The carbon cycle is the means by which carbon atoms are exchanged between living things, the ground, the oceans, and the skies; or biosphere, geosphere, hydrosphere, and atmosphere, respectively. There is about 1,000,000 gigatons of carbon on Earth, most of which is locked in sedimentary rocks and never reaches the surface. At the surface, carbon is continuously engaging in a dynamic exchange of consumption and production. This active exchange is referred to as the carbon cycle.

The atmosphere contains approximately 750 gigatons of carbon, mostly in the form of carbon dioxide (CO2). At the consumption end of the carbon cycle, carbon is continuously being drained by 1) the process of photosynthesis, which uses carbon dioxide to create carbohydrates, and 2) the colder parts of the oceans, which are able to absorb carbon dioxide. At the production end of the carbon cycle, atmospheric carbon is continuously being replenished by the following processes: respiration of plants and animals and their decay; the burning of fossil fuels; limestone reactions; the release of carbon dioxide by warm areas of the ocean; and volcanic eruptions. The human burning of fossil fuels has slightly knocked the global carbon cycle or budget out of balance, resulting in an atmosphere with increasing levels of carbon dioxide.

The biosphere contains approximately 600 gigatons of carbon, representing the most condensed carbon pool on Earth. Through the carbon cycle, carbon circulates in and out of the biosphere as plants and animals respirate, excrete, perish, and perform photosynthesis. Soil in the form of organic matter, which plants and animals become when they die, contains about 1,500 gigatons of carbon. Some of this carbon sinks deep into the Earth into sedimentary rocks, never to surface again. Some of it reaches the oceans and dissolves. The deep oceans contain a substantial amount of carbon, about 38,000 gigatons.

About 5.5 gigatons of fossil fuel emissions are released into the atmosphere each year by human industry, creating a figurative top-heavy carbon cycle. International coalitions have been formed to limit these emissions, with moderate success so far. Remaining coal deposits total about 3,000 gigatons of carbon, and remaining oil and gas reserves make up about 300.