The measure of acidity in chemical systems is the pH value. pH values can range from 0 to 14. Values in the range less than 7 are acidic, pH of 7 is neutral, and a pH greater than 7 is basic (alkaline). The oceans are normally slightly alkaline, with a pre-industrial pH value of approximately 8.179. The current pH value of the oceans is approx 8.07, which represents a decline in alkalinity in the last 2-3 centuries.
Increased ocean acidity is caused by increased concentrations of CO2 in the atmosphere, the result of massive combustion of fossil fuels for the generation of energy for modern civilizations. Atmospheric CO2 is absorbed by the oceans and in its reaction with water, releases hydrogen ions that lower ocean pH levels.
The chemistry of CO2 dissolved in ocean water results in a decline of carbonate ions, which causes a decrease in the availability of calcium carbonate, the primary constituent of shells and other exoskeletons of marine organisms. This is a major issue, threatening the viability of numerous ocean ecosystems and resulting in the marked decline of numerous marine species.