TCThe CryosphereTCThe Cryosphere1994-0424Copernicus GmbHGöttingen, Germany10.5194/tc-9-945-2015Future climate and surface mass balance of Svalbard glaciers in an RCP8.5
climate scenario: a study with the regional climate model MAR forced by MIROC5LangC.charlotte.lang@ulg.ac.beFettweisX.https://orcid.org/0000-0002-4140-3813ErpicumM.Département de Géographie, Université de Liège, Liège, BelgiumC. Lang (charlotte.lang@ulg.ac.be)7May20159394595629October20143December2014This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://tc.copernicus.org/articles/9/945/2015/tc-9-945-2015.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/9/945/2015/tc-9-945-2015.pdf
We have performed a future projection of the climate and surface mass balance
(SMB) of Svalbard with the MAR (Modèle Atmosphérique Régional) regional climate model forced by
MIROC5 (Model for Interdisciplinary Research on Climate), following the RCP8.5 scenario at a spatial resolution of
10 km. MAR predicts a similar evolution of increasing surface melt
everywhere in Svalbard followed by a sudden acceleration of melt around
2050, with a larger melt increase in the south compared to the north of the
archipelago. This melt acceleration around 2050 is mainly
driven by the albedo–melt feedback associated with the expansion of the
ablation/bare ice zone. This effect is dampened in part as the solar
radiation itself is projected to decrease due to a cloudiness increase. The
near-surface temperature is projected to increase more in winter than in
summer as the temperature is already close to 0 ∘C in summer. The
model also projects a stronger winter west-to-east temperature gradient,
related to the large decrease of sea ice cover around Svalbard.
By 2085, SMB is projected to become negative over all of Svalbard's glaciated regions,
leading to the rapid degradation of the firn layer.
Introduction
Worldwide, glaciers and ice caps are currently observed to be retreating. At present, they
contribute to sea level rise (SLR) as much as the Antarctic and Greenland ice
sheets . Arctic glaciers have been the second
contributor to SLR among glaciers and ice caps between 1961 and 2004
. However, contrary to what was previously estimated
, glaciers and ice caps (as found over Svalbard for
example) are no longer believed to be the dominant contributors to SLR in the
next decades, as the melt of the Antarctic and Greenland ice sheets has been
accelerating . Nevertheless, the vanishing of Svalbard
glaciers could have huge impacts on the fauna and flora, permafrost
, tourism and even possibly the development of
agriculture. Future
projections of Svalbard's climate have been made but the
future evolution of the glaciers of Svalbard themselves have been little
studied and most studies have focussed on past and present surface mass balance
(, and references therein). studied the impact
of the future sea ice decline on
the temperature, precipitation and surface mass balance (SMB) of Svalbard
while , and evaluated
the contribution of Svalbard glaciers to future sea level rise. These SMB calculations
are based on empirical models and are rarely forced by
outputs from high-resolution atmospheric models but rather by global ones.
Therefore, we propose a more extensive study at high resolution (10km) of the
future of Svalbard glaciers and ice caps using the regional climate model MAR
(Modèle Atmosphérique Régional) evaluated over the current
Svalbard climate in the companion paper .
For the first time, this study uses an atmospheric model fully coupled
to a snow module, which explicitly solves the energy and mass balance of
glaciated regions. This coupling allows us to take atmosphere–surface feedback explicitly into account in future SMB projections. This computationally
intensive approach currently allows only a single scenario and forcing model
to be used, but we provide evidence in Sect. that this is
representative of a combination of models.
In Sect. , we present the future SMB of Svalbard
and its regional evolution through the 21st century. In Sect. , we
investigate the temperature change and how it should be impacted by the sea
ice cover decrease. In Sect. , we describe the evolution of the
melt season and, finally, the sensitivity of the energy balance components to
rising temperatures is investigated in Sect. before concluding in
Sect. .
