Increasing atmospheric CO2 is absorbed by the ocean and leads to changes in the ocean’s carbonate chemistry. Ocean acidification occurs when CO2 in the atmosphere reacts with water to create carbonic acid, decreasing both the pH of seawater (increasing seawater’s acidity) and the concentration of the carbonate ion. The carbonate ion is essential for calcification, a process needed for all marine animals that create a calcium carbonate skeleton, like corals.
Although the chemistry of this effect is well understood, the extent of the impacts of ocean acidification on marine ecosystems and human well-being are not well known. Warming seas and ocean acidification are already affecting reefs by causing mass coral bleaching events and slowing the growth of coral skeletons, which threatens coral reef resilience. ref
Recent research demonstrates that severe acidification and warming alone can reduce reef resilience (through impaired coral growth and increased coral mortality). ref In addition, ocean acidification is likely to make corals more susceptible to breakage from tropical storm impacts. The ability of corals to keep pace with sea-level rise may also be reduced due to decreased growth rates caused by ocean acidification. Finally, reefs that are already threatened by local stressors are likely to be more vulnerable to ocean acidification , thus management of local-scale stressors will be critical to keep reefs healthy in the face of increasing global stressors.ref
Guidance on managing ocean acidification can be found here.