The Arctic

The Arctic is the northernmost region of the Earth, centred on the North Pole and characterized by distinctively polar conditions of climate, plant and animal life, and other physical features. The term is derived from the Greek arktos ("bear"), referring to the northern constellation of the Bear. It has sometimes been used to designate the area within the Arctic Circle—a mathematical line that is drawn at latitude 66°30' N, marking the southern limit of the zone in which there is at least one annual period of 24 hours during which the sun does not set and one during which it does not rise. This line, however, is without value as a geographic boundary, since it is not keyed to the nature of the terrain.

While no dividing line is completely definitive, a generally useful guide is the irregular line marking the northernmost limit of the stands of trees. The regions north of the tree line include Greenland (Kalaallit Nunaat),

Svalbard, and other polar islands; the northern parts of the mainlands of Siberia, Alaska, and Canada; the coasts of Labrador; the north of Iceland; and a strip of the Arctic coast of Europe. The last-named area, however, is classified as subarctic because of other factors.

Conditions typical of Arctic lands are extreme fluctuations between summer and winter temperatures; permanent snow and ice in the high country; grasses, sedges, and low shrubs in the lowlands; and permanently frozen ground (permafrost), the surface layer of which is subject to summer thawing. Three-fifths of the Arctic terrain is outside the zones of permanent ice. The brevity of the Arctic summer is partly compensated by the long daily duration of summer sunshine.

International interest in the Arctic and subarctic regions has steadily increased during the 20th century, particularly since World War II. Three major factors are involved: the advantages of the North Pole route as a shortcut between important centres of population, the growing realization of economic potentialities such as mineral (especially petroleum) and forest resources and grazing areas, and the importance of the regions in the study of global meteorology

Continentai Ice Sheets of the Past

Little is known about the climate of the northern lands in early Cenozoic times. It is possible that the tree line was at least 1.6 km (1,000 miles) farther north than at present. During the Cenozoic, however, the polar lands became cooler and permanent land ice formed, first in the Alaskan mountain ranges and subsequently, by the end of the Pliocene (2.6 million years ago), in Greenland. By the onset of the Quaternary Period, glaciers were widespread in northern latitudes. Throughout the Quaternary, continental-scale ice sheets expanded and decayed on at least eight occasions in response to major climatic oscillations in high latitudes. Detailed information available for the final glaciation (80,000 to 10,000 years ago) indicates that in North America the main ice sheet developed on Baffin Island and swept south and west across Canada, amalgamating with smaller glaciers to form the Laurentide Ice Sheet, covering much of the continent between the Atlantic Ocean and the Rocky Mountains and between the Arctic Ocean and the Ohio and Missouri river valleys. A smaller ice cap formed in the Western Cordillera. The northern margin of the ice lay along the Brooks Range (excluding the Yukon Basin) and across the southern islands of the Canadian Archipelago. To the north the Queen Elizabeth Islands supported small, probably thin, ice caps. Glacier ice from Greenland crossed Nares Strait to reach Ellesmere Island during maximum glaciation.

The Atlantic Arctic islands were covered with ice except where isolated mountain peaks (nunataks) projected through the ice. In Europe the Scandinavian Ice Sheet covered most of northern Europe between Severnaya Zemlya in Russia and the British Isles. Northeastern Siberia escaped heavy glaciation, although, as in northern Canada, the ice sheet had been more extensive in an earlier glaciation.

As the ice sheets melted, unique landforms developed by the ice were revealed. Although not restricted to the present Arctic, they are often prominent there and, in the absence of forests, are clearly visible. In areas of crystalline rocks, including large parts of the northern Canadian Shield and Finland, the ice left disarranged drainage and innumerable lakes. In the lowlands deep glacial deposits filled eroded surfaces and produced a smoother landscape, often broken by low ridges and hills of glacial material, drumlins, rogen (ribbed) moraines, and eskers. In the uplands the characteristic glacial landforms are U-shaped valleys. Near the polar coasts these have been submerged to produce fjords, which are well developed in southern Alaska, along the east coast of Canada, around Greenland, in east and west Iceland, along the coast of Norway, and on many of the Arctic islands.

Because of their enormous weight, continental ice sheets depress the Earth's crust. As the ice sheets melted at the close of the Pleistocene Epoch (11,700 years ago), the land slowly recovered its former altitude, but before this was completed the sea flooded the coastal areas. Subsequent emergence has elevated marine beaches and

Polar bears walk along the rocky shores of Sillem Island in Canada. The weight of ice sheets and flooding over time has resulted in raised beaches in many Arctic locations. Pete Ryan/National Geographic/Getty Images

sediments to considerable heights in many parts of the Arctic, where their origin is easily recognized from the presence of marine shells, the skeletons of sea mammals, and driftwood. The highest strandlines are found 500 to 900 feet above contemporary sea level in many parts of the western and central Canadian Arctic and somewhat lower along the Baffin Bay and Labrador coasts. Comparable emergence is found on Svalbard, Greenland, the northern Urals, and on the Franz Josef Archipelago, where it reaches more than 1,500 feet. In many emerged lowlands, such as those south and west of Hudson Bay, the raised beaches are the most conspicuous features in the landscape, forming hundreds of low, dry, gravel ridges in the otherwise ill-drained plains. Emergence is still continuing, and in parts of northern Canada and northern Sweden uplift of two to three feet a century has occurred during the historical period. In contrast, a few Arctic coasts, notably around the Beaufort Sea, are experiencing submergence at the present time.

Polar continental shelves in areas that escaped glaciation during the ice ages were exposed during periods of low sea level, especially in the Bering Strait and Sea (Beringia), which facilitated migration of people to North America from Asia, and in the Laptev and East Siberian seas.

Terrain

Although the detail of the terrain in many parts of the Arctic is directly attributable to the Pleistocene glaciations, the major physiographic divisions reveal close correlation with geologic structure. The two largest shield areas, the Canadian and the Baltic, have developed similar landscapes. West of Hudson Bay, in southwestern Baffin Island, and in Karelia the land is low and rocky with countless lakes and disjointed drainage. Uplands, generally

1,000 to 2,000 feet above sea level and partially covered with glacial deposits, are more widely distributed. They form the interior of Quebec-Labrador and parts of the Northwest Territories in Canada, and the Lapland Plateau in northern Scandinavia. The eastern rim of the Canadian Shield in Canada from Labrador to Ellesmere Island has been raised by crustal changes and then dissected by glaciers to produce fjords that separate mountain peaks more than 6,000 feet high. The surface of the shield in Greenland has the shape of an elongated basin, with the central part, which is below sea level, buried beneath the Greenland ice cap. Around the margins, on the east and west coasts, the mountainous rim is penetrated by deep troughs through which local and inland-ice glaciers flow to the sea. The mountains are highest in the east, where they exceed 10,000 feet.

In shield areas where sedimentary rocks mantle the crystalline variety, as in north-central Siberia, the southern sector of the Canadian archipelago, and Peary Land, the topography varies from plains to plateaus, with the latter deeply dissected by narrow valleys. Far beyond the margins of the shields, extensive plains have evolved on soft sedimentary rocks. In North America these form the Mackenzie Lowlands, Banks and Prince Patrick islands, and the Arctic Plains section of northern Alaska; in northern Europe they form the Severnaya Dvina and Pechora Plains. In Siberia the Ob delta, its northeastern extension to the Laptev Sea, the North Siberian Lowland, the West Siberian Plain, and farther east the Lena-Kolyma plains (including the New Siberian Islands) have also developed on sedimentary rocks. Although there are differences in degree, these terrains are essentially flat, occasionally broken by low rock scarps, and covered with numerous shallow lakes. The plains are crossed by large rivers that have laid down deep alluvial deposits.

