Ocean acidification is the decreasing in the basic value (the pH value) of the earth's oceans. This is mostly the result of the formation of carbonic acid in the water from increased carbon dioxide in the atmosphere, which is, in turn, caused by the burning of fossil fuels. An estimated 30 to 40 percent of the carbon dioxide released into the atmosphere from human activity dissolves into the oceans, lakes and other water bodies.
Seawater is slightly basic, and ocean acidification involves a shift towards pH-neutral conditions rather than an actual transition to an acidic condition. This decrease in the basic value can harm marine animals in a variety of ways, including interfering with the growing of their calcium carbonate shells and making it difficult for them to reproduce. The result is damage to marine ecosystems as a whole and thus also to commercial fisheries.
Acidification is only one of several ways in which the large-scale use of coal and petroleum is damaging the ocean ecosystems. Others include an increase in the toxic metal mercury from absorption of mercury released into the atmosphere by burning coal and contamination from plastics and synthetic fibers.