Accumulation And Ablation Of The Ice Sheets

The size and thickness of an ice sheet depends on the amount of precipitation it receives and the amount of material it loses to melting, calving, and other processes. An ice sheet grows in years where accumulation exceeds ablation and declines in years where ablation exceeds accumulation.

The rate of precipitation on the Antarctic Ice Sheet is so low that it may be called a cold desert. Snow accumulation over much of the vast polar plateau is less than five cm (two inches) water equivalent per year. Only around the margin of the continent, where cyclonic storms penetrate frequently, does the accumulation rise to values of more than 30 cm (12 inches). The mean for Antarctica is 15 cm (6 inches) or less. In Greenland values are higher: less than 15 cm in a comparatively small area of north-central Greenland, 30 cm along the crests of the domes, and more than 80 cm (32 inches) along the southeast and southwest margins; the mean annual snow accumulation is about 37 cm (15 inches) of water equivalent.

Snow accumulation occurs mainly as direct snowfall when cyclonic storms move inland. At high altitudes on the Greenland Ice Sheet and in central Antarctica, ice crystals form in the cold air during clear periods and slowly settle out as fine "diamond dust." Hoarfrost and rime deposition are generally minor items in the snow-accumulation totals. It is almost impossible to measure the precipitation directly in these climates; precipitation gauges are almost useless for the measurement of blowing snow, and the snow is blown about almost constantly in some areas. The thickness and density of snow deposited on the ground equals precipitation plus hoarfrost and rime deposition, less evaporation, less snow blown away, and plus snow blown in from somewhere else. The last two phenomena are thought to cancel each other approx-imately—except in the coastal areas, where fierce drainage, or katabatic, winds move appreciable quantities of snow out to sea.

The snow surface may be smooth where soft powder snow is deposited with little wind, or very hard packed and rough when high winds occur during or after snowfall. Two features are prominent: snow dunes are depositional features resembling sand dunes in their several shapes; sastrugi are jagged erosional features (often cut into snow dunes) caused by strong prevailing winds that occur after snowfall. Sharp, rugged sastrugi, which can be one to two metres (6.5 feet) high, make travel by vehicle or on foot difficult. The annual snow layers exposed in the side of a snow pit can usually be distinguished by a low density layer (depth hoar) that forms by the burial of surface hoarfrost or by metamorphism of the snow deposited in the fall at a time when the temperature is changing rapidly

Almost all of the Antarctic Ice Sheet lies within the dry-snow zone. The percolation, soaked, and superimposed ice zones occur only in a very narrow strip in a small area along the coast. In Greenland only the central part of the northern half of the ice sheet, or about 30 percent of the total area, is within the dry-snow zone. Almost half of the area of the Greenland ice sheet is considered to be in the percolation zone. In flat areas near the equilibrium line, especially in west-central Greenland, there are notorious snow swamps, or slush fields, in summer; some of this water runs off, but much of it refreezes.

The ice sheets lose material by several processes, including surface melting, evaporation, wind erosion (deflation), iceberg calving, and the melting of the bottom surfaces of floating ice shelves by warmer seawater.

In Antarctica, calving of ice shelves and outlet glacier tongues clearly predominates among all the processes of ice loss, but calving is very episodic and cannot be measured accurately. The amount of surface melt and evaporation is small, amounting to about 22 cm (about 9 inches) of ice lost from a 5 km (3 mile) ring around half the continent. Wind erosion is difficult to evaluate but probably accounts for only a very small loss in the mass balance. The undersides of ice shelves near their outer margins are subject to melting by the ocean water. The rate of melting decreases inland, and at that point some freezing of seawater onto the base of the ice shelves must occur, but farther inland, near the grounding line, the tidal circulation of warm seawater may produce basal melting.

In Greenland, surface melt is more important, calving is less so, and undershelf melting is important only on floating glacier tongues (seaward projections of a glacier). Most of the calving is from the termini of a relatively few large, fast-moving outlet glaciers. In Greenland, vertical-walled melt pits in the ice are a well-known feature of the ice surface at the ablation zone. Ranging from a few millimetres to a metre (3 feet) in diameter, these pits are floored with a dark, silty material called cryoconite, once thought to be of cosmic origin but now known to be largely terrestrial dust. The vertical melting of the holes is due to the absorption of solar radiation by the dark silt, possibly augmented by biological activity.

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