Natural carbon cycle

This page discusses the Earth's natural cycle of carbon - to read about human effects on this cycle click here

The natural carbon cycle is the flow of carbon naturally throughout across the globe in various forms, such as carbon dioxide or methane. This carbon moves through the atmosphere, ocean, terrestrial biosphere, and lithosphere. The natural carbon cycle is kept very nearly in balance; animals and plants emit CO2 into the atmosphere through respiration, while plants absorb it through photosynthesis. The ocean also cycles CO2 with the atmosphere, in an almost perfect balance. This process cycles carbon rapidly, with a typical molecule of CO2 spending only around 5 years in the atmosphere.[1]

Within the broad view of the carbon cycle are various key cycles, ranging from land to air to ocean. Figure 1 below shows all of the cycles that will be talked about on this page.


The two main cycles are the land-atmosphere cycle, and the ocean-atmosphere cycle. They both occur on very different timescales, with the land cycle occurring at a high rate while the ocean cycle is much slower.

  • Land-atmosphere - this cycle occurs via two main drivers; photosynthesis and respiration. In photosynthesis, carbon dioxide is absorbed from the atmosphere to create a fuel for the plant, while respiration consumes oxygen and produces carbon dioxide. Respiration also occurs via decaying matter, in which the compounds making up the matter decompose by bacteria which consumes oxygen along with the matter to produce energy and carbon dioxide.[2]
  • Ocean-atmosphere - the driving mechanism for this cycle is the difference in carbon dioxide's partial pressure between the ocean and atmosphere (partial pressure is the pressure the gas would have if it occupied the entire volume of the mixed solutions, in this case the volume of the ocean and atmosphere). This pressure varies with both the temperature of the ocean, and the local marine photosynthesis. The lower the ocean temperature is the less carbon is emitted (much like how a warm glass of pop becomes flat more quickly). Therefore some regions of the ocean take in carbon (carbon sink) while some emit carbon (carbon source).[2]
Figure 1. The carbon cycle of the Earth. Numbers represent the mass of carbon in gigatonnes (not the molecules, just carbon alone) that is cycled in a year. Yellow text is the natural carbon cycle, with red text showing human effects.[3] Notice that the 9 gigatonnes of carbon that humans are emitting (~35 gigatonnes of carbon dioxide) becomes an extra 4 gigatonnes in the atmosphere, an extra 3 gigatonnes of photosynthesis and an extra 2 gigatonnes in the ocean every year. This is how humans are changing the natural carbon cycle.

Human effect on this natural cycle

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Human activities are known to be negatively affecting this cycle. As can be seen in the figure above, human input (all red numbers) are causing a net increase in the atmosphere, soil, and oceans. This net increase stems from the fact that humans pull carbon-rich fossil fuels from underground that would otherwise not be part of the cycle, and introduce them into it via combustion. To learn more about this, visit the main article.

For Further Reading


  1. R. Wolfson, "Carbon: A Closer Look" in Energy, Environment, and Climate, 2nd ed., New York, NY: W.W. Norton & Company, 2012, ch. 13, sec. 5, pp. 357-361
  2. 2.0 2.1 M. Melieres and C. Marechal, "Warming in the 20th century," in Climate Change: Past, Present and Future 1st ed., U.K.: Wiley, 2015, ch.29, sec.1, pp. 298-301
  3. Wikimedia Commons [Online]. (June 5 2015). Available: