Editing Permafrost (section)

== Classification and extent ==
[[File:Vertical Temperature Profile in Permafrost (English Text).jpg|thumb|upright=1.3|Permafrost temperature profile. Permafrost occupies the middle zone, with the active layer above it, while [[geothermal activity]] keeps the lowest layer above freezing. The vertical {{convert|0|°C|disp=or}} line denotes the average annual temperature that is crucial for the upper and lower limit of the permafrost zone, while the red lines represent seasonal temperature changes and seasonal temperature extremes. Solid curved lines at the top show seasonal maximum and minimum temperatures in the active layer, while the red dotted-to-solid line depicts the average temperature profile with depth of soil in a permafrost region.]]
Permafrost is [[soil]], [[Rock (geology)|rock]] or [[sediment]] that is frozen for more than two consecutive years. In practice, this means that permafrost occurs at a mean annual temperature of {{convert|0|°C|1}} or below. In the coldest regions, the depth of continuous permafrost can exceed {{convert|1400|m|ft|abbr=on}}.<ref name="Desonie2008" /> It typically exists beneath the so-called [[active layer]], which freezes and thaws annually, and so can support plant growth, as the [[root]]s can only take hold in the soil that's thawed.<ref name="IPADefinition" /> Active layer thickness is measured during its maximum extent at the end of summer:<ref>{{cite journal |last1=Zhang |first1=Caiyun |last2=Douglas |first2=Thomas A. |last3=Anderson |first3=John E. |title=Modeling and mapping permafrost active layer thickness using field measurements and remote sensing techniques |journal=International Journal of Applied Earth Observation and Geoinformation |date=27 July 2021 |volume=102 |doi=10.1016/j.jag.2021.102455 |bibcode=2021IJAEO.10202455Z }}</ref> as of 2018, the average thickness in the [[Northern Hemisphere]] is ~{{convert|145|cm|ft}}, but there are significant regional differences. Northeastern [[Siberia]], [[Alaska]] and [[Greenland]] have the most solid permafrost with the lowest extent of active layer (less than {{convert|50|cm|ft}} on average, and sometimes only {{convert|30|cm|ft}}), while southern [[Norway]] and the [[Mongolian Plateau]] are the only areas where the average active layer is deeper than {{convert|600|cm|ft}}, with the record of {{convert|10|m|ft}}.<ref name="Li2022">{{cite journal |last1=Li |first1=Chuanhua |last2=Wei |first2=Yufei |last3=Liu |first3=Yunfan |last4=Li |first4=Liangliang |last5=Peng |first5=Lixiao |last6=Chen |first6=Jiahao |last7=Liu |first7=Lihui |last8=Dou |first8=Tianbao |last9=Wu |first9=Xiaodong |date=14 June 2022 |title=Active Layer Thickness in the Northern Hemisphere: Changes From 2000 to 2018 and Future Simulations |journal= Journal of Geophysical Research: Atmospheres|volume=127 |issue=12 |pages=e2022JD036785 |doi=10.1029/2022JD036785 |bibcode=2022JGRD..12736785L |s2cid=249696017 }}</ref><ref>{{cite journal |last1=Luo |first1=Dongliang |last2=Wu |first2=Qingbai |last3=Jin |first3=Huijun |last4=Marchenko |first4=Sergey S. |last5=Lü |first5=Lanzhi |last6=Gao  |first6=Siru |title=Recent changes in the active layer thickness across the northern hemisphere |journal=Environmental Earth Sciences |date=26 March 2016 |volume=75 |issue=7 |page=555 |doi=10.1007/s12665-015-5229-2 |bibcode=2016EES....75..555L |s2cid=130353989 }}</ref> The border between active layer and permafrost itself is sometimes called permafrost table.<ref name="Lacelle2022" />

