What is the phenomenon where increasing CO2 makes oceans more acidic, harming corals?

The phenomenon where increasing carbon dioxide makes oceans more acidic and harms corals is called ocean acidification.

When the ocean absorbs atmospheric carbon dioxide, the gas reacts with seawater and forms carbonic acid. That reaction increases hydrogen ions, lowering ocean pH and reducing the concentration of carbonate ions. Seawater remains slightly alkaline, but it becomes progressively less alkaline—a change scientists call acidification.

Corals and other marine organisms are vulnerable because they use calcium carbonate to build skeletons and shells. Reduced carbonate availability can make coral growth more difficult and can leave existing structures more susceptible to erosion. Ocean warming and acidification often occur together, adding different stresses to reef ecosystems.

This is not the same as global warming, although both are driven largely by human greenhouse-gas emissions. Global warming refers primarily to rising temperatures; ocean acidification describes a chemical change caused by carbon dioxide uptake. Since about 1750, oceans have absorbed roughly 30% of human-caused carbon dioxide emissions.