Permafrost
Permafrost is soil or underwater sediment which continuously remains below for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below a meter, the deepest is greater than. Similarly, the area of individual permafrost zones may be limited to narrow mountain summits or extend across vast Arctic regions. 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.
Around 15% of the Northern Hemisphere or 11% of the global surface is underlain by permafrost, covering a total area of around. This includes large areas of Alaska, Canada, Greenland, and Siberia. It is also located in high mountain regions, with the Tibetan Plateau being 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.
Permafrost contains large amounts of dead biomass that has accumulated throughout millennia without having had the chance to fully decompose and release its carbon, making tundra soil a carbon sink. As global warming heats the ecosystem, frozen soil thaws and becomes warm enough for decomposition to start anew, accelerating the permafrost carbon cycle. Depending on conditions at the time of thaw, decomposition can release either carbon dioxide or methane, and these greenhouse gas emissions act as a climate change feedback. The emissions from thawing permafrost will have a sufficient impact on the climate to impact global carbon budgets. It is difficult to accurately predict how much greenhouse gases the permafrost releases because the different thaw processes are still uncertain. There is widespread agreement that the emissions will be smaller than human-caused emissions and not large enough to result in runaway warming. Instead, the annual permafrost emissions are likely comparable with global emissions from deforestation, or to annual emissions of large countries such as Russia, the United States or China.
Apart from its climate impact, permafrost thaw brings more 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. 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. Furthermore, between 13,000 and 20,000 sites contaminated with toxic waste are present in the permafrost, as well as natural mercury deposits, which are all liable to leak and pollute the environment as the warming progresses. Lastly, concerns have been raised about the potential for pathogenic microorganisms surviving the thaw and contributing to future pandemics. However, this is considered unlikely, and a scientific review on the subject describes the risks as "generally low".
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 or below. In the coldest regions, the depth of continuous permafrost can exceed. 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. Active layer thickness is measured during its maximum extent at the end of summer: as of 2018, the average thickness in the Northern Hemisphere is ~, but there are significant regional differences. Northeastern Siberia, Alaska and Greenland have the most solid permafrost with the lowest extent of active layer, while southern Norway and the Mongolian Plateau are the only areas where the average active layer is deeper than, with the record of. The border between active layer and permafrost itself is sometimes called permafrost table.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. 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. 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".
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 due to the geothermal crustal gradient. Thus, if the mean annual air temperature is only slightly below, permafrost will form only in spots that are sheltered 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 even be 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 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
A range of elevations in both the Northern and Southern Hemisphere are cold enough to support perennially frozen ground: some of the best-known examples include the Canadian Rockies, the European Alps, Himalaya and the Tien Shan. In general, it has been found that extensive alpine permafrost requires mean annual air temperature of, though this can vary depending on local topography, and some mountain areas are known to support permafrost at. It is also possible for subsurface alpine permafrost to be covered by warmer, vegetation-supporting soil.Alpine permafrost is particularly difficult to study, and systematic research efforts did not begin until the 1970s. 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. In 2014, a collection of regional estimates of alpine permafrost extent had established a global extent of . However, by 2014, alpine permafrost in the Andes had not been fully mapped, although its extent has been modeled to assess the amount of water bound up in these areas.