The carbon cycle is a biogeochemical cycle that corresponds to all carbon exchanges on the planet.
Carbon is an essential element for all life forms. Two types of carbon are found in nature. First, carbon is the basis of complex molecules (proteins, lipids, carbohydrates) that are used to build the tissues of living organisms; this is known as organic carbon. Inorganic carbon is when the carbon is not bound to living organisms. This is the case of carbon dioxide |\left( CO_2 \right)| and methane |\left( CH_4 \right)|, two greenhouse gases that have a major impact on the planet's climate.
There is a constant exchange of carbon between the hydrosphere, lithosphere, biosphere, and atmosphere. However, most of the short-term cycle takes place between the atmosphere, the soil’s surface layers and the oceans, and the biosphere. Two large carbon reservoirs store it for a long period: the lithosphere and the hydrosphere through sediments, rocks, and oceans.
The chemical recycling of carbon is a critical element in maintaining our planet’s balance. Indeed, this cycle directly affects biological productivity and climate. Some of the processes that allow the chemical recycling of carbon are very rapid (biochemical processes) while others take place over several hundred years (geochemical processes).
The main biochemical processes taking place during the carbon cycle are:
The main geochemical processes taking place during the carbon cycle are:
In addition, certain factors, mainly of anthropogenic origin, can modify the carbon cycle.
Photosynthesis takes place in both terrestrial and aquatic environments. Through this process, plants store carbon of atmospheric origin or carbon dissolved in water. Plants use solar energy to transform carbon dioxide |\left( CO_2 \right)| into glucose by producing oxygen. Glucose will thus be used as organic matter to make plant tissues.
Consumption takes place both in terrestrial and aquatic environments. Herbivorous animals obtain the carbon necessary for their growth by consuming plants. Carnivorous animals, on the other hand, absorb carbon from the animals they eat. Carbon is thus transferred from one level to another in a food chain.
Respiration takes place both in terrestrial and aquatic environments. Carbon is returned to the atmosphere through the respiratory process. All living organisms, whether plant or animal, breathe. Therefore, they release part of the carbon they initially ingested into the atmosphere or the hydrosphere, in the form of carbon dioxide.
The portion of carbon that is not released through respiration is eliminated in plant and animal waste (urine, stool, dead organisms, etc.). In the soils and sediments of lakes and oceans, the waste is broken down by microorganisms. Depending on the presence or absence of oxygen, decomposers will decompose or ferment organic matter. These processes release carbon dioxide |\left( CO_2 \right)| and methane |\left( CH_4 \right)|, and convert organic matter into inorganic matter.
Under the action of combustion, the carbon inside tree trunks and foliage is transformed into carbon dioxide |\left( CO_2 \right)|. Deforestation, on the other hand, decreases the number of trees present that can carry out photosynthesis. Usually, in the absence of these two phenomena, the quantity of carbon fixed on a planetary scale by the organisms which carry out the photosynthesis is in balance with that released by the respiration and decomposition of other organisms. However, when both of these two phenomena occur, more carbon dioxide will be released into the atmosphere.
Carbon dissolution takes place in the hydrosphere. A large proportion of atmospheric carbon dioxide is dissolved in the oceans. In fact, the oceans are carbon sinks, because overall they remove more carbon from the atmosphere than they give back to it. Some of the carbon dioxide dissolved in water reacts with water molecules and then with calcium to become calcium carbonate |\left( CaCO_3 \right)|. Calcium carbonate is found in shell and skeleton composition of marine organisms.
Sedimentation takes place mainly in the hydrosphere. The shells and skeletons of dead marine organisms accumulate on the ocean floor. Calcium carbonate therefore accumulates in sediments and gives rise to carbonate rocks. These rocks follow the movement of tectonic plates. They sink under the Earth's mantle during the subduction event and can eventually be brought to the surface. They can also be buried in the Earth's crust and remain there for many years.
Volcanic eruptions can occur on the Earth's surface or underwater. In both cases, when the magma comes in contact with the carbon inside carbonate rocks, it can be released and return to the atmosphere. Volcanoes and geysers release carbon dioxide and methane into the atmosphere.
Dead organisms that fall to the bottom of the ocean form a layer of sediment. They can sometimes turn into fossil fuels such as coal or petroleum if they remain buried in sediments for hundreds of millions of years. Humans burn these fossil fuels (petroleum, coal, natural gas) to meet their energy needs. As a result, it increases the amount of carbon dioxide released into the atmosphere and disrupts the carbon cycle.
As can be seen, the carbon geological phases, such as subduction and fossil fuel formation, take place over a period of millions of years while the biological phases of the carbon cycle, such as photosynthesis, respiration, and decomposition, can occur over a period ranging from a few days to thousands of years.
Before the industrial era, at the end of the 19th century, the carbon cycle involved only the atmosphere, the oceans, and the terrestrial biomass. At that time, fossil fuels were not part of the carbon cycle, and this cycle was balanced. But through the destruction of vegetation and the use of fossil fuels, humans are responsible for the imbalance in the global carbon cycle. Fossil fuels are used too quickly without the time to regenerate. The burning of fossil materials means that there is a surplus of carbon entering the atmosphere and the oceans.
It is estimated that the carbon dioxide content of the Earth's atmosphere has increased by almost 30% since the mid-19th century. Human activities now release more carbon than the oceans. Forests cannot absorb everything. As a result, the carbon cycle is out of balance. This imbalance can cause significant climate change. For example, it is the primary cause of the enhanced greenhouse effect.
Data from direct measurements taken in ice cores show an increase in atmospheric carbon concentration since the mid-1900s.