The strongly folded rocks associated with the two oro-genic periods in the Arctic form separate physiographic regions. The original mountains of the older, Paleozoic folding were long ago destroyed by erosion, but the rocks have been elevated in recent geologic time, and renewed erosion, often by ice, has produced a landscape of plateaus, hills, and mountains very similar to the higher parts of the shields. In Ellesmere Island the mountains are nearly 10,000 feet high. In Peary Land and Spitsbergen, maximum elevations are about 6,000 feet, while in eastern Svalbard and on Novaya Zemlya and Severnaya Zemlya the uplands rarely exceed 2,000 feet. The younger groups of fold mountains of northeast Siberia and Alaska are generally higher. Peaks of 10,000 feet are found in the Chersky Mountains, 15,000 feet in Kamchatka, and even higher in southern Alaska. Characteristic of this physiographic division are wide intermontane basins drained by large rivers, including the Yukon and Kolyma.

Throughout the Arctic, excluding a few maritime areas, the winter cold is so intense that the ground remains permanently frozen except for a shallow upper zone, called the active layer, which thaws during the brief summer. Permanently frozen ground (permafrost) covers nearly one-quarter of the Earth's surface. In northern Alaska and Canada scattered observations suggest that permafrost is 800 to 1,500 feet deep; it is generally deeper in northern Siberia. The deepest known permafrost is in northern Siberia, where it exceeds 2,000 feet. The depth of the permafrost depends on the site, climate, vegetation, and recent history of the area, particularly whether it was covered by sea or glacier ice. Very deep permafrost was probably formed in unglaciated areas during the extreme cold of the ice ages. To the south in the subarctic, the permafrost thins and eventually becomes discontinuous, although locally it may still be 200 to 400 feet thick; along its southern boundary, permafrost survives under peat and in muskeg. In areas of continuous permafrost the active layer may be many feet thick in sandy well-drained soils with little vegetation but is usually less than six inches thick beneath peat.

Permafrost occurs in both bedrock and surface deposits. It has little effect in most rocks, but in fine-grained, unconsolidated sediments, particularly silts, lenses of ice, called ground ice, grow by migration of moisture, and in extreme cases half the volume of Arctic silts may be ice. Ground ice is often exposed in riverbanks and sea cliffs, where it may be 6 to 9 metres (20 to 30 feet) thick. In northern Siberia fossil ice has been reported up to 60 metres (200 feet) thick, although it may be glacier or lake ice that has subsequently been buried under river deposits. If ground ice melts, owing to a change in climate, hollows develop on the surface and quickly fill with water to form lakes and ponds. When frozen the silts have considerable strength, but if they thaw they change in volume, lose their strength, and may turn to mud. Variations in volume and bearing capacity of the ground due to changes in the permafrost constitute one of the major problems in Arctic construction.

Drainage and Soils

Continuous permafrost inhibits underground drainage. Consequently, shallow lakes are numerous over large areas of the Arctic, and everywhere in early summer there is a wet period before the saturated upper layers of the ground dry out. During the summer waterlogged active layers on slopes may flow downhill over the frozen ground, a phenomenon known as solifluction. It is ubiquitous in the Arctic but is particularly intense where the soils are finegrained, as in the coastal plain of northern Alaska, or where the precipitation is heavy, as on Bear Island in the

Norwegian Sea. The effect of solifluction is to grade slopes so that long, smooth profiles are common; slopes are normally covered with vegetation, but if the soil movement is too rapid plants may not be able to survive. Under these conditions the surface material is often graded, with narrow strips of pebbles and boulders separated by broader strips of finer particles.

The surface of many soils in northern areas show distinctive patterns produced by complex processes of freezing and thawing, which cause frost heaving and sorting of debris; although permafrost is not essential to these formations, it is usually present. There are many different types of patterned ground. In some, coarser material, pebbles, and boulders form polygonal nets, with the finer materials concentrated in the centre. When sorting is widely spaced, stone circles develop. Another variety of pattern, formed in sands and muds, is outlined by frost-crack fissures or strips of vegetation. Individual polygons vary from about half a metre (1.6 feet) to more than 90 metres (300 feet) in diameter. Mounds due to frost heaving in the soil also are widespread. They grow rapidly, disrupting leveled fields in a few years and limiting the use of farm machinery for haying. Elsewhere, notably in the Mackenzie valley and in parts of Alaska, removal of the natural vegetation—and, in isolated cases, plowing—has modified the soil climate. The ground ice has thawed, leading to disruption of drainage. Where the ice was wedge-shaped and in polygonal patterns, soil mounds several feet high may result. All Arctic terrains are sensitive to human-induced thermal disturbance, especially by vehicular traffic or oil-pipeline operations, and the preservation of the original soil climate is of great environmental importance.

The largest ice-covered mounds, which may reach 60 metres (200 feet) in height, are known in North America as pingos. Although they are widely distributed in the

Arctic and subarctic, major concentrations are restricted to the Mackenzie delta, the Arctic slope of Alaska, and coastal areas near the deltas of the Ob, Lena, and Indigirka rivers. Submarine landforms resembling pingos are found beneath the Beaufort Sea.

Arctic soils are closely related to vegetation. Unlike soils farther south, they rarely develop strong zonal characteristics. By far the most common are the tundra soils, which are circumpolar in distribution. They are badly drained and strongly acid and have a variable, undecom-posed organic layer over mineral horizons. Some of the drier heath and grassland tundras overlie Arctic brown soils, which have a dark-brown upper horizon with gray and yellowish brown lower horizons. The active layer in the permafrost is normally deep in them.

Many exposed rock surfaces in the Arctic have been broken up by frost action so that the bedrock is buried under a cover of angular shattered boulders. These mantles are known as felsenmeer (German: "sea of rock") and are found principally on Arctic uplands. Their continuity and depth varies with climate, vegetation, and rock type, but they may be as much as 4 metres (12 feet) deep. Felsenmeer are especially well-developed on basalts and are consequently numerous on the basaltic Icelandic plateaus. They also develop quickly on sedimentary rocks and are wides-pread in the Canadian Arctic, where they occur down to sea level.

Present-Day Glaciation

Although the Arctic is commonly thought to be largely ice-covered, less than two-fifths of its land surface in fact supports permanent ice. The remainder is ice-free because of either relatively warm temperatures or scant snowfall. Glaciers are formed when the annual accumulation of snow, rime, and other forms of solid precipitation exceeds that removed by summer melting. The excess snow is converted slowly into glacier ice, the rate depending on the temperature and annual accumulation of snow. In the Arctic, where most glaciers have temperatures far below the freezing point, the snow changes into ice slowly. In northwestern Greenland a hole 427 metres (1,400 feet) deep was drilled into the ice sheet without reaching glacier ice. The hole showed more than 800 annual snow layers, from which it was possible to determine precipitation changes for the past eight centuries. An ice core 1,400 metres (4,560 feet) deep was recovered in the mid-1960s from Camp Century in northwestern Greenland, and a core 2,000 metres (6,683 feet) deep from Dye 3, southeastern Greenland, was recovered in 1981. The ice cores have been analyzed for paleoclimatic and paleoatmospheric information covering the 100,000 years since the last interglacial.

The elevation at which accumulation and melting of glacier ice are equal is known as the equilibrium line and is roughly equivalent to the snow line. It frequently varies greatly over short distances and from year to year on a specific glacier. On Baffin Island the equilibrium line is a little more than 2,000 feet above sea level in the extreme southeast, rising to more than 4,500 feet in the Penny Ice Cap 300 miles to the north and descending to about 2,000 feet in the north of the island. In Greenland the line is at about 6,000 feet in the south and decreases irregularly to about 3,300 feet in the north. The summits of some ice caps are well below the snow line, but they continue to survive because of their low internal temperatures; the winter snowfall melts completely but refreezes in contact with the cold ice before flowing off the glacier. This phenomenon, first observed on the Barnes Ice Cap of Baffin Island, is now known to be widespread in the high Arctic.

GLacier Groups

In Arctic Canada glacier ice is restricted, with few exceptions, to the northeast as a consequence of the greater relief and precipitation around Baffin Bay and Davis Strait. The most southerly ice is found in the Torngat Mountains of northern Labrador, where there are small cirque glaciers at the base of the mountains. Immediately north of Hudson Strait on the plateau south of Frobisher Bay, there are two small ice caps. Larger ice caps and highland ice (through which mountains project) are present farther north along the east of Baffin Island and on Bylot Island; only the Barnes Ice Cap lies west of the coastal group. North of Lancaster Sound the ice is more extensive, and large parts of Devon, Ellesmere, and Axel Heiberg islands are glacier-ized. In many ways these ice caps are small versions of the Greenland Inland Ice, with a central dome-shaped section and outlet glaciers flowing through the mountains toward the sea. The ice cap on Meighen Island, the most westerly of the group, is an exception, as it is circular in shape and lies on low ground. Except for three small glaciers on Melville Island, there are no glaciers in the Canadian western Arctic. Few Canadian glaciers reach the sea and form icebergs. In the Arctic Ocean off northwestern Ellesmere Island there is an area of floating shelf ice that may at one time have been joined by glaciers, but the glaciers no longer reach the sea. This shelf ice has been the principal source of the ice islands of the Arctic Ocean.

Other glaciers are found north and east of the Atlantic Ocean and its continuation in the Norwegian and Barents seas. Iceland has five major ice caps, the largest of which, Vatna Glacier, covers more than 4,800 sq km (3,000 square miles). All have small outlet glaciers, although none reaches the sea. The ice caps owe their survival to heavy snowfall.

A grouping of glaciers on Spitsbergen, the largest island in Norway's Svalbard archipelago. The abundance of glaciers in the area make it a prime ecotourism destination. Chris Jackson/Getty Images

The western part of Vatna Glacier buries a volcano, Grimsvôtn (Grims Depression), which erupts every 6 to 10 years; the heat of the eruption forms a subglacial lake that bursts in great floods over the margins of the glacier.

North of Iceland, Jan Mayen Island supports a glacier on the volcano Mount Beeren. The glaciers of Svalbard cover about 90 percent of the land. On the largest island, Spitsbergen, the plateaus are covered with highland ice from which outlet glaciers reach the sea; there are also numerous independent valley and cirque glaciers. North East Land, the second largest island, supports two ice caps on its plateaus. On the east side of the Norwegian Sea, precipitation is heavy over the Scandinavian highlands, but temperatures are also high, and the total area of ice is only about 3,200 sq km (2,000 square miles), a small part of which is in northern Sweden and the remainder in Norway To the northeast beyond the Barents Sea, precipitation is less, but the summer is shorter and permanent ice is widespread.

Farthest north in this group are the islands of the Franz Josef archipelago. Although at no point are they higher than 762 metres (2,500 feet), probably more than 90 percent of their area is covered with ice; some of the smaller islands are completely buried by glaciers. The southern island of Novaya Zemlya supports a few small glaciers; on the northern island they are more numerous, and the northern four-fifths of the island is ice-covered, with large outlet glaciers reaching the sea. Cyclonic depressions penetrate from the Barents Sea into the Kara Sea beyond Novaya Zemlya and produce sufficient snow for glaciers to form on Severnaya Zemlya. There are four major and many minor islands in the group. Although they are low-lying, consisting primarily ofplateaus less than 610 metres (2,000 feet) high, all the larger islands have ice caps that cover less than half the total area. Outlet glaciers reach the sea and are an occasional source of icebergs. Elsewhere the Russian northern areas are remarkably free of glacier ice. Small cirque glaciers are found in the northern Ural Mountains and the Byrranga Mountains of the Taymyr Peninsula.

The glaciers around the North Pacific are concentrated in Alaska. The glaciers of southern Alaska are Alpine rather than Arctic and include some of the most spectacular mountain glaciers in the world. All types of ice are present, from small valley glaciers to highland ice that almost buries mountain ranges, with piedmont glaciers spreading out in the lowlands. The largest ice fields are around the Fairweather Range, the St. Elias Mountains, and the Chugach Mountains. Glaciers in these areas include the Hubbard, 145 km (90 miles) long, intermontane glaciers such as the Seward, and piedmont glaciers such as the Malaspina. Smaller glaciers also occur inland on the Alaska Range and in the Brooks Range of northern Alaska. There is more ice farther east in the Romanzof Mountains, where one glacier, the Okailak, is 10 miles long, and in a similar situation in the Selwyn and Ogilvie mountains of Canada's Yukon. There are a few small glaciers in the Aleutian Range and on the Aleutian Islands. On the northwest side of the Pacific basin there are small glaciers in the East Siberian Mountains and on the volcanic peaks of the Kamchatka Peninsula.

