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date: 17 February 2018

Arctic Warming

Source:
The Oxford Companion to Global Change
Author(s):

Susan Joy Hassol,

Robert W. Corell

Arctic Warming 

Climate change is being experienced particularly intensely in the Arctic. The average temperature in the Arctic has, in the past few decades, risen at almost twice the rate as that of the rest of the world. Widespread melting of glaciers and sea ice and thawing of permafrost are additional evidence of strong arctic warming (ACIA, 2004, 2005). These changes in the Arctic provide an early indication of the environmental and societal significance of global warming.

An acceleration of these climatic trends is projected to occur during this century, caused by increases in the concentrations of greenhouse gases in the Earth's atmosphere. While greenhouse gas emissions do not originate primarily in the Arctic, they are projected to bring wide-ranging changes to the region. These Arctic changes will, in turn, impact the planet as a whole. For this reason, people outside the Arctic have a great stake in what is happening there. For example, climatic processes unique to the Arctic have significant effects on global and regional climate. The Arctic also provides important natural resources to the rest of the world (such as oil, gas, and fish) that will be affected by climate change. The melting of land-based ice in the Arctic is one of the factors contributing to global sea-level rise.

Climate change is also projected to have major effects inside the Arctic, some of which are already under way. Whether a particular impact is perceived as negative or positive often depends on one's interests. For example, the reduction in sea ice is very likely to have devastating consequences for polar bears, ice-dependent seals, and local people for whom these animals are a primary food source. On the other hand, reduced sea ice is likely to increase marine access to the region's resources, expanding opportunities for shipping and possibly for offshore oil extraction (although operations could be hampered initially by increasing movement of ice in some areas). Further complicating the issue, possible increases in environmental damage that often accompanies shipping and resource extraction could harm the marine habitat and negatively affect the health and traditional lifestyles of indigenous people.

Climate change is taking place concurrently with many other changes in the Arctic, including an increase in chemical contaminants entering the Arctic from other regions, overfishing, land-use changes that result in habitat destruction and fragmentation, an increase in ultraviolet radiation reaching the surface, rapid growth in the human population, and cultural, governance, and economic changes. Impacts on the environment and society result not from climate change alone, but from the interplay of all of these changes. The combination of climate change and other stresses presents a range of potential problems for human health and well-being as well as risks to other Arctic species and ecosystems.

Changes in Arctic climate including shorter warmer winters, increased precipitation, and substantial decreases in snow and ice cover, are projected to persist for centuries. Unexpected and even larger changes are also possible.

Why the Arctic Warms Faster Than Lower Latitudes

First, as Arctic snow and ice melt, the darker land and ocean surfaces that are revealed absorb more of the Sun's energy, increasing warming. Second, in the Arctic, a greater fraction of the extra energy received at the surface due to increasing concentrations of greenhouse gases goes directly into warming the atmosphere, whereas in the tropics, a greater fraction goes into evaporation. Third, the depth of the atmospheric layer that has to warm in order to cause warming of near-surface air is much shallower in the Arctic than in the tropics, resulting in a larger Arctic temperature increase. Fourth, as warming reduces the extent of sea ice, solar heat absorbed by the oceans in the summer is more easily transferred to the atmosphere in the winter, making the air temperature warmer than it would be otherwise.

Observed and Possible Future Changes in Arctic Climate

Increasing temperatures, melting glaciers, reductions in the extent and thickness of sea ice, thawing permafrost, and rising sea level all provide strong evidence of recent warming in the Arctic.

Increasing temperatures and precipitation

There are regional variations due to atmospheric winds and ocean currents, with some areas showing more warming than others and a few areas even showing a slight cooling; but for the Arctic as a whole, there is a clear warming trend. Annual average temperature changes range from a 2–3°C warming in Alaska and Siberia to a cooling of up to 1°C in southern Greenland. There are also patterns within this overall trend; for example, in most places, temperatures in winter are rising more rapidly than in summer. In Siberia, Alaska, and western Canada, winter temperatures have increased as much as 4°C in the past 50 years.

