Editing Permafrost (section)

=== 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>