The overwhelming majority of Arctic glaciers for which precise data are available have experienced negative mass balances (i.e., reduction in mass) in the 20th century broken only by temporary cool phases in the 1960s and 1970s. The effect has been a general retreat of glacier fronts and thinning of ice around the margins. The Greenland Inland Ice may be an important exception to this generalization.

In Iceland, where glacier fluctuations are well recorded, the ice appears to have been restricted from the 10 th until about the 16 th century The ice then advanced, reaching a maximum about 1750. A second advance followed a minor retreat, culminating about 1850, and a major retreat set in about 1890. The recession was slow at first, but by the 1930s it was generally rapid and has continued since, except locally for a brief interruption in the 1970s.

Climate

The climates of polar lands vary greatly depending on their latitude, proximity of the sea, elevation, and topography; even so, they all share certain "polar" characteristics. Owing to the high latitudes, solar energy is limited to the summer months. Although it may be considerable, its effectiveness in raising surface temperatures is restricted by the high reflectivity of snow and ice. Only in the central polar basin does the annual net radiation fall below zero. In winter, radiative cooling at the surface is associated with extreme cold, but, at heights a few thousand feet above the surface, temperatures as much as 20 to 30T (11 to i7°C) warmer can often be found. Temperature inversions such as this occur more than 90 percent of the time in midwinter in northwestern Siberia and over much of the polar basin. They also are common over the Greenland Ice Cap and in the sheltered mountain valleys of the Yukon and Yakutia. The lowest surface temperature ever recorded in North America was observed at Snag, Yukon (-8i°F, -63°C), and even lower temperatures have been observed in Yakutia (-90T, -68°C) and northern Greenland (-94T, -70°C).

It has been customary to divide polar climates into two large groups, those corresponding to the climate of ice caps, in which no mean monthly temperature exceeds 32°F (0°C), and the tundra climates, with at least one month above 32T but no month above 50T (i0°C). A more satisfactory division is to classify them as polar maritime climates, located principally on the northern islands and the adjacent coasts of the Atlantic and Pacific oceans, in which winter temperatures are rarely extremely low and snowfall is high; and the polar continental climates, as in northern Alaska, Canada, and Siberia, where winters are intensely cold and snowfall is generally light. Included in the polar continental climate type are the islands of the Canadian Arctic Archipelago, which are influenced only slightly by the sea in winter because of thick, unbroken sea ice. In addition to these two climates, there are smaller transitional zones, limited areas of "ice" climates, the climate of the polar basin, and, on the south side of the tree line, the subarctic climates.

In the polar continental areas, winter sets in toward the end of August in the far north and about a month later nearer the tree line. Temperatures continue to drop rapidly until about December. January, February, and early March have uniform conditions with mean temperatures about -35 °F (-37 °C) in the central Siberian Arctic and -30 to -20 °F (-34 to -29 °C) in North America. The lowest extreme temperatures in the winter are between -65 and -50 °F (-54 and -46 °C). A better indication of low temperatures as they affect humans is given by the windchill, a measurement of the cooling power of the atmosphere on human skin. It reaches a maximum north of Hudson Bay, where strong and persistent northwest winds, typical of the Canadian eastern Arctic, are combined with low air temperatures. This area is stormy in winter, with moderately high snowfall (1,300 to 2,500 mm [50 to 100 inches]), rapidly changing temperatures, and even occasional rain.

Elsewhere the winter continental climate is quiet, with long periods of clear sky and low snowfall. Visibility may be poor locally if there are open channels of water in the sea ice, and it is universally reduced when the wind blows drifting snow. The lowest snowfall is in the polar deserts of the northern Canadian islands and northern Greenland, where the total annual precipitation is frequently less than the equivalent of four inches of water.

Winter in the maritime Arctic (the Aleutians, coastal southwestern Greenland, Iceland, and the European Arctic) is a period of storminess, high winds, heavy precipitation in the form of either snow or rain (the latter at sea level), and moderate temperatures. The mean temperature of the coldest month is rarely below 20°F (-7°C), and extremely low temperatures are unknown.

