Editing Permafrost (section)

=== Climate change feedback ===
{{Main|Permafrost carbon cycle}}
[[File:Hugelius 2020 peatland projections.jpg|thumb|Permafrost peatlands (a smaller, carbon-rich subset of permafrost areas) under varying extent of global warming, and the resultant emissions as a fraction of anthropogenic emissions needed to cause that extent of warming.<ref name="Hugelius2020">{{Cite journal |last1=Hugelius |first1=Gustaf |last2=Loisel |first2=Julie |last3=Chadburn |first3=Sarah |display-authors=etal |date=10 August 2020 |title=Large stocks of peatland carbon and nitrogen are vulnerable to permafrost thaw |journal=Proceedings of the National Academy of Sciences |volume=117 |issue=34 |pages=20438–20446 |bibcode=2020PNAS..11720438H |doi=10.1073/pnas.1916387117 |pmc=7456150 |pmid=32778585 |doi-access=free}}</ref>
]]

As recent warming deepens the active layer subject to permafrost thaw, this exposes formerly stored [[carbon]] to biogenic processes which facilitate its entrance into the atmosphere as [[carbon dioxide]] and [[methane]].<ref name="Schuur2022" /> Because carbon emissions from permafrost thaw contribute to the same warming which facilitates the thaw, it is a well-known example of a [[Climate change feedback#Positive feedbacks|positive climate change feedback]].<ref name="Natali2020">{{Cite journal |last1=Natali |first1=Susan M. |last2=Holdren |first2=John P. |last3=Rogers |first3=Brendan M. |last4=Treharne |first4=Rachael |last5=Duffy |first5=Philip B. |last6=Pomerance |first6=Rafe |last7=MacDonald |first7=Erin |date=10 December 2020 |title=Permafrost carbon feedbacks threaten global climate goals |journal=Proceedings of the National Academy of Sciences |volume=118 |issue=21 |doi=10.1073/pnas.2100163118 |pmc=8166174 |pmid=34001617 |doi-access=free}}</ref> Permafrost thaw is sometimes included as one of the major [[tipping points in the climate system]] due to the exhibition of local thresholds and its effective irreversibility.<ref name="ArmstrongMcKay2022">{{Cite journal |last1=Armstrong McKay |first1=David |last2=Abrams |first2=Jesse |last3=Winkelmann |first3=Ricarda |last4=Sakschewski |first4=Boris |last5=Loriani |first5=Sina |last6=Fetzer |first6=Ingo |last7=Cornell |first7=Sarah |last8=Rockström |first8=Johan |last9=Staal |first9=Arie |last10=Lenton |first10=Timothy |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points |url=https://www.science.org/doi/10.1126/science.abn7950 |journal=Science |language=en |volume=377 |issue=6611 |pages=eabn7950 |doi=10.1126/science.abn7950 |issn=0036-8075 |pmid=36074831 |s2cid=252161375 |hdl-access=free |hdl=10871/131584}}</ref> However, while there are self-perpetuating processes that apply on the local or regional scale, it is debated as to whether it meets the strict definition of a global tipping point as in aggregate permafrost thaw is gradual with warming.<ref>{{Cite journal |last1=Nitzbon |first1=J. |last2=Schneider von Deimling |first2=T. |last3=Aliyeva |first3=M. |date=2024 |title=No respite from permafrost-thaw impacts in the absence of a global tipping point. |url=https://doi.org/10.1038/s41558-024-02011-4 |journal=Nature Climate Change |volume=14 |issue=6 |pages=573–585|doi=10.1038/s41558-024-02011-4 |bibcode=2024NatCC..14..573N }}</ref>
[[File:Fig 1.2.15 Schematic showing feedback processes related to land and subsea permafrost..png|thumb|Feedback processes related to land and subsea permafrost.]]
