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Carbon Cycle : Definition, Steps, & Diagram

Jul 9, 2022

Carbon Cycle

Plants need carbon dioxide to manufacture their food. Humans give off carbon dioxide from respiration. Burning fossil fuels releases carbon. With a multitude of sources pumping carbon and its compounds into the atmosphere, how does all of it get balanced? Via carbon cycle. But what is the carbon cycle? The carbon cycle demonstrates the movement of elemental and combined carbon on earth. Graphite and diamond are the common elemental carbon forms, whereas the combined state forms include carbonates, carbon dioxide, and more. Learn in detail about the cycling of carbon, carbon cycle definition, the steps involved in the process, and other details through the cycle diagram.

What is the Carbon Cycle?

Carbon is amongst the most important elements of life on earth. The human body contains 18% carbon by mass; food products contain carbon; economies, homes, and transport—are all built on carbon. Carbon is a must, but it is entwined with serious climate change issues. However, an efficient carbon cycle has been maintaining the level of the element in nature. 


Carbon cycle definition: The process of interchange of carbon compounds among the geosphere, biosphere, atmosphere, hydrosphere, and pedosphere of the earth.  

So there are carbon reservoirs in nature, and a change that shifts carbon from one reservoir adds more carbon to the other reservoirs. For instance, changes that liberate more carbon into the atmosphere cause a rise in the earth’s temperature. 


What does fast and slow carbon cycling mean?

A carbon cycle has fast and slow components depending upon the amount of carbon cycling per year. 

Short-term/ Slow carbon cycle: The movement of carbon between soil, rocks, oceans, and the atmosphere via a series of chemical reactions consuming up to 100-200 million years is said to be a slow carbon cycle. Approximately 1013 to 1014 grams of carbon move annually via the slow carbon cycle.  


Long-term/ Fast carbon cycle: The movement of carbon amidst life forms on earth is called the fast carbon cycle. The main components of the fast carbon cycle are plants, animals, and phytoplanktons (microscopic organisms in the ocean). Approximately 1016 to 1017 grams of carbon move annually via the fast carbon cycle. 

Steps of Carbon Cycle 

The plants take up atmospheric carbon dioxide, absorb it into their cells, and use it to manufacture starch while utilising energy from the sun and water from the soil. The chemical reaction in which carbon is fixed is given as follows:


6CO2 + 6H2O → C6H12O6 + 6O2                   

The movement of carbon from plants to the atmosphere can be in the following ways:

  • Breakdown of sugar in plants for energy to grow. 
  • Animals eat the plants and break down the plant sugar for energy. 
  • Plants die and decay. Bacteria eat them. 
  • Fires flare up and consume plants. 

In the four cases mentioned above, oxygen combines with sugar releasing water, carbon dioxide, and energy. The chemical reaction for this procedure can be represented as follows:

CO2 + H2O → C6H12O6 + 6O2 + energy


When sugar is consumed anaerobically, lactic acid is formed, which further breaks down into carbon dioxide and water on the availability of oxygen. The equation for the reaction is as follows:



The above processes involved in the cycling of carbon can be summarised in the following steps:

  • Atmospheric carbon gets absorbed by plants for photosynthesis.
  • Animals consume the plants, and carbon gets bioaccumulated into their bodies.
  • When animals and plants die and decompose, the carbon is released into the atmosphere.
  • The unreleased carbon eventually becomes fossil fuels.
  • Fossil fuels are used as fuels for several man-made activities. Hence, carbon is pumped back into the atmosphere. 

Oceans and seas take up more carbon than they give out. Hence, they are also called carbon sinks. In the ocean and seas, the carbon cycle operates as follows:

  • Marine animals convert carbon to calcium carbonate.
  • This calcium carbonate becomes a part of their hard shells, such as in clams and oysters.
  • When these shelled organisms die, their bodies decompose, leaving the hard shell behind.  
  • It accumulates on the seafloor, gets broken down by sea waves and is compacted under enormous pressure to form limestone.
  • When limestone rocks get air exposure, they get weathered, releasing carbon into the atmosphere as carbon dioxide.

Carbon Cycle Diagram

The following diagram shows the various components of the carbon cycle and its movement across land, water, and the atmosphere. 

