Theory:

The circulation of carbon from the atmosphere to the earth is known as the carbon cycle. This is a biogeochemical process.
 
The amount of carbon present in the atmosphere is \(0.04\%\). This is also released by the respiration process and on the complete combustion process. Carbon occurs in its elemental forms as graphite and diamond in nature.
 
Carbon-cycle-full.jpg
 
The following is the process involved in the carbon cycle
  • Plants take in atmospheric CO2 for photosynthesis.
  • Animals feed on primary producers, and carbon is accumulated in them.
  • The carbon is released back to the atmosphere on the decomposition of dead plants and animals.
  • Some amount of carbon is retained in the soil and becomes fossil fuel.
  • The combustion of fossil fuels by human activities releases carbon back to the atmosphere.
Screenshot 2021-06-11 094153.png
 
The intake of CO2 from the atmosphere for preparing food with the help of sunlight is known as photosynthesis.
 
CO2+H2O+LightenergyC6H12O6+O2CarbondioxideGlucose
 
The carbon is passed from producers to consumers in the ecosystem from this process.
Fact: \(18.5\%\) of carbon is present in the human body.
The carbon present in the human body is not in pure form. It is found in proteins, carbohydrates, fats etc., which is the building blocks for these compounds. When plants and animals die, the carbon content is released back to the atmosphere by decomposition.
Carbon sink: The intake of  CO2 is more compared to the release of  CO2.
The carbon cycle.png
 
The plants, soil and ocean act as a natural carbon sink. We know the shells of marine organisms are made of calcium carbonates. So when the marine organisms die, they get accumulated in the seafloor, and decomposition takes place.
 
Apart from the natural release, there is also an additional amount of CO2 which is released into the atmosphere due to modernisation techniques, burning, industrialisation etc., which can alter the carbon cycle. If the carbon cycle is altered, it will have a drastic effect on earth, which concerns the life on earth and other conditions such as global warming, climate change, etc.
Reference:
Harry C, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons