Let1991a

Let1991a
Journal of Glaciology, Vol. 37, No. 125, 1991
Steady-state characteristics of the Greenland
ice sheet under different climates
ANNE LETRÉGUILLY,*
Alfred-Wegener-Institut für Polar- und Meeresforschung, D-2850 Bremerhaven, Germany
PHILIPPE HUYBRECHTS
Geografisch Instituut, Vrije Universiteit Brussel, B-1050 Brussel, Belgium
NIELS REEH
Alfred-Wegener-Institut für Polar- und Meeresforschung, D-2850 Bremerhaven, Germany
ABSTRACT. The Greenland ice sheet is modelled to simulate its extent and
volume in warmer climates, and to find out whether the ice sheet would re-form on
the ice-free bedrock under present climatic conditions. The ice-sheet model is a three-
dimensional thermo-mechanical model with a fine-resolution grid. The bedrock
surface beneath the ice sheet was mapped using radio-echo-sounding measurements
by the Electromagnetic Institute, Copenhagen. The model experiments show that
increased temperature will result in ice-margin retreat, but the ice sheet is relatively
stable; it takes a temperature rise of at least 6 deg for the ice sheet to. disappear
completely, which indicates that the ice sheet probably survived the last interglacial.
Furthermore, it appears that the Greenland ice sheet is not a mere relict ice mass from
a previously colder climate but that the ice sheet will still re-form on the bare bedrock
under the present, or even slightly warmer, climatic conditions.
INTRODUCTION
In Greenland, the processes limiting the size of the ice
sheet are the ablation (melting of ice) and calving of ice
into the sea, in roughly equal proportions. Large ablation
areas can presently be found along the ice-sheet edge.
Because of this, it is generally believed that the Greenland
ice sheet is vulnerable to a climatic warming. Further-
more, the present ice mass raises the surface to more than
3000 m above the bedrock, which creates a climate very
different from that which would have existed on bare
bedrock if the ice sheet were to have been removed. On
these lower elevations without ice cover, it has been
suggested (Oerlemans and Van der Veen, 1984) that the
present climate would be too warm to allow the ice sheet
to re-form. Since the Greenland ice sheet survived the
transition from the last ice age to the present warmer
climate, it would then be a relict ice mass remaining from
a colder climate, in the sense that the ice sheet owes its
existence solely to its past mass-balance history. Then, how
* Present address: Laboratoire de Glaciologie, B.P. 96, F-
38402 Saint-Martin-d’Hères Cedex, France.
stable is the ice sheet, and how would the ice sheet react to
a warmer climate than today? Is the present Greenland ice
sheet indeed a relict ice mass? If the ice sheet did not
already exist, would it re-form under present climatic
conditions?
The following study attempts to answer these
questions by modelling the evolution of the Greenland
ice sheet in present and warmer climates. We start with
two different initial situations: first, with the present ice
sheet, to find out under what climatic conditions it will
disappear and, secondly, with bare bedrock, to investigate
whether the ice sheet would reappear. For such a study, a
good map of the Greenland ice sheet is essential, as well as
a map of the bedrock topography. The existing maps of
the Greenland ice sheet do not agree well with each other.
A smoothed bedrock map, based on radio-echo-soundings
performed by the Electromagnetic Institute (EMI),
Technical University of Denmark, has been published by
Bogorodsky and others (1985, p. 155). This map, however,
is not sufficiently detailed for our work. In the text that
follows, we will first describe the data sets we used to
produce the maps; then, we will give an overview of the ice
sheet and mass-balance models and, finally, we will
describe the experiments carried out to test the sensitivity
of the ice sheet under various climatic conditions.
149
Journal of Glaciology
THE MAPS
We used data of different origins for our maps. In order to
minimize the number of interpolation procedures, we
chose data that were already in digitized format, rather
than printed maps. The elevations of the ice-free areas
come from the ETOP5 World Data Base. For the ice sheet
and bedrock elevations, we used data obtained by radio-
echo-sounding flights undertaken by the Electromagnetic
Institute (EMI) of the Technical University of Denmark.
There are some problems associated with the EMI
measurements because, at some spots where the flight
lines cross each other, differences of up to 200 m in the
surface altitude can be found. However, by comparing the
EMI measurements from the southern part of the ice sheet
with the surface-elevation data from Seasat and Geosat,
Eckholm (unpublished) found that the differences are
randomly distributed. By applying a smoothing filter, the
map produced agrees reasonably well with that published
by Ohmura (1987). We did not use the satellite altimetry
that exists south of 72°N. However, the map we obtained
reproduces rather well the main features of the satellite-
altimetry map of Bindschadler and Zwally (1989).
NO
L- 70
=
20
— ES
— 60
50 40 30
Fig. 1. Surface-elevation contours of Greenland: the ice-free
areas and the bathymetry are described by the ETOP data;
the ice-sheet surface is obtained from smoothing the data from
the EMI radio-echo-sounding flights. The triangles indicate
the three deep-drilling locations: Camp Century, Summit and
Dye 3.
150
50 30 10 0 80
ИХ \ \ NE N
907
80
70 —
\
50 40 30
Fig. 2. Bedrock elevations, derived from the EMI radio-echo
sounding of surface elevation and ice thickness.
Furthermore, the EMI data have the advantage of
having the same origin as the ice-thickness measurements
(which are the only extensive source of information on ice
thickness), so that the two data sets can be expected to be
coherent when used together.
The radio-echo-sounding data (along flight tracks)
and the ETOP5 data (every 5 min of arc) were
interpolated on to a regular grid and combined, and
subsequently smoothed to produce a map of Greenland
(Fig. 1). A map for the bedrock was obtained by
subtracting the ice-thickness data from the surface
elevation (Fig. 2). The central part of the ice-sheet base
is at present below sea level and is much lower than the
surrounding mountains near the coast. However, since the
ice thickness in the area approaches values of up to
3000 m, the ice-sheet base would rise by up to 1000 m if the
ice load were to be removed. We computed the ice-free,
unloaded bedrock after isostatic rebound, by attributing a
density of 0.910 Mg m™ to the ice, and 3.370 Mg m” to the
mantle (assuming that the bedrock is now in equilibrium).
The result is plotted in Figure 3. This changes the mean
altitude of the Greenland bedrock considerably (Table 1).
In order to evaluate the accuracy of our data, we
compared their values with measurements at a few points.
Stations of known elevation are easy to find on land as well
as on ice. Ice-thickness measurements, however, are not so
90 70 50 30 10 O 80
/ 44 | \ \ NX /
907
80
N
80” 9
7S
Ss Ap
70
70
~~
70 x
\
20
so / L- 65
65 —
60 —_ — 60
7 T T ‘
50 40 30
Fig. 3. Uplifted bedrock elevations after isostatic rebound.
abundant. As a matter of fact, they are limited to the sites
of Dye 3 and Camp Century, where deep drillings have
reached the bedrock. The result of the comparison is given
in Table 2.
At Camp Century, the altitude of the surface appears
to be overestimated by 107 m. Since the surface slope of
this area is of the order of 100 m per 20 km, this difference
corresponds to the uncertainty in the location of the
station in the gridded data. 100m also seems to be the
order of magnitude of the errors in other points, also on
land. Many of the ETO elevations for the coastal stations
are slightly below sea level, but this is probably because
Table 1, Simple information on the maps
Mean altitude of the ice sheet 1788m
Mean altitude of the bedrock
At present, with the ice load 440 m
Without ice, after isostatic uplift 812m
Letréguilly and others: Characteristics of the Greenland ice sheet
the topography is averaged inside each grid cell. The ice
thickness, on the other hand, is underestimated at Camp
Century by 124 m, and at Dye 3 by 40 m. From this, it can
be concluded that both surface altitudes and ice
thicknesses are probably accurate within about 100 m.
