Reservoir absorbing more carbon from, than emitting to, the air
This article is about storage reservoirs for carbon. For the processes involved for storing carbon for a long time, see
carbon sequestration.
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]
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,
sequestration,
source and uptake".[2]: 2249 The same publication defines carbon pool as "a reservoir in the Earth system where elements, such as carbon [...], reside in various chemical forms for a period of time."[2]: 2244
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:
Public awareness of the significance of CO2 sinks has grown since passage of the 1997
Kyoto Protocol, which promotes their use as a form of
carbon offset.[12]
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.[17]
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]
Wetlands are a natural carbon sink, however climate change can cause these biomes to turn into a carbon source. The high temperature and low water resulted from climate change causes the wetland to transform into a carbon source. [19] This can be seen in peatbogs which are a type of wetland. They undergoes slow
anaerobic decomposition below the surface. This process is slow enough that in many cases the bog grows rapidly and
fixes more carbon from the atmosphere than is released. Over time, the peat grows deeper. Peat bogs hold approximately one-quarter of the carbon stored in land plants and soils.[20]
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]
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]
Carbon offset programs are planting millions of fast-growing trees per year to reforest tropical lands, for as little as $0.10 per tree. Over their typical 40-year lifetime, one million of these trees can sequester up to one million tons of carbon dioxide.[37][38]
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. Please help update this article to reflect recent events or newly available information.(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.[43][44][45][46]
August 2023 research, drawing from 176 flux stations globally, reveals a climate trade-off: increased carbon uptake from
afforestation results in reduced
albedo. Initially, this reduction may lead to moderate
global warming over a span of approximately 20 years, but it is expected to transition into significant cooling thereafter.[47]
Deep ocean, tidal marshes, mangroves and seagrasses
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]
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"[55]: 37 The report described the economic mitigation potential from relevant activities around forests and ecosystems as follows: "the conservation, improved management, and restoration of forests and other ecosystems (coastal wetlands,
peatlands, savannas and grasslands)". A high mitigation potential is found for reducing deforestation in tropical regions. The economic potential of these activities has been estimated to be 4.2 to 7.4 gigatonnes of carbon dioxide equivalent (GtCO2 -eq) per year.[55]: 37
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.[9] 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.[9]
^"Global Carbon Budget 2021"(PDF). Global Carbon Project. 4 November 2021. p. 57.
Archived(PDF) from the original on 11 December 2021. The cumulative contributions to the global carbon budget from 1850. The carbon imbalance represents the gap in our current understanding of sources & sinks. ... Source: Friedlingstein et al 2021; Global Carbon Project 2021