Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya) derived from stereo imagery

Abstract

Mass loss of Himalayan glaciers has wide-ranging consequences such as changing runoff distribution, sea level rise and an increasing risk of glacial lake outburst floods (GLOFs). The assessment of the regional and global impact of glacier changes in the Himalaya is, however, hampered by a lack of mass balance data for most of the range. Multi-temporal digital terrain models (DTMs) allow glacier mass balance to be calculated. Here, we present a time series of mass changes for ten glaciers covering an area of about 50 km2 south and west of Mt. Everest, Nepal, using stereo Corona spy imagery (years 1962 and 1970), aerial images and recent high resolution satellite data (Cartosat-1). This is the longest time series of mass changes in the Himalaya. We reveal that the glaciers have been significantly losing mass since at least 1970, despite thick debris cover. The specific mass loss for 1970-2007 is 0.32 ± 0.08 m w.e. a-1, however, not higher than the global average. Comparisons of the recent DTMs with earlier time periods indicate an accelerated mass loss. This is, however, hardly statistically significant due to high uncertainty, especially of the lower resolution ASTER DTM. The characteristics of surface lowering can be explained by spatial variations of glacier velocity, the thickness of the debris-cover, and ice melt due to exposed ice cliffs and ponds.

Full text

The Cryosphere, 5, 349–358, 2011
www.the-cryosphere.net/5/349/2011/
doi:10.5194/tc-5-349-2011
© Author(s) 2011. CC Attribution 3.0 License.
The Cryosphere
Multi-decadal mass loss of glaciers in the Everest area (Nepal
Himalaya) derived from stereo imagery
T. Bolch
1,3
, T. Pieczonka
1
, and D. I. Benn
2,4
1
Institut f
¨
ur Kartographie, Technische Universit
¨
at Dresden, Germany
2
The University Centre in Svalbard, Norway
3
Geographisches Institut, Universit
¨
at Z
¨
urich, Switzerland
4
School of Geography and Geosciences, University of St Andrews, UK
Received: 1 December 2010 – Published in The Cryosphere Discuss.: 20 December 2010
Revised: 28 March 2011 – Accepted: 30 March 2011 – Published: 20 April 2011
Abstract. Mass loss of Himalayan glaciers has wide-ranging
consequences such as changing runoff distribution, sea level
rise and an increasing risk of glacial lake outburst floods
(GLOFs). The assessment of the regional and global im-
pact of glacier changes in the Himalaya is, however, ham-
pered by a lack of mass balance data for most of the range.
Multi-temporal digital terrain models (DTMs) allow glacier
mass balance to be calculated. Here, we present a time series
of mass changes for ten glaciers covering an area of about
50km
2
south and west of Mt. Everest, Nepal, using stereo
Corona spy imagery (years 1962 and 1970), aerial images
and recent high resolution satellite data (Cartosat-1). This is
the longest time series of mass changes in the Himalaya. We
reveal that the glaciers have been significantly losing mass
since at least 1970, despite thick debris cover. The specific
mass loss for 1970–2007 is 0.32±0.08mw.e.a
−1
, however,
not higher than the global average. Comparisons of the re-
cent DTMs with earlier time periods indicate an accelerated
mass loss. This is, however, hardly statistically significant
due to high uncertainty, especially of the lower resolution
ASTER DTM. The characteristics of surface lowering can be
explained by spatial variations of glacier velocity, the thick-
ness of the debris-cover, and ice melt due to exposed ice cliffs
and ponds.
1 Introduction
Recent debate on whether Himalayan glaciers are shrinking
faster than those in other parts of the world (Cogley et al.,
2010) highlighted the lack of knowledge about the glaciers
in this region. The best measure of glacier health is mass bal-
ance, which can be directly linked to climate and compared
Correspondence to: T. Bolch
(tobias.bolch@geo.uzh.ch)
to other regions. The topographic setting and the glacier
hypsometry modify the way the climatic signal is translated
into glacier mass balance, but this can calculated using a
DTM (Paul and Haeberli, 2008). However, only a few in-situ
mass balance measurements have been made on Himalayan
glaciers, and existing data series are short (Kulkarni, 1992;
Fujita et al., 2001; Wagnon et al., 2007; Dobhal et al., 2008).
