The Cryosphere 5 349ndash358 2011wwwthe-cryospherenet53492011doi105194tc-5-349-2011copy Author(s) 2011 CC Attribution 30 License

The Cryosphere

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

T Bolch13 T Pieczonka1 and D I Benn24

1Institut fur Kartographie Technische Universitat Dresden Germany2The University Centre in Svalbard Norway3Geographisches Institut Universitat Zurich Switzerland4School of Geography and Geosciences University of St Andrews UK

Received 1 December 2010 ndash Published in The Cryosphere Discuss 20 December 2010Revised 28 March 2011 ndash Accepted 30 March 2011 ndash Published 20 April 2011

Abstract Mass loss of Himalayan glaciers has wide-rangingconsequences such as changing runoff distribution sea levelrise 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 rangeMulti-temporal digital terrain models (DTMs) allow glaciermass balance to be calculated Here we present a time seriesof mass changes for ten glaciers covering an area of about50 km2 south and west of Mt Everest Nepal using stereoCorona spy imagery (years 1962 and 1970) aerial imagesand recent high resolution satellite data (Cartosat-1) This isthe longest time series of mass changes in the Himalaya Wereveal that the glaciers have been significantly losing masssince at least 1970 despite thick debris cover The specificmass loss for 1970ndash2007 is 032plusmn 008 m we aminus1 howevernot higher than the global average Comparisons of the re-cent DTMs with earlier time periods indicate an acceleratedmass loss This is however hardly statistically significantdue to high uncertainty especially of the lower resolutionASTER DTM The characteristics of surface lowering can beexplained by spatial variations of glacier velocity the thick-ness of the debris-cover and ice melt due to exposed ice cliffsand ponds

1 Introduction

Recent debate on whether Himalayan glaciers are shrinkingfaster than those in other parts of the world (Cogley et al2010) highlighted the lack of knowledge about the glaciersin this region The best measure of glacier health is mass bal-ance which can be directly linked to climate and compared

Correspondence toT Bolch(tobiasbolchgeouzhch)

to other regions The topographic setting and the glacierhypsometry modify the way the climatic signal is translatedinto glacier mass balance but this can calculated using aDTM (Paul and Haeberli 2008) However only a few in-situmass balance measurements have been made on Himalayanglaciers and existing data series are short (Kulkarni 1992Fujita et al 2001 Wagnon et al 2007 Dobhal et al 2008)Comparisons of digital terrain models for different years cancomplement field measurements and allow regional massbalance to be estimated (Bamber and Rivera 2007) How-ever to date it has only been applied to some glaciers in thewestern Himalaya for 1999 to 2004 (Berthier et al 2007)and to four glaciers at Mt Everest for 1962 to 2002 (Bolchet al 2008b) Broader and more detailed knowledge ofglacier mass balance is also needed to decrease the highuncertainty about the importance of Himalayan glaciers forwater resources (eg Immerzeel et al 2010) and sea levelrise (eg Braithwaite and Raper 2002) Finally improvedknowledge of glacier recession is needed to better estimatehazards of glacier lake outburst floods (GLOFs Richardsonand 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 033plusmn 024 m aminus1 by analysing a 1962 Corona andan ASTER DTM generated based on 2001 2002 and 2003data but had high uncertainties The second aim is to presentmass balance estimates for a larger sample of glaciers aroundMt Everest including Imja Glacier which is of high interestdue to the proglacial lake which formed in the 1960s and hasrapidly grown since (Bolch et al 2008a Fujita et al 2009)In addition the mass balance of the entire Khumbu Glacierwill be presented for the first time Thirdly we wanted to pro-duce the first time-series of glacier volume changes aroundMt 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 dataBackground 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 debrisarea of about 363 km2 and an average debris thickness in-creasing downglacier (Nakawo et al 1999 Hambrey et al2008) The glaciers are mainly nourished by snow and iceavalanches which accumulate cones below the steep head-walls Only Khumbu Glacier has an extensive accumu-lation area (called ldquoWestern Cwmrdquo) Glacier equilibriumline altitudes (ELAs) are roughly estimated to be situatedabove 5600 m (Asahi 2001) Ice velocities typically de-crease downglacier from the ELA and most of the studiedglaciers have extensive stagnant ice in their lower reaches(Bolch et al 2008a Scherler et al 2008 Quincey et al2009) Between 1962 and 2005 the overall glacier arealoss in the study area wassim5 with an increasing debris-covered area (sim25) 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 Grun 1990) and 2007 Cartosat-1 (Indian RemoteSensing Satellite IRS P5) images (Table 1) In addition weused previously generated 1962 Corona and 2002 ASTERDTMs (Bolch et al 2008b) We did not consider the DTMdata from the Shuttle Radar Topography Mission (SRTM)due to large data gaps especially at the clean ice area ofKhumbu Glacier and the coarser spatial resolution (sim90 m)in comparison to the ASTER DTM (30 m) In addition us-ing only stereo optical data results in a methodologicallyconsistent data set The SRTM DTM data in contrast arebased on C-band adar beams that penetrate into snow and cancause higher uncertainties in the snow-covered accumulationareas 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 STDVDTM diff diff

15 Dec 1962 Corona KH-4 DS009050054DF173173 76 20 minus530 209 minus01 197DS009050054DA175175

20 Nov 1970 Corona KH-4B DS1112-1023DA163163 52 15 minus90 272 minus05 188DS1112-1023DF157157

20 Dec 1984 Wild RC-10 1440 144114761477 05 15 82 112 minus18 78

20 Dec 2001 ASTER L1A0032005569609 15 30 128 295 minus15 10121 Nov 2002 L1A003200931688123 Oct 2003 L1A0032018198969

13 May 2007 Cartosat-1 097071200102 (BAND A) 25 10 Reference Reference Reference Reference097071200101 (BAND F)

Fig 2 Elevation difference of 1970 Corona and 2007 Cartosat DTM before (left) and after adjustment (right)

Graz (RSGhttpdibjoanneumatrsg) 613 for processingCorona PCI Geomatica OrthoEngine 102 for Cartosat andLeica Photogrammetry Suite (LPS) 91 for the aerial imagesWe used 14 non-differential GPS points acquired in 2006 and2008 andsim200 points from the National Geographic 150ktopographic map (Altherr and Grun 1990) as ground con-trol points (GCPs) The GPS points were mostly measuredalong the main trekking routes within a height range between3900 m and 5600 m Their horizontal accuracy is about 79 mand their vertical accuracy in comparison to topographic mapheight points is about 206 m The GCPs based on the mapare almost randomly distributed in accordance with the un-

systematic distribution of significant peaks and other geo-morphological forms marked as height points In additionthese points represent different slope angles and aspects well(Pieczonka et al 2011) The RMSEz and RSMExy of themap were computed based on the GPS points to be 206 mand 178 m respectively This matches the results achievedby Altherr and Grun (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 appearspromising as the glacier tongues are almost fully represented(Figs 2 3) Data gaps occur mainly due to snow cover andcast shadow

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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 STDVDTM diff diff

15 Dec 1962 Corona KH-4 DS009050054DF173173 76 20 minus530 209 minus01 197DS009050054DA175175

20 Nov 1970 Corona KH-4B DS1112-1023DA163163 52 15 minus90 272 minus05 188DS1112-1023DF157157

20 Dec 1984 Wild RC-10 1440 144114761477 05 15 82 112 minus18 78

20 Dec 2001 ASTER L1A0032005569609 15 30 128 295 minus15 10121 Nov 2002 L1A003200931688123 Oct 2003 L1A0032018198969

13 May 2007 Cartosat-1 097071200102 (BAND A) 25 10 Reference Reference Reference Reference097071200101 (BAND F)

Fig 2 Elevation difference of 1970 Corona and 2007 Cartosat DTM before (left) and after adjustment (right)

Graz (RSGhttpdibjoanneumatrsg) 613 for processingCorona PCI Geomatica OrthoEngine 102 for Cartosat andLeica Photogrammetry Suite (LPS) 91 for the aerial imagesWe used 14 non-differential GPS points acquired in 2006 and2008 andsim200 points from the National Geographic 150ktopographic map (Altherr and Grun 1990) as ground con-trol points (GCPs) The GPS points were mostly measuredalong the main trekking routes within a height range between3900 m and 5600 m Their horizontal accuracy is about 79 mand their vertical accuracy in comparison to topographic mapheight points is about 206 m The GCPs based on the mapare almost randomly distributed in accordance with the un-

systematic distribution of significant peaks and other geo-morphological forms marked as height points In additionthese points represent different slope angles and aspects well(Pieczonka et al 2011) The RMSEz and RSMExy of themap were computed based on the GPS points to be 206 mand 178 m respectively This matches the results achievedby Altherr and Grun (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 appearspromising as the glacier tongues are almost fully represented(Figs 2 3) Data gaps occur mainly due to snow cover andcast 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 1970ndash2007 and 2002ndash2007

In order to measure glacier elevation changes as preciselyas possible it is recommended to adjust the DTMs relativeto each other (Nuth and Kaab 2011) Tilts which occurredespecially in the Corona DTM were corrected using trendsurfaces calculated based on stable non-glacierised areas ofthe DTMs (Fig 2 Pieczonka et al 2011) We observedslight horizontal shifts of the generated DTMs although weused the same GCPs for all images whenever possible Inorder to avoid biases introduced thereby and to improve thez-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 (59 m) and standard deviation(183 m) relative to the SRTM3 DTM We co-registered theother DTMs to it by minimizing the standard deviation ofthe elevation differences (Berthier et al 2007) The appliedshifts varied between 5 and 30 m Altitudinal differenceswhich exceededplusmn100 m (usually around data gaps and nearDTM edges) were omitted assuming that these values repre-sent outliers Similar assumptions were made by Berthier etal (2010) We resampled all DTMs bilinearly to the pixelsize of the coarsest DTM (30 m) in order to reduce the effectof different resolutions

The relative uncertainties of the DTMs prior to and afterthe adjustments were calculated based on the ice free terrainrelative to the 2007 master DTM The mean difference be-tween the final adjusted DTMs was in the rangeminus01 tominus18 m while the RMSEz was 78 to 198 m (Table 1) Toaddress the uncertainty of the elevation differences of theglacierised areas we calculated statistical parameters for thedifferences of ice covered and the non-ice covered areas sep-arately (Table 2) The standard deviation of the non glacierarea (STDVnoglac) or the RSMEz can be used as a first esti-

mate of the uncertainty but would probably overestimate itas it does not account for the reduction of the error due toaveraging over larger regions (Berthier et al 2007 2010)Another suitable measure for the uncertainty is the standarderror of the mean (SE) defined as

SE=STDVno glac

radicn

(1)

while n is the number of the included pixels We choose adecorrelation length of 600 m for the ASTER DTMs with aneffective spatial resolution of 30 m and a length of 400 m forall other higher resolution DTMs to minimize the effect ofauto-correlation These numbers are slightly more conserva-tive than the average value of 500 m utilized by Berthier atel (2010) for DTMs with coarser resolution (mostly 40 m)Koblet et al (2010) suggested a decorrelation length of100 m for DTMs based on aerial images of a spatial reso-lution of 5 m

We used the standard error (SE) and the mean elevationdifference (MED) of the non glacier area (Table 2) as anestimate of the uncertainty according to the law of errorpropagation

e =

radicSE2

+MED2 (2)

Volume change was calculated for each glacier assuming thatthe density profile remains unchanged and that only ice is lostor gained (Paterson 1994 Zemp et al 2010) To convertvolume changes into mass change we assumed an ice den-sity of 900 kg mminus3 and assigned an additional uncertainty of7 due to lack of ground truth (Zemp et al 2010) We inter-polated small data voids (lt10 pixel) 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 SEcoverage coverage diff no glac diff glac glac no glac no glac

study area glac no glac(km2) (km2) (m) (m) (m) (m) (m)

