The Carbon Cycle Explained

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The carbon cycle is the process that moves carbon around the Earth, through different physical, chemical and biological stores. Carbon is temporarily stored in the atmosphere, oceans, rocks and also in plants, animals and all other life.

Did you know that there is a word for each of the above stores that surround the Earth?

  • The atmosphere is the air
  • The hydrosphere is the water
  • The lithosphere is the rocks
  • The biosphere is the biological life

And as a bonus, the cryosphere is the ice surrounding the Earth!

What is the carbon cycle?

Photosynthesis and the carbon cycle

Just as with the water cycle and nitrogen cycle, carbon is cycled around the Earth through physical, chemical and biological stores. Trees and vegetation are perhaps where the cycle begins as they perform photosynthesis and take in carbon dioxide from the atmosphere (or the water in the case of aquatic or marine plants) this is then turned into biomass and stored before being eaten by animals where it is moved into another temporary store. 

Vegetation could be consumed whilst it is still alive or it could be consumed by organisms called decomposers after the plant dies. At these stages, carbon dioxide is produced through the breakdown of glucose during respiration and released back into the atmosphere. 

Remember that these processes also happen in the oceans and remove carbon dioxide from the water. Photosynthesising organisms make glucose and this eventually ends up sinking to the bottom of the oceans. Layers and layers of dead organic matter build up over time and become part of the Earth’s crust, producing oil and gases and being locked away in rocks.

Learn more about where photosynthesis takes place in cells!

Oceans, rocks and the carbon cycle

The carbon cycle also has non-biological components such as the interactions between the oceans and the atmosphere where carbon dioxide dissolves, making the oceans slightly more acidic. Carbon dioxide dissolving in the oceans is a form of dynamic equilibrium in order to balance the carbon in the atmosphere with the oceans. More carbon in the air means more carbon moving to the water and vice-versa.

Another mechanism by which carbon is stored and moved around is through the interaction with slightly acidic rainwater which reacts with rocks during weathering and locks away the carbon in more stable forms. The carbon which ends up underground or as part of the lithosphere is buried for millions of years before mixing deep in the mantle of the Earth and returning to the surface again in the form of volcanic emissions. 

Volcanoes also release vast amounts of carbon into the atmosphere which was stored in the rocks of the Earth’s crust. 

Carbon cycle diagram gcse

Diagram of the carbon cycle – Shows each of the major stores of carbon and transfers between stores.

Human emissions are considered to be responsible for the short-term climate change that has been seen over the past 100 years. Increasing levels of carbon dioxide and other greenhouse gases in the atmosphere cause warming of the atmosphere as they trap radiation which would normally escape the Earth, cooling it down.

Humans and the carbon cycle

Humans play an important role in the carbon cycle because as a species we change the 4 major stores of carbon and influence the exchange between these stores. 

These stores as listed at the top of this page are:

  • The atmosphere – the air and gases
  • The hydrosphere – oceans, rivers and lakes
  • The lithosphere – rocks and Earth’s crust
  • The biosphere – all biological life

We influence the atmosphere by burning fossil fuels and through our interactions with the rest of the stores:

  1. Combustion of fossil fuels directly adds carbon dioxide into the atmosphere. Before fossil fuels were burned they were stored for millions of years in the lithosphere, deep underground.
  2. Deforestation affects the biosphere, limiting the amount of carbon dioxide plants can take up during photosynthesis. The result is increased carbon dioxide in the atmosphere. 
  3. Global warming causes an increase in the temperature of oceans, warm water can dissolve less carbon dioxide than cold water and this reduces the exchange between the atmosphere and oceans. 

There are many more interactions by humans in the carbon cycle and it is not important at the GCSE level to understand them all. Just remember the basics of the carbon cycle and a few important transfers between stores such as volcanism, burning fossil fuels, photosynthesis and respiration.

Try one of the quizzes below to test your knowledge of material cycles!