Carbon is fundamental to life and regulates Earth’s climate. It continuously moves through the atmosphere, oceans, land, and rocks in the carbon cycle. This movement makes carbon available for biological processes and influences global temperatures.
Atmosphere
The atmosphere acts as a carbon reservoir, primarily holding carbon as carbon dioxide (CO2) and methane (CH4). Although its carbon content is relatively small, this atmospheric carbon is dynamic and rapidly exchanges with other systems. Atmospheric CO2 is a greenhouse gas, absorbing and re-radiating heat to maintain Earth’s surface temperature. Its concentration has increased by 50% since the pre-industrial era, largely due to human activities. This amplifies the natural greenhouse effect, leading to rising global temperatures.
Oceans
The oceans represent a vast carbon reservoir, holding approximately 38,000 gigatons (Gt) of carbon, about 60 times more than the pre-industrial atmosphere. Carbon exists in the ocean in several forms, including dissolved inorganic carbon like bicarbonate and carbonate ions, dissolved organic carbon, and within marine organisms. The surface ocean actively exchanges carbon dioxide with the atmosphere, acting as a significant sink for atmospheric CO2. This absorption helps regulate atmospheric CO2 levels, but it also leads to ocean acidification as CO2 reacts with water to form carbonic acid.
The deep ocean holds a much larger, long-term store of carbon, with circulation processes moving carbon from the surface to greater depths over centuries. When marine organisms die, their carbon-rich bodies sink, leading to carbon being dissolved into deep waters or settling on the seafloor where it can be sequestered for millennia. This deep ocean storage is a significant component of the global carbon cycle.
Terrestrial Ecosystems
Carbon is extensively stored across Earth’s land-based ecosystems, encompassing living organisms and soils. Living biomass, particularly in forests, sequesters substantial carbon through photosynthesis, where plants absorb CO2 from the atmosphere to build their tissues. Forests alone store a large percentage of the planet’s terrestrial above-ground carbon.
Soils represent an even larger terrestrial carbon pool than living biomass, holding more carbon than vegetation and the atmosphere combined. This soil carbon consists of organic matter from decomposed plants and animals, which can remain stored for varying periods. Peatlands and permafrost regions are rich in stored organic carbon, acting as long-term reservoirs. The balance between carbon uptake by plants and its release through decomposition and respiration determines the overall carbon storage capacity of these land ecosystems.
Geological Reservoirs
Geological reservoirs contain the largest and longest-term stores of carbon on Earth. The single largest reservoir is found in sedimentary rocks, primarily as limestone (calcium carbonate), where carbon has been locked away over millions of years through geological processes. This carbon is removed from active, faster-cycling carbon reservoirs for vast periods.
Fossil fuels (coal, oil, natural gas) represent another significant geological carbon store. These fuels formed over geological timescales from ancient organic matter that was buried and transformed under heat and pressure. Their combustion rapidly releases carbon sequestered for millions of years back into the atmosphere. Geologic carbon sequestration efforts aim to store industrial CO2 emissions deep underground in depleted oil and gas reservoirs or saline formations.