Models and climate forcings
MAR is a regional climate atmospheric model fully
coupled to a surface model resolving the energy balance at the surface of the
snow pack and has been described in . The version and forcings of
the model are the same as those used over the present era in . We ran MAR over the period 2006–2100 at a spatial resolution of
10 km. The lateral and upper (tropopause) boundaries (temperature,
humidity, wind speed and surface pressure) as well as oceanic boundaries (sea
surface temperature and sea ice cover) were forced every 6 h by the
MIROC5 global model (Model for Interdisciplinary Research on Climate;
) using the RCP8.5 scenario .
MIROC5 has been successfully evaluated over Svalbard in the companion paper
. MIROC5 performs as one of the best CMIP5 GCMs (general
circulation models) over Greenland
. Over Svalbard, MIROC5 also performs well and
the near-surface temperature bias from MIROC5 is no longer significant over land
in MAR forced by MIROC5. As a result, SMB, precipitation and runoff modelled
by MAR forced by ERA-Interim and MIROC5 are not significantly different over
the present era.
Melt increases non-linearly with temperature, so it is very important to
realistically simulate the present climate, especially the elevation of
the 0∘C isotherm. Of course, simulating a realistic
current climate does not necessarily mean that future changes are also robust.
CMIP5 GCMs do not project significant circulation changes in the Arctic
so that projected temperature changes dominate the SMB change
. The temperature increase projected by MIROC5 follows the CMIP5
ensemble mean until 2060 (Fig. ) and exceeds the ensemble mean
after that. Our projection for 2100 with this forcing may therefore be representative
for later decades, and does not alter the main results. The extreme scenario RCP8.5
was chosen to have a forced warming signal that significantly exceeds
natural interannual variability.
(a) 1980–2100 evolution of the
JJA near-surface temperature (TASJJA, ∘C)
anomaly over Svalbard with respect to the 1980–2005 mean simulated by MIROC5 (red curve), the
CMIP5 GCMs (grey curves), the ensemble mean (black curve) and MAR forced by MIROC5 under the RCP8.5 scenario (yellow curve).
(b) Same as (a) but for the annual near-surface temperature.
Surface mass balance
Figure a shows that MAR SMB is projected to be
negative on average over 2070–2099 over the entire archipelago, according to
the MIROC5-based RCP8.5 scenario. MARRCP8.5 predicts that the
greatest losses will mostly happen in the southern part of Spitsbergen with
values lower than -4000 mmw.e.yr-1 in the most
extreme cases, where we also have the largest differences compared to the
1980–2005 average (Fig. b and Fig. 14 in (MARhisto)).
This suggests that the surface mass loss from
small southern glaciers will be higher than over the ice caps and large ice
fields of northern Spitsbergen. The mean 2070–2099 meltwater runoff anomaly
is largely positive (Fig. c), and the largest anomalies
(>5000mmw.e.yr-1) are also located in the south of the
archipelago. The snowfall will mostly increase
(Fig. d) but not nearly enough to compensate for the
increase in meltwater runoff, as also simulated by MAR over the Greenland ice
sheet and by RACMO2 (Regional Atmospheric Climate Model)
in the Canadian Arctic Archipelago
. At lower elevations, however, the snowfall anomaly is
mostly negative because the winter solid precipitation increase will not be able to
compensate for the summer decrease as a large part of the current snowfall
is projected to become rainfall at the end of this century.
(a) 2070–2099 mean SMB (mmw.e.yr-1) as
simulated by MAR forced by the MIROC5-based RCP8.5 scenario. (b)
Difference between (a) and the 1980–2005 mean shown in Fig. 14 in
. (c) Same as (b) but for runoff. (d)
Same as (b) but for precipitation.
Figure , showing the temporal evolution of the annual
SMB for five different regions around the archipelago, confirms that the
surface mass loss acceleration after 2050 is larger in the south of the
archipelago than in the north. MARhisto and MARRCP8.5 project a similar SMB
evolution for all our five regions until 2050. After 2050, the acceleration of
surface mass loss is projected to increase suddenly and be more pronounced in
the south of Spitsbergen and on Barentsøya and Edgeøya (BE) than in
west/east Spitsbergen and on Austfonna and Vestfonna (AV). After 2085, the
surface mass loss is projected to stabilise and even to decrease slightly
according to the MIROC5-based RCP8.5 scenario. The SMB future evolution is
primarily determined by the significant runoff increase (Fig. a)
as the snowfall remains much more constant in time and very similar
from region to region (Fig. S1a in the Supplement).
