Carbon sinks: reservoirs to protect.

What is a carbon sink?

Carbon sinks are defined as reservoirs containing biomass (living organic matter) or necromass (dead organic matter or otherwise the waste produced by living things) that absorb carbon. In our biosphere, the different carbon sinks are forests, soils and oceans. Today, we can say that there are natural wells like those mentioned above but also artificial wells created by humans. It is also possible to improve the phenomenon of carbon sequestration by natural wells. Nowadays, carbon capture and storage (CCS) technologies are the main example of an anthropogenic carbon sink. CO2 is captured in the air or at industrial chimneys and then stored in different ways. Gradually, CO2 recovery technologies, as developed by the Revcoo project, even replace the simple storage of CO2.

The ocean, the largest natural carbon sink

Today, 1/3 of the CO2 emitted each year into the atmosphere is absorbed by the ocean. The ocean captures CO2 from the atmosphere that reacts with water molecules according to the CO2 + H2O = carbonic acid reaction. This causes formation of bicarbonate ions and release of hydrogen (H + ions). The more hydrogen ions in the ocean, the more it becomes acidic. The carbonate ions formed are used by certain marine organisms to make the limestone of their shell or skeleton following the reaction: bicarbonate + calcium = CO2 + limestone. This reaction produces a small amount of CO2 that rises to the surface to re-enter the atmosphere, but most of the captured CO2 dives with the cold waters to the deep sea where it will remain for several hundred years. At the surface level, about 60% of the CO2 captured by the ocean participates in photosynthesis. Thanks to the energy of the sun, vegetable plankton transforms CO2 into organic matter and releases some of the CO2 to the atmosphere by breathing. Plankton is eaten by many predators. The carbon stored by plankton is found in many marine organisms. These organisms then produce necromass by emitting waste and dying. All the corpses and the droppings then fall towards the bottom of the ocean by dragging the consumed carbon. The carbon moving towards the funds will be found with the part of the carbon which directly plunged with the cold currents and will remain in these depths during several hundreds of years. Only 1/1000 of the mass of this carbon will be buried in the ocean floor for millions of years. In short, CO2 carbon is dissolved in the ocean, used to make calcareous material by organisms and used during photosynthesis to make organic matter.

Forests, just behind the oceans in ranking

Forests currently cover more than 31% of the land area. In France, forests capture 70 million tonnes of CO2 each year, which represents about 15% of French CO2 emissions, and a tree of 5 cubic meters can absorb about 5 tonnes of CO2 per year, which corresponds to 5 tonnes of CO2. round trip Paris – New York by plane per passenger. This figure may seem huge but in reality, it is nothing compared to the 32 billion tonnes issued by humans in one year. Forests are actively involved in the fight against global warming because, in addition to these millions of tons of CO2 captured, forests offer us an ecological resource, wood, whose transformation allows us to store CO2 in useful products and the combustion allows us to substitute the use of fossil fuels. Thanks to the mechanism of photosynthesis, the trees draw the carbon dioxide present in the atmosphere, water and the energy of the sun to produce organic matter. They store CO2 in their trunks, branches, roots and leaves and release oxygen into the air. The carbon stored by the forests is found in biomass but also in the necromass ie all the dead branches, dead leaves and dead trees which, by decomposing, will guide the carbon to a third well, the soil.

Soil, a well that also uses photosynthesis.

In France, about 760 kg of carbon are stored under 1 hectare of grassland used by cows, sheep, horses and goats. Grassland grass grows through the process of photosynthesis, ie they use carbon dioxide that accumulates in plant tissue, water and sun to grow. When plants die and produce necromass, the carbon they contain accumulates in the soil. This is why natural or permanent meadows, that is to say that have not been modified or manipulated by man, store carbon in the soil and thus behave in carbon well. On the other hand, if the land of the grasslands is turned over, for example for agricultural use, then the carbon stored by the soil organic matter that comes into contact with the air is re-emitted into the atmosphere in the form of CO2. On a global scale, these grasslands store about 30% of the world’s soil carbon. Their role is therefore essential in the fight against global warming.

Anthropogenic carbon sinks

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