In the northern circumpolar region, permafrost contains organic matter equivalent to 1400–1650&nbsp;billion tons of pure carbon, which  was built up over thousands of years. This amount equals almost half of all organic material in all [[soil]]s,<ref name="Tarnocai2009">{{cite journal |last1=Tarnocai, C. |author2=Canadell, J. G. |author3=Schuur, E. A. G. |author4=Kuhry, P. |author5=Mazhitova, G. |author6=Zimov, S. |date=June 2009 |title=Soil organic carbon pools in the northern circumpolar permafrost region |journal=Global Biogeochemical Cycles |volume=23 |issue=2 |page=GB2023 |bibcode=2009GBioC..23.2023T |doi=10.1029/2008gb003327 |doi-access=free}}</ref><ref name="Schuur2022" /> and it is about twice the carbon content of the [[atmosphere]], or around four times larger than the human emissions of carbon between the start of the [[Industrial Revolution]] and 2011.<ref name="Schuur2011">{{cite journal |last1=Schuur |display-authors=etal |year=2011 |title=High risk of permafrost thaw |url=https://digital.library.unt.edu/ark:/67531/metadc836756/ |journal=Nature |volume=480 |issue=7375 |pages=32–33 |bibcode=2011Natur.480...32S |doi=10.1038/480032a |pmid=22129707 |s2cid=4412175 |doi-access=free}}</ref> Further, most of this carbon (~1,035&nbsp;billion tons) is stored in what is defined as the near-surface permafrost, no deeper than {{convert|3|m|ft}} below the surface.<ref name="Tarnocai2009" /><ref name="Schuur2022" /> However, only a fraction of this stored carbon is expected to enter the atmosphere.<ref name="Bockheim2007">{{Cite journal |author1=Bockheim, J.G. |author2=Hinkel, K.M. |name-list-style=amp |year=2007 |title=The importance of "Deep" organic carbon in permafrost-affected soils of Arctic Alaska |url=http://soil.scijournals.org/cgi/content/abstract/71/6/1889 |url-status=dead |journal=Soil Science Society of America Journal |volume=71 |issue=6 |pages=1889–92 |bibcode=2007SSASJ..71.1889B |doi=10.2136/sssaj2007.0070N |archive-url=https://web.archive.org/web/20090717063627/http://soil.scijournals.org/cgi/content/abstract/71/6/1889 |archive-date=17 July 2009 |access-date=5 June 2010}}</ref> In general, the volume of permafrost in the upper 3&nbsp;m of ground is expected to decrease by about 25% per {{convert|1|C-change|F-change}} of global warming,<ref name="AR6_WG1_Chapter922" />{{rp|1283}} yet even under the [[Representative Concentration Pathway#RCP8.5|RCP8.5]] scenario associated with over {{convert|4|C-change|F-change}} of global warming by the end of the 21st century,<ref name="ar5 21st century projections">IPCC: Table SPM-2, in: [http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf Summary for Policymakers] (archived [https://web.archive.org/web/20140716042158/http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf 16 July 2014]), in: {{harvnb|IPCC AR5 WG1|2013|p=21}}</ref> about 5% to 15% of permafrost carbon is expected to be lost "over decades and centuries".<ref name="Schuur2022" />

The exact amount of carbon that will be released due to warming in a given permafrost area depends on depth of thaw, carbon content within the thawed soil, physical changes to the environment, and microbial and vegetation activity in the soil.<ref name="Nowinski2010">{{Cite journal |vauthors=Nowinski NS, Taneva L, [[Susan Trumbore|Trumbore SE]], Welker JM |date=January 2010 |title=Decomposition of old organic matter as a result of deeper active layers in a snow depth manipulation experiment |journal=Oecologia |volume=163 |issue=3 |pages=785–92 |bibcode=2010Oecol.163..785N |doi=10.1007/s00442-009-1556-x |pmc=2886135 |pmid=20084398}}</ref> Notably, estimates of carbon release alone do not fully represent the impact of permafrost thaw on climate change. This is because carbon can be released through either [[aerobic respiration|aerobic]] or [[anaerobic respiration]], which results in carbon dioxide (CO<sub>2</sub>) or methane (CH<sub>4</sub>) emissions, respectively. While methane lasts less than 12 years in the atmosphere, its [[global warming potential]] is around 80 times larger than that of CO<sub>2</sub> over a 20-year period and about 28 times larger over a 100-year period.