Area and Mass Changes of Glaciers in the West Kunlun Mountains Based on the Analysis of Multi-Temporal Remote Sensing Images and Dems from 1970 to 2018

remote sensing

Article Area and Mass Changes of Glaciers in the West Kunlun Mountains Based on the Analysis of Multi-Temporal Remote Sensing Images and DEMs from 1970 to 2018

Bo Cao 1, Weijin Guan 1, Kaiji Li 1, Zhenling Wen 1, Hui Han 2 and Baotian Pan 1,*

1 Key Laboratory of Western China’s Environmental Systems (MOE), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730070, China; [email protected] (B.C.); [email protected] (W.G.); [email protected] (K.L.); [email protected] (Z.W.) 2 Faculty of geomatics, Lanzhou Jiaotong University, Lanzhou 730070, China; [email protected] * Correspondence: [email protected]

Received: 12 July 2020; Accepted: 13 August 2020; Published: 14 August 2020

Abstract: Most of the world’s glaciers have retreated significantly against the background of recent climate warming, while reports have indicated that the glaciers in the West Kunlun Mountains (WKL) may be in a relatively stable state, although there are some gaps in previous research. Based on Landsat series data, topographic maps, SRTM and TanDEM-x data, this paper extracts detailed glacial area information and glacial mass balance during different time periods from 1970 to 2018. We found that, the total area of glaciers in the WKL decreased by 8.0 km2 from 1972 to 2018. The area decreased by 12.0 km2 from 1972 to 1991 and increased by 4.7 km2 from 2010 to 2018. Glacier surface elevation change results in the WKL showed that the overall glacier thickness slightly decreased from 1970 to 2016, with an average of 1.9 1.0 m. The glaciers thinned by approximately 2.5 1.0 m ± ± from 1970 to 2000, while from 2000 to 2016, the glaciers thickened approximately by 0.6 1.0 m. ± Overall, the glaciers in the WKL showed very slight retreat. In addition, the mass changes of glaciers were affected by glacial surging.

Keywords: West Kunlun Mountains; glacier changes; glacier surging

1. Introduction Most of the glaciers in the Tibetan Plateau (TP) have retreated significantly against the background of recent climate warming [1–6]. In contrast, Karakoram glaciers have remained stable or advanced during the same period [5,7]. This pattern is known as the “Karakoram Anomaly” [7]. This phenomenon of heterogeneous glacier variations in TP regions is mainly attributable to different spatial patterns of climate change [5]. Glaciers in the Karakoram and West Kunlun Mountains (WKL) region are mainly affected by mid-latitude westerlies and receive minor influences from the Asian summer monsoons [1,5]. According to the China Glacier Inventory (CGI) [8], the WKL region is the most concentrated area of large mountain glaciers in China, attracting the attention of many scholars. The results of studies on the glacier mass balance in this region differ. Some studies have shown that the glacier is thinning, some have shown that it has remained basically the same, and some have shown that it is thickening. Wang et al. [9] showed that the mass balance was 0.06 0.13 m w.e. a 1 from the 1970s–1999 in the − ± − WKL through topographic maps and Shuttle Radar Topography Mission (SRTM) data. For the same time period, Zhou et al. [10] used the results of glacier surface elevation obtained by the KeyHole-9 photographic satellite mission (KH-9)and SRTM to show that some glaciers in the region had a mass