Comparisons of digital terrain models for different years can
complement field measurements, and allow regional mass
balance to be estimated (Bamber and Rivera, 2007). How-
ever, to date it has only been applied to some glaciers in the
western Himalaya for 1999 to 2004 (Berthier et al., 2007)
and to four glaciers at Mt. Everest for 1962 to 2002 (Bolch
et al., 2008b). Broader and more detailed knowledge of
glacier mass balance is also needed to decrease the high
uncertainty about the importance of Himalayan glaciers for
water resources (e.g. Immerzeel et al., 2010) and sea level
rise (e.g. Braithwaite and Raper, 2002). Finally, improved
knowledge of glacier recession is needed to better estimate
hazards of glacier lake outburst floods (GLOFs; Richardson
and Reynolds, 2000).
The aims of this study are threefold. First, we aim to eval-
uate the results of the pilot study by Bolch et al. (2008b) us-
ing independent data sets. This study revealed surface low-
ering of 0.33 ±0.24 m a
−1
by analysing a 1962 Corona and
an ASTER DTM generated based on 2001, 2002 and 2003
data but had high uncertainties. The second aim is to present
mass balance estimates for a larger sample of glaciers around
Mt. Everest, including Imja Glacier which is of high interest
due to the proglacial lake which formed in the 1960s and has
rapidly grown since (Bolch et al., 2008a; Fujita et al., 2009).
In addition, the mass balance of the entire Khumbu Glacier
will be presented for the first time. Thirdly, we wanted to pro-
duce the first time-series of glacier volume changes around
Mt. Everest to show the suitability of different optical im-
agery to derive mass balance variability over time and to dis-
cuss the possible causes of the surface changes.
Published by Copernicus Publications on behalf of the European Geosciences Union.

350 T. Bolch et al.: Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya)
Fig. 1. Study area; location, names and debris-covered portion of the glaciers in the study area, coverage of the utilized satellite data.
Background: SRTM3 CGIAR, Vers. 4, study area: ASTER DTM; glacier outlines based on Bolch et al. (2008b).
The tongues of nine of the ten studied glaciers are heav-
ily covered by supraglacial debris (Fig. 1), with a debris
area of about 36.3km
2
and an average debris thickness in-
creasing downglacier (Nakawo et al., 1999; Hambrey et al.,
2008). The glaciers are mainly nourished by snow and ice
avalanches which accumulate cones below the steep head-
walls. Only Khumbu Glacier has an extensive accumu-
lation area (called “Western Cwm”). Glacier equilibrium
line altitudes (ELAs) are roughly estimated to be situated
above 5600m (Asahi, 2001). Ice velocities typically de-
crease downglacier from the ELA and most of the studied
glaciers have extensive stagnant ice in their lower reaches
(Bolch et al., 2008a; Scherler et al., 2008; Quincey et al.,
2009). Between 1962 and 2005 the overall glacier area
loss in the study area was ∼5% with an increasing debris-
covered area (∼2.5%) but almost stable terminus positions
(Bolch et al., 2008b).
2 Data and methodology
We used 1970 Corona KH-4B (declassified US spy imagery)
data, 1984 aerial photographs (camera: Wild RC 10) (Al-
therr and Gr
¨
un, 1990) and 2007 Cartosat-1 (Indian Remote
Sensing Satellite, IRS P5) images (Table 1). In addition, we
used previously generated 1962 Corona and 2002 ASTER
DTMs (Bolch et al., 2008b). We did not consider the DTM
data from the Shuttle Radar Topography Mission (SRTM)
due to large data gaps especially at the clean ice area of
Khumbu Glacier and the coarser spatial resolution (∼90 m)
in comparison to the ASTER DTM (30 m). In addition, us-
ing only stereo optical data results in a methodologically
consistent data set. The SRTM DTM data in contrast are
based on C-band adar beams that penetrateinto snow and can
cause higher uncertainties in the snow-covered accumulation
areas. We applied the Remote Sensing Software Package
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T. Bolch et al.: Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya) 351
Table 1. Utilized imagery and derived DTM characteristics based on the ice free area relative to the 2007 master DTM.
