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The permafrost carbon cycle or Arctic carbon cycle is a sub-cycle of the larger global
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major compon ...
.
Permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...
is defined as subsurface material that remains below 0o C (32o F) for at least two consecutive years. Because permafrost soils remain frozen for long periods of time, they store large amounts of carbon and other nutrients within their frozen framework during that time. Permafrost represents a large carbon reservoir that is seldom considered when determining global terrestrial carbon reservoirs. Recent and ongoing scientific research however, is changing this view. The permafrost carbon cycle deals with the transfer of carbon from permafrost soils to terrestrial vegetation and microbes, to the atmosphere, back to vegetation, and finally back to permafrost soils through burial and sedimentation due to cryogenic processes. Some of this carbon is transferred to the ocean and other portions of the globe through the global carbon cycle. The cycle includes the exchange of
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
and
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
between terrestrial components and the atmosphere, as well as the transfer of carbon between land and water as methane,
dissolved organic carbon Dissolved organic carbon (DOC) is the fraction of organic carbon operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometers. The fraction remaining on the filter is called partic ...
, dissolved inorganic carbon,
particulate inorganic carbon Particulate inorganic carbon (PIC) can be contrasted with dissolved inorganic carbon (DIC), the other form of inorganic carbon found in the ocean. These distinctions are important in chemical oceanography. Particulate inorganic carbon is somet ...
and particulate organic carbon.


Storage

Soils, in general, are the largest reservoirs of carbon in
terrestrial ecosystem Terrestrial ecosystems are ecosystems which are found on land. Examples include tundra, taiga, temperate deciduous forest, tropical rain forest, grassland, deserts. Terrestrial ecosystems differ from aquatic ecosystems by the predominant pres ...
s. This is also true for soils in the Arctic that are underlain by permafrost. In 2003, Tarnocai, et al. used the Northern and Mid Latitudes Soil Database to make a determination of carbon stocks in cryosols—soils containing permafrost within two meters of the soil surface. Permafrost affected soils cover nearly 9% of the earth's land area, yet store between 25 and 50% of the soil organic carbon. These estimates show that permafrost soils are an important carbon pool. These soils not only contain large amounts of carbon, but also sequester carbon through
cryoturbation In gelisols ( permafrost soils), cryoturbation (frost churning) refers to the mixing of materials from various horizons of the soil down to the bedrock due to freezing and thawing. Cryoturbation occurs to varying degrees in most gelisols. The c ...
and cryogenic processes.


Processes

Carbon is not produced by permafrost. Organic carbon derived from terrestrial vegetation must be incorporated into the soil column and subsequently be incorporated into permafrost to be effectively stored. Because permafrost responds to climate changes slowly, carbon storage removes carbon from the atmosphere for long periods of time.
Radiocarbon Carbon-14, C-14, or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and c ...
dating techniques reveal that carbon within permafrost is often thousands of years old. Carbon storage in permafrost is the result of two primary processes. *The first process that captures carbon and stores it is
syngenetic permafrost growth Syngenetic permafrost growth is a mode of the growth of permafrost whereby additional material is deposited to a permafrost site during freezing conditions, causing the permafrost layer to build upwards. It is cited as an efficient mode of permafro ...
. This process is the result of a constant active layer where thickness and energy exchange between permafrost, active layer, biosphere, and atmosphere, resulting in the vertical increase of the soil surface elevation. This aggradation of soil is the result of aeolian or
fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...
sedimentation and/or
peat Peat (), also known as turf (), is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. The peatland ecosystem covers and ...
formation. Peat accumulation rates are as high as 0.5mm/yr while sedimentation may cause a rise of 0.7mm/yr. Thick silt deposits resulting from abundant loess deposition during the
last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eu ...
form thick carbon-rich soils known as yedoma. As this process occurs, the organic and mineral soil that is deposited is incorporated into the permafrost as the permafrost surface rises. *The second process responsible for storing carbon is
cryoturbation In gelisols ( permafrost soils), cryoturbation (frost churning) refers to the mixing of materials from various horizons of the soil down to the bedrock due to freezing and thawing. Cryoturbation occurs to varying degrees in most gelisols. The c ...
, the mixing of soil due to freeze-thaw cycles. Cryoturbation moves carbon from the surface to depths within the soil profile. Frost heaving is the most common form of cryoturbation. Eventually, carbon that originates at the surface moves deep enough into the active layer to be incorporated into permafrost. When cryoturbation and the deposition of sediments act together carbon storage rates increase.


