The carbon cycle is a complex series of processes in which all the carbon atoms present in the earth circulate. Carbon is one of the more than 100 known elements. While the earth constantly receives light from the sun, it does not receive chemicals from outside except from occasional meteorites (and this is negligible). Thus, the carbon atoms (as also other elements) are part of a closed system, that is, they cannot be lost or replenished. These fixed number of carbon atoms are recycled in various processes that use chemicals containing carbon. These processes form a cycle that involves the living sphere called the biosphere, and the nonliving spheres – lithosphere, atmosphere and hydrosphere.
The carbon cycle is thus a kind of Biogeochemical Cycle in which carbon moves between the biosphere, lithosphere, hydrosphere, and atmosphere of the earth. There are also other biogeochemical cycles like the Oxygen Cycle and the Water Cycle. The astronomical systems outside the earth may also have some carbon cycles but nothing is known about them.
 Continuous Recycling
The carbon cycle is the continuous process of recycling of carbon atoms. This continuous recycling of carbon atoms implies that the same carbon atoms that are, for example, now part of our bodies have been used in innumerable other molecules earlier. Carbon is one of the constituents of wood, when this wood is burned it produces carbon dioxide that mixes in the atmosphere. Plants use this carbon dioxide during photosynthesis, and these carbon atoms become parts of the plants. When we eat these plants, the carbon atoms are metabolised by our bodies and they become parts of our bodies. The carbon cycle involves both living and nonliving processes and sites.
Many such processes and inter-related cycles form the carbon cycle. The chemicals that are part of the carbon cycle sometimes exist for long periods of time in one place. Such a place is called a reservoir; an example would be a deposit of Natural Gas that will store carbon-containing chemicals for a long period of time. Places where carbon-containing chemicals are held for short periods of time are called exchange pools. Examples of exchange pools include plants and animals, which temporarily use carbon in their systems and release it back into the air or surrounding medium. Carbon is held for a relatively short time in plants and animals when compared to coal deposits. The amount of time that a chemical is held in one place is called its residence. Generally, reservoirs are abiotic factors while exchange pools are biotic factors.
 Major Carbon Reservoirs
The carbon cycle is usually considered to consist of four major reservoirs of carbon; and these reservoirs are interconnected by pathways of exchange. The four reservoirs are
- The atmosphere
- The terrestrial biosphere – besides the living beings on land, this includes freshwater systems and non-living organic material, such as carbon in the soil
- The oceans – these include the dissolved inorganic carbon in the seas and the living and non-living marine biota
- The sediments – these include the fossil fuels.
The various chemical, physical, geological, and biological processes taking place on the earth make the carbon move between these reservoirs.
Without the proper functioning of the carbon cycle, every aspect of life could be changed dramatically. It is vital to understand how the carbon cycle works in order to see the danger of it not working. Understanding the carbon cycle has implications for many processes like the greenhouse effect, global warming and climate change.
 Carbon Cycle and The Greenhouse Effect
It is believed that the increase in carbon dioxide levels in the atmosphere coincides with the start of the industrial revolution in the 18th century. This theory gains more support when samples of air trapped for centuries in glaciers showed no change in carbon dioxide content until 300 years ago.
Since the measuring of atmospheric carbon dioxide began in late 19th century, its level has increased by over 20%. Experts believes that this increase is a direct result of human activities such as
- Burning of fossil fuels (coal, oil, natural gas) which is actually returning to the atmosphere carbon that has been locked within the earth for millions of years.
- Reduction in the total forest area. Forest lands in many countries have been cleared for agriculture and cattle grazing.
The rise in the concentration of carbon dioxide is a cause of concern. Though CO2 is transparent to light, it is opaque to heat rays. As a result, CO2 in the atmosphere traps heat radiated from the earth back into space, resulting in the "greenhouse effect". Experts believe that average temperatures have increased slightly in the last century. A doubling of the CO2 concentration (expected by the end of this century) will increase the temperatures by 2.5–3.5°C.
- Carbon Cycle
- Vision Learning
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