Table 2. Comparison of the surface altitude, ice thickness and
bedrock altitude of some stations in Greenland. (1)
Terrestrial altimetry measurement (m). (2) Drilling to
the bedrock. (3) Averaged ice thickness, bedrock and surface
altitude of the 20km x 20 km grid cell where the station is
located, as obtained by radio-echo-sounding measurements
(EMI). (4) Altitude (m) of the 20 km x 20 km grid cell
where the station is located (ETOPS5 data)
Lat N Long. W (1),(2) (3)
Ice stations
Camp Century 77.18 61.15
Surface altitude (m) 1885 1992
Bedrock altitude (m) — 498 729
Ice thickness (m) 1387 1263
Dye 3 65.18 43.83
Surface altitude (m) 2438 2531
Bedrock altitude (m) 408 461
Ice thickness (m) 2030 2070
Summit 72.28 39.97
Surface altitude (m) 3204
3287
Land stations Lat. N Long. W (1) (4)
Thule AFB 76.52 68.83 11 —6
Upernavik 72.78 50.17 63 —32
Jakobshavn 62.22 51.05 40 —54
Prins Christian
Sund 60.03 43.12 76 —56
Angmassalik 62.62 37.57 36 74
Danmarkshavn 76.77 18.77 12 —34
151
Journal of Glaciology
THE ICE-SHEET MODEL
The ice-sheet model is three-dimensional, time-dependent
and calculates the fully coupled temperature and velocity
fields. Ice deformation is assumed to result from shear
strain and longitudinal stresses are disregarded. Calving of
ice into the ocean is simulated by letting the ice disappear
at the coast, i.e. the ice thickness is set to 0 at the coast
edge. Other than that, there are no constraints on the ice-
sheet geometry, which is freely generated by the model in
response to the model inputs. Model inputs are bed
topography, surface temperature, mass balance, thermal
parameters and an initial state. The model also includes
the response of the underlying bedrock to changing ice
load. The Earth deformation model is based on a viscous
asthenosphere (Oerlemans and Van der Veen, 1984) and
lithosphere deflection is given by considering local isostatic
equilibrium. The model was originally developed for the
Antarctic ice sheet, but has been adapted to simulate
conditions on Greenland. A complete description of the
model has been given by Huybrechts (1986, in press), and
Huybrechts and Oerlemans (1988).
The equations are solved numerically, using finite
differences, on a horizontal grid with squared cells of
20 km. With 14 layers in the vertical for the ice flow and
thermodynamic calculations, this adds up to a total of
nearly 200000 grid points. We used a scaled vertical
coordinate in order to avoid boundary problems in the
temperature calculations. The upper layer has a thickness
of 10% of the ice thickness, and the lowermost layer is 2%.
This allows a much more refined description of ice
deformation in the basal layers, where the shear
concentrates. The diffusion equations are solved by using
an Alternating-Direction-Implicit scheme. This approach
has the advantage that larger time steps can be taken than
the more conventional explicit integration schemes. For
the fine grid that was chosen, the time step is set at 2 years,
which means that only a powerful computer can handle
the number of operations involved. We used the CRAY-2
computer of the University of Stuttgart. On this machine,
the model needs 40 min CPU time for a 10000 years
integration.
THE MASS BALANCE
The mass balance is the most important factor determin-
ing the state of the ice sheet. In the model, the components
of the mass balance (accumulation and ablation) are
calculated separately and its perturbations are parameter-
ized in terms of temperature. Temperature is thus the
principal forcing variable. Although run-off and accumu-
lation result from quite complex processes, involving the
general circulation pattern in the atmosphere and the
energy balance at the ice-sheet surface, such a simplifica-
tion is necessary to keep calculation times on the fine grid
in use within acceptable bounds.
In particular, precipitation is a difficult process to
model. It depends not only on a cyclonic activity,
depression paths and moisture content, but accumulation
rates over the Greenland ice sheet are also determined by
such factors as temperature (since colder air can carry less
precipitable moisture), continentality and orientation of
152
the ice sheet with respect to prevailing winds (orographic
effect). These factors are much too complicated to be
parameterized. Alternatively, one could think of using a
general circulation model. However, those models only
describe the climate of the whole planet, and for a period
limited to a few years only. Also, they are not sufficiently
detailed for a specific area such as Greenland, and cannot
handle evolution over many thousands of years. So, a
more practical approach is to use observed data and to
perturb the resulting accumulation distribution by a
prescribed change in the temperature. We used a map
by Ohmura and Reeh (1991) for the mean annual
precipitation rate. The accumulation rate is then
prescribed to vary by changes in surface temperature
DT according to
acc(DT) = acc(0) x 1.0533PT, (1)
The fact that part of the precipitation falls as rain is
neglected. This is however not considered a serious
constraint, since only a small fraction of the annual
precipitation is involved, during the summer months, and
a large part of the rainfall may be expected to refreeze into
superimposed ice, before it can run off to the coast.
