One of the most certain outcomes from increasing CO2 concentrations in the atmosphere is the acidification of the world’s oceans. Roughly one-quarter of the CO2 currently released by human activities is absorbed in the sea. While some of the CO2 is taken up by marine organisms, most if it combines with water to form carbonic acid. The result has been a roughly 30 percent increase in ocean acidity since preindustrial times.
Global trends and projections
Since the beginning of the industrial era, the pH of surface seawater has decreased from about 8.2 to 8.1. While the ocean is “basic” because it is above 7 on the pH scale, dissolved CO2 acidifies the ocean by increasing the concentration of hydrogen ion. The pH change from 8.2 to 8.1 corresponds to a 26 percent increase in acidity (or hydrogen ion concentration) and is likely the fastest acidification rate in 300 million years.
Without concerted action to reduce global CO2 emissions, oceanic pH could drop to 7.8 by the end of this century. That would be a huge change, representing a 150-percent increase in acidity. Such an alteration in the marine environment could have devastating results, for any marine species that extracts calcium carbonate to build its shell or skeleton, and for the people who depend upon them.
A January 2018 study looked at the skeletal growth of Porites—a dominant reef-building coral. The study found that ocean acidification directly and negatively affects the coral's density and that skeletal density of Porites corals could decline by up to 20.3 percent over the 21st century solely due to ocean acidification.