Permafrost

Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the

ocean

. Most common in the Northern Hemisphere, around 15% of the

Northern Hemisphere

or 11% of the global surface is underlain by permafrost, including substantial areas of

Alaska

,

Greenland

,

Canada

and

Siberia

. It can also be located on mountaintops in the

Southern Hemisphere

and beneath ice-free areas in the

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. As global warming heats the ecosystem and causes soil thawing, the permafrost carbon cycle 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

(1815–1894). However, Russian scientists also realized, that

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 and became a member of the St Petersburg Academy of Sciences. 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 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. Thus in 2001 the discovery of the typescript from 1843 in the library

archive

s of the University of Giessen 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 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 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

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 (edited by J. Brown et al.).

Beginning in 1942, Siemon William Muller delved into the relevant Russian literature held by the

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

, rock or

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 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

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

where the mean annual air temperature is lower than the freezing point of

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, where snow acts as an insulating blanket. Glaciated areas may also be exceptions. Since all glaciers are warmed at their base by geothermal heat,

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. "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 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 (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

and

Alaska

where the present depth of permafrost is a relic 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

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

Antarctic continent

is overlain by glaciers, under which much of the terrain is subject to basal melting. 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

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

, 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 and

Yana River

basins in

Siberia

.

The geothermal gradient is the rate of increasing temperature with respect to increasing depth in the

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 was over a half-million years. This extended over several glacial and interglacial cycles of the Pleistocene 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 (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.