Carbon Cycle

Carbon Cycle
The carbon cycle describes the storage and exchange of carbon between the Earth's biosphere, atmosphere, hydrosphere, and geosphere. NASA

The carbon cycle describes the storage and exchange of carbon between the Earth's biosphere (living matter), atmosphere (air), hydrosphere (water), and geosphere (earth). The main reservoirs of carbon are the atmosphere, biosphere, ocean, sediments, and interior of the Earth. Both natural and human activities transfer carbon between the reservoirs.

Key Takeaways: The Carbon Cycle

  • The carbon cycle is the process through which the element carbon moves through the atmosphere, land, and ocean.
  • The carbon cycle and nitrogen cycle are key to Earth's sustainability of life.
  • The main reservoirs of carbon are the atmosphere, biosphere, ocean, sediments, and Earth's crust and mantle.
  • Antoine Lavoisier and Joseph Priestly were the first to describe the carbon cycle.

Why Study the Carbon Cycle?

There are two important reasons the carbon cycle is worth learning about and understanding.

Carbon is an element that is essential for life as we know it. Living organisms obtain carbon from their environment. When they die, carbon is returned to the non-living environment. However, the concentration of carbon in living matter (18%) is about 100 times higher than the concentration of carbon in the earth (0.19%). The uptake of carbon into living organisms and return of carbon to the non-living environment are not in balance.

The second big reason is the the carbon cycle plays a key role in the global climate. Although the carbon cycle is huge, humans are able to effect it and modify the ecosystem. Carbon dioxide released by fossil fuel burning is about double the net uptake from plants and the ocean.

Forms of Carbon in the Carbon Cycle

Hand holding a green plant
Photoautotrophs take carbon dioxide and turn it into organic compounds.

sarayut Thaneerat / Getty Images

Carbon exists in several forms as it moves through the carbon cycle.

Carbon in the Non-Living Environment

The non-living environment includes substances that never were alive as well as carbon-bearing materials that remain after organisms die. Carbon is found in the non-living part of the hydrosphere, atmosphere, and geosphere as:

  • Carbonate (CaCO3) rocks: limestone and coral
  • Dead organic matter, such as humus in soil
  • Fossil fuels from dead organic matter (coal, oil, natural gas)
  • Carbon dioxide (CO2) in the air
  • Carbon dioxide dissolved in water to form HCO3

How Carbon Enters Living Matter

Carbon enters living matter through autotrophs, which are organisms capable of making their own nutrients from inorganic materials.

  • Photoautotrophs are responsible for most of the conversion of carbon into organic nutrients. Photoautotrophs, primarily plants, and algae, use light from the sun, carbon dioxide, and water to make organic carbon compounds (e.g., glucose).
  • Chemoautotrophs are bacteria and archaea that convert carbon from carbon dioxide into an organic form, but they get the energy for the reaction through oxidation of molecules rather than from sunlight.

How Carbon Is Returned to the Non-Living Environment

Carbon returns to the atmosphere and hydrosphere through:

  • Burning (as elemental carbon and several carbon compounds)
  • Respiration by plants and animals (as carbon dioxide, CO2)
  • Decay (as carbon dioxide if oxygen is present or as methane, CH4, if oxygen is not present)

Deep Carbon Cycle

The carbon cycle generally consists of carbon movement through the atmosphere, biospheres, ocean, and geosphere, but the deep carbon cycle between the mantle and crust of the geosphere is not as well understood as the other parts. Without the movement of tectonic plates and volcanic activity, carbon would eventually become trapped in the atmosphere. Scientists believe the quantity of carbon stored in the mantle is about a thousand times greater than the amount found on the surface.


  • Archer, David (2010). The Global Carbon Cycle. Princeton: Princeton University Press. ISBN 9781400837076.
  • Falkowski, P.; Scholes, R. J.; Boyle, E.; et al. (2000). "The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System". Science. 290 (5490): 291–296. doi:10.1126/science.290.5490.291
  • Lal, Rattan (2008). "Sequestration of atmospheric CO2 in global carbon pools". Energy and Environmental Science. 1: 86–100. doi:10.1039/b809492f
  • Morse, John W.; MacKenzie, F. T. (1990). "Chapter 9 the Current Carbon Cycle and Human Impact". Geochemistry of Sedimentary Carbonates. Developments in Sedimentology. 48. pp. 447–510. doi:10.1016/S0070-4571(08)70338-8. ISBN 9780444873910.
  • Prentice, I.C. (2001). "The carbon cycle and atmospheric carbon dioxide". In Houghton, J.T. (ed.). Climate change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.
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Your Citation
Helmenstine, Anne Marie, Ph.D. "Carbon Cycle." ThoughtCo, Oct. 29, 2020, Helmenstine, Anne Marie, Ph.D. (2020, October 29). Carbon Cycle. Retrieved from Helmenstine, Anne Marie, Ph.D. "Carbon Cycle." ThoughtCo. (accessed April 15, 2021).