Precipitation has increased by roughly 8% across the Arctic over the past 100 years. In addition to the overall increase, there have been changes in the characteristics of precipitation. Much of the increase appears to be coming as rain, mostly in winter, and to a lesser extent in autumn and spring. The increasing winter rains, which fall on top of existing snow, cause faster snowmelt and, when the rainfall is intense, can result in flash flooding. Rain-on-snow events have increased significantly across much of the Arctic, for example, by 50% over the past 50 years in western Russia. Snow-cover extent over Arctic land areas has declined by about 10% over the past 30 years, with much of the decrease taking place in spring, resulting in a shorter snow-cover season.

Global climate model projections suggest that the Arctic will warm roughly twice as much as the globe over the course of the twenty-first century. Under one moderate emissions scenario run by five climate models, this would result in about 3 to 5°C of warming over land areas and up to 7°C over the oceans by 2100, averaged over the Arctic region (see Figure 1; the full set of emissions scenarios suggests a much wider range of possible outcomes). Winter temperature increases are projected to be substantially greater than the annual averages, with warming of 4 to 7°C over land areas and 7 to 10°C over the oceans for a moderate emissions scenario. Higher emissions scenarios yield larger increases.

Precipitation is also projected to increase significantly. Over the Arctic as a whole, annual total precipitation is projected to increase by about 20% by the end of the twenty-first century, with most of the increase coming as rain. The overall increase is projected to be most concentrated over coastal regions and in winter and autumn; increases in these seasons are projected to exceed 30%.

Snow-cover extent, which has already declined by 10% over the past 30 years, is projected to decline an additional 10–20% before the end of the twenty-first century. The decreases in snow-covered area are expected to be greatest in April and May, suggesting a further shortening of the snow season and an earlier pulse of river runoff to the Arctic Ocean and coastal seas. Important snow quality changes are also projected, such as an increase in thawing and freezing in winter that leads to ice formation which in turn restricts the access of some land animals to food and nesting sites.

Declining sea ice

Arctic sea ice is a key indicator and agent of climate change, affecting surface reflectivity, cloudiness, humidity, exchanges of heat and moisture at the ocean surface, and ocean currents. Changes in sea ice also have enormous environmental, economic, and societal implications. [See Arctic Sea Ice.]

Over the past 30 years, the annual average Arctic sea-ice extent has decreased by about 8%, or nearly one million square kilometers, an area larger than all of Norway, Sweden, and Denmark combined, and the melting trend is accelerating. Sea-ice extent in summer has declined far more dramatically than the annual average and is now declining at about 10% per decade or 72,000 square kilometers (28,000 square miles) per year. The rate of melting is accelerating. The 2007 record low sea-ice extent shattered the previous 2005 record low by 23%, and was 39% below the long-term average from 1970–2000. Scientists monitoring the sea ice now conclude the Arctic Ocean could be ice-free in summer by 2030 (NSIDC, 2007). Sea ice also has become thinner in recent decades, with Arctic-wide average thickness reductions estimated at 10–15%, and with particular areas showing reductions of up to 40% between the 1960s and the late 1990s.

Continuing declines in annual average sea-ice extent are projected, with the loss of sea ice in summer projected to be even greater (see projected shift in summer sea-ice boundary in Figure 2). Some recent analyses suggest a nearly complete disappearance of summer sea ice as early as the middle of this century.

Impacts of Arctic Climate Change on the Globe

Because the Arctic plays a special role in global climate, Arctic changes have global implications. Here we focus on three of these: increased global warming due to a reduction in Arctic surface reflectivity, increases in global sea level due to melting of land-based ice in the Arctic, and possible releases of greenhouse gases from permafrost and ocean sediments. Arctic changes will also reverberate throughout the rest of the planet through potential alterations in ocean circulation patterns and changes in the availability of Arctic resources including oil, gas, fish, and migratory bird habitat.