Date Sensor Image IDs Spatial Resolution (m) Vertical accuracy (m) Vertical accuracy (m)
before adjustment after adjustment
Imagery Original Mean elev. STDV Mean elev. STDV
DTM diff. diff.
15 Dec 1962 Corona KH-4 DS009050054DF173 173, 7.6 20 −53.0 20.9 −0.1 19.7
DS009050054DA175 175
20 Nov 1970 Corona KH-4B DS1112-1023DA163 163, 5.2 15 −9.0 27.2 −0.5 18.8
DS1112-1023DF157 157
20 Dec 1984 Wild RC-10 1440, 1441,1476,1477 0.5 15 8.2 11.2 −1.8 7.8
20 Dec 2001 ASTER L1A.003:2005569609 15 30 12.8 29.5 −1.5 10.1
21 Nov 2002 L1A.003:2009316881
23 Oct 2003 L1A.003:2018198969
13 May 2007 Cartosat-1 097071200102 (BAND A), 2.5 10 Reference Reference Reference Reference
097071200101 (BAND F)
Fig. 2. Elevation difference of 1970 Corona and 2007 Cartosat DTM before (left) and after adjustment (right).
Graz (RSG, http://dib.joanneum.at/rsg/) 6.13 for processing
Corona, PCI Geomatica OrthoEngine 10.2 for Cartosat, and
Leica Photogrammetry Suite (LPS) 9.1 for the aerial images.
We used 14 non-differential GPS points acquired in 2006 and
2008 and ∼200 points from the National Geographic 1:50k
topographic map (Altherr and Gr
¨
un, 1990) as ground con-
trol points (GCPs). The GPS points were mostly measured
along the main trekking routes within a height range between
3900m and 5600m. Their horizontal accuracy is about 7.9m
and their vertical accuracy in comparison to topographic map
height points is about 20.6m. The GCPs based on the map
are almost randomly distributed in accordance with the un-
systematic distribution of significant peaks and other geo-
morphological forms marked as height points. In addition,
these points represent different slope angles and aspects well
(Pieczonka et al., 2011). The RMSE
z
and RSME
x,y
of the
map were computed based on the GPS points to be 20.6 m
and 17.8m, respectively. This matches the results achieved
by Altherr and Gr
¨
un (1990). In addition, we used automat-
ically selected tie points (TPs) to improve the sensor mod-
els. The overall quality of the generated raw DTMs appears
promising as the glacier tongues are almost fully represented
(Figs. 2, 3). Data gaps occur mainly due to snow cover and
cast shadow.
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352 T. Bolch et al.: Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya)
Fig. 3. DTM differences of the study area 1970–2007 and 2002–2007.
In order to measure glacier elevation changes as precisely
as possible it is recommended to adjust the DTMs relative
to each other (Nuth and K
¨
a
¨
ab, 2011). Tilts which occurred
especially in the Corona DTM were corrected using trend
surfaces calculated based on stable non-glacierised areas of
the DTMs (Fig. 2, Pieczonka et al., 2011). We observed
slight horizontal shifts of the generated DTMs although we
used the same GCPs for all images whenever possible. In
order to avoid biases introduced thereby and to improve the
z-accuracy, we choose the Cartosat-1 DTM as the master ref-
erence as it has a high spatial resolution and showed the low-
est mean elevation difference (5.9m) and standard deviation
(18.3m) relative to the SRTM3 DTM. We co-registered the
other DTMs to it by minimizing the standard deviation of
the elevation differences (Berthier et al., 2007). The applied
shifts varied between 5 and 30m. Altitudinal differences
which exceeded ±100 m (usually around data gaps and near
DTM edges) were omitted assuming that these values repre-
sent outliers. Similar assumptions were made by Berthier et
al. (2010). We resampled all DTMs bilinearly to the pixel
size of the coarsest DTM (30m) in order to reduce the effect
of different resolutions.
The relative uncertainties of the DTMs prior to and after
the adjustments were calculated based on the ice free terrain
relative to the 2007 master DTM. The mean difference be-
tween the final adjusted DTMs was in the range −0.1 to
−1.8m while the RMSE
z
was 7.8 to 19.8m (Table 1). To
address the uncertainty of the elevation differences of the
glacierised areas we calculated statistical parameters for the
differences of ice covered and the non-ice covered areas sep-
arately (Table 2). The standard deviation of the non glacier
area (STDV
noglac
) or the RSME
z
can be used as a first esti-
mate of the uncertainty, but would probably overestimate it
as it does not account for the reduction of the error due to
averaging over larger regions (Berthier et al., 2007, 2010).
