Ocean acidification is the continuous reduction of the Earth's oceans pH levels. This is caused by the oceans uptake of carbon dioxide derived from human activities from the atmosphere. In the last 243 years the ocean surface pH as been reduced from about 8.179 to 8.104.
The carbon cycle is the constant change of carbon dioxide between the terrestrial biosphere, atmosphere, lithosphere and the oceans. Human activities such as combustion of fossil fuels, land-use changes and cement production have let to this constant change of CO2 into the Earth's atmosphere. Some CO2 has been taken by terrestrial plants, the oceans have absorbed some CO2 and some has remained in the atmosphere. There are two forms of the carbon cycle: inorganic carbon cycle and organic carbon cycle.
The decrease in ocean pH most often stems from dissolving CO2 in the oceans saltwater. By dissolving this it increases the hydrogen ion concentration leading to a reduction in pH. Ocean acidification will continue to decrease as more carbon dioxide derived from human activities is absorbed by the oceans. How much acidification takes place in the future ultimately depends on the emissions pathways and mitigation that society chooses to take.
A continuous reduction in the ocean's pH will have negative consequences. These consequences will primarily affect oceanic calcifying organisms. These organisms rely on the calcite of calcium carbonate or aragonite polymorphs to construct their skeletons or cell coverings. Calcifying organisms include organisms such as molluscs, coccolithophores, crustaceans, corals, echinoderms and foraminifera.
When pH levels are within a normal range, aragonite and calcite are stable in the ocean surface because carbonate ion experiences supersaturating conditions. This ions concentration falls as the ocean pH falls resulting in structures made of calcium becoming undersaturated. When undersaturated they are vulnerable to dissolution.
It is also thought that if ocean acidification causes the coccolithophore population to decline climate will be affected. This affect of ocean acidification will stem from the oceanic cloud cover being affected leading to a reduction of the Earth's albedo. Other adverse effects of ocean acidification include organisms facing a variety of physiological and reproductive effects, hypercapnia, reduction in food sources, increase in ocean noise, elevated ocean alkalinity and climate change.
Ocean: http://www.flickr.com/photos/lanacar/243681251/