SMB 10-year running mean (mw.e.yr-1) for five
different regions (Austfonna and Vestfonna, west Spitsbergen, east
Spitsbergen, south Spitsbergen and Barentsøya and Edgeøya) as simulated
by MAR forced by the MIROC5-based historical scenario over 1980–2005 and
RCP8.5 afterwards. The units are in mw.e.yr-1 (rather than
Gtyr-1) to be independent of the different areas of the regions. The
permanent ice mask of each region defined for the regional evolution is shown
in the inset.
The increasing summer near-surface temperature
(TASJJA, JJA for June–July–August) explains in part the acceleration of melt
around 2050 but not the regional differences (Fig. b),
which result rather from the surface JJA albedo–melt feedback
(Fig. c) associated with the expansion of the
ablation/bare ice zone as also projected over the Greenland ice sheet .
However, the JJA albedo–melt feedback is partly reduced in the west by the
decrease of the solar flux at the surface caused by a larger cloud optical depth
in west and south
Spitsbergen in summer, compared to the northeast and the AV ice caps
(Fig. S1b and c). The larger cloud optical depth in the west and the south
is caused by a warmer and therefore more humid atmosphere. As a result, despite
a larger decrease of JJA surface albedo in west Spitsbergen than in the other
northern regions, the amount of net shortwave radiation absorbed by the surface
in west Spitsbergen is closer to the amount over the other regions
(Fig. d).
Figure shows the projected
yearly anomaly (with respect to the historical mean) of SMB integrated
over the 21st century. This gives an estimate of the impact on the ice
caps topography of the SMB changes integrated over this century (by assuming
that there is no change in ice dynamics).
In the south and along the west coast, some glaciers could lose more than
200 mw.e. over the 21st century. BE is projected to be the first
of our five regions to undergo net ablation as MARRCP8.5
projects that the accumulation zone on BE will disappear
by 2065 and will be reduced to less than 5 % of the total glaciated
area of BE as early as 2035 (Fig. ).
In south Spitsbergen, the vanishing of the accumulation zone is projected
to happen around 2065 and even Austfonna and Vestfonna will undergo net
ablation at the end of the 21st century, leading to rapid degradation
of firn. However, on Austfonna, given the large ice thickness
, we expect that a great part (in area) of the ice cap
will still remain at the end of the century even if the SMB is negative
everywhere and that the retreat will only concern the margins in 2100.
(a) 10-year running mean of the meltwater runoff
(RU, mw.e.yr-1) over 1980–2099 for the five regions shown in
Fig. . (b) Same as (a) but for the
near-surface JJA temperature (TASJJA, ∘C).
(c) Same as (a) but for the JJA albedo (ALJJA).
(d) Same as (a) but for the JJA net solar radiation
absorbed by the surface (SWnetJJA,Wm-2).
Over the whole 21st century, the integrated Svalbard MARRCP8.5-based
SMB decrease corresponds to a mass loss of 2600 km3w.e.
(i.e. 2827 km3 of ice) with respect to the historical mean.
The MARRCP8.5 SMB decrease compared to the present value is therefore
projected to contribute 7.2 mm to the 21st century sea level rise (SLR),
according to MIROC5-based
MARRCP8.5. calculated a mean value of the sea level
rise associated with the 21st century SMB changes of Svalbard with a positive
degree-day (PDD) model based on the outputs of an ensemble of 14 GCMs for the
RCP8.5 scenario. Their projected SLR at the end of the century is more than
twice as large as ours (15.81 mm). projected a SLR
between 15 and 25 mm for Svalbard for the RCP8.5 scenario, with an empirical model based on
the outputs of climatologies and CMIP5 GCMs.
However, these values were based
on large-scale temperature and precipitation changes from global models, in
most of which the topography of Svalbard is not explicitly represented given
their huge spatial resolution.
