When humans burn fossil fuels, the oceans absorb roughly one-third of that additional carbon dioxide. This process staves off (some) global warming, but it also makes the seas more acidic, as the carbon dissolves in water to form carbonic acid. That’s ocean acidification.
Since the Industrial Revolution, the oceans have become 30 percent more acidic (that is, the pH of ocean surface water has dropped from roughly 8.18 to 8.07). And that process is expected to continue if humans continue emitting greenhouse gases, with the rate of change expected to be the fastest in 300 million years.
More acidic seawater can chew away at coral reefs and kill oysters by making it harder for them to form protective shells. Acidification can also interfere with the food supply for key species like Alaska’s salmon. One study in the journal Climatic Change estimated that the loss of mollusks alone could cost the world as much as $100 billion per year by the end of the century.
Scientists are still trying to understand exactly how acidification will affect different species and the marine food chain, both through lab experiments and by looking at past acidification events. About 55 million years ago, during the Paleocene-Eocene Thermal Maximum, the oceans became warmer and more acidic. As a result, coral reefs became scarcer and the food chain had difficulty supporting larger predators.