The permafrost carbon cycle or Arctic carbon cycle is a sub-cycle of the larger global carbon cycle. Permafrost 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, one which was often neglected in the initial research determining global terrestrial carbon reservoirs. Since the start of the 2000s, however, far more attention has been paid to the subject, with an enormous growth both in general attention and in the scientific research output.
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 and methane between terrestrial components and the atmosphere, as well as the transfer of carbon between land and water as methane, dissolved organic carbon, dissolved inorganic carbon, particulate inorganic carbon, and particulate organic carbon.
Storage Soils, in general, are the largest reservoirs of carbon in terrestrial ecosystems. 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 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 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. 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 sedimentation and/or peat 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 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, 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 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 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 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. In 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 in parts of Northern Alaska (early 1980s to mid-2000s) and up to 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, 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 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 and aluminum can adsorb 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.
A 1993 study suggested that while the tundra was a carbon sink until the end of the 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 northern 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 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 and 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 releases carbon dioxide, while anaerobic respiration releases methane. This is a substantial difference, as while biogenic methane lasts less than 12 years in the atmosphere, its global warming potential 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