The global carbon cycle describes the continuous movement of carbon atoms through Earth’s various systems, including the atmosphere, oceans, land, and deep underground. This natural process forms the foundation for all life on Earth, as carbon is a building block for complex molecules like DNA, proteins, sugars, and fats. Beyond its role in living organisms, the carbon cycle also plays a significant part in regulating Earth’s temperature and overall climate. The balance of carbon cycling influences the concentration of carbon dioxide in the atmosphere, which in turn affects the planet’s heat retention.
Where Carbon Resides
Carbon is stored in several major natural reservoirs across the Earth’s systems. The atmosphere holds carbon primarily as carbon dioxide (CO2) gas, though it represents a relatively small fraction of the total carbon on Earth compared to other reservoirs.
The oceans constitute the largest active carbon reservoir, storing carbon in various forms. Dissolved carbon dioxide, carbonic acid, bicarbonates, and carbonates are all present in ocean waters. Marine organisms, from microscopic phytoplankton to larger animals, also contain carbon as part of their biomass.
On land, the terrestrial biosphere and soils act as significant carbon storage sites. Living organisms like plants and animals contain organic carbon within their structures. Dead organic matter, as well as the vast amounts of carbon stored in soils, contribute to this terrestrial reservoir. Forests, in particular, are large carbon sinks.
The Earth’s crust, encompassing sediments and rocks, holds the largest amount of carbon globally. Here, carbon is stored over geological timescales, often as fossil fuels such as coal, oil, and natural gas, which formed from ancient organic remains over millions of years. Limestone and other carbonate rocks also contain substantial quantities of carbon.
How Carbon Moves
Carbon continuously moves between these reservoirs through a variety of natural processes, known as fluxes. One of the primary ways carbon enters the terrestrial and oceanic biospheres is through photosynthesis. Plants on land and algae in water absorb carbon dioxide from the atmosphere or dissolved in water, converting it into organic compounds like glucose using sunlight. This process transfers carbon from an inorganic atmospheric form into organic matter.
Living organisms, including plants, animals, and microbes, release carbon back into the atmosphere or water through respiration. During respiration, organisms break down organic compounds for energy, producing carbon dioxide as a byproduct. When plants and animals die, decomposers like bacteria and fungi break down their organic matter, releasing stored carbon back into the atmosphere, soil, or water as carbon dioxide.
The exchange of carbon dioxide between the ocean and the atmosphere is a continuous process. Carbon dioxide from the atmosphere dissolves into surface ocean waters, forming carbonic acid, bicarbonates, and carbonates. Conversely, the ocean can release carbon dioxide back into the atmosphere when conditions change, such as warming temperatures. This oceanic absorption and release help regulate atmospheric carbon levels.
Over much longer timescales, geological processes play a role in carbon movement. Sedimentation occurs when marine organisms with calcium carbonate shells, like plankton and corals, die and sink to the ocean floor. Over millions of years, these accumulated shells can form carbonate rocks like limestone, effectively transferring carbon from the ocean to the Earth’s crust. The formation of fossil fuels also locks away carbon over millions of years, as dead organic matter from ancient plants and animals is buried and transformed under high pressure and temperature. Volcanic outgassing, on the other hand, can release carbon that has been trapped in rocks back into the atmosphere as carbon dioxide.
Human Influence on the Cycle
Human activities have significantly altered the natural balance of the global carbon cycle, primarily by releasing large quantities of stored carbon into the atmosphere. The burning of fossil fuels, including coal, oil, and natural gas, is a major contributor to increased atmospheric carbon dioxide. These fuels represent carbon that was stored underground for millions of years, and their combustion releases this carbon at a much faster rate than it was sequestered.
Deforestation and other land-use changes also play a substantial role in disrupting the carbon cycle. When forests are cleared, the removal of trees reduces the amount of carbon dioxide absorbed from the atmosphere through photosynthesis. When trees are burned or left to decompose, their stored carbon is released into the atmosphere. Agricultural practices, such as tilling soil, can also lead to the rapid decomposition of organic matter in soils, releasing additional carbon.
Industrial processes, beyond fossil fuel combustion, contribute to carbon emissions. For instance, the production of cement involves the chemical conversion of limestone, which releases carbon dioxide as a byproduct. These various human-driven transfers of carbon from long-term reservoirs to the atmosphere have led to a rapid increase in atmospheric carbon dioxide concentrations. This increase has significant implications for Earth’s climate system.