Carbon sink
A carbon sink is a natural or artificial process that "removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere".[2]: 2249 These sinks form an important part of the natural carbon cycle. An overarching term is carbon pool, which is all the places where carbon on Earth can be, i.e. the atmosphere, oceans, soil, plants, and so forth. A carbon sink is a type of carbon pool that has the capability to take up more carbon from the atmosphere than it releases.
Globally, the two most important carbon sinks are vegetation and the ocean.[3] Soil is an important carbon storage medium. Much of the organic carbon retained in the soil of agricultural areas has been depleted due to intensive farming. Blue carbon designates carbon that is fixed via certain marine ecosystems. Coastal blue carbon includes mangroves, salt marshes and seagrasses. These make up a majority of ocean plant life and store large quantities of carbon. Deep blue carbon is located in international waters and includes carbon contained in "continental shelf waters, deep-sea waters and the sea floor beneath them".[4]
For
Definition
In the context of climate change and in particular mitigation, a sink is defined as "Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere".[2]: 2249
In the case of non-CO2 greenhouse gases, sinks need not store the gas. Instead they can break it down into substances that have a reduced effect on global warming. For example, nitrous oxide can be reduced to harmless N2.[6][7]
Related terms are "carbon pool, reservoir,
Both carbon pools and carbon sinks are important concepts in understanding the carbon cycle, but they refer to slightly different things. A carbon pool can be thought of as the overarching term, and carbon sink is then a particular type of carbon pool: A carbon pool is all the places where carbon can be (for example the atmosphere, oceans, soil, plants, and fossil fuels).[2]: 2244 A carbon sink, on the other hand, is a type of carbon pool that has the capability to take up more carbon from the atmosphere than it releases.[citation needed]
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.[8]
- 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).[9][10] No major artificial systems remove carbon from the atmosphere on a large scale yet.[11]
Public awareness of the significance of CO2 sinks has grown since passage of the 1997
Natural carbon sinks
Soils
Organic matter tends to accumulate in litter and soils of colder regions such as the
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 to the soil in the form of biochar that does not significantly degrade back to carbon dioxide.[18]
Enhancing soil carbon sinks
Much organic carbon retained in many agricultural areas worldwide has been severely depleted due to intensive farming practices.[21] 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 include no-till farming, residue mulching, cover cropping, and crop rotation, all of which are more widely used in organic farming than in conventional farming.[22][23]
Forests
Favourable factors and carbon sink saturation in forests
Forests are generally carbon dioxide sinks when they are high in diversity, density or area. However, they can also be carbon sources if diversity, density or area decreases due to deforestation, selective logging, climate change, wildfires or diseases.[25][26][27] One study in 2020 found that 32 tracked Brazilian non-Amazon seasonal tropical forests declined from a carbon sink to a carbon source in 2013 and concludes that "policies are needed to mitigate the emission of greenhouse gases and to restore and protect tropical seasonal forests".[28][29] In 2019 forests took up a third less carbon than they did in the 1990s, due to higher temperatures, droughts and deforestation. The typical tropical forest may become a carbon source by the 2060s.[30]
An assessment of European forests found early signs of carbon sink saturation, after decades of increasing strength.[31] The Intergovernmental Panel on Climate Change (IPCC) concluded that a combination of measures aimed at increasing forest carbon stocks, andsustainable timber offtake will generate the largest carbon sequestration benefit.[32]
Life expectancy of forests varies throughout the world, influenced by tree species, site conditions and natural disturbance patterns. In some forests, carbon may be stored for centuries, while in other forests, carbon is released with frequent stand replacing fires. Forests that are harvested prior to stand replacing events allow for the retention of carbon in manufactured forest products such as lumber.[33] However, only a portion of the carbon removed from logged forests ends up as durable goods and buildings. The remainder ends up as sawmill by-products such as pulp, paper and pallets, which often end with incineration (resulting in carbon release into the atmosphere) at the end of their lifecycle. For instance, of the 1,692 megatonnes of carbon harvested from forests in Oregon and Washington from 1900 to 1992, only 23% is in long-term storage in forest products.[34]