Current estimates

It is estimated that the total soil organic carbon (SOC) stock in northern circumpolar permafrost region equals around 1,460-1,600 Pg. (1 Pg = 1 Gt = 1015g) With the
Tibetan Plateau The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the Ti ...
carbon content included, the total carbon pools in the permafrost of the Northern Hemisphere is likely to be around 1832 Gt. This estimation of the amount of carbon stored in permafrost soils is more than double the amount currently in the atmosphere. Soil column in the permafrost soils is generally broken into three horizons, 0–30 cm, 0–100 cm, and 1–300 cm. The uppermost horizon (0–30 cm) contains approximately 200 Pg of organic carbon. The 0–100 cm horizon contains an estimated 500 Pg of organic carbon, and the 0–300 cm horizon contains an estimated 1024 Pg of organic carbon. These estimates more than doubled the previously known carbon pools in permafrost soils. Additional carbon stocks exist in yedoma (400 Pg), carbon rich
loess Loess (, ; from german: Löss ) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits. Loess is a periglacial or aeoli ...
deposits found throughout Siberia and isolated regions of North America, and deltaic deposits (240 Pg) throughout the Arctic. These deposits are generally deeper than the 3 m investigated in traditional studies. Many concerns arise because of the large amount of carbon stored in permafrost soils. Until recently, the amount of carbon present in permafrost was not taken into account in climate models and global carbon budgets.


