Greenland Ice Sheet
The Greenland Ice Sheet is by far the largest
terrestrial ice mass in the Arctic and is the
second largest in the world (following the
Antarctic Ice Sheet).
The ice sheet covers an area of 1.71 × 106
km2 and reaches a maximum elevation of
3208 m at Summit (72.6oN, 37.5oW).
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.
1
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).
Climate data for Greenland are available
from several field programs. 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
2
temperatures although latitude and distance
inland are also involved.
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.
A prominent feature of the Greenland
climate, just as in Antarctica, is its katabatic
wind regime; dynamically, katabatic winds
3
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, when not great
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.
Measurements at Swiss Camp during 199099 yield a maximum monthly mean wind
speed of 9-11 m s-1 during NovemberJanuary, 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.
4
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 (< 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.
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
5
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.
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.
6
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.
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
7
reversed humidity gradient or
nighttime due to radiative cooling.
during
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.
Large parts of the Greenland ice sheet
experience surface melt in summer, a
process which can be assessed using satellite
passive microwave brightness temperatures.
8
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.
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
9
inland from the estimated equilibrium line
(the line along which the net mass balance is
zero).
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.
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.
10
Table 1: Estimates of the terms in the mass
budget of the Greenland Ice Sheet (IPCC)
Term
Mass Rate
Uncertainty
(Gt/year)
(Gt/year)
Accumulation
520
±26
520
Total
Accumulation
(Ma)
Ablation
Calving
-235
±33
(Mc)
Sub-ice
-32
±3
Melting (Mb)
Surface
-297
±32
Runoff (Mm)
-564
Total
Ablation
-44
±53
Net
Mass
Balance (Bn)
11
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 sealevel 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.
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
12
(assumed constant) yielded an estimated net
mass loss of 78 Gt/year.
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.
GRACE has therefore been applied to
examine mass balance variations in both the
Greenland and Antarctic ice sheets.
13
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.
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.
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
14
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.
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.
15
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.
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.
16
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.
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.
Recent observations of startling changes at
the margins of the Greenland and Antarctic
17
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.
For the full range of climate scenarios and
model uncertainties, average 21st-century
sea-level contributions are –0.6 ± 0.6 mm yr1
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.
Looking further into the future, inland-ice
models raise concerns about the Greenland
Ice Sheet.
18
At present, mass loss by surface meltwater
runoff is similar to iceberg-calving 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.
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.
19
Topographic map of southern Greenland and vicinity. The locations of 136 glacial
earthquakes defining seven groups are indicated with red circles: DJG, Daugaard Jensen
Glacier (5 events); KG, Kangerdlugssuaq Glacier (61); HG, Helheim Glacier (26); SG,
southeast Greenland glaciers (6); JI, Jakobshavn Isbrae (11); RI, Rinks Isbrae (10); NG,
northwest Greenland glaciers (17). Owing to the tight clustering of the earthquakes, many
of the individual symbols on the map overlap. (Source: Ekstrom et al., 2006).
(A) Histogram showing seasonality of glacial earthquakes on Greenland. Green bars
show the number of detected Greenland glacial earthquakes in each month during the
period 1993 to 2004. Gray bars show the number of earthquakes of similar magnitude
detected elsewhere north of 45°N during the same period. (B) Histogram showing the
increasing number of Greenland glacial earthquakes (green bars) since at least 2002. No
general increase in the detection of earthquakes north of 45°N (gray bars) is observed
during this time period. (Source: Ekstrom et al., 2006).
20
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).
21
Source: Velicogna and Wahr (2005)
22
Source: Velicogna and Wahr (2005)
23
Relation between estimated atmospheric CO2 and the ice contribution to eustatic sea level
indicated by geological archives and referenced to modern (pre-Industrial Era) conditions
[CO2 = 280 parts per million by volume (ppmV), eustatic sea level = 0 m]. The most
recent time when no permanent ice existed on the planet (sea level = +73 m) occurred
>35 million years ago when atmospheric CO2 was 1250 ± 250 ppmV. In the early
Oligocene ( 32 million years ago), atmospheric CO2 decreased to 500 ± 150 ppmV,
which was accompanied by the first growth of permanent ice on the Antarctic continent,
with an attendant eustatic sea-level lowering 45 ± 5 m. The most recent time of low
atmospheric CO2 (185 ppmV) corresponds to the Last Glacial Maximum 21,000 years
ago, when eustatic sea level was –130 ± 10 m. Error bars show means ± SD. (Source:
Alley et al., 2005).
24
(Source: IPCC AR4)
25
Time series of key variables encompassing the last interval of significant global warming
(last deglaciation) (left) compared with the same variables projected for various scenarios
of future global warming (right). (A) Atmospheric CO2 from Antarctic ice cores. (B) Sea
surface temperature in the western equatorial Pacific based on Mg/Ca measured in
planktonic foraminifera. (C) Relative sea level as derived from several sites far removed
from the influence of former ice-sheet loading. MWP, meltwater pulse. (D) Atmospheric
CO2 over the past millennium (circles) and projections for future increases (solid lines).
Records of atmospheric CO2 are from Law Dome, Antarctica, and direct measurements
since 1958 are from Mauna Loa, Hawaii. Also shown are three emission scenarios for
time evolution of atmospheric CO2 over the course of the 21st century and subsequent
stabilization through the 22nd century. (E) Temperature reconstruction for Northern
Hemisphere from 1000 to 2000 AD (gray time series), global temperature based on
historic measurements, 1880 to 2004 (blue time series), and projected warming based on
simulations with two global coupled three-dimensional (3D) climate models with the use
of three emission scenarios (orange time series). (F) Relative sea-level rise during the
19th and 20th centuries from tide gauge record at Brest, France (green time series),
projections for contributions from combined Greenland and Antarctic ice sheets (dark
blue time series), and projections for sea-level rise from thermal expansion based on
climate simulations shown in (E) (light blue time series). (Source: Alley et al., 2005).
26
Future evolution of the Greenland Ice Sheet calculated from a 3D ice-sheet model forced
by three greenhouse gas stabilization scenarios. The warming scenarios correspond to the
average of seven IPCC models in which the atmospheric carbon dioxide concentration
stabilizes at levels between 550 and 1000 ppm after a few centuries and is kept constant
after that. For a sustained average summer warming of 7.3°C (1000 ppm), the Greenland
Ice Sheet is shown to disappear within 3000 years, raising sea level by about 7.5 m. For
lower carbon dioxide concentrations, melting proceeds at a slower rate, but even in a
world with twice as much CO2 (550 ppm or a 3.7° C summer warming) the ice sheet will
eventually melt away apart from some residual glaciation over the eastern mountains.
(Source: Alley et al., 2005).
27