Around 15% of [[Northern Hemisphere]] land that is not completely covered by ice is directly underlain by permafrost; 22% is defined as part of a permafrost zone or region.<ref name="Obu2021" /> This is because only slightly more than half of this area is defined as a continuous permafrost zone, where 90%–100% of the land is underlain by permafrost. Around 20% is instead defined as discontinuous permafrost, where the coverage is between 50% and 90%. Finally, the remaining <30% of permafrost regions consists of areas with 10%–50% coverage, which are defined as sporadic permafrost zones, and some areas that have isolated patches of permafrost covering 10% or less of their area.<ref name="Brown1997">{{cite report |last1=Brown |first1=J. |last2=Ferrians Jr. |first2=O. J. |last3=Heginbottom |first3=J. A. |last4=Melnikov |first4=E. S. |title=Circum-Arctic map of permafrost and ground-ice conditions |year=1997 |publisher=[[USGS]] |doi=10.3133/cp45 |doi-access=free }}</ref><ref>{{cite report |last1=Heginbottom |first1=J. Alan |last2=Brown |first2=Jerry |last3=Humlum |first3=Ole |last4=Svensson |first4=Harald |editor-first1=Richard S. |editor-first2=Jane G. |editor-last1=Williams |editor-last2=Ferrigno |title=State of the Earth's Cryosphere at the Beginning of the 21st Century: Glaciers, Global Snow Cover, Floating Ice, and Permafrost and Periglacial Environments |year=2012 |publisher=[[USGS]] |url=https://pubs.usgs.gov/pp/p1386a/pdf/pp1386a-5-web.pdf |doi=10.3133/pp1386A }}</ref>{{rp|435}} Most of this area is found in Siberia, northern Canada, Alaska and Greenland. Beneath the active layer annual temperature swings of permafrost become smaller with depth. The greatest depth of permafrost occurs right before the point where geothermal heat maintains a temperature above freezing. Above that bottom limit there may be permafrost with a consistent annual temperature—"isothermal permafrost".<ref name="Degradation">{{cite journal|last=Delisle |first=G. |title=Near-surface permafrost degradation: How severe during the 21st century? |journal=Geophysical Research Letters |volume=34 |issue=L09503 |pages=4 |date=10 May 2007 |doi=10.1029/2007GL029323 |bibcode=2007GeoRL..34.9503D| doi-access=free }}</ref>

=== Continuity of coverage ===
Permafrost typically forms in any [[climate]] where the mean annual air temperature is lower than the freezing point of water. Exceptions are found in [[taiga|humid boreal forests]], such as in Northern [[Scandinavia]] and the North-Eastern part of [[European Russia]] west of the [[Ural Mountains|Urals]], where snow acts as an insulating blanket. Glaciated areas may also be exceptions. Since all glaciers are warmed at their base by geothermal heat, [[Glacier#Types|temperate glaciers]], which are near the [[pressure melting point]] throughout, may have liquid water at the interface with the ground and are therefore free of underlying permafrost.<ref>{{cite book |last=Sharp |first=Robert Phillip |author-link=Robert P. Sharp |title=Living Ice: Understanding Glaciers and Glaciation |publisher=Cambridge University Press |page=[https://archive.org/details/livingiceunderst0000shar/page/27 27] |url=https://archive.org/details/livingiceunderst0000shar |url-access=registration |isbn=978-0-521-33009-1 |date=1988 }}</ref> "Fossil" cold anomalies in the [[geothermal gradient]] in areas where deep permafrost developed during the Pleistocene persist down to several hundred metres. This is evident from temperature measurements in [[borehole]]s in North America and Europe.<ref>{{Cite journal |last=Majorowicz |first=Jacek |title=Permafrost at the ice base of recent pleistocene glaciations – Inferences from borehole temperatures profiles |journal=Bulletin of Geography. Physical Geography Series |series=Physical Geography Series |date=28 December 2012 |doi=10.2478/v10250-012-0001-x |volume=5 |pages=7–28 |doi-access = free }}</ref>

==== Discontinuous permafrost ====
[[File:Digging in permafrost.jpg|thumb|left|Excavating ice-rich permafrost with a [[jackhammer]] in [[Alaska]].]]