A 5.3% change in accumulation rate for every | deg
change in mean annual temperature is suggested by
correlating annual accumulation rates measured in
shallow ice cores in central Greenland with the corre-
sponding 8'%0 values (Clausen and others, 1988), and
converting 8'*0 to temperature by means by means of the
factor 0.62% x 8'80/deg valid for present-day central
Greenland conditions (Dansgaard, 1961). For a 5deg
temperature shift, the corresponding accumulation rates
change by around 30%. The highest degree of uncertainty
in the present approach is connected with the evolution of
precipitation in warmer climates and/or different ice-sheet
geometries; if the ice sheet changes substantially, or even
disappears, this may well affect not only the intensity but
also the pattern of the accumulation distribution.
However, very little is known in that respect. In order to
evaluate the sensitivity of the model results to the above
accumulation parameterization, a different relation was
also used, where the increase in accumulation is limited to
the present value:
acc(DT) = acc(0) x 1.0533°T for DT < 0 and
acc(DT) = acc(0) for DT > 0. (2)
The ablation model (Reeh, unpublished) is based on
the degree-day method. As shown by Braithwaite and
Olesen (1989), there is a high correlation between positive
degree days and melt rates at West Greenland ice-margin
locations. The annual number of positive degree days that
represents a melt potential is then calculated from the July
temperature and the mean annual temperature, that is
parameterized in terms of altitude and latitude using data
compiled by Ohmura (1987). The positive degree-day
model also accounts for the daily cycle and for random
temperature variations from the regular, long-term annual
cycle. The calculated number of positive degree days
(PDD) is then used to melt, in the following order, snow,
superimposed ice and ice, with degree-day factors of
70
\
20
—60
'30
Fig. 4. Modelled ice-sheet elevations for the present climate.
0.003 т ice melt/PDD for snow, and 0.008 for ice
(Braithwaite and Thomsen, 1984). The first 60% of
melted snow forms superimposed ice, and the rest runs off.
In the event that superimposed ice remains at the end of
the melting season, the warming effect due to the release of
Letréguilly and others: Characteristics of the Greenland ice sheet
latent heat during its formation is included in the ice-sheet
surface-temperature calculations.
In order to test the model, as well as to have a
reference run against which to compare the ice-sheet
evolution for different climates, Figure 4 shows the present
ice sheet simulated in steady state. The modelled ice sheet
then appears to be slightly larger and thicker than the
actual ice sheet, occupying 1.78 x 10* km” and containing
3.21 x 10° km* ofice as compared to 1.67 x 10% km” and
2.83 x 10°km® for the present ice sheet. However,
considering that the complete ice-sheet geometry is
internally generated and that response of the ice sheet to
increased accumulation rates since the beginning of the
Holocene is not precisely known, the similarity with the
present ice sheet is actually quite good.
THE MODEL EXPERIMENTS
The purpose of the model experiments is to evaluate the
reaction of the Greenland ice sheet to a climatic warming.
More specifically, we want to study (1) what would be left
of the ice sheet in a warmer climate, (2) what increase of
temperature would make the ice sheet disappear, and (3)
conditions in which the ice sheet would re-form if it had to
start from bare, ice-free ground.
