Carbon is an abundant element that is necessary for life on Earth. The carbon cycle is the exchange of carbon between all of the earth’s components—the atmosphere, oceans and rivers, rocks and sediments, and living things.
The processes of photosynthesis and respiration are the basis of the carbon cycle. In photosynthesis, plants use energy from the sun and carbon dioxide (CO2) gas from the atmosphere to create carbohydrates and oxygen (O2). Carbohydrates are then stored in their biomass (living parts, such as leaves, stems, and roots) as plants live and grow. Stored carbohydrates can be used as energy. To use the energy, carbohydrates need to be broken down in respiration and CO2 is released back into the atmosphere. The rate at which CO2 is produced is variable. For example, decomposition— where fungi and microorganisms break down carbohydrates to gather energy— is a slow but significant way that carbon is returned to the atmosphere.
The carbon cycle involves the flux, or flow, of carbon between different earth systems. An object or process that absorbs and stores carbon is called a sink, while one that releases carbon faster than it is absorbed is termed a source. For example, a healthy plant is a carbon sink because it is taking in CO2 from the air and storing it in new leaves and roots and a larger stem. However, a plant can become a source of carbon if the amount of CO2 going out exceeds the amount taken in. This might happen if CO2 is sent back into the atmosphere through decomposition or fire.
(Source: NASA Earth Observatory)
Humans have a large impact on the worldwide carbon cycle. Fossil fuels, including coal, oil, and natural gas, all contain large amounts of carbon that was formed during the decomposition of plants and animals over millions of years. Burning fossil fuels releases large amounts of CO2 and other greenhouse gases into the atmosphere faster than natural processes. Changes in land use, especially deforestation, also contribute to elevated levels of atmospheric CO2. Although plants absorb some of the additional CO2, most of the gases remain in the atmosphere and contribute to climate change.
Scientists throughout the world are working to determine the amounts of carbon stored in different components of the earth and the movements between them. Carbon can be measured using two different approaches. One approach is to use satellites and metrological instruments to measure the flux of CO2 in the air. The other approach is to measure the amount of carbon present in samples from plants, trees, soil, and other components and scale those up to a regional or worldwide level. Using these methods scientists have been able to determine the approximate quantities and fluxes involved in the global carbon cycle (see figure above). However, the precise size of many sources and sinks is still unclear because of the complexity of the systems involved.
- North American Carbon Program
- United States Carbon Cycle Science Program
- NASA Earth Observatory: Carbon Cycle
- US Geological Survey (USGS): Carbon Cycle Research
- Canadian Forest Service: Carbon Cycle
Last Modified: 03/30/2012