Carbon release from the permafrost

Carbon is continually cycling between soils, vegetation, and the atmosphere. As climate change increases mean annual air temperatures throughout the Arctic, it extends permafrost thaw and deepens the active layer, exposing old carbon that has been in storage for decades to millennia to biogenic processes which facilitate its entrance into the atmosphere. In general, the volume of permafrost in the upper 3 m of ground is expected to decrease by about 25% per 1 °C of global warming.Fox-Kemper, B., H.T. Hewitt, C. Xiao, G. Aðalgeirsdóttir, S.S. Drijfhout, T.L. Edwards, N.R. Golledge, M. Hemer, R.E. Kopp, G.  Krinner, A. Mix, D. Notz, S. Nowicki, I.S. Nurhati, L. Ruiz, J.-B. Sallée, A.B.A. Slangen, and Y. Yu, 2021
Chapter 9: Ocean, Cryosphere and Sea Level Change
I
Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
[Masson-Delmotte, V., P. Zhai, A. Pirani, S.L.  Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1211–1362, doi:10.1017/9781009157896.011.
According to the IPCC Sixth Assessment Report, there is high confidence that global warming over the last few decades has led to widespread increases in permafrost temperature. Observed warming was up to 3 °C in parts of Northern Alaska (early 1980s to mid-2000s) and up to 2 °C in parts of the Russian European North (1970-2020), and active layer thickness has increased in the European and Russian Arctic across the 21st century and at high elevation areas in Europe and Asia since the 1990s. In
Yukon Yukon (; ; formerly called Yukon Territory and also referred to as the Yukon) is the smallest and westernmost of Canada's three territories. It also is the second-least populated province or territory in Canada, with a population of 43,964 as ...
, the zone of continuous permafrost might have moved poleward since 1899, but accurate records only go back 30 years. Based on high agreement across model projections, fundamental process understanding, and paleoclimate evidence, it is virtually certain that permafrost extent and volume will continue to shrink as global climate warms. Carbon emissions from permafrost thaw contribute to the same warming which facilitates the thaw, making it a positive climate change feedback. The warming also intensifies Arctic water cycle, and the increased amounts of warmer rain are another factor which increases permafrost thaw depths. The amount of carbon that will be released from warming conditions depends on depth of thaw, carbon content within the thawed soil, physical changes to the environment and microbial and vegetation activity in the soil. Microbial respiration is the primary process through which old permafrost carbon is re-activated and enters the atmosphere. The rate of microbial decomposition within organic soils, including thawed permafrost, depends on environmental controls, such as soil temperature, moisture availability, nutrient availability, and oxygen availability. In particular, sufficient concentrations of iron oxides in some permafrost soils can inhibit microbial respiration and prevent carbon mobilization: however, this protection only lasts until carbon is separated from the iron oxides by Fe-reducing bacteria, which is only a matter of time under the typical conditions. Depending on the soil type,
Iron(III) oxide Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally a ...
can boost oxidation of methane to carbon dioxide in the soil, but it can also amplify methane production by acetotrophs: these soil processes are not yet fully understood. Altogether, the likelihood of the entire carbon pool mobilizing and entering the atmosphere is low despite the large volumes stored in the soil. Although temperatures will increase, this does not imply complete loss of permafrost and mobilization of the entire carbon pool. Much of the ground underlain by permafrost will remain frozen even if warming temperatures increase the thaw depth or increase thermokarsting and permafrost degradation. Moreover, other elements such as
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
and
aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
can
adsorb Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a f ...
some of the mobilized soil carbon before it reaches the atmosphere, and they are particularly prominent in the mineral sand layers which often overlay permafrost. On the other hand, once the permafrost area thaws, it will not go back to being permafrost for centuries even if the temperature increase reversed, making it one of the best-known examples of
tipping points in the climate system In climate science, a tipping point is a critical threshold that, when crossed, leads to large and often irreversible changes in the climate system. If tipping points are crossed, they are likely to have severe impacts on human society. Tippin ...
. A 1993 study suggested that while the tundra was a carbon sink until the end of 1970s, it had already transitioned to a net carbon source by the time the study concluded. The 2019 Arctic Report Card estimated that Arctic permafrost releases between 0.3 and 0.6 Pg C per year. That same year, a study settled on the 0.6 Pg C figure, as the net difference between the annual emissions of 1,66 Pg C during the winter season (October–April), and the model-estimated vegetation carbon uptake of 1 Pg C during the growing season. It estimated that under RCP 8.5, a scenario of continually accelerating greenhouse gas emissions, winter emissions from the norther permafrost domain would increase 41% by 2100. Under the "intermediate" scenario RCP 4.5, where greenhouse gas emissions peak and plateau within the next two decades, before gradually declining for the rest of the century (a rate of mitigation deeply insufficient to meet the
Paris Agreement The Paris Agreement (french: Accord de Paris), often referred to as the Paris Accords or the Paris Climate Accords, is an international treaty on climate change. Adopted in 2015, the agreement covers climate change mitigation, adaptation, and ...
goals) permafrost carbon emissions would increase by 17%. In 2022, this was challenged by a study which used a record of atmospheric observations between 1980 to 2017, and found that permafrost regions have been gaining carbon on net, as process-based models underestimated net CO2 uptake in the permafrost regions and overestimated it in the forested regions, where increased respiration in response to warming offsets more of the gains than was previously understood. Notably, estimates of carbon release alone do not fully represent the impact of permafrost thaw on climate change. This is because carbon can either be released as carbon dioxide (CO2) or methane (CH4).
Aerobic respiration Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
releases carbon dioxide, while
anaerobic respiration Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. In aerobic organisms undergoing r ...
releases methane. This is a substantial difference, as while biogenic methane lasts less than 12 years in the atmosphere, its
global warming potential Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (). GWP is 1 for . For other gases it depends on the gas and the time ...
is around 80 times larger than that of CO2 over a 20-year period and between 28 and 40 times larger over a 100-year period.