Moreover, the surface temperature of glaciated regions is limited
to 0∘C in MAR, damping the MAR near-surface temperature
increase (Fig. ), whereas there is no limitation in most GCMs .
Additionally, those studies are based on empirical calculations of the energy balance
while ours are physically based, which also explains part of the differences in SMB values.
Finally, there is also an error in our estimation due to the use of a fixed
ice mask and topography. However, we estimate this error to be small (10 %
of the SMB anomaly, see discussion below) and the SLR contribution from MAR would
still have been twice as small as the and
estimations had we not used a fixed ice mask and topography.
Projected cumulated anomaly
of SMB changes (m w.e.) over the 21st century. The SMB
anomaly is the difference with the 1980–2005 mean and has
been summed over 2000–2100.
10-year running mean of the accumulation area ratio (AAR)
over 1980–2099 for the five regions shown in Fig. .
AAR represents the ratio of the area of the accumulation zone of a region
compared to the total area of the region, i.e. the proportion of a
region that is in the accumulation area.
estimated
the total present ice volume of Svalbard to be 9089 km3, which
corresponds to a potential sea level rise of 23 mm. Our projection
therefore suggests that 31 % of their present estimated volume will
disappear by 2100. According to a previous estimate of 7000 km3
(equivalent to a sea level rise of 20 mm) by ,
about 40 % of the ice mass is projected to disappear by 2100 in our
projection, due to surface mass loss only.
As shown in , a resolution of 10 km smoothes the
topography, especially on Spitsbergen where the topography is very steep. As
a result, the elevation is underestimated over a large part of Svalbard and
some low altitude glaciers should not even exist in our 10 km grid,
causing a likely overestimation of the surface mass loss in our projection.
Moreover, glaciers are typically concentrated at higher elevations, where the
negative elevation bias in MAR is largest, leading to further
overestimated mass loss.
The topography is also fixed in our simulations,
which is an acceptable approximation under the present climate but will
likely introduce an underestimation of the melt increase in the future, as
a result of surface lowering. On the other hand, glaciers
are going to retreat in the future, and using a fixed ice mask like we do overestimates
the melt, as some areas should not
be covered with permanent ice under the future warmer climate. The
contribution of these areas (with relatively high mass loss) to the sea
level rise should be removed in our projection. As the aforementioned
effects partly compensate for each other, we expect a relatively minor
impact on our future projection. According to , the additional SMB
changes coming from topography changes are about 10 times lower than SMB changes
directly induced by climate warming. Over the Greenland ice sheet,
those effects are projected to contribute to about only 5–10 % of
the SMB anomaly by the end of the century and we assume their contribution to be of
the same order of magnitude in Svalbard. However, only a high-resolution
simulation coupled with an ice sheet model could yield insight in the magnitude of this
contribution. In southern Spitsbergen, given the very negative values of SMB and
the fact that glaciers rather than ice caps prevail, we expect the retreat
effect to be dominant and MARRCP8.5 probably overestimates the
surface mass loss in this area. On Austfonna, on the other hand, we expect
the retreat to be limited only to the proximity of the margins, but the
elevation decrease towards the centre of the ice cap is also expected to be
limited. We therefore expect that, on Austfonna, both effects will balance each other out, or at
least that none of them will be largely dominant.
(a) 1980–2005 mean summer (JJA) near-surface temperature
(∘C). (b) Same as (a) but for winter (DJF).
(c) 2070–2099 mean summer (JJA) near-surface temperature
anomaly (∘C) with respect to the 1980–2005 mean.
(d) Same as (c) but for winter (DJF).
Near-surface temperature
MARRCP8.5 predicts a rather small near-surface temperature increase
in summer (TASJJA increase of 3.0 to 6.5 ∘C) compared
to the winter increase (TASDJF (December–January–February) increase
of 11 to 25∘C) (Figs. c, d and
a). The spatial range of temperature increase over our domain
is also much smaller in summer than in winter (3 ∘C vs. almost
15 ∘C), due to the presence of a 10 ∘C west-to-east winter gradient
projected by MAR.
