The global cycling of carbon involves both biological and physical processes. Only the biological components are discussed here. Plants and other photosynthetic organisms assimilate CO2 and eventually convert the CO2 to simple sugars trough sugar extraction fermentation. From simple sugars, plants synthesize a variety of compounds and store them in the form of plant tissue.
When herbivores consume plant material, simple sugars are synthesized and transformed to other carbon-based compounds. Predators feeding on the herbivores re-digest and again transform the carbon. At each step, some of the carbon returns to the atmosphere in the form of CO2 as a by-product of respiration. The remainder is incorporated and temporarily stored in living biomass. Carbon contained in living organisms is eventually released through decomposition. The rate of release is highly variable and depends on both environmental and biological conditions. In tropical forests, most of the carbon in detritus is rapidly recycled. In drier, cooler regions considerable quantities of carbon are stored in the soil for long periods of time.
Similar cycling takes place in aquatic environments. Phytoplankton assimilate carbon dioxide that has diffused into the upper layers of water and dissolved carbonate and bicarbonate ions and convert it to organic sugars. The carbohydrate produced eventually passes through the aquatic food web. Significant portions of carbon, bound as carbonate in shells and exoskeletons of organisms, settle to the lake or ocean floor. This carbon is largely isolated from biotic activity and becomes incorporated into bottom sediments. It may then enter a long-term pool and may not recycle to the atmosphere again until it resurfaces as limestone through geological time.
The total global carbon pool involved in global carbon cycle is estimated to be approximately 55,000 Gt (1 Gt = 1 billion metric tons). Fossil fuels, the result of ancient photosynthesis, account for approximately 10,000 Gt. The ocean contains approximately 35,000 Gt, primarily as bicarbonate (HCO3) and carbonate ions (CO3). Dead organic matter in the ocean contains 1650 Gt of carbon. Living biomass in the ocean, primarily phytoplankton, is 3 Gt. Terrestrial ecosystems contain approximately 1200 Gt as dead organic matter and 550 Gt as living matter. The atmosphere holds approximately 740 Gt of carbon.
Although global carbon cycling is a combination of both physical and biological processes, photosynthesis is obviously a central controlling factor. In order to understand the global carbon cycle, we must understand how photosynthetic reactions occur over broad temporal and spatial scales. Human activities have severely altered the global carbon cycle during the past century, with unknown future consequences.