Carbon dioxide emissions

Most of the permafrost soil are oxic and provide a suitable environment for aerobic microbial respiration. As such, carbon dioxide emissions account for the overwhelming majority of permafrost emissions and of the Arctic emissions in general. There's some debate over whether the observed emissions from permafrost soils primarily constitute microbial respiration of ancient carbon, or simply greater respiration of modern-day carbon (i.e. leaf litter), due to warmer soils intensifying microbial metabolism. Studies published in the early 2020s indicate that while soil microbiota still primarily consumes and respires modern carbon when plants grow during the spring and summer, these microorganisms then sustain themselves on ancient carbon during the winter, releasing it into the atmosphere. On the other hand, former permafrost areas consistently see increased vegetation growth, or primary production, as plants can set down deeper roots in the thawed soil and grow larger and uptake more carbon. This is generally the main counteracting feedback on permafrost carbon emissions. However, in areas with streams and other waterways, more of their leaf litter enters those waterways, increasing their dissolved organic carbon content. Leaching of soil organic carbon from permafrost soils is also accelerated by warming climate and by erosion along river and stream banks freeing the carbon from the previously frozen soil. Moreover, thawed areas become more vulnerable to wildfires, which alter landscape and release large quantities of stored organic carbon through combustion. As these fires burn, they remove organic matter from the surface. Removal of the protective organic mat that insulates the soil exposes the underlying soil and permafrost to increased
solar radiation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...
, which in turn increases the soil temperature, active layer thickness, and changes soil moisture. Changes in the soil moisture and saturation alter the ratio of oxic to anoxic decomposition within the soil. A hypothesis promoted by Sergey Zimov is that the reduction of herds of large herbivores has increased the ratio of energy emission and energy absorption tundra (energy balance) in a manner that increases the tendency for net thawing of permafrost. He is testing this hypothesis in an experiment at Pleistocene Park, a nature reserve in northeastern Siberia. On the other hand, warming allows the
beaver Beavers are large, semiaquatic rodents in the genus ''Castor'' native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers a ...
s to extend their habitat further north, where their dams impair boat travel, impact access to food, affect water quality, and endanger downstream fish populations. Pools formed by the dams store heat, thus changing local
hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...
and causing localized permafrost thaw.


Methane emissions

Global warming in the Arctic accelerates methane release from both existing stores and
methanogenesis Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. Organisms capable of producing methane for energy conservation have been identified only from the domain Archaea, a group ...
in rotting
biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bio ...
. Methanogenesis requires thoroughly anaerobic environments, which slows down the mobilization of old carbon. A 2015 ''
Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
'' review estimated that the cumulative emissions from thawed anaerobic permafrost sites were 75-85% lower than the cumulative emissions from aerobic sites, and that even there, methane emissions amounted to only 3% to 7% of CO2 emitted in situ. While they represented between 25% to 45% of the CO2's potential impact on climate over a 100-year timescale, the review concluded that aerobic permafrost thaw still had a greater warming impact overall. In 2018, however, another study in '' Nature Climate Change'' performed seven-year incubation experiments and found that methane production became equivalent to CO2 production once a methanogenic microbial community became established at the anaerobic site. This finding had substantially raised the overall warming impact represented by anaerobic thaw sites. Since methanogenesis requires anaerobic environments, it is frequently associated with Arctic lakes, where the emergence of bubbles of methane can be observed. Lakes produced by the thaw of particularly ice-rich permafrost are known as thermokarst lakes. Not all of the methane produced in the sediment of a lake reaches the atmosphere, as it can get oxidized in the water column or even within the sediment itself: However, 2022 observations indicate that at least half of the methane produced within thermokarst lakes reaches the atmosphere. Another process which frequently results in substantial methane emissions is the
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...
of permafrost-stabilized hillsides and their ultimate collapse. Altogether, these two processes - hillside collapse (also known as retrogressive thaw slump, or RTS) and thermokarst lake formation - are collectively described as abrupt thaw, as they can rapidly expose substantial volumes of soil to microbial respiration in a matter of days, as opposed to the gradual, cm by cm, thaw of formerly frozen soil which dominates across most permafrost environments. This rapidity was illustrated in 2019, when three permafrost sites which would have been safe from thawing under the "intermediate" Representative Concentration Pathway 4.5 for 70 more years had undergone abrupt thaw. Another example occurred in the wake of a 2020 Siberian heatwave, which was found to have increased RTS numbers 17-fold across the northern
Taymyr Peninsula The Taymyr Peninsula (russian: Таймырский полуостров, Taymyrsky poluostrov) is a peninsula in the Far North of Russia, in the Siberian Federal District, that forms the northernmost part of the mainland of Eurasia. Administrat ...
- from 82 to 1404, while the resultant soil carbon mobilization increased 28-fold, to an average of 11 grams of carbon per square meter per year across the peninsula (with a range between 5 and 38 grams). Until recently, Permafrost carbon feedback (PCF) modeling had mainly focused on gradual permafrost thaw, due to the difficulty of modelling abrupt thaw, and because of the flawed assumptions about the rates of methane production. Nevertheless, a study from 2018, by using field observations, radiocarbon dating, and remote sensing to account for thermokarst lakes, determined that abrupt thaw will more than double permafrost carbon emissions by 2100. And a second study from 2020, showed that under the scenario of continually accelerating emissions (RCP 8.5), abrupt thaw carbon emissions across 2.5 million km2 are projected to provide the same feedback as gradual thaw of near-surface permafrost across the whole 18 million km2 it occupies. Thus, abrupt thaw adds between 60 and 100 gigatonnes of carbon by 2300, increasing carbon emissions by ~125–190% when compared to gradual thaw alone. However, there is still scientific debate about the rate and the trajectory of methane production in the thawed permafrost environments. For instance, a 2017 paper suggested that even in the thawing peatlands with frequent thermokarst lakes, less than 10% of methane emissions can be attributed to the old, thawed carbon, and the rest is anaerobic decomposition of modern carbon. A follow-up study in 2018 had even suggested that increased uptake of carbon due to rapid peat formation in the thermokarst wetlands would compensate for the increased methane release. Another 2018 paper suggested that permafrost emissions are limited following thermokarst thaw, but are substantially greater in the aftermath of wildfires. In 2022, a paper demonstrated that peatland methane emissions from permafrost thaw are initially quite high (82 milligrams of methane per square meter per day), but decline by nearly three times as the permafrost bog matures, suggesting a reduction in methane emissions in several decades to a century following abrupt thaw.


