Greenland Ice Sheet

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Greenland Ice Sheet
Slides courtesy of Jason E. Box Department of
Geography Byrd Polar Research Center The Ohio
State University Columbus, Ohio, USA
Research supported by
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Orientation
Greenland

• 2.16 x 106 km2
• 81 ice covered
• 3 x Texas
• 10 global land ice
• 7.4 m sea level equivalent
• Max elevation of 3208 m _at_ Summit

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http//en.wikipedia.org/wiki/FileGeography-of-gre
enland.svg
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• The surface slope over most of the Greenland Ice
  Sheet is barely 1o, but is much greater at the
  margins which is also characterized by numerous
  fiords and associated valley glaciers that drain
  the ice sheet.
• Greenland has an estimated ice volume of is 2.93
  106 km3 and is the source of most of the
  icebergs found in the North Atlantic.

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• With adjustment for isostatic rebound, the water
  locked up in the Greenland Ice Sheet corresponds
  to an approximate global sea level equivalent of
  7.2 m.
• At present, 88 of the coterminous ice sheet lies
  in the accumulation zone (where annual mass gains
  exceed mass losses), with the other 12 lying in
  the ablation zone (where annual mass losses
  exceed more than loss gains).

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• Beginning in 1987, an automatic weather station
  (AWS) network was established in Greenland. Data
  from these stations provide a valuable addition
  to the few previous expedition measurements.
• The high elevation, large extent and high albedo
  of the ice sheet are significant factors for
  local and regional surface air temperatures
  although latitude and distance inland are also
  involved.

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• For both the eastern and western slopes of the
  ice sheet, surface air temperatures (SATs)
  decrease by about 0.8oC per degree of latitude
  and by about 0.71oC per 100 m.
• The ice sheet is characterized by pronounced
  low-level inversions, which are most strongly
  expressed during winter.
• February tends to be the coldest month in
  Greenland. For instance, at Summit, summer maxima
  reach -8oC, whereas winter minima attain -53oC
  however, there is strong daily variability in
  winter, which is associated with synoptic
  activity and katabatic winds.

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Coastal Weather Stations
Greenland Weather Station, 1945
Upernavik, 2005
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Greenland Climate Network (GC-Net) Automatic
Weather Stations (AWS)
Steffen, K. and J.E. Box, 2001 Surface
climatology of the Greenland ice sheet Greenland
Climate Network 1995-1999, J. Geophys. Res.,
106(D24), 33951-33964.
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NGRIP
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(No Transcript)
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Box, J.E., Survey of Greenland instrumental
temperature records 1873-2001, International
Journal of Climatology, 22, 1829-1847, 2002.
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Annual Surface Air Temperature
Box, J.E., Survey of Greenland instrumental
temperature records 1873-2001, International
Journal of Climatology, 22, 1829-1847, 2002.
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January Surface Air Temperature
Box, J.E., Survey of Greenland instrumental
temperature records 1873-2001, International
Journal of Climatology, 22, 1829-1847, 2002.
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Source Serreze and Barry (2005)
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• A prominent feature of the Greenland climate,
  just as in Antarctica, is its katabatic wind
  regime dynamically, katabatic winds in Greenland
  are the same as those found in Antarctica.
• They relate to flows that are forced by
  radiational cooling of the lower atmosphere
  adjacent to the sloping terrain on the ice sheet.
  
• Greenlands katabatic winds, while not greatly
  influenced by topography, tend to flow with a
  pronounced component across the fall line because
  of the Coriolis force however, winds near the
  coast are channeled by valleys and fiords.

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• Measurements at Swiss Camp during 1990-99 yield a
  maximum monthly mean wind speed of 9-11 m s-1
  during November-January, and a minimum of 5 m s-1
  in July, with the prevailing wind direction is
  from 120-130o, reflecting a katabatic regime.
• Winds show strong directional constancy over most
  of the ice sheet.

