Ocean acidification

Causes and chemistry

When atmospheric carbon dioxide (CO2) dissolves into seawater it reacts with water to form carbonic acid, which then releases hydrogen ions and bicarbonate. The increase in hydrogen ions lowers pH and reduces carbonate ion concentrations. Because carbonate ions are a key component of calcium carbonate minerals, organisms such as corals, mollusks, and some plankton can struggle to form and maintain their shells. Roughly one quarter to one third of human-produced CO2 is absorbed by the ocean, linking emissions from human activities to changes in marine chemistry.

Historical context and trends

Ocean acidification is a consequence of rising atmospheric CO2 since the Industrial Revolution. Global surface waters have become measurably less alkaline over the past century as oceans take up a large fraction of emitted CO2. The change is ongoing and varies by region and depth because of circulation, temperature, and biological activity, so coastal and polar areas can experience different rates and seasonal extremes.

Ecological and economic impacts

The most visible biological effects are on calcifying species whose shells or skeletons are made of calcium carbonate. Early life stages of many animals are often most sensitive. Broader ecosystem consequences cascade through food webs and can alter habitat structure (for example, coral reefs). Economically important fisheries, aquaculture, and tourism may be affected where key species decline.

• Vulnerable organisms: corals, oysters, mussels, and some plankton
• Possible ecosystem effects: reef loss, altered species interactions, and reduced biodiversity
• Societal consequences: threats to fisheries, livelihoods, and coastal protection

Monitoring, responses and distinctions

Scientists track ocean chemistry with ships, buoys and autonomous sensors that measure pH, alkalinity and carbonate saturation. Responses fall into mitigation and adaptation: cutting CO2 emissions is the only long-term solution to halt ocean acidification; local measures such as reducing pollution and protecting habitats can increase resilience. Ocean acidification is related to but distinct from ocean warming—both are driven by greenhouse gas emissions and together pose combined stresses on marine life.

Research priorities and notable facts

Key research needs include understanding species-specific sensitivities, regional vulnerability, interactive effects with warming and deoxygenation, and potential socio-economic impacts. The process is gradual but persistent: even small changes in seawater chemistry can have outsized effects on processes such as calcification, nutrient cycling and food web dynamics. For further information and monitoring data see resources and programs linked at ocean monitoring networks and detailed carbon-cycle summaries at carbon research portals.