Remote Sens. 2020, 12, 2632; doi:10.3390/rs12162632 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 2632 2 of 16 Remote Sens. 2020, 12, x FOR PEER REVIEW 2 of 16 balance closeclose toto 00 oror slightslight thickening.thickening. According to the ICESat altimeter, glaciers in this regionregion thickenedthickened signiﬁcantlysignificantly duringduring thethe periodperiod fromfrom 20032003 toto 20092009 [[4,8,11].4,8,11]. Studies onon glacier area changechange havehave shown that the glacier area in the WKL has been decreasing over thethe pastpast fewfew decades.decades. However, in laterlater periods,periods, especiallyespecially after 2010, thethe glaciersglaciers exhibitedexhibited aa tendencytendency to advance. Shangguan Shangguan et et al. al. [12] [12 ]showed showed that that the the total total glacier glacier area area decreased decreased from from 1970 1970 to to2001 2001 although although with with a slight a slight increase increase from from 1990 1990 to 2001. to 2001. The The results results from from Bao Baoet al. et [11] al. [concluded11] concluded that thatthe glaciers the glaciers in the in WKL the WKL decreased decreased from from1970 1970to 2010 to 2010overall, overall, while while the glacier the glacier area on area the on south the southslope slopeincreased increased from from1990 1990to 2010. to 2010. Wang Wang et al. et [9] al. [found9] found that that from from 1970 1970 to to 2016, 2016, the the glacier glacier area area in thisthis regionregion alsoalso showedshowed aa decreasingdecreasing trend.trend. According toto HanHan [[13],13], thethe glacier area in the WKL decreased by 2.95% from 1977 to 2013, while Ma [[14]14] showed thatthat the glacier area showed a slight increase from 2010 toto 2017.2017. Bao etet al.al. [[11]11] obtainedobtained aa slightlyslightly positivepositive massmass trendtrend ofof 0.230.23 ± 0.24 m through ICESat from 2003 ± toto 2009.2009. However, However, also also based on ICESat, Ke et al.al. [[8]8] foundfound thatthat thethe surfacesurface heightheight changeschanges werewere heterogeneous overover 2003–2008. 2003–2008. In In addition, addition, ICESat ICESat can can provide provide information information on only on only the changes the changes of some of footprints,some footprints, and the and time the frame time frame is limited is limited to 2003–2009. to 2003–2009. Zhou Zhou et al. [et10 ]al. used [10] KH-9used KH-9 and SRTM and SRTM to show to show that glaciers in WKL exhibited an almost 0 or slight positive mass balance of 0.02 ± 0.10 m w.e.1 that glaciers in WKL exhibited an almost 0 or slight positive mass balance of 0.02 0.10 m w.e. a− −1 ± froma from 1970 1970 to 2000.to 2000. However, However, because because of of data data problems, problems, therethere areare extensiveextensive spatialspatial data gaps. However, WangWang etet al.al. [[9]9] showedshowed that thethe massmass balancebalance waswas −0.060.06 ± 0.130.13 mm w.e.w.e. aa−1 in the same timetime − ± − periodperiod throughthrough topographictopographic mapsmaps andand SRTMSRTM data.data. The WKLWKL is locatedlocated inin thethe northwestnorthwest of the TPTP andand isis oneone ofof thethe highesthighest elevationelevation areasareas inin thethe 2 worldworld (Figure(Figure1 1).). AccordingAccording toto CGI-2,CGI-2, therethere areare 433433 glaciersglaciers inin WKLWKL withwith anan areaarea ofof 2961.32961.3 kmkm2 [6].[6]. TheThe glaciersglaciers have limited debris cover, accounting for approximately 3% of the total glacier area [15]. [15]. Of all the glaciers in High Mountain Asia, the WKL glaciers are located in the most continental climate because thethe WKLWKL isis thethe coldestcoldest andand driestdriest regionregion onon andand aroundaround thethe TibetanTibetan PlateauPlateau [16[16].].

Figure 1.1. Location of the Western KunlunKunlun MountainsMountains (WKL)(WKL) on remote sensingsensing images from GoogleGoogle Earth. TheThe insetinset indicates indicates the the locations locations of the of WKLthe WKL and Tibetan and Tibetan Plateau. Plateau. The black The dotted black line dotted indicates line theindicates dividing the linedividing between line monsoonbetween monsoon and westerlies and we [17sterlies]. The pink[17]. The dotted pink line dotted indicates line theindicates dividing the linedividing between line dibetweenﬀerent glacier different types glacier [18]. types The WKL [18]. glaciers The WKL are continentalglaciers are (polar)-typecontinental glaciers(polar)-type and inﬂuencedglaciers and by influenced westerlies. by westerlies.