In the first experiment, the steady state of the ice sheet
was computed for a range of temperature rises between 1
and 8deg relative to the present climate. The ice sheet
appears sensitive to a climate warming: even a small
increase of temperature will cause a decrease of the ice
volume. However, on the map scale, for an increase up to
2 deg, those changes are hardly detectable. For warmings
of up to 3 deg, most of the changes occur at the ice margin,
particularly in the southwestern part, which retreats by up
to 60km (Fig. 5a). The surface elevations in the central
part of the ice sheet, on the other hand, appear to be
relatively unaffected. It takes a temperature increase of
4deg for changes in the ice sheet to become more
apparent: the ice sheet then splits up into two parts, one
a b
Fig. 5. Modelled steady-state ice sheet for temperature increases of 3—6 deg as compared to the present. The ice-sheet
boundaries are not explicitly drawn, but the steep front can be recognized where three or four elevation contours (every
200m) run close together.
153
Journal of Glaciology
№
а b
С
Fig. 6. The ice sheet after 50 000 years of modelled evolution for temperature increases of 0—4 deg relative to the
present. Here, the ice-free topography after isostatic rebound served as an initial configuration.
large part covering central and northern Greenland, and a
much smaller ice cap over the southern mountains. It is
interesting to note that the deep-drilling site of Dye 3
becomes ice-free. With a 5 deg temperature rise, the main
ice sheet is shown to shrink even more, and northern
Greenland, including the area of the Camp Century deep-
drilling site, then also becomes ice-free. For a temperature
increase of 6deg, the remaining ice sheet disappears
completely, leaving behind only four small ice caps on the
southern and eastern mountains. It then takes a
temperature increase of 2 deg more for the remaining ice
masses to disintegrate completely. |
In the second experiment (Fig. 6), we investigated the
conditions under which the ice sheet would re-form. These
runs started from the bare bedrock (after isostatic
rebound) for various temperature variations between —10
and +4 deg. Surprisingly, under present conditions, the
ice sheet re-forms into an ice sheet very similar to the one
simulated by the model when starting with the present ice
sheet. The ice sheet will even re-form for climates warmer
4e+6
=
E 4 "€
X, 3e+6 7 —
y Ш
с
: 22
<
„|| - T
© 2e+6 O
LA —
La Ш
a LJ
= -
wn 1e+6 + !
5 -6 <
Ш
2 N N
0e+0 ” т " T " + A A
0 2 4 6 8
TEMPERATURE INCREASE ABOVE THE PRESENT (°C)
Fig. 7. Dependence of ice-sheet volume on a temperature rise
relative to the present. Open triangles: present ice sheet used
as initial configuration. Solid circles: ice-free Greenland used
as initial configuration.
154
than today: it re-forms completely for a climate up to 2 deg
warmer, and possibly also for a climate 3 deg warmer (in
this case, equilibrium is still not reached after 50 000
years). The Greenland ice sheet is thus clearly not a relict
ice mass left over from a previously colder climate.
The volume changes are summarized in Figure 7. As
seen already in Figures 5 and 6, the temperature-ice-
volume curve is not a one-to-one relationship, but presents
a hysteresis. At all temperature increases between 0 and
6 deg, we have two possible steady states of the ice sheet, a
situation that has been theoretically suggested by Oerle-
mans and Van der Veen (1984), although not for the high
temperature range found in our study. At a 4deg
temperature increase, for example, one of the steady
states consists of an almost ice-free Greenland, while the
other consists of a large central-northern ice sheet and a
small southern ice cap. Which situation will occur (large
ice sheet or no ice sheet) is then not only determined by
climatic conditions but also by the preceding mass-balance
history.
As mentioned before, the accumulation parameteriza-
tion is probably the most delicate part of the mass-balance
model. The present distribution on the ice sheet is based
on existing measurements in Greenland, and 1s well
described. In a landscape with little or no ice, however,
it is very possible that the precipitation pattern would be
different. The accumulation is parameterized only in
terms of temperature changes, which means that changes
in the accumulation distribution due to orographic
changes are not accounted for. As long as the ice sheet
exists in Greenland, these changes are not important and
can be safely neglected. In situations without an ice sheet,
this may introduce some errors. Such errors are likely to be
found near the western coast, where the presence or
absence of ice means a large difference in the surface
topography. In southern Greenland and along the eastern
coast are high mountains. Whether they are covered with
ice or not does not affect the altitude distribution much,
and changes of accumulation due to orographic changes
are expected to be minor there. By neglecting accumula-
Fig. 8. Steady-state ice sheet for a temperature increase of
3deg above the present, with the accumulation—temperature
relationship given by Equation (2).
tion changes due to orographic changes in the simulations
with a bare bedrock as an initial condition, we may then
be overestimating the rate of development of the ice sheet.
