The carbon cycle is the continuous movement of carbon atoms through Earth’s major storage areas, known as reservoirs. These reservoirs include the atmosphere, the oceans, all living organisms, and the rocks and sediments of the Earth’s crust. Carbon is the chemical backbone of all life and its cycling regulates the planet’s temperature and climate. For millions of years, the natural processes of the carbon cycle maintained a relative balance of the element across all spheres.
Carbon Assimilation
The first major step in the carbon cycle is assimilation, which draws inorganic carbon from the atmosphere into the living world. This is primarily accomplished by photoautotrophs, such as plants, algae, and cyanobacteria, through photosynthesis. These organisms absorb atmospheric carbon dioxide (\(\text{CO}_2\)) using light energy to drive the conversion.
The process incorporates \(\text{CO}_2\) into an existing organic compound through the Calvin-Benson cycle. This fixation converts simple inorganic carbon into complex organic compounds, such as glucose and carbohydrates, which are used for energy and building the organism’s biomass. Approximately 250 billion tons of carbon dioxide are fixed by photosynthesis annually, forming the base of the global food web. This stored carbon is then transferred to heterotrophs, such as animals and fungi, when they consume the plants.
Release Through Biological Processes
Once carbon is incorporated into living biomass, a rapid exchange begins to return it to the atmosphere through various biological mechanisms. The primary pathway is cellular respiration, performed by nearly all living organisms, including the plants that initially fixed the carbon. Respiration breaks down complex organic carbon compounds, such as sugars, to release stored energy, with carbon dioxide as a byproduct released into the surroundings.
Another significant release mechanism is decomposition, which occurs when dead organic matter and waste products are broken down by microbes and fungi. These decomposers also perform respiration as they consume the decaying material, returning its stored carbon directly to the atmosphere or soil as \(\text{CO}_2\). This fast biological cycle ensures that carbon atoms are rapidly exchanged between the atmosphere and the terrestrial biosphere over short timescales.
Oceanic Cycling
The ocean represents the largest active reservoir of carbon, holding about 50 times more carbon than the atmosphere. Carbon dioxide is continually exchanged between the atmosphere and the ocean’s surface waters through a physical process known as the solubility pump. This exchange is driven by the fact that \(\text{CO}_2\) readily dissolves in seawater, with solubility increasing in colder water.
Once dissolved, \(\text{CO}_2\) reacts with water to form carbonic acid and subsequently bicarbonate and carbonate ions, collectively known as dissolved inorganic carbon (DIC). The ocean’s circulation, particularly the thermohaline circulation, transports this DIC from the surface to the deep ocean, where it can be sequestered for hundreds to thousands of years. The biological pump involves marine organisms like phytoplankton fixing carbon through photosynthesis; when they die, their remains sink as “marine snow,” transferring carbon to the deep sea floor.
Long-Term Geological Storage
The slowest component of the cycle involves the transfer of carbon into the lithosphere, where it can be stored for millions of years. This long-term storage begins when marine organisms use dissolved carbon to create calcium carbonate shells. Upon death, these shells sink and accumulate on the seafloor, eventually forming thick layers of carbonate rock, such as limestone, which constitute the largest carbon reservoir on Earth.
On land, organic matter from dead plants and plankton that escapes decomposition can become buried under layers of sediment. Over millions of years, heat and pressure transform this buried carbon into fossil fuels, including coal, oil, and natural gas. The natural release of this geologically stored carbon back into the atmosphere is extremely slow, occurring through processes like volcanic eruptions and the weathering of rocks.
Human Influence and Accelerated Release
The final step involves the modern, accelerated release of carbon, which has significantly disrupted the natural balance of the cycle. This disruption began with the Industrial Revolution and is primarily driven by the combustion of fossil fuels. When humans burn coal, oil, and natural gas for energy, they quickly release vast amounts of carbon sequestered over millions of years of geological time.
Burning these long-term stores releases \(\text{CO}_2\) at a rate 100 to 300 times greater than natural volcanic activity, overwhelming the planet’s natural absorption mechanisms. Land-use changes, particularly deforestation, further accelerate the release by removing plants that assimilate carbon and by burning forests, which immediately returns stored carbon to the atmosphere. This rapid injection of carbon has led to a significant rise in atmospheric \(\text{CO}_2\) concentrations. Approximately half of the anthropogenic carbon released is absorbed by the ocean and land, with the remaining half staying in the atmosphere, driving global climate change.