Ocean Acidification: How Rising CO₂ Is Changing Sea Chemistry

The chemistry of CO₂ in seawater

When CO₂ from the atmosphere dissolves in the ocean, it undergoes a sequence of reactions that drive down pH:

Each step is an equilibrium. As atmospheric CO₂ rises, the first equilibrium shifts right — more CO₂ dissolves — driving every subsequent reaction right too, increasing H⁺ concentration and decreasing pH. For a detailed treatment of acid equilibria, see our acids and bases guide and the pH glossary entry.

The carbonate chemistry of the ocean is governed by Le Chatelier's principle — the same principle that governs industrial processes like the Haber process. Our reaction rates guide covers equilibrium and Le Chatelier's principle in full.

What the numbers mean

Pre-industrial average ocean surface pH was approximately 8.2. In 2026 it is approximately 8.08 — a change of 0.12 pH units. Because pH is a logarithmic scale (pH = −log₁₀[H⁺]), this corresponds to a 26% increase in hydrogen ion concentration. Projections under high-emission scenarios forecast pH reaching 7.95 by 2100 — a 150% increase in acidity relative to pre-industrial levels.

The ocean's natural buffering system — primarily the bicarbonate/carbonate equilibrium — partially moderates these changes, but cannot fully offset the rate of CO₂ input.

Biological consequences

The most directly affected organisms are those that build calcium carbonate (CaCO₃) structures:

• Corals — reef-building corals calcify aragonite skeletons. Lower pH decreases the aragonite saturation state; at undersaturation, skeletons dissolve faster than they grow. Coral reefs support about 25% of all marine species despite covering less than 1% of the ocean floor.
• Molluscs and echinoderms — oysters, mussels, sea urchins and sea stars show reduced calcification rates and thinner shells in acidified water.
• Pteropods — small free-swimming snails that are a critical food source for salmon, herring and whales. Shell dissolution has been directly observed in Southern Ocean pteropods.
• Fish sensory behaviour — elevated CO₂ interferes with the GABA neurotransmitter system in some fish, impairing predator avoidance and habitat navigation.

Intervention chemistry

Ocean alkalinity enhancement (OAE) proposes adding alkaline minerals — such as calcium hydroxide, calcium oxide or olivine (MgFeSiO₄) — to seawater to consume CO₂ and raise pH:

The net effect is to convert dissolved CO₂ into bicarbonate ions — stable in seawater — and raise pH. The challenge is scale: to meaningfully affect global ocean chemistry would require gigatonne-scale mineral processing, with significant energy and land-use implications.

For the acid-base chemistry of neutralisation reactions used in interventions like these, see our balancing equations guide and our molarity and dilutions guide.