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National Assessment of
Thawing and Melting
Throughout Alaska, the landscape and human activities are fundamentally affected by the presence of ice, snow, and permafrost. All components of the cryosphere (the frozen portions of the Earth) in the Arctic are experiencing change, including snow cover, mountain and continental glaciers, permafrost, sea ice, and lake and river ice. For example, glaciers in Alaska, as throughout the Arctic, have retreated through most of the 20th century. Estimated losses in Alaskan glaciers are of the order of 30 feet in thickness over the past 40 years, even while some have gained thickness in their upper regions. Melting of glaciers is contributing to rising sea levels worldwide, while melting of Alaskan glaciers can have pronounced regional effects through the contribution of their runoff to ocean currents and marine ecosystems in the Gulf of Alaska and Bering Sea.
Thawing of permafrost, retreat and thinning of sea ice, and reduction of the river and lake-ice seasons are underway and are projected to continue. Permafrost is a frozen layer of soil of variable depth found beneath Earth's surface in frigid regions. Ice-rich permafrost is found beneath much of Alaska (on the order of 80%) and supports much of the state's infrastructure and natural ecosystems. Although its name suggests the permanence of this crucial subterranean layer, recent thawing of Alaska's permafrost proves that it too is subject to the effects of climate change. Thawing and melting are likely to continue to bring widespread changes in ecosystems, increased erosion, harm to subsistence livelihoods, and damage to buildings, roads, and other infrastructure. Infrastructure includes installations and facilities necessary to support essential services. The term also encompasses the resources available to many in modern society, such as food, water, habitation, waste disposal, power, transportation, communication, education, health care, safety, and other goods and services provided to a community.
In fact, there are already numerous ecosystem changes observed due to permafrost thawing. They include: destruction of trees and loss of boreal forests; expansion of thaw lakes, grasslands, and wetlands; loss of habitat for caribou and terrestrial birds and mammals; additional habitat for aquatic birds and mammals; increased coastal and riverine (along the banks of rivers) erosion; blocking of streams important for salmon spawning; increased slope and soil instability, landslides, erosion; and development of talik (a year-round thawed layer of what was formerly permafrost), and increased water table depth.
In the longer term, longer ice-free seasons are likely to bring substantial benefits to marine transport and offshore operations in the petroleum industry, and will likely have major implications for international trade and national defense.
The discussion here concentrates on melting and thawing of permafrost and sea ice, where the ecological, hydrological, economic, and social effects of these changes to the cryosphere will be large and the impacts on people and ecosystems were judged to be most direct and important. Strategies that could be helpful in addressing these impacts conclude the discussion.
Permafrost underlies about 85% of Alaska, everywhere except for a narrow belt along the southern coast. It varies widely, in depth, continuity, and ice content. Permafrost has profound effects on hydrology (the land-based water cycle), erosion, vegetation, and human activities. It limits movement of ground water and the rooting depth of plants. On slopes, it allows fluid-like movement of surface soil and deposits. Seasonal thawing over continuous permafrost creates a saturated surface layer in which pools of meltwater accumulate, conducive to marsh and tundra ecosystems and peat formation. Thawing permafrost creates thermokarst terrain -- uneven surface topography that includes pits, troughs, mounds, and depressions, which can fill with water. Thermokarst damages agricultural fields and ecosystems such as forests by drying in mounded areas and flooding in low-lying zones. Further, it can contribute to erosion and increased sedimentation and siltation of rivers, which poses additional environmental concerns.
Permafrost in Alaska has been warming for more than a century. Continuous permafrost on the North Slope of Alaska has warmed 4-7°F over the last century. Because temperatures at the upper surface of continuous permafrost are still low, typically below 23°F, no significant loss of continuous permafrost is projected over the 21st century. The discontinuous permafrost to the south is warmer, usually above 28°F and increased warming at multiple sites (from 1 -- 3°F since the late 1980s) suggests that much of the discontinuous permafrost south of the Yukon River and on the south side of the Seward Peninsula must already be thawing. At some sites, the discontinuous permafrost is thawing at the top and the bottom.
