Ocean carbonate saturation, 1861, Global map showing the saturation state of aragonite, a form of calcium carbonate, in the world's oceans in 1861. Higher levels of saturation are blue, with saturation decreasing through yellow and orange to very low levels (red). More than a quarter of the carbon dioxide released into the atmosphere is absorbed by the oceans, where it reacts with water to form carbonic acid. This reaction lowers the concentration of carbonate ions, which are used by many marine organisms, such as coral, to form shells. As carbon dioxide levels rise and more is absorbed by the oceans, those organisms that need carbonate to make their shells will find it harder to survive. Computer model based on past CO2 emissions. For a sequence showing the changes in ocean carbonate saturation between 1861 and 2100, see images C047/5613 to C047/5619.