Increased global warming due to reduced surface reflectivity

The bright white snow and ice that cover much of the Arctic reflect back to space most of the solar energy that reaches the surface. As greenhouse gas concentrations rise and warm the lower atmosphere and surface, snow and ice form later in the autumn and melt earlier in the spring. The melting of snow and ice reveals the land and water surfaces beneath, which are much darker and thus absorb more of the Sun's energy. This warms the surface further, causing faster melting, which in turn causes more warming, and so on, creating a self-reinforcing cycle that amplifies warming. This process is already underway in the Arctic, and accelerates global warming. [See Albedo.]

Sea-level rise

Climate change causes sea level to rise by affecting both the density and the amount of water in the oceans. First and most significantly, water expands as it warms, and less-dense water takes up more space. Second, warming increases melting of glaciers (land-based ice), adding to the amount of water flowing into the oceans.

The total volume of land-based ice in the Arctic corresponds to a global sea level equivalent of about eight meters. Most Arctic glaciers have been in decline since the early 1960s, with this trend accelerating in the 1990s. Projections from global climate models suggest that the contribution of arctic glaciers to global sea-level rise will accelerate over the next 100 years.

The Greenland ice sheet dominates land ice in the Arctic. [See Greenland Ice Sheet.] The area of surface melt on the ice sheet has been increasing rapidly in recent decades and studies suggest that Greenland is losing ice mass much more rapidly than anticipated.

Beyond this century, the Arctic contribution to global sea-level rise is projected to grow as ice sheets continue to respond to climate change and to contribute to sea-level rise for thousands of years. Climate models indicate that the local warming over Greenland is likely to be up to two to three times the global average. Ice sheet models project that sustained local warming of that magnitude would eventually lead to a virtually complete melting of the Greenland ice sheet, with a resulting sea-level rise of about seven meters.

Sea-level rise is projected to have serious implications for coastal communities and industries, islands, river deltas, harbors, and the large fraction of people living in coastal areas worldwide.

Release of greenhouse gases from permafrost and ocean sediments

Carbon is currently trapped as organic matter in the permafrost (frozen soil) that underlies much of the Arctic. Large amounts of carbon accumulate in the vast waterlogged peat bogs of Siberia and parts of North America. During the summer, when the surface layer of the permafrost thaws, organic matter in this layer decomposes, releasing methane and carbon dioxide to the atmosphere. Warming increases these releases, and can create an amplifying feedback loop whereby more warming causes additional releases, which causes more warming, and so on (see projected change in the permafrost boundary in Figure 2). The magnitude of these releases is affected by soil moisture and other factors.

Vast amounts of methane, in an icy form called methane hydrates or clathrates, are trapped at shallow depths in cold ocean sediments, and to a lesser extent in some permafrost areas on land. [See Methane Hydrates.] If the temperature of this permafrost or the water at the seabed rises beyond certain thresholds, it could initiate the decomposition of these hydrates, releasing methane to the atmosphere and amplifying global warming.

Impacts of Climate Change in the Arctic

Climate-induced changes in Arctic landscapes are important to local people and animals in terms of food, fuel, culture, and habitat. These changes may also have global impacts because many processes operating on Arctic landscapes affect global climate and resources. Some changes in Arctic landscapes are already occurring, and future changes are projected to be considerably greater.

Shifting vegetation zones

The major Arctic vegetation zones include the polar deserts, tundra, and the northern part of the boreal forest. [See Boreal Forests and Climate Change.] Climate change is projected to cause vegetation shifts because rising temperatures favor taller, denser vegetation and will thus promote the expansion of forests into the Arctic tundra (see the projected shift in treeline in Figure 2), and tundra into the polar deserts. The timeframe of these shifts will vary around the Arctic. Where suitable soils and other conditions exist, changes are likely to be apparent in this century. Where they do not, the changes can be expected to take longer. These vegetation changes, along with rising sea levels, are projected to shrink the area of tundra, greatly reducing the breeding area for many birds and the grazing areas for land animals that depend on the open landscape of tundra and polar desert habitats. Not only are some threatened species very likely to become extinct, some currently widespread species are projected to decline sharply.