According to previousprevious studies,studies, there maymay alsoalso bebe aa phenomenonphenomenon similarsimilar toto thethe “Karakoram“Karakoram Anomaly”Anomaly” inin thethe glaciersglaciers inin thethe WKLWKL [[9].9]. However, the results of research by didifferentﬀerent scholarsscholars provideprovide somesome contrastingcontrasting results.results. In addition, previousprevious studies have covered a short timetime spanspan andand lackedlacked aa continuouscontinuous long-sequencelong-sequence studystudy ofof glaciers.glaciers. Furthermore, duedue toto data reasons, manymany studiesstudies have studied only part of the glaciers in the study area and did not cover all glaciers in the WKL. Therefore, how the glaciers have changed in the WKL area over the past few decades requires a more

Remote Sens. 2020, 12, 2632 3 of 16 have studied only part of the glaciers in the study area and did not cover all glaciers in the WKL. Therefore, how the glaciers have changed in the WKL area over the past few decades requires a more comprehensive study of the area and mass changes over a long period of time. Hence, the purpose of this article is to (1) obtain continuous changes in the area of glaciers in the WKL area since 1970 through multi-source remote sensing images. (2) Obtain the changes in glacial mass in the WKL area since 1970 through multi-phase DEM.

2. Materials and Methods This study calculated the glacier area change from the band ratio method and modiﬁed the results manually based on multitemporal Landsat series imagery. The ice elevation changes were calculated by diﬀerentiating digital elevation models (DEMs) constructed using topographic maps, SRTM and TanDEM-X.

2.1. Data

2.1.1. Landsat Images Landsat series data (e.g., Landsat 1/2/3 MSS, 4/5 TM, 7 ETM+, 8 OLI) are most frequently used for glacier mapping. The time series of the Landsat scenes have been processed through standard terrain correction (level 1T). L1T satellite images were chosen from 1972 onward (Table1) from the United States Geological Survey (USGS). As far as possible, images without cloud and snow inﬂuences were selected to depict the glaciers. As precipitation mainly occurred in the summer, the wintertime images were considered acceptable in the interest of minimizing seasonal snow and cloud cover. To obtain as many continuous changes in glacier area as possible, 18 Multispectral Scanner System (MSS)/Thematic Mapper (TM)/Enhanced Thematic Mapper Plus (ETM+)/Operational Land Imager (OLI) images (Table1) were used to extract the glacier outlines from the USGS.

Table 1. The remote sensing data selected in this study.

Sensors Path/Row Date Resolution (m) Area DEM Landsat MSS P145/R35 1972/12/01 60 √ Landsat MSS P145/R36 1972/12/01 60 √ Landsat MSS P145/R35 1976/11/1 60 √ Landsat MSS P145/R36 1976/11/1 60 √ Landsat TM P145/R35 1991/2/3 30 √ Landsat TM P145/R36 1991/2/3 30 √ Landsat TM P145/R35 1995/12/31 30 √ Landsat TM P145/R36 1995/12/31 30 √ Landsat ETM+ P145/R35 2000/12/4 30(Multi)15(Pan) √ Landsat ETM+ P145/R36 2000/12/4 30(Multi)15(Pan) √ Landsat ETM+ P145/R35 2005/11/16 30(Multi)15(Pan) √ Landsat ETM+ P145/R36 2005/11/16 30(Multi)15(Pan) √ Landsat TM P145/R35 2010/12/8 30 √ Landsat TM P145/R36 2010/12/8 30 √ Landsat OLI P145/R35 2010/12/8 30(Multi)15(Pan) √ Landsat OLI P145/R36 2010/12/8 30(Multi)15(Pan) √ Landsat OLI P145/R35 2015/12/6 30(Multi)15(Pan) √ Landsat OLI P145/R36 2015/12/6 30(Multi)15(Pan) √ Topographic maps 1970.10 √ SRTM 2000.2 30 √ TanDEM-X 2014–2018 12 √

2.1.2. Topographic Maps Eight topographic maps drawn by the China Military Geodetic Survey (CMGS) in October 1970 at a scale of 1:100,000 (Figure1) was used here to generated the oldest DEM. The outline was digitalized Remote Sens. 2020, 12, 2632 4 of 16 manually, and then the Thiessen polygon method was used to convert it to a grid DEM (TopoDEM) with 30 m grid cells. The nominal vertical accuracy of these topographic maps is 3–5 m in ﬂat and hilly areas (slopes < 6) and about 8–14 m in mountainsides and high mountain areas (slopes 6–25 and >25, respectively).

