Greenland Ice Sheet
Research supported by
Slides courtesy of Jason E. Box
Department of Geography
Byrd Polar Research Center
The Ohio State University
Columbus, Ohio, USA
Orientation
Greenland
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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 @
Summit
http://en.wikipedia.org/wiki/File:Geography-of-greenland.svg
3
• 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.
4
• 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).
5
• 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.
6
• 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.
7
Coastal
Weather
Stations
Greenland Weather Station, 1945
8
Upernavik, 2005
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.
9
NGRIP
10
Box, J.E., Survey of Greenland instrumental temperature records: 18732001, International Journal of Climatology, 22, 1829-1847, 2002.
12
Annual Surface Air Temperature
Box, J.E., Survey of Greenland instrumental temperature records: 18732001, International Journal of Climatology, 22, 1829-1847, 2002.
13
January Surface Air Temperature
Box, J.E., Survey of Greenland instrumental temperature records: 18732001, International Journal of Climatology, 22, 1829-1847, 2002.
14
Source: Serreze
and Barry (2005)
15
• 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.
• Greenland’s 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.
16
• 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.
17
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.
18
• Direct observations of Greenland
precipitation are particularly scant, as 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 (<100 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.
19
• 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.
20
Source: Serreze
and Barry (2005)
21
Precipitation
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.
22
• 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.
23
• 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.
24
• 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.
25
• 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.
26
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
27
• 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.
28
• 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 refreeze.
• 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).
29
Source: Serreze
and Barry (2005)
30
31
8 July 2012
12 July 2012
Runoff
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.
32
Zwally et al. 2002: Surface MeltInduced Acceleration of Greenland
Ice-Sheet Flow, Science
Zwally et al. 2002: Surface Melt-Induced Acceleration of Greenland Ice-Sheet Flow,
Science
• 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.
35
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.
36
• 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.
37
Term
Accumulation
Grounded ice
Total
Ablation
Calving
Sub-ice melting
Surface runoff
Total
Net mass
balance
Mass Rate
(Gt/yr)
Uncertainty (%)
520
520
±5
-235
-32
-297
-564
±14
±10
±10
-44
38
• 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.
39
• 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.
40
• 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.
41
GRACE (Gravity Recovery
and Climate Experiment)
43
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).
44
45
Source: Velicogna and Wahr (2005)
Surface
Mass Balance
1988-2004
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.
46
Source: Velicogna and Wahr (2005)
47
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.
48
• 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.
49
• 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.
50
51
(Source: Ekstrom et al., 2006)
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.
52
• 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.
53
• 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
5°C.
54
• The future of the West Antarctic Ice Sheet
(WAIS) remains uncertain, with its marinebased 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.
55
• 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 icesheet growth in Antarctica will offset
enhanced melting in Greenland.
56
• 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.
57
• 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 icebergcalving loss plus sub–ice-shelf melting,
with total loss only slightly larger than
snow accumulation.
• For warming of more than about 3°C 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 >5 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.
59
Estimates of Global Sea Level
Rise from Tide Gauge Records
1.5
( IPCC, 2001)
The University of Texas at Austin, Center for Space Research
Leuliette, E. W, R. S. Nerem, and G. T. Mitchum, 2004:
Calibration of TOPEX/Poseidon and Jason altimeter data to construct a
continuous record of mean sea level change. Marine Geodesy, 27(1-2), 79-94.
(Source: Alley et al., 2005).
62
(Source: Alley et al., 2005).
63