With climate change, warming ranging from 3 -- 18° F over the next 100 years is possible and will be accompanied by more precipitation in the summer and winter. Thawing of Alaska's discontinuous permafrost will likely result from continued warming and predicted increases in snow depth. This thawing, resulting in warmer soils, will speed decomposition reactions and release carbon dioxide and methane, both greenhouse gases, into the atmosphere. Thawing of any permafrost increases groundwater mobility, increases susceptibility to erosion and landslides, and can affect soil storage of carbon dioxide. If the thawed soil drains and dries it could release carbon dioxide to the atmosphere, or storage of carbon dioxide in the soil could increase if the soil remains flooded.
In the central Canadian Arctic, a general northward retreat of the southernmost margin of discontinuous permafrost by about 60 miles over the 20th century has been reported. Continued climate warming is highly likely to bring accelerated thawing of warm discontinuous permafrost. Over the 21st century the top 30 feet is likely to thaw throughout much of the ice-rich discontinuous permafrost zone, although complete thawing is likely to take centuries. Canadian studies have projected that even present surface temperatures will cause an eventual further retreat of the southernmost permafrost fringe in the Canadian subarctic by 60 to 100 miles. Further retreat of 200 to 300 miles could be anticipated under doubled-CO2 equilibrium. The actual pattern of loss of permafrost will, of course, be more complex than a simple uniform retreat.
Because annual average temperatures in much of the southern portion of the state are near 32°F (0°C) -- e.g., 28°F (-2°C) in Fairbanks -- a small warming can transform the landscape through thawing of permafrost, melting of ice, and reduction of snow cover. Thawing is likely to benefit some activities (e. g., construction, transport, and agriculture) after it is completed, but the transitional period of decades or longer is likely to bring many disruptions and few benefits.
Building on permafrost can incur a significant cost because it requires that structures be stabilized in permanently frozen ground below the active layer, and that they limit their heat transfer to the ground, usually by elevating them on piles. For example, to prevent thawing of permafrost from transport of heated oil in the Trans-Alaska pipeline, 400 miles of pipeline were elevated on thermosyphon piles (to keep the ground frozen), at an additional cost of $800 million. The pipeline was completed at a cost of $7 billion because of ice-rich permafrost along the route. This figure is eight times the estimated cost of installing the traditional in-ground pipeline.
Breaks in the pipeline and other repair costs due to melting permafrost could become even more significant in the future. The near-term risk of disruption to operations of the Trans-Alaska pipeline is judged to be small, although costly increases in maintenance due to increased ground instability are likely. The pipeline's support structures are designed for specific ranges of ground temperatures, and are subject to heaving or collapse if the permafrost thaws. Replacing them, if required, would cost about $2 million per mile.
Where permafrost has a high ice content, typically in about half the area of discontinuous permafrost, thawing can induce severe, uneven sinking of the surface, called thermokarst, observed in some cases to exceed 16 feet. This subsidence or sinking from thawing can also destroy the substrate (base) of present ecosystems, destroying them or transforming them to other types of ecosystems, for example, changing forests to grasslands or bogs. Forests that have collapsed from permafrost melting are sometimes referred to as “drunken forests. -- Where large-scale thawing of ground ice has occurred, the landscape has been transformed through mudslides, formation of flat-bottomed valleys, and formation of melt ponds, which can enlarge for decades to centuries.
Thawing of ice-rich discontinuous permafrost has already damaged houses, roads, airports, pipelines, and military installations; required costly road replacements and increased maintenance expenditures for pipelines and other infrastructure; and increased landscape erosion, slope instabilities and landslides. Because of melting permafrost, buildings already have been abandoned, including homes, a radio transmitter site near Fairbanks, and a hospital at Kotzebue, to name a few. The impact on subsistence communities has also been seen, is expected to increase, and is difficult to quantify in dollars. Alaska's warming climate has, for example, thawed traditional ice cellars in several northern villages, rendering them useless.
Present costs of thaw-related damage to structures and infrastructure in Alaska have been estimated at about $35 million per year, of which repair of permafrost-damaged roads is the largest component. Longer seasonal thaw of the active layer could disrupt petroleum exploration and extraction and increase associated environmental damage in the tundra, by shortening the season for minimal-impact operations on ice roads and pads.