2.1.3. SRTM The C-band and X-band SRTM DEMs were obtained from the USGS and the German Aerospace Center (DLR), respectively. The X-band SAR system had a swath width of 45 km, leaving large data gaps in the resulting X-band DEM [19]. Unfortunately, approximately only half of the WKL glaciers were covered by this data set. Therefore, the C-band SRTM DEMs were used to provide a reference altitude to monitor changes in glacial altitude. X-band SRTM DEMs were used to estimate the penetration depth of C-band radar in glacial areas. The SRTM C-band DEM data can represent the height of the glacier surface after the 2000 ablation period, and there is a slight seasonal variation [3,20,21]. The vertical errors indicative of SRTM C-band data range between 7.2 and 12.6 m [22].

2.1.4. TanDEM-X The Tandem-X mission established the first spaceborne bistatic interferometer on 21 June 2010 and used two close flying satellites to generate global DEM with a resolution of 12 m. It has a height accuracy of 90% of 2 m on the medium terrain [23]. The first verification in Germany shows that the accuracy of 1–2 m can be achieved in relatively flat terrain [24]. The TanDEM-X data used multiple image pairs to generate DEM, and we chose the middle time of the collection cycle to represent the data [25].

2.2. Glacier Outline Mapping, Changes and Uncertainty All images from USGS have been processed into standard L1T products through standard terrain correction for further pixel-level analysis [26]. Various techniques for automatic glacier mapping using optical satellite images have been successfully applied in different ice regions and the simple band ratio method emerged as the most suitable method [27]. Therefore, outlines of glaciers were derived from Landsat images using the band ratio method except for the MSS images. To retrieve glacial regions from MSS images without short-wave bands, ISODATA unsupervised classification was used, which iteratively clustered the pixel classes using the minimum distance technique [27]. Then all glacier outlines were modified by manual interpretation. The uncertainty of terrestrial satellite images is estimated to be half a pixel (15 m) as a buffer zone [28–31]. The difference between the buffer zone and the glacier polygon is between 1.2% and 5.4%, which can be regarded as a rough assessment of the accuracy of mapping [26]. According to the error propagation law, the final uncertainty of the area change (EA) calculated according to Formula (1) [31] is 1.7–7.7%: p 2 2 EA = EA1 + EA2 (1) where EA1 and EA2 represent the uncertainties of the glacier outlines of time 1 and 2, respectively.

2.3. Glacial Elevation Changes and Uncertainty The glacier elevation changes were derived from three DEMs (TopoDEM, SRTM-C and TanDEM-X). Bilinear resampling was performed on all DEM to achieve the same horizontal resolution of 30 m. Before any differential analysis can be performed, the DEM needs to be integrated to eliminate both horizontal and vertical offsets [20]. A method proposed by Nuth and Kääb [32] and Paul et al. [27] was used in this study to co-registration the DEMs. The relationship between the observed height difference and aspect ratio was used for relative adjustment (Figure2). Depending on the e ffect, multiple iterations will be performed during the correction process. In addition, in order to remove the influence of extreme values on the results, it is necessary to delete them using thresholding [3,33]. Remote Sens. 2020, 12, 2632 5 of 16 Remote Sens. 2020, 12, x FOR PEER REVIEW 5 of 16

Significant statistical features of these height differences are used to exclude pixels representing height pixels representing height differences outside the 5–95 percent range to correct for potential differences outside the 5–95 percent range to correct for potential deviations [20,34]. deviations [20,34].

Figure 2. ScatterScatter plot plot of of slope-standardized slope-standardized altitudinal altitudinal di differencesﬀerences in in the the terrain terrain aspect aspect for for off-glacier oﬀ-glacier areas. ( (aa)) Before Before co-registration co-registration and and ( b) after co-registration.