The below-ground temperature varies less from season to season than the air temperature, with mean annual temperatures tending to increase with depth due to the geothermal crustal gradient. Thus, if the mean annual air temperature is only slightly below {{convert|0|°C|°F|abbr=on}}, permafrost will form only in spots that are sheltered (usually with a northern or southern [[aspect (geography)|aspect]], in the north and south hemispheres respectively) creating discontinuous permafrost. Usually, permafrost will remain discontinuous in a climate where the mean annual soil surface temperature is between {{convert|-5|and|0|C|F}}. In the moist-wintered areas mentioned before, there may not even be discontinuous permafrost down to {{convert|-2|°C|°F}}. Discontinuous permafrost is often further divided into extensive discontinuous permafrost, where permafrost covers between 50 and 90 percent of the landscape and is usually found in areas with mean annual temperatures between {{convert|-2|and|-4|C|F}}, and sporadic permafrost, where permafrost cover is less than 50 percent of the landscape and typically occurs at mean annual temperatures between {{convert|0|and|-2|C|F}}.<ref name="BrownPéwé">{{Cite journal |last1=Brown |first1=Roger J. E. |last2=Péwé |first2=Troy L. |title=Distribution of permafrost in North America and its relationship to the environment: A review, 1963–1973 |journal=Permafrost: North American Contribution – Second International Conference |volume=2 |pages=71–100 |year=1973 |isbn=978-0-309-02115-9 |url=https://books.google.com/books?id=SjErAAAAYAAJ&pg=PA72}}</ref>

In soil science, the sporadic permafrost zone is abbreviated '''SPZ''' and the extensive discontinuous permafrost zone '''DPZ'''.<ref>{{Cite report |first= S. D. |last= Robinson |editor-last=Phillips |contribution= Permafrost and peatland [[carbon sink]] capacity with increasing latitude |title= Permafrost |year=2003 |pages=965–970 |publisher=Swets & Zeitlinger |url=http://www.arlis.org/docs/vol1/ICOP/55700698/Pdf/Chapter_169.pdf |isbn=90-5809-582-7 |display-authors=etal |display-editors=etal |access-date=18 August 2023 |archive-url=https://web.archive.org/web/20140302190815/http://www.arlis.org/docs/vol1/ICOP/55700698/Pdf/Chapter_169.pdf |archive-date=2 March 2014 |url-status=live }}</ref> Exceptions occur in un-glaciated [[Siberia]] and [[Alaska]] where the present depth of permafrost is a [[Relict (geology)|relic]] of climatic conditions during glacial ages where winters were up to {{convert|11|C-change}} colder than those of today.

==== Continuous permafrost ====
{| class="wikitable floatright"
|+ Estimated extent of alpine permafrost by region<ref name="BockhMunr" />
|-
! Locality
! Area<br />
|-
| [[Qinghai-Tibet Plateau]]
| style="text-align:right;" | {{convert|1300000|km2|mi2|abbr=on}}
|-
| [[Khangai Mountains|Khangai]]-[[Altai Mountains]]
| style="text-align:right;" | {{convert|1000000|km2|mi2|abbr=on}}
|-
| [[Brooks Range]]
| style="text-align:right;" | {{convert|263000|km2|mi2|abbr=on}}
|-
| [[Siberia#Mountain ranges|Siberian Mountains]]
| style="text-align:right;" | {{convert|255000|km2|mi2|abbr=on}}
|-
| [[Greenland]]
| style="text-align:right;" | {{convert|251000|km2|mi2|abbr=on}}
|-
| [[Ural Mountains]]
| style="text-align:right;" | {{convert|125000|km2|mi2|abbr=on}}
|-
| [[Andes]]
| style="text-align:right;" | {{convert|100000|km2|mi2|abbr=on}}
|-
| [[Rocky Mountains]] (US and Canada)
| style="text-align:right;" | {{convert|100000|km2|mi2|abbr=on}}
|-
| [[Alps]]
| style="text-align:right;" | {{convert|80000|km2|mi2|abbr=on}}
|-
| [[Fennoscandian]] mountains
| style="text-align:right;" | {{convert|75000|km2|mi2|abbr=on}}
|-
| Remaining
| style="text-align:right;" | <{{convert|50000|km2|mi2|abbr=on}}
|}