The pattern and magnitude of the temperature increase modelled by MARRCP8.5
are similar to estimates. projected a temperature
increase in Longyearbyen of 2.8 and 10.4 ∘C in JJA and DJF by
the end of the century using B2, A1B and A2 scenarios while our temperature
is projected to increase by 6 ∘C in JJA and 14 ∘C
in DJF. Considering that and worked with B2,
A1B and A2 scenarios and we used RCP8.5, it is to be expected that our temperature
increase is larger , and we can conclude that our results are
in qualitative agreement with those of and .
In summer, TAS is already close to 0 ∘C over the historical
period (Fig. a) and can not increase very much because the
excess energy available at the surface is used to melt snow/ice. According to
our MIROC5-based RCP8.5 scenario, JJA temperature is projected to increase by
3.75 to 4.75 ∘C over the glaciated areas
(Fig. c) and the only regions where the TAS increase is
larger (up to 6.5 ∘C) are regions with small permanent ice
area at present, i.e. BE and Nordenskiöld Land (orange/red area separating
the north and south of Svalbard in Fig. c).
The higher temperature increase in winter is due to (i) very low present-day
DJF temperatures (Fig. b) allowing it to increase much more
before reaching freezing point and (ii) the projected decrease of the winter
sea ice cover (SIC) (also highlighted by and ),
that is also responsible for the large west-to-east temperature gradient.
At present, there is a large west-to-east SIC gradient, caused by the North
Atlantic Drift, preventing sea ice from forming west of Svalbard. In a warming
climate, the SIC gradient will decrease, hence strongly reducing the west-to-east
gradient in near-surface air temperature.
In the future, near-surface temperature will increase more in areas where
sea ice can decrease. Therefore, in the west, as
there is already no significant sea ice cover in the present climate,
the projected temperature increase is much lower than in
the east. We have shown in that the ocean has a large influence
on the climate in Svalbard, even quite far inland. In Fig. a, showing the
1980–2005 mean JJA TAS, the temperature follows the topography whereas in
winter (Fig. b), the most dominant temperature gradient is the west-to-east
gradient due to the presence or absence of sea ice. At the end of the
century, the effect of topography is projected to become dominant in winter
(Fig. S2b) as most of the sea ice will have disappeared according to the
MIROC5-based RCP8.5 scenario. The DJF east coast maximum temperature
increase in is located on the east coast of Nordaustlandet,
whereas ours is on BE and our Nordaustlandet anomaly lies rather around
+16/17∘C, compared to +21∘C in
using HadRM3 (Hadley regional climate model). This is probably due to the fact that MIROC5
overestimates the present sea ice extent and still has up to 40 % of
sea ice cover on the east coast of Nordaustlandet over the period 2070–2099
(Fig. S3), whereas HadGEM1 (Hadley Centre Global Environmental Model;
used as forcings in ) ocean is mostly
ice-free at the end of this century.
(a) Mean annual cycle of the surface melt (mm w.e. d-1, 30-day running mean) for the listed
decades. The 1980–2005 mean is shown in black as comparison. (b)
Annual cycle of the surface melt in the 2050s (solid line) as well as, in
dashed lines, the cycle if it were symmetrical with respect to its maximum.
(c) Same as (b) but for the 2090s. (d) Mean annual
cycle of melt (solid line) and runoff (dashed line)
(mmw.e.d-1) during the 1980–2005 period. (e) Same as
(d) in the 2020s. (f) Same as (d) in the 2060s.
Melt season
During the first half of this century, MARRCP8.5 projects that
the beginning of the melt season (Fig. a) will not vary much
(melt season will start 0.2 days earlier per year) because the effect
of the temperature increase bringing more energy for the melt
(Fig. a) will be compensated by the albedo effect
(Fig. c) induced by increasing winter snowfall
(Fig. b). As the amount of snowfall
increases, so does the winter snowpack height above bare ice/old dirty snow
at the beginning of the summer. The appearance of low albedo zones in summer
is therefore delayed and the SWnet (net shortwave radiation flux) available for the melt in the energy budget
is reduced. After the 2050s, the temperature increase is projected to
dominate the effect of heavier snowfall accumulation and the melt season is
expected to start significantly sooner (1.5 days earlier per year).