Subsea permafrost

Subsea permafrost occurs beneath the seabed and exists in the continental shelves of the polar regions. Thus, it can be defined as "the unglaciated continental shelf areas exposed during the
Last Glacial Maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eu ...
(LGM, ~26 500 BP) that are currently inundated". Large stocks of organic matter (OM) and methane () are accumulated below and within the subsea permafrost deposits.This source of methane is different from
methane clathrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...
s, but contributes to the overall outcome and feedbacks in the Earth's climate system. The size of today's subsea permafrost has been estimated at around 2 million km2 (~1/5 of the terrestrial permafrost domain size), which constitutes a 30-50% reduction since the LGM. Containing around 560 GtC in OM and 45 GtC in CH4, with a current release of 18 and 38 MtC per year respectively, which is due to the warming and thawing that the subsea permafrost domain has been experiencing since after the LGM (~14000 years ago). In fact, because the subsea permafrost systems responds at millennial timescales to climate warming, the current carbon fluxes it is emitting to the water are in response to climatic changes occurring after the LGM. Therefore, human-driven climate change effects on subsea permafrost will only be seen hundreds or thousands of years from today. According to predictions under a business-as-usual emissions scenario RCP 8.5, by 2100, 43 GtC could be released from the subsea permafrost domain, and 190 GtC by the year 2300. Whereas for the low emissions scenario RCP 2.6, 30% less emissions are estimated. This constitutes a significant anthropogenic-driven acceleration of carbon release in the upcoming centuries.