18
Snow Transport 1991-2000
Box, J.E., D. H. Bromwich, L-S Bai, 2004
Greenland ice sheet surface mass balance for
1991-2000 application of Polar MM5 mesoscale
model and in-situ data, J. Geophys. Res., Vol.
109, No. D16, D16105, 10.1029/2003JD004451.
19

• Direct observations of Greenland precipitation
  are particularly scant, with long records are
  limited to the coasts.
• In recent years, data over the ice sheet have
  been acquired from automatic stations.
• The main features of precipitation distribution
  over Greenland are very low accumulation (lt100 mm
  yr-1) over the northern portions of the island
  with the highest values along the southeast coast
  where it exceeds 2000 mm yr-1.

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• Fairly high values are also found along the
  western coast related to orographic uplift and
  cyclone activity in Baffin Bay.
• Accumulation basically represents the net effects
  of direct precipitation, its redistribution on
  the surface via wind scour and drifting, and mass
  losses due to melt and evaporation/ sublimation,
  and is typically assessed via snow pits or ice
  cores.
• Based on coastal station observations of
  precipitation, adjusted for wind speed and
  accumulation data from recent ice cores, the
  annual precipitation averaged over the ice sheet
  is estimated to be 340 mm yr-1.

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Source Serreze and Barry (2005)
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Precipitation1991-2000
Box, J.E., D. H. Bromwich, L-S Bai, 2004
Greenland ice sheet surface mass balance for
1991-2000 application of Polar MM5 mesoscale
model and in-situ data, J. Geophys. Res., Vol.
109, No. D16, D16105, 10.1029/2003JD004451.
23

• There are zones of maximum precipitation
  exceeding 2000 mm yr-1 in the southeast coastal
  area and 600 mm yr-1 in the northwest. Amounts in
  the north-central area are around 100 mm yr-1.
• The southeastern maximum is strongly influenced
  by orographic uplift of southeasterly flow
  associated with traveling cyclones whereas the
  northwestern maximum is related to flow off
  northern Baffin Bay and uplift.

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• Sublimation refers to the exchange of water
  vapour between the surface and the overlying
  atmosphere during sub-freezing conditions
  (typical of Greenland) in which water molecules
  are transferred directly from the solid to the
  gas phase.
• In the ablation area of the ice, estimates of
  annual sublimation are between 60 and 70 mm yr-1,
  whereas over the higher parts of the ice sheet,
  it is probably 20-30 mm during the summer months.

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• Sublimation over the ice sheet is highly variable
  in both space and time.
• Maximum sublimation rates from the surface to the
  atmosphere tend to occur when temperatures are
  close to 0oC and winds are strong.
• Deposition (vapour to solid) can occur under
  favourable synoptic conditions with a reversed
  humidity gradient or during nighttime due to
  radiative cooling.

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• An annual map of sublimation shows positive
  values over most of the ice sheet, and greatest
  in the warmer lower elevations during the summer
  season.
• The highest elevations show a small vapour
  transfer from the atmosphere to the surface.
• Overall, the estimated mass losses by sublimation
  account from possibly 12 to 23 of the annual
  precipitation, such that sublimation emerges as a
  fairly important term for the Greenland Ice Sheet
  mass budget.

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Box, J. E. and K. Steffen, 2001 Sublimation on
the Greenland ice sheet from automated weather
station observations J. Geophys. Res., Vol. 106
, No. D24 , p. 33,965
28

• Large parts of the Greenland ice sheet experience
  surface melt in summer, a process which can be
  assessed using satellite passive microwave
  brightness temperatures.
• The melt areas shows a general association with
  latitude and elevation melt occurs in the
  southern and coastal regions of the ice sheet,
  but not in the highest and hence coldest parts.
• For the ice sheet as a whole, the area undergoing
  surface melt correlates strongly with surface air
  temperature anomalies.

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• The presence of melt inferred from passive
  microwave data does not imply that runoff is
  actually occurring.
• In higher regions where melt is observed, it may
  only be occurring in a near-surface layer,
  whereas at lower elevations, meltwater that is
  formed will percolate to lower depths and
  re-freeze.
• It is only near the coast that actual runoff is
  observed. In the southern part of the ice sheet,
  the area experiencing melt extends inland from
  the estimated equilibrium line (the line along
  which the net mass balance is zero).