1962ndash1970 1378 254 minus09 223 minus19 157 319 131970ndash1984 838 244 minus36 263 minus99 161 163 211984ndash2002 834 257 minus22 265 minus54 182 107 262002ndash2007 1745 598 +22 228 minus32 135 214 161984ndash2007 806 209 +24 158 minus92 154 109 151970ndash2007 1529 465 +22 194 minus132 156 293 11

glac glacier area no glac non glacier areaN number of considered pixels

Table 3 Glacier volume loss and mass balance 1970ndash2007 and 2002ndash2007

Period 1970ndash2007 Period 2002ndash2007

Glacier Glacier Glacier area Average SpecificGlacier area Average SpecificSize covered elev mass covered elev mass

by DTM diff balance Diff by DTM balance(km2) (km2) (m) (m we aminus1) (km2) (m) (m we aminus1)

Changri SharNup 130 685 minus116plusmn 25 minus028plusmn 008 693 minus16plusmn 27 minus029plusmn 052Khumbu accumulation area1 62 43 minus62plusmn 25 466 +12plusmn 27Khumbu ablation area1 108 100 minus139plusmn 25 101 minus40plusmn 27Whole Khumbu 17 1426 minus111plusmn 25 minus027plusmn 008 147 minus25plusmn 27 minus045plusmn 052Nuptse 40 345 minus94plusmn 25 minus025plusmn 008 352 minus22plusmn 27 minus040plusmn 053Lhotse Nup 195 186 minus76plusmn 25 minus018plusmn 007 186 minus57plusmn 27 minus103plusmn 051Lhotse 65 671 minus107plusmn 25 minus026plusmn 008 671 minus61plusmn 27 minus110plusmn 052Lhotse SharImja2 107 865 minus206plusmn 25 minus050plusmn 009 887 minus81plusmn 27 minus145plusmn 052Amphu Laptse 15 105 minus100plusmn 25 minus024plusmn 008 108 minus43plusmn 27 minus077plusmn 052Chukhung 38 188 minus53plusmn 25 minus030plusmn 008 32 +02plusmn 27 +004plusmn 054Amadablam 22 186 minus120plusmn 25 minus029plusmn 008 25 minus31plusmn 27 minus056plusmn 052Duwo 10 037 minus122plusmn 25 minus030plusmn 008 037 minus109plusmn 27 minus196plusmn 053

SumAverage 617 469 minus133plusmn 25 minus032plusmn 008 496 minus44plusmn 27 minus079plusmn 052

1 We assumed an ELA of 5700 m based on Ashai (2001) and interpretation of the satellite images2 For the year 2007 the estimated volume of Imja lake (378times 106 m3 calculated based on the area extent 091 km2 and the average depth of 414 m) was added to the elevationdifference of 1970ndash2007 For 2002ndash2007 we added the volume difference between 2007 and 2002 (20times 106 m3) Data is based on Fujita et al (2009)

areas using a spline algorithm We did not fill the larger datagaps eg on steep slopes The glacier tongues the avalanchecones and Western Cwm are represented in the DTMs of1970 2002 and 2007 (Fig 3 Tables 3 4) which allow es-timation of the mass balance for the entire glacier OnlyChangri Nup Duwo and the debris-free Chukhung Glacierhave larger data gaps Detailed investigations on KhumbuGlacier are limited to the tongue belowsim5700 m (mainly theablation area) due to the small coverage of the aerial images

3 Volume changes and mass losses

31 Periods 1970ndash2007 and 2002ndash2007 for the wholestudy area

Between 1970 and 2007 significant surface lowering oc-curred on all investigated glaciers (Fig 3 Table 2) Thegreatest lowering was on ImjaLhotse Shar Glacier Exceptfor this glacier which displays surface lowering throughoutthe terminus most glaciers show maximum lowering in theirmid ablation zones with a negligible change near their ter-mini Overall ice loss is estimated to begt06 km3 with anaverage surface lowering of 036plusmn 007 m aminus1 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 ofminus032plusmn 008 m we aminus1 between 1970 and2007 (Table 3) The surface lowering for the debris-coveredparts only isminus039plusmn007 m aminus1 clearly showing that signif-icant thickness loss occurs despite thick debris cover Mostglaciers also experienced surface lowering between 2002 and2007 (Fig 3 Table 3) The specific mass balance of all10 glaciers has possibly doubled compared to 1970ndash2007(minus079plusmn052 m we aminus1) However the uncertainty is high

32 Detailed multi-temporal investigations on KhumbuGlacier

The surface of the ablation area of Khumbu Glacier loweredin all investigated time periods (Table 3) DTM differenc-ing (Figs 3 and 4) and longitudinal profiles in particular for1970ndash2007 show almost no surface lowering in the clean icezone below Khumbu Icefall (Figs 5 and 6 section A) anincreasing downwasting in the debris-covered part with thehighest lowering between 2 and 8 km from the terminus (B

Table 4 Volume loss of the ablation area of Khumbu Glacier1962ndash2007

Time DTM coverage Average down- Downwasting rate(km2) wasting (m) (m aminus1)

1962ndash1970 49 minus274plusmn 154 minus034plusmn 0191970ndash1984 99 minus253plusmn 416 minus018plusmn 0291984ndash2002 98 minus672plusmn 278 minus037plusmn 0162002ndash2007 100 minus395plusmn 268 minus079plusmn 0521984ndash2007 98 minus1300plusmn 284 minus056plusmn 013

C) and almost no lowering withinsim15 km of the terminus(D) For 1970ndash1984 only lowering betweensim15 and 55 kmof the terminus is significant (Fig 5) Between 1970 and2007 average surface lowering rate in the ablation area wasminus038plusmn007 m aminus1 The rate for 1984ndash2002 is higher thanfor 1962ndash1970 and 1970ndash1984 and comparison of the re-cent DTMs (2002ndash2007) 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 imageincluding the profile and the different sections of surface lowering(see Fig 6A)(B C) Terminus of Khumbu Glacier based on the1970 Corona KH-4B and on the 2007 Cartosat-1 image

lowering (Table 4) However these differences are statisti-cally not significant Comparing the periods 1970ndash1984 and1984ndash2007 however shows an almost statistically signifi-cant increase in the surface lowering rate (018plusmn 029 m aminus1

in comparison to 056plusmn013 m aminus1) The accumulation zoneof Khumbu Glacier has possibly also thinned during the in-vestigated time while there might be a slight thickening inrecent time (2002ndash2007)

4 Discussion

Stereo capability acquisition in the 1960s and 1970s andrelatively 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 alreadyfound to be suitable for this task (Bolch et al 2008b) wehave shown that the later higher resolution KH-4 results inlower uncertainties The accuracy and even distribution ofthe ground control points used for the rectification of theimagery are crucial for the resultant accuracy of the DTMHence it could be expected that an even better accuracy thanours could be achieved if more precise GCPs are availableOther reconnaissance images such as Hexagon KH-9 fromthe 1970s and early 1980s are also suitable for this task ifcoverage with no clouds and little snow cover are available(Surazakov and Aizen 2010) Surazakov and Aizen (2010)have shown a RSMEz for mountainous terrain ofsim20 mwhich is the same range as our results using KH-4B datadespite lower resolution of the KH-9 data Reseau marks onthe KH-9 images facilitate minimizing the distortion Coronaand Hexagon data are also of high value for mapping glaciersand investigating area changes (Bhambri et al 2011 Bolchet 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 KhumbuGlacier 1970 and 2007 See Figs 3 and 5 for the location of theprofiles

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 onaerial imagery highlighted by a clear positive-negative trendon hillslopes with opposing aspects for the periods 1970ndash1984 and 1984ndash2002 (visible on the lower right of the im-ages on Fig 4) However a shifting to adjust this areawould worsen the accuracy of the steep slope in the northernpart of the DTM Hence the distortion of the 1984 DTM ismore complex and could not be fully adjusted relative to theothers Although inaccuracies remain on steep slopes withall DTMs most of the glacier area is not affected by thesebiases The inclusion of the lower resolution 2002 ASTERDTM provides some insight in the recent volume loss of theglaciers However the volume change of the glaciers is toosmall for a significant signal to be detected In addition thelarge scatter in the volume change amongst the investigatedglaciers needs further investigation

The quality of the DTMs however is supported by lo-cal details such as the area on Khumbu Glacier arrowed onFig 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 lakebasin visible on the 1984 aerial photograph (SupplementFig 4) Melting and calving around the margins of lakesis well known to produce locally high ablation rates (Sakai etal 2000 Benn et al 2001) Thickening occurred in this re-gion between 1984 and 2002 attributable to drainage of thelake reducing ablation and ice inflow from upglacier

The utilized imagery also allows length changes of theglaciers to be examined in detail Bajracharya and Mool(2009) argued that Khumbu Glacier has undergone termi-nus retreat based on comparison of recent imagery withold topographic maps Our images however indicate thatthe terminus region of Khumbu Glacier has undergone verylittle change in recent decades (Fig 5b c and Supple-ment Fig 1) This highlights the fact that debris-coveredglaciers may have stable terminus positions even thoughthey are in negative balance and that volume changes aremore reliable indicators of glacier health than area changes(cf Scherler et al 2011)

The calculated average 1970ndash2007 thickness changes forthe whole study area ofminus036plusmn 007 m aminus1 based on in-dependent data sets is very close to the value ofminus033plusmn

024 m aminus1 presented by Bolch et al (2008b) for 1962ndash2002but with smaller uncertainty The wider coverage of thisstudy including Imja Glacier and the accumulation areaof Khumbu Glacier as well as lower uncertainties and themulti-temporal coverage allow greater insight into decadalglacier changes and the influence of debris cover

The longitudinal profile of glacier thinning of KhumbuGlacier shows similar characteristics to those presented byNakawo et al (1999) based on estimated ice flow and thermalproperties derived from Landsat data Based on field mea-surements Kadota et al (2000) found a slight surface lower-ing (sim5 m) about 1 km upglacier of the terminus and highersurface loss (more than 10 m in places) up to the pinnaclezone close to Everest base camp for the period 1978ndash1995This 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 canbe explained in terms of ice dynamics and surface meltrates Sustained high rates of ice delivery below the icefalllargely offset melt in the upper ablation area where debriscover is thin or absent (Figs 5 and 6a section A) Furtherdownglacier 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 remainhigh downglacier despite an increasing debris thickness dueto very low glacier velocities and ablation associated withsupraglacial lakes and exposed ice cliffs (Sakai et al 20002002) (section C Supplement Fig 2) Almost no thinningwas observed within 1 km of the terminus (section D) whichmay reflect either a thick complete debris cover or indicatethat ice loss is already complete The possible slight surfacelowering in the accumulation area of the glacier from 1970

to 2007 might be due to less snowfall This is consistentwith an ice core record at the East Rongbuk Glacier north ofMt Everest that indicates decreasing snow accumulation for1970ndash2001 (Kaspari et al 2008)

The pattern of downwasting has resulted in the devel-opment of a concave glacier profile on Khumbu Glacierwith very low surface gradients in the lower ablation zone(Fig 5b) This trend indicates that a glacial lake could de-velop about 15 to 3 km upstream of the terminus as was pre-dicted in simulations using a 1D-coupled mass balance andflow model by Naito et al (2000) The formation of a largelake on Khumbu Glacier would have major consequences foroutburst flood hazards downstream

The greatest amount of mass loss occurred on ImjaGlacier which can be at least partly attributed to theproglacial Imja Lake which enhances ice losses by calvingThis lake grew significantly since its formation in the late1960s reachingsim09 km2 in 2008 (Supplement Fig 3 Fu-jita et al 2009 Bolch et al 2008a) The comparatively thindebris cover of Imja Glacier apparent in exposed ice cliffsis likely to be another reason for the higher mass loss AtImja Glacier a slight thinning is also observed on the relictice-cored moraine situated below Imja Lake This is in linewith recent field measurements in this area which revealeda lowering of about 1 m aminus1 (Fujita et al 2009) The massloss of the smaller glaciers such as Amphu Laptse Glacieramounts to half of that of Imja Glacier but is still significant

