Ocean acidification is the reduction in the pH of the ocean over an extended period, typically taking decades or longer. The primarily cause is the uptake of carbon dioxide from the atmosphere into seawater, but can also be caused by other chemical additions or subtractions from the ocean. Examples of ocean acidification are recorded in the geologic record associated with major periods of geologic eruptions and massive extraterrestrial impacts (such as the event that wiped out the dinosaurs along with many groups of marine organisms with shells about 66 million years ago).
Anthropogenic ocean acidification refers to the component of pH reduction that is caused by human activity. In the last 250 years, the concentration of CO2 in the atmosphere has increased from 280 parts per million to over 394 parts per million. Most of this is due to the burning of fossil fuels ( coal, gas, and oil) and also by CO2 and other acid-forming compounds released by land use changes (such as burning off forests to be replaced by agriculture). Ocean acidification has potentially devastating ramifications for all forms of ocean life, from microscopic plankton to the largest animals at the top of the food chain. See Environmental consequences of ocean acidification (United Nations).
Ocean acidification, increased temperatures, and changing oxygen level are all related, all of which can have catastrophic impacts of marine ecosystems (example in Figure 9.41).
Figure 9.41. Bleaching of coral results death of symbiotic algae living within the corals. This also kills the coral, and is resulting in the collapse of local ecosystems. Elevated water temperatures and acidification are contributing factors.