Voyager: How does ocean acidification directly change coral reef formations?


Research into the impacts of ocean acidification on coral reefs and other marine organisms is still in its beginning stages. However, understanding how higher concentrations of carbon dioxide in the oceans affect corals and their formation is quickly gaining new ground.

It is well documented that the oceans are absorbing atmospheric CO2 released from the burning of fossil fuels. And the amount being emitted into the atmosphere is only growing. This is evidenced in the Keeling Curve, started at Scripps Institution of Oceanography at UC San Diego by geochemist Charles David Keeling. When he began his measurements in 1958, there were 315 parts per million (ppm) of carbon dioxide in the air. Today there are roughly 385 ppm. When CO2 is absorbed by seawater, chemical reactions occur that result in an increase of hydrogen ions or acidity, and a decrease in pH and a decrease in the concentration of carbonate ion. Carbonate ions are an important building block of the calcium carbonate shells and skeletons produced by many marine animals and algae.  

Experimental evidence suggests that rising acidity will reduce the ability of corals to build their skeletons. At some point, growth of coral reefs may not keep up with their rates of erosion. In addition, global warming is causing tropical ocean temperatures to increase, resulting in coral bleaching in many regions.  The stress of higher temperatures and decreased pH may be happening faster than corals can adapt. If the health of corals deteriorates, this will in turn affect other animals such as molluscs, anemones, and fish, which depend on reefs for food, protection, and shelter.

The rising ocean acidity of surface waters is another aspect of fossil fuel use. The extent and rate of change projected suggests that at least some species may be adversely impacted, thus strengthening the call to reduce carbon emissions.

— Victoria Fabry, Visiting Research Scientist, Marine Physical Laboratory

To learn more about how scientists study acid in the oceans, read our story “Carbonated Oceans” at

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