Cumulative

In 2011, preliminary computer analyses suggested that permafrost emissions could be equivalent to around 15% of anthropogenic emissions. A 2018 perspectives article discussing
tipping points in the climate system In climate science, a tipping point is a critical threshold that, when crossed, leads to large and often irreversible changes in the climate system. If tipping points are crossed, they are likely to have severe impacts on human society. Tippin ...
activated around 2 degrees Celsius of global warming suggested that at this threshold, permafrost thaw would add a further 0.09 °C to global temperatures by 2100, with a range between 0.04 °C and 0.16 °C In 2021, another study estimated that in a future where zero emissions were reached following a emission of a further 1000 Pg C into the atmosphere (a scenario where temperatures ordinarily stay stable after the last emission, or start to decline slowly) permafrost carbon would add 0.06 °C (with a range between 0.02 °C and 0.14 °C) 50 years after the last anthropogenic emission, 0.09 °C (with a range between 0.04 °C to 0.21 °C) 100 years later and 0.27 °C (ranging between 0.12 to 0.49 °C) 500 years later. However, neither study was able to take abrupt thaw into account. In 2020, a study of the northern permafrost peatlands (a smaller subset of the entire permafrost area, covering 3.7 million km2 out of the estimated 18 million km2) would amount to ∼1% of anthropogenic
radiative forcing Radiative forcing (or climate forcing) is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change as measured by watts / metre2. It is a scientific concept used to quantify and compare the extern ...
by 2100, and that this proportion remains the same in all warming scenarios considered, from 1.5 °C to 6 °C. It had further suggested that after 200 more years, those peatlands would have absorbed more carbon than what they had emitted into the atmosphere. The IPCC Sixth Assessment Report estimates that carbon dioxide and methane released from permafrost could amount to the equivalent of 14–175 billion tonnes of carbon dioxide per 1 ºC of warming. For comparison, by 2019 the anthropogenic emission of all carbon dioxide into the atmosphere stood around 40 billion tonnes. A 2021 assessment of the economic impact of climate tipping points estimated that permafrost carbon emissions would increase the social cost of carbon by about 8.4% However, the methods of that assessment have attracted controversy: when researchers like
Steve Keen Steve Keen (born 28 March 1953) is an Australian economist and author. He considers himself a post-Keynesian, criticising neoclassical economics as inconsistent, unscientific and empirically unsupported. The major influences on Keen's thinking ...
and Timothy Lenton had accused it of underestimating the overall impact of tipping points and of higher levels of warming in general, the authors have conceded some of their points. In 2021, a group of prominent permafrost researchers like
Merritt Turetsky Merritt Turetsky is an American ecosystem ecologist and a professor at the University of Colorado Boulder. She currently serves as the Director of Arctic Security for the University of Colorado. She served as the first woman Director of the Insti ...
had presented their collective estimate of permafrost emissions, including the abrupt thaw processes, as part of an effort to advocate for a 50% reduction in anthropogenic emissions by 2030 as a necessary milestone to help reach net zero by 2050. Their figures for combined permafrost emissions by 2100 amounted to 150–200 billion tonnes of carbon dioxide equivalent under 1.5 degrees of warming, 220–300 billion tonnes under 2 degrees and 400–500 billion tonnes if the warming was allowed to exceed 4 degrees. They compared those figures to the extrapolated present-day emissions of
Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
, the
European Union The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are located primarily in Europe, Europe. The union has a total area of ...
and the
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or
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, respectively. The 400–500 billion tonnes figure would also be equivalent to the today's remaining budget for staying within a 1.5 degrees target. One of the scientists involved in that effort, Susan M. Natali of Woods Hole Research Centre, had also led the publication of a complementary estimate in a
PNAS ''Proceedings of the National Academy of Sciences of the United States of America'' (often abbreviated ''PNAS'' or ''PNAS USA'') is a peer-reviewed multidisciplinary scientific journal. It is the official journal of the National Academy of Scie ...
paper that year, which suggested that when the amplification of permafrost emissions by abrupt thaw and wildfires is combined with the foreseeable range of near-future anthropogenic emissions, avoiding the exceedance (or "overshoot") of 1.5 degrees warming is already implausible, and the efforts to attain it may have to rely on negative emissions to force the temperature back down. An updated 2022 assessment of climate tipping points concluded that abrupt permafrost thaw would add 50% to gradual thaw rates, and would add 14 billion tons of carbon dioxide equivalent emissions by 2100 and 35 by 2300 per every degree of warming. This would have a warming impact of 0.04 °C per every full degree of warming by 2100, and 0.11 °C per every full degree of warming by 2300. It also suggested that at between 3 and 6 degrees of warming (with the most likely figure around 4 degrees) a large-scale collapse of permafrost areas could become irreversible, adding between 175 and 350 billion tons of equivalent emissions, or 0.2–0.4 degrees, over about 50 years (with a range between 10 and 300 years).


See also

* Fire and carbon cycling in boreal forests *
Carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major compon ...


References


External links


International Permafrost Association

Center for Permafrost

Carbon in Arctic Reservoirs Vulnerability Experiment
{{DEFAULTSORT:Permafrost Carbon Cycle Climate change feedbacks Carbon cycle Permafrost