The Food and Agriculure Organization (FAO) reported that: "The total carbon stock in forests decreased from 668 gigatonnes in 1990 to 662 gigatonnes in 2020".[24]: 11 However, another study finds that the leaf area index has increased globally since 1981, which was responsible for 12.4% of the accumulated terrestrial carbon sink from 1981 to 2016. The CO2 fertilization effect, on the other hand, was responsible for 47% of the sink, while climate change reduced the sink by 28.6%.[35] In Canada's boreal forests as much as 80% of the total carbon is stored in the soils as dead organic matter.[36]
Changes in albedo effect
This section needs to be updated. The reason given is: the references used are quite old; there must be more updated information available in the IPCC Sixth Assessment Report. (March 2023) |
Forests generally have a low albedo because the majority of the ultraviolet and visible spectrum is absorbed through photosynthesis. For this reason, the greater heat absorption by trees could offset some of the carbon benefits of afforestation (or offset the negative climate impacts of deforestation). In other words: The climate change mitigation effect of carbon sequestration by forests is partially counterbalanced in that reforestation can decrease the reflection of sunlight (albedo).[39]
In the case of evergreen forests with seasonal snow cover, albedo reduction may be significant enough for deforestation to cause a net cooling effect.[40] Trees also impact climate in extremely complicated ways through evapotranspiration. The water vapor causes cooling on the land surface, causes heating where it condenses, acts as strong greenhouse gas, and can increase albedo when it condenses into clouds.[41] Scientists generally treat evapotranspiration as a net cooling impact, and the net climate impact of albedo and evapotranspiration changes from deforestation depends greatly on local climate.[42]
Mid-to-high-latitude forests have a much lower albedo during snow seasons than flat ground, thus contributing to warming. Modeling that compares the effects of albedo differences between forests and grasslands suggests that expanding the land area of forests in temperate zones offers only a temporary mitigation benefit.Deep ocean, tidal marshes, mangroves and seagrasses
The methods for blue carbon management fall into the category of "ocean-based biological carbon dioxide removal (CDR) methods".[50]: 764 They are a type of biological carbon fixation.
Scientists are looking for ways to further develop the blue carbon potential of ecosystems.[51] However, the long-term effectiveness of blue carbon as a carbon dioxide removal solution is under debate.[52][51][53]
The term deep blue carbon is also in use and refers to storing carbon in the deep ocean waters.[54]Enhancing natural carbon sinks
Purpose in the context of climate change
An important mitigation measure is "preserving and enhancing carbon sinks".[55] This refers to the management of Earth's natural carbon sinks in a way that preserves or increases their capability to remove CO2 from the atmosphere and to store it durably. Scientists call this process also carbon sequestration. In the context of climate change mitigation, the IPCC defines a sink as "Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere".[56]: 2249 Globally, the two most important carbon sinks are vegetation and the ocean.[57]
To enhance the ability of ecosystems to sequester carbon, changes are necessary in agriculture and forestry.[58] Examples are preventing deforestation and restoring natural ecosystems by reforestation.[59]: 266 Scenarios that limit global warming to 1.5 °C typically project the large-scale use of carbon dioxide removal methods over the 21st century.[60]: 1068 [61]: 17 There are concerns about over-reliance on these technologies, and their environmental impacts.[61]: 17 [62]: 34 But ecosystem restoration and reduced conversion are among the mitigation tools that can yield the most emissions reductions before 2030.[55]: 43
Land-based mitigation options are referred to as "AFOLU mitigation options" in the 2022 IPCC report on mitigation. The abbreviation stands for "agriculture, forestry and other land use"Carbon 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:
Artificial carbon sinks
Geologic carbon sequestration
Wooden buildings
Broad-base adoption of
See also
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