Permafrost is an important aspect of the cryosphere, fundamental and integral part of the climate system in periglacial landscapes. Here we present the general concept, and especially the climatic conditions leading to the formation and preservation of permafrost in different Earth’s environments.
Permafrost, by definition, refers to the ground that remains below 0°C for at least two consecutive years. More briefly, we can define it as perennially cryotic ground. The term cryotic, better than frozen which implies the presence of ice, suggests a ground temperature below 0°C.
In fact, water in its solid phase is not necessary to characterize the Permafrost, which instead is exclusively defined by the thermal state of the ground. For this reason alone, it is important to bear in mind that permafrost thaws, while ice melts.
Except under very special circumstances, permafrost does not extend to the ground surface due to solar radiation and above-freezing temperature thaw the uppermost layer of ground during summer. Exceptions exist under perennial snow-beds or cold-based glaciers. The uppermost layer, which freezes and thaws on a seasonal basis, is called the Active Layer.
To understand how the temperature in the ground behaves in areas interested by permafrost, we drew the sketch above. The “Y” diagram explains how the ground temperature behaves from the surface to depth. Annual extremes are of course the greatest close to the surface, becoming gradually smaller moving down.
At a certain depth, the temperature is constant all year round and this is the ‘Zero Annual Temperature’ depth. From this depth, the temperature starts rising constantly following the geothermal gradient at the rate of 25-30°C per kilometer (72-87 °F per mile).
When the ‘Y’ intersects the 0°C isotherms in the ground this is the Permafrost table located just below the Active Layer. Below this depth, we find the Perennially Cryotic layer or the Permafrost layer. When the ‘Y’ intersects for the second time the 0°C isotherms in the ground, we reached the Permafrost base. From this depth, the ground is perennially NON-cryotic and always thawed.
There is a well-defined difference in the behavior of the ground in permafrost or non-permafrost terrains. The term seasonally frozen ground, or more strictly seasonally cryotic ground, is used to describe ground undergoing a seasonal cycle of freezing and thawing.
Usually, it refers to the uppermost layer of the ground in cold environments where permafrost is not present. The images above (seasonally frozen ground in a permafrost-free environment) and below (above the permafrost) are of course oversimplifications but simply highlight what happens in the ground during the seasonal cycles.
HOW PERMAFROST IS REACTING TO GLOBAL WARMING
Permafrost is a key component of the cryosphere and occupies around a quarter of the Earth’s land area in the Northern Hemisphere. The change in surface energy balance triggering permafrost degradation may be caused by regional changes in climate such as longer or warmer summer or increased winter snowfalls which insulate the ground from the atmosphere. Another cause could be deforestation both human-induced or natural, like a forest fire.
If the ground warms for one of these reasons, how does the permafrost react? To understand what happens when such a circumstance occurs, we created the GIF animation below. If ground warms up, the surface temperature extremes will rise. The same will happen proceeding in depth. As a consequence, the ‘Y’-diagram will move on the right while permafrost is warming. As you can easily see in the animated GIF, the Active layer will deepen and the permafrost becomes thinner.
PERMAFROST AND PERIGLACIAL ENVIRONMENTS
Originally, the term periglacial was used to describe the climatic and geomorphic conditions of areas peripheral to Pleistocene ice sheets and glaciers. Modern usage refers, however, to a wider range of cold climatic conditions regardless of their proximity to a glacier, either in space or time. In addition, many, but not all, periglacial environments possess permafrost; but they appear all to be dominated by frost action processes.
In line with this view, the English version of the Multi-Language Glossary of Permafrost and Related Ground-Ice Terms compiled by the International Permafrost Association Terminology Working Group defines the term periglacial as the conditions, processes, and landforms associated with cold, nonglacial environments.
A typical Arctic periglacial landscape with patterned ground in the Spitzbergen-Svalbard credits dreamstime
The geographical extent of present-day periglacial activity is not easily distinguished, but the general view is that the actual periglacial zones include all non-glaciated regions where frozen ground or/and thawing and freezing of the ground considerably affect landform development. Starting from this assumption, we understand how this model is strongly related to the geomorphological concept.