Many animal species from around the world depend on summer breeding and feeding grounds in the Arctic, and climate change will alter some of these habitats significantly. For example, several hundred million birds migrate to the Arctic each summer, and their success in the Arctic determines their populations elsewhere. Important breeding and nesting areas are projected to decrease sharply as the treeline advances northward, encroaching on tundra, and because the timing of bird arrival in the Arctic might no longer coincide with the availability of their insect food sources. At the same time, sea-level rise will diminish the tundra extent from the north in many areas, further shrinking important habitat. A number of bird species, including several globally endangered seabird species, are projected to lose more than 50% of their breeding area during this century.

Forest disturbances: insects and fires

Increased insect outbreaks due to climate warming are already occurring and are almost certain to continue. Increasing climate-related outbreaks of spruce-bark beetles and spruce budworms in the North American Arctic provide two important examples. Over the past decade, areas of Alaska and Canada have experienced the largest and most intense outbreaks of spruce-bark beetles on record. There has also been an upsurge in spruce-budworm outbreaks, and the entire range of white-spruce forests in North America is considered vulnerable to such outbreaks under projected warming.

Fire will also have pervasive ecological effects. The area burned in boreal western North America has doubled over the past thirty years, and it is forecast to increase by as much as 80% over the next 100 years under projected warming. The area of boreal forest burned annually in Russia averaged four million hectares over the last three decades and more than doubled in the 1990s. As the climate continues to warm, the forest-fire season will begin earlier and last longer. Models of forest fire in parts of Siberia suggest that a summer temperature increase of 5.5°C would double the number of years in which there are severe fires, and increase the area of forest burned annually by nearly 150%.

Animal species

Many animal species will be affected by increasing arctic temperatures. In the marine environment, the sharp decline in sea ice is likely to be devastating to polar bears. Impacts have already been documented at James and Hudson Bays in Canada, the southern limits of the polar bear's distribution. The condition of adult polar bears has declined during the last two decades in the Hudson Bay area, as have the number of live births and the proportion of first-year cubs in the population. Polar bears in that region suffered 15% declines in both average weight and number of cubs born between 1981 and 1998. Other ice-dependent species at risk include ringed seals, walrus, and some species of marine birds.

Terrestrial animal species also face threats from warming. Caribou and reindeer herds depend on abundant tundra vegetation and good foraging conditions, especially during the calving season. Climate-induced changes are projected to reduce the area of tundra and the traditional forage (such as mosses and lichens) for these herds. Freeze-thaw cycles and freezing rain are also projected to increase, reducing the ability of caribou and reindeer populations to obtain food and raise calves. Future climate change could thus mean a decline in caribou and reindeer populations, threatening human nutrition and the whole way of life of some indigenous Arctic communities.

Warming also leads to other cascading impacts on Arctic land animals. For example, in winter, lemmings and voles live and forage in the space between the frozen ground of the tundra and the snow, almost never appearing on the surface. The snow provides critical insulation. Mild weather and wet snow lead to the collapse of the under-snow spaces, destroying the animals’ burrows, while ice-crust formation reduces the insulating properties of the snow pack vital to their survival. Well-established population cycles are no longer seen in some areas. Declines in populations of lemmings, for example, would be very likely to result in even stronger declines in the populations of predators that specialize in preying on them, such as snowy owls, skuas, weasels, and ermine. More generalist predators, such as the Arctic fox, switch to other prey when lemming populations are low. Thus, a decline in lemmings can also indirectly result in a decline in populations of other prey species such as waders and other birds.

Freshwater species face climate-related changes to their environments that include increasing water temperatures, thawing permafrost, and reduced ice cover on rivers and lakes. Southern species are projected to shift northward, competing with northern species for resources. The broad whitefish, Arctic char, and Arctic cisco are particularly vulnerable to displacement as they are fundamentally northern in their distribution. As water temperatures rise, spawning grounds for cold-water species will shift northward and probably diminish. As southern fish species move northward, they may introduce new parasites and diseases to which Arctic fish are not adapted. The implications of these changes for both commercial and subsistence fishing in far northern areas are potentially devastating, as the most vulnerable species are often the only fishable species present.

