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.
The ice sheet has 3 distinct morphological
zones: East Antarctica, West Antarctic, and
the Antarctic Peninsula, with its highest
point being Vinson Massif (at 5440 m),
located in the Ellsworth Mountains.
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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.
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.
2
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.
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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.
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.
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The resulting flow, known as a “katabatic
wind”, will be turned by the Coriolis force
and retarded by surface friction.
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.
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.
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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.
The saturation vapour pressure, through the
Clausius-Clapeyron relation, is by far the
most important parameter in determining the
distribution of precipitation.
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.
Meltwater may refreeze within the snow
instead of being lost, and some snow may
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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.
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.
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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.
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).
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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Table 1: Estimates of the terms in the mass
budget of the Antarctic Ice Sheets (Jacobs et
al. 1992)
Term
Mass Rate
Uncertainty
(Gt/year)
(%)
Accumulation
Grounded
1528
20
Ice
Ice Shelves
616
2144
Total
Accumulation
(Ma)
Ablation
Calving
-2016
33
(Mc)
Sub-ice
-554
50
Melting (Mb)
Surface
-53
50
Runoff (Mm)
-2623
Total
Ablation
-479
Net
Mass
Balance (Bn)
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Mass loss by iceberg calving is clearly the
largest negative term in the budget, but subice-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.
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
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that, on a continental scale, is considerably
less than the others listed in Table 1.
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.
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
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volume of approximately 5,400 km3 of
water.
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.
Thus the lakes may harbor an ancient and
alien ecosystem adapted to life beneath the
ice sheet.
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