Khumbu Glacier for which data from the year 1984 wasavailable showed a higher surface lowering for the period1984ndash2007 in comparison to 1962ndash1984 The recent trendof more negative mass balances since 2002 however needsfurther investigation as it is not statistically significant Ac-celerated thinning could reflect decreasing velocity (Quinceyet 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 withdebris-cover as BC does not lead into changes in the albedothere The catchment topography plays an important role forthe glacier flow regimes in Khumbu Himalaya (Quincey etal 2009) For example Kangshung Glacier with a large highaltitude accumulation area flowing from Mt Everest towardsthe east shows flow activity across the entire snout while allother 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 ofthe mass balance amongst the investigated mainly avalanchefed glaciers in the same study region Mass balance esti-mate of Kangshung Glacier would reveal how the catchmenttopography and the higher flow influence the mass balanceUnfortunately only the upper part of Kangshung Glacier iscovered by our multi-temporal DTMs making a mass balanceestimate impossible

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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 1970ndash2007 atminus027plusmn008 m we aminus1 is lower than that observedon several other Himalayan glaciers including Chhota Shi-gri Glacier (minus098 m we aminus1 2002ndash2006 Wagnon et al2007 and ndash102 to ndash112 m we aminus1 1999ndash2004 Berthier etal 2007) or the small debris-free Glacier AX010 (minus06 tominus08 m we aminus1 1978ndash1999 Fujita et al 2001) but similarto Dokriani Glacier (minus032 m we aminus1 1992ndash2000 Dobhalet al 2008) However the different observation times andglacier sizes have to be considered and Khumbu Glacier alsohas possibly a more negative mass balance in recent yearsThe tendency towards increased mass loss has also been ob-served worldwide and for the few other Himalayan glacierswith mass balance estimates (Cogley 2011) The mass lossof the investigated glaciers is similar to the average massloss of the 30 reference glaciers worldwide for 1976ndash2005(minus032 m we aminus1) (Zemp et al 2009)

5 Conclusions

This study presents the longest time series of geodeticallyderived mass-balance and glacier volume change estimatesobtained to date in the Himalaya Geodetic mass-balanceestimates based on early stereo Corona and recent satellitedata are suitable for tracking glacier changes through timethus 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 sourceswith 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 predictionof future glacier change and its influence on water resourcesriver runoff sea level rise and glacial hazards

Glaciers south of Mt Everest have continuously lost massfrom 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 whichterminates into a lake The specific mass balance of the in-vestigated glaciers ofminus032plusmn008 m we aminus1 is not higherthan the global average

Supplementary material related to thisarticle is available online athttpwwwthe-cryospherenet53492011tc-5-349-2011-supplementpdf

AcknowledgementsWe thank M Buchroithner for the support andB Shreshta S Bajracharja and P Mool and the staff from ICIMODNepal for cooperation We are grateful for the thorough reviewsof A Racoviteanu G Cogley and an anonymous reviewer on anearlier version of this paper The comments by the anonomousreviewer D Quincey and the short comments of M Pelto and thescientific editor A Kaab improved it further H Raggam helped

with RSG software GAF AG offered Cartosat-1 data at a reducedprice and D Quincey provided the 1984 images DFG (DeutscheForschungsgemeinschaft) provided financial support (Codes BU94915-1 and BO 31992-1) T Bolch was partly funded throughthe ESA project GlobGlacier (2108807I-EC)

Edited by A Kaab

References

Altherr W and Grun A Mt Everest Karte 150 000 ndash Faszinationeines Kartenprojektes Schweizerische Zeitschrift fur Vermes-sung Kulturtechnik und Photogrammetrie Febr 90ndash97 1990

Asahi K Inventory and recent variations of glaciers in the easternNepal Himalayas Seppyo 159ndash169 2001

Bajracharya S R and Mool P K Glaciers glacial lakes andglacial lake outburst floods in the Mount Everest region NepalAnn Glaciol 50 81ndash86 2009

Bamber J L and Rivera A A review of remote sensing methodsfor glacier mass balance determination Global Planet Change59 138ndash148 2007

Benn D I Wiseman S and Hands K Growth and drainageof supraglacial lakes on debris-mantled Ngozumpa GlacierKhumbu Himal Nepal J Glaciol 47(159) 626ndash638 2001

Berthier E Arnaud Y Rajesh K Sarfaraz A Wagnon P andChevallier P Remote sensing estimates of glacier mass bal-ances in the Himachal Pradesh (Western Himalaya India) Re-mote Sens Environ 108 327ndash338 2007

Berthier E Schiefer E Clarke G K C Menounos Band Remy F Contribution of Alaskan glaciers to sea-levelrise derived from satellite imagery Nat Geosci 3 92ndash95doi101038NGEO737 2010

Bhambri R Bolch T Chaujar R K and Kulshreshtha S CGlacier changes in the Garhwal Himalayas India 1968ndash2006based on remote sensing J Glaciol 97(203) 14 pp 2011

Bolch T Buchroithner M F Peters J Baessler M and Ba-jracharya S Identification of glacier motion and potentiallydangerous glacial lakes in the Mt Everest regionNepal usingspaceborne imagery Nat Hazards Earth Syst Sci 8 1329ndash1340doi105194nhess-8-1329-2008 2008a

Bolch T Buchroithner M F Pieczonka T and Kunert A Plani-metric and volumetric glacier changes in Khumbu Himalayasince 1962 using Corona Landsat TM and ASTER data JGlaciol 54 592ndash600 2008b

Bolch T Yao T Kang S Buchroithner M F Scherer D Maus-sion F Huintjes E and Schneider C A glacier inventory forthe western Nyainqentanglha Range and the Nam Co Basin Ti-bet and glacier changes 1976-2009 The Cryosphere 4 419ndash433doi105194tc-4-419-2010 2010

Braithwaite R J and Raper S C B Glaciers and their contri-bution to sea level change Phys Chem Earth 27 1445ndash14542002

Cogley J G Himalayan glaciers now and in 2035 in Encyclopae-dia of Snow Ice and Glaciers edited by Singh V P Singh Pand Haritashya U K Springer in press 2011

Cogley J G Kargel J S Kaser G and Van der Veen C JTracking the source of glacier misinformation Science 327p 522 2010

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 2011

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 SEcoverage coverage diff no glac diff glac glac no glac no glac

study area glac no glac(km2) (km2) (m) (m) (m) (m) (m)

1962ndash1970 1378 254 minus09 223 minus19 157 319 131970ndash1984 838 244 minus36 263 minus99 161 163 211984ndash2002 834 257 minus22 265 minus54 182 107 262002ndash2007 1745 598 +22 228 minus32 135 214 161984ndash2007 806 209 +24 158 minus92 154 109 151970ndash2007 1529 465 +22 194 minus132 156 293 11

glac glacier area no glac non glacier areaN number of considered pixels

Table 3 Glacier volume loss and mass balance 1970ndash2007 and 2002ndash2007

Period 1970ndash2007 Period 2002ndash2007

Glacier Glacier Glacier area Average SpecificGlacier area Average SpecificSize covered elev mass covered elev mass

by DTM diff balance Diff by DTM balance(km2) (km2) (m) (m we aminus1) (km2) (m) (m we aminus1)

Changri SharNup 130 685 minus116plusmn 25 minus028plusmn 008 693 minus16plusmn 27 minus029plusmn 052Khumbu accumulation area1 62 43 minus62plusmn 25 466 +12plusmn 27Khumbu ablation area1 108 100 minus139plusmn 25 101 minus40plusmn 27Whole Khumbu 17 1426 minus111plusmn 25 minus027plusmn 008 147 minus25plusmn 27 minus045plusmn 052Nuptse 40 345 minus94plusmn 25 minus025plusmn 008 352 minus22plusmn 27 minus040plusmn 053Lhotse Nup 195 186 minus76plusmn 25 minus018plusmn 007 186 minus57plusmn 27 minus103plusmn 051Lhotse 65 671 minus107plusmn 25 minus026plusmn 008 671 minus61plusmn 27 minus110plusmn 052Lhotse SharImja2 107 865 minus206plusmn 25 minus050plusmn 009 887 minus81plusmn 27 minus145plusmn 052Amphu Laptse 15 105 minus100plusmn 25 minus024plusmn 008 108 minus43plusmn 27 minus077plusmn 052Chukhung 38 188 minus53plusmn 25 minus030plusmn 008 32 +02plusmn 27 +004plusmn 054Amadablam 22 186 minus120plusmn 25 minus029plusmn 008 25 minus31plusmn 27 minus056plusmn 052Duwo 10 037 minus122plusmn 25 minus030plusmn 008 037 minus109plusmn 27 minus196plusmn 053

SumAverage 617 469 minus133plusmn 25 minus032plusmn 008 496 minus44plusmn 27 minus079plusmn 052

1 We assumed an ELA of 5700 m based on Ashai (2001) and interpretation of the satellite images2 For the year 2007 the estimated volume of Imja lake (378times 106 m3 calculated based on the area extent 091 km2 and the average depth of 414 m) was added to the elevationdifference of 1970ndash2007 For 2002ndash2007 we added the volume difference between 2007 and 2002 (20times 106 m3) Data is based on Fujita et al (2009)

areas using a spline algorithm We did not fill the larger datagaps eg on steep slopes The glacier tongues the avalanchecones and Western Cwm are represented in the DTMs of1970 2002 and 2007 (Fig 3 Tables 3 4) which allow es-timation of the mass balance for the entire glacier OnlyChangri Nup Duwo and the debris-free Chukhung Glacierhave larger data gaps Detailed investigations on KhumbuGlacier are limited to the tongue belowsim5700 m (mainly theablation area) due to the small coverage of the aerial images

3 Volume changes and mass losses

31 Periods 1970ndash2007 and 2002ndash2007 for the wholestudy area

Between 1970 and 2007 significant surface lowering oc-curred on all investigated glaciers (Fig 3 Table 2) Thegreatest lowering was on ImjaLhotse Shar Glacier Exceptfor this glacier which displays surface lowering throughoutthe terminus most glaciers show maximum lowering in theirmid ablation zones with a negligible change near their ter-mini Overall ice loss is estimated to begt06 km3 with anaverage surface lowering of 036plusmn 007 m aminus1 or a specific

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 2011

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 ofminus032plusmn 008 m we aminus1 between 1970 and2007 (Table 3) The surface lowering for the debris-coveredparts only isminus039plusmn007 m aminus1 clearly showing that signif-icant thickness loss occurs despite thick debris cover Mostglaciers also experienced surface lowering between 2002 and2007 (Fig 3 Table 3) The specific mass balance of all10 glaciers has possibly doubled compared to 1970ndash2007(minus079plusmn052 m we aminus1) However the uncertainty is high

32 Detailed multi-temporal investigations on KhumbuGlacier

The surface of the ablation area of Khumbu Glacier loweredin all investigated time periods (Table 3) DTM differenc-ing (Figs 3 and 4) and longitudinal profiles in particular for1970ndash2007 show almost no surface lowering in the clean icezone below Khumbu Icefall (Figs 5 and 6 section A) anincreasing downwasting in the debris-covered part with thehighest lowering between 2 and 8 km from the terminus (B

Table 4 Volume loss of the ablation area of Khumbu Glacier1962ndash2007

Time DTM coverage Average down- Downwasting rate(km2) wasting (m) (m aminus1)

1962ndash1970 49 minus274plusmn 154 minus034plusmn 0191970ndash1984 99 minus253plusmn 416 minus018plusmn 0291984ndash2002 98 minus672plusmn 278 minus037plusmn 0162002ndash2007 100 minus395plusmn 268 minus079plusmn 0521984ndash2007 98 minus1300plusmn 284 minus056plusmn 013

C) and almost no lowering withinsim15 km of the terminus(D) For 1970ndash1984 only lowering betweensim15 and 55 kmof the terminus is significant (Fig 5) Between 1970 and2007 average surface lowering rate in the ablation area wasminus038plusmn007 m aminus1 The rate for 1984ndash2002 is higher thanfor 1962ndash1970 and 1970ndash1984 and comparison of the re-cent DTMs (2002ndash2007) suggests further accelerated surface

The Cryosphere 5 349ndash358 2011 wwwthe-cryospherenet53492011

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 imageincluding the profile and the different sections of surface lowering(see Fig 6A)(B C) Terminus of Khumbu Glacier based on the1970 Corona KH-4B and on the 2007 Cartosat-1 image

lowering (Table 4) However these differences are statisti-cally not significant Comparing the periods 1970ndash1984 and1984ndash2007 however shows an almost statistically signifi-cant increase in the surface lowering rate (018plusmn 029 m aminus1

in comparison to 056plusmn013 m aminus1) The accumulation zoneof Khumbu Glacier has possibly also thinned during the in-vestigated time while there might be a slight thickening inrecent time (2002ndash2007)