Solid-waste generation could increase as communities impacted by climate change abandon and rebuild infrastructure. The availability of landfill sites would increase over time if Alaska's interior changes from permafrost to wetlands to settled drier lands.
Human health could suffer as a result of changes in the volume and timing of precipitation and inter-related impacts on thawing permafrost and infrastructure. For example, water-borne diseases could increase through pollution of water supplies by leaching of sewage or solid-waste sites. Compromised water quality could result in outbreaks of diseases now present in northern regions or provide opportunities for new diseases to be introduced to the region.
As permafrost is a prominent feature of the Alaskan landscape, sea ice is a prominent feature of its coasts and the adjoining marine ecosystems. Present for six months along the Bering Sea coast and ten months along most of the Chukchi and Beaufort Seas, sea ice strongly influences coastal climate, ecosystems, and human activities in the region. The area of Arctic sea ice varies up to 50% seasonally, and also shows strong year-to-year variation. Significant reductions in summer sea ice, which have been proposed as an early signal of global climate change, are evident in recent decades. Recent reports show sea ice area declines of about 3% per decade since the late 1970s. Comparison of two satellite records suggests that the rate of area loss increased from 2.8% per decade in the 1980s to 4.5% per decade in the 1990s. Record low values of summer ice extent have been set repeatedly since 1980.
Arctic sea ice has also grown thinner over the past few decades. Local observations of sea ice thinning by 3.3 to 6.5 feet have been reported for several years. A recent analysis of submarine ice data, however, has provided the first persuasive evidence of large-scale thinning over the entire Arctic basin. A loss of 4.1 feet was found when ice depth from six trans-Arctic submarine cruises from 1958 to 1976 was compared with three similar cruises between 1993 and 1997. In addition to the 4.1 feet of average thinning between the two sets of cruises, the recent cruises also found continued thinning at a rate of 4 inches per year from 1993 to 1997. Evidence of widespread sea-ice melting is corroborated by substantial recent increase in freshwater content of the Arctic Ocean -- on the order of three times more freshwater. Under further climate change, further large reductions in sea ice are projected, although there is substantial variation in estimates of the amount and timing. Most models suggest large reductions (to complete losses) in Arctic summer sea ice area accompanied by an increase in the duration of the open-water season by 2100.
Sea-ice retreat allows larger storm surges to develop in the increased open-water areas, increasing erosion from increased waves, sedimentation, and the risk of inundation in coastal areas. Moreover, coastline where permafrost has thawed is made more vulnerable, which in combination with increased wave action can cause severe erosion.
Local coastal losses to erosion of the order of 100 feet per year have been observed in some locations in both Siberia and Canada. Aerial photo comparison has revealed total erosive losses up to 1,500 feet over the past few decades along some stretches of the Alaskan coast. Several villages on Alaska's west coast are sufficiently threatened by increased erosion and inundation that they must be protected or relocated. Present plans include constructing a $4-6 million sea wall in Shishmaref (a 10-15 year interim solution), and relocating Kivalina on higher ground at an estimated cost of $54 million. An increase in natural disasters resulting from climate change could increase insurance costs, especially for coastal properties, limiting the creation of new infrastructure and jobs.
Loss of sea ice threatens large-scale change in marine ecosystems, including threats to populations of marine mammals and polar bears that depend on the ice, and to the subsistence livelihoods that depend on them. Further retreat of sea ice could also bring some benefits, principally by facilitating water transport as a result of more ice-free days and a longer shipping season including the possibility of routine summer navigation through both the Northeast and Northwest Passages (North of the Eurasian and North American continents). These changes are likely to have major implications for both trade and national security.
The effect on Alaska's economy from yet other impacts is less clear. For example:
While no adaptation options are likely to be available for terrestrial ecosystems threatened by permafrost thawing, or marine ecosystems threatened by sea-ice retreat, strategic planning and research could mitigate some of the potential impacts to the human/built environment of climate change from thawing and melting. Options could include:
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