The ocean absorbs about 30% of anthropogenic CO₂. The CO₂ reacts with seawater to form carbonic acid, reducing pH. Ocean pH has dropped by 0.1 units since industrialization — a 26% increase in acidity (the pH scale is logarithmic).

The consequences for calcifying organisms (corals, oysters, pteropods) are significant: at lower pH, building and maintaining calcium carbonate shells requires more energy, and above certain thresholds, shells dissolve.

This is a slower-moving, less-visible threat than sea level rise or extreme weather, which is probably why it gets less attention.