Why is the ocean turning more acidic, and why do oysters care?

The ocean soaks up about a quarter of the carbon dioxide humans release. That gas reacts with seawater to form an acid. Between 1950 and 2020, the average pH of the ocean surface slid from about 8.15 to 8.05. Seawater is still alkaline, not acidic in the everyday sense. But today's pH is unprecedented for at least 26,000 years, and so is the speed of the change.

The chemistry is short. Dissolved carbon dioxide forms carbonic acid. Carbonic acid splits and releases hydrogen ions, and more hydrogen ions means a lower pH. The pH scale is logarithmic, so a drop of just 0.1 means 26 percent more hydrogen ions in the water.

Oysters care because that chemistry takes away the mineral they build with. Shells and skeletons are made of calcium carbonate. A more acidic ocean holds fewer of the carbonate ions needed to make it. The Pacific oyster, the most farmed oyster in the world, shows the strain. Acidifying water cuts its reproduction, lowers the survival of young oysters and makes oyster farms less profitable. Corals, tiny swimming sea snails called pteropods, and plankton with chalky shells face the same squeeze. This is a different problem from coral bleaching, which is caused by heat.

About a billion people depend at least partly on coral reefs for fishing, tourism or coastal protection. Scientists describe cutting carbon dioxide emissions as one of the only solutions that tackles the root cause. And once a shellfish population is gone, even careful conservation may not bring it back.