Present-day (2021) atmospheric carbon dioxide (CO) levels of around 415 ppm are around 50% higher than preindustrial concentrations. The current elevated levels and rapid growth rates are unprecedented in the past 55 million years of the geological record. The sources of this excess CO are clearly established as human driven: they include anthropogenic fossil fuel, industrial, and land-use/land-change emissions. The ocean acts as a carbon sink for anthropogenic CO and takes up roughly a quarter of total anthropogenic CO emissions. However, the additional CO in the ocean results in a wholesale shift in seawater acid-base chemistry toward more acidic, lower pH conditions and lower saturation states for carbonate minerals used in many marine organism shells and skeletons. 
Accumulated since 1850, the ocean sink holds up to 175 ± 35 gigatons of carbon, with more than two-thirds of this amount (120 GtC) being taken up by the global ocean since 1960. Over the historical period, the ocean sink increased in pace with the exponential anthropogenic emissions increase. From 1850 until 2022, the ocean has absorbed 26 % of total anthropogenic emissions. Emissions during the period 1850–2021 amounted to 670 ± 65 gigatons of carbon and were partitioned among the atmosphere (41 %), ocean (26 %), and land (31 %).
The carbon cycle describes the fluxes of carbon dioxide () between the oceans, terrestrial biosphere, lithosphere, and atmosphere. The carbon cycle involves both organic compounds such as cellulose and inorganic carbon compounds such as carbon dioxide, carbonate ion, and bicarbonate ion, together referenced as dissolved inorganic carbon (DIC). These inorganic compounds are particularly significant in ocean acidification, as they include many forms of dissolved  present in the Earth's oceans. 
When  dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free carbon dioxide (), carbonic acid (), bicarbonate () and carbonate (). 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 oceans surface to its interior by the oceans solubility pump. The resistance of an area of ocean to absorbing atmospheric  is known as the Revelle factor.