Carbon Dioxide in Earth's Atmosphere - Relationship With Oceanic Concentration

Relationship With Oceanic Concentration

See also: Solubility pump and Ocean acidification

The Earth's oceans contain a huge amount of carbon dioxide in the form of bicarbonate and carbonate ions — much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide. One example is the dissolution of calcium carbonate:

CaCO₃ + CO₂ + H₂O ⇌ Ca2+ + 2 HCO−
3

Reactions like this tend to buffer changes in atmospheric CO₂. Since the right-hand side of the reaction produces an acidic compound, adding CO₂ on the left-hand side decreases the pH of sea water, a process which has been termed ocean acidification (pH of the ocean becomes acidic although the pH value remains in the alkaline range). Reactions between carbon dioxide and non-carbonate rocks also add bicarbonate to the seas. This can later undergo the reverse of the above reaction to form carbonate rocks, releasing half of the bicarbonate as CO₂. Over hundreds of millions of years this has produced huge quantities of carbonate rocks.

Ultimately, most of the CO₂ emitted by human activities will dissolve in the ocean; however, the rate at which the ocean will take it up in the future is less certain. Even if equilibrium is reached, including dissolution of carbonate minerals, the increased concentration of bicarbonate and decreased or unchanged concentration of carbonate ion will give rise to a higher concentration of un-ionized carbonic acid and dissolved carbon dioxide gas. This, along with higher temperatures, would mean a higher equilibrium concentration of carbon dioxide in the air.