In other words, the distribution of resultant landforms derived from frost-action processes defines the extent of the periglacial domain. Depth and frequency of seasonal ground freezing and thawing, the depth of winter snow cover, the seasonal availability of liquid water in the upper level of the ground, erosion, and deposition processes are some of the main parameters in a periglacial environment, all driven by the climate
Periglacial landscape in Iceland with patterned ground and polygons credits dreamstime
The periglacial domain refers by definition to the global extent of the so-called periglacial zone. Based upon the spatial association of certain microforms and their climate threshold values, several different periglacial zones can be recognized. They occur not only as tundra zones in the high latitudes but also as forested zones south of treeline and in high-altitude alpine regions of temperature latitudes.
In the northern hemisphere, they include polar desert and semi-desert of the High Arctic, tundra, boreal forest, the sub-arctic areas of either maritime or continental nature, and the vast high-elevation Qinghai-Xizang plateau in Tibet. In the southern hemisphere, a similar map would include the higher elevations and southern tip of South America, the sub-antarctic islands, the Antarctic Peninsula, and the various ice-free zones on the Antarctic continent, namely the Dry Valleys.
The typical periglacial environment of a sub-antarctic island. Photo Renato R. Colucci
Indeed, there is no perfect spatial correlation between areas of intense frost and areas underlain by permafrost. In fact, a number of sub-arctic, maritime, and alpine locations experience recursive freeze-thaw oscillations but lack permafrost. More, relict permafrost coming from the last ice age roughly ended 20 thousand years ago, underlies vast areas of the boreal forest both in North America and Siberia.
In this view, any simple delimitation of periglacial environments is difficult. Nevertheless, using the diagnostic criteria above, a conservative estimation is that around 25% of the Earth’s land surface currently experiences periglacial conditions.
The boundary between periglacial and non-periglacial conditions is indeed rather arbitrary and, to a large extent, varies according to different criteria. Indeed, no single climatic parameter adequately defines the limit of periglacial climates, though a mean annual air temperature of +3°C might be considered a good approximation.
Even within particular areas there may be marked climatic variation relating to such factors as altitude, slope aspect, and distance from the coast. A fourfold classification of periglacial climates comprises high arctic, low-arctic, and subarctic continental interiors, maritime periglacial with low annual temperature range, and alpine climates. The Qinghai-Tibet Plateau and Antarctica do not fall readily into any of these classes.
An image of the Southern Qinghai-Tibet Plateau with Mount-Everest in the background. Credits dreamstime
High Arctic climates in polar latitudes show the extremely weak diurnal patterns and strong seasonal patterns. Therefore, temperature shows a small daily and large annual range. Extremely low winter temperatures occur for periods of several months, when there is perpetual darkness and the monthly average temperature may fall to between -20°C and -30°C, or even lower.
There are actually three key factors. The first one is a strongly negative annual radiation budget at the top of the atmosphere in arctic areas. This is equal to roughly -60 to -120 Watts per square meter implying that outward radiative losses exceed inputs. This is in part due to the fact that north of 66.5 degrees, the Arctic Circle, there is a period of complete darkness in winter becoming progressively longer with increasing latitude.
Global distribution of surface net radiation averaged from 2001 to 2015 from the CERES EBAF product
A second key factor is the development of a semi-permanent high-pressure system over much of the Arctic, especially between autumn and spring. This pattern prevents frontal systems incursions thus limiting associated movement of relatively warm and moist air into northern polar latitudes.
The North Atlantic cyclone track is the only exception, and extends eastwards from a zone of recursive low pressure called the Icelandic Low, centered south of Greenland at about 60 degrees latitude, and to a lesser extent the Aleutian Low in the Pacific. In the image below, the two lows are highlighted in the 1991-2020 winter map of mean sea level pressure.
The most obvious effect of such a pattern is low mean annual precipitation in the Arctic. Extensive areas of the Arctic or sub-Arctic continental interiors have mean annual precipitation smaller than 400 millimeters of water equivalent (equal to liters per square meters). This is particularly the situation of the Canadian Arctic and northern Asia.
A belt of much higher precipitation with values greater than 600 millimeters of water equivalent, interests Baffin Island across southern Greenland and Iceland into the Barents and Kara Seas. In southeastern Greenland, there is a sector with mean annual precipitation up to about 4 meters of water equivalent, likely the highest in the northern hemisphere. In the map below, the mean annual precipitation in meters averaged over the period 1991-2020.
