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The Carbon Cycle


The amount of carbon we have on Earth doesn’t change. It is the same as it is now as it was millions of years ago when the dinosaurs roamed the Earth. Most carbon is stored in reservoirs, or sinks, such as: 








Fern leaf


Carbon is a crucial element for all life on Earth. Carbon is the basic building block of life, form the bodies of complex living organisms. Its compounds form solids, liquids and gases.

Carbon may be either ‘organic’ or ‘inorganic’. Organic carbon

is found in: 

  • living or dead organisms

  • fossil fuels

  • small deposits in rocks

  • dissolved in water 

  • dispersed in the atmosphere


The majority of the inorganic carbon exists as carbon dioxide, carbonate and hydrogen carbonate. 


There is a continuous two-way flow of carbon between the organic and inorganic forms. Inorganic carbon is found in what is known as an oxidised state, which is reduced to organic carbon during photosynthesis. Organic carbon can be oxidised by atmospheric oxygen, usually through respiration (breathing). 

The energy released during respiration is used both by plants and animals to maintain their bodily functions. A similar process in which oxygen cycles between the atmosphere, the Earth and living organisms interlinks with the carbon cycle, and photosynthesis (the process plants use to produce energy from their food) and respiration are central to both.

More than 99 per cent of the carbon in the carbon cycle is found in the Earth’s crust. Most of this has a biological origin, deposited on the ocean floor from the remains of the many marine creatures that use calcium carbonate in their skeletons and shells. After consolidation, these deposits may form a rock known as limestone.


Carbon dioxide is a gas that contains carbon and oxygen; it has twice as much oxygen as carbon. Carbon dioxide levels in the atmosphere depend on a balance between Carbon dioxide sources and sinks: sources give out Carbon dioxide and sinks absorb and store Carbon dioxide. 

Geological sources of Carbon dioxide in the atmosphere are: 

  • volcanoes

  • gases escaping from the Earth’s mantle 

  • erosion of rocks containing carbon, such as limestone


By contrast, the erosion of silicate rocks is a process that tends to remove carbon dioxide from the atmosphere. Rain water is slightly acidic because it incorporates carbon dioxide from the atmosphere. The result of this rain falling on silicate rocks is the formation of chemicals called bicarbonates, which are washed into the oceans and eventually incorporated into the mantle through the process of subduction. 

Volcanoes can increase Carbon dioxide in the atmosphere. Volcanism was much greater during the middle part of the Cretaceous Period, around 100 million years ago, which is why Carbon dioxide levels in the atmosphere were much higher then, resulting in a much warmer climate. 

Above the Clouds
Detail of arctic permafrost thawing in Alaska and Canada, global warming climate change co



Methane is a gas that is composed of carbon and hydrogen; it contains four times as much hydrogen as carbon. It is actually a much more powerful greenhouse gas than carbon dioxide but is less concentrated in the atmosphere. 

Frozen methane forms an ice known as methane hydrate. It's found in permafrost (ground that is frozen all year round) and below the sea floor..  The structure of methane hydrate binds molecules much closer together than in the gaseous form. This means that methane hydrate can potentially hold huge quantities of this powerful greenhouse gas. Methane in these reservoirs can be released by surface temperature changes or integrity failure of seafloor sediments eg. submarine landslides.

The amount of methane hydrate in permafrost soils is poorly known, with estimates ranging from 7.5 – 400 gigatonnes of carbon. If all this methane hydrate were to melt, there could be catastrophic changes to the global climate.

Methane hydrates are a component of the cryosphere; anywhere on Earth where water is in solid form is known as the cryosphere: this includes snow, ice and permanently frozen ground. These all play different roles within the climate system. For example, the continental ice sheets of Antarctica and Greenland actively influence global climate over geological time scales, but they may also have more rapid effects on sea level. Changes in the cryosphere are connected to global-scale feedbacks, including solar reflectivity and ocean circulation.

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