However, the effect on the final steady state is probably
minor.
Changes in the general circulation pattern may also
influence the precipitation pattern. To include those
effects, however, is beyond the scope of this work.
However, some experiments with a different precipita-
tion-temperature relationship (constant precipitation in
warmer climates; relation 2) were performed, in order to
estimate the sensitivity of the accumulation variation on
the results (Fig. 8). It appears that the resulting ice-sheet
configurations are only moderately affected: in the
warming experiment, the splitting of the ice sheet now
occurs for a temperature rise of 3 deg instead of 4 deg. In
the experiment with initially bare bedrock, the ice sheet
will re-form for temperatures up to 1 deg warmer than at
present instead of 2deg. Both of these accumulation—
temperature relations probably represent extreme situa-
tions, and the different results give an idea of the
uncertainty caused by the lack of knowledge of how the
Letréguilly and others: Characteristics of the Greenland ice sheet
accumulation rate will change in a warmer climate. It is
certainly comforting that the results appear to be only
moderately influenced.
The isostatic adjustment rate of the bedrock due to
changing ice load is on the order of 20000 years for
Greenland. A different value will influence the rate at
which equilibrium is reached in the simulation. However,
when equilibrium is reached, the deflection of the bedrock
is independent of this value.
In the experiments, we tried to run the model ice sheet
to a steady-state equilibrium, in order to reach a reference
state for each temperature. To do that, we computed the
evolution of the ice sheet during 50000 years for each
temperature change. For the warming experiments
(starting from the present ice sheet) shown in Figure 7,
most of the ice-sheet volume variations occur during the
first 10000 years, and equilibrium is approximately
reached after 30000 years. However, for the initially
bare bedrock experiments (see Fig. 9), 50000 years was
not always sufficient to reach equilibrium: for tempera-
tures of 2 and 3 deg higher than present, the equilibrium is
not reached even after 50 000 years. The building of an ice
sheet is clearly a much slower process than its destruction.
What mostly influences the evolution of the ice sheet is the
mass balance, which depends on temperature. It is limited
upwards, because the accumulation decreases with
decreasing temperature, while it is not limited down-
wards, because for a climatic warming, the ablation
increases faster than the accumulation. There is then a
limit to the rate of formation while, in principle, there is no
limit to the rate of destruction.
The rate of formation of the ice sheet also depends on
the initial extent of the accumulation area on the ice-free
bedrock, and hence on the topography. For DT = 0 and
—10 deg the ice sheet builds up much faster than for DT =
2 and 3deg. In the latter cases, the accumulation area at
the start is restricted to the high elevations of the eastern
mountain tops, so that the initial ice caps are very small.
They can grow only by advecting ice formed at higher
elevations, and this in turn creates an altitude-mass-
balance feed-back by raising the overall elevation and thus
by enlarging the accumulation area. This process is slow
and may well last more than 100000 years. For DT = 0,
the larger part of Greenland is in the accumulation zone
4e+6
DT=-10°C
_
3e+6 - DT=0°C
© Г
Е >
= a O
= 2e+6 1 OT=2°C 0
>
= Г |
O C
>
ul 1e+6 7 С р
O 4 DT=2°C
o OT=4°C
0е+0 BE :
0 10090 20900 30000 40000 50000
TIME (years)
Fig. 9. Growth of the ice sheet for various climatic conditions,
starting with an ice-free Greenland as an initial condition.
155
Journal of Glaciology
right from the start, not only the eastern mountains but
also the lower western mountains. The total volume of ice
that can accumulate during the first 1000 years is much
larger for DT = 0 than it is for a higher DT. As for DT =
—10 deg, all of Greenland has a positive mass balance. This
illustrates the effect of the topography. For climatic
conditions similar to today or colder, a model with a
lower resolution that does not reproduce the details of the
mountains would still be able to initiate an ice sheet on the
ice-free bedrock. However, for climatic conditions 2 or
3deg warmer than at present, the mountains are
responsible for the initiation of the ice sheet. Although
the lower reaches of Greenland may not have a climate
suitable for the development of an ice sheet, the mountains
act like a source of ice that can cover most of Greenland
with ice, given enough time. With less detailed topogra-
phy, it would be impossible to model such an effect.