The last of the three key features in the Arctic is the heat transfer from the ocean which acts in reducing temperature extremes in winter in surroundings landmasses. Heat loss to the Arctic Ocean also slightly depresses summer temperature in coastal locations.
The map below with winter and summer average temperatures in the Arctic highlights the effect of the Arctic Ocean on the mean summer temperature of the Arctic coasts. Although summer is short (2-3 months) and rather cool with an average rarely greater than 5 degrees Celsius, a few summer days can be remarkably mild. Winter is definitely milder in the North Atlantic sector of the Arctic and Svalbard represents the warmest place in the Arctic given the same latitude.
Low-arctic and subarctic continental interiors experience a much wide range of temperatures than High Arctic areas. The mean values disguise excessively low temperatures in winter and remarkably high temperatures in summer. Mean annual air temperature, in the end, is the same or slightly higher than in areas north of the treeline.
This temperature pattern reflects the absence of moderating oceanic influences. A zone of high pressure over eastern Eurasia builds during the autumn in response to radiative cooling and persists until spring. This reflects in the lowest winter temperatures anywhere in the northern hemisphere.
As an example, Verkhoyansk at 67 degrees latitude has a mean temperature of -48 degrees Celsius in January and almost no precipitation, but for five months in the warm season, temperatures are above 0 degrees Celsius, peaking at 17 degrees Celsius in July.
Canadian locations like Churchill have similar, if less extreme, characteristics with cold winters and moderately warm summers. On average, 4-5 months reports a temperature higher than 0 degrees Celsius.
Where permafrost is present, warm summers allow seasonal thaw up to 2-3 meters depth, while where permafrost is absent, seasonal ground freezing can reach similar depths. In the map below-average summer (JJA) and Winter (DJF) for Eurasia.
Precipitation in subarctic continental interiors is low and ranges from about 180 to 600 millimeters per year. Nevertheless, precipitation is greater than in High Arctic regions because disturbances associated with the Arctic and polar fronts are more frequent at these latitudes.
The majority of precipitation falls in the form of rain and mostly during the summer. Nevertheless, the relatively high summer temperature also promotes high rates of evapotranspiration, so there is a seasonal soil moisture deficit. In the image below, higher summer precipitation between about latitudes 55 and 70 degrees North in contrast with high precipitation in the North Atlantic and North Pacific induced by the Icelandic and Aleutian Lows.
Maritime periglacial environments experience mean annual temperature below +3 degrees Celsius and a low annual temperature range. Such climatic conditions occur in two types of areas: subarctic oceanic locations and alpine locations in low latitude. Those areas are strongly influenced by the proximity of relatively warm oceanic waters and frequent cyclonic activity.
Extreme cold is generally absent, but precipitation is high due to frequent cloudy conditions and strong winds associated with deep depressions, especially in winter. Permafrost and deep seasonal ground freezing are usually absent, but air freeze-thaw cycles at ground level are frequent. Such temperature variability comes from the passage of alternating warm and cold air masses between autumn and spring.
As an example, such conditions are particularly evident in northwest Norway. In the area of Tromsø, mean annual precipitation is slightly above 1000 millimeters, and the mean annual air temperature is 2.8 degrees Celsius. Much of Iceland experiences a similar climate as the south coasts of Alaska do.
Other maritime periglacial conditions are typical of mountains in ocean-proximal locations such as southwest Norway, the Faroe Islands, and Scotland at elevations above 800 meters in the northern hemisphere. In the southern hemisphere, similar conditions are present in sub-antarctic islands such as Kerguelen, South Georgia, and the high ground in the Falkland Islands.
The maritime periglacial environment of South Georgia. Photo credits Renato R. Colucci
Alpine periglacial climates are those where altitude and slope aspect and shading control the annual temperature cycle. They are not as extensive as any of the previous climatic environments. In the European Alps and North American Rockies, the timberline rises generally to a maximum altitude between 2000 and 4000 meters.
If in a stationary atmosphere the environmental lapse rate averages roughly 0.65 degrees Celsius every 100 meters, in practice it may periodically and regionally reverse due to inversions. Precipitation tends to increase with altitude on mid-latitudes mountains thanks to orographic processes. In fact, air masses and frontal structures rise and cool adiabatically forced by the wind flowing against the massifs. This induces condensations, cloud formation, and orographically-enhanced precipitation. Conversely, on the lee sides, compression and adiabatic warming give birth to dry föhn winds resulting in marked precipitation contrasts between the windward and lee sides of mountain ranges.
