Ocean acidification is the lowering of the ocean's pH due to the rising concentration of CO2 in the atmosphere. Because CO2 in the atmosphere and surface ocean is in equilibrium, increasing atmospheric CO2 pushes CO2 into the ocean, where it combines with H2O to form carbonic acid (H2CO3). Since pre-industrial times, the average pH of the ocean has decreased by roughly 0.1 units (from about 8.2 to 8.1), a 30 percent increase in acidity. About one-third of the CO2 emitted by humans since the start of the Industrial Revolution has been absorbed by the ocean.
Often referred to as global warming’s evil twin, ocean acidification is potentially damaging to marine ecosystems. Many organisms that have calcium carbonate shells or skeletons have more difficulty building them as pH decreases below pre-industrial levels; damage to these organisms threatens the base of the marine food chain. Potential levels of acidification have also been shown to be directly hazardous to other living things, such as fish.
The ocean is buffered against pH change by CaCO3 derived from continental erosion. Global pH can only experience a sustained drop when the rise of atmospheric CO2 outpaces the replenishment of the carbonate buffer. This has kept ocean pH constant over long periods of geologic time, even during times when atmospheric CO2 was higher than it is today. It has probably been between 3 and 30 million years since ocean pH last dropped below its current levels.