<ref>{{Cite book |last1=Forster |first1=Piers |title={{Harvnb|IPCC AR6 WG1|2021}} |last2=Storelvmo |first2=Trude |year=2021 |chapter=Chapter 7: The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity |ref={{harvid|IPCC AR6 WG1 Ch7|2021}} |chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf}}</ref><ref>{{Cite journal |last1=Allen |first1=Robert J. |last2=Zhao |first2=Xueying |last3=Randles |first3=Cynthia A. |last4=Kramer |first4=Ryan J. |last5=Samset |first5=Bjørn H. |last6=Smith |first6=Christopher J. |date=16 March 2023 |title=Surface warming and wetting due to methane's long-wave radiative effects muted by short-wave absorption |journal=Nature Geoscience |volume=16 |issue=4 |pages=314–320 |bibcode=2023NatGe..16..314A |doi=10.1038/s41561-023-01144-z |s2cid=257595431}}</ref> While only a small fraction of permafrost carbon will enter the atmosphere as methane, those emissions will cause 40–70% of the total warming caused by permafrost thaw during the 21st century. Much of the uncertainty about the eventual extent of permafrost methane emissions is caused by the difficulty of accounting for the recently discovered abrupt thaw processes, which often increase the fraction of methane emitted over carbon dioxide in comparison to the usual gradual thaw processes.<ref>{{Cite journal |last1=Miner |first1=Kimberley R. |last2=Turetsky |first2=Merritt R. |last3=Malina |first3=Edward |last4=Bartsch |first4=Annett |last5=Tamminen |first5=Johanna |last6=McGuire |first6=A. David |last7=Fix |first7=Andreas |last8=Sweeney |first8=Colm |last9=Elder |first9=Clayton D. |last10=Miller |first10=Charles E. |date=11 January 2022 |title=Permafrost carbon emissions in a changing Arctic |url=https://www.nature.com/articles/s43017-021-00230-3 |journal=Nature Reviews Earth & Environment |volume=13 |issue=1 |pages=55–67 |bibcode=2022NRvEE...3...55M |doi=10.1038/s43017-021-00230-3 |s2cid=245917526}}</ref><ref name="Schuur2022" />
[[File:Permafrost thaw ponds in Hudson Bay Canada near Greenland.jpg|thumb|left|Permafrost thaw ponds on peatland in [[Hudson Bay]], Canada in 2008.<ref>{{cite journal |last1=Dyke |first1=Larry D. |last2=Sladen |first2=Wendy E. |date=3 December 2010 |title=Permafrost and Peatland Evolution in the Northern Hudson Bay Lowland, Manitoba |journal=Arctic |volume=63 |issue=4 |pages=429–441 |doi=10.14430/arctic3332 |doi-access=free}}</ref>]]
Another factor which complicates projections of permafrost carbon emissions is the ongoing "greening" of the Arctic. As climate change warms the air and the soil, the region becomes more hospitable to plants, including larger [[shrub]]s and trees which could not survive there before. Thus, the Arctic is losing more and more of its [[tundra]] biomes, yet it gains more plants, which proceed to absorb more carbon. Some of the emissions caused by permafrost thaw will be offset by this increased plant growth, but the exact proportion is uncertain. It is considered very unlikely that this greening could offset all of the emissions from permafrost thaw during the 21st century, and even less likely that it could continue to keep pace with those emissions after the 21st century.<ref name="Schuur2022" /> Further, climate change also increases the risk of [[wildfire]]s in the Arctic, which can substantially accelerate emissions of permafrost carbon.<ref name="Natali2020" /><ref>{{Cite journal |last1=Estop-Aragonés |first1=Cristian |last2=Czimczik |first2=Claudia I |last3=Heffernan |first3=Liam |last4=Gibson |first4=Carolyn |last5=Walker |first5=Jennifer C |last6=Xu |first6=Xiaomei |last7=Olefeldt |first7=David |date=13 August 2018 |title=Respiration of aged soil carbon during fall in permafrost peatlands enhanced by active layer deepening following wildfire but limited following thermokarst |journal=Environmental Research Letters |volume=13 |issue=8 |doi=10.1088/1748-9326/aad5f0|bibcode=2018ERL....13h5002E |s2cid=158857491 |doi-access=free }}</ref>

