Oceanic pH, 24 million years BP - present. The rapid change in ocean pH since the Industrial Revolution is likely the fastest in Earth’s history. Turley, C., et al. 2006

The oceans absorb large amounts of carbon dioxide emissions each day. As a result, their pH has declined by 30 percent since the beginning of the Industrial Revolution. This rapid change in ocean chemistry, called ocean acidification, is already threatening habitats like coral reefs, and the future of shellfish like oysters, clams, and mussels is also in jeopardy. This means that nations that rely heavily on threatened types of fisheries as a primary food source could be hit hardest.

Ocean-Based Food Security Threatened in a High CO2 World [pdf]