At mean annual soil surface temperatures below {{convert|-5|C}} the influence of aspect can never be sufficient to thaw permafrost and a zone of continuous permafrost (abbreviated to '''CPZ''') forms. A line of continuous permafrost in the [[Northern Hemisphere]]<ref>{{cite book |title=Frozen ground engineering |first1=Orlando B. |last1=Andersland |first2=Branko |last2=Ladanyi |publisher=Wiley |year=2004 |page=5 |isbn=978-0-471-61549-1 |edition=2nd}}</ref> represents the most southern border where land is covered by continuous permafrost or glacial ice. The line of continuous permafrost varies around the world northward or southward due to regional climatic changes. In the [[southern hemisphere]], most of the equivalent line would fall within the [[Southern Ocean]] if there were land there. Most of the [[Antarctica|Antarctic continent]] is overlain by glaciers, under which much of the terrain is subject to basal [[pressure melting point|melting]].<ref>{{Cite journal |last=Zoltikov |first=I. A. |title=Heat regime of the central Antarctic glacier |journal=Antarctica, Reports of the Commission, 1961 |pages=27–40 |year=1962 |language=ru }}</ref> The exposed land of Antarctica is substantially underlain with permafrost,<ref>{{Cite book |first1=Iain B. |last1=Campbell |first2=Graeme G. C. |last2=Claridge |editor-last=Margesin |editor-first=Rosa |contribution=Antarctic Permafrost Soils |isbn=978-3-540-69370-3  |title = Permafrost Soils |volume=16 |year=2009 |pages=17–31 |place = Berlin |publisher=Springer |doi=10.1007/978-3-540-69371-0_2 |series=Soil Biology }}</ref> some of which is subject to warming and thawing along the coastline.<ref>{{Cite news |last=Heinrich |first=Holly |title=Permafrost Melting Faster Than Expected in Antarctica |publisher=[[National Public Radio]] |date=25 July 2013 |url=https://stateimpact.npr.org/texas/2013/07/25/permafrost-melting-faster-than-expected-in-antarctica/ |access-date=23 April 2016 |archive-url=https://web.archive.org/web/20160503120018/https://stateimpact.npr.org/texas/2013/07/25/permafrost-melting-faster-than-expected-in-antarctica/ |archive-date=3 May 2016 |url-status=live }}</ref>

=== Alpine permafrost ===
A range of elevations in both the [[Northern Hemisphere|Northern]] and [[Southern Hemisphere]] are cold enough to support perennially frozen ground: some of the best-known examples include the [[Canadian Rockies]], the [[European Alps]], [[Himalaya]] and the [[Tien Shan]]. In general, it has been found that extensive alpine permafrost requires mean annual air temperature of {{cvt|-3|C|F}}, though this can vary depending on local [[topography]], and some mountain areas are known to support permafrost at {{cvt|-1|C|F}}. It is also possible for subsurface alpine permafrost to be covered by warmer, vegetation-supporting soil.<ref name="Haeberli2010">{{cite journal |last1=Haeberli |first1=Wilfried |last2=Noetzli |first2=Jeannette |last3=Arenson |first3=Lukas |last4=Delaloye |first4=Reynald |last5=Gärtner-Roer |first5=Isabelle |last6=Gruber |first6=Stephan |last7=Isaksen |first7=Ketil |last8=Kneisel |first8=Christof |last9=Krautblatter |first9=Michael |last10=Phillips |first10=Marcia |year=2010 |title=Mountain permafrost: development and challenges of a young research field |journal=Journal of Glaciology |volume=56 |issue=200 |pages=1043–1058 |publisher=Cambridge University Press |doi=10.3189/002214311796406121  |bibcode=2010JGlac..56.1043H |s2cid=33659636 }}</ref>