Coastal erosion

The effects of rising temperatures are already altering the Arctic coastline, and much larger changes are projected to occur during this century as a result of reduced sea ice, thawing permafrost, and sea-level rise. Thinner, less extensive sea ice creates more open water, allowing the wind to generate larger waves, thus increasing wave-induced erosion along Arctic shores. Sea-level rise, increasing storm surge heights, and thawing of coastal permafrost exacerbate this problem. Dozens of Arctic communities are threatened by these changes, and some are already planning to relocate. Hundreds more could be at risk in the future. The costs of protecting or relocating these communities will be enormous. Coastal erosion will also pose increasing problems for some ports, tanker terminals, and other industrial facilities around the Arctic.

Thawing permafrost

Transportation and industry on land, including oil and gas extraction and forestry, will be increasingly disrupted by the shortening of the periods during which ice roads and tundra are frozen sufficiently to permit travel. For example, warming has caused the number of days per year in which travel on the tundra is allowed under Alaska Department of Natural Resources rules to drop from over 200 to about 100 in the past 30 years, resulting in a 50% reduction in days that oil and gas exploration and extraction equipment can be used. In addition, as frozen ground thaws, many existing buildings, roads, and pipelines are likely to be destabilized, requiring costly repair and replacement (see the projected shift in permafrost boundary in Figure 2). Projected warming and its effects will need to be taken into account in the design of all new construction, thus increasing costs.

Marine access

Observed and projected reductions in sea ice suggest that the Arctic Ocean will have longer seasons of less sea-ice cover of reduced thickness, implying improved ship accessibility around the margins of the Arctic Basin. As summer sea ice retreats further from most Arctic landmasses, new shipping routes will open, and the period during which shipping is feasible via existing routes will expand (see the projected change in summer sea ice boundary in Figure 2).

The Northern Sea Route north of Eurasia could provide up to a 40% savings in distance for journeys from northern Europe to northeastern Asia and the northwest coast of North America compared to southern routes via the Suez or Panama canals. The navigation season for the Northern Sea Route is projected to increase from the current 20–30 days per year to 90–100 days by 2080; and for ships with ice-breaking capability the season could expand to 150 days. This could have major implications for transportation as well as for access to natural resources.

On the Canadian side of the Arctic, home to the fabled Northwest Passage, near-term benefits are less clear. Recent sea ice changes could, in fact, make the Northwest Passage less predictable for shipping. Studies indicate that sea ice conditions in the Canadian Arctic during the past three decades have had high year-to-year variability, making planning for transport very difficult. In addition, research suggests a warming climate could lead to more icebergs and greater ice movement in the Northwest Passage, presenting additional hazards to navigation. Thus, despite widespread retreat of sea ice around the Arctic Basin, the Canadian Arctic Archipelago is likely to have complex and challenging ice conditions for the decades ahead. [See Arctic Sea Ice.]

While increased marine access to Arctic resources will benefit some, it will also raise new issues relating to sovereignty, security, and safety. For example, the risk of oil spills and other industrial accidents in the challenging Arctic environment raises concerns.

Indigenous communities

Across the Arctic, indigenous people are already reporting the effects of climate change. Local landscapes, seascapes, and ice-scapes are becoming unfamiliar. Climate change is occurring faster than indigenous knowledge can adapt and is strongly affecting people in many communities. Unpredictable weather, snow, and ice conditions make travel hazardous, endangering lives. Impacts of climate change on wildlife, from caribou on land, to fish in the rivers, to seals and polar bears on the sea ice, are having enormous effects, not only on the diets of indigenous peoples, but also on their cultures and their very identities.

Assessment

Despite the fact that a relatively small percentage of the world's greenhouse gas emissions originate in the Arctic, changes in Arctic climate are among the largest on Earth. As a consequence, the changes already underway in Arctic landscapes, communities, and unique features provide an early indication for the rest of the world of the environmental and societal significance of global climate change. Changes in climate and their impacts in the Arctic are already being widely noticed and felt, and are projected to become much greater. These changes will also reach far beyond the Arctic, affecting global climate, sea level, biodiversity, and many aspects of human social and economic systems.

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Susan Joy Hassol and Robert W. Corell