The carbon story

In this section:

• The carbon cycle
• Carbon dioxide levels in the atmosphere
• Methane hydrates

The carbon cycle

Carbon is a crucial element for all life on Earth. Carbon is the basic building block of life and helps form the bodies of 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 or 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 in an oxidised state, which is reduced to organic carbon during photosynthesis. Organic carbon can be oxidised by atmospheric oxygen, usually through respiration. 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 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. After consolidation, these deposits may form limestone.

A carbon atom could spend millions of years moving through the carbon cycle. The ocean plays the most critical role in regulating Earth’s carbon balance, and understanding how the carbon cycle is changing is key to understanding Earth’s changing climate. Source: NASA

Carbon dioxide levels in the atmosphere

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 atmospheric carbon dioxide are: volcanism, escaped gases from the Earth’s mantle and the erosion of rocks containing carbons such as limestone. 

By contrast the erosion of silicate rocks is a process which tends to remove carbon dioxide from the atmosphere. Rain water is slightly acidic as it incorporates carbon dioxide from the atmosphere. The result of this rainfall falling on silicate rocks is the formation of bicarbonates, which are washed into the oceans and eventually incorporated into the mantle through the process of subduction. Volcanoes, on the other hand, can increase carbon dioxide in the atmosphere. Volcanism was much greater during the middle part of the Cretaceous Period, around 100 million years ago, and so this is why carbon dioxide levels in the atmosphere were much higher, and resulted in a much warmer climate. 

Methane hydrates

Methane is composed of carbon and hydrogen (CH₄). It is actually a much more powerful greenhouse gas than carbon dioxide but is less concentrated in the atmosphere. There is a large amount of methane frozen into a type of ice called methane hydrate, which is found in permafrost i.e. ground that remain frozen throughout the year. The structure of methane hydrate holds methane molecules much closer together than if they were in their gaseous form. This means that solid methane hydrate can potentially hold a lot of methane gas.

There is a large quantity of methane below the sea floor and in permafrost soils. Methane in these reservoirs can be released by such things as temperature changes or by disturbing the sediment as a result of 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 our 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.

Further information

Stephenson, M. (2018) Energy and Climate Change. Elsevier.