How does ocean acidification affect coral reef ecosystems?

Ocean acidification undermines the chemical foundation that allows reef-building corals to build skeletons and sustain complex ecosystems. As the ocean absorbs carbon dioxide from the atmosphere, carbonate chemistry shifts so that fewer carbonate ions are available for organisms that form calcium carbonate shells and skeletons. The National Oceanic and Atmospheric Administration explains that reduced carbonate ion concentration and a lower aragonite saturation state make calcification energetically more difficult for corals and many reef-associated calcifiers.

How ocean chemistry changes
Laboratory experiments and field observations dating back decades document these basic mechanisms. Research led by Ove Hoegh-Guldberg at the University of Queensland synthesizes evidence that rising CO2 both reduces the rate at which corals deposit aragonite skeleton and alters physiological processes such as photosynthesis and respiration in the coral animal and its symbiotic algae. Work by Ken Caldeira at the Carnegie Institution for Science and others models how continued CO2 uptake will progressively shift seawater chemistry in tropical reef regions, with the most vulnerable areas reaching corrosive conditions for aragonite sooner than previously estimated.

Ecological and physiological impacts
Corals under acidified conditions commonly show slower growth, thinner skeletons, and reduced structural complexity. Weaker skeletons make reefs more susceptible to physical damage from storms and bioerosion by boring organisms, a pattern reported in field studies by Katharina Fabricius at the Australian Institute of Marine Science. Early life stages are especially sensitive; acidified seawater can impair larval development, settlement onto reef substrate, and recruitment of juvenile corals, reducing the capacity of reefs to recover after disturbances. Beyond corals, acidification affects calcifying algae, bivalves, and some plankton, changing competitive relationships and often favoring non-calcifying algae or soft-bodied organisms. These shifts simplify habitat structure and can lower overall biodiversity.

Interactions with other stressors and consequences for people
Ocean acidification rarely acts alone. Rising sea temperature, pollution, overfishing, and coastal development magnify effects of acidification by increasing coral bleaching, disease, and algal overgrowth. The Intergovernmental Panel on Climate Change highlights that combined stressors lead to more frequent and severe reef degradation than any single factor. Consequences extend beyond ecology: reefs provide food security, livelihoods, cultural identity, and shoreline protection for millions, particularly in island and coastal communities. Declines in reef condition reduce fisheries productivity, threaten tourism economies, and diminish natural coastal defenses, increasing vulnerability to erosion and storm surge.

Pathways for resilience
Scientific authors and institutions emphasize that reducing CO2 emissions is the primary means to limit long-term acidification and its systemic impacts on reefs. Local measures such as improved water quality management, sustainable fisheries practices, and protection of herbivore populations can increase resilience in the near term by reducing cumulative stresses. Continued monitoring and targeted restoration informed by research from institutions like the National Oceanic and Atmospheric Administration and university research groups are essential to track changes and guide conservation strategies that also respect the cultural and territorial rights of reef-dependent peoples.