(a) Mean annual cycle of TAS (∘C, 30-day running mean) over the permanent ice covered area for the
listed decades. The 1980–2005 mean is given in black as comparison.
(b) Same as (a) but for the snowfall
(mmw.e.d-1). As the daily variability of precipitation is very
high, we have applied here a 60-day running mean instead of 30 days
(like in Figs. and a and c) in order
to make the figure more clear. (c) Same as (a) but for the
albedo.
The seasonal melt maximum happens around 15–20 July through the whole
21st century and coincides with the temperature maximum. Before 2050, the
temperature seasonal cycle is more or less symmetrical with respect to its
maximum value. The seasonal melt (albedo) cycle is also symmetrical with respect
to its maximum (minimum) (Figs. b and
c). In the second half of the century, the temperature
and therefore the melt are projected to increase more after their seasonal
maximum than at the beginning of summer. The melt asymmetry is also partly
explained by changing snowfall that is projected to increase before June
but to significantly decrease in late summer, impacting the melt through
positive albedo feedback.
As early as the 2030s, the MARRCP8.5 time of runoff maximum
coincides with the time of melt maximum (Fig. d, e and f).
The 5- to 8-day delay visible in
Fig. d, e and f corresponds to the time needed in MAR for the
meltwater to runoff from the glaciers to the sea as parametrised in
. The runoff maximum is also projected to be
equal to (or near to) the melt maximum. This agreement in time is due to
the fact that, from the 2030s, at the time of the melt maximum, a smaller
fraction of the melting area is covered with snow (retaining part of the
meltwater and delaying the runoff) and large areas are covered with bare ice or
impermeable snowpack (snow becomes impermeable when its density reaches
830 kgm-3 and prevents meltwater from percolating and refreezing)
damping the meltwater retention capacity of the glaciers. During the
historical period and up until the 2020s on the other hand, the presence of snow
above ice in the ablation zone allows part of meltwater to be stored in the
snowpack and refreeze in winter without running off.
A rapid decrease of the refreezing capacity of the Greenland ice sheet and its buffering role in the
future was also projected by van Angelen et al. (2013).
Conversely, at the beginning of the melt season, there will still be a small delay between the melt
and runoff seasons as the bare ice will be covered by the winter snowpack even at
the end of the century. However, this delay will decrease steadily with
time as the water storage and refreezing capacity will also decrease, as
a consequence of the snow cover decrease in the enlarging ablation zone.
Energy balance
Studying energy balance components anomaly vs. temperature anomaly (rather
than vs. time) offers the advantage that results do not depend on the choice
of a particular future scenario, as shown by .
The net energy available at the surface for the melt (NET) can be calculated
as follows:
NET=SWnet+LWnet+SHF+LHF(Wm-2),
where
SWnet = SWD×(1-a) is the net shortwave
radiation, i.e. the amount of the downward shortwave (solar
radiation) energy flux (SWD) that is absorbed by the surface following
its albedo (a).
LWnet = LWD - LWU is the net long-wave radiation, i.e. the difference
between the downward long-wave radiation coming from the atmosphere (LWD) and the
upward long-wave radiation emitted by the surface (LWU).
SHF and LHF are the sensible and latent heat
fluxes.
Two other net shortwave radiation fluxes have also been estimated (Fig. c and
Table ) in
order to distinguish between the effects of the albedo change and the solar radiation change
alone on SWnet, as done in :
SWalb = SWDave× (1-a)
SWswd = SWD × (1-aave)
where the subscript “ave” denotes the 1980–2005 mean value. SWalb represents the effect of
the varying albedo alone on SWnet and has been computed by keeping constant the amount of
solar radiation reaching the surface (1980–2005 mean value of SWD) and allowing the
albedo to vary throughout the investigated period (1980–2100). SWswd, on the other hand,
represents the effect of the varying amount of solar radiation alone at the surface and has
been computed by keeping the albedo constant and allowing SWD to vary (1980–2100).
