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Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the
ocean The ocean (also the or the world ocean) is the body of that covers approximately 70.8% of the surface of and contains 97% of . Another definition is "any of the large bodies of water into which the great ocean is divided".
. Most common in the Northern Hemisphere, around 15% of the
Northern Hemisphere The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remain ...

Northern Hemisphere
or 11% of the global surface is underlain by permafrost, including substantial areas of
Alaska Alaska (; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Central Alaskan Yup'ik language, Yup'ik: ''Alaskaq''; tli, Anáaski) is a U.S. state in the Western United States, on the northwest extremity of the country's West Coast of the United State ...

Alaska
,
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an autonomous territory An autonomous administrative division (also referred to as an autonomous area, entity, unit, region, subdivision, or territory) is a subnational administra ...

Greenland
,
Canada Canada is a country in the northern part of North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean, Atlantic to the Pacific Ocean, Pacific and northward into the Arctic Oce ...

Canada
and
Siberia Siberia (; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region spanning much of Northern Asia. Siberia has been Russian conquest of Siberia, part of modern Russia since the latter half of th ...

Siberia
. It can also be located on mountaintops in the
Southern Hemisphere The Southern Hemisphere is the half (hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere of Earth ** Northern Hemisphere ** Southern Hemisphere ** Eastern Hemisphere ** Western He ...

Southern Hemisphere
and beneath ice-free areas in the
Antarctic The Antarctic (US English or , UK English or and or ) is a around 's , opposite the region around the . The Antarctic comprises the continent of , the and other located on the or south of the . The Antarctic region includes the , wa ...

Antarctic
. Permafrost does not have to be the first layer that is on the ground. It can be from an inch to several miles deep under the Earth's surface. It frequently occurs in ground ice, but it can also be present in non-porous bedrock. Permafrost is formed from ice holding various types of soil, sand, and rock in combination. Permafrost contains large amounts of biomass and decomposed biomass that has been stored as methane and carbon dioxide, making tundra soil a
carbon sink A carbon sink is any reservoir, natural or otherwise, that accumulates and stores some carbon Carbon (from la, carbo "coal") is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic tabl ...
. As global warming heats the ecosystem and causes soil thawing, the
permafrost carbon cycle The permafrost 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 ...
accelerates and releases much of these soil-contained greenhouse gases into the atmosphere, creating a feedback cycle that increases climate change.


Study of permafrost

In contrast to the relative dearth of reports on frozen ground in North America prior to World War II, a vast literature on basic permafrost science and the engineering aspects of permafrost was available in Russian. Some Russian authors relate permafrost research with the name
Alexander von Middendorff Alexander Theodor von Middendorff (russian: Алекса́ндр Фёдорович Ми́ддендорф; Romanization of Russian, tr. ; 18 August 1815 – 24 January 1894) was a zoologist and explorer of Baltic German and Estonians, Estonian ex ...

Alexander von Middendorff
(1815–1894). However, Russian scientists also realized, that
Karl Ernst von Baer Karl Ernst Ritter von Baer Edler von Huthorn ( – ) was a Baltic German scientist and explorer. Baer is also known in Russia as Karl Maksímovich Ber (russian: Карл Макси́мович Бэр). Baer was a naturalist Natural history ...

Karl Ernst von Baer
must be given the attribute "founder of scientific permafrost research". In 1843, Baer’s original study “materials for the study of the perennial ground-ice” was ready to be printed. Baer’s detailed study consists of 218 pages and was written in German language, as he was a Baltic German scientist. He was teaching at the
University of Königsberg The University of Königsberg (german: Albertus-Universität Königsberg) was the university A university ( la, universitas, 'a whole') is an educational institution, institution of higher education, higher (or Tertiary education, tertiary) educ ...
and became a member of the
St Petersburg Academy of Sciences The Russian Academy of Sciences (RAS; russian: Росси́йская акаде́мия нау́к (РАН) ''Rossíiskaya akadémiya naúk'') consists of the national academy of Russia; a network of scientific research institutes from across th ...
. This world's first permafrost textbook was conceived as a complete work and ready for printing in 1843. But it remained lost for around 150 years. However, from 1838 onwards, Baer published several individual publications on permafrost. The
Russian Academy of Sciences The Russian Academy of Sciences (RAS; russian: Росси́йская акаде́мия нау́к (РАН) ''Rossíiskaya akadémiya naúk'') consists of the national academy#REDIRECT National academy A national academy is an organizational bo ...
honoured Baer with the publication of a tentative Russian translation of his study in 1942. These facts were completely forgotten after the
Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
. Thus in 2001 the discovery of the typescript from 1843 in the library
archive An archive is an accumulation of Historical document, historical records – in any media – or the physical facility in which they are located. Archives contain primary source documents that have accumulated over the course of an individual ...

