Ocean Acidification — When CO₂ Meets Seawater
The ocean has absorbed roughly 30% of all CO₂ humans have emitted since industrialisation — about 525 billion tonnes. This chemical gift has triggered a chain reaction in marine chemistry that threatens organisms from oysters to tropical reef fish.
1. CO₂ + H₂O: The Chemistry
When CO₂ dissolves in seawater it reacts with water to form carbonic acid (H₂CO₃), which rapidly dissociates:
CO₂(aq) + H₂O ⇌ H₂CO₃ (carbonic acid)
H₂CO₃ ⇌ H⁺ + HCO₃⁻ (bicarbonate, pKa ≈ 6.3)
HCO₃⁻ ⇌ H⁺ + CO₃²⁻ (carbonate, pKa ≈ 10.3)
The net result: dissolved CO₂ releases hydrogen ions (H⁺), lowering ocean pH. This is not new — the ocean has always done this. What is new is the rate: current pH reduction is approximately 100× faster than any natural transition in the geological record.
2. The Carbonate System
Seawater contains a carbonate buffering system that resists pH change — but at a cost. Excess H⁺ ions react with carbonate (CO₃²⁻), converting it to bicarbonate:
This absorption of H⁺ resists acidification, but it depletes the carbonate ion concentration that shell-building organisms depend on. The ocean loses its ability to buffer as CO₂ concentrations rise.
3. pH Change — Small Numbers, Big Consequences
pH is a logarithmic scale: pH = −log₁₀[H⁺]. A decrease of 0.1 pH units means [H⁺] increases by 10^0.1 ≈ 26%. A projected decrease of 0.3–0.4 by 2100 (RCP 8.5) corresponds to [H⁺] roughly doubling.
Normal seawater at pH 8.1 is technically basic, not acid — "ocean acidification" refers to the direction of change, not the absolute state. However, organisms evolved for pH 8.2 experience 8.0 as physiologically stressful.
4. Aragonite and Calcite Saturation
Marine organisms build shells from two calcium carbonate (CaCO₃) minerals: calcite (more stable) and aragonite (less stable, higher solubility). The saturation state Ω determines whether these minerals dissolve:
Ω > 1 → precipitation favoured (shells grow)
Ω = 1 → equilibrium
Ω < 1 → dissolution (shells dissolve)
As CO₂ rises and [CO₃²⁻] falls, Ω_arag decreases. Arctic and Southern
Ocean surface waters are projected to become aragonite-undersaturated
(Ω < 1) within decades under high-emission scenarios.
When that happens, aragonite shells dissolve faster than they can form.
5. Biological Impacts
Corals
Coral reefs occupy 0.1% of the ocean floor but support ~25% of all marine species. Corals build their skeletons from aragonite. Under acidification, calcification rates decline — reefs grow slower than they erode. Combined with warming-induced bleaching, the IPCC projects most tropical reefs are lost above 2 °C warming.
Shellfish and Pteropods
Oysters, mussels, clams, and sea urchins are already seeing thinner shells in regions where upwelling brings corrosive deep water to the surface (Pacific Northwest, US). Pacific oyster larvae in some Oregon hatcheries failed completely in 2007–2008 before the cause was identified and the water chemistry adjusted.
Pteropods (small sea snails) are a critical link in polar food webs. Their aragonite shells visibly dissolve within 45 days in seawater at pH 7.8 — conditions already encountered seasonally in parts of the Southern Ocean.
Fish
Higher CO₂ impairs the sense of smell in some fish (disrupts the receptor protein GABA-A), reducing their ability to avoid predators. Clownfish larvae have difficulty locating reef habitat by chemical cues.
6. The Arctic Is Hardest Hit
Cold water absorbs more CO₂ than warm water (Henry's Law: gas solubility increases with decreasing temperature). Arctic and Antarctic waters are therefore more acidic than tropical waters, and acidification is proceeding faster there.
7. Ocean–Atmosphere Feedbacks
As the ocean acidifies and biology is disrupted, carbon cycle feedbacks may amplify warming:
- Reduced biological pump: Calcareous organisms lock carbon into shells that sink to the deep ocean. Fewer shells → less biological export → more carbon stays at the surface.
- DMS reduction: Phytoplankton produce dimethylsulphide (DMS), which nucleates cloud droplets. Acidification stresses phytoplankton → less DMS → fewer clouds → warmer surface. Estimated feedback +0.1 to +0.5 W/m².
- Positive feedback: Warmer, more acidic oceans absorb less CO₂, leaving more in the atmosphere.
8. Projections and Tipping Points
Under RCP 2.6 (aggressive mitigation), ocean pH stabilises around 8.05 by 2100 — uncomfortable but survivable for most organisms. Under RCP 8.5 (business as usual), surface pH reaches ~7.75 — a level not seen for 20–30 million years, faster than the ocean system can adapt.
Unlike temperature, ocean acidification cannot be reversed on human timescales. Even if emissions reached zero tomorrow, the 30% of CO₂ already absorbed would remain dissolved for centuries. The ocean is an enormous but finite buffer.