The Carbon Cycle

The carbon cycle refers to the constant movement of carbon atoms from one place to another. The cycling and movement of carbon plays a key role in supporting life on Earth. Physical processes control this cycle over a range of time, space and scale. If there is an imbalance between these processes, the outcome can have a detrimental effect on the Earth and its inhabitants.

Where can carbon be found?

Carbon is the building blocks of all living and non-living organisms. It can be found in the Earth in a number of stores such as the Earth, Oceans and Atmosphere. Most of the Earth’s carbon is geological and found in “stores” such as soils, vegetation and bedrock.

The Carbon Cycle

When viewing the carbon cycle as a system, these stores such as soil, rock and oceans are referred to as pools. Any movement of carbon between these pools is called a flux. In any system, fluxes connect pools together, creating the carbon cycle. The carbon cycle is in a constant state of motion where carbon is being transferred between the pools.

carbon cycle
Carbon flowing through the carbon cycle. Source: serc.carleton.edu.

Soils hold carbon in their organic matter which is manufactured by plants upon taking in carbon dioxide through the process of photosynthesis. Once animals consume the plants the carbon compounds travel through the food chain until it is released via respiration. Once the animal dies and decomposes, the carbon is released back into the atmosphere and the soil, where the cycle continues. In addition, some plants shed leaves, roots and branches each year. As all parts of the plant are made up of carbon, the loss is a transfer of carbon from the plant to the soil where decomposition will begin, adding organic matter back into the soil, continuing the cycle.

coal power plant
Coal power plants are a major source of carbon emission into Earth’s atmosphere. Credit: Clay Gilliland, Flickr.

Some living organisms are not decomposed but may be fossilized. These sources of carbon, e.g. coal and oil, are used as a source of fuel. Humans may burn these fuels, therefore releasing the carbon back into the atmosphere as carbon dioxide.

In the sea, marine animal shells, a source of carbon, collect on the seabed. Over time these shells are cemented with sediment storing the carbon in rocks such as limestone. Due to uplift, the limestone may eventually become exposed to air, where it is broken down and weathered, releasing carbon back into the atmosphere as carbon dioxide. The rocks can also be exposed to rain, which when exposed to carbon can combine with water to form acid rain. Upon contact, chemical weathering can occur, releasing carbon onto the surface or into the atmosphere.

The atmosphere contains carbon in the form of carbon dioxide, methane and various other smaller compounds, such as carbon monoxide and chlorofluorocarbons. These gases make up the greenhouse gases, which have a large role in maintaining the temperature of the Earth.



Volcanic eruptions also release gases such as carbon dioxide into the atmosphere from melted rocks that have been subducted at plate boundaries .

Time Scales

The carbon cycle can vary over time scales, moving from one store to another. Short term carbon cycles include fluxes that occur over a number of years. These fluxes include photosynthesis and respiration. Long term processes occur over thousands to millions of years, such as the transformation of carbon into limestone, weathering of rocks, uplift of the rocks and melting of the rock in volcanoes.

Why is carbon so important?

The amount of carbon in our atmosphere is extremely important. The carbon cycle plays a vital role in the regulation of the Earth’s climate by controlling the concentration of the carbon dioxide in the atmosphere. Once mixed with air in the atmosphere, the carbon dioxide acts like a blanket trapping long wave radiation, maintaining a constant temperature on the Earth for us to survive. Changes in the carbon cycle due to both natural causes, e.g. wild fires or volcanic activity, and human causes, e.g. burning fossil fuels or deforestation, are having a huge impact on the greenhouse effect.

With global population and affluence increasing, the combustion of fossil fuels continues to rise along with high rates of deforestation, causing the levels of carbon dioxide in the atmosphere to increase, enhancing the greenhouse effect. This leads to an increase in the amount of carbon dioxide, along with other greenhouses gases, trapping infrared radiation from the sun and blocking the re-radiation of infrared waves back out into space. Global temperatures rise as a result, which leads to climate changes at both global and local scales.  With the observed trend of global temperatures rising, climate change continues to occur at a fast rate. The rate at which this rise will continue depends on future levels of carbon dioxide and, in short, the effect of people on the Earth’s carbon cycle.

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