Circulation through nature of carbon in the form of the simple element and its compounds. The source of carbon in living things is carbon dioxide (CO2) from air or dissolved in water. Algae and green plants (producers) use CO2 in photosynthesis to make carbohydrates, which in turn are used in the processes of metabolism to make all other compounds in their tissues and those of animals that consume them. The carbon may pass through several levels of herbivores and carnivores (consumers). Animals and, at night, plants return the CO2 to the atmosphere as a by-product of respiration. The carbon in animal wastes and in the bodies of organisms is released as CO2 in a series of steps by decay organisms (decomposers), chiefly bacteria and fungi (see fungus). Some organic carbon (the remains of organisms) has accumulated in Earth's crust in fossil fuels, limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric times, is being returned in vast quantities as CO2 via industrial and agricultural processes, some accumulating in the oceans as dissolved carbonates and some staying in the atmosphere (see greenhouse effect).
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Reduction in photosynthesis between 1850-2000: 156 / (540 + 156) = ~22%
120 = (100-22=78)% of 154 GtCpa - a difference of ~34 GtCpa in 2000
Given the increase of 0.5 GtCpa over the past eight years since 2000, this figure would be closer to 38GtCpa in 2008.
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