archive
s of the
University of Giessen Image:2004-JLU Gießen Sicherlich.jpg, 249px, Main building University of Giessen, official name Justus Liebig University Giessen (German: ''Justus-Liebig-Universität Gießen''), is a large public research university in Giessen, Hesse, Germany. ...
and its annotated publication was a scientific sensation. The full text of Baer's original work is available online (234 pages). The editor added to the
facsimile '', a famous illuminated manuscript An illuminated manuscript is a formally prepared document A document is a written Writing is a medium of human communication Communication (from Latin ''communicare'', meaning "to share") is the a ...
reprint a preface in English, two colour permafrost maps of Eurasia and some figures of permafrost features. Baer's text is introduced with detailed comments and references on additional 66 pages written by the
Estonian Estonian may refer to: *Something of, from, or related to Estonia, a country in the Baltic region in northern Europe *Estonians, people from Estonia, or of Estonian descent *Estonian language *Estonian cuisine *Estonian culture See also

* * La ...
historian Erki Tammiksaar. The work is fascinating to read, because both Baer's observations on permafrost distribution and his periglacial morphological descriptions are largely still correct today. With his compilation and analysis of all available data on ground ice and permafrost, Baer laid the foundation for the modern permafrost terminology. Baer’s southern limit of permafrost in
Eurasia Eurasia () is the largest continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth description") is a ...

Eurasia
drawn in 1843 corresponds well with the actual southern limit on the ''Circum-Arctic Map of Permafrost and Ground Ice Conditions'' of the
International Permafrost Association The International Permafrost Association (IPA), founded in 1983, has as its objectives to foster the dissemination of knowledge concerning permafrost and to promote cooperation among persons and national or international organisations engaged in s ...
(edited by J. Brown et al.). Beginning in 1942,
Siemon William Muller Siemon William Muller (May 9, 1900 – September 9, 1970) was an American paleontologist and geologist, known for his studies on Triassic The Triassic ( ) is a geologic period and system A system is a group of Interaction, interacting or interr ...
delved into the relevant Russian literature held by the
Library of Congress The Library of Congress (LC) is the research library A library is a collection of materials, books or media that are easily accessible for use and not just for display purposes. It is responsible for housing updated information in order ...

Library of Congress
and the U.S. Geological Survey Library so that he was able to furnish the government an engineering field guide and a technical report about permafrost by 1943", the year in which he coined the term as a contraction of permanently frozen ground. Although originally classified (as U.S. Army. Office of the Chief of Engineers, ''Strategic Engineering Study'', no. 62, 1943), in 1947 a revised report was released publicly, which is regarded as the first North American treatise on the subject.


Classification and extent

Permafrost is
soil Soil is a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, comp ...

soil
,
rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in w ...
or
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...

sediment
that is frozen for more than two consecutive years. In areas not covered by ice, it exists beneath a layer of soil, rock or sediment, which freezes and thaws annually and is called the "active layer". In practice, this means that permafrost occurs at an mean annual temperature of or below. Active layer thickness varies with the season, but is 0.3 to 4 meters thick (shallow along the Arctic coast; deep in southern Siberia and the
Qinghai-Tibetan Plateau The Tibetan Plateau (), also known in China as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau in South Asia, Central Asia and East Asia, covering most of the Tibet Autono ...
). The extent of permafrost is displayed in terms of permafrost zones, which are defined according to the area underlain by permafrost as continuous (90%–100%), discontinuous (50%–90%), sporadic (10%–50%), and isolated patches (10% or less). These permafrost zones cover together approximately 22% of the Northern Hemisphere. Continuous permafrost zone covers slightly more than half of this area, discontinuous permafrost around 20 percent, and sporadic permafrost together with isolated patches little less than 30 percent. Because permafrost zones are not entirely underlain by permafrost, only 15% the ice-free area of the
Northern Hemisphere The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remain ...

