What is a "Carbon Sink"?

Carbon sink, carbon cycle, carbon sources: different meanings

Carbon is essential for all life on Earth. It moves naturally through many processes: the dissolution of carbon dioxide (CO2) in the ocean, the uptake of CO2 by plants during photosynthesis, the uptake of carbohydrates stored in plants by animals, or the release of CO2 through respiration.

In this respect, the carbon cycle lies in the constant movement of carbon from land and water through the atmosphere and living organisms.

Processes that add additional carbon to the atmosphere are called carbon sources. It refers to an area or ecosystem that releases more carbon dioxide than it takes in. For example, burning fossil fuels or volcanic eruptions release carbon dioxide into the atmosphere. Besides, any process that uses fossil fuels - like burning coal to generate electricity - releases much carbon into the atmosphere. In the fight against climate change, humans are trying to counteract the effects of global warming with mitigation and adaptation measures. However, nature is also helping to prevent the planet's average temperature from rising.

Processes that absorb and shop CO2 from the atmosphere are carbon sinks reducing its concentration in the air. They act like a sponge, soaking up carbon compounds like carbon dioxide as an artificial carbon sink (technologies and chemicals) or natural sinks like the oceans or mangroves.

Forest as natural sinks

In forests, carbon sequestration is done through photosynthesis. Under normal conditions, there is a net absorption of CO2 and a net release of oxygen in plants. Most of the forest carbon can be found in living biomass and soil organic matter (thanks to the remainder in dead wood and litter).

Forests can act as either carbon sources or carbon sinks:

• A forest represents a carbon source if it releases more carbon than it takes in. Trees can burn or rot after dying due to insect infestation, age, or fire, for instance.
• A forest becomes a carbon sink if it absorbs more carbon from the atmosphere than it releases. Photosynthesis deposits it in the forest biomass - branches, roots and leaves, trunks, dead organic material such as litter and deadwood, or even soils. Such a process of carbon absorption and deposition entails carbon storage. An ecosystem can be a carbon source or a sink - depending on the net balance of all these carbon exchanges.

Forests can be carbon stores and carbon dioxide sinks when they increase in density or area. In that capacity, the world has three largest tropical rainforests: the Amazon, the Congo Basin, and Southeast Asia. They sequester more carbon from the atmosphere overall than temperate or boreal forests. For example, Congo's tropical rainforest sequesters 600 million tonnes more carbon dioxide than it emits, equivalent to about one-third of CO2 emissions from all US transportation. The Amazon basin is still a net carbon sink but is at risk of becoming a net carbon source if forest loss continues at the current rate because of deforestation. Temperate forests absorb between 10 and 20 tonnes of carbon dioxide per hectare each year through photosynthetic conversion to starch, cellulose, lignin, and wood biomass. Besides, the oceans are the most crucial carbon sink and absorb up to 50% of CO2.

Preserving ecosystems for mitigating climate change

Management measures can influence carbon sequestration by stimulating specific processes and mitigating the effects of negative factors. For example, whether forests are carbon sources or sink depends on project management. It means time passed between harvest cycles, forest cut down, ages, and the total area over which flows are calculated. On top of it, considering that 27% of the world's net forest carbon sink is in "protected areas," recognizing local communities as owners of their land, particularly indigenous peoples, and enforcing their rights helps protect existing ecosystems and enhance the carbon they shop. Protected areas and indigenous reserves are among the most valuable tools in the climate change toolbox, combined with monitoring and governance policies.

Natural Carbon sinks

Implementing terrestrial techniques should help mitigate climate change on land. Terrestrial carbon sequestration represent the cycle where trees and plants absorb CO2 from the atmosphere through photosynthesis and store it as carbon in soils and biomass (tree trunks, branches, foliage, and roots).

Some projects also help increase carbon sequestration, for instance:

• Small-grower afforestation and restoration of sub-tropical thickets and forests can be encouraged. Furthermore, management approaches should secure the carbon storage potential of coastal ecosystems and oceans thanks to buffer zones to allow inland migration of coastal carbon sinks, ecosystem rehabilitation, marine protected areas, marine spatial planning, area-based fisheries management approaches, and regulated coastal development, for instance. Restoration and management of grasslands are especially noteworthy. It has substantial rural livelihood and ecological infrastructure benefits. Moreover, planning and regulation encourage reduced deforestation and deforestation emissions (REDD+).
• Biochar production and application can help create a carbon-rich material that can be mixed into soils to increase soil carbon. It represents charcoal-like solid material made from biomass's carbonization (thermochemical conversion) in an oxygen-limited environment. Indeed, such a light black residue, made of carbon and ashes, remains after the pyrolysis of biomass.
• Reducing or eliminating tillage is one of the farming practices most frequently touted to improve carbon sequestration in soil. This is because it can leave sufficient biomass residue to cover a minimum of 30% of the soil surface after planting.

Artificial Techniques

Eventually, artificial techniques also help enhance and accelerate the natural process of carbon sequestration. They extract carbon from the atmosphere and store it in the Earth's crust. Investigations have also been made concerning the ocean. A growing number of artificial carbon-trapping technologies efficiently capture large amounts of carbon and store it for a long time.

Unfortunately, all these technologies have not acquired the efficiency and maturity needed to cope with extreme climate change, and sometimes, in critical cases, CO2 escapes the artificial sinks - carbon leakage. Therefore, we still need to urgently and significantly. Managing and protecting our natural carbon sinks is key to determining whether they can act as a source of CO2 to the atmosphere or a sink under future climates.

Photo: © Reforest'Action / Anne-Lise.