evidence: That careful balance has survived over time because of a near equilibrium among the acids emitted by volcanoes and the bases liberated by the weathering of rock. The pH of seawater has remained steady for millions of years. Before the industrial era began, the average pH at the ocean surface was about 8.2 (slightly basic; 7.0 is neutral). Today it is about 8.1. Although the change may seem small, similar natural shifts have taken 5,000 to 10,000 years. We have done it in 50 to 80 years. Ocean life survived the long, gradual change, but the current speed of acidification is very worrisome. Emissions could reduce surface pH by another 0.4 unit in this century alone and by as much as 0.7 unit beyond 2100. We are hurtling toward an ocean different than the earth has known for more than 25 million years. [...] We are now carrying out an extraordinary chemical experiment on a global scale. Our fossil-fuel emissions raise the dissolved CO2 levels in the ocean, which reduces carbonate ion concentrations and lowers pH. The ocean’s sunlit surface layer (the top 100 yards or so) could easily lose 50 percent of its carbonate ion by the end of this century unless we reduce emissions dramatically. Marine animals will find it harder to build skeletons, construct reefs, or simply to grow and breathe. Compared with past geologic events, the speed and scale of this conversion is astonishing.