4 Discussion

Stereo capability acquisition in the 1960s and 1970s andrelatively 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 alreadyfound to be suitable for this task (Bolch et al 2008b) wehave shown that the later higher resolution KH-4 results inlower uncertainties The accuracy and even distribution ofthe ground control points used for the rectification of theimagery are crucial for the resultant accuracy of the DTMHence it could be expected that an even better accuracy thanours could be achieved if more precise GCPs are availableOther reconnaissance images such as Hexagon KH-9 fromthe 1970s and early 1980s are also suitable for this task ifcoverage with no clouds and little snow cover are available(Surazakov and Aizen 2010) Surazakov and Aizen (2010)have shown a RSMEz for mountainous terrain ofsim20 mwhich is the same range as our results using KH-4B datadespite lower resolution of the KH-9 data Reseau marks onthe KH-9 images facilitate minimizing the distortion Coronaand Hexagon data are also of high value for mapping glaciersand investigating area changes (Bhambri et al 2011 Bolchet 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 KhumbuGlacier 1970 and 2007 See Figs 3 and 5 for the location of theprofiles

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 onaerial imagery highlighted by a clear positive-negative trendon hillslopes with opposing aspects for the periods 1970ndash1984 and 1984ndash2002 (visible on the lower right of the im-ages on Fig 4) However a shifting to adjust this areawould worsen the accuracy of the steep slope in the northernpart of the DTM Hence the distortion of the 1984 DTM ismore complex and could not be fully adjusted relative to theothers Although inaccuracies remain on steep slopes withall DTMs most of the glacier area is not affected by thesebiases The inclusion of the lower resolution 2002 ASTERDTM provides some insight in the recent volume loss of theglaciers However the volume change of the glaciers is toosmall for a significant signal to be detected In addition thelarge scatter in the volume change amongst the investigatedglaciers needs further investigation

The quality of the DTMs however is supported by lo-cal details such as the area on Khumbu Glacier arrowed onFig 4 A large thinning occurred in this area between 1970

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 2011

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 lakebasin visible on the 1984 aerial photograph (SupplementFig 4) Melting and calving around the margins of lakesis well known to produce locally high ablation rates (Sakai etal 2000 Benn et al 2001) Thickening occurred in this re-gion between 1984 and 2002 attributable to drainage of thelake reducing ablation and ice inflow from upglacier

The utilized imagery also allows length changes of theglaciers to be examined in detail Bajracharya and Mool(2009) argued that Khumbu Glacier has undergone termi-nus retreat based on comparison of recent imagery withold topographic maps Our images however indicate thatthe terminus region of Khumbu Glacier has undergone verylittle change in recent decades (Fig 5b c and Supple-ment Fig 1) This highlights the fact that debris-coveredglaciers may have stable terminus positions even thoughthey are in negative balance and that volume changes aremore reliable indicators of glacier health than area changes(cf Scherler et al 2011)

The calculated average 1970ndash2007 thickness changes forthe whole study area ofminus036plusmn 007 m aminus1 based on in-dependent data sets is very close to the value ofminus033plusmn

024 m aminus1 presented by Bolch et al (2008b) for 1962ndash2002but with smaller uncertainty The wider coverage of thisstudy including Imja Glacier and the accumulation areaof Khumbu Glacier as well as lower uncertainties and themulti-temporal coverage allow greater insight into decadalglacier changes and the influence of debris cover

The longitudinal profile of glacier thinning of KhumbuGlacier shows similar characteristics to those presented byNakawo et al (1999) based on estimated ice flow and thermalproperties derived from Landsat data Based on field mea-surements Kadota et al (2000) found a slight surface lower-ing (sim5 m) about 1 km upglacier of the terminus and highersurface loss (more than 10 m in places) up to the pinnaclezone close to Everest base camp for the period 1978ndash1995This 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 canbe explained in terms of ice dynamics and surface meltrates Sustained high rates of ice delivery below the icefalllargely offset melt in the upper ablation area where debriscover is thin or absent (Figs 5 and 6a section A) Furtherdownglacier 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 remainhigh downglacier despite an increasing debris thickness dueto very low glacier velocities and ablation associated withsupraglacial lakes and exposed ice cliffs (Sakai et al 20002002) (section C Supplement Fig 2) Almost no thinningwas observed within 1 km of the terminus (section D) whichmay reflect either a thick complete debris cover or indicatethat ice loss is already complete The possible slight surfacelowering in the accumulation area of the glacier from 1970

to 2007 might be due to less snowfall This is consistentwith an ice core record at the East Rongbuk Glacier north ofMt Everest that indicates decreasing snow accumulation for1970ndash2001 (Kaspari et al 2008)

The pattern of downwasting has resulted in the devel-opment of a concave glacier profile on Khumbu Glacierwith very low surface gradients in the lower ablation zone(Fig 5b) This trend indicates that a glacial lake could de-velop about 15 to 3 km upstream of the terminus as was pre-dicted in simulations using a 1D-coupled mass balance andflow model by Naito et al (2000) The formation of a largelake on Khumbu Glacier would have major consequences foroutburst flood hazards downstream

The greatest amount of mass loss occurred on ImjaGlacier which can be at least partly attributed to theproglacial Imja Lake which enhances ice losses by calvingThis lake grew significantly since its formation in the late1960s reachingsim09 km2 in 2008 (Supplement Fig 3 Fu-jita et al 2009 Bolch et al 2008a) The comparatively thindebris cover of Imja Glacier apparent in exposed ice cliffsis likely to be another reason for the higher mass loss AtImja Glacier a slight thinning is also observed on the relictice-cored moraine situated below Imja Lake This is in linewith recent field measurements in this area which revealeda lowering of about 1 m aminus1 (Fujita et al 2009) The massloss of the smaller glaciers such as Amphu Laptse Glacieramounts to half of that of Imja Glacier but is still significant

Khumbu Glacier for which data from the year 1984 wasavailable showed a higher surface lowering for the period1984ndash2007 in comparison to 1962ndash1984 The recent trendof more negative mass balances since 2002 however needsfurther investigation as it is not statistically significant Ac-celerated thinning could reflect decreasing velocity (Quinceyet 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 withdebris-cover as BC does not lead into changes in the albedothere The catchment topography plays an important role forthe glacier flow regimes in Khumbu Himalaya (Quincey etal 2009) For example Kangshung Glacier with a large highaltitude accumulation area flowing from Mt Everest towardsthe east shows flow activity across the entire snout while allother 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 ofthe mass balance amongst the investigated mainly avalanchefed glaciers in the same study region Mass balance esti-mate of Kangshung Glacier would reveal how the catchmenttopography and the higher flow influence the mass balanceUnfortunately only the upper part of Kangshung Glacier iscovered by our multi-temporal DTMs making a mass balanceestimate impossible

The Cryosphere 5 349ndash358 2011 wwwthe-cryospherenet53492011

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 1970ndash2007 atminus027plusmn008 m we aminus1 is lower than that observedon several other Himalayan glaciers including Chhota Shi-gri Glacier (minus098 m we aminus1 2002ndash2006 Wagnon et al2007 and ndash102 to ndash112 m we aminus1 1999ndash2004 Berthier etal 2007) or the small debris-free Glacier AX010 (minus06 tominus08 m we aminus1 1978ndash1999 Fujita et al 2001) but similarto Dokriani Glacier (minus032 m we aminus1 1992ndash2000 Dobhalet al 2008) However the different observation times andglacier sizes have to be considered and Khumbu Glacier alsohas possibly a more negative mass balance in recent yearsThe tendency towards increased mass loss has also been ob-served worldwide and for the few other Himalayan glacierswith mass balance estimates (Cogley 2011) The mass lossof the investigated glaciers is similar to the average massloss of the 30 reference glaciers worldwide for 1976ndash2005(minus032 m we aminus1) (Zemp et al 2009)

5 Conclusions

This study presents the longest time series of geodeticallyderived mass-balance and glacier volume change estimatesobtained to date in the Himalaya Geodetic mass-balanceestimates based on early stereo Corona and recent satellitedata are suitable for tracking glacier changes through timethus 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 sourceswith 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 predictionof future glacier change and its influence on water resourcesriver runoff sea level rise and glacial hazards

Glaciers south of Mt Everest have continuously lost massfrom 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 whichterminates into a lake The specific mass balance of the in-vestigated glaciers ofminus032plusmn008 m we aminus1 is not higherthan the global average

Supplementary material related to thisarticle is available online athttpwwwthe-cryospherenet53492011tc-5-349-2011-supplementpdf

AcknowledgementsWe thank M Buchroithner for the support andB Shreshta S Bajracharja and P Mool and the staff from ICIMODNepal for cooperation We are grateful for the thorough reviewsof A Racoviteanu G Cogley and an anonymous reviewer on anearlier version of this paper The comments by the anonomousreviewer D Quincey and the short comments of M Pelto and thescientific editor A Kaab improved it further H Raggam helped

with RSG software GAF AG offered Cartosat-1 data at a reducedprice and D Quincey provided the 1984 images DFG (DeutscheForschungsgemeinschaft) provided financial support (Codes BU94915-1 and BO 31992-1) T Bolch was partly funded throughthe ESA project GlobGlacier (2108807I-EC)

Edited by A Kaab

References

Altherr W and Grun A Mt Everest Karte 150 000 ndash Faszinationeines Kartenprojektes Schweizerische Zeitschrift fur Vermes-sung Kulturtechnik und Photogrammetrie Febr 90ndash97 1990

Asahi K Inventory and recent variations of glaciers in the easternNepal Himalayas Seppyo 159ndash169 2001

Bajracharya S R and Mool P K Glaciers glacial lakes andglacial lake outburst floods in the Mount Everest region NepalAnn Glaciol 50 81ndash86 2009

Bamber J L and Rivera A A review of remote sensing methodsfor glacier mass balance determination Global Planet Change59 138ndash148 2007

Benn D I Wiseman S and Hands K Growth and drainageof supraglacial lakes on debris-mantled Ngozumpa GlacierKhumbu Himal Nepal J Glaciol 47(159) 626ndash638 2001

Berthier E Arnaud Y Rajesh K Sarfaraz A Wagnon P andChevallier P Remote sensing estimates of glacier mass bal-ances in the Himachal Pradesh (Western Himalaya India) Re-mote Sens Environ 108 327ndash338 2007

Berthier E Schiefer E Clarke G K C Menounos Band Remy F Contribution of Alaskan glaciers to sea-levelrise derived from satellite imagery Nat Geosci 3 92ndash95doi101038NGEO737 2010

Bhambri R Bolch T Chaujar R K and Kulshreshtha S CGlacier changes in the Garhwal Himalayas India 1968ndash2006based on remote sensing J Glaciol 97(203) 14 pp 2011

Bolch T Buchroithner M F Peters J Baessler M and Ba-jracharya S Identification of glacier motion and potentiallydangerous glacial lakes in the Mt Everest regionNepal usingspaceborne imagery Nat Hazards Earth Syst Sci 8 1329ndash1340doi105194nhess-8-1329-2008 2008a

Bolch T Buchroithner M F Pieczonka T and Kunert A Plani-metric and volumetric glacier changes in Khumbu Himalayasince 1962 using Corona Landsat TM and ASTER data JGlaciol 54 592ndash600 2008b

Bolch T Yao T Kang S Buchroithner M F Scherer D Maus-sion F Huintjes E and Schneider C A glacier inventory forthe western Nyainqentanglha Range and the Nam Co Basin Ti-bet and glacier changes 1976-2009 The Cryosphere 4 419ndash433doi105194tc-4-419-2010 2010

Braithwaite R J and Raper S C B Glaciers and their contri-bution to sea level change Phys Chem Earth 27 1445ndash14542002

Cogley J G Himalayan glaciers now and in 2035 in Encyclopae-dia of Snow Ice and Glaciers edited by Singh V P Singh Pand Haritashya U K Springer in press 2011