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Source Serreze and Barry (2005)
31
Runoff1991-2000
Box, J.E., D. H. Bromwich, L-S Bai, 2004
Greenland ice sheet surface mass balance for
1991-2000 application of Polar MM5 mesoscale
model and in-situ data, J. Geophys. Res., Vol.
109, No. D16, D16105, 10.1029/2003JD004451.
32
Zwally et al. 2002 Surface Melt-Induced
Acceleration of Greenland Ice-Sheet Flow, Science
33
Zwally et al. 2002 Surface Melt-Induced
Acceleration of Greenland Ice-Sheet Flow, Science
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• For Greenland, runoff is an important term but
  net ablation has only been measured directly at a
  few locations and therefore has to be calculated
  from models, which have considerable sensitivity
  to the surface elevation data set and the
  parameters of the melt and refreezing methods
  used.
• Recent studies have suggested a loss of mass in
  the ablation zone and have brought to light the
  important role played by bottom melting below
  floating glaciers neglect of this term led to
  erroneous results in earlier analyses.

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Mass Balance

• For Greenland, updated estimates based on repeat
  altimetry, and the incorporation of atmospheric
  and runoff modeling, indicate increased net mass
  loss, with most change toward the coasts.

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• Between 1993 to 1994 and 1998 to 1999, the ice
  sheet was losing 54 14 gigatons per year
  (Gt/year) of ice, equivalent to a sea-level rise
  of 0.15 mm yr-1 (where 360 Gt of ice 1 mm sea
  level).
• The excess of meltwater runoff over surface
  accumulation was about 32 5 Gt/year, leaving
  ice-flow acceleration responsible for loss of 22
  Gt/year.
• Summers were warmer from 1997 to 2003 than from
  1993 to 1999, which likely explains the increased
  surface melt.

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Term Mass Rate (Gt/yr) Uncertainty ()
Accumulation
Grounded ice 520 5
Total 520
Ablation
Calving -235 14
Sub-ice melting -32 10
Surface runoff -297 10
Total -564

Net mass balance -44

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• These results are broadly similar to those from a
  meso-scale atmospheric model used to simulate the
  surface mass balance of the Greenland Ice Sheet
  from 1991 to 2000.
• Accounting for additional mass loss from iceberg
  discharge and basal melting (assumed constant)
  yielded an estimated net mass loss of 78 Gt/year.

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• Large interannual variability did not obscure
  significant simulated trends toward increased
  melting and snowfall consistent with
  reconstructed warming, especially in west
  Greenland.
• GRACE provides monthly estimates of Earth's
  global gravity field at scales of a few hundred
  kilometers and larger.
• Time variations in the gravity field can be used
  to determine changes in Earth's mass
  distribution.

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• GRACE has therefore been applied to examine mass
  balance variations in both the Greenland and
  Antarctic ice sheets.
• Dramatic new evidence has emerged of the speed of
  climate change in the polar regions which
  scientists fear is causing huge volumes of ice to
  melt far faster than predicted.

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GRACE (Gravity Recovery and Climate Experiment)
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(No Transcript)
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Monthly ice mass changes and their best-fitting
linear trends for WAIS (red) and EAIS (green) for
April 2002 to August 2005. The GRACE data have
been corrected for hydrology leakage and for PGR.
(Source Velicogna and Wahr, 2006).
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Source Velicogna and Wahr (2005)
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Surface Mass Balance1988-2004
Box, J.E., D.H. Bromwich, B.A. Veenhuis, L-S Bai,
J.C. Stroeve, J.C. Rogers, K. Steffen, T. Haran,
S-H Wang, Greenland ice sheet surface mass
balance variability (1988-2004) from calibrated
Polar MM5 output, J. Climate, accepted Sept 27
2005.
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Source Velicogna and Wahr (2005)
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Glacial Earthquakes

• Scientists have recorded a significant and
  unexpected increase in the number of "glacial
  earthquakes" caused by the sudden movement of
  Manhattan-sized blocks of ice in Greenland.
• The rise in the number of glacial earthquakes
  over the past four years lends further weight to
  the idea that Greenland's glaciers and its ice
  sheet are beginning to move and melt on a scale
  not seen for perhaps thousands of years.

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• The annual number of glacial earthquakes recorded
  in Greenland between 1993 and 2002 was between
  six and 15. In 2003 seismologists recorded 20
  glacial earthquakes. In 2004 they monitored 24
  and for the first 10 months of 2005 they recorded
  32.
• The latest seismic study found that in a single
  area of north-western Greenland scientists
  recorded just one quake between 1993 and 1999.
  But they monitored more than two dozen quakes
  between 2000 and 2005.

