfish, some seagrasses) expand while calcifying species contract. This restructuring of food webs has unpredictable cascading effects throughout ocean ecosystems. There is also a positive feedback loop: as acidification reduces marine biological productivity, the ocean absorbs less CO₂ biologically, potentially increasing atmospheric concentrations.
Political: Ocean acidification is increasingly a diplomatic issue. Pacific Island nations, whose shallow reefs are especially vulnerable, have been among the most vocal advocates for strict emissions reductions in international climate negotiations. The UN Sustainable Development Goal 14 specifically addresses ocean acidification.
How does ocean acidification interact with other pressures on marine environments?
Ocean acidification rarely acts alone. It combines with:
- Warming: Higher temperatures bleach corals; acidification weakens them. Together they are more damaging than either alone.
- Deoxygenation: Warmer water holds less dissolved oxygen, creating hypoxic (low-oxygen) dead zones. Acidification can worsen the physiological stress on marine animals already dealing with oxygen depletion.
- Overfishing: Healthy fish populations can help maintain reef ecosystems; depleted populations remove this resilience.
- Pollution: Nutrient pollution from agriculture causes algal blooms that further depress oxygen levels and block light from reef-building corals.
This combination of simultaneous stressors — sometimes called the "deadly trio" (warming, acidification, deoxygenation) — makes ocean ecosystems significantly more fragile than any single pressure would suggest.
What can be done to address ocean acidification?
The fundamental response must be reducing CO₂ emissions — ocean acidification is a direct chemical consequence of atmospheric CO₂, and there is no technological fix that can de-acidify the global ocean at scale. However, several approaches can help at local or intermediate levels:
- Protecting and restoring seagrass meadows and kelp forests: These absorb CO₂ and can raise local pH, providing refugia for shellfish and reef organisms.
- Reducing local pollution: Lowering nutrient pollution reduces local stress on reef systems.
- Marine protected areas: Reducing fishing pressure and physical damage improves reef resilience.
- Breeding research: Selective breeding of acid-tolerant oyster and shellfish strains offers some commercial resilience.
- Carbon capture and storage: Large-scale removal of CO₂ from the atmosphere would slow acidification, but no technology exists at the scale required.
Frequently asked questions
Is ocean acidification the same as coral bleaching?
No — they are separate processes, though both are driven by CO₂ emissions and both threaten coral reefs. Coral bleaching is caused by thermal stress: when sea temperature rises above a threshold (typically 1–2°C above the seasonal maximum), corals expel their symbiotic algae (zooxanthellae), which normally provide up to 90% of the coral's energy through photosynthesis, causing the coral to turn white. If temperatures do not return to normal quickly, the coral starves and dies. Ocean acidification weakens corals by making it harder to build their calcium carbonate skeletons. A coral reef stressed by acidification is more vulnerable to bleaching events; the two threats compound each other.
Why do some areas of the ocean acidify faster than others?
Cold water absorbs more CO₂ than warm water — just as a cold fizzy drink holds its carbonation better than a warm one. This means polar oceans (the Arctic and Antarctic) are acidifying faster than tropical oceans. The Southern Ocean around Antarctica has been absorbing CO₂ at an especially rapid rate, and pteropod shells have already been found dissolving there. Coastal upwelling zones — where deep, naturally CO₂-rich water rises to the surface — are also vulnerable, which explains why oyster farms on the US Pacific Northwest coast have been affected despite not being near coral reef systems.
How does ocean acidification affect the UK?
The UK's marine environment — particularly the cold-water coral reefs off the coast of Scotland (Lophelia pertusa reefs in the Faeroe-Shetland Channel) — is vulnerable to acidification because cold North Atlantic waters are absorbing CO₂ particularly rapidly. The UK's oyster, mussel and scallop aquaculture industries face risk. The UK fishing industry, which depends on healthy marine food webs that include pteropods and other calcifying organisms, faces long-term structural risk. Scotland's salmon farms may be affected if krill and pteropod populations — key salmon food sources — decline in the North Atlantic.
What does ocean acidification mean for the carbon cycle?
The oceans are the largest active carbon sink on Earth, absorbing approximately 26 per cent of annual human CO₂ emissions. This absorption moderates climate change — without it, atmospheric CO₂ would be significantly higher. However, as the ocean acidifies and warms, its capacity to absorb CO₂ may decrease. Biological carbon pumps — the processes by which marine organisms, including calcifying organisms, capture carbon and sink it to the deep ocean — may be disrupted. If the ocean sink weakens, a larger proportion of human CO₂ emissions will remain in the atmosphere, accelerating climate change. This feedback loop is one of the reasons scientists regard ocean acidification as a risk beyond the marine environment alone.
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