As noted in previous studies [35–37], [35–37], the altitude-related vertical deviation in mountains can be attributed toto thethe di differencefference in in the the original original spatial spatial resolution resolution between between the twothe DEMs.two DEMs. Coarse Coarse DEM DEM tends tendsto underestimate to underestimate the height the height of peaks of orpeaks ridges or ridges with high with topographic high topographic curvature curvature because because of their of limited their limitedability toability represent to represent frequent changesfrequent inchanges slope. Thisin slope. deviation This candeviation be corrected can be by corrected the relationship by the relationshipbetween elevation between diff elevationerence and difference maximum and curvature maximum on curvature a glacier on stabilized a glacier topography stabilized topography [3,37]. [3,37].The penetration depth of the SRTM C-band radar beam in snow and ice should be taken intoThe account penetration when evaluating depth of the glacier SRTM height C-band change radar [2 ,beam20,35]. in Depending snow and ice on should different be parameters, taken into accountthe penetration when evaluating depth is generallyglacier height 0–10 mchange [38,39 [2,20,35].]. As the Depending approximation, on different since the parameters, penetration the of penetrationSRTM X-band depth radar is beamgenerally is close 0–10 to m 0, [38,39]. the elevation As the di approximation,fference between since SRTM the c-band penetration and x-band of SRTM can X-bandgenerally radar be regarded beam is asclose the penetrationto 0, the elevation depth of difference SRTM C-band between radar SRTM beam againstc-band snowand x-band and ice can [2]. generallyTherefore, be the regarded average as penetration the penetration depth depth of SRTX of SRTM C-band C-band in the radar research beam against is about snow 2.8 m, and which ice [2]. is Therefore,basically consistent the average with penetration the results depth of Wang of etSRTX al. [9 C-band]. in the research is about 2.8 m, which is basicallyAfter consistent adjustment, with we the use results the normalizedof Wang et al. median [9]. absolute deviation (NMAD; represented by 1.4826AfterMED adjustment, ex xi we|; x i:use elevational the normalized difference; medianex: median) absolute of deviation non-glacier (NMAD areas; represented to estimate theby × − 1.4826uncertainty × MED of( the|𝒙 −𝒙 diff𝒊erence|)|; 𝒙𝒊: between elevational DEMs difference; [21,40,41 ].𝒙: In median) order to of convert non-glacier surface areas height to estimate changes intothe uncertaintymass changes, of densitythe difference assumptions between are needed.DEMs [21,40 The volume,41]. In order averaged to convert density surface used is 850height60 changes kg m 3 ± − into(see [mass42]). changes, The final density uncertainty assumptions of mass balance are needed (E) is. The calculated volume as averaged [40]: density used is 850 ± 60 −3 kg m (see [42]). The final uncertaintys of mass balance (E) is calculated as [40]: !2 !2 NMAD ∆ρ NMAD ρI E = 𝑁𝑀𝐴𝐷 ∆𝜌 + 𝑁𝑀𝐴𝐷 𝜌 (2) 𝐸= ( t ×× ρW) +( t ×× ρW) (2) 𝑡 𝜌 𝑡 𝜌 3 where t is the observation period, ρI is the ice density (850 kg m−3 ), ∆ρ is the ice density uncertainty where t is 3the observation period, 𝜌 is the ice density3 (850 kg m ), ∆𝜌 is the ice density uncertainty (60 kg m−3 ) and ρW is the water density (1000 kg m−−3 ). (60 kg m ) and 𝜌 is the water density (1000 kg m ). 3. Results 3. Results 3.1. Glacial Area Changes 3.1. Glacial Area Changes After band ratio and manual digitization, the glacial boundary between 1972 and 2018 in the WKL was obtainedAfter band approximately ratio and manual every digitization, five years. Inthe 1972, glacial the boundary area of glaciers between was 1972 2965.4 and km 20182. In in 2018, the WKLthe area was was obtained 2953.4 kmapproximately2. From 1972 every to 2018, five the years. total areaIn 1972, of glaciers the area in theof glaciers WKL decreased was 2965.4 by 8.0km km2. In2 . 2018,The area the decreasedarea was 2953.4 by 12.0 km km2.2 Fromfrom 1972 to 19912018, and the increasedtotal area byof glaciers 4.7 km2 fromin the 2010 WKL to decreased 2018 (Table by2, 8.0Figure km23.). The area decreased by 12.0 km2 from 1972 to 1991 and increased by