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• Some of Greenland's glaciers can move 10 metres
  in less than a minute, a jolt that is sufficient
  to generate moderate seismic waves.
• As the glacial meltwater seeps down it lubricates
  the bases of the "outlet" glaciers of the
  Greenland ice sheet, causing them to slip down
  surrounding valleys towards the sea.
• Of the 136 glacial quakes analysed by the
  scientists, more than a third occurred during
  July and August.

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(Source Ekstrom et al., 2006)
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Sea-level rise

• Because a heavy concentration of the population
  lives along coastlines, even small amounts of
  sea-level rise would have substantial societal
  and economic impacts through coastal erosion,
  increased susceptibility to storm surges,
  groundwater contamination by salt intrusion, and
  other effects.

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• Over the last century, sea level rose 1.0 to 2.0
  mm yr-1, with water expansion from warming
  contributing 0.5 0.2 mm (steric change) and the
  rest from the addition of water to the oceans
  (eustatic change) due mostly to melting of land
  ice.
• By the end of the 21st century, sea level is
  projected to rise by 0.5 0.4 m in response to
  additional global warming, with potential
  contributions from the Greenland and Antarctic
  ice sheets dominating the uncertainty of that
  estimate.

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• These projections emphasize surface melting and
  accumulation in controlling ice-sheet mass
  balance, with different relative contributions
  for warmer Greenland and colder Antarctica.
• The Greenland Ice Sheet may melt entirely from
  future global warming, whereas the East Antarctic
  Ice Sheet (EAIS) is likely to grow through
  increased accumulation for warmings not exceeding
  5C.

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• The future of the West Antarctic Ice Sheet (WAIS)
  remains uncertain, with its marine-based
  configuration raising the possibility of
  important losses in the coming centuries.
• Despite these uncertainties, the geologic record
  clearly indicates that past changes in
  atmospheric CO2 were correlated with substantial
  changes in ice volume and global sea level.

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• Recent observations of startling changes at the
  margins of the Greenland and Antarctic ice sheets
  indicate that dynamical responses to warming may
  play a much greater role in the future mass
  balance of ice sheets than previously considered.
  
• Longterm climate projections show that up to the
  year 2100, warming-induced ice-sheet growth in
  Antarctica will offset enhanced melting in
  Greenland.

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• For the full range of climate scenarios and model
  uncertainties, average 21st-century sea-level
  contributions are 0.6 0.6 mm yr-1 from
  Antarctica and 0.5 0.4 mm yr-1 from Greenland,
  resulting in a net contribution not significantly
  different from zero, but with uncertainties
  larger than the peak rates from outlet glacier
  acceleration during the past 5 to 10 years.

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• Looking further into the future, inland-ice
  models raise concerns about the Greenland Ice
  Sheet.
• At present, mass loss by surface meltwater runoff
  is similar to iceberg-calving loss plus
  subice-shelf melting, with total loss only
  slightly larger than snow accumulation.
• For warming of more than about 3C over
  Greenland, surface melting is modeled to exceed
  snow accumulation, and the ice sheet would shrink
  or disappear.

58

• This loss of the Greenland Ice Sheet would be
  irreversible without major cooling.
• In contrast, important mass loss from surface
  melting of Antarctic ice is not expected in
  existing scenarios, although grounding-line
  retreat along the major ice shelves is modeled
  for basal melting rates gt5 to 10 m yr-1, causing
  the demise of WAIS ice shelves after a few
  centuries and retreat of coastal ice toward more
  firmly grounded regions after a few millennia,
  with implied rates of sea-level rise of up to 3
  mm yr-1.

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Estimates of Global Sea Level Rise from Tide
Gauge Records
1.5 ( IPCC, 2001)
The University of Texas at Austin, Center for
Space Research
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Leuliette, E. W, R. S. Nerem, and G. T. Mitchum,
2004Calibration of TOPEX/Poseidon and Jason
altimeter data to construct a continuous record
of mean sea level change. Marine Geodesy,
27(1-2), 79-94.
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(Source Alley et al., 2005).
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(Source Alley et al., 2005).