How does ocean acidification affect coral reefs?

When atmospheric carbon dioxide rises, much of the excess is absorbed by the surface ocean. Ken Caldeira at Carnegie Institution for Science and colleagues established that this uptake alters seawater chemistry by increasing hydrogen ion concentration and reducing the availability of carbonate ions that corals use to build calcium carbonate skeletons. Richard A. Feely at the National Oceanic and Atmospheric Administration Pacific Marine Environmental Laboratory has measured these chemical changes globally, documenting declines in aragonite saturation states that directly challenge the mineral form corals need.

Effects on coral calcification and growth

Coral organisms deposit aragonite to create the three-dimensional framework of reefs. Lower carbonate ion concentrations translate into reduced calcification rates, producing thinner, weaker skeletons that grow more slowly. Ove Hoegh-Guldberg at the University of Queensland and collaborators have shown that acidification impairs early life stages, including larval development and settlement, which undermines natural reef recovery. Weaker skeletons are more vulnerable to physical breakage and to bioeroders such as boring sponges and parrotfish whose actions remove skeletal material more quickly than it is replaced. The combination of diminished calcification and increased bioerosion can shift reefs from net accreting systems to net eroding systems, reducing structural complexity that many reef species depend on.

Interactions with warming and local stressors amplify impacts. Hoegh-Guldberg’s research emphasizes that ocean warming increases the frequency of coral bleaching events; when bleaching and acidification occur together, corals show compounded declines in health and survival. Poor water quality, overfishing, and destructive coastal development further reduce resilience by limiting nutrients, increasing disease prevalence, and removing species that help control algae and bioerosion.

Ecological, cultural, and societal consequences

Coral reefs support exceptional biodiversity and provide ecosystem services that sustain coastal communities. Declines in reef structure and live coral cover reduce habitat for fish and invertebrates, with cascading effects on fisheries and food security for subsistence and commercial fishers. Reefs buffer shorelines from waves and storms; weakening of reef frameworks increases coastal erosion and risk to infrastructure for island nations and low-lying coasts. Cultural practices, tourism economies, and livelihoods tied to reef ecosystems are therefore at risk, a reality underscored in assessments by the National Academies of Sciences, Engineering, and Medicine which recommend integrating scientific evidence into policy and management.

Management and policy responses must address both the root cause and local drivers. Global reductions in carbon dioxide emissions are essential to slow further acidification and limit warming. Simultaneously, local actions such as improving water quality, protecting herbivores, and reducing overfishing can enhance reef resilience and buy time for adaptation. Monitoring programs led by institutions such as the National Oceanic and Atmospheric Administration provide the observational basis to track changes and inform management. Scientific work from Caldeira, Feely, Hoegh-Guldberg, and others establishes that without coordinated global and local action, ocean acidification will continue to impair coral reef formation, biodiversity, and the human communities that depend on them.