Another suitable measure for the uncertainty is the standard
error of the mean (SE) defined as
SE=
STDV
noglac
√
n
(1)
while n is the number of the included pixels. We choose a
decorrelation length of 600m for the ASTER DTMs with an
effective spatial resolution of 30m and a length of 400m for
all other higher resolution DTMs to minimize the effect of
auto-correlation. These numbers are slightly more conserva-
tive than the average value of 500m utilized by Berthier at
el. (2010) for DTMs with coarser resolution (mostly 40m).
Koblet et al. (2010) suggested a decorrelation length of
100m for DTMs based on aerial images of a spatial reso-
lution of 5m.
We used the standard error (SE) and the mean elevation
difference (MED) of the non glacier area (Table 2) as an
estimate of the uncertainty according to the law of error
propagation:
e =
q
SE
2
+MED
2
(2)
Volume change was calculated for each glacier assuming that
the density profile remains unchanged and that only ice is lost
or gained (Paterson, 1994; Zemp et al., 2010). To convert
volume changes into mass change, we assumed an ice den-
sity of 900kgm
−3
and assigned an additional uncertainty of
7% due to lack of ground truth (Zemp et al., 2010). We inter-
polated small data voids (<10pixel) within the ice covered
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T. Bolch et al.: Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya) 353
Table 2. Statistics of the DTM differences for the investigated periods.
Period DTM DTM Mean elev. STDV Mean elev. STDV N SE
coverage coverage diff. no glac. diff. glac. glac. no glac. no glac.
study area glac. no glac.
(km
2
) (km
2
) (m) (m) (m) (m) (m)
1962–1970 137.8 25.4 −0.9 22.3 −1.9 15.7 319 1.3
1970–1984 83.8 24.4 −3.6 26.3 −9.9 16.1 163 2.1
1984–2002 83.4 25.7 −2.2 26.5 −5.4 18.2 107 2.6
2002–2007 174.5 59.8 +2.2 22.8 −3.2 13.5 214 1.6
1984–2007 80.6 20.9 +2.4 15.8 −9.2 15.4 109 1.5
1970–2007 152.9 46.5 +2.2 19.4 −13.2 15.6 293 1.1
glac.: glacier area, no glac.: non glacier area, N : number of considered pixels.
Table 3. Glacier volume loss and mass balance 1970–2007, and 2002–2007.
Period 1970–2007 Period 2002–2007
Glacier Glacier Glacier area Average Specific Glacier area Average Specific
Size covered elev. mass covered elev. mass
by DTM diff. balance Diff. by DTM balance
(km
2
) (km
2
) (m) (mw.e.a
−1
) (km
2
) (m) (mw.e.a
−1
)
Changri Shar/Nup 13.0 6.85 −11.6±2.5 −0.28±0.08 6.93 −1.6±2.7 −0.29±0.52
Khumbu accumulation area
1
6.2 4.3 −6.2±2.5 4.66 +1.2±2.7
Khumbu ablation area
1
10.8 10.0 −13.9±2.5 10.1 −4.0±2.7
Whole Khumbu 17 14.26 −11.1±2.5 −0.27±0.08 14.7 −2.5±2.7 −0.45±0.52
Nuptse 4.0 3.45 −9.4±2.5 −0.25±0.08 3.52 −2.2±2.7 −0.40±0.53
Lhotse Nup 1.95 1.86 −7.6±2.5 −0.18±0.07 1.86 −5.7±2.7 −1.03±0.51
Lhotse 6.5 6.71 −10.7±2.5 −0.26±0.08 6.71 −6.1±2.7 −1.10±0.52
Lhotse Shar/Imja
2
10.7 8.65 −20.6±2.5 −0.50 ±0.09 8.87 −8.1±2.7 −1.45±0.52
Amphu Laptse 1.5 1.05 −10.0±2.5 −0.24±0.08 1.08 −4.3±2.7 −0.77±0.52
Chukhung 3.8 1.88 −5.3±2.5 −0.30±0.08 3.2 +0.2±2.7 +0.04±0.54
Amadablam 2.2 1.86 −12.0±2.5 −0.29±0.08 2.5 −3.1±2.7 −0.56±0.52
Duwo 1.0 0.37 −12.2±2.5 −0.30±0.08 0.37 −10.9±2.7 −1.96±0.53
Sum/Average 61.7 46.9 −13.3±2.5 −0.32±0.08 49.6 −4.4±2.7 −0.79±0.52
1
We assumed an ELA of 5700m based on Ashai (2001) and interpretation of the satellite images.