Carbon Cycle

Processes Involved in Carbon Cycle

There are main processes involved in the carbon cycle: 

  • Photosynthesis
  • Exchange
  • Respiration
  • Sedimentation
  • Combustion 
  • Extraction

Importance of Carbon Cycle 

  • The carbon cycle regulates the presence of carbon dioxide in the atmosphere.
  • It plays a vital role in creating an energy balance by trapping long-wave radiations from the sun, enabling the atmosphere to act as a blanket over the planet. Thus, stabilising the climate.
  • Along with other biogeochemical cycles such as the nitrogen cycle and oxygen cycle, it plays a vital role in the existence of life on earth.

Factors Affecting Carbon Cycle

The carbon cycle is influenced by different factors such as:

  • Burning fossil fuels: Human activities like burning coal and petroleum release enormous amounts of carbon into the atmosphere. 
  • Deforestation: Plants fix carbon. However, cutting down trees puts a lot of free carbon into the atmosphere. Changing land use or urbanisation leads to increased deforestation.  
  • Using limestone to manufacture concrete: Concrete is high in demand for building materials. As more and more limestone is utilised to make concrete, higher amounts of carbon are liberated, creating a disbalance in the carbon cycle. 
  • Carbon sequestration: High carbon sequestration helps reduce the rising amounts of carbon in the atmosphere. 
  • Volcanic eruptions: Volcanoes release high amounts of gases, including sulphur dioxide and carbon dioxide, that upsets the natural carbon cycle.

Effects of Changing Carbon Cycles               

When left unperturbed, the carbon cycle maintains a steady carbon concentration in the atmosphere, ocean, and land. However, when changes are introduced via inappropriate practices in one reservoir, the adverse effects ripple through the other reservoirs. 

Temperature Change: The atmosphere holds a significant amount of carbon as it plays an important part in controlling the earth’s temperature. Methane, carbon dioxide, and halocarbons are greenhouse gases that keep the earth warm by absorbing a wide range of energy and then re-emitting it. This re-emitted energy moves out in all directions, and some of it returns to earth to heat its surface. 

It is important to understand that a lack of greenhouse gases would lead to frozen earth at -18 degrees Celsius. At the same time, too many greenhouse gases would make the earth hot like Venus, with a temperature of around 400 degrees Celsius. 

Currently, an increasing amount of carbon gases in the atmosphere has led to global warming. Higher the carbon dioxide concentration in the atmosphere, more radiation will be returned to the earth. Consequently, the earth’s temperature would rise too. If practices are not checked soon, the temperature will rise and lead to many serious scenarios such as melting ice in polar regions, milder winters, a decrease in moisture in the soil, and more. 

Frequently Asked Questions

Q1. What is carbon sequestration?

The removal of carbon dioxide from the air and the process of storing it is called carbon sequestration. Farms, grasslands, and forests serve as sinks of carbon dioxide. Human activities, both positive and negative, affect the amount of carbon dioxide removal from the atmosphere via trees and plants.

Q2. What are the main sources of carbon in the non-living world?

The four main sources of carbon in the non-lining world are as follows:

  • Carbon dioxide in the air and that dissolved in water
  • Rocks in the earth’s crust contain carbonates such as dolomites and limestones.
  • Fossil fuels like coal and petroleum
  • Volcanic eruptions release carbon dioxide into the atmosphere.

Q3. How is the carbon cycle significant to humans?

Humans are dependent on carbon for various purposes, and the carbon cycle regulates the carbon available to them. The carbon cycle balances the earth’s temperature, makes up the food that sustains life, and provides energy that fuels the global economy. 

Q4. What efforts can help fix the carbon cycle?

The most promising way to maintain the carbon cycle is by avoiding fossil fuel consumption. Instead of burning fossil fuels, we can opt for more solar energy or wind power for electricity. Trees also participate in the cycling of carbon by using carbon dioxide for photosynthesis. Therefore, we can maintain the cycle by cutting down fewer forests or, better still, by afforestation. 


The earth and its atmosphere have plenty of carbon which is balanced via carbon cycles. The availability of carbon in the atmosphere is a crucial factor in maintaining living organisms and life forms on the earth. Nearly every compound involved in the metabolic activities of living beings contains carbon. In nature, the recycling of carbon is essentially an automated feedback mechanism. However, human activities interfere with the mechanism. Therefore, it is vital to check practices that can hinder the optimal working of the carbon cycle.

carbon cycle

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