1 Antarctic Ice Sheet • The Antarctic and Greenland ice sheets contribute directly to sea level and ocean circulation, and hence potentially to climate change. • It is estimated that the Greenland and Antarctic ice sheets together contain enough water to raise sea level by almost 70 m. • The continent itself makes up about 10% of the land surface of the Earth with the combined area of the ice sheets and ice shelves covering 14 × 10 6 km 2 .
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Antarctic Ice Sheet
• The Antarctic and Greenland ice sheets contribute directly to sea level and ocean circulation, and hence potentially to climate change.
• It is estimated that the Greenland and Antarctic ice sheets together contain enough water to raise sea level by almost 70 m.
• The continent itself makes up about 10% of the land surface of the Earth with the combined area of the ice sheets and ice shelves covering 14 × 106 km2.
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• The ice sheet has 3 distinct morphological zones: East Antarctica, West Antarctica, and the Antarctic Peninsula, with its highest point being Vinson Massif (at 5440 m), located in the Ellsworth Mountains.
• The vast majority of the surface of Antarctica is covered with ice, with the continent as a whole containing around 30 × 106 km3 or 90% of the world’s freshwater.
• The ice thickness varies across the continent, with its maximum being 4776 m about 400 km south of Dumont d’Urville.
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Antarctica
Source: www.geology.com
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• The mass of ice conceals the details of the land below, which is made up of sub-glacial mountains and lower elevation topographic features.
• The ice that builds up in the interior of the Antarctic flows down to the edge of the continent in ice streams that move at speeds of up to 500 m per year.
• There is a strong correlation between surface air temperature (SAT) and elevation over Antarctica.
• There is a “dipole” of cold air temperature on the East Antarctica plateau where mean annual SATs are < -55oC, whereas the South Pole experiences an annual SAT of -50oC.
• Only in the northern part of the Antarctic Peninsula in summer do monthly mean SATs exceed freezing; hence, over the greater part of the Antarctic ice sheets, there is little or no direct ablation of the snow surface.
• In the Antarctic Peninsula and around the coast of East Antarctica, the annual cycle of temperature takes a familiar form with a broad summer maximum and a minimum in July or August.
• However, moving southward onto the polar plateau, the form of the cycle changes to a short, peaked summer season and a “coreless” winter during which the temperature varies relatively little.
• Contributing to this cycle are the abrupt changes in solar radiation at the beginning and end of winter darkness, whereas during the dark period, the surface radiation balance is determined only by net longwave radiation.
• In addition, warm air advection onto Antarctica has a strong semi-annual component, and the snowpack acts as a heat reservoir, smoothing out extremes of temperature variations.
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Source: King and Turner (1997)
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• As the surface of the Antarctic continent cools radiatively, the air close to the surface also cools relative to the air aloft.
• The near-surface air is thus negatively buoyant and, over a sloping surface, will accelerate down the slope in response to the buoyancy force.
• The resulting flow, known as a “katabatic wind”, will be turned by the Coriolis force and impeded by surface friction.
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• Even on some of the modest slopes of Antarctica, the surface wind is primarily determined by katabatic forcing.
• In Antarctica and Greenland, the combination of extensive sloping surfaces and uninterrupted surface cooling for much of the year allows a large-scale katabatic circulation to develop.
• Knowledge of precipitation formation mechanisms, snowfall distribution over the Antarctic continent, and the synoptic origins of the precipitating air masses is important for the investigation of whether the ice sheets are growing or shrinking (mass balance), how accumulation may change in an environment of higher mean SAT, etc.
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• It has been estimated that about 2300 km3 yr-1 of snow falls on the continent each year, but it is extremely difficult to make measurements of precipitation in Antarctica, not least because of the difficulty in distinguishing blowing snow from snowfall.
• With the strong winds experienced over the continent, conventional snow gauges give poor results so that other means of assessing accumulation must be found, including the use of snow stakes, stratigraphy, a knowledge of snowdrift density, etc.
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• Net accumulation of snow at the surface is often taken as equivalent to precipitation in Antarctica.
• The largest accumulations are found slightly inland of the coast, where the steepest slopes are found and where air masses are lifted and cooled as they move poleward.
• The largest accumulations are over 1000 mm yr-1, found near southeastern Bellingshausen Sea, whereas the lowest values of snowfall accumulation (< 50 mm yr-1) occur over the Antarctic Plateau; this minimum in precipitation is strongly correlated to air temperature.
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Annual Precipitation (mm swe)http://en.w
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• The saturation vapour pressure, through the Clausius-Clapeyron relation, is by far the most important parameter in determining the distribution of precipitation in Antarctica.