2
For the year 2007, the estimated volume of Imja lake (37.8 ×10
6
m
3
, calculated based on the area extent 0.91 km
2
and the average depth of 41.4m) was added to the elevation
difference of 1970–2007. For 2002–2007 we added the volume difference between 2007 and 2002 (2.0×10
6
m
3
). Data is based on Fujita et al. (2009).
areas using a spline algorithm. We did not fill the larger data
gaps e.g. on steep slopes. The glacier tongues, the avalanche
cones, and Western Cwm are represented in the DTMs of
1970, 2002, and 2007 (Fig. 3, Tables 3, 4) which allow es-
timation of the mass balance for the entire glacier. Only
Changri Nup, Duwo, and the debris-free Chukhung Glacier
have larger data gaps. Detailed investigations on Khumbu
Glacier are limited to the tongue below ∼5700 m (mainly the
ablation area) due to the small coverage of the aerial images.
3 Volume changes and mass losses
3.1 Periods 1970–2007 and 2002–2007 for the whole
study area
Between 1970 and 2007 significant surface lowering oc-
curred on all investigated glaciers (Fig. 3, Table 2). The
greatest lowering was on Imja/Lhotse Shar Glacier. Except
for this glacier, which displays surface lowering throughout
the terminus, most glaciers show maximum lowering in their
mid ablation zones, with a negligible change near their ter-
mini. Overall ice loss is estimated to be >0.6km
3
with an
average surface lowering of 0.36 ± 0.07m a
−1
or a specific
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354 T. Bolch et al.: Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya)
Fig. 4. DTM differences on Khumbu Glacier for different times.
mass balance of −0.32 ±0.08 m w.e. a
−1
between 1970 and
2007 (Table 3). The surface lowering for the debris-covered
parts only is −0.39±0.07ma
−1
, clearly showing that signif-
icant thickness loss occurs despite thick debris cover. Most
glaciers also experienced surface lowering between 2002 and
2007 (Fig. 3, Table 3). The specific mass balance of all
10 glaciers has possibly doubled compared to 1970–2007
(−0.79±0.52mw.e.a
−1
). However, the uncertainty is high.
3.2 Detailed multi-temporal investigations on Khumbu
Glacier
The surface of the ablation area of Khumbu Glacier lowered
in all investigated time periods (Table 3). DTM differenc-
ing (Figs. 3 and 4) and longitudinal profiles, in particular for
1970–2007, show almost no surface lowering in the clean ice
zone below Khumbu Icefall (Figs. 5 and 6, section A); an
increasing downwasting in the debris-covered part, with the
highest lowering between 2 and 8km from the terminus (B,
Table 4. Volume loss of the ablation area of Khumbu Glacier
1962–2007.
Time DTM coverage Average down- Downwasting rate
(km
2
) wasting (m) (ma
−1
)
1962–1970 4.9 −2.74±1.54 −0.34±0.19
1970–1984 9.9 −2.53±4.16 −0.18±0.29
1984–2002 9.8 −6.72±2.78 −0.37±0.16
2002–2007 10.0 −3.95±2.68 −0.79±0.52
1984–2007 9.8 −13.00±2.84 −0.56±0.13
C), and almost no lowering within ∼1.5km of the terminus
(D). For 1970–1984 only lowering between ∼1.5 and 5.5 km
of the terminus is significant (Fig. 5). Between 1970 and
2007, average surface lowering rate in the ablation area was
−0.38±0.07ma
−1
. The rate for 1984–2002 is higher than
for 1962–1970 and 1970–1984, and comparison of the re-
cent DTMs (2002–2007) suggests further accelerated surface
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T. Bolch et al.: Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya) 355