Cogley J G Kargel J S Kaser G and Van der Veen C JTracking the source of glacier misinformation Science 327p 522 2010

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 2011

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 ofminus032plusmn 008 m we aminus1 between 1970 and2007 (Table 3) The surface lowering for the debris-coveredparts only isminus039plusmn007 m aminus1 clearly showing that signif-icant thickness loss occurs despite thick debris cover Mostglaciers also experienced surface lowering between 2002 and2007 (Fig 3 Table 3) The specific mass balance of all10 glaciers has possibly doubled compared to 1970ndash2007(minus079plusmn052 m we aminus1) However the uncertainty is high

32 Detailed multi-temporal investigations on KhumbuGlacier

The surface of the ablation area of Khumbu Glacier loweredin all investigated time periods (Table 3) DTM differenc-ing (Figs 3 and 4) and longitudinal profiles in particular for1970ndash2007 show almost no surface lowering in the clean icezone below Khumbu Icefall (Figs 5 and 6 section A) anincreasing downwasting in the debris-covered part with thehighest lowering between 2 and 8 km from the terminus (B

Table 4 Volume loss of the ablation area of Khumbu Glacier1962ndash2007

Time DTM coverage Average down- Downwasting rate(km2) wasting (m) (m aminus1)

1962ndash1970 49 minus274plusmn 154 minus034plusmn 0191970ndash1984 99 minus253plusmn 416 minus018plusmn 0291984ndash2002 98 minus672plusmn 278 minus037plusmn 0162002ndash2007 100 minus395plusmn 268 minus079plusmn 0521984ndash2007 98 minus1300plusmn 284 minus056plusmn 013

C) and almost no lowering withinsim15 km of the terminus(D) For 1970ndash1984 only lowering betweensim15 and 55 kmof the terminus is significant (Fig 5) Between 1970 and2007 average surface lowering rate in the ablation area wasminus038plusmn007 m aminus1 The rate for 1984ndash2002 is higher thanfor 1962ndash1970 and 1970ndash1984 and comparison of the re-cent DTMs (2002ndash2007) suggests further accelerated surface

The Cryosphere 5 349ndash358 2011 wwwthe-cryospherenet53492011

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 imageincluding the profile and the different sections of surface lowering(see Fig 6A)(B C) Terminus of Khumbu Glacier based on the1970 Corona KH-4B and on the 2007 Cartosat-1 image

lowering (Table 4) However these differences are statisti-cally not significant Comparing the periods 1970ndash1984 and1984ndash2007 however shows an almost statistically signifi-cant increase in the surface lowering rate (018plusmn 029 m aminus1

in comparison to 056plusmn013 m aminus1) The accumulation zoneof Khumbu Glacier has possibly also thinned during the in-vestigated time while there might be a slight thickening inrecent time (2002ndash2007)

4 Discussion

Stereo capability acquisition in the 1960s and 1970s andrelatively 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 alreadyfound to be suitable for this task (Bolch et al 2008b) wehave shown that the later higher resolution KH-4 results inlower uncertainties The accuracy and even distribution ofthe ground control points used for the rectification of theimagery are crucial for the resultant accuracy of the DTMHence it could be expected that an even better accuracy thanours could be achieved if more precise GCPs are availableOther reconnaissance images such as Hexagon KH-9 fromthe 1970s and early 1980s are also suitable for this task ifcoverage with no clouds and little snow cover are available(Surazakov and Aizen 2010) Surazakov and Aizen (2010)have shown a RSMEz for mountainous terrain ofsim20 mwhich is the same range as our results using KH-4B datadespite lower resolution of the KH-9 data Reseau marks onthe KH-9 images facilitate minimizing the distortion Coronaand Hexagon data are also of high value for mapping glaciersand investigating area changes (Bhambri et al 2011 Bolchet 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 KhumbuGlacier 1970 and 2007 See Figs 3 and 5 for the location of theprofiles

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 onaerial imagery highlighted by a clear positive-negative trendon hillslopes with opposing aspects for the periods 1970ndash1984 and 1984ndash2002 (visible on the lower right of the im-ages on Fig 4) However a shifting to adjust this areawould worsen the accuracy of the steep slope in the northernpart of the DTM Hence the distortion of the 1984 DTM ismore complex and could not be fully adjusted relative to theothers Although inaccuracies remain on steep slopes withall DTMs most of the glacier area is not affected by thesebiases The inclusion of the lower resolution 2002 ASTERDTM provides some insight in the recent volume loss of theglaciers However the volume change of the glaciers is toosmall for a significant signal to be detected In addition thelarge scatter in the volume change amongst the investigatedglaciers needs further investigation

The quality of the DTMs however is supported by lo-cal details such as the area on Khumbu Glacier arrowed onFig 4 A large thinning occurred in this area between 1970

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 2011

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 lakebasin visible on the 1984 aerial photograph (SupplementFig 4) Melting and calving around the margins of lakesis well known to produce locally high ablation rates (Sakai etal 2000 Benn et al 2001) Thickening occurred in this re-gion between 1984 and 2002 attributable to drainage of thelake reducing ablation and ice inflow from upglacier

The utilized imagery also allows length changes of theglaciers to be examined in detail Bajracharya and Mool(2009) argued that Khumbu Glacier has undergone termi-nus retreat based on comparison of recent imagery withold topographic maps Our images however indicate thatthe terminus region of Khumbu Glacier has undergone verylittle change in recent decades (Fig 5b c and Supple-ment Fig 1) This highlights the fact that debris-coveredglaciers may have stable terminus positions even thoughthey are in negative balance and that volume changes aremore reliable indicators of glacier health than area changes(cf Scherler et al 2011)

The calculated average 1970ndash2007 thickness changes forthe whole study area ofminus036plusmn 007 m aminus1 based on in-dependent data sets is very close to the value ofminus033plusmn

024 m aminus1 presented by Bolch et al (2008b) for 1962ndash2002but with smaller uncertainty The wider coverage of thisstudy including Imja Glacier and the accumulation areaof Khumbu Glacier as well as lower uncertainties and themulti-temporal coverage allow greater insight into decadalglacier changes and the influence of debris cover

The longitudinal profile of glacier thinning of KhumbuGlacier shows similar characteristics to those presented byNakawo et al (1999) based on estimated ice flow and thermalproperties derived from Landsat data Based on field mea-surements Kadota et al (2000) found a slight surface lower-ing (sim5 m) about 1 km upglacier of the terminus and highersurface loss (more than 10 m in places) up to the pinnaclezone close to Everest base camp for the period 1978ndash1995This 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 canbe explained in terms of ice dynamics and surface meltrates Sustained high rates of ice delivery below the icefalllargely offset melt in the upper ablation area where debriscover is thin or absent (Figs 5 and 6a section A) Furtherdownglacier 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 remainhigh downglacier despite an increasing debris thickness dueto very low glacier velocities and ablation associated withsupraglacial lakes and exposed ice cliffs (Sakai et al 20002002) (section C Supplement Fig 2) Almost no thinningwas observed within 1 km of the terminus (section D) whichmay reflect either a thick complete debris cover or indicatethat ice loss is already complete The possible slight surfacelowering in the accumulation area of the glacier from 1970

to 2007 might be due to less snowfall This is consistentwith an ice core record at the East Rongbuk Glacier north ofMt Everest that indicates decreasing snow accumulation for1970ndash2001 (Kaspari et al 2008)

The pattern of downwasting has resulted in the devel-opment of a concave glacier profile on Khumbu Glacierwith very low surface gradients in the lower ablation zone(Fig 5b) This trend indicates that a glacial lake could de-velop about 15 to 3 km upstream of the terminus as was pre-dicted in simulations using a 1D-coupled mass balance andflow model by Naito et al (2000) The formation of a largelake on Khumbu Glacier would have major consequences foroutburst flood hazards downstream

The greatest amount of mass loss occurred on ImjaGlacier which can be at least partly attributed to theproglacial Imja Lake which enhances ice losses by calvingThis lake grew significantly since its formation in the late1960s reachingsim09 km2 in 2008 (Supplement Fig 3 Fu-jita et al 2009 Bolch et al 2008a) The comparatively thindebris cover of Imja Glacier apparent in exposed ice cliffsis likely to be another reason for the higher mass loss AtImja Glacier a slight thinning is also observed on the relictice-cored moraine situated below Imja Lake This is in linewith recent field measurements in this area which revealeda lowering of about 1 m aminus1 (Fujita et al 2009) The massloss of the smaller glaciers such as Amphu Laptse Glacieramounts to half of that of Imja Glacier but is still significant

Khumbu Glacier for which data from the year 1984 wasavailable showed a higher surface lowering for the period1984ndash2007 in comparison to 1962ndash1984 The recent trendof more negative mass balances since 2002 however needsfurther investigation as it is not statistically significant Ac-celerated thinning could reflect decreasing velocity (Quinceyet 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 withdebris-cover as BC does not lead into changes in the albedothere The catchment topography plays an important role forthe glacier flow regimes in Khumbu Himalaya (Quincey etal 2009) For example Kangshung Glacier with a large highaltitude accumulation area flowing from Mt Everest towardsthe east shows flow activity across the entire snout while allother 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 ofthe mass balance amongst the investigated mainly avalanchefed glaciers in the same study region Mass balance esti-mate of Kangshung Glacier would reveal how the catchmenttopography and the higher flow influence the mass balanceUnfortunately only the upper part of Kangshung Glacier iscovered by our multi-temporal DTMs making a mass balanceestimate impossible

The Cryosphere 5 349ndash358 2011 wwwthe-cryospherenet53492011

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 1970ndash2007 atminus027plusmn008 m we aminus1 is lower than that observedon several other Himalayan glaciers including Chhota Shi-gri Glacier (minus098 m we aminus1 2002ndash2006 Wagnon et al2007 and ndash102 to ndash112 m we aminus1 1999ndash2004 Berthier etal 2007) or the small debris-free Glacier AX010 (minus06 tominus08 m we aminus1 1978ndash1999 Fujita et al 2001) but similarto Dokriani Glacier (minus032 m we aminus1 1992ndash2000 Dobhalet al 2008) However the different observation times andglacier sizes have to be considered and Khumbu Glacier alsohas possibly a more negative mass balance in recent yearsThe tendency towards increased mass loss has also been ob-served worldwide and for the few other Himalayan glacierswith mass balance estimates (Cogley 2011) The mass lossof the investigated glaciers is similar to the average massloss of the 30 reference glaciers worldwide for 1976ndash2005(minus032 m we aminus1) (Zemp et al 2009)

5 Conclusions

This study presents the longest time series of geodeticallyderived mass-balance and glacier volume change estimatesobtained to date in the Himalaya Geodetic mass-balanceestimates based on early stereo Corona and recent satellitedata are suitable for tracking glacier changes through timethus 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 sourceswith 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 predictionof future glacier change and its influence on water resourcesriver runoff sea level rise and glacial hazards

Glaciers south of Mt Everest have continuously lost massfrom 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 whichterminates into a lake The specific mass balance of the in-vestigated glaciers ofminus032plusmn008 m we aminus1 is not higherthan the global average

Supplementary material related to thisarticle is available online athttpwwwthe-cryospherenet53492011tc-5-349-2011-supplementpdf

AcknowledgementsWe thank M Buchroithner for the support andB Shreshta S Bajracharja and P Mool and the staff from ICIMODNepal for cooperation We are grateful for the thorough reviewsof A Racoviteanu G Cogley and an anonymous reviewer on anearlier version of this paper The comments by the anonomousreviewer D Quincey and the short comments of M Pelto and thescientific editor A Kaab improved it further H Raggam helped

with RSG software GAF AG offered Cartosat-1 data at a reducedprice and D Quincey provided the 1984 images DFG (DeutscheForschungsgemeinschaft) provided financial support (Codes BU94915-1 and BO 31992-1) T Bolch was partly funded throughthe ESA project GlobGlacier (2108807I-EC)

Edited by A Kaab

References

Altherr W and Grun A Mt Everest Karte 150 000 ndash Faszinationeines Kartenprojektes Schweizerische Zeitschrift fur Vermes-sung Kulturtechnik und Photogrammetrie Febr 90ndash97 1990

