Permafrost
Permafrost | |
---|---|
Used in | International Permafrost Association |
Climate | High latitudes, alpine regions |
Permafrost (from perma- 'permanent', and frost) is soil or underwater sediment which continuously remains below 0 °C (32 °F) for two years or more: the oldest permafrost had been continuously frozen for around 700,000 years.[1] While the shallowest permafrost has a vertical extent of below a meter (3 ft), the deepest is greater than 1,500 m (4,900 ft).[2] Similarly, the area of individual permafrost zones may be limited to narrow mountain summits or extend across vast Arctic regions.[3] The ground beneath glaciers and ice sheets is not usually defined as permafrost, so on land, permafrost is generally located beneath a so-called active layer of soil which freezes and thaws depending on the season.[4]
Around 15% of the Northern Hemisphere or 11% of the global surface is underlain by permafrost,[5] with the total area of around 18 million km2 (6.9 million sq mi).[6] This includes substantial areas of Alaska, Greenland, Canada, and Siberia. It is also located in high mountain regions, with the Tibetan Plateau a prominent example. Only a minority of permafrost exists in the Southern Hemisphere, where it is consigned to mountain slopes like in the Andes of Patagonia, the Southern Alps of New Zealand, or the highest mountains of Antarctica.[3][1]
Permafrost contains large amounts of dead
In addition to its climate impact, permafrost thaw brings additional risks. Formerly frozen ground often contains enough ice that when it thaws, hydraulic saturation is suddenly exceeded, so the ground shifts substantially and may even collapse outright. Many buildings and other infrastructure were built on permafrost when it was frozen and stable, and so are vulnerable to collapse if it thaws.[12] Estimates suggest nearly 70% of such infrastructure is at risk by 2050, and that the associated costs could rise to tens of billions of dollars in the second half of the century.[13] Furthermore, between 13,000 and 20,000 sites contaminated with toxic waste are present in the permafrost,[14] as well as the natural mercury deposits,[15] which are all liable to leak and pollute the environment as the warming progresses.[16] Lastly, there have been concerns about potentially pathogenic microorganisms surviving the thaw and contributing to future epidemics and pandemics,[17][18] although this risk is speculative and is considered implausible by much of the scientific community.[19][20][21]
Classification and extent
Permafrost is soil, 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 −2 °C (28.4 °F) or below. In the coldest regions, the depth of continuous permafrost can exceed 1,400 m (4,600 ft).[22] It typically exists beneath the so-called active layer, which freezes and thaws annually, and so can support plant growth, as the roots can only take hold in the soil that's thawed.[2] Active layer thickness is measured during its maximum extent at the end of summer:[23] as of 2018, the average thickness in the Northern Hemisphere is ~145 centimetres (4.76 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 50 centimetres (1.6 ft) on average, and sometimes only 30 centimetres (0.98 ft)), while southern Norway and the Mongolian Plateau are the only areas where the average active layer is deeper than 600 centimetres (20 ft), with the record of 10 metres (33 ft).[24][25] The border between active layer and permafrost itself is sometimes called permafrost table.[26]
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.[5] 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.[27][28]: 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".[29]
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.[30] "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.[31]
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 0 °C (32 °F), 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 −5 and 0 °C (23 and 32 °F). In the moist-wintered areas mentioned before, there may not be even discontinuous permafrost down to −2 °C (28 °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 −2 and −4 °C (28 and 25 °F), and sporadic permafrost, where permafrost cover is less than 50 percent of the landscape and typically occurs at mean annual temperatures between 0 and −2 °C (32 and 28 °F).[32]
In soil science, the sporadic permafrost zone is abbreviated SPZ and the extensive discontinuous permafrost zone DPZ.[33] 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 11 °C (20 °F) colder than those of today.
Continuous permafrost
Locality | Area |
---|---|
Qinghai-Tibet Plateau
|
1,300,000 km2 (500,000 sq mi) |
Khangai-Altai Mountains | 1,000,000 km2 (390,000 sq mi) |
Brooks Range | 263,000 km2 (102,000 sq mi) |
Siberian Mountains | 255,000 km2 (98,000 sq mi) |
Greenland | 251,000 km2 (97,000 sq mi) |
Ural Mountains | 125,000 km2 (48,000 sq mi) |
Andes | 100,000 km2 (39,000 sq mi) |
Rocky Mountains (US and Canada) | 100,000 km2 (39,000 sq mi) |
Alps | 80,000 km2 (31,000 sq mi) |
Fennoscandian mountains
|
75,000 km2 (29,000 sq mi) |
Remaining | <50,000 km2 (19,000 sq mi) |
At mean annual soil surface temperatures below −5 °C (23 °F) 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
Alpine permafrost
A range of elevations in both the
Alpine permafrost is particularly difficult to study, and systematic research efforts did not begin until the 1970s.[39] 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 (4,700 m (15,400 ft) above sea level and approximately 3° south of the equator).[40] In 2014, a collection of regional estimates of alpine permafrost extent had established a global extent of 3,560,000 km2 (1,370,000 sq mi).[34] Yet, by 2014, alpine permafrost in the Andes has not been fully mapped,[41] although its extent has been modeled to assess the amount of water bound up in these areas.[42]
Subsea permafrost
Subsea permafrost occurs beneath the
Past extent of permafrost
At the
Manifestations
Time (yr) | Permafrost depth |
---|---|
1 | 4.44 m (14.6 ft) |
350 | 79.9 m (262 ft) |
3,500 | 219.3 m (719 ft) |
35,000 | 461.4 m (1,514 ft) |
100,000 | 567.8 m (1,863 ft) |
225,000 | 626.5 m (2,055 ft) |
775,000 | 687.7 m (2,256 ft) |
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 (32 °F).
Base depth is affected by the underlying geology, and particularly by
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.[55] First recorded North American observations of this phenomenon were by European scientists at Canning River (Alaska) in 1919.[56] Russian literature provides an earlier date of 1735 and 1739 during the Great North Expedition by P. Lassinius and Khariton Laptev, respectively. Russian investigators including I.A. Lopatin, B. Khegbomov, S. Taber and G. Beskow had also formulated the original theories for ice inclusion in freezing soils.[57]
While there are four categories of ice in permafrost – pore ice, ice wedges (also known as vein ice), buried surface ice and intrasedimental (sometimes also called constitutional
Buried surface ice may derive from snow, frozen lake or
Intrasedimental or constitutional ice has been widely observed and studied across Canada. It forms when subterranean waters freeze in place, and is subdivided into intrusive, injection and segregational ice. The latter is the dominant type, formed after crystallizational differentiation in wet
Landforms
Permafrost processes such as
-
A group of palsas, as seen from above, formed by the growth of ice lenses.
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Helicopter view of ground polygons and ice lenses at Padjelanta National Park, Sweden
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Ice wedges seen from top
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Contraction crack (ice wedge) polygons on Arctic sediment.
Ecology
Only plants with shallow
While permafrost soil is frozen, it is not completely inhospitable to microorganisms, though their numbers can vary widely, typically from 1 to 1000 million per gram of soil.[70][71] The
Most of the bacteria and fungi found in permafrost cannot be cultured in the laboratory, but the identity of the microorganisms can be revealed by
Construction on permafrost
There are only two large cities in the world built in areas of continuous permafrost (where the frozen soil forms an unbroken, below-zero sheet) and both are in Russia –
A common solution is placing
Two other approaches are building on an extensive
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A building on elevated piles in permafrost zone.
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Trans-Alaska Pipeline that are at risk of thawing.[82]
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Pile foundations in Yakutsk, a city underlain with continuous permafrost.
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District heating pipes run above ground in Yakutsk.
Impacts of climate change
Globally, permafrost warmed by about 0.3 °C (0.54 °F) between 2007 and 2016, with stronger warming observed in the continuous permafrost zone relative to the discontinuous zone. Observed warming was up to 3 °C (5.4 °F) in parts of
Permafrost thaw is associated with a wide range of issues, and International Permafrost Association (IPA) exists to help address them. It convenes International Permafrost Conferences and maintains Global Terrestrial Network for Permafrost, which undertakes special projects such as preparing databases, maps, bibliographies, and glossaries, and coordinates international field programmes and networks.[85]
Climate change feedback
As recent warming deepens the active layer subject to permafrost thaw, this exposes formerly stored
In the northern circumpolar region, permafrost contains organic matter equivalent to 1400–1650 billion tons of pure carbon, which was built up over thousands of years. This amount equals almost half of all organic material in all soils,[89][11] and it is about twice the carbon content of the atmosphere, or around four times larger than the human emissions of carbon between the start of the Industrial Revolution and 2011.[90] Further, most of this carbon (~1,035 billion tons) is stored in what is defined as the near-surface permafrost, no deeper than 3 metres (9.8 ft) below the surface.[89][11] However, only a fraction of this stored carbon is expected to enter the atmosphere.[91] In general, the volume of permafrost in the upper 3 m of ground is expected to decrease by about 25% per 1 °C (1.8 °F) of global warming,[83]: 1283 yet even under the RCP8.5 scenario associated with over 4 °C (7.2 °F) of global warming by the end of the 21st century,[92] about 5% to 15% of permafrost carbon is expected to be lost "over decades and centuries".[11]
The exact amount of carbon that will be released due to warming in a given permafrost area depends on depth of thaw, carbon content within the thawed soil, physical changes to the environment, and microbial and vegetation activity in the soil.
Another factor which complicates projections of permafrost carbon emissions is the ongoing "greening" of the Arctic. As climate change warms the air and the soil, the region becomes more hospitable to plants, including larger shrubs and trees which could not survive there before. Thus, the Arctic is losing more and more of its tundra biomes, yet it gains more plants, which proceed to absorb more carbon. Some of the emissions caused by permafrost thaw will be offset by this increased plant growth, but the exact proportion is uncertain. It is considered very unlikely that this greening could offset all of the emissions from permafrost thaw during the 21st century, and even less likely that it could continue to keep pace with those emissions after the 21st century.[11] Further, climate change also increases the risk of wildfires in the Arctic, which can substantially accelerate emissions of permafrost carbon.[87][98]
Impact on global temperatures
Altogether, it is expected that cumulative greenhouse gas emissions from permafrost thaw will be smaller than the cumulative anthropogenic emissions, yet still substantial on a global scale, with some experts comparing them to emissions caused by deforestation.[11] The IPCC Sixth Assessment Report estimates that carbon dioxide and methane released from permafrost could amount to the equivalent of 14–175 billion tonnes of carbon dioxide per 1 °C (1.8 °F) of warming.[83]: 1237 For comparison, by 2019, annual anthropogenic emissions of carbon dioxide alone stood around 40 billion tonnes.[83]: 1237 A major review published in the year 2022 concluded that if the goal of preventing 2 °C (3.6 °F) of warming was realized, then the average annual permafrost emissions throughout the 21st century would be equivalent to the year 2019 annual emissions of Russia. Under RCP4.5, a scenario considered close to the current trajectory and where the warming stays slightly below 3 °C (5.4 °F), annual permafrost emissions would be comparable to year 2019 emissions of Western Europe or the United States, while under the scenario of high global warming and worst-case permafrost feedback response, they would approach year 2019 emissions of China.[11]
Fewer studies have attempted to describe the impact directly in terms of warming. A 2018 paper estimated that if global warming was limited to 2 °C (3.6 °F), gradual permafrost thaw would add around 0.09 °C (0.16 °F) to global temperatures by 2100,[99] while a 2022 review concluded that every 1 °C (1.8 °F) of global warming would cause 0.04 °C (0.072 °F) and 0.11 °C (0.20 °F) from abrupt thaw by the year 2100 and 2300. Around 4 °C (7.2 °F) of global warming, abrupt (around 50 years) and widespread collapse of permafrost areas could occur, resulting in an additional warming of 0.2–0.4 °C (0.36–0.72 °F).[88][100]
Thaw-induced ground instability
As the water drains or evaporates, soil structure weakens and sometimes becomes viscous until it regains strength with decreasing moisture content. One visible sign of permafrost degradation is the random displacement of trees from their vertical orientation in permafrost areas.[101] Global warming has been increasing permafrost slope disturbances and sediment supplies to fluvial systems, resulting in exceptional increases in river sediment.[102] On the other hands, disturbance of formerly hard soil increases drainage of water reservoirs in northern wetlands. This can dry them out and compromise the survival of plants and animals used to the wetland ecosystem.[103]
In high mountains, much of the structural stability can be attributed to
Permafrost thaw can also result in the formation of frozen debris lobes (FDLs), which are defined as "slow-moving landslides composed of soil, rocks, trees, and ice".