Fig. 5. (A): Khumbu Glacier based on the 2007 Cartosat-1 image
including the profile and the different sections of surface lowering
(see Fig. 6A). (B, C): Terminus of Khumbu Glacier based on the
1970 Corona KH-4B and on the 2007 Cartosat-1 image.
lowering (Table 4). However, these differences are statisti-
cally not significant. Comparing the periods 1970–1984 and
1984–2007, however, shows an almost statistically signifi-
cant increase in the surface lowering rate (0.18 ±0.29 m a
−1
in comparison to 0.56±0.13 m a
−1
). The accumulation zone
of Khumbu Glacier has possibly also thinned during the in-
vestigated time, while there might be a slight thickening in
recent time (2002–2007).
4 Discussion
Stereo capability, acquisition in the 1960s and 1970s and
relatively high spatial resolution make Corona KH-4, KH-
4A and B imagery a valuable source for geodetic mass bal-
ance estimations. While the earlier KH-4 data were already
found to be suitable for this task (Bolch et al., 2008b) we
have shown that the later higher resolution KH-4 results in
lower uncertainties. The accuracy and even distribution of
the ground control points used for the rectification of the
imagery are crucial for the resultant accuracy of the DTM.
Hence, it could be expected that an even better accuracy than
ours could be achieved if more precise GCPs are available.
Other reconnaissance images such as Hexagon KH-9 from
the 1970s and early 1980s are also suitable for this task if
coverage with no clouds and little snow cover are available
(Surazakov and Aizen, 2010). Surazakov and Aizen (2010)
have shown a RSME
z
for mountainous terrain of ∼20 m,
which is the same range as our results using KH-4B data
despite lower resolution of the KH-9 data. Reseau marks on
the KH-9 images facilitate minimizing the distortion. Corona
and Hexagon data are also of high value for mapping glaciers
and investigating area changes (Bhambri et al., 2011; Bolch
et al., 2010; Narama et al., 2010). The generation of mass
Fig. 6. (A): Profiles of the DTM differences of Khumbu Glacier.
(B): Longitudinal profiles of the surface elevation of Khumbu
Glacier 1970 and 2007. See Figs. 3 and 5 for the location of the
profiles.
balance time series estimation of different data sets with dif-
ferent resolution requires careful co-registration and adjust-
ment. This is especially true for the 1984 DTM based on
aerial imagery, highlighted by a clear positive-negative trend
on hillslopes with opposing aspects for the periods 1970–
1984 and 1984–2002 (visible on the lower right of the im-
ages on Fig. 4). However, a shifting to adjust this area
would worsen the accuracy of the steep slope in the northern
part of the DTM. Hence, the distortion of the 1984 DTM is
more complex and could not be fully adjusted relative to the
others. Although inaccuracies remain on steep slopes with
all DTMs, most of the glacier area is not affected by these
biases. The inclusion of the lower resolution 2002 ASTER
DTM provides some insight in the recent volume loss of the
glaciers. However, the volume change of the glaciers is too
small for a significant signal to be detected. In addition, the
large scatter in the volume change amongst the investigated
glaciers needs further investigation.
The quality of the DTMs, however, is supported by lo-
cal details, such as the area on Khumbu Glacier arrowed on
Fig. 4. A large thinning occurred in this area between 1970
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356 T. Bolch et al.: Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya)
and 1984, which can be attributed to the growth of a lake
basin visible on the 1984 aerial photograph (Supplement,
Fig. 4). Melting and calving around the margins of lakes
is well known to produce locally high ablation rates (Sakai et
al., 2000; Benn et al., 2001). Thickening occurred in this re-
gion between 1984 and 2002, attributable to drainage of the
lake reducing ablation and ice inflow from upglacier.
The utilized imagery also allows length changes of the
glaciers to be examined in detail. Bajracharya and Mool
(2009) argued that Khumbu Glacier has undergone termi-
nus retreat, based on comparison of recent imagery with
old topographic maps. Our images, however, indicate that
the terminus region of Khumbu Glacier has undergone very
little change in recent decades (Fig. 5b, c and Supple-
ment Fig. 1). This highlights the fact that debris-covered
glaciers may have stable terminus positions even though
they are in negative balance, and that volume changes are
more reliable indicators of glacier health than area changes
(cf. Scherler et al., 2011).
The calculated average 1970–2007 thickness changes for
the whole study area of −0.36 ±0.07 m a
−1
, based on in-
dependent data sets, is very close to the value of −0.33±
0.24ma
−1
presented by Bolch et al. (2008b) for 1962–2002,
but with smaller uncertainty. The wider coverage of this
study including Imja Glacier, and the accumulation area
of Khumbu Glacier, as well as lower uncertainties and the
multi-temporal coverage, allow greater insight into decadal
glacier changes and the influence of debris cover.
The longitudinal profile of glacier thinning of Khumbu
Glacier shows similar characteristics to those presented by
Nakawoet al. (1999) based on estimated ice flowand thermal
properties derived from Landsat data. Based on field mea-
surements, Kadota et al. (2000) found a slight surface lower-
ing (∼5m) about 1 km upglacier of the terminus and higher
surface loss (more than 10m in places) up to the pinnacle
zone close to Everest base camp for the period 1978–1995.
This is in good agreement with our data. These observa-
tions increase confidence in the observed patterns of down-
wasting, despite the remaining uncertainties.
The pattern of surface lowering on Khumbu Glacier can
be explained in terms of ice dynamics and surface melt
rates. Sustained high rates of ice delivery below the icefall
largely offset melt in the upper ablation area, where debris
cover is thin or absent (Figs. 5 and 6a, section A). Further
downglacier, thin debris cover increases the ice melt (sec-
tion B), in line with field measurements of increased sur-
face lowering (Takeuchi et al., 2000). Thinning rates remain
high downglacier despite an increasing debris thickness, due
to very low glacier velocities and ablation associated with
supraglacial lakes and exposed ice cliffs (Sakai et al., 2000,
2002) (section C, Supplement, Fig. 2). Almost no thinning
was observed within 1km of the terminus (section D), which
may reflect either a thick, complete debris cover or indicate
that ice loss is already complete. The possible slight surface
lowering in the accumulation area of the glacier from 1970
to 2007 might be due to less snowfall. This is consistent
with an ice core record at the East Rongbuk Glacier north of
Mt. Everest that indicates decreasing snow accumulation for
1970–2001 (Kaspari et al., 2008).
The pattern of downwasting has resulted in the devel-
opment of a concave glacier profile on Khumbu Glacier,
with very low surface gradients in the lower ablation zone
(Fig. 5b). This trend indicates that a glacial lake could de-
velop about 1.5 to 3 km upstream of the terminus, as was pre-
dicted in simulations using a 1D-coupled mass balance and
flow model by Naito et al. (2000). The formation of a large
lake on Khumbu Glacier would have major consequences for
outburst flood hazards downstream.
The greatest amount of mass loss occurred on Imja
Glacier, which can be at least partly attributed to the
proglacial Imja Lake, which enhances ice losses by calving.
This lake grew significantly since its formation in the late
1960s, reaching ∼0.9km
2
in 2008 (Supplement, Fig. 3, Fu-
jita et al., 2009; Bolch et al., 2008a). The comparatively thin
debris cover of Imja Glacier, apparent in exposed ice cliffs,
is likely to be another reason for the higher mass loss. At
Imja Glacier, a slight thinning is also observed on the relict
ice-cored moraine situated below Imja Lake. This is in line
with recent field measurements in this area which revealed
a lowering of about 1ma
−1
(Fujita et al., 2009). The mass
loss of the smaller glaciers, such as Amphu Laptse Glacier,
amounts to half of that of Imja Glacier but is still significant.
Khumbu Glacier, for which data from the year 1984 was
available, showed a higher surface lowering for the period
1984–2007 in comparison to 1962–1984. The recent trend
of more negative mass balances since 2002, however, needs
further investigation, as it is not statistically significant. Ac-
celerated thinning could reflect decreasing velocity (Quincey
et al. 2009), higher air temperatures (Prasad et al., 2009), de-
creasing snow accumulation (Kaspari et al., 2008), or a com-
bination of these factors. The influence of black carbon (BC)
as summarized by Ramanathan and Carmichael (2008) can-
not be excluded but is negligible for the ablation zones with
debris-cover as BC does not lead into changes in the albedo
there. The catchment topography plays an important role for
the glacier flow regimes in Khumbu Himalaya (Quincey et
al., 2009). Forexample, Kangshung Glacier with a largehigh
altitude accumulation area flowing from Mt. Everest towards
the east, shows flow activity across the entire snout while all
other avalanche fed glaciers and glaciers with a lower accu-
mulation area such as Khumbu Glacier contain large stag-
nant parts. Our result shows comparatively little scatter of
the mass balance amongst the investigated, mainly avalanche
fed glaciers in the same study region. Mass balance esti-
mate of Kangshung Glacier would reveal how the catchment
topography and the higher flow influence the mass balance.
Unfortunately, only the upper part of Kangshung Glacier is
covered by our multi-temporal DTMs making a mass balance
estimate impossible.
The Cryosphere, 5, 349–358, 2011 www.the-cryosphere.net/5/349/2011/

T. Bolch et al.: Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya) 357
The total mass balance of Khumbu Glacier for 1970–
2007, at −0.27±0.08mw.e.a
−1
is lower than that observed
on several other Himalayan glaciers including Chhota Shi-
gri Glacier (−0.98 mw.e.a
−1
, 2002–2006 Wagnon et al.,
2007 and –1.02 to –1.12 m w.e. a
−1
, 1999–2004 Berthier et
al., 2007) or the small debris-free Glacier AX010 (−0.6 to
−0.8mw.e.a
−1
, 1978–1999 Fujita et al., 2001), but similar
to Dokriani Glacier (−0.32mw.e. a
−1
, 1992–2000 Dobhal
et al., 2008). However, the different observation times and
glacier sizes have to be considered, and Khumbu Glacier also
has possibly a more negative mass balance in recent years.
The tendency towards increased mass loss has also been ob-
served worldwide and for the few other Himalayan glaciers
with mass balance estimates (Cogley, 2011). The mass loss
of the investigated glaciers is similar to the average mass
loss of the 30 reference glaciers worldwide for 1976–2005
(−0.32mw.e.a
−1
) (Zemp et al., 2009).
5 Conclusions
This study presents the longest time series of geodetically
derived mass-balance and glacier volume change estimates
obtained to date in the Himalaya. Geodetic mass-balance
estimates based on early stereo Corona and recent satellite
data are suitable for tracking glacier changes through time,
thus filling major gaps in glaciological knowledge of the Hi-
malaya and other mountain regions. However, careful rela-
tive adjustments of the DTMs are necessary to obtain suit-
able accuracies of DTMs based on different data sources
with different resolutions. Mass balance information is ur-
gently needed to improve knowledge of the response of Hi-
malayan glaciers to climate change and to allow prediction
of future glacier change and its influence on water resources,
river runoff, sea level rise, and glacial hazards.
Glaciers south of Mt. Everest have continuously lost mass
from 1970 until 2007, with a possibly increasing rate in re-
cent years. All glaciers lost mass despite partly thick debris-
cover. The highest loss was observed at Imja Glacier which
terminates into a lake. The specific mass balance of the in-
vestigated glaciers of −0.32±0.08mw.e. a
−1
is not higher
than the global average.
Supplementary material related to this
article is available online at:
http://www.the-cryosphere.net/5/349/2011/
tc-5-349-2011-supplement.pdf.
Acknowledgements. We thank M. Buchroithner for the support and
B. Shreshta, S. Bajracharja and P. Mool and the staff from ICIMOD,
Nepal for cooperation. We are grateful for the thorough reviews
of A. Racoviteanu, G. Cogley and an anonymous reviewer on an
earlier version of this paper. The comments by the anonomous
reviewer, D. Quincey and the short comments of M. Pelto and the
scientific editor A. K
¨
a
¨
ab improved it further. H. Raggam helped
with RSG software. GAF AG offered Cartosat-1 data at a reduced
price and D. Quincey provided the 1984 images. DFG (Deutsche
Forschungsgemeinschaft) provided financial support (Codes BU
949/15-1 and BO 3199/2-1). T. Bolch was partly funded through
the ESA project GlobGlacier (21088/07/I-EC).
Edited by: A. K
¨
a
¨
ab
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