(a) Melt and runoff anomalies (Gtyr-1) vs.
TASJJA anomaly (∘C). The anomalies are
differences with respect to the 1980–2005 mean. (b) Same as
(a) but for the JJA net energy flux at the surface
(Wm-2). (c) Same as (a) but for the JJA energy
balance components. The solid lines are quadratic regression curves.
In summer, the snowpack melts and the subsurface heat flux is therefore
negligible. In the future, it will become even more negligible as larger
and larger parts of the glaciated area will start melting and most of the
snowpack will have a temperature of 0∘C.
We therefore do not take this flux into account in the energy balance equation.
Figure b
shows that the JJA net energy flux at the surface (and therefore melt and
runoff, Fig. a) quadratically increase with the
JJA TAS projected changes, as also projected over Greenland .
Figure c shows the evolution of the anomaly of
each energy balance component (JJA) as a function of the TASJJA
anomaly. In order to distinguish between the albedo and solar radiation effects in
SWnet, we estimated two additional variables for the net solar radiation,
as done in . First, we computed SWswd, reflecting the effect of SWD on
SWnet, by using the 1980–2099 SWD outputs and the 1980–2005 mean value of the surface
albedo. Secondly, we computed SWalb, reflecting the effect of the albedo on SWnet, by
using the 1980–2099 albedo outputs and the 1980–2005 mean value of SWD.
Anomaly of the energy balance components (Wm-2) and
relative contribution of the energy balance components to the NET anomaly
(2080–2099 mean compared to the historical period).
Energy balanceAnomaly% of NETcomponent(Wm-2)anomalySWnet2533SWalb3849SWswd-6-7.5SHF1924LHF1722LWnet1621NET77
MARRCP8.5 predicts that, at the end of the century (2080–2099
mean), the anomaly of SWnet will represent 33 % of the NET anomaly,
while the SWalb anomaly, reflecting the effect of the albedo on SWnet, will
account for 50 % of the NET anomaly (Table ). The
expected increase in cloud optical depth will decrease the incident solar radiation
at the surface (Fig. c), and it partly compensates
for the increase of SWalb associated with the decreasing albedo,
leading to a positive and increasing
SWnet, as also projected over Greenland .
The second contribution to the NET increase is the sensible heat
flux, whose anomaly at the end of the century is projected to represent
24 % of the NET anomaly, as a consequence of the advection of warmer
(oceanic) air over the cold ice/snow surface. At present, the modelled TAS is
negative on average in summer and therefore lower than the snow/ice
temperature (0 ∘C as the surface snow/ice is melting). SHF is
thus also negative and the surface loses energy to the atmosphere.
MARRCP8.5 predicts that, around 2030, the summer near-surface temperature will
become positive and consequently higher than the melting snow/ice
temperature. The JJA SHF averaged over the entire Svalbard will also become
positive.
The third contribution to the NET change is the latent heat flux, counting for
22 % over Svalbard, whereas it is the smallest contributor of the
energy fluxes over Greenland . LHF is currently negative as
evaporation and sublimation, requiring energy, are the dominant processes, but
they will decrease in the future in favour of condensation and deposition
(giving energy to the surface) as more and
more humid and warm air due to the reduction of sea ice during summer will be
advected towards the cold ice surface. On the other hand, condensation and
deposition will also directly contribute to accumulation (10 % of
the mean 2080–2099 accumulation) and act to oppose mass loss. In contrast
to the Greenland ice sheet , which is
higher in altitude, the oceanic conditions around Svalbard have a larger impact on
its climate. In Svalbard, the katabatic winds, weaker than in Greenland, can
not prevent the warm oceanic air from penetrating up to the central regions, and the
SHF and LHF increase will take place over the entire land area instead of along
the ice sheet margins as in Greenland .
Finally, the weakest contribution will come from the net long-wave radiation flux
(LWnet, 21 % of the 2080–2099 NET anomaly). The increase in
long-wave radiation emitted downward by the warmer and wetter atmosphere
following the increase of the greenhouse gases concentration will partly be
counterbalanced by the increase in upward long-wave radiation emitted by the
surface, due to the surface temperature increase.