Modelling equilibrium states of an ice sheet are useful
to assess its sensitivity to climate change. However, they
are unlikely to be encountered often in the glacial history.
A temperature history derived from an ice-margin oxygen-
18 record from central West Greenland (Reeh and others,
in press) indicates that the temperature in the last
interglacial was about 3-4 deg above the present value
for a duration of about 10000 years. The resulting ice
sheet would then have looked like something in between
those shown in Figure 5a and b, and would not have been
far from the equilibrium state. Furthermore, Figure 10
shows that a temperature increase of at least 6 deg during
a period of 20000 years is required for the ice sheet to
disappear completely. This is a much higher temperature
increase as well as a much longer time period than the
Eemian interglacial. This suggests that the Greenland ice
sheet must have survived the Eemian interglacial.
However, large areas may have been ice-free, and it is
questionable whether the sites of Camp Century in
northwest Greenland and particularly Dye 3 in southern
Greenland were still ice-covered during that period, since
they are located in areas of large margin fluctuations. This
could explain the difficulties in interpreting the basal part
of those ice cores. Central Greenland, on the other hand,
seems to be the most favourable location for retrieving pre-
Eemian ice, either by future deep drilling in the Summit
area (GISP and GRIP), or by ice-margin studies (Reeh
and others, 1987).
We also conducted a short-term experiment to
evaluate the response of the ice sheet to the anticipated
greenhouse warming (Huybrechts and others, in press).
Forcing the model with a temperature scenario rising
exponentially to 8 deg by the year AD 2100, the ice volume
would decrease by 68 500 km®. This is only a small fraction
of the total ice volume in Greenland, however such
wastage would be clearly observed; it corresponds to a
mean ice-margin retreat of 3.2 km, and a worldwide sea-
level rise of 17 cm.
CONCLUSIONS
The ice sheet is only moderately vulnerable to a climate
warming: it would take a temperature increase of 6 deg,
sustained over 20000 years, or a temperature increase of
8 deg, sustained over 5000 years, to allow the ice sheet to
156
4e+6
ICE-SHEET VOLUME (km3)
SEA-LEVEL CHANGE (m)
\ DT=8*C
0 10900
0e+0
20000 30000 40000 50000
TIME (years)
Fig. 10. Ice-volume evolution of the Greenland ice sheet for
warmer than present conditions, starting with the present ice
sheet as an initial configuration.
disappear completely. This increase of 6 deg is higher than
the 3—4 deg for the previous interglacial (Eemian). The
model results then suggest that the ice sheet probably
survived this period. This means that the Greenland ice
sheet still contains pre-Eemian ice, which is available for
palaeoclimatic studies. Furthermore, under present-day
climatic conditions, if the ice sheet did not already exist, it
would re-form on the ice-free Greenland topography. This
indicates that the ice sheet is not a mere relict from a
previously colder climate, but an active ice sheet that
sustains itself. Although a climatic warming would
immediately result in increased melting of the Greenland
ice sheet and retreat of the ice margin, it seems that, from
the more general point of view of the existence of the ice
sheet, it is somewhat resistant to a climatic warming.
ACKNOWLEDGEMENTS
This paper is published with the approval of the Director,
Alfred-Wegener-Institut. P. Huybrechts is supported by
the Belgian National Fund for Scientific Research
(NFWO) and is sponsored in part by the Belgian Office
of Science Policy under contract ANTAR/04. This study
would not have been possible without the excellent
computing facilities (CRAY 2, University of Stuttgart)
which were made available to us at the Alfred-Wegener-
Institut. We thank A. Ohmura (ETH Ziirich, Switzer-
land) for communicating data that were unpublished at
the time of the study, and the Electromagnetic Institute,
Technical University of Denmark, Copenhagen, for
providing the radio-echo-sounding data of the Greenland
ice sheet. The Geodetic Section of the Alfred-Wegener-
Institut assisted with the ETOP5 data. Useful comments
from C. Lorius on an earlier draft of this paper were also
much appreciated.
