|
Permafrost is soil, rock or sediment that is frozen more than two consecutive years. In areas not overlain by ice, it exists beneath a layer of soil, rock or sediment, which freezes and thaws annually and is called the "active layer".[4] In practice, this means that permafrost occurs at an average air temperature of -2°C or colder. 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). In the Northern Hemisphere, 24% of the ice-free land area, equivalent to 19 million square kilometers,[5] is more or less influenced 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, whose temperature doesn't change annually—"isothermal permafrost".[6] Cracks forming at the edges of the Storflaket permafrost plateau bog near Abisko in northern Sweden, due to thawing of the permafrost. The extent of permafrost varies with the climate. Today, a considerable area of the Arctic is covered by permafrost (including discontinuous permafrost). Overlying permafrost is a thin active layer that seasonally thaws during the summer. Plant life can be supported only within the active layer since growth can occur only in soil that is fully thawed for some part of the year. Thickness of the active layer varies by year and location, but is typically 0.6–4 m (2.0–13.1 ft) thick. In areas of continuous permafrost and harsh winters, the depth of the permafrost can be as much as 1,493 m (4,898 ft) in the northern Lena and Yana River basins in Siberia. Permafrost can also store carbon, both as peat and as methane. The most recent work investigating the permafrost carbon pool size estimates that 1400–1700 Gt of carbon is stored in permafrost soils worldwide.[7] This large carbon pool represents more carbon than currently exists in all living things and twice as much carbon as exists in the atmosphere. In Alaska excavation of ice-rich permafrost can require a jackhammer Continuous and discontinuous permafrost Permafrost typically forms in any climate where the mean annual air temperature is less than the freezing point of water. Exceptions are found in moist-wintered forest climates, 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 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.[8] Typically, the below-ground temperature varies less from season to season than the air temperature, with temperatures tending to increase with depth. 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 northerly aspect. This creates what is known as 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).[citation needed] Time required for permafrost to reach depth at Prudhoe Bay, Alaska[9] 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) In soil science, the sporadic permafrost zone is abbreviated SPZ and the extensive discontinuous permafrost zone DPZ.[10] 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. 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. "Fossil" cold anomalies in the Geothermal gradient in areas where deep permafrost developed during the Pleistocene persist down to several hundred metres. The Suwałki cold anomaly in Poland led to the recognition that similar thermal disturbances related to Pleistocene-Holocene climatic changes are recorded in boreholes throughout Poland.[11] A line of continuous permafrost in the Northern Hemisphere[12] represents the most southerly 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. In the Andes along the Atacama Desert permafrost extends down to an altitude of 4,400 metres (14,400 ft) and is continuous above 5,600 metres (18,400 ft). Subsea permafrost Subsea permafrost occurs beneath the seabed and exists in the continental shelves of the polar regions.[13] Time to form deep permafrost Calculations indicate that the time required to form the deep permafrost underlying Prudhoe Bay, Alaska was over a half-million years.[9][14] 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.[9] The table to the right shows that the first hundred metres of permafrost forms relatively quickly but that deeper levels take progressively longer. Landforms Permafrost processes manifest themselves in large-scale land forms, such as palsas and pingos[15] and smaller-scale phenomena, such as patterned ground found in arctic, periglacial and alpine areas.[16] A group of palsas, as seen from above, formed by the growth of ice lenses. Pingos near Tuktoyaktuk, Northwest Territories, Canada Polygons on the ground Stone rings on Spitsbergen Ice wedges seen from top Solifluction on Svalbard Contraction crack polygons on Arctic sediment. Carbon cycle in permafrost The permafrost carbon cycle (Arctic Carbon Cycle) deals with the transfer of carbon from permafrost soils to terrestrial vegetation and microbes, to the atmosphere, back to vegetation, and finally back to permafrost soils through burial and sedimentation due to cryogenic processes. Some of this carbon is transferred to the ocean and other portions of the globe through the global carbon cycle. The cycle includes the exchange of carbon dioxide and methane between terrestrial components and the atmosphere, as well as the transfer of carbon between land and water as methane, dissolved organic carbon, dissolved inorganic carbon, particulate inorganic carbon and particulate organic carbon.[17] Global climate change effects on permafrost extent See also: Climate change in the Arctic and Arctic methane release Historical changes in permafrost Recently thawed Arctic permafrost and coastal erosion on the Beaufort Sea, Arctic Ocean, near Point Lonely, AK. Photo Taken in August, 2013 At the Last Glacial Maximum, continuous permafrost covered a much greater area than it does today, covering all of ice-free Europe south to about Szeged (southeastern Hungary) and the Sea of Azov (then dry land)[18] and East Asia south to present-day Changchun and Abashiri.[19] In North America, only an extremely narrow belt of permafrost existed south of the ice sheet at about the latitude of New Jersey through southern Iowa and northern Missouri, but permafrost was more extensive in the drier western regions where it extended to the southern border of Idaho and Oregon.[20] In the southern hemisphere, there is some evidence for former permafrost from this period in central Otago and Argentine Patagonia, but was probably discontinuous, and is related to the tundra. Alpine permafrost also occurred in the Drakensberg during glacial maxima above about 3,000 metres (9,840 ft).[21][22] According to IPCC Fifth Assessment Report there is high confidence that permafrost temperatures have increased in most regions since the early 1980s. Observed warming was up to 3°C in parts of Northern Alaska (early 1980s to mid-2000s) and up to 2°C in parts of the Russian European North (1971–2010).[23] In Yukon, the zone of continuous permafrost might have moved 100 kilometres (62 mi) poleward since 1899, but accurate records only go back 30 years. It is thought that permafrost thawing could exacerbate global warming by releasing methane and other hydrocarbons, which are powerful greenhouse gases.[24][25][26] It also could encourage erosion because permafrost lends stability to barren Arctic slopes. Predicted rate of temperature change in Arctic Arctic temperatures are expected to increase at roughly twice the global rate.[27] The Intergovernmental Panel on Climate Change (IPCC) will in their fifth report establish scenarios for the future, where the temperature in the Arctic will rise between 1.5 and 2.5°C by 2040 and with 2 to 7.5°C by 2100. Estimates vary on how many tons of greenhouse gases are emitted from thawed permafrost soils. One estimate suggests that 110-231 billion tons of CO2 equivalents (about half from carbon dioxide and the other half from methane) will be emitted by 2040, and 850-1400 billion tons by 2100.[28] This corresponds to an average annual emission rate of 4-8 billion tons of CO2 equivalents in the period 2011-2040 and annually 10-16 billion tons of CO2 equivalents in the period 2011-2100 as a result of thawing permafrost. For comparison, the anthropogenic emission of all greenhouse gases in 2010 is approximately 48 billion tons of CO2 equivalents.[29] Release of greenhouse gases from thawed permafrost to the atmosphere may increase global warming. Permafrost thaw versus melt The ground can consist of many substrate materials, including bedrock, sediment, organic matter, water or ice. Frozen ground is that which is below the freezing point of water, whether or not water is present in the substrate. Ground ice is not always present, as may be the case with nonporous bedrock, but it frequently occurs and may be present in amounts exceeding the potential hydraulic saturation of the thawed substrate. By definition, permafrost is ground that remains frozen for two or more years. Since frozen soil, including permafrost, comprises a large percentage of substrate materials other than ice, it thaws rather than melts even as any ice content melts.[30] An analogy is when a freezer door is left open, although the ice in the freezer may change phase to a liquid, the food solids will not experience a phase change. In aggregate, the food thaws but does not melt. Melting implies the phase change of all solids to liquid. One visible sign of permafrost degradation is the random displacement of trees from their vertical orientation in permafrost areas.[31][32] Ecological consequences World-wide, permafrost contains 1700 billion tons of organic material equaling almost half of all organic material in all soils.[5] This pool was built up over thousands of years and is only slowly degraded under the cold conditions in the Arctic. The amount of carbon sequestered in permafrost is four times the carbon that has been released to the atmosphere due to human activities in modern time.[28] One manifestation of this is yedoma, which is an organic-rich (about 2% carbon by mass) Pleistocene-age loess permafrost with ice content of 50–90% by volume.[33] Formation of permafrost has significant consequences for ecological systems, primarily due to constraints imposed upon rooting zones, but also due to limitations on den and burrow geometries for fauna requiring subsurface homes. Secondary effects impact species dependent on plants and animals whose habitat is constrained by the permafrost. One of the most widespread examples is the dominance of Black Spruce in extensive permafrost areas, since this species can tolerate rooting pattern constrained to the near surface.[34] One gram of soil from the active layer may include more than one billion bacteria cells. If placed along each other, bacteria from one kilogram of active layer soil will form a 1000 km long chain. The number of bacteria in permafrost soil varies widely, typically from 1 to 1000 million per gram of soil.[35] Most of these bacteria and fungi in permafrost soil can not be cultured in the laboratory, but the identity of the microorganisms can be revealed by DNA-based techniques. Should a substantial amount of the carbon enter the atmosphere, it would accelerate planetary warming. A significant proportion will emerge as methane, which is produced when the breakdown occurs in lakes or wetlands. Although it does not remain in the atmosphere for long, methane traps more of the sun’s heat. The Arctic region is one of the many natural sources of the greenhouse gas methane.[36] Global warming accelerates its release, due to both release of methane from existing stores, and from methanogenesis in rotting biomass.[37] Large quantities of methane are stored in the Arctic in natural gas deposits, permafrost, and as submarine clathrates. Permafrost and clathrates degrade on warming, thus large releases of methane from these sources may arise as a result of global warming.[38][39] Other sources of methane include submarine taliks, river transport, ice complex retreat, submarine permafrost and decaying gas hydrate deposits.[40] Preliminary computer analyses suggest that permafrost could produce carbon equal to 15 percent or so of today’s emissions from human activities.[41] Climate Change and Slope Stability Over the past century, an increasing number of alpine rock slope failure events in mountain ranges around the world have been recorded. It is expected that the high number of structural failures is due to permafrost thawing, which is thought to be linked to climate change. In mountain ranges, much of the structural stability can be attributed to glaciers and permafrost. As climate warms, permafrost thaws, which results in a less stable mountain structure, and ultimately more slope failures.[42] Other permafrost issues The International Permafrost Association (IPA) is an integrator of issues, regarding permafrost. It convenes International Permafrost Conferences, undertake special projects such as preparing databases, maps, bibliographies, and glossaries, and coordinate international field programmes and networks. Among others issues addressed by the IPA and other organizations are: Problems for construction on permafrost, owing to the change of soil properties of ground on which structures are placed and the biological processes in permafrost, e.g. the preservation of organisms frozen in situ. Construction on permafrost Building on permafrost is difficult because the heat of the building (or pipeline) can thaw the permafrost and destabilize the structure. Three common solutions include: using foundations on wood piles; building on a thick gravel pad (usually 1–2 metres/3.3–6.6 feet thick); or using anhydrous ammonia heat pipes.[43] The Trans-Alaska Pipeline System uses heat pipes built into vertical supports to prevent the pipeline from sinking and the Qingzang railway in Tibet employs a variety of methods to keep the ground cool, both in areas with frost-susceptible soil. Permafrost may necessitate special enclosures for buried utilities, called "utilidors". The Permafrost Research Institute in Yakutsk, found that the sinking of large buildings into the ground can be prevented by using pile foundations extending down to 15 metres (49 ft) or more. At this depth the temperature does not change with the seasons, remaining at about −5 °C (23 °F). Thawed permafrost under this house caused it to settle unevenly—Tomsk, Siberia, Russia. Modern buildings in permafrost zones may be built on piles to avoid permafrost-thaw foundation failure from the heat of the building. Heat pipes in vertical supports maintain a frozen bulb around portions of the Trans-Alaska Pipeline that are at risk of thawing. Above-ground utility lines in a permafrost zone avoid thawing of ground. Pile foundations in Yakutsk, a city underlain with continuous permafrost. Revival of organisms preserved in permafrost In 2012, Russian researchers have proved that permafrost can serve as a natural depository for ancient life forms by the reviving of Silene stenophylla from a tissue as the oldest plant ever to be generated from a burrow in the Siberian permafrost for over 30,000 years. The plant is fertile, producing white flowers and viable seeds. The study has demonstrated that tissue can survive ice preservation for tens of thousands of years.[44] In geology, permafrost or cryotic soil is soil at or below the freezing point of water 0 °C (32 °F) for two or more years. Most permafrost is located in high latitudes (i.e. land close to the North and South poles), but alpine permafrost may exist at high altitudes in much lower latitudes. Ground ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of the ground material. Permafrost accounts for 0.022% of total water on earth[1] and exists in 24% of exposed land in the Northern Hemisphere.[2][3] A global temperature rise of 1.5 °C (2.7 °F) above current levels would be enough to start the thawing of permafrost in Siberia, according to one group of scientists.[3] |
About us|Jobs|Help|Disclaimer|Advertising services|Contact us|Sign in|Website map|Search|
GMT+8, 2015-9-11 21:14 , Processed in 0.121155 second(s), 16 queries .
