There is a fixed amount of carbon on planet earth which is distributed among several carbon reservoirs or pools in the atmosphere, biosphere, hydrosphere, and lithosphere. In the grand scheme, carbon is neither created nor destroyed but continually moves between these various pools owing to the operation of natural and human-induced processes. The root cause of global climate change is that human activity has shifted massive quantities of carbon to the atmosphere from forests, soil, and fossil deposits.

“Diagram of the carbon cycle. The black numbers indicate how much carbon is stored in various reservoirs, in billions of tons ("GtC" stands for GigaTons of Carbon). The blue numbers indicate how much carbon moves between reservoirs each year. The sediments, as defined in this diagram, do not include the ~70 million GtC of carbonate rock and kerogen^”1.

In the atmosphere carbon is stored as CO₂, methane (CH₄), and other organic compounds. Carbon moves into the atmosphere from decomposition of organic matter, respiration by living organisms, combustion, volcanic activity, burning fossil fuels, degassing of waterbodies, etc. Carbon moves out of the atmosphere via photosynthesis, rock weathering, dissolution in water, etc. All plants, including forests and many micro-organisms, use photosynthesis which takes CO₂ out of the air to build sugars that can be used by the cell to build cellulose or other complex carbon molecules that comprise plant biomass. This process is called “primary production” and it feeds the bottom of the global food chain. Virtually all life on earth, including humans, relies directly or indirectly on photosynthesis. Most terrestrial plants share a significant portion of their photosynthate with soil organisms, a cooperative relationship that builds a large and complex underground ecosystem that also stores carbon. Plants shed dead leaves and wood which also builds carbon stores in the soil².

 In the hydrosphere (e.g. the oceans) carbon is stored mostly as dissolved CO₂ and other dissolved organic compounds that originated in some photosynthetic life form. Carbon moves into the ocean from the atmosphere and biosphere via dissolving of gaseous CO₂ in cold seas, leaching from soil, and input of organic matter from river systems and the biosphere. Carbon moves out of the ocean primarily via photosynthesis (e.g. phytoplankton and cyanobacteria), degassing of warm seas, and deposition in marine sediments³.

In the biosphere carbon is stored as live or recently dead plants, animals, and micro-organisms both in the ocean and on land (e.g., forests and soils). Forests dominate the terrestrial carbon cycle, harboring 86% of the planet’s above-ground carbon and 73% of the planet’s soil carbon⁴.  Carbon enters into the biomass pool via photosynthesis, then becomes entrained and cycled through the entire global food chain. Carbon moves out of the biomass pool through decomposition and respiration or through deposition in long-term storage in soil or geologic and fossil deposits. 

In fossil deposits, the carbon from long-dead plants and animals are stored as coal, oil, “natural gas,” or kerogen. These can be thought of as both “ancient sunlight” and “ancient atmosphere.” Carbon moves into the fossil pool via deposition and storage in low-oxygen conditions⁵.  Carbon moves out of fossil pool mostly via industrial exploitation and combustion. 

In the non-fossil lithosphere carbon is stored in carbonate rocks such as limestone and chalk. Carbon moves into these geologic structures mostly through ocean deposition. A portion of the oceanic carbon is taken up to make the shells of marine organisms that fall to the deep ocean floor where they may be subducted beneath the earth’s crust and end up in long-term geologic storage, e.g. the Cliffs of Dover. Carbon moves out of the lithosphere mostly via volcanic activity and human industry such as the manufacture of cement which heats limestone and releases significant quantities of CO₂⁶. 

The advent and diversity of life on earth has had a profound impact on the global carbon cycle and now plays a fundamental role in determining whether or not we have a livable climate. The abiotic carbon cycle that existed before the proliferation of life was less stable than the carbon cycle that developed after marine organisms started to make calcium carbonate shells and deposit carbon in deep storage which has helped buffer CO₂ extremes over long time scales⁷.  Scientists have found a correlation between biodiversity and levels of atmospheric CO₂ over the last 370 million years⁸.

Human activity, mostly in just the recent era, has dramatically reallocated global carbon stores from the other carbon reservoirs into the atmosphere where it can influence our climate. For example, burning fossil fuels and heating limestone to make cement move carbon from long-term fossil and geologic storage into the atmosphere. Logging kills trees - stops carbon-uptake via photosynthesis, and moves carbon from living forests and soil into the atmosphere. Land uses such as agriculture, livestock grazing, and draining swamps move carbon from the soil to the atmosphere.