Carbon cycle and Ocean Acidification

The carbon cycle describes the fluxes of carbon dioxide (CO 2) between the oceans, terrestrial biosphere, lithosphere, and atmosphere. Human activities such as the combustion of fossil fuels and land-use changes have led to a new flux of CO 2 into the atmosphere. About 45% has remained in the atmosphere; most of the rest has been taken up by the oceans, with some taken up by terrestrial plants.

Distribution of (A) aragonite and (B) calcite saturation depth in the global oceans This map shows changes in the aragonite saturation level of ocean surface waters between the 1880s and the most recent decade (2006–2015).

Aragonite is a form of calcium carbonate that many marine animals use to build their skeletons and shells. The lower the saturation level, the more difficult it is for organisms to build and maintain their skeletons and shells. A negative change represents a decrease in saturation.

The carbon cycle involves both organic compounds such as cellulose and inorganic carbon compounds such as carbon dioxide, carbonate ion, and bicarbonate ion.

The inorganic compounds are particularly relevant when discussing ocean acidification for they include many forms of dissolved CO 2 present in the Earth's oceans. When CO 2 dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free carbon dioxide (CO 2(aq)), carbonic acid (H 2CO 3), bicarbonate (HCO− 3) and carbonate (CO2− 3).

The ratio of these species depends on factors such as seawater temperature, pressure and salinity (as shown in a Bjerrum plot). These different forms of dissolved inorganic carbon are transferred from an ocean's surface to its interior by the ocean's solubility pump. The resistance of an area of the ocean to absorbing atmospheric CO 2 is known as the Revelle factor.