Carbon sink

Types

The amount of carbon dioxide varies naturally in a dynamic equilibrium with photosynthesis of land plants. The natural carbon sinks are:

• Soil is a carbon store and active carbon sink.
• Photosynthesis by terrestrial plants with grass and trees allows them to serve as carbon sinks during growing seasons.
• Absorption of carbon dioxide by the oceans via solubility and biological pumps.

Artificial carbon sinks are those that store carbon in building materials or deep underground (geologic carbon sequestration). No major artificial systems remove carbon from the atmosphere on a large scale yet.

Public awareness of the significance of sinks has grown since passage of the 1997 Kyoto Protocol, which promotes their use as a form of carbon offset.

Natural carbon sinks

Soils

Soils represent a short to long-term carbon storage medium and contain more carbon than all terrestrial vegetation and the atmosphere combined. Plant litter and other biomass including charcoal accumulates as organic matter in soils, and is degraded by chemical weathering and biological degradation. More recalcitrant organic carbon polymers such as cellulose, hemi-cellulose, lignin, aliphatic compounds, waxes and terpenoids are collectively retained as humus.

Organic matter tends to accumulate in litter and soils of colder regions such as the boreal forests of North America and the Taiga of Russia. Leaf litter and humus are rapidly oxidized and poorly retained in sub-tropical and tropical climate conditions due to high temperatures and extensive leaching by rainfall. Areas, where shifting cultivation or slash and burn agriculture are practiced, are generally only fertile for two to three years before they are abandoned. These tropical jungles are similar to coral reefs in that they are highly efficient at conserving and circulating necessary nutrients, which explains their lushness in a nutrient desert.

Grasslands contribute to soil organic matter, stored mainly in their extensive fibrous root mats. Due in part to the climatic conditions of these regions (e.g., cooler temperatures and semi-arid to arid conditions), these soils can accumulate significant quantities of organic matter. This can vary based on rainfall, the length of the winter season, and the frequency of naturally occurring lightning-induced grass-fires. While these fires release carbon dioxide, they improve the quality of the grasslands overall, in turn increasing the amount of carbon retained in the humic material. They also deposit carbon directly into the soil in the form of biochar that does not significantly degrade back to carbon dioxide.

Much organic carbon retained in many agricultural areas worldwide has been severely depleted due to intensive farming practices. Since the 1850s, a large proportion of the world's grasslands have been tilled and converted to croplands, allowing the rapid oxidation of large quantities of soil organic carbon. Methods that significantly enhance carbon sequestration in soil are called carbon farming. They include for example no-till farming, residue mulching, cover cropping, and crop rotation.

Forests

Deep ocean, tidal marshes, mangroves and seagrasses

Enhancing natural carbon sinks

Main article: Carbon sequestration

Purpose in the context of climate change

Carbon sequestration techniques in oceans

Main article: Carbon sequestration#Sequestration techniques in oceans

To enhance carbon sequestration processes in oceans the following technologies have been proposed but none have achieved large scale application so far: Seaweed farming, ocean fertilisation, artificial upwelling, basalt storage, mineralization and deep sea sediments, adding bases to neutralize acids. The idea of direct deep-sea carbon dioxide injection has been abandoned.

Artificial carbon sinks

Geologic carbon sequestration

Main article: Carbon sequestration#Geologic carbon sequestration

Wooden buildings

Broad-base adoption of mass timber and their role in substituting steel and concrete in new mid-rise construction projects over the next few decades has the potential to turn timber buildings into carbon sinks, as they store the carbon dioxide taken up from the air by trees that are harvested and used as mass timber. This could result in storing between 10 million tons of carbon per year in the lowest scenario and close to 700 million tons in the highest scenario. For this to happen, the harvested forests would need to be sustainably managed and wood from demolished timber buildings would need to be reused or preserved on land in various forms.

Using rapidly renewable plant materials like bamboo, straw or hempcrete can be further carbon sinks.

References

References

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