Carbon is an element that is essential to all life on Earth and is continually moving among Earth’s lithosphere, hydrosphere, biosphere, and atmosphere in various forms: as carbon dioxide (CO2) in the atmosphere, sugars or carbohydrates (CnH2nOn) in living organisms, and calcium carbonate (CaCO3) in rocks and minerals, to name just a few. The movement of carbon among Earth’s spheres is known as the carbon cycle. Carbon dioxide (CO2) and it’s sister greenhouse gas, Methane (CH4), are continually recycled on Earth.
The carbon cycle describes the flow of carbon between each of these places. For example, carbon continually flows in and out of the atmosphere and also living things. As plants photosynthesize, they absorb carbon dioxide from the atmosphere. When plants die, the carbon goes into the soil, and microbes can release the carbon back into the atmosphere through decomposition. Forests, soil, oceans, the atmosphere, and fossil fuels are important stores of carbon. Carbon is constantly moving between these different stores, that act as either “sinks” or “sources.”
A carbon sink is any natural reservoir that absorbs more carbon than it releases and thereby lowers the concentration of CO2 from the atmosphere. Globally, the two most important carbon sinks are vegetation and the ocean. Processes that release CO2 to the atmosphere are called carbon “sources”, while processes that absorb it are called carbon “sinks”. A sink absorbs more carbon than it gives off, while a source emits more than it absorbs.
The amount of carbon in the atmosphere at any one time depends on the balance that exists between the sinks and sources. This system of sinks and sources operates all over th e planet and is known as the carbon cycle. Natural sources of atmospheric CO2 include volcanoes, fires, decomposition, respiration, digestion, and, under certain conditions, oceans and freshwater bodies. The latter can release large amounts of dissolved CO2 when waters warm-up or are disturbed by storms or tremors.
An increase in atmospheric carbon dioxide means an increase in global temperature. The amount of carbon dioxide varies naturally in a dynamic equilibrium with the photosynthesis of land plants. The natural sinks are:
- SOIL: is the Earth’s greatest carbon store and active carbon sink
- FORESTS (TREES): Photosynthesis: by terrestrial plants with grass and trees serving as carbon sinks during growing seasons. Photosynthesis accounts for about half of the carbon extracted from the atmosphere. Photosynthesis equation: 6H2O (water) + 6CO2 + sunlight energy = C6H12O6 (glucose) + 6O2 (oxygen). Land plants take most of their carbon dioxide from the air around them while aquatic plants in lakes, seas and oceans uses carbon dioxide dissolved in water. Phytoplankton is one of these important plants as they produce up to 50% of the atmospheric oxygen through photosynthesis
- OCEANS: Absorption of carbon dioxide by the oceans via physicochemical and minor biological processes. The world’s oceans are absorbing an unprecedented amount of carbon dioxide (CO2), which is increasing their acidity and possibly threatening the long-term survival of many marine species, especially calcifying organisms including corals, shellfish, and phytoplankton (UNESCO, 2004).
Carbon sources include the combustion of fossil fuels (coal, natural gas, and oil) by humans for energy and transportation and farmland (by animal respiration), forest fires, landfills, and respiration. Human perturbations to the carbon cycle are impacting the maintenance of the concentration of atmospheric carbon. This is leading to an increase in the amount of carbon in the atmosphere as humans produce carbon dioxide and methane far faster than the natural sinks can absorb it. The extraction, processing, and use of oil, gas, and coal are the greatest contributors to this carbon loading. Methane (CH4) is an important trace gas in Earth’s atmosphere. Even though it only makes up 0.00017% (1.7 parts per million by volume) of the atmosphere, methane traps a significant amount of heat, helping the planet remain warm and habitable. The amount of methane in the atmosphere is the result of a balance between production on the surface and destruction in the atmosphere. Methane forms when organic matter decomposes in oxygen-poor environments, such as marshes, rice paddies, or the digestive systems of cattle. It also comes from the combustion (burning) of carbon-based fuels. Each methane molecule holds about 23 times more heat than 1 molecule of carbon dioxide. Methane is 23 times more potent than carbon dioxide as a greenhouse gas, but CO2 is much more abundant than methane and the predicted growth rate is far greater. In addition to contributing significantly to carbon sources, human activities are also interfering with natural carbon sinks. For example, the clearing of tropical rainforests for agriculture and logging represents a significant loss to the earth’s ability to absorb and store carbon.
As a greenhouse gas, CO2 increases the rate of global climate change. This is because CO2 contributes so greatly to the greenhouse effect.