Atmospheric carbon dioxide plays an integral role in the Earth's carbon cycle whereby  is removed from the atmosphere by some natural processes such as photosynthesis and deposition of carbonates, to form limestones for example, and added back to the atmosphere by other natural processes such as respiration and the acid dissolution of carbonate deposits. There are two broad carbon cycles on Earth: the fast carbon cycle and the slow carbon cycle. The fast carbon cycle refers to movements of carbon between the environment and living things in the biosphere whereas the slow carbon cycle involves the movement of carbon between the atmosphere, oceans, soil, rocks, and volcanism. Both cycles are intrinsically interconnected and atmospheric  facilitates the linkage.
Natural sources of atmospheric  include volcanic outgassing, the combustion of organic matter, wildfires and the respiration processes of living aerobic organisms. Man-made sources of  include the burning of fossil fuels for heating, power generation and transport, as well as some industrial processes such as cement making. It is also produced by various microorganisms from fermentation and cellular respiration. Plants, algae and cyanobacteria convert carbon dioxide to carbohydrates by a process called photosynthesis. They gain the energy needed for this reaction from absorption of sunlight by chlorophyll and other pigments. Oxygen, produced as a by-product of photosynthesis, is released into the atmosphere and subsequently used for respiration by heterotrophic organisms and other plants, forming a cycle with carbon.
Most sources of  emissions are natural, and are balanced to various degrees by similar  sinks. For example, the decay of organic material in forests, grasslands, and other land vegetation - including forest fires - results in the release of about 436 gigatonnes of  (containing 119 gigatonnes carbon) every year, while  uptake by new growth on land counteracts these releases, absorbing 451 Gt (123 Gt C).  Although much  in the early atmosphere of the young Earth was produced by volcanic activity, modern volcanic activity releases only 130 to 230 megatonnes of  each year. Natural sources are more or less balanced by natural sinks, in the form of chemical and biological processes which remove  from the atmosphere.
Overall, there is a large natural flux of atmospheric  into and out of the biosphere, both on land and in the oceans. In the pre-industrial era, each of  these fluxes were in balance to such a degree that little net  flowed between the land and ocean reservoirs of carbon, and little change resulted in the atmospheric concentration.  From the human pre-industrial era to 1940, the terrestrial biosphere represented a net source of atmospheric  (driven largely by land-use changes), but subsequently switched to a net sink with growing fossil carbon emissions. In 2012, about 57% of human-emitted , mostly from the burning of fossil carbon, was taken up by land and ocean sinks.
The ratio of the increase in atmospheric  to emitted  is known as the airborne fraction. This ratio varies in the short-term and is typically about 45% over longer (5-year) periods. Estimated carbon in global terrestrial vegetation increased from approximately 740 gigatonnes in 1910 to 780 gigatonnes in 1990.