A carbon pool is a reservoir within the Earth system that accumulates and stores carbon in various chemical forms. These pools hold the planet’s carbon in dynamic storage, ranging from short-term sequestration in living matter to storage lasting millions of years in rock. The movement of carbon between these reservoirs constitutes the global carbon cycle, a continuous exchange that regulates the amount of carbon available to the atmosphere, oceans, and land. Understanding the size and capacity of these major pools is fundamental to tracking carbon movement and comprehending the Earth’s climate system.
The Major Carbon Reservoirs
The planet’s carbon is distributed across four primary reservoirs: the lithosphere, the oceans, the terrestrial biosphere, and the atmosphere. The lithosphere, which includes the Earth’s crust and upper mantle, holds the largest quantity of carbon, locked away in carbonate-rich sedimentary rocks like limestone. This geological pool contains an estimated 66 to 100 million gigatons of carbon (GtC), with only a tiny fraction stored in fossil fuels.
The ocean represents the largest active carbon pool, holding approximately 38,000 GtC, primarily as dissolved inorganic carbon such as bicarbonate and carbonate ions. This reservoir is stratified, with the vast majority of carbon stored in the cold, deep ocean layers, away from immediate contact with the atmosphere.
The terrestrial biosphere holds carbon in all living and dead organic matter on land, including plants, animals, and soil. The terrestrial pool consists of roughly 540 to 610 GtC in biomass, mainly in forests, and 1,500 to 1,600 GtC stored in soils and permafrost. The atmosphere is the smallest of the major pools, containing about 800 GtC of carbon, primarily as carbon dioxide. Although small, its gaseous form makes it the most significant regulator of Earth’s surface temperature.
Movement Between Pools (The Carbon Cycle)
The carbon cycle describes the continuous exchange of carbon between these major reservoirs through natural physical and biological processes, known as fluxes. On land, the exchange between the atmosphere and the terrestrial biosphere is governed by two main biological actions. Photosynthesis, carried out by plants, removes carbon dioxide from the atmosphere to build organic matter. Respiration and decomposition by plants, animals, and microbes release carbon back into the atmosphere and soil.
The oceans facilitate carbon transfer through both physical and biological mechanisms. The solubility pump is a physicochemical process where carbon dioxide dissolves into cold surface waters at high latitudes. These waters then sink due to increased density, transporting dissolved inorganic carbon to the deep ocean. This process is driven by the thermohaline circulation, the global conveyor belt of ocean currents.
The biological pump involves marine life, starting with phytoplankton, which use photosynthesis to convert dissolved carbon dioxide into organic matter in the surface waters. When these organisms die or are consumed, their remains sink as “marine snow,” transferring carbon to the deep ocean floor. On geological timescales, the lithosphere exchanges carbon through the chemical weathering of silicate rocks, which removes atmospheric carbon dioxide when it reacts with rainwater to form carbonic acid.
Time Scales of Carbon Storage
The movement of carbon is divided into the fast and slow carbon cycles, which operate over vastly different time scales. The fast carbon cycle involves the atmosphere, the terrestrial biosphere, and the surface ocean, with carbon moving between these pools on time scales of years to centuries. Carbon in living biomass, for example, is cycled within decades as plants grow and decay.
The slow carbon cycle includes the deep ocean and the lithosphere, where carbon is sequestered for millennia to millions of years. Deep ocean circulation can hold carbon for hundreds to thousands of years before upwelling brings it back to the surface. The longest-term storage is found in the formation of sedimentary rocks, a process that can take 100 to 200 million years to complete before carbon is naturally returned to the atmosphere through volcanic outgassing or metamorphic reactions.
Anthropogenic Impacts on Pool Balance
Human activity has perturbed the balance of the carbon cycle by rapidly accelerating the transfer of carbon from the slow pool into the fast, active pools. The primary disruption comes from extracting and burning fossil fuels, which are carbon compounds stored in the geological pool for millions of years. Combusting coal, oil, and natural gas introduces a massive flux of carbon dioxide directly into the atmosphere.
This rate of release far exceeds the slow natural rate of geological carbon cycling. Land-use change, particularly large-scale deforestation, represents the second major anthropogenic impact, rapidly moving carbon from the terrestrial biosphere and soil pools into the atmosphere. When forests are cleared or burned, the carbon stored in the wood and soil organic matter is quickly oxidized and released as carbon dioxide.
These rapid additions of carbon overwhelm the natural capacity of the oceans and land to absorb the excess, disrupting the equilibrium that existed for millennia. While the oceans and terrestrial ecosystems have absorbed over half of the human-caused carbon dioxide, this uptake is not instantaneous or limitless. The resulting increase in atmospheric carbon dioxide drives changes to the Earth’s climate system.