Conclusions
Over the 21st century, according to MARRCP8.5,
the warming induced SMB decrease will be amplified by the snow/ice albedo feedback
related to the extension of the ablation area that will increase the net shortwave
radiation absorbed by the surface (and thus increase the energy available for the
melt) and will decrease the meltwater retention capacity. The projected rapid
decrease of the albedo will cause an acceleration of mass loss around 2050.
MARRCP8.5 simulates a larger acceleration of mass loss in
the south of the archipelago compared to the north. This regional difference
is due to a larger increase of JJA SWnet in the south, related to the
larger decrease of the JJA surface albedo. SWnet is the component of the
energy balance the most sensitive to an increase in temperature because of
the decreasing surface albedo. However, the downward shortwave radiation
itself also decreases with increasing temperature due to an increase in
cloud optical depth which partly counterbalances the effect of the melt–albedo positive
feedback.
The summer sensible and latent heat fluxes are both negative at
present but will increase with increasing temperature and become positive in the
future thereby heating the surface. The LHF increase will be caused by the
decreasing SIC allowing for more evaporation around Svalbard and warmer and more
humid air to be advected over the cold ice surface, showing the significant
impact of the oceanic conditions on Svalbard, even far inland. The SHF will become
positive when the temperature of the warmer oceanic air advected over the
cold ice/snow surface will become positive, causing the atmosphere to give
energy to the surface.
The temperature is projected to increase more
in winter than in summer as (i) the surface temperature is limited
to 0∘C, damping the temperature increase in summer
and (ii) sea ice retreat is higher in winter than in summer since
a large part of the ocean surrounding Svalbard is already ice free
in the current climate ().
Because of the larger present sea ice cover east of the archipelago
than west of it, the winter temperature increase will be larger in
the east than in the west.
All glaciated areas of the archipelago are projected to undergo net ablation
by the end of the century. The disappearance of the accumulation zone is projected to
happen much earlier in the south and northwest of Spitsbergen than in the
northeast and on the ice caps. However, even in these regions, the accumulation
area is projected to completely disappear by the end of the
century. The contribution of Svalbard 21st century SMB changes to sea level rise under the
RCP8.5 scenario will be about 7.1 mm, according to MIROC5-forced MAR.
The increase of snowfall accumulation during winter and spring and the small
increase in temperature at the beginning of the melt season explain why,
during the first half of this century, the melt season is not expected to
start much earlier than now, as the low albedo zones will be covered by
a thicker winter snowpack. However, as the melt area is projected to be no
longer covered with melting snow but rather with bare ice at the time
of the melt maximum as early as the 2030s, the meltwater retention and
refreezing capacity of the ice sheet will decrease
greatly, and the runoff maximum will be equal in magnitude to the melt maximum and
there will not be any delay between them.
Finally, it should be noted that the ice caps topography is fixed during our
simulation, suggesting that we underestimate the surface mass loss in our
projection as glacier thinning is not taken into account. On the other hand,
our ice sheet mask is also fixed, suggesting that our projected integrated
surface melt includes ice areas that will disappear in the near future and
therefore that we overestimate the contribution of Svalbard to the sea
level rise. This drives the necessity of coupling MAR with an ice sheet
model in further development to evaluate if not taking into account the
glaciers thinning is counterbalanced by the use of a fixed permanent ice mask
or not. In addition, a 10 km resolution results in an underestimation of
the topography over most of the archipelago and an increased melt.
Future projections at higher resolution (∼5km) are therefore
required to better resolve the altitude of small glaciers.
The Supplement related to this article is available online at doi:10.5194/tc-9-945-2015-supplement.
Acknowledgements
C. Lang work is supported by a PhD grant from the Fonds pour la formation à la
Recherche dans l'Industrie et l'Agriculture (FRIA), Belgium.
The authors wish to thank the two anonymous reviewers as well as Marco Möller and
Michiel van den Broeke, whose comments helped improve this manuscript greatly.
Edited by: M. van den Broeke
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