Asahi K Inventory and recent variations of glaciers in the easternNepal Himalayas Seppyo 159ndash169 2001

Bajracharya S R and Mool P K Glaciers glacial lakes andglacial lake outburst floods in the Mount Everest region NepalAnn Glaciol 50 81ndash86 2009

Bamber J L and Rivera A A review of remote sensing methodsfor glacier mass balance determination Global Planet Change59 138ndash148 2007

Benn D I Wiseman S and Hands K Growth and drainageof supraglacial lakes on debris-mantled Ngozumpa GlacierKhumbu Himal Nepal J Glaciol 47(159) 626ndash638 2001

Berthier E Arnaud Y Rajesh K Sarfaraz A Wagnon P andChevallier P Remote sensing estimates of glacier mass bal-ances in the Himachal Pradesh (Western Himalaya India) Re-mote Sens Environ 108 327ndash338 2007

Berthier E Schiefer E Clarke G K C Menounos Band Remy F Contribution of Alaskan glaciers to sea-levelrise derived from satellite imagery Nat Geosci 3 92ndash95doi101038NGEO737 2010

Bhambri R Bolch T Chaujar R K and Kulshreshtha S CGlacier changes in the Garhwal Himalayas India 1968ndash2006based on remote sensing J Glaciol 97(203) 14 pp 2011

Bolch T Buchroithner M F Peters J Baessler M and Ba-jracharya S Identification of glacier motion and potentiallydangerous glacial lakes in the Mt Everest regionNepal usingspaceborne imagery Nat Hazards Earth Syst Sci 8 1329ndash1340doi105194nhess-8-1329-2008 2008a

Bolch T Buchroithner M F Pieczonka T and Kunert A Plani-metric and volumetric glacier changes in Khumbu Himalayasince 1962 using Corona Landsat TM and ASTER data JGlaciol 54 592ndash600 2008b

Bolch T Yao T Kang S Buchroithner M F Scherer D Maus-sion F Huintjes E and Schneider C A glacier inventory forthe western Nyainqentanglha Range and the Nam Co Basin Ti-bet and glacier changes 1976-2009 The Cryosphere 4 419ndash433doi105194tc-4-419-2010 2010

Braithwaite R J and Raper S C B Glaciers and their contri-bution to sea level change Phys Chem Earth 27 1445ndash14542002

Cogley J G Himalayan glaciers now and in 2035 in Encyclopae-dia of Snow Ice and Glaciers edited by Singh V P Singh Pand Haritashya U K Springer in press 2011

Cogley J G Kargel J S Kaser G and Van der Veen C JTracking the source of glacier misinformation Science 327p 522 2010

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 2011

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 imageincluding the profile and the different sections of surface lowering(see Fig 6A)(B C) Terminus of Khumbu Glacier based on the1970 Corona KH-4B and on the 2007 Cartosat-1 image

lowering (Table 4) However these differences are statisti-cally not significant Comparing the periods 1970ndash1984 and1984ndash2007 however shows an almost statistically signifi-cant increase in the surface lowering rate (018plusmn 029 m aminus1

in comparison to 056plusmn013 m aminus1) The accumulation zoneof Khumbu Glacier has possibly also thinned during the in-vestigated time while there might be a slight thickening inrecent time (2002ndash2007)

4 Discussion

Stereo capability acquisition in the 1960s and 1970s andrelatively 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 alreadyfound to be suitable for this task (Bolch et al 2008b) wehave shown that the later higher resolution KH-4 results inlower uncertainties The accuracy and even distribution ofthe ground control points used for the rectification of theimagery are crucial for the resultant accuracy of the DTMHence it could be expected that an even better accuracy thanours could be achieved if more precise GCPs are availableOther reconnaissance images such as Hexagon KH-9 fromthe 1970s and early 1980s are also suitable for this task ifcoverage with no clouds and little snow cover are available(Surazakov and Aizen 2010) Surazakov and Aizen (2010)have shown a RSMEz for mountainous terrain ofsim20 mwhich is the same range as our results using KH-4B datadespite lower resolution of the KH-9 data Reseau marks onthe KH-9 images facilitate minimizing the distortion Coronaand Hexagon data are also of high value for mapping glaciersand investigating area changes (Bhambri et al 2011 Bolchet 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 KhumbuGlacier 1970 and 2007 See Figs 3 and 5 for the location of theprofiles

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 onaerial imagery highlighted by a clear positive-negative trendon hillslopes with opposing aspects for the periods 1970ndash1984 and 1984ndash2002 (visible on the lower right of the im-ages on Fig 4) However a shifting to adjust this areawould worsen the accuracy of the steep slope in the northernpart of the DTM Hence the distortion of the 1984 DTM ismore complex and could not be fully adjusted relative to theothers Although inaccuracies remain on steep slopes withall DTMs most of the glacier area is not affected by thesebiases The inclusion of the lower resolution 2002 ASTERDTM provides some insight in the recent volume loss of theglaciers However the volume change of the glaciers is toosmall for a significant signal to be detected In addition thelarge scatter in the volume change amongst the investigatedglaciers needs further investigation

The quality of the DTMs however is supported by lo-cal details such as the area on Khumbu Glacier arrowed onFig 4 A large thinning occurred in this area between 1970

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 2011

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 lakebasin visible on the 1984 aerial photograph (SupplementFig 4) Melting and calving around the margins of lakesis well known to produce locally high ablation rates (Sakai etal 2000 Benn et al 2001) Thickening occurred in this re-gion between 1984 and 2002 attributable to drainage of thelake reducing ablation and ice inflow from upglacier

The utilized imagery also allows length changes of theglaciers to be examined in detail Bajracharya and Mool(2009) argued that Khumbu Glacier has undergone termi-nus retreat based on comparison of recent imagery withold topographic maps Our images however indicate thatthe terminus region of Khumbu Glacier has undergone verylittle change in recent decades (Fig 5b c and Supple-ment Fig 1) This highlights the fact that debris-coveredglaciers may have stable terminus positions even thoughthey are in negative balance and that volume changes aremore reliable indicators of glacier health than area changes(cf Scherler et al 2011)

The calculated average 1970ndash2007 thickness changes forthe whole study area ofminus036plusmn 007 m aminus1 based on in-dependent data sets is very close to the value ofminus033plusmn

024 m aminus1 presented by Bolch et al (2008b) for 1962ndash2002but with smaller uncertainty The wider coverage of thisstudy including Imja Glacier and the accumulation areaof Khumbu Glacier as well as lower uncertainties and themulti-temporal coverage allow greater insight into decadalglacier changes and the influence of debris cover

The longitudinal profile of glacier thinning of KhumbuGlacier shows similar characteristics to those presented byNakawo et al (1999) based on estimated ice flow and thermalproperties derived from Landsat data Based on field mea-surements Kadota et al (2000) found a slight surface lower-ing (sim5 m) about 1 km upglacier of the terminus and highersurface loss (more than 10 m in places) up to the pinnaclezone close to Everest base camp for the period 1978ndash1995This 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 canbe explained in terms of ice dynamics and surface meltrates Sustained high rates of ice delivery below the icefalllargely offset melt in the upper ablation area where debriscover is thin or absent (Figs 5 and 6a section A) Furtherdownglacier 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 remainhigh downglacier despite an increasing debris thickness dueto very low glacier velocities and ablation associated withsupraglacial lakes and exposed ice cliffs (Sakai et al 20002002) (section C Supplement Fig 2) Almost no thinningwas observed within 1 km of the terminus (section D) whichmay reflect either a thick complete debris cover or indicatethat ice loss is already complete The possible slight surfacelowering in the accumulation area of the glacier from 1970

to 2007 might be due to less snowfall This is consistentwith an ice core record at the East Rongbuk Glacier north ofMt Everest that indicates decreasing snow accumulation for1970ndash2001 (Kaspari et al 2008)

The pattern of downwasting has resulted in the devel-opment of a concave glacier profile on Khumbu Glacierwith very low surface gradients in the lower ablation zone(Fig 5b) This trend indicates that a glacial lake could de-velop about 15 to 3 km upstream of the terminus as was pre-dicted in simulations using a 1D-coupled mass balance andflow model by Naito et al (2000) The formation of a largelake on Khumbu Glacier would have major consequences foroutburst flood hazards downstream

The greatest amount of mass loss occurred on ImjaGlacier which can be at least partly attributed to theproglacial Imja Lake which enhances ice losses by calvingThis lake grew significantly since its formation in the late1960s reachingsim09 km2 in 2008 (Supplement Fig 3 Fu-jita et al 2009 Bolch et al 2008a) The comparatively thindebris cover of Imja Glacier apparent in exposed ice cliffsis likely to be another reason for the higher mass loss AtImja Glacier a slight thinning is also observed on the relictice-cored moraine situated below Imja Lake This is in linewith recent field measurements in this area which revealeda lowering of about 1 m aminus1 (Fujita et al 2009) The massloss of the smaller glaciers such as Amphu Laptse Glacieramounts to half of that of Imja Glacier but is still significant

Khumbu Glacier for which data from the year 1984 wasavailable showed a higher surface lowering for the period1984ndash2007 in comparison to 1962ndash1984 The recent trendof more negative mass balances since 2002 however needsfurther investigation as it is not statistically significant Ac-celerated thinning could reflect decreasing velocity (Quinceyet 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 withdebris-cover as BC does not lead into changes in the albedothere The catchment topography plays an important role forthe glacier flow regimes in Khumbu Himalaya (Quincey etal 2009) For example Kangshung Glacier with a large highaltitude accumulation area flowing from Mt Everest towardsthe east shows flow activity across the entire snout while allother 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 ofthe mass balance amongst the investigated mainly avalanchefed glaciers in the same study region Mass balance esti-mate of Kangshung Glacier would reveal how the catchmenttopography and the higher flow influence the mass balanceUnfortunately only the upper part of Kangshung Glacier iscovered by our multi-temporal DTMs making a mass balanceestimate impossible

The Cryosphere 5 349ndash358 2011 wwwthe-cryospherenet53492011

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 1970ndash2007 atminus027plusmn008 m we aminus1 is lower than that observedon several other Himalayan glaciers including Chhota Shi-gri Glacier (minus098 m we aminus1 2002ndash2006 Wagnon et al2007 and ndash102 to ndash112 m we aminus1 1999ndash2004 Berthier etal 2007) or the small debris-free Glacier AX010 (minus06 tominus08 m we aminus1 1978ndash1999 Fujita et al 2001) but similarto Dokriani Glacier (minus032 m we aminus1 1992ndash2000 Dobhalet al 2008) However the different observation times andglacier sizes have to be considered and Khumbu Glacier alsohas possibly a more negative mass balance in recent yearsThe tendency towards increased mass loss has also been ob-served worldwide and for the few other Himalayan glacierswith mass balance estimates (Cogley 2011) The mass lossof the investigated glaciers is similar to the average massloss of the 30 reference glaciers worldwide for 1976ndash2005(minus032 m we aminus1) (Zemp et al 2009)

5 Conclusions

This study presents the longest time series of geodeticallyderived mass-balance and glacier volume change estimatesobtained to date in the Himalaya Geodetic mass-balanceestimates based on early stereo Corona and recent satellitedata are suitable for tracking glacier changes through timethus 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 sourceswith 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 predictionof future glacier change and its influence on water resourcesriver runoff sea level rise and glacial hazards

Glaciers south of Mt Everest have continuously lost massfrom 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 whichterminates into a lake The specific mass balance of the in-vestigated glaciers ofminus032plusmn008 m we aminus1 is not higherthan the global average

Supplementary material related to thisarticle is available online athttpwwwthe-cryospherenet53492011tc-5-349-2011-supplementpdf

AcknowledgementsWe thank M Buchroithner for the support andB Shreshta S Bajracharja and P Mool and the staff from ICIMODNepal for cooperation We are grateful for the thorough reviewsof A Racoviteanu G Cogley and an anonymous reviewer on anearlier version of this paper The comments by the anonomousreviewer D Quincey and the short comments of M Pelto and thescientific editor A Kaab improved it further H Raggam helped

with RSG software GAF AG offered Cartosat-1 data at a reducedprice and D Quincey provided the 1984 images DFG (DeutscheForschungsgemeinschaft) provided financial support (Codes BU94915-1 and BO 31992-1) T Bolch was partly funded throughthe ESA project GlobGlacier (2108807I-EC)

