Carbon Cycle

The carbon cycle describes the movement and storage of carbon on earth. Knowledge of the carbon cycle helps us to understand the impacts of anthropogenic additions of carbon to the atmosphere on the storage and movement of carbon.

Places where carbon is stored are called pools or reservoirs. The five pools of carbon are sedimentary rock, terrestrial soils, the atmosphere, land vegetation, and oceans.

By far, the largest amount of carbon is buried in sedimentary rock, and this quantity is considered inactive in the carbon cycle. The active (or surface) pools constitute less than 1 percent of the carbon on earth, and they contain 40 × 1018 g C. Within this active carbon, the oceans constitute the largest pool (38,000 × 1015 g C, or 38,000 gigatons of carbon [GtC]). Movement (or fluxes) of carbon to and from the ocean happen primarily with the atmosphere, and the input and output of carbon to the ocean are almost balanced. Oceans take up carbon dioxide through diffusion, which is then used by photosynthetic plankton to produce sugars, or is taken up by organisms to produce shells. Some of the carbon, particularly from shell-producing organisms, sinks to the ocean floor and is buried in the sediments, which can eventually become part of the inactive carbon pool. The largest flux of carbon out of the ocean is back into the atmosphere, and this occurs via respiration of ocean organisms.

Terrestrial soils make up the second-largest pool of active carbon (1,500 GtC). Inputs of carbon (made up mostly of sugars) to soils occur when terrestrial vegetation sheds litter or dies. This organic matter is decomposed by organisms within the soil and becomes part of the soil carbon pool. Output of carbon from soils happens through the respiration of carbon dioxide from soil organisms, which, together with the respiration of land plants, generally equals the amount of carbon taken up by land plants, thus balancing the carbon budget for terrestrial systems. Land plants constitute the smallest pool of carbon (560 GtC). Vegetation takes up carbon dioxide from the atmosphere to make sugars and other carbon-based compounds. Such carbon compounds are either used within the plant for storage, reproduction, or respiration or they are transferred to the soil through litterfall or when plants die. This process, like any other that removes carbon from the atmosphere is an example of carbon sequestration.

The atmosphere is the third-largest pool of carbon (750 GtC), and the only two forms of carbon found here are methane and carbon dioxide. As we have seen, the atmosphere exchanges carbon with the ocean and land plants and receives carbon from soils. In addition, the atmosphere is the only pool known to be increasing with anthropogenic additions to the carbon cycle. Fossil fuel emissions and land clearing constitute an additional flux to the atmosphere of about 7 GtC/year. The combustion of fossil fuels and the burning of forests emit carbon dioxide, while the draining and clearing of wetlands release methane. It is known that half of this additional flux (about 3.2 GtC/year) remains in the atmosphere, slowly increasing the concentrations of carbon dioxide and methane. The fate of the other half of this additional flux (or carbon “sink”), however, is not fully known. It is possible that the oceans are absorbing some of the extra carbon.

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