Northern Hemisphere
is actually underlain by permafrost. 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 deepest depth of permafrost occurs where geothermal heat maintains a temperature above freezing. Above that bottom limit there may be permafrost with a consistent annual temperature—"isothermal permafrost".


Continuity of coverage

Permafrost typically forms in any
climate Climate is the long-term pattern of weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a la ...

climate
where the mean annual air temperature is lower than the freezing point of
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
. Exceptions are found in humid boreal forests, such as in Northern Scandinavia and the North-Eastern part of European Russia west of the
Urals The Ural Mountains (; rus, Ура́льские го́ры, r=Uralskiye gory, p=ʊˈralʲskʲɪjə ˈgorɨ; ba, Урал тауҙары, ''Ural tauźarı'') or simply the Urals, are a Mountain range, mountain range that runs approximately fr ...
, where snow acts as an insulating blanket. Glaciated areas may also be exceptions. Since all glaciers are warmed at their base by geothermal heat, , 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. "Fossil" cold anomalies in the
Geothermal gradient ). Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consi ...
in areas where deep permafrost developed during the Pleistocene persist down to several hundred metres. This is evident from temperature measurements in boreholes in North America and Europe.


Discontinuous permafrost

The below-ground temperature varies less from season to season than the air temperature, with mean annual temperatures tending to increase with depth as a result of the geothermal crustal gradient. Thus, if the mean annual air temperature is only slightly below , permafrost will form only in spots that are sheltered—usually with a northern or southern
aspect Aspect or Aspects may refer to: Entertainment * ''Aspect magazine ASPECT Volume 9: Performance ''ASPECT'' was a biannual DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data stor ...
(in 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 . In the moist-wintered areas mentioned before, there may not be even discontinuous permafrost down to . 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 , and ''sporadic permafrost'', where permafrost cover is less than 50 percent of the landscape and typically occurs at mean annual temperatures between . In soil science, the sporadic permafrost zone is abbreviated SPZ and the extensive discontinuous permafrost zone DPZ. Exceptions occur in ''un-glaciated''
Siberia Siberia (; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region spanning much of Northern Asia. Siberia has been Russian conquest of Siberia, part of modern Russia since the latter half of th ...

Siberia
and
Alaska Alaska (; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Central Alaskan Yup'ik language, Yup'ik: ''Alaskaq''; tli, Anáaski) is a U.S. state in the Western United States, on the northwest extremity of the country's West Coast of the United State ...

Alaska
where the present depth of permafrost is a
relic In religion, a relic is an object or article of religious significance from the past, it usually consists of the physical remains of a saint or the personal effects of the saint or venerated person preserved for purposes of veneration Veneratio ...
of climatic conditions during glacial ages where winters were up to colder than those of today.


Continuous permafrost

At mean annual soil surface temperatures below 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 The Northern Hemisphere is the half of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remain ...

Northern Hemisphere
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 The Southern Hemisphere is the half (hemisphere Hemisphere may refer to: * A half of a sphere As half of the Earth * A hemispheres of Earth, hemisphere of Earth ** Northern Hemisphere ** Southern Hemisphere ** Eastern Hemisphere ** Western He ...

southern hemisphere
, most of the equivalent line would fall within the
Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. As such, it is regarded as the second-smallest of t ...

Southern Ocean
if there were land there. Most of the is overlain by glaciers, under which much of the terrain is subject to basal
melting Melting, or Enthalpy of fusion, fusion, is a physical process that results in the phase transition of a chemical substance, substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the applic ...
. The exposed land of Antarctica is substantially underlain with permafrost, some of which is subject to warming and thawing along the coastline.


Alpine permafrost

Alpine permafrost occurs at elevations with low enough average temperatures to sustain perennially frozen ground; much alpine permafrost is discontinuous. Estimates of the total area of alpine permafrost vary. Bockheim and Munroe combined three sources and made the tabulated estimates by region, totaling . Alpine permafrost in the
Andes The Andes, Andes Mountains or Andean Mountains ( es, Cordillera de los Andes) are the List of mountain ranges#Mountain ranges by length, longest continental mountain range in the world, forming a continuous highland along the western edge of Sou ...

Andes
has not been mapped. Its extent has been modeled to assess the amount of water bound up in these areas. In 2009, a researcher from Alaska found permafrost at the level on Africa's highest peak,
Mount Kilimanjaro Mount Kilimanjaro () is a dormant volcano in Tanzania. It has three volcanic cones: Kibo, Mawenzi, and Shira. It is the highest African mountains, mountain in Africa and the highest single free-standing mountain in the world: above sea le ...

