A1.2.4.1 Carbon Cycle


Carbon is the fourth most abundant element in the Universe. 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.

The carbon cycle is a complex series of processes through which all of the carbon atoms in existence rotate. It involves the earth’s atmosphere, fossil fuels, the oceans, vegetation and the soils of the earth’s terrestrial ecosystem.


Source: http://thoughtsonglobalwarming.blogspot.com/2007/02/factsheet.html


The carbon cycle is sometimes thought of as a series of carbon reservoirs, where carbon may move from one to another (figures are in billion tonnes of carbon):

  • Marine sediments and sedimentary rocks (66,000 000 – 100,000 000)
  • Deep oceans (40,000)
  • Surface oceans (1,000)
  • Vegetation (540-610)
  • Soils and organic matter (1,600)
  • Coal (3,000)
  • Oil, gas (300)
  • Atmosphere (750)


At a glance we can see that the amount of carbon stored in sedimentary rocks and the deep oceans is enormous and far greater than that stored in the atmosphere or in the earth’s vegetation. These large reservoirs of carbon are stable and long-term.

In contrast, the atmospheric reservoir of carbon is less stable. Different processes (such as photosynthesis, respiration and the absorption and release of carbon by the oceans) operate to cycle carbon between the atmosphere, biosphere and oceans. Processes that release carbon into the atmosphere are called sources (e.g. respiration), while those that remove carbon from the atmosphere are called sinks (e.g., photosynthesis). This exchange of carbon between the different reservoirs is simply called ‘the carbon cycle’.


A1.2.4.1 carbon cycle

Source: http://earthobservatory.nasa.gov


Carbon flows between each reservoir in an exchange called the carbon cycle, which has slow and fast components. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Changes that put carbon gases into the atmosphere result in warmer temperatures on Earth.

During the carbon cycle plants absorb carbon dioxide from the atmosphere and use it, combined with water from the soil, to make the substances they need for growth during photosynthesis. This process incorporates the carbon atoms from carbon dioxide into sugars. Animals then eat the plants and use the carbon to build their own tissues. The animals return the carbon dioxide into the air when they breathe and then when they die, carbon is returned to the soil during decomposition. The carbon atoms in the soil may then be used in a new plant.

Over the long term, the carbon cycle seems to maintain a balance that prevents all of Earth’s carbon from entering the atmosphere (as is the case on Venus) or from being stored entirely in rocks. This balance helps keep Earth’s temperature relatively stable, like a thermostat.


Carbon enters the atmosphere through:

  • Respiration when animals breathe out Carbon Dioxide.
  • Burning.
  • Decomposition.


Carbon is taken out of the atmosphere by:

  • Plants and algae through photosynthesis.


Two key processes involved in the carbon cycle are:

  • Photosynthesis.
  • Combustion.


An activity on the Carbon Cycle is available here (this is voluntary, but you may find it useful): http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/energy_biomass/thecarboncycleact.shtml