Carbon exists in the atmosphere, oceans, soil, rocks, fossil fuels, and living organisms, and is continually cycled through the Earth system. For example, the oceans absorb carbon dioxide from the atmosphere, and organisms exhale, digest, and decompose carbon compounds. These and other processes of carbon storage, transformation, and release back into the atmosphere make up the
In addition to the carbon cycle, other key
Key Messages About Biogeochemical Cycles from the National Climate Assessment
Human activities have increased atmospheric carbon dioxide by about 40% over pre-industrial levels and more than doubled the amount of nitrogen available to ecosystems. Similar trends have been observed for phosphorus and other elements, and these changes have major consequences for biogeochemical cycles and
In total, land in the U.S. absorbs and stores an amount of carbon equivalent to about 17% of annual U.S. fossil fuel emissions. U.S. forests and associated wood products account for most of this land
Altered biogeochemical cycles together with climate change increase the
To learn more about the carbon cycle and other biogeochemical cycles, explore the resources in the sidebar.