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Paleoclimatic history from the Eocene to the Anthropocene is summarized. First, the variation of temperatures over geologic time is reviewed. Geological records from ocean sediment cores and ice cores are described. The main climate drivers associated with orbital variations of the Earth and the global carbon cycle are noted. Then the climatic conditions during the major geologic epochs from the Eocene to present are discussed. During the early Eocene (56-34 million years ago, MYA) there was a Paleocene-Eocene Thermal Maximum attributed to high carbon dioxide concentrations. Cooling around 34 MYA due to reduced atmospheric carbon dioxide led to Antarctic glaciation with major buildup around 15 MYA. The Plio-Pleistocene starting around 5 MYA, saw 40-kyr glacial cycles that switched to 100-kyr at 0.8 MYA. Large ice sheets formed and retreated over North America, Fenno-Scandinavia and the British Isles and West Antarctica. The post-glacial Holocene started at 11.7 kyr. The time when human influence began to dominate- the Anthropocene – is still debated. Polar amplification of global warming since the late 20th century is discussed, as is the role of poleward transport of heat and moisture by planetary waves.
The climate of the Greenland ice sheet is determined by latitude and altitude. Mean annual accumulation is ~337 mm, but >2000 mm in the southeast. There are four snow/ice facies. Melt area is irregularly expanding. Supraglacial streams carry melt water to surface lakes and moulins. Surface mass balance became negative from 1990. Mass loss for 2011-14 averaged 296 Gt/yr. The Antarctic ice sheet is mainly grounded on bedrock in the east, but the West Antarctic ice sheet (WAIS) is grounded below sea level making it potentially unstable. The climate is extremely cold and arid. Annual melt affects ~10 percent of the ice sheet. Blue ice areas cover 1.7 percent of the ice sheet. Megadunes and glazed areas are common on the East Antarctic plateau. Mass loss of ice has been pronounced in the Amundsen-Bellingshausen Sea sector and the Antarctic Peninsula. Ice shelves are mainly a feature of the Antarctic. There are eleven with the largest being the Ross and Weddell. Thickness decreases from 1600 m at the grounding line to 300 m at the seaward edge. Hydro-fracture of water-filled crevasses probably caused the breakup of Larsen B. Basal melt is related to basal channels transporting upwelled Circumpolar Deep Water.
The atmospheric and oceanic circulations of the polar regions and their climatic conditions are discussed. Both hemispheres feature a large upper tropospheric polar vortex surrounded by westerly airflows, but the low-level circulations differ greatly. Around Antarctica there is a circumpolar trough, while in the northern hemisphere there is the year-round Icelandic low, with an Aleutian low and Siberian high in winter. The Southern Ocean has a continuous Antarctic Circumpolar Current (ACC) flowing eastward while the North Atlantic Current enters the Arctic Ocean via the Norwegian Sea and the East Greenland Current carries cold water and sea ice southward. Despite the polar night / day, the radiative regimes differ owing to persistent low clouds in the Arctic summer, a variable surface albedo, and winter cyclones. The Antarctic plateau presents a persistent ice surface. Arctic temperatures range from ~-30 °C in January to near 0 °C in summer, while at South Pole they are about 30 °C lower. Surface temperature inversions are prevalent in both regions. Annual precipitation in the Arctic is <200 mm, but most of Antarctica has even less. The Arctic and Antarctic Peninsula have warmed at twice the global average since the 1950's due to polar amplification.
In the Southern Ocean the Antarctic Circumpolar Current flows continuously eastward, except near the Antarctic coast. It has multiple fronts. The Ross Sea and Weddell Sea embayments are half covered by ice shelves. The mainly ice-covered Arctic Ocean has wide continental shelves and receives large quantities of river runoff resulting in low salinities. Eight seas surround the central Arctic - the Barents, Kara, Laptev, East Siberian, Chukchi, Beaufort, Lincoln and Greenland, as well as the channels of the Canadian Arctic Archipelago. North Atlantic water enters the Barents Sea and cold water and ice exit via the East Greenland Current. The Labrador Sea links Baffin Bay to the North Atlantic. The Bering Sea and Sea of Okhotsk are marginal seas of the North Pacific and Pacific water enters the Arctic via Bering Strait. Ocean warming caused 1.1 mm/yr of sea level rise from 1992 – 2010, with glacier melt accounting for 0.86 mm and Greenland and Antarctica 0.60 mm. Arctic ice is ~60 percent first year and 40 percent multiyear. Its extent has decreased dramatically since the 1980s, especially in September. Antarctic sea ice is mainly seasonal. Recently, it had been increasing until 2016. Coastal polynyas are major sea ice producers.
