The carbon cycle is a fundamental component of the biogeochemical dynamics of Earth. Carbon is exchanged and cycled between atmosphere, ocean layers, land, and geosphere through processes such as photosynthesis, decomposition, respiration, and mineralization. In the atmosphere, carbon exists in the form of gaseous atmospheric CO2, which constitutes a small but very important portion of total terrestrial carbon.
On land, living biota and decaying organic matter are phases of the cycle. A significant fraction of carbon occurs in minerals, especially in calcium and magnesium carbonates (CaCO3). A fraction has been buried in the Earth in the form of solid coal, liquid petroleum, and natural gas by biogeochemical processes. Photosynthesis is the milestone for life on Earth. Early organisms developed the capacity, aided by sunlight, to use CO2 and water from their surroundings to build the organic molecules they required for growth. Land hosts a large percentage of the photosynthesis, and the biggest contribution comes from forests in the tropical belt. Carbon is fixed by plants as CO2 and is sooner or later returned to the air as CO2 or to the sea as organic material. This return occurs by two pathways: respiration of consuming organisms (including humans) and the action of organisms which decompose dead organic matter and eventually return it to the mineral state. Photosynthesis and respiration cause daily fluctuations of carbon dioxide in the atmospheric reservoir.
In the ocean, the carbon is dissolved as HCO3~ or exists in form of molecular CO2 or in microscopic plants such as phytoplankton. The carbon cycle is somewhat different in oceans where the agent of photosynthesis is phytoplankton. CO2 fixed in the surface layers of water initiates a downward flow of carbon. Organic sediment is used by the decomposing organisms of the ocean floor, again producing CO2 which is partly absorbed in the depths of the sea and partly released to the atmosphere. The marine reservoir absorbs CO2 from the other two systems (the Earth and atmosphere) through the rain cycle and surface absorption.
About 38 000-40 000 Gt of carbon is stored in the ocean depths, while 2000 Gt on land and in soil, 750 Gt in the atmosphere, and 800 Gt in the surface layer of the ocean.
The atmosphere, biota, soil, and surface layer of the ocean are closely linked, continuous, relatively rapid carbon exchanges of carbon occurring between them. Exchange of carbon between this fast-responding system and the ocean depths takes much longer (of the order of thousands of years). In other words, exchange with the depths of the ocean limits absorption of CO2; the overall result is accumulation of CO2 in the atmosphere. This means that the ocean depths cannot help to mitigate CO2 buildup.
Although all these fluxes, mostly driven by solar energy, are approximately balanced each year, imbalances are possible and feedbacks may occur that significantly affect atmospheric CO2 concentration in the time horizon ranging from years to centuries. This variability is mainly caused by variations in land and ocean uptake, by climatic phenomena such as El Nino, as well as by human behavior. It is often difficult to distinguish changes due to human activity from natural variations.
Human activities contribute to CO2 accumulation in two main ways: through combustion of fossil fuels (coal, oil, natural gas) and through deforestation, especially of tropical rain forests. Biomass and fossil fuels are burned to meet mankind's growing demand of energy, releasing CO2 into atmosphere. This input causes a significant perturbation in the delicate equilibria of the biosphere, especially because fossil fuels are being burned in an infinitesimal time compared to the aeons taken by slow sedimentary processes to form this resource.
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