The carbon cycle describes the flow of carbon between carbon sources, from which carbon is released into the atmosphere, and carbon sinks, where carbon is stored on earth (Tribal Energy and Environmental Information Clearinghouse [TEEIC], n.d.). The total amount of carbon on earth and in the atmosphere always remains the same, but molecules of carbon are constantly moving through the carbon cycle as they are transferred between sources, sinks, and the atmosphere (TEEIC, n.d.). Plants are one example of a carbon sink. During the process of photosynthesis, they remove carbon from the atmosphere in the form of carbon dioxide (TEEIC, n.d.). The carbon is then integrated into compounds that are stored in the shoots, roots, and leaves of plants (TEEIC, n.d.). When animals eat plants, they store some of the carbon in their bodies, and the also release some carbon back into the atmosphere as carbon dioxide (TEEIC, n.d.). At the same time, carbon is constantly being exchanged between the atmosphere and the ocean (Earth System Research Laboratory [ESRL], n.d.). In the ocean, dissolved organic carbon is present in both the surface ocean and the deep ocean (TEEIC, n.d.). Dissolved carbon in the deep ocean is sometimes deposited into sediment at the bottom of the ocean (TEEIC, n.d.)
When animals and plants die, the process of decomposition may lead to the release of carbon back into the atmosphere as methane, or carbon may be stored in the soil (TEEIC, n.d.). Some of this carbon may also become part of stable carbon sinks, which are carbon sinks through which the flow of carbon is much slower than it is for other sources and sinks (TEEIC, n.d.). Examples of stable sinks include underground deposits of hydrocarbons such as oil, gas, and coal (TEEIC, n.d.). Stable carbon sinks may be periodically released into the atmosphere through natural geologic processes such as volcanic eruptions (TEEIC, n.d.).
At the same time, human activity is having an important effect on the carbon cycle today. One of the most significant ways in which humans are currently affecting the carbon cycle is through the burning of fossil fuels for the production of energy (ESRL, n.d.). Fossil fuels like oil, natural gas, and coal are stable carbon sources, so the carbon that they store is rarely released into the atmosphere through natural processes. Thus, fossil fuel combustion and industrial processes significantly increase the amount of carbon dioxide that enters the atmosphere from carbon sinks.
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The release of carbon dioxide into the atmosphere through the burning of fossil fuels has also impacted the carbon cycle by altering the exchange of carbon dioxide between the oceans and the atmosphere. When there is more carbon dioxide in the atmosphere, more carbon dioxide dissolves into the water (ESRL, n.d.). At the same time, warming temperatures can lead to the release of higher amounts of carbon dioxide into the atmosphere (TEEIC, n.d.). Human activities such as the burning of fossil fuels have recently contributed to significantly higher global temperatures, which may be speeding the rate of the release of carbon dioxide from the world’s oceans (ESRL, n.d.).
Another way in which human activity affects the carbon cycle is through changing land use practices (ESRL, n.d.). For example, when ecosystems with high concentrations of plants, such as rainforests, are clear cut for agricultural production or human settlement, it can reduce the amount of carbon dioxide that is sequestered from the atmosphere by these carbon sinks. (ESRL, n.d.). In addition, when this land is used to graze food animals like cattle, it can lead to even more carbon being released into the atmosphere as methane.
It is important to note that human impacts on the carbon cycle are not yet fully understood. While scientists have been able to describe global trends, there is still ongoing research on the impacts of human activity on the carbon cycle at the regional level (ESRL, n.d.). Future studies may make it possible for scientists to more fully characterize the role of anthropogenic factors in the carbon cycle in different parts of the world. (ESRL, n.d.).
- Carbon cycle science. (n.d.). Earth System Research Laboratory [ESRL]. Retrieved from https://www.esrl.noaa.gov/research/themes/carbon/
- What is the carbon cycle? (n.d.). Tribal Energy and Environmental Information Clearinghouse [TEEIC]. Retrieved from https://teeic.indianaffairs.gov/er/carbon/carboninfo/cycle/
