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 carbon cycle.
In addition to the carbon cycle, other key biogeochemical cycles include the nitrogen, sulfur, and phosphorus cycles.
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 climate change.
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 sink. The effect of this carbon storage is to partially offset warming from emissions of CO2 and other greenhouse gases.
Altered biogeochemical cycles together with climate change increase the vulnerability of biodiversity, food security, human health, and water quality to changing climate. However, natural and managed shifts in major biogeochemical cycles can help limit rates of climate change.
To learn more about the carbon cycle and other biogeochemical cycles, explore the resources in the sidebar.