The Ocean as a Carbon Sink
The world's oceans have absorbed a significant portion of the carbon dioxide released by human activities since the Industrial Revolution. This buffering role has slowed the rate of atmospheric warming — but it comes at a cost. When CO₂ dissolves in seawater, it forms carbonic acid, and this is fundamentally altering the chemistry of the marine environment in a process known as ocean acidification.
The Chemistry Behind the Change
When CO₂ enters the ocean, a series of chemical reactions takes place:
- CO₂ + H₂O → H₂CO₃ (carbonic acid)
- H₂CO₃ → H⁺ + HCO₃⁻ (bicarbonate)
- The increase in H⁺ ions lowers the pH of the water
- H⁺ ions also react with carbonate ions (CO₃²⁻), reducing their availability
The pH of ocean surface water has measurably decreased since the pre-industrial era. Even seemingly small changes in pH represent significant shifts in acidity because pH is measured on a logarithmic scale — a drop of 0.1 pH units means roughly a 26% increase in acidity.
Why Carbonate Matters So Much
Carbonate ions are the building blocks that many marine organisms use to construct their shells and skeletons. When carbonate availability falls, it becomes harder — and eventually impossible — for these organisms to build and maintain their structures. Affected species include:
- Corals: Tropical and cold-water coral reefs are built from calcium carbonate. More acidic water not only slows growth but can begin to dissolve existing reef structures.
- Molluscs: Oysters, clams, mussels, and scallops rely on calcium carbonate for their shells. Larval stages are particularly vulnerable.
- Pteropods: These tiny free-swimming sea snails are a critical food source for many fish and whales. Their thin shells dissolve measurably in water that is already more acidic than historical norms.
- Sea urchins and starfish: Echinoderms use calcite structures that become weaker in lower pH conditions.
Effects on Marine Ecosystems
The impact of acidification extends beyond shell-building organisms. Some fish species show altered behaviour in more acidic water — including impaired ability to detect predators — due to changes in how their nervous systems respond to water chemistry. This has downstream effects on population dynamics and food webs.
Coral reefs, which support an estimated quarter of all marine species despite covering less than 1% of the ocean floor, face a dual threat from acidification and warming. Bleaching events caused by heat stress weaken reefs, while acidification slows recovery and undermines structural integrity.
Polar Regions: The Frontline
Cold water absorbs CO₂ more efficiently than warm water, which means polar seas are acidifying faster than tropical ones. The Arctic and Southern Oceans are already approaching corrosive conditions for some carbonate shell structures. This is particularly concerning because polar ecosystems are foundational — small crustaceans like krill, which underpin the entire food chain from penguins to whales, depend on stable chemical conditions.
Quick Reference: Ocean Acidification at a Glance
| Factor | Detail |
|---|---|
| Primary cause | Absorption of atmospheric CO₂ |
| Chemical process | Formation of carbonic acid, release of H⁺ ions |
| Most vulnerable organisms | Corals, molluscs, pteropods, echinoderms |
| Fastest-changing regions | Arctic and Southern Oceans |
| Broader ecosystem risk | Disruption of food webs, reef degradation |
What Can Be Done?
The most direct solution is reducing CO₂ emissions — there is no chemical fix that can be applied to the ocean at scale without significant unintended consequences. Local actions such as reducing coastal pollution and protecting seagrass and mangrove habitats (which absorb carbon and buffer local acidity) can help improve ocean health. But fundamentally, ocean acidification is a global problem that requires a global response at the level of energy systems and emissions.