==== Impact on global temperatures ====
[[File:Schuur 2022 century-scale permafrost projections.jpeg|thumb|Nine probable scenarios of [[greenhouse gas emission]]s from permafrost thaw during the 21st century, which show a limited, moderate and intense {{CO2}} and {{CH4}} emission response to low, medium and high-emission [[Representative Concentration Pathway]]s. The vertical bar uses emissions of selected large countries as a comparison: the right-hand side of the scale shows their cumulative emissions since the start of the [[Industrial Revolution]], while the left-hand side shows each country's cumulative emissions for the rest of the 21st century if they remained unchanged from their 2019 levels.<ref name="Schuur2022" />]]

Altogether, it is expected that cumulative greenhouse gas emissions from permafrost thaw will be smaller than the cumulative anthropogenic emissions, yet still substantial on a global scale, with some experts comparing them to emissions caused by [[deforestation]].<ref name="Schuur2022" /> The [[IPCC Sixth Assessment Report]] estimates that carbon dioxide and methane released from permafrost could amount to the equivalent of 14–175&nbsp;billion tonnes of carbon dioxide per {{convert|1|C-change|F-change}} of warming.<ref name="AR6_WG1_Chapter922" />{{rp|1237}}  For comparison, by 2019, annual anthropogenic emissions of carbon dioxide alone stood around 40&nbsp;billion tonnes.<ref name="AR6_WG1_Chapter922" />{{rp|1237}} A major review published in the year 2022 concluded that if the goal of preventing {{convert|2|C-change|F-change}} of warming was realized, then the average annual permafrost emissions throughout the 21st century would be equivalent to the year 2019 annual emissions of Russia. Under RCP4.5, a scenario considered close to the current trajectory and where the warming stays slightly below {{convert|3|C-change|F-change}}, annual permafrost emissions would be comparable to year 2019 emissions of Western Europe or the United States, while under the scenario of high global warming and worst-case permafrost feedback response, they would approach year 2019 emissions of China.<ref name="Schuur2022" />

Fewer studies have attempted to describe the impact directly in terms of warming. A 2018 paper estimated that if global warming was limited to {{convert|2|C-change|F-change}}, gradual permafrost thaw would add around {{convert|0.09|C-change|F-change}} to global temperatures by 2100,<ref>{{Cite journal |last1=Schellnhuber |first1=Hans Joachim |last2=Winkelmann |first2=Ricarda |last3=Scheffer |first3=Marten |last4=Lade |first4=Steven J. |last5=Fetzer |first5=Ingo |last6=Donges |first6=Jonathan F. |last7=Crucifix |first7=Michel |last8=Cornell |first8=Sarah E. |last9=Barnosky |first9=Anthony D. |author-link9=Anthony David Barnosky |date=2018 |title=Trajectories of the Earth System in the Anthropocene |journal=[[Proceedings of the National Academy of Sciences]] |volume=115 |issue=33 |pages=8252–8259 |bibcode=2018PNAS..115.8252S |doi=10.1073/pnas.1810141115 |issn=0027-8424 |pmc=6099852 |pmid=30082409 |doi-access=free}}</ref> while a 2022 review concluded that every {{convert|1|C-change|F-change}} of global warming would cause {{convert|0.04|C-change|F-change}} and {{convert|0.11|C-change|F-change}} from abrupt thaw by the year 2100 and 2300. Around {{convert|4|C-change|F-change}} of global warming, abrupt (around 50 years) and widespread collapse of permafrost areas could occur, resulting in an additional warming of {{convert|0.2-0.4|C-change|F-change}}.<ref name="ArmstrongMcKay2022" /><ref>{{Cite web |last=Armstrong McKay |first=David |date=9 September 2022 |title=Exceeding 1.5°C global warming could trigger multiple climate tipping points – paper explainer |url=https://climatetippingpoints.info/2022/09/09/climate-tipping-points-reassessment-explainer/ |access-date=2 October 2022 |website=climatetippingpoints.info |language=en}}</ref>