Observations in polar environments are discussed, beginning with International Polar Years, the International Geophysical Year, the Tundra Biome project and the Long-term Ecological Research Program. The development of observing networks for climate, frozen ground, and glaciers is traced. In situ measurements of meteorological conditions in both polar regions, North Pole Drifting Stations, and the use of icebreakers are described. Oceanographic observation from ARGO floats and buoys are examined. Upward looking sonar measurements of sea ice draft are described. Remote sensing, beginning with aerial, followed by satellite photography, and mapping of glaciers and snow cover from Landsat, is described. Data from passive microwave sensors and their applications to mapping sea ice, snow water equivalent, and frozen ground are detailed. Radar applications to map sea ice and glaciers at high resolution from satellite, and the application of ground penetrating radar to determine ice sheet thickness and permafrost depth are noted. Interferometric synthetic aperture radar (InSAR) is used to map ice stream motion. Radar and laser altimetry are used to map ice sheet elevation and sea ice freeboard and are combined with data from the Gravity Recovery and Climate Experiment (GRACE) satellite to calculate ice sheet mass balance. Finally, reanalysis products are described.
The geographical setting, history of scientific studies, and the climatic role of the cryosphere are discussed. Definitions of the Arctic, Antarctic and the Central Asian Third Pole are given Their similarities of climate and ice cover and contrasts (ice-covered ocean and massive ice sheet, and latitude/altitude) are noted. Scientific study of the Arctic began with the First International Polar Year, 1882-3 and the Fram expedition of F. Nansen. The contributions of Norwegian, Russian, Canadian and U.S. scientific programs in the Arctic Ocean and across fthe tundra are discussed. Antarctic research dates from R. Byrd's work in the 1920's -30s, but was mainly post World War II. The International Geophysical Year, 1957-8 was a major milestone with the establishment of permanent bases. Research in Central Asia and Tibet began in the 1950's from the USSR and China, respectively, focused on glaciers. The role of polar snow, land ice and sea ice in the climate system and important feedbacks are described.
The ‘term Third Pole’ refers to its cold climate, extensive ice cover and permafrost. It comprises the 4500 m-high Tibetan Plateau with the Kun Lun mountains, the mountain ranges of Central Asia (Pamir, Tien Shan, Qilian Shan) to the north and the Karakorum-Himalaya to the south, with extensive ice cover. The climate is very cold in winter and 0-10 °C in summer. There has been a warming trend since the 1970s. Precipitation decreases from southeast to northwest. The monsoon affects the south and mid-latitudes westerlies the north. Glaciers and ice caps cover 113,000 km2, and are generally retreating. Shrinkage was greatest in the Himalaya and least in the Pamir. The Karakorum glaciers are in near balance. Glacier melt in headwaters provides 32-58 percent of annual flow, but this drops to 7-9 percent 40 km from the termini. The snow line is ~5800 m over the Tibet Plateau. The Plateau has 60-150 days with snow cover, which is mostly shallow and decreasing over much of the Plateau. The Plateau is underlain by continuous permafrost in the north and discontinuous in the south. Most is warm, thin and ice-poor.
Polar terrestrial environments are predominantly polar desert or tundra. Polar desert occurs in the Canadian High Arctic, northern Greenland, the Eurasian Arctic islands and the Antarctic Dry Valleys. Tundra is widespread, comprising barrens, graminoids, shrub tundra, and wetlands. Both environments are widely underlain by permafrost (12-18 percent of the northern hemisphere). Subsea permafrost is widespread on Arctic continental shelves. Ground ice forms palsas and pingos and melting ground ice forms thermokarst. Periglacial features include patterned ground and rock glaciers. Permafrost temperatures are rising due to global warming. Arctic lakes are frozen for 7-9 months of the year. Thermokarst lakes are widespread in tundra regions. Proglacial lakes are common in the Himalaya. Perennially frozen and subglacial lakes occur in Antarctica. Lake and river breakup/freeze up is occurring 5-6 days earlier/later. Arctic rivers flow mainly into the Arctic Ocean and have extensive deltas. Glaciers and ice caps are mainly in the eastern Canadian Arctic, around Greenland, in the western Eurasian Arctic, in the Antarctic Peninsula, and around the ice sheet. Recent mass loss in the Arctic was 73 percent of the global total. Glacial landscapes are erosional – scoured bedrock to fiords - and depositional – moraines and drumlins; eskers and kames are fluvioglacial.
Projections of future polar environments by 2050-2100 are described. The roles of greenhouse gases and four Representative Concentration Pathways (RCPs) are discussed. Half of the 1985-2012 warming was anthropogenically-caused. Aerosols led to significant cooling. The Antarctic ozone hole will persist until the 2050s The Arctic Ocean will be ice-free in September before 2050 leading to increased shipping. Sea ice loss leads to less severe winter cold air outbreaks. Winter snowfall will increase over much of the northern hemisphere in the late 21st century. Near-surface permafrost will shrink considerably and thermokarst landscapes will expand. Extensive greening of the tundra has already occurred with positive and negative consequences. Few glaciers will remain in 2100 if warming is 4.8 °C and the Antarctic ice sheet can cause a 1m sea level rise. Impacts include Arctic coastal erosion. Arctic precipitation and runoff to the Arctic Ocean will increase. Arctic Observing Networks have been organized, the Year of Polar Prediction is underway and future Arctic research includes a planned icebreaker drift in the Arctic in 2019.