Carbon is exchanged, or “cycled” among Earth’s oceans, atmosphere, ecosystem, and geosphere. All living organisms are built of carbon compounds. It is the fundamental building block of life and an important component of many chemical processes. It is present in the atmosphere primarily as carbon dioxide (CO2), but also as other less abundant but climatically significant gases, such as methane (CH4).

Because life processes are fueled by carbon compounds which are oxidized to CO2, the latter is exhaled by all animals and plants. Conversely, CO2 is assimilated by plants during photosynthesis to build new carbon compounds. CO2 is produced by the burning of fossil fuels, which derive from the preserved products of ancient photosynthesis. The atmosphere exchanges CO2 continuously with the oceans. Regions or processes that predominately produce CO2 are called sources of atmospheric CO2, while those that absorb CO2 are called sinks.

Most of Earth’s carbon—about 65,500 billion metric tons—is stored in rocks. The rest is in the ocean, atmosphere, plants, soil, and fossil fuels. Forests, oceans, and soil are the main carbon sinks on Earth. Carbon flows between each reservoir in the exchange called the carbon cycle, which has slow and fast components. Through a series of chemical reactions and tectonic activity, carbon takes 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. On average, 1013 to 1014 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year. In comparison, the fast carbon cycle moves 1016 to 1017 grams of carbon per year. Plants and phytoplankton are the main components of the fast carbon cycle.

Human actions cause emissions of carbon to the atmosphere, at about 1015 grams of carbon per year. The atmosphere now contains more carbon than at any time in at least two million years, due to carbon moved from deep in the earth into the atmosphere. So far, land plants and the ocean have taken up about 55 percent of the extra carbon human actions have put into the atmosphere while about 45 percent has stayed in the atmosphere. Eventually, the land and oceans will take up most of the extra carbon dioxide, but as much as 20 percent may remain in the atmosphere for many thousands of years.

Note: The nitrogen cycle and the halocarbon cycle (linked with the ozone cycle) are also natural parts of the complex human influence on the environment.