Alpine permafrost is particularly difficult to study, and systematic research efforts did not begin until the 1970s.<ref name="Haeberli2010" /> Consequently, there remain uncertainties about its geography As recently as 2009, permafrost had been discovered in a new area – Africa's highest peak, [[Mount Kilimanjaro]] ({{Convert|4700|m|ft|abbr=on}} above sea level and approximately 3° south of the [[equator]]).<ref>{{Cite news |last=Rozell |first=Ned |title=Permafrost near equator; hummingbirds near subarctic |newspaper=Capitol City Weekly |place=Juneau, Alaska |date=18 November 2009 |url=http://www.capitalcityweekly.com/stories/111809/out_522300722.shtml| archive-url=https://web.archive.org/web/20180305063150/http://www.capitalcityweekly.com/stories/111809/out_522300722.shtml | archive-date=5 March 2018}}</ref> In 2014, a collection of regional estimates of alpine permafrost extent had established a global extent of  {{convert|3560000|km2|mi2|abbr=on}}.<ref name=BockhMunr>{{cite journal |last1=Bockheim |first1=James G. |last2=Munroe |first2=Jeffrey S. |title=Organic Carbon Pools and Genesis of Alpine Soils with Permafrost: A Review |journal=Arctic, Antarctic, and Alpine Research |date=November 2014 |volume=46 |issue=4 |pages=987–1006 |doi=10.1657/1938-4246-46.4.987 |bibcode=2014AAAR...46..987B |s2cid=53400041 |doi-access=free }}</ref> However, by 2014, alpine permafrost in the [[Andes]] had not been fully mapped,<ref>{{cite thesis |last1=Azocar |first1=Guillermo |title=Modeling of Permafrost Distribution in the Semi-arid Chilean Andes |date=2 January 2014 |hdl=10012/8109 }}</ref> although its extent has been modeled to assess the amount of water bound up in these areas.<ref>{{Cite report |last1=Ruiz |first1= Lucas |last2=Liaudat |first2 =Dario Trombotto |title=Mountain Permafrost Distribution in the Andes of Chubut (Argentina) Based on a Statistical Model |place= Mendoza, Argentina |publisher=Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales |series=Tenth International Conference on Permafrost |year=2012 |pages=365–370 |url=http://www.geocriologia.com.ar/wp-content/uploads/2011/11/Pages-from-TICOP2012.pdf |access-date=24 April 2016 |archive-url =https://web.archive.org/web/20160513180525/http://www.geocriologia.com.ar/wp-content/uploads/2011/11/Pages-from-TICOP2012.pdf |archive-date=13 May 2016 |url-status=live }}</ref>

=== Subsea permafrost ===
[[File:Sayedi 2020 post LGM permafrost.jpg|thumb|Changes in subsea permafrost extent and structure between the Last Glacial Maximum and 2020<ref name="Sayedi2020" />]]
Subsea permafrost occurs beneath the [[seabed]] and exists in the [[continental shelf|continental shelves]] of the polar regions.<ref name="IPADefinition">{{cite web |title=What is Permafrost? |url=https://www.permafrost.org/what-is-permafrost/ |access-date=27 September 2023 |publisher=International Permafrost Association}}</ref> These areas formed during the last [[Last Glacial Period|Ice Age]], when a larger portion of Earth's water was bound up in [[ice sheet]]s on land and when sea levels were low. As the ice sheets melted to again become seawater during the [[Holocene glacial retreat]], coastal permafrost became submerged shelves under relatively warm and salty boundary conditions, compared to surface permafrost. Since then, these conditions led to the gradual and ongoing decline of subsea permafrost extent.<ref name="Sayedi2020" /> Nevertheless, its presence remains an important consideration for the "design, construction, and operation of coastal facilities, structures founded on the seabed, [[artificial island]]s, [[submarine pipeline|sub-sea pipelines]], and [[oil well|wells]] drilled for [[mineral exploration|exploration]] and production".<ref name="Osterkamp2001">{{Cite book |last=Osterkamp |first=T. E. |chapter=Sub-Sea Permafrost |title=Encyclopedia of Ocean Sciences |pages=2902–12 |year=2001 |doi=10.1006/rwos.2001.0008 |isbn=978-0-12-227430-5 |chapter-url=https://archive.org/details/encyclopediaofoc0000unse }}</ref> Subsea permafrost can also overlay deposits of [[methane clathrate]], which were once speculated to be a major [[tipping points in the climate system|climate tipping point]] in what was known as a [[clathrate gun hypothesis]], but are now no longer believed to play any role in projected climate change.