How it works
When CO2 dissolves in seawater, it reacts with water to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions. The increase in hydrogen ions lowers the pH of seawater — making it more acidic. Ocean surface pH has fallen from approximately 8.2 in pre-industrial times to around 8.1 today. A 0.1 unit drop in pH represents a 26% increase in hydrogen ion concentration.
Who is most affected
Calcifying organisms — those that build shells or skeletons from calcium carbonate — are particularly vulnerable. Higher acidity lowers the saturation of aragonite (a form of calcium carbonate) in seawater, making it harder for corals, oysters, mussels, sea urchins and some plankton to build and maintain their structures.
Coral reefs at risk
Coral reefs already face existential pressure from warming-induced bleaching. Acidification compounds this threat. Studies suggest that at 2 degrees C of warming, combined with acidification, the conditions suitable for reef-building corals will shrink dramatically. Some reefs may shift from net calcification to net dissolution — literally dissolving.
The outlook
Ocean acidification is directly tied to CO2 emissions. Unlike warming, where there is some lag between emissions and temperature response, acidification tracks CO2 concentrations closely. Reducing emissions is the only meaningful way to slow or halt the acidification trend.