Carbon Cycle

The carbon cycle describes the biogeochemical cycle, or routes by which carbon atoms are exchanged through nested networks of environmental systems from the atmosphere into the biosphere, through photosynthesis and back again with respiration, decomposition, and biomass burning. Elemental carbon is a traditional component of the hydrosphere, atmosphere, geosphere (rocks, such as limestone; coal; and soils), as well as the biosphere (all living things).

The carbon cycle also involves the process of removal and uptake of carbon on a global scale. This process involves components in food chains; the atmosphere, as carbon dioxide; the hydrosphere; and the geosphere. The major movement of carbon results from photosynthesis and respiration. Carbon is present in the planet in the following major reservoirs: as the gas carbon dioxide (CO2) in the atmosphere; as organic matter in soils; as organic molecules in living and dead organisms found in the biosphere; in the lithosphere as fossil fuels and sedimentary rock deposits such as limestone, dolomite, and chalk; and in the oceans as dissolved atmospheric CO2 and calcium carbonate shells in marine organisms.

The carbon cycle has a large impact on Earth, both globally and locally. At the global scale, the carbon cycle influences Earth’s climate by regulating the amount of CO2, a principal greenhouse gas, in the atmosphere. Terrestrial ecosystems store as much carbon as the atmosphere, so plants and soils play an important role in regulating climate. The carbon cycle also plays a primary role in keeping ecological systems in balance, since it is involved in basic ecological processes such as plant growth and accumulation, and the death and decay of plant material.

As a principal building block of organic matter, carbon is utilized by biotic components of an ecosystem, especially by organisms for structural growth: a portion of elemental carbon that a living thing takes in is usually incorporated into its tissues. Thus, the carbon cycle is one of the most important biogeochemical cycles to humans, because it is a vehicle through which one of the primary elements required for the formation of human tissues is cycled, and also because it is a means through which elemental carbon is introduced to plants, the basis of human food.

The carbon cycle is also important to the human climate system because it sets the background for the environment, through CO2 and methane (CH4), which are major drivers of global climate temperatures. The carbon atoms present in the atmosphere, hydrosphere, biosphere, and the geosphere are able to move from one of these environmental systems to another as part of the carbon cycle.

The carbon cycle is sometimes conceptualized as four major carbon sinks, interrelated by nested systems of pathways for transport of carbon atoms. The carbon reservoirs are the air (atmosphere), considered as the starting point of the cycle; seawater and oceans (hydrosphere), which include biotic and abiotic marine biota; sediments (fossil fuels); and terrestrial biosphere, which includes freshwater systems (lenthic or lotic), as well as nonliving organic substances, such as soil carbon. The carbon cycle is primarily controlled by a series of biological, physical, chemical, and geological processes within the environment.

In the atmosphere, carbon exists primarily as the gas CO2. It is this form that plants transform into carbohydrates via photosynthesis, releasing oxygen in the process. This process is mainly carried out in autotrophs (terrestrial and aquatic plants such as algae and cyanobacteria). They produce their organic compounds using atmospheric CO2, with solar radiation providing the source of energy for the process. However, a minor group of autotrophs harness chemical energy sources for the production of their organic compounds through a process called chemosynthesis.

Through the food chain, carbon is transferred as autotrophs (producers) and eaten by heterotrophs or as heterotrophs feed on other organisms. When the plants and animals die, their carcasses, stems, or leaves decompose, releasing the carbon trapped in them into the geosphere. Some could be buried, and over time, will become fossil fuels. Food webs serve as part of a carbon atom’s journey through the carbon cycle. Carbon is returned to the biosphere during cellular respiration.

Biomass burning has the ability to transport substantial amounts of carbon into the atmosphere. When humans burn fossil fuels to run industries, power plants, cars, airplanes, and jets, a significant portion of carbon is transferred directly into the atmosphere as CO2. When autotrophs and heterotrophs die, their remains may settle as sediments in freshwater and marine ecosystems. The carbon trapped in sediments is eventually released into the aquatic environments through geological and chemical processes. Marine animals use the released carbon to build their skeletal materials.

Ultimately, the stored carbon compounds in these organisms up the web are broken down by decomposition, and the carbon is released as CO2 back into the cycle to be used by plants. The world’s oceans act as sinks for CO2, and, in general, contain more than 90 percent of the carbon involved in the global cycle. The concentration of atmospheric CO2, which is an important driver of climate change, influences the temperature of the Earth’s surface, which is largely determined by the ocean. The levels of CO2 in surface water, and hence in the atmosphere, are usually kept lower than that in deep water by two principal natural processes: the solubility pump and the biological pump.