REFERENCES
Bindschadler, R.A., H.J. Zwally, J. A. Major and A.C.
Brenner. 1989. Surface topography of the Greenland ice sheet
Jrom satellite radar altimetry. Washington, DC, National
Aeronautics and Space Administration. Scientific and
Technical Information Division. (NASA SP-503.)
Bogorodskiy, V. V., C. R. Bentley and P. E. Gudmandsen.
1985. Radioglaciology. Dordrecht, etc., D. Reidel Publish-
ing Company, Kluwer Academic Publishers.
Braithwaite, R. J. and O.B. Olesen. 1989. Calculation of
glacier ablation from air temperature, West Greenland.
In Oerlemans, J., ed. Glacier fluctuations and climatic change.
Proceedings of the Symposium on Glacier Fluctuations and
Climatic Change, held in Amsterdam, 1-5 June 1987.
Dordrecht, etc., Kluwer Academic Publishers, 219-233.
Braithwaite, К.]. and H.H. Thomsen. 1984. Runoff
conditions at Paakitsup Akuliarusersua, Jakobshavn,
estimated by modelling. Granl. Geol. Undersagelse Glet-
scher-Hydrol. Medd., 84 (3).
Clausen, H.B., N.S. Gundestrup, S.J. Johnsen, R.A.
Bindschadler and H.J. Zwally. 1988. Glaciological
investigations in the Crête area, central Greenland: a
search for a new deep-drilling site. Ann. Glaciol., 10, 10-
Dansgaard, W. 1961. The isotopic composition of natural
waters with special reference to the Greenland ice cap.
Medd. Granl., 165(2).
Eckholm, S. Unpublished. Investigation of the Green-
landic icecap using satellite altimetry. Internal Report.
Copenhagen, Danish Geodetic Institute.
Huybrechts, P. 1986. À three dimensional time dependent
numerical model for polar ice sheets: some basic testing with a
stable and efficient finite difference scheme. Brussels, Vrije
Universiteit Brussel. Geografisch Instituut. (Report 86-
1.)
Huybrechts, P. In press. A 3-D model for the Antarctic ice
Letréguilly and others: Characteristics of the Greenland ice sheet
sheet: a sensitivity study on the glacial-interglacial
contrast. Climate Dyn.
Huybrechts, P. and J. Oerlemans. 1988. Evolution of the
East Antarctic ice sheet: a numerical study of thermo-
mechanical response patterns with changing climate.
Ann. Glaciol., 11, 52-59.
Huybrechts, P., A. Letréguilly and N. Reeh. In press. The
Greenland ice sheet and the greenhouse warming. Global
and Planetary Change.
Oerlemans, J. and C.J. Van der Veen. 1984. Ice sheet and
climate. Dordrecht, etc., D. Reidel Publishing Company.
Ohmura, A. 1987. New temperature distribution maps for
Greenland. {. Gletscherkd. Glazialgeol., 23(1), 1-45.
Ohmura, A. and N. Reeh. 1991. New precipitation and
accumulation maps for Greenland. J. Glactiol., 37(125),
140-148.
Reeh, N. Unpublished. Parameterization of melt rate and
surface temperature on the Greenland ice sheet. Manu-
script. Bremerhaven, Alfred Wegener Institute for Polar
and Marine Research.
Reeh, N., H. H. Thomsen and H.B. Clausen. 1987. The
Greenland ice sheet margin — a mine of ice for paleo-
environmental studies. Palaeogeogr. Palaeoclimatol.
Palaeoecol, 58, 229-234.
Reeh, N. H. Oerter, A. Letréguilly, H. Miller and H.-W.
Hubberten. In press. A new detailed ice-age oxygen-18
record from the ice-sheet margin in central West
Greenland. Global and Planetary Change.
The accuracy of references in the text and in this list is the
responsibility of the authors, to whom queries should be addressed.
MS recewed 21 May 1990 and in revised form 5 November 1990
157
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement