Life is composed primarily of carbon, so estimates of the global production and destruction of organic carbon give us an overall index of the health of the biosphere. Photosynthetic organisms capture sunlight energy in organic compounds that fuel the biosphere and account for the presence of molecular O2 in our atmosphere. Thus, the carbon and oxygen cycles on Earth are inextricably linked, and the presence of O2 in Earth's atmosphere sets the redox potential for organic metabolism in most habitats. When we understand the carbon cycle, we can make good first approximations of the movement of other elements in global cycles, recognizing the predictable stoichiometry of the chemical elements in organic matter. Currently humans harvest about 20% of the annual production of organic carbon on land. In many areas we have destroyed land vegetation, while in other areas we have planted productive crops and forests. At the moment, it appears that humans have created a net sink for carbon in the terrestrial biosphere, which mitigates some of the anticipated rise in atmospheric CO2 from fossil fuel combustion.
William H. Schlesinger, Emily S. Bernhardt, Chapter 11 - The Global Carbon and Oxygen Cycles, Editor(s): William H. Schlesinger, Emily S. Bernhardt, Biogeochemistry (Fourth Edition), Academic Press, 2020, Pages 453-481, 9780128146088,