Summer temperatures are more uniform across the whole of the Arctic. On the southern margin the monthly mean temperature reaches 50T (i0°C), and in continental situations short spells of hot weather with temperatures in the 80s°F (27-32^), continuous sunshine, and calm weather are not uncommon; such weather often ends with thunderstorms. In the maritime climates, along the coasts, and on the northern islands when there is open water in the sea ice, the summer is relatively cool. In the south the temperatures are about 45T (7°C), decreasing north to 40°F (4°C) or less; a maximum of 60°F (i6°C) is hardly ever reached except at the heads of fjords as in southwestern Greenland, where marine influences are less marked. Fog and low clouds are widespread in maritime areas, and at this time of the year these areas are the cloudiest in the world. In lands that experience continental winters, precipitation is heaviest during the summer months; light rain and snow showers are frequent, but the average fall is low. The summer is everywhere a time of sudden changes. Calm, clear weather with sunshine and temperatures of about 50°F (i0°C) will be followed by sudden winds, often causing a temperature drop of 20 to 30T (11 to i7°C) and accompanied by cloud and fog.

Frost-free and growing periods are relatively short throughout the Arctic. For the most part there is no true frost-free period; frost and some snow have been recorded in every month of the year. At a few places near the tree line, notably in the Canadian western Arctic, the frostfree period may be the same as the less favourable parts of the prairies.

South of the tree line in the subarctic, differences between continental (Mackenzie Basin, interior Yukon, and Alaska and northeastern Siberia) and oceanic (northern Quebec-Labrador, northern Scandinavia, and northern Russia) situations are marked. A summer maximum of precipitation and frequent high summer temperatures (July means exceeding 60°F [i6°C} in northeastern Siberia) in the continental regions contrast with heavier precipitation, often with a fall maximum, and lower summer temperatures in the oceanic regions.

The central polar ocean, together with the Beaufort and East Siberian seas, have winters comparable to northern Alaska and northeastern Siberia. Conditions are stable for extended periods of low wind velocities, clear skies— especially bordering Siberia—and temperatures ranging from -20 to -40°F (-30 to -40°C). Occasional storms originating in the Barents and Bering seas may penetrate the adjacent sectors of the polar basin and bring a temporary rise in temperature accompanied by snow or blowing snow. There is a negligible area (less than 1 percent) of open water in the central polar basin in winter. By April, air temperatures are rising until in June melting of the snow and underlying sea ice begins. Mean summer temperatures fail to rise above 34T (i°C) and are accompanied by almost continuous low cloud cover and fog.

The only extensive ice climate in the Northern Hemisphere is in Greenland. In the south the climate of the inland ice cap has maritime characteristics with heavy precipitation, mainly snow from passing cyclone disturbances. In the centre and north a continental situation develops, and the snowfall is less. Although the air temperature may sometimes rise to 32T (0°C), the mean temperature is much lower than in the south. Strong winds blowing off the ice cap are common in all parts of the island.

The evidence from glacier fluctuations suggests significant climatic change in polar latitudes in the past millennium. The first half of the 20th century saw climatic amelioration in the Arctic, with higher temperatures found particularly in winter and especially around the Norwegian Sea. In general, the magnitude of the warming increased with latitude, and in Svalbard winter temperatures rose by 14T (8°C). Associated with climatic changes were a radical reduction of sea ice around Svalbard and off southwestern Greenland. Birds, animals, and especially fish appeared farther north than before; in Greenland this led to a change in the economy, as its traditional dependence on seals yielded to dependence on fishing, particularly cod, which were caught north of the 70th parallel.

In the early 1940s, however, there was a downturn in polar temperatures. This widespread climatic cooling continued intermittently into the early 1970s. At this time sea ice failed to leave coastal areas in the summer in the eastern Canadian Arctic for the first time in living memory. A reversal of this trend followed in the next two decades, with the most noticeable temperature increases occurring in the lands to the north of the Pacific Ocean and around the Barents and Greenland seas (a change of +2.7 °F [+1.5 °C] in annual temperatures).

The underlying cause of the changes is not known, although they result directly from increased penetration of southerly winds into the polar regions.

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