Edited by A Kaab

References

Altherr W and Grun A Mt Everest Karte 150 000 ndash Faszinationeines Kartenprojektes Schweizerische Zeitschrift fur Vermes-sung Kulturtechnik und Photogrammetrie Febr 90ndash97 1990

Asahi K Inventory and recent variations of glaciers in the easternNepal Himalayas Seppyo 159ndash169 2001

Bajracharya S R and Mool P K Glaciers glacial lakes andglacial lake outburst floods in the Mount Everest region NepalAnn Glaciol 50 81ndash86 2009

Bamber J L and Rivera A A review of remote sensing methodsfor glacier mass balance determination Global Planet Change59 138ndash148 2007

Benn D I Wiseman S and Hands K Growth and drainageof supraglacial lakes on debris-mantled Ngozumpa GlacierKhumbu Himal Nepal J Glaciol 47(159) 626ndash638 2001

Berthier E Arnaud Y Rajesh K Sarfaraz A Wagnon P andChevallier P Remote sensing estimates of glacier mass bal-ances in the Himachal Pradesh (Western Himalaya India) Re-mote Sens Environ 108 327ndash338 2007

Berthier E Schiefer E Clarke G K C Menounos Band Remy F Contribution of Alaskan glaciers to sea-levelrise derived from satellite imagery Nat Geosci 3 92ndash95doi101038NGEO737 2010

Bhambri R Bolch T Chaujar R K and Kulshreshtha S CGlacier changes in the Garhwal Himalayas India 1968ndash2006based on remote sensing J Glaciol 97(203) 14 pp 2011

Bolch T Buchroithner M F Peters J Baessler M and Ba-jracharya S Identification of glacier motion and potentiallydangerous glacial lakes in the Mt Everest regionNepal usingspaceborne imagery Nat Hazards Earth Syst Sci 8 1329ndash1340doi105194nhess-8-1329-2008 2008a

Bolch T Buchroithner M F Pieczonka T and Kunert A Plani-metric and volumetric glacier changes in Khumbu Himalayasince 1962 using Corona Landsat TM and ASTER data JGlaciol 54 592ndash600 2008b

Bolch T Yao T Kang S Buchroithner M F Scherer D Maus-sion F Huintjes E and Schneider C A glacier inventory forthe western Nyainqentanglha Range and the Nam Co Basin Ti-bet and glacier changes 1976-2009 The Cryosphere 4 419ndash433doi105194tc-4-419-2010 2010

Braithwaite R J and Raper S C B Glaciers and their contri-bution to sea level change Phys Chem Earth 27 1445ndash14542002

Cogley J G Himalayan glaciers now and in 2035 in Encyclopae-dia of Snow Ice and Glaciers edited by Singh V P Singh Pand Haritashya U K Springer in press 2011

Cogley J G Kargel J S Kaser G and Van der Veen C JTracking the source of glacier misinformation Science 327p 522 2010

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 2011

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 lakebasin visible on the 1984 aerial photograph (SupplementFig 4) Melting and calving around the margins of lakesis well known to produce locally high ablation rates (Sakai etal 2000 Benn et al 2001) Thickening occurred in this re-gion between 1984 and 2002 attributable to drainage of thelake reducing ablation and ice inflow from upglacier

The utilized imagery also allows length changes of theglaciers to be examined in detail Bajracharya and Mool(2009) argued that Khumbu Glacier has undergone termi-nus retreat based on comparison of recent imagery withold topographic maps Our images however indicate thatthe terminus region of Khumbu Glacier has undergone verylittle change in recent decades (Fig 5b c and Supple-ment Fig 1) This highlights the fact that debris-coveredglaciers may have stable terminus positions even thoughthey are in negative balance and that volume changes aremore reliable indicators of glacier health than area changes(cf Scherler et al 2011)

The calculated average 1970ndash2007 thickness changes forthe whole study area ofminus036plusmn 007 m aminus1 based on in-dependent data sets is very close to the value ofminus033plusmn

024 m aminus1 presented by Bolch et al (2008b) for 1962ndash2002but with smaller uncertainty The wider coverage of thisstudy including Imja Glacier and the accumulation areaof Khumbu Glacier as well as lower uncertainties and themulti-temporal coverage allow greater insight into decadalglacier changes and the influence of debris cover

The longitudinal profile of glacier thinning of KhumbuGlacier shows similar characteristics to those presented byNakawo et al (1999) based on estimated ice flow and thermalproperties derived from Landsat data Based on field mea-surements Kadota et al (2000) found a slight surface lower-ing (sim5 m) about 1 km upglacier of the terminus and highersurface loss (more than 10 m in places) up to the pinnaclezone close to Everest base camp for the period 1978ndash1995This 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 canbe explained in terms of ice dynamics and surface meltrates Sustained high rates of ice delivery below the icefalllargely offset melt in the upper ablation area where debriscover is thin or absent (Figs 5 and 6a section A) Furtherdownglacier 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 remainhigh downglacier despite an increasing debris thickness dueto very low glacier velocities and ablation associated withsupraglacial lakes and exposed ice cliffs (Sakai et al 20002002) (section C Supplement Fig 2) Almost no thinningwas observed within 1 km of the terminus (section D) whichmay reflect either a thick complete debris cover or indicatethat ice loss is already complete The possible slight surfacelowering in the accumulation area of the glacier from 1970

to 2007 might be due to less snowfall This is consistentwith an ice core record at the East Rongbuk Glacier north ofMt Everest that indicates decreasing snow accumulation for1970ndash2001 (Kaspari et al 2008)

The pattern of downwasting has resulted in the devel-opment of a concave glacier profile on Khumbu Glacierwith very low surface gradients in the lower ablation zone(Fig 5b) This trend indicates that a glacial lake could de-velop about 15 to 3 km upstream of the terminus as was pre-dicted in simulations using a 1D-coupled mass balance andflow model by Naito et al (2000) The formation of a largelake on Khumbu Glacier would have major consequences foroutburst flood hazards downstream

The greatest amount of mass loss occurred on ImjaGlacier which can be at least partly attributed to theproglacial Imja Lake which enhances ice losses by calvingThis lake grew significantly since its formation in the late1960s reachingsim09 km2 in 2008 (Supplement Fig 3 Fu-jita et al 2009 Bolch et al 2008a) The comparatively thindebris cover of Imja Glacier apparent in exposed ice cliffsis likely to be another reason for the higher mass loss AtImja Glacier a slight thinning is also observed on the relictice-cored moraine situated below Imja Lake This is in linewith recent field measurements in this area which revealeda lowering of about 1 m aminus1 (Fujita et al 2009) The massloss of the smaller glaciers such as Amphu Laptse Glacieramounts to half of that of Imja Glacier but is still significant

Khumbu Glacier for which data from the year 1984 wasavailable showed a higher surface lowering for the period1984ndash2007 in comparison to 1962ndash1984 The recent trendof more negative mass balances since 2002 however needsfurther investigation as it is not statistically significant Ac-celerated thinning could reflect decreasing velocity (Quinceyet 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 withdebris-cover as BC does not lead into changes in the albedothere The catchment topography plays an important role forthe glacier flow regimes in Khumbu Himalaya (Quincey etal 2009) For example Kangshung Glacier with a large highaltitude accumulation area flowing from Mt Everest towardsthe east shows flow activity across the entire snout while allother 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 ofthe mass balance amongst the investigated mainly avalanchefed glaciers in the same study region Mass balance esti-mate of Kangshung Glacier would reveal how the catchmenttopography and the higher flow influence the mass balanceUnfortunately only the upper part of Kangshung Glacier iscovered by our multi-temporal DTMs making a mass balanceestimate impossible

The Cryosphere 5 349ndash358 2011 wwwthe-cryospherenet53492011

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 1970ndash2007 atminus027plusmn008 m we aminus1 is lower than that observedon several other Himalayan glaciers including Chhota Shi-gri Glacier (minus098 m we aminus1 2002ndash2006 Wagnon et al2007 and ndash102 to ndash112 m we aminus1 1999ndash2004 Berthier etal 2007) or the small debris-free Glacier AX010 (minus06 tominus08 m we aminus1 1978ndash1999 Fujita et al 2001) but similarto Dokriani Glacier (minus032 m we aminus1 1992ndash2000 Dobhalet al 2008) However the different observation times andglacier sizes have to be considered and Khumbu Glacier alsohas possibly a more negative mass balance in recent yearsThe tendency towards increased mass loss has also been ob-served worldwide and for the few other Himalayan glacierswith mass balance estimates (Cogley 2011) The mass lossof the investigated glaciers is similar to the average massloss of the 30 reference glaciers worldwide for 1976ndash2005(minus032 m we aminus1) (Zemp et al 2009)

5 Conclusions

This study presents the longest time series of geodeticallyderived mass-balance and glacier volume change estimatesobtained to date in the Himalaya Geodetic mass-balanceestimates based on early stereo Corona and recent satellitedata are suitable for tracking glacier changes through timethus 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 sourceswith 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 predictionof future glacier change and its influence on water resourcesriver runoff sea level rise and glacial hazards

Glaciers south of Mt Everest have continuously lost massfrom 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 whichterminates into a lake The specific mass balance of the in-vestigated glaciers ofminus032plusmn008 m we aminus1 is not higherthan the global average

Supplementary material related to thisarticle is available online athttpwwwthe-cryospherenet53492011tc-5-349-2011-supplementpdf

AcknowledgementsWe thank M Buchroithner for the support andB Shreshta S Bajracharja and P Mool and the staff from ICIMODNepal for cooperation We are grateful for the thorough reviewsof A Racoviteanu G Cogley and an anonymous reviewer on anearlier version of this paper The comments by the anonomousreviewer D Quincey and the short comments of M Pelto and thescientific editor A Kaab improved it further H Raggam helped

with RSG software GAF AG offered Cartosat-1 data at a reducedprice and D Quincey provided the 1984 images DFG (DeutscheForschungsgemeinschaft) provided financial support (Codes BU94915-1 and BO 31992-1) T Bolch was partly funded throughthe ESA project GlobGlacier (2108807I-EC)

Edited by A Kaab

References

Altherr W and Grun A Mt Everest Karte 150 000 ndash Faszinationeines Kartenprojektes Schweizerische Zeitschrift fur Vermes-sung Kulturtechnik und Photogrammetrie Febr 90ndash97 1990

Asahi K Inventory and recent variations of glaciers in the easternNepal Himalayas Seppyo 159ndash169 2001

Bajracharya S R and Mool P K Glaciers glacial lakes andglacial lake outburst floods in the Mount Everest region NepalAnn Glaciol 50 81ndash86 2009

Bamber J L and Rivera A A review of remote sensing methodsfor glacier mass balance determination Global Planet Change59 138ndash148 2007

Benn D I Wiseman S and Hands K Growth and drainageof supraglacial lakes on debris-mantled Ngozumpa GlacierKhumbu Himal Nepal J Glaciol 47(159) 626ndash638 2001

Berthier E Arnaud Y Rajesh K Sarfaraz A Wagnon P andChevallier P Remote sensing estimates of glacier mass bal-ances in the Himachal Pradesh (Western Himalaya India) Re-mote Sens Environ 108 327ndash338 2007

Berthier E Schiefer E Clarke G K C Menounos Band Remy F Contribution of Alaskan glaciers to sea-levelrise derived from satellite imagery Nat Geosci 3 92ndash95doi101038NGEO737 2010

Bhambri R Bolch T Chaujar R K and Kulshreshtha S CGlacier changes in the Garhwal Himalayas India 1968ndash2006based on remote sensing J Glaciol 97(203) 14 pp 2011

Bolch T Buchroithner M F Peters J Baessler M and Ba-jracharya S Identification of glacier motion and potentiallydangerous glacial lakes in the Mt Everest regionNepal usingspaceborne imagery Nat Hazards Earth Syst Sci 8 1329ndash1340doi105194nhess-8-1329-2008 2008a