<ref name="IPCC AR6 WG1 Ch.5">{{Cite journal |last1=Fox-Kemper |first1=B. |last2=Hewitt |first2=H. T.|author2-link=Helene Hewitt |last3=Xiao |first3=C. |last4=Aðalgeirsdóttir |first4=G. |last5=Drijfhout |first5=S. S. |last6=Edwards |first6=T. L. |last7=Golledge |first7=N. R. |last8=Hemer |first8=M. |last9=Kopp |first9=R. E. |last10=Krinner |first10=G. |last11=Mix |first11=A. |date=2021 |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Pirani |editor3-first=A. |editor4-last=Connors |editor4-first=S. L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |title=Chapter 5: Global Carbon and other Biogeochemical Cycles and Feedbacks |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter05.pdf |publisher=Cambridge University Press, Cambridge, UK and New York, NY, USA |page=5 |doi=10.1017/9781009157896.011 |quote=It is very unlikely that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century. }}</ref>

=== Past extent of permafrost ===
At the [[Last Glacial Maximum]], continuous permafrost covered a much greater area than it does today, covering all of ice-free Europe south to about [[Szeged]] (southeastern [[Hungary]]) and the [[Sea of Azov]] (then dry land)<ref>{{cite journal |last1=Sidorchuk |first1=Aleksey |last2=Borisova |first2=Olga |last3=Panin |first3=Andrey |date=20 February 2001 |title=Fluvial response to the late Valdai/Holocene environmental change on the East European plain |url=http://www.fluvial-systems.net/present_en/global.pdf |archive-url=https://web.archive.org/web/20131226230314/http://www.fluvial-systems.net/present_en/global.pdf |archive-date=26 December 2013 |journal=Quaternary International |volume=118–119 |issue=1–4 |pages=13–22 |doi=10.1016/S0921-8181(00)00081-3 |bibcode=2001GPC....28..303S }}</ref> and East Asia south to present-day [[Changchun]] and [[Abashiri, Hokkaidō|Abashiri]].<ref>{{cite journal |last1=Ono |first1=Yugo |last2=Irino |first2=Tomohisa |date=16 September 2003 |title=Southern migration of westerlies in the Northern Hemisphere PEP II transect during the Last Glacial Maximum |journal=Quaternary International |volume=118–119 |pages=13–22 |doi=10.1016/S1040-6182(03)00128-9 }}</ref> In North America, only an extremely narrow belt of permafrost existed south of the [[ice sheet]] at about the latitude of [[New Jersey]] through southern [[Iowa]] and northern [[Missouri]], but permafrost was more extensive in the drier western regions where it extended to the southern border of [[Idaho]] and [[Oregon]].<ref>{{cite journal |last1=Malde |first1=Harold E. |date=1 March 1964 |title=Patterned Ground in the Western Snake River Plain, Idaho, and Its Possible Cold-Climate Origin |journal=Geological Society of America Bulletin |url=https://core.ac.uk/download/pdf/159286331.pdf |volume=75 |issue=3 |pages=191–208 |doi=10.1130/0016-7606(1964)75[191:PGITWS]2.0.CO;2 }}</ref> In the [[Southern Hemisphere]], there is some evidence for former permafrost from this period in central [[Otago]] and [[Argentina|Argentine]] [[Patagonia]], but was probably discontinuous, and is related to the tundra. Alpine permafrost also occurred in the [[Drakensberg]] during glacial maxima above about {{convert|3000|m|ft|-1}}.<ref>{{cite journal |last1=Grab |first1=Stefan |date=17 December 2001 |title=Characteristics and palaeoenvironmental significance of relict sorted patterned ground, Drakensberg plateau, southern Africa |journal=Quaternary Science Reviews |volume=21 |issue=14–15 |pages=1729–1744 |doi=10.1016/S0277-3791(01)00149-4 }}</ref><ref>{{cite journal |last1=Trombotto |first1=Dario |date=17 December 2001 |title=Inventory of fossil cryogenic forms and structures in Patagonia and the mountains of Argentina beyond the Andes |journal=South African Journal of Science |url=https://core.ac.uk/download/pdf/159286331.pdf |volume=98 |pages=171–180 }}</ref>