Mount Kilimanjaro
, approximately 3° south of the equator.


Subsea permafrost

Subsea permafrost occurs beneath the seabed and exists in the continental shelves of the polar regions. These areas formed during the last ice age, when a larger portion of Earth's water was bound up in ice sheets on land and when sea levels were low. As the ice sheets melted to again become seawater, the permafrost became submerged shelves under relatively warm and salty boundary conditions, compared to surface permafrost. Therefore, subsea permafrost exists in conditions that lead to its diminishment. According to Osterkamp, subsea permafrost is a factor in the "design, construction, and operation of coastal facilities, structures founded on the seabed, artificial islands, sub-sea pipelines, and wells drilled for exploration and production." It also contains gas hydrates in places, which are a "potential abundant source of energy" but may also destabilize as subsea permafrost warms and thaws, producing large amounts of methane gas, which is a potent greenhouse gas.


Manifestations


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 0 °C. The base depth of permafrost reaches in the northern
Lena Lena or LENA may refer to: Places * Léna Department, Burkina Faso * Lena, Manitoba, Canada * Lena, Norway * Lena, Asturias, Spain Russia * Lena (river), Siberia * Lena, Russia, name of several rural localities * Lena Plateau United States * Lena ...
and
Yana River 265px, Map showing the two Yana Rivers in the Russian Far East. The river of this article is the northern one which It flows into the Laptev Sea. The Yana ( rus, Я́на, p=ˈjanə; sah, Дьааҥы, ''Caañı'') is a river in Sakha Republic, Sa ...

Yana River
basins in
Siberia Siberia (; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region spanning much of Northern Asia. Siberia has been Russian conquest of Siberia, part of modern Russia since the latter half of th ...

Siberia
. The
geothermal gradient ). Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consi ...
is the rate of increasing temperature with respect to increasing depth in the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
's interior. Away from tectonic plate boundaries, it is about 25–30 °C/km (124–139 °F/mi) near the surface in most of the world. It varies with the thermal conductivity of geologic material and is less for permafrost in soil than in bedrock. Calculations indicate that the time required to form the deep permafrost underlying
Prudhoe Bay, Alaska Prudhoe Bay or Sagavanirktok is a census-designated place (CDP) located in North Slope Borough, Alaska, North Slope Borough in the U.S. state of Alaska. As of the 2010 United States Census, 2010 census, the population of the CDP was 2,174 people, ...
was over a half-million years. This extended over several glacial and interglacial cycles of the
Pleistocene The Pleistocene ( , often referred to as the ''Ice Age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the earth’s most recent period of repeated glaciations. Before a change finally ...
and suggests that the present climate of Prudhoe Bay is probably considerably warmer than it has been on average over that period. Such warming over the past 15,000 years is widely accepted. The table to the right shows that the first hundred metres of permafrost forms relatively quickly but that deeper levels take progressively longer.


Massive ground ice

When the ice content of a permafrost exceeds 250 percent (ice to dry soil by mass) it is classified as ''massive'' ice. Massive ice bodies can range in composition, in every conceivable gradation from icy mud to pure ice. Massive icy beds have a minimum thickness of at least 2 m and a short diameter of at least 10 m. First recorded North American observations were by European scientists at Canning River, Alaska in 1919. Russian literature provides an earlier date of 1735 and 1739 during the Great North Expedition by P. Lassinius and Kh. P. Laptev, respectively. Two categories of massive ground ice are ''buried surface ice'' and ''intrasedimental ice'' (also called ''constitutional ice''). Buried surface ice may derive from snow, frozen lake or sea ice,
aufeis Aufeis, ( ), ( for "ice on top") is a sheet-like mass of layered that forms from successive flows of ground water during freezing temperatures. This form of ice is also called overflow, icings, or the term, naled. The term was first used in 185 ...
(stranded river ice) and—probably the most prevalent—buried glacial ice. Intrasedimental ice forms by in-place freezing of subterranean waters and is dominated by segregational ice which results from the crystallizational differentiation taking place during the freezing of wet sediments, accompanied by water migrating to the freezing front. Intrasedimental or constitutional ice has been widely observed and studied across Canada and also includes intrusive and injection ice.