Bolch T Buchroithner M F Pieczonka T and Kunert A Plani-metric and volumetric glacier changes in Khumbu Himalayasince 1962 using Corona Landsat TM and ASTER data JGlaciol 54 592ndash600 2008b

Bolch T Yao T Kang S Buchroithner M F Scherer D Maus-sion F Huintjes E and Schneider C A glacier inventory forthe western Nyainqentanglha Range and the Nam Co Basin Ti-bet and glacier changes 1976-2009 The Cryosphere 4 419ndash433doi105194tc-4-419-2010 2010

Braithwaite R J and Raper S C B Glaciers and their contri-bution to sea level change Phys Chem Earth 27 1445ndash14542002

Cogley J G Himalayan glaciers now and in 2035 in Encyclopae-dia of Snow Ice and Glaciers edited by Singh V P Singh Pand Haritashya U K Springer in press 2011

Cogley J G Kargel J S Kaser G and Van der Veen C JTracking the source of glacier misinformation Science 327p 522 2010

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 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 1970ndash2007 atminus027plusmn008 m we aminus1 is lower than that observedon several other Himalayan glaciers including Chhota Shi-gri Glacier (minus098 m we aminus1 2002ndash2006 Wagnon et al2007 and ndash102 to ndash112 m we aminus1 1999ndash2004 Berthier etal 2007) or the small debris-free Glacier AX010 (minus06 tominus08 m we aminus1 1978ndash1999 Fujita et al 2001) but similarto Dokriani Glacier (minus032 m we aminus1 1992ndash2000 Dobhalet al 2008) However the different observation times andglacier sizes have to be considered and Khumbu Glacier alsohas possibly a more negative mass balance in recent yearsThe tendency towards increased mass loss has also been ob-served worldwide and for the few other Himalayan glacierswith mass balance estimates (Cogley 2011) The mass lossof the investigated glaciers is similar to the average massloss of the 30 reference glaciers worldwide for 1976ndash2005(minus032 m we aminus1) (Zemp et al 2009)

5 Conclusions

This study presents the longest time series of geodeticallyderived mass-balance and glacier volume change estimatesobtained to date in the Himalaya Geodetic mass-balanceestimates based on early stereo Corona and recent satellitedata are suitable for tracking glacier changes through timethus 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 sourceswith 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 predictionof future glacier change and its influence on water resourcesriver runoff sea level rise and glacial hazards

Glaciers south of Mt Everest have continuously lost massfrom 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 whichterminates into a lake The specific mass balance of the in-vestigated glaciers ofminus032plusmn008 m we aminus1 is not higherthan the global average

Supplementary material related to thisarticle is available online athttpwwwthe-cryospherenet53492011tc-5-349-2011-supplementpdf

AcknowledgementsWe thank M Buchroithner for the support andB Shreshta S Bajracharja and P Mool and the staff from ICIMODNepal for cooperation We are grateful for the thorough reviewsof A Racoviteanu G Cogley and an anonymous reviewer on anearlier version of this paper The comments by the anonomousreviewer D Quincey and the short comments of M Pelto and thescientific editor A Kaab improved it further H Raggam helped

with RSG software GAF AG offered Cartosat-1 data at a reducedprice and D Quincey provided the 1984 images DFG (DeutscheForschungsgemeinschaft) provided financial support (Codes BU94915-1 and BO 31992-1) T Bolch was partly funded throughthe ESA project GlobGlacier (2108807I-EC)

Edited by A Kaab

References

Altherr W and Grun A Mt Everest Karte 150 000 ndash Faszinationeines Kartenprojektes Schweizerische Zeitschrift fur Vermes-sung Kulturtechnik und Photogrammetrie Febr 90ndash97 1990

Asahi K Inventory and recent variations of glaciers in the easternNepal Himalayas Seppyo 159ndash169 2001

Bajracharya S R and Mool P K Glaciers glacial lakes andglacial lake outburst floods in the Mount Everest region NepalAnn Glaciol 50 81ndash86 2009

Bamber J L and Rivera A A review of remote sensing methodsfor glacier mass balance determination Global Planet Change59 138ndash148 2007

Benn D I Wiseman S and Hands K Growth and drainageof supraglacial lakes on debris-mantled Ngozumpa GlacierKhumbu Himal Nepal J Glaciol 47(159) 626ndash638 2001

Berthier E Arnaud Y Rajesh K Sarfaraz A Wagnon P andChevallier P Remote sensing estimates of glacier mass bal-ances in the Himachal Pradesh (Western Himalaya India) Re-mote Sens Environ 108 327ndash338 2007

Berthier E Schiefer E Clarke G K C Menounos Band Remy F Contribution of Alaskan glaciers to sea-levelrise derived from satellite imagery Nat Geosci 3 92ndash95doi101038NGEO737 2010

Bhambri R Bolch T Chaujar R K and Kulshreshtha S CGlacier changes in the Garhwal Himalayas India 1968ndash2006based on remote sensing J Glaciol 97(203) 14 pp 2011

Bolch T Buchroithner M F Peters J Baessler M and Ba-jracharya S Identification of glacier motion and potentiallydangerous glacial lakes in the Mt Everest regionNepal usingspaceborne imagery Nat Hazards Earth Syst Sci 8 1329ndash1340doi105194nhess-8-1329-2008 2008a

Bolch T Buchroithner M F Pieczonka T and Kunert A Plani-metric and volumetric glacier changes in Khumbu Himalayasince 1962 using Corona Landsat TM and ASTER data JGlaciol 54 592ndash600 2008b

Bolch T Yao T Kang S Buchroithner M F Scherer D Maus-sion F Huintjes E and Schneider C A glacier inventory forthe western Nyainqentanglha Range and the Nam Co Basin Ti-bet and glacier changes 1976-2009 The Cryosphere 4 419ndash433doi105194tc-4-419-2010 2010

Braithwaite R J and Raper S C B Glaciers and their contri-bution to sea level change Phys Chem Earth 27 1445ndash14542002

Cogley J G Himalayan glaciers now and in 2035 in Encyclopae-dia of Snow Ice and Glaciers edited by Singh V P Singh Pand Haritashya U K Springer in press 2011

Cogley J G Kargel J S Kaser G and Van der Veen C JTracking the source of glacier misinformation Science 327p 522 2010

wwwthe-cryospherenet53492011 The Cryosphere 5 349ndash358 2011

358 T Bolch et al Multi-decadal mass loss of glaciers in the Everest area (Nepal Himalaya)

Dobhal D P Gergan J T and Thayyen R J Mass balancestudies of the Dokriani Glacier from 1992 to 2000 Garhwal Hi-malaya India Bull Glaciol Res 25 9ndash17 2008

Fujita K Kadota T Rana B Shresta R B and AgetaY Shrinkage of Glacier AX010 in shorong region Nepal Hi-malayas in the 1990s Bull Glaciol Res 18 51ndash54 2001

Fujita K Sakai A Nuimura T Yamaguchi S and SharmaR R Recent changes in Imja Glacial Lake and its dammingmoraine in the Nepal Himalaya revealed by in situ surveys andmulti-temporal ASTER imagery Environ Res Lett 4 045205doi1010881748-932644045205 2009

Hambrey M Quincey D Glasser N F Reynolds J M Richard-son S J and Clemmens S Sedimentological geomorpho-logical and dynamic context of debris-mantled glaciers MountEverest (Sagarmatha) region Nepal Quaternary Sci Rev 272341ndash2360 2008

Immerzeel W W van Beek L P H and Bierkens M F P Cli-mate change will affect the Asian water towers Science 3281382ndash1385 2010

Kadota T Seko K Aoki T Iwata S and Yamaguchi SShrinkage of Khumbu Glacier east Nepal from 1978 to 1995IAHS Publ 264 235ndash243 2000

Kaspari S Hooke R L B Mayewski P A Kang S Hou Sand Qin D Snow accumulation rate on Qomolangma (MountEverest) Himalaya synchroneity with sites across the TibetanPlateau on 50ndash100 year timescales J Glaciol 54 343ndash3522008

Koblet T Gartner-Roer I Zemp M Jansson P Thee P Hae-berli W and Holmlund P Reanalysis of multi-temporal aerialimages of Storglaciren Sweden (1959-99) - Part 1 Determina-tion of length area and volume changes The Cryosphere 4333ndash343doi105194tc-4-333-2010 2010

Kulkarni A V Mass balance of Himalayan glaciers using AARand ELA methods J Glaciol 38 101ndash104 1992

Naito N Nakawo M Kadota T and Raymond C F Numer-ical simulation of recent shrinkage of Khumbu Glacier NepalHimalayas IAHS Publ 264 245ndash254 2000

Nakawo M Yabuki H and Sakai A Characteristics of KhumbuGlacier Nepal Himalaya recent changes in the debris-coveredarea Ann Glaciol 28 118ndash122 1999

Nuth C and Kaab A Co-registration and bias corrections of satel-lite elevation data sets for quantifying glacier thickness changeThe Cryosphere 5 271ndash290doi105194tc-5-271-2011 2011

Paterson W S B The Physics of Glaciers 3rd ed New York Perg-amon 1994

Paul F and Haeberli W Spatial variability of glacier ele-vation changes in the Swiss Alps obtained from two dig-ital elevation models Geophys Res Lett 35 L21502doi1010292008GL034718 2008

Pieczonka T Bolch T and Buchroithner M F Generation andevaluation of multi-temporal digital elevation models from dif-ferent optical sensors J Photogramm Rem Sens in revision2011

Prasad A K Yang K-H S El-Askary H M and Kafatos MMelting of major glaciers in the western Himalayas evidenceof climatic changes from long term MSU derived tropospherictemperature trend (1979-2008) Ann Geophys 27 4505ndash4519doi105194angeo-27-4505-2009 2009

Quincey D Luckman A and Benn D Quantification of Everestregion glacier velocities between 1992 and 2002 using satelliteradar interferometry and feature tracking J Glaciol 55 596ndash606 2009

Ramanathan V and Carmichael G Global and regional climatechanges due to black carbon Nat Geosci 1 221ndash227 2008

Richardson S D and Reynolds J M An overview of glacial haz-ards in the Himalayas Quatern Int 6566 31ndash47 2000

Sakai A Takeuchi N Fujita K and Nakawo M Role ofsupraglacial ponds in the ablation process of a debris-coveredglacier in the Nepal Himalayas IAHS Publ 264 119ndash1302000

Sakai A Nakawo M and Fujita K Distribution characteristicsand energy balance of ice cliffs on debris-covered glaciers NepalHimalaya Arc Antarc Alp Res 34 12ndash19 2002

Scherler D Leprince S and Strecker M R Glacier-surface ve-locities in alpine terrain from optical satellite imageryndashAccuracyimprovement and quality assessment Remote Sens Environ112 3806ndash3819 2008

Scherler D Bookhagen B and Strecker M R Spatially variableresponse of Himalayan glaciers to climate change affected bydebris cover Nat Geosci 4156ndash159 2011

Surazakov A and Aizen V B Positional accuracy evaluationof declassified Hexagon KH-9 mapping camera imagery Pho-togramm Eng Rem S 76 603ndash608 2010

Takeuchi Y Kayastha R B and Nakawo M Characteristicsof ablation and heat balance in debris-free and debris-coveredareas on Khumbu Glacier Nepal Himalayas in the pre-monsoonseason IAHS Publ 264 53ndash61 2000

Wagnon P Linda A Arnaud Y Kumar R Sharma P VincentC Pottakkal J G Berthier E Ramanathan A Hasnain S Iand Chevallier P Four years of mass balance on Chhota ShigriGlacier Himachal Pradesh India a new benchmark glacier inthe western Himalaya J Glaciol 53 603ndash611 2007

Zemp M Hoelzle M and Haeberli W Six decades of glaciermass balance observations ndash a review of the worldwide monitor-ing network Ann Glaciol 50 101ndash111 2009

Zemp M Jansson P Holmlund P Gartner-Roer I Koblet TThee P and Haeberli W Reanalysis of multi-temporal aerialimages of Storglaciaren Sweden (1959-99) - Part 2 Comparisonof glaciological and volumetric mass balances The Cryosphere4 345ndash357doi105194tc-4-345-2010 2010

The Cryosphere 5 349ndash358 2011 wwwthe-cryospherenet53492011