Carbon exists in a continuous state of movement throughout the planet’s systems. A carbon reservoir is a natural storage location that accumulates and stores carbon-containing chemical compounds over various timescales. These reservoirs function as a planetary thermostat, regulating the concentration of carbon dioxide in the atmosphere, which controls Earth’s temperature and climate stability. Understanding the size and capacity of these major storage pools is fundamental to comprehending the global carbon cycle and the forces that drive environmental change. The movement of carbon between these reservoirs, measured in gigatons of carbon (GtC), determines the overall balance of the Earth system.
Defining Earth’s Carbon Storage Pools
The planet’s carbon is distributed across four primary reservoirs: the geological lithosphere, the oceans, the terrestrial biosphere, and the atmosphere. The geological reservoir, primarily within sedimentary rocks like limestone and chalk, holds up to 100 million GtC. This massive pool represents the long-term sequestration of carbon over hundreds of millions of years, making it the largest, yet slowest-moving, component of the global cycle.
The ocean represents the largest active reservoir, storing an estimated 38,000 to 41,000 GtC, predominantly as dissolved inorganic carbon. This oceanic storage is stratified into two main layers: the surface ocean, which rapidly exchanges carbon with the atmosphere, and the deep ocean, where carbon can remain sequestered for centuries or longer. The solubility of carbon dioxide in cold deep water is a major factor in this vast storage capacity.
The terrestrial biosphere, encompassing all life on land, holds a comparatively smaller but highly active amount of carbon, estimated to be around 2,000 to 3,170 GtC. This storage is split between living biomass, such as the wood and leaves of forests (around 550 GtC), and the soil, which contains a much larger pool of organic matter (around 1,500 GtC). The carbon stored in soil, derived from decomposed plant matter, can remain sequestered for hundreds to thousands of years.
The atmospheric reservoir, though the smallest of the four, holds approximately 750 to 900 GtC, mostly in the form of carbon dioxide and methane. Despite its small size, this pool is the most relevant for climate regulation because its composition directly controls the greenhouse effect. Small changes in the amount of carbon held here can disproportionately influence global temperatures.
The Dynamics of Carbon Exchange
Carbon constantly moves between these reservoirs through a set of processes known as the carbon cycle, which operates on both rapid and extremely slow timescales. The Fast Carbon Cycle involves exchanges that occur over weeks, months, or years, primarily between the atmosphere, the terrestrial biosphere, and the surface ocean. Photosynthesis is the primary mechanism that removes about 120 GtC from the atmosphere annually, transforming it into organic matter in plants.
This carbon is then returned to the atmosphere through respiration by plants and animals, as well as through decomposition of dead organic matter by microbes. The ocean surface also engages in a two-way exchange with the atmosphere, absorbing and releasing carbon dioxide through simple gas diffusion, with a flux of approximately 90 GtC moving in each direction annually. While these exchanges are large, they are nearly balanced in the natural cycle, maintaining a relatively stable atmospheric concentration.
The Slow Carbon Cycle governs the movement of carbon into and out of the geological reservoir over millions of years. This cycle involves processes like the weathering of silicate rocks on land, which slowly pulls carbon dioxide out of the atmosphere. The dissolved carbon is transported by rivers to the ocean, where marine organisms use it to build shells of calcium carbonate. When these organisms die, their shells sink and eventually form carbon-rich sedimentary rock on the ocean floor, effectively sequestering the carbon. This slow, geological process is balanced by carbon release from volcanoes and mid-ocean ridges, where carbon dioxide is vented back into the atmosphere and ocean. The slow cycle operates at a rate of only about 0.1 GtC per year.
Anthropogenic Influence on Reservoir Stability
Human activities have fundamentally altered the balance of the carbon cycle by rapidly transferring carbon from the slow geological and fast terrestrial reservoirs into the small atmospheric pool. The combustion of fossil fuels—coal, oil, and natural gas—extracts carbon sequestered millions of years ago and releases it as carbon dioxide gas. This process introduces approximately 9.7 GtC of new carbon into the active cycle each year. Land-use change, particularly deforestation, acts as a secondary source of atmospheric carbon by removing standing biomass and disturbing carbon-rich soil, adding about 1.1 GtC annually.
These combined human emissions represent a sudden and massive injection of carbon that far exceeds the natural capacity of the slow cycle to remove it. Consequently, the atmospheric reservoir has experienced a net gain, leading to the measurable increase in the greenhouse effect. Fortunately, the ocean and the terrestrial biosphere have acted as net sinks, temporarily absorbing a portion of this excess carbon, thereby slowing the rate of atmospheric increase.
Over the past decade, the ocean has absorbed about 27% of anthropogenic emissions, while the terrestrial biosphere has absorbed roughly 30%. This absorption leads to the dissolution of excess carbon dioxide in seawater, which lowers the ocean’s pH and results in ocean acidification, impacting marine ecosystems. The capacity of these natural sinks is finite and may diminish as warming reduces the ocean’s ability to absorb gas.