Infrastructure
As of 2021, there are 1162 settlements located directly atop the Arctic permafrost, which host an estimated 5 million people. By 2050, permafrost layer below 42% of these settlements is expected to thaw, affecting all their inhabitants (currently 3.3 million people).[114] Consequently, a wide range of infrastructure in permafrost areas is threatened by the thaw.[12][115]: 236 By 2050, it's estimated that nearly 70% of global infrastructure located in the permafrost areas would be at high risk of permafrost thaw, including 30–50% of "critical" infrastructure. The associated costs could reach tens of billions of dollars by the second half of the century.[13] Reducing greenhouse gas emissions in line with the Paris Agreement is projected to stabilize the risk after mid-century; otherwise, it'll continue to worsen.[113]
In
In Canada, Northwest Territories have a population of only 45,000 people in 33 communities, yet permafrost thaw is expected to cost them $1.3 billion over 75 years, or around $51 million a year. In 2006, the cost of adapting Inuvialuit homes to permafrost thaw was estimated at $208/m2 if they were built at pile foundations, and $1,000/m2 if they didn't. At the time, the average area of a residential building in the territory was around 100 m2. Thaw-induced damage is also unlikely to be covered by home insurance, and to address this reality, territorial government currently funds Contributing Assistance for Repairs and Enhancements (CARE) and Securing Assistance for Emergencies (SAFE) programs, which provide long- and short-term forgivable loans to help homeowners adapt. It is possible that in the future, mandatory relocation would instead take place as the cheaper option. However, it would effectively tear the local Inuit away from their ancestral homelands. Right now, their average personal income is only half that of the median NWT resident, meaning that adaptation costs are already disproportionate for them.[118]
By 2022, up to 80% of buildings in some Northern Russia cities had already experienced damage.
Outside of the Arctic,
Release of toxic pollutants
For much of the 20th century, it was believed that permafrost would "indefinitely" preserve anything buried there, and this made deep permafrost areas popular locations for hazardous waste disposal. In places like Canada's
A notable example of pollution risks associated with permafrost was the
Another issue associated with permafrost thaw is the release of natural
Revival of ancient organisms
Microorganisms
Bacteria are known for being able to
At the same time, notable pathogens like influenza and smallpox appear unable to survive being thawed,[20] and other scientists argue that the risk of ancient microorganisms being both able to survive the thaw and to threaten humans is not scientifically plausible.[19] Likewise, some research suggests that antimicrobial resistance capabilities of ancient bacteria would be comparable to, or even inferior to modern ones.[126][21]
Plants
In 2012, Russian researchers proved that permafrost can serve as a natural repository for ancient life forms by reviving a sample of
History of scientific research
Between the middle of the 19th century and the middle of the 20th century, most of the literature on basic permafrost science and the engineering aspects of permafrost was written in Russian. One of the earliest written reports describing the existence of permafrost dates to
Baer is also known to have composed the world's first permafrost textbook in 1843, "materials for the study of the perennial ground-ice", written in his native language. However, it was not printed at the time, and a Russian translation wasn't ready until 1942. The original German textbook was believed to be lost until the typescript from 1843 was discovered in the library archives of the University of Giessen. The 234-page text was made available online, with additional maps, preface and comments.[128] Notably, Baer's southern limit of permafrost in Eurasia drawn in 1843 corresponds well with the actual southern limit verified by modern research.[27][128]
Beginning in 1942,
Between 11 and 15 November 1963, the First International Conference on Permafrost took place on the grounds of
In the recent decades, permafrost research has attracted more attention than ever due to the role it plays in climate change. Consequently, there has been a massive acceleration in published scientific literature. Around 1990, almost no papers were released containing the words "permafrost" and "carbon": by 2020, around 400 such papers were published every year.[11]
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