Editing Permafrost (section)

=== Base depth ===
Permafrost extends to a base depth where geothermal heat from the Earth and the mean annual temperature at the surface achieve an equilibrium temperature of {{convert|0|°C|°F|abbr=on}}.<ref name="Ostercamp-Burn2003">{{Cite book |last1=Osterkamp |first1=T. E. |last2=Burn |first2=C. R. |contribution=Permafrost |title=Encyclopedia of Atmospheric Sciences |year=2003 |editor-last=North |editor-first=Gerald R. |editor-last2=Pyle |editor-first2=John A. |editor-last3=Zhang |editor-first3=Fuqing |volume=4 |pages=1717–1729 |url=http://curry.eas.gatech.edu/Courses/6140/ency/Chapter11/Ency_Atmos/Permafrost.pdf |publisher=Elsevier |isbn=978-0-12-382226-0 |access-date=8 March 2016 |archive-url=https://web.archive.org/web/20161130124841/http://curry.eas.gatech.edu/Courses/6140/ency/Chapter11/Ency_Atmos/Permafrost.pdf |archive-date=30 November 2016 |url-status=live }}</ref> This base depth of permafrost can vary wildly – it is less than a meter (3&nbsp;ft) in the areas where it is shallowest,<ref name="IPADefinition" /> yet reaches {{convert|1493|m|ft|abbr=on}} in the northern [[Lena River|Lena]] and [[Yana River]] basins in [[Siberia]].<ref name="Desonie2008">{{cite book| last=Desonie |first=Dana |title=Polar Regions: Human Impacts |publisher=Chelsea Press |year=2008 |location= New York |isbn=978-0-8160-6218-8 |url=https://archive.org/details/polarregionshuma0000deso }}</ref> Calculations indicate that the formation time of permafrost greatly slows past the first several metres. For instance, over half a million years was required to form the deep permafrost underlying [[Prudhoe Bay, Alaska]], a time period extending over several glacial and interglacial cycles of the [[Pleistocene]].<ref name="Lunardini1995">{{cite report |last=Lunardini |first=Virgil J. |title=Permafrost Formation Time |date=April 1995 |work=CRREL Report 95-8 |publisher=US Army Corps of Engineers Cold Regions Research and Engineering Laboratory |id={{DTIC|ADA295515}} |location=Hanover NH }}</ref>{{rp|18}}

Base depth is affected by the underlying geology, and particularly by [[thermal conductivity]], which is lower for permafrost in soil than in [[bedrock]].<ref name="Ostercamp-Burn2003" /> Lower conductivity leaves permafrost less affected by the [[geothermal gradient]], which is the rate of increasing temperature with respect to increasing depth in the Earth's interior. It occurs as the Earth's internal [[thermal energy]] is generated by [[radioactive decay]] of unstable [[isotope]]s and flows to the surface by conduction at a rate of ~47 [[terawatts]] (TW).<ref>{{cite journal |last1=Davies |first1=J. H. |last2=Davies |first2=D. R. |date=22 February 2010 |title=Earth's surface heat flux |volume=1 |issue=1 |pages=5–24 |journal=Solid Earth |doi=10.5194/se-1-5-2010 |bibcode=2010SolE....1....5D |doi-access=free }}</ref> Away from tectonic plate boundaries, this is equivalent to an average heat flow of 25–30&nbsp;°C/km (124–139&nbsp;°F/mi) near the surface.<ref name="IPCC2008">{{cite report |first1=Ingvar B. |last1=Fridleifsson |first2=Ruggero |last2=Bertani |first3=Ernst |last3=Huenges |first4=John W. |last4=Lund |first5=Arni |last5=Ragnarsson |first6=Ladislaus |last6=Rybach |date=11 February 2008 |title=The possible role and contribution of geothermal energy to the mitigation of climate change |editor=O. Hohmeyer and T. Trittin |location=IPCC Scoping Meeting on Renewable Energy Sources, Luebeck, Germany |pages=59–80 |url=https://www.researchgate.net/publication/284685252 |access-date=27 September 2023 |archive-url=https://web.archive.org/web/20130312182133/http://ipcc.ch/pdf/supporting-material/proc-renewables-lubeck.pdf | archive-date=12 March 2013 }}</ref>