Carbon Cycle Diagram

Updated February 27, 2018
Basic carbon cycle graphic

All the elements on earth, including carbon move in cycles, as part of a closed system. There is no loss or input of carbon from space. The carbon cycle diagram shows various steps in the recycling of carbon in the 21st century.

The Diagram

The global carbon diagram by the University of New Hamsphire depicts pools and fluxes that make up the carbon cycle. Carbon pools store large quantities of carbon for long periods of time and are in blue. Fluxes are the processes which move carbon from one pool to the next and are in red. Fluxes have two parts: one that removes carbon from the air and one that releases the fixed carbon back as CO2 into the atmosphere.

The Global Carbon Cycle

Carbon Pools

The amounts of carbon the pools store is mentioned in Petagram of carbon (PgC). One Pg is equal to one billion tons and is also called Gigatons (Gt).

  • Rocks: Most of the carbon is locked away as sedimentary rocks.
  • Ocean bed: The second largest carbon pool is under the oceans in the form of carbon dioxide (CO2) dissolved in water.
  • Fossil fuels: The third largest carbon pool are the fossil fuels, such as coal, lignite, natural gas, and oil, which are formed from remains of land and marine plants and animals under special temperature and pressure.
  • Ocean surface: Carbon is stored for a short time in the surface water as CO2 dissolved in water or in the bodies of living marine plants and animals.
  • Terrestrial pools: All the carbon that accumulates in trees and soils form another short-term pool, and are released after some decades or centuries, for example, when trees are cut or die.
  • Carbon dioxide: Carbon present in air in its gaseous form, CO2, helps keep the earth warm. Without this life as it exists would not be possible on earth. There is a constant addition and uptake from this carbon pool.

Carbon Removal in Fluxes

The amounts of carbon that are moved every year are shown as PgC per year in the diagram. CO2 is removed from the air and is fixed by quick daily processes. Formation of organic matter and carbon sinks are slower and require time.

  • Photosynthesis - Green plants use CO2, along with water and energy from the sun in a process called photosynthesis to form simple sugars and then the nutrients plants need.
  • Oceans uptake - Atmospheric CO2 is taken in and used for photosynthesis also in oceans. Here phyto-planktons are the equivalents of plants, on whom all life in the oceans depends. In addition, CO2 dissolved in water is converted to calcium carbonate and used in shells and skeletons of marine animals.
  • Food chain - When herbivores eat plants, or carnivores and omnivores eat other animals this carbon is passed along the food-chain to help animals grow, live and multiple.
  • Addition of organic matter and litter - When plants and animals die, they are decomposed by microbes to form humus or organic matter that becomes part of the soil. Litter that is formed every year when trees shed twigs and leaves, and recycle carbon constantly to the soil. This is partly used for the growth of plants and keeps the carbon circulating, while the rest forms soil carbon.

Formation of Carbon Pools

The amounts of CO2 being used and the length of time they remain stored as fixed carbon varies with different organisms and processes.

  • Since trees are long-lived and accumulate carbon in their stem, leaves and roots, they act as carbon sinks.
  • Soils accumulate carbon as organic matter and dead roots that remain in soil long after a plant or tree dies; there are enormous quantities of biomass in the form of the growing live roots of trees and grasslands in the soil. Soils form another important carbon sink.
  • Some shells and skeletons of marine animals accumulate at the bottom of the oceans to produce limestone.

Carbon sinks are an important flux or process that eventually result in carbon pools. On a short term they produce terrestrial carbon pools, and on the long term fossil fuels and rocks.

Land to Ocean Flow

When rivers flow into oceans they carry sediments rich in organic matter with them. Marshes and tidal flooding also move carbon in the form of organic matter into oceans each year.

Natural Release of Carbon Dioxide

In the natural carbon cycle, carbon is released back into the atmosphere mainly through respiration and decomposition.

  • Plant respiration - Most of the living beings microbes, plants, and animals on land respire. They breathe in oxygen, and breathe out out CO2, by breaking down food they have eaten. This is one of the shortest cycling of carbon.
  • Soil decomposition and respiration - All the decaying matter on land is not converted to organic matter. Some of the carbon is released directly into the air as CO2. The microbes and small aminals living in soil also release CO2 every day when they respire.
  • Ocean loss - Respiration and decaying of marine plants and animals also release CO2 to the carbon pool in the atmosphere.
  • Volcanoes - A small amount of carbon is released into the atmosphere by volcanic activity.

Human Activities Releasing Carbon Dioxide

Oil pumpjack and air pollution

In addition to natural fluxes, there are many human activities that release fixed carbon back into the atmosphere as CO2.

  • Burning fossil fuels - Combustion of carbon sinks wood, coal, natural gas, gasoline for electricity, heating, cooking, or transport is one of the main ways that carbon is released back into air. Many of the fossil fuels are also used for industrial purposes and further add CO2 into the atmosphere.
  • Land use changes - Deforestation, clearing away grasslands to create settlements, farms replacing natural growth, and machinery use that leads to emissions, have long-term consequences. It leads to addition of CO2 to the atmospheric carbon pool.

Different Perspectives From Additional Diagrams

There are many types of carbon cycle diagrams and they provide different information about this vital cycle.

  • Simple cycle: The diagram by BBC depicts a simple carbon cycle. This was what the carbon cycle looked like in pre-industrial times, until 150 years ago, when quantities of carbon movement were not an issue.
  • Climate change: The Carbon Cycle by the University of Calgary is a pictoral analysis of how modern human activities have changed the delicate balance in the carbon cycle.
  • Chemical processes: The carbon cycle by Britannica focuses on the various chemical reactions that influence carbon fluxes and pools, and not quantities of carbon recycling. This cycle is interesting for people who want to know the different forms in which carbon exists and how it changes.

Using Carbon Cycle

There is a 30% increase in CO2 in the atmosphere, due to human activities in the last 150 years. Since CO2 in air causes warming, adding more CO2 to the atmosphere also increases its warming effect. This has resulted in global warming and climate change. Understanding the carbon cycle, and how and where human activities change it, can help in finding efficient ways and methods to tackle the problem of climate change.

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Carbon Cycle Diagram