Carbon dioxide is an atmospheric gas composed of one carbon and two oxygen atoms. One of the best known of chemical compounds, it is frequently called by its formula:
CO2 (pronunciation: "see oh two")
Carbon dioxide results from the combustion of organic matter if sufficient amounts of oxygen are present. It is also produced by various microorganisms from fermentation and cellular respiration. Plants utilize carbon dioxide during photosynthesis, using both the carbon and the oxygen to construct carbohydrates. In addition, plants also release oxygen to the atmosphere which is subsequently used for respiration by heterotrophic organisms, forming a cycle. It is present in the Earth's atmosphere at a low concentration and acts as a greenhouse gas. It is a major component of the carbon cycle.
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 HCO3-
Reactions like this tend to buffer changes in atmospheric CO2. Reactions between carbon dioxide and non-carbonate rocks also add bicarbonate to the seas, which 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. If all the carbonate rocks in the earth's crust were to be converted back into carbon dioxide, the resulting carbon dioxide would weigh 40 times as much as the rest of the atmosphere.
The vast majority of CO2 added to the atmosphere will eventually be absorbed by the oceans and become bicarbonate ion, but the process takes on the order of a hundred years because most seawater rarely comes near the surface.