• An ice sheet gains mass by accumulating snow that is transformed into ice by densification processes.
• It loses mass (ablation) mainly by melting at the surface or base with subsequent runoff or evaporation of the meltwater.
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• Meltwater may refreeze within the snow instead of being lost, and some snow may sublimate from the surface back to the atmosphere.
• Ice may also be removed by flowing into a floating ice shelf, from which it is lost by basal melting and calving of icebergs.
• In general, net mass ablation at lower altitudes are balanced by the downhill flow of ice under internal deformation and by the sliding and bed deformation at the base.
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• This balance is expressed usually as the rate of change of the equivalent volume of liquid water, in cubic metres per year; for a steady state the mass balance is zero.
• Mass balances are computed for both the whole year and individual seasons (winter and summer) with the specific mass balance being the net summer and winter mass balances averaged over the surface area, in metres per year.
• Ice-covered regions are dynamic environments that are characterized by forcing responses at variable timescales, which may not always be synchronous with external weather and climate forcing factors.
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• While seasonal accumulation and ablation processes might balance in approximate terms, internal ice dynamical processes occurring at both shorter and longer timescales contribute to balance inequalities.
• Antarctic temperatures are so low that there is virtually no surface runoff; the ice sheet mainly loses mass by ice discharge into floating ice shelves, which experience melting and freezing at their underside and eventually break up to form icebergs.
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• The mass balance equation is:
• Bn = Ma – Mm – Mc ± Mb
• Where Bn is the mass balance at the end of the balance year, Ma is annual surface accumulation; Mm is annual loss by glacial surface runoff, Mc is annual loss by calving of icebergs, and Mb is the annual balance at the bottom (melting or freezing-on of ice).
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• This equation suggests that the mass balance can be inferred from two methods: a) by direct measurement of the change in volume by monitoring surface elevation change and b) by the budget method, determining each term on the right-hand side of the equation separately.
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Term Mass Rate (Gt/yr)
Uncertainty (%)
Accumulation
Grounded ice 1528 20
Ice shelves 616
Total 2144
Ablation
Calving -2016 33
Sub-ice melting -554 50
Surface runoff -53 50
Total -2623
Net mass balance
-479
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• Mass loss by iceberg calving is clearly the largest negative term in the budget, but sub-ice-shelf melting cannot be neglected.
• Runoff of meltwater is largely controlled by the small amount of melting which takes place in coastal regions during the summer, with a small additional contribution from subglacial melting.
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• If the terms in the mass balance are summed, we obtain an imbalance in the net mass budget of the Antarctic ice sheet; however, given the large uncertainties in some of the terms, particularly in the amount of calving, the significance of this imbalance is not very high.
• Blowing snow also plays a role in the mass balance of Antarctic, both through the divergence/convergence of mass and through sublimation; however, it is a term that, on a continental scale, is considerably less than the others listed in Table 1.
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Blowing snow frequency in July 2009 simulated by a regional climate model
(Source: Lenaerts et al. 2012)
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Mean annual blowing snow transport (Mg/m)
Source: Dery & Yau (2002)
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• For long-term studies (up to 250,000 years) and to reconstruct previous accumulation rates, snow pit studies and ice core measurements have been used widely.
• Snow pits are used to quantify vertical variations in snow density and to identify accumulation layers in the snow that can be used to calculate densification rates.
• Ice core studies are used to reconstruct past climate variations, particularly temperature variations, using O18/O16 ratios from ice cores.
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• Lake Vostok, a lake as big as Lake Ontario, lies in the heart of the Antarctic continent hidden beneath 4 kilometers of ice.
• It is believed that Lake Vostok has been covered by the vast Antarctic ice sheet for up to 25 million years.
• The lake was named for the Russian research station that sits above its southern tip - a place where in 1983 the temperature fell to -89°C, the coldest ever recorded temperature on Earth.
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• More than 145 lakes have been identified beneath the thick Antarctic ice sheet, with most covered by 3-4 kilometers of ice and being several kilometers long.
• Lake Vostok, is an order of magnitude larger than all other known subglacial lakes, at 14,000 km2 in area, 900 m deep, and a volume of approximately 5,400 km3 of water.
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• Antarctic ice effectively floats on the surface of the lakes such that the ice sheet exhibits slight depressions over the lakes that appear in radar and laser elevations.
• The combination of heat from below and a thick layer of insulating ice above keeps the water temperature at the top of the lakes at a relatively balmy -2oC.
• Since the lakes are bounded by faults, it is likely the lakes receive flows of nutrients that could support unique ecosystems, perhaps an ancient and alien ecosystem adapted to life beneath the ice sheet.