Such difference is beautifully illustrated by the Southern Alps in New Zealand as highlighted in the image above. Here, western windward slopes receive more than 6,000 millimeters of water equivalent of rainfalls while eastern lee side slopes receive less than 1000 millimeters.
In alpine areas, permafrost is generally absent below the treeline but exists discontinuously at higher altitudes. Only on the higher summits and particularly on shaded slopes permafrost is continuous. In the image below the presence of permafrost in the Alps according to the recently realized Alpine Permafrost Index map.
Antarctica and the Qinghai-Xizang (Tibet) Plateau are special cases. In Antarctica, glacier ice covers more than 99% of the continent and unglaciated areas are restricted to a few coastal areas, Transantarctic mountains, nunataks, and the dry valleys. A layer of extremely cold and dense air flows radially outwards from the Antarctic ice sheet having a few hundreds of meters in thickness.
The cooling effect of winds blowing from the plateau results in mean summer temperatures between -5 and 0 degrees Celsius at the coastal margins, and summer temperatures as low as between -20 and -30 degrees Celsius. For this reason, Permafrost interests all free-of-ice terrain except a few coastal fringes of the Antarctic Peninsula.
In the image above permafrost temperature maps of the Transantarctic Mountains east of 90 degrees E according to a recent paper by Jaroslav Obu et al. The Transantarctic Mountains are the largest ice-free region and experiences the harshest conditions thanks to their altitude and the impact of strong winds.
The region consists of numerous mountain ranges extending from the Ross Ice Shelf and the Ross Sea up to more than 4000 m elevations and from 69 to 85 degrees S latitude. The Earth’s lowest permafrost temperature of −36 degrees Celsius is modeled here at Mount Markham in Queen Elizabeth Range.
Precipitation is very low and mostly falls in form of snow, although rain on coastal slopes can occur during incursions of summer weather systems from lower latitudes. An area of extreme aridity is the McMurdo Dry Valleys. This area occupies 4800 square kilometers in Victoria Land and mean annual air temperature range from -14.8 to -30 degrees Celsius.
A combination of low surface albedo, dry katabatic winds, and very low solid precipitation relative to potential evaporation make this one of the aridest parts of the Earth. Equally important in Antarctica is the exceptional strength and duration of wind. The station located on Inexpressible Island records near-continuous wind flowing outwards from the Priestley glacier at a speed of in excess of 50 kilometers per hour for over 51% of the time.
Permafrost zonation, permafrost extent and difference between the borehole and modeled MAGT for the Qinghai-Xizang (Tiber) plateau according to a recent paper by Jaroslav Obu et al.
The Qubghai.Xizang (Tibetan) Plateau averages 4950 meters in altitude and low latitude between 29 and 37 degrees N. The mean annual air temperature ranges between -2.0 and -6.0 degrees Celsius. In contrast to the alpine environment, precipitation is low, being more similar to high latitudes, because the Himalayas act as a barrier to the deep penetration of moist air from the south. Nevertheless, almost all precipitation falls between May and September. Continuous Permafrost is widespread as it is clearly visible in the image above.
So far, the best estimate of the permafrost area in the Northern Hemisphere is 13.9 million square kilometers representing the total area where the mean annual ground temperature is below 0 degrees Celsius. This extent corresponds to 14.6% of the exposed land area.
Average mean annual ground temperature for the Northern Hemisphere according to a recent paper by Jaroslav Obu et al.
There is indeed a lot of concern related to Permafrost degradation, mainly because massive positive feedback related to CO2 and Methane release in the atmosphere might eventually speed up Global Warming. As the global surface temperature has continued to rise over the recent decades, the risk of permafrost degradation has also increased.
The degradation of the permafrost may affect the climate system via many factors such as local ecological balance, hydrological processes, energy exchange, and the carbon cycle, as well as the engineering infrastructure in cold regions and even extreme weather events.
We will talk about this and much more related to periglacial environments and permafrost in other sections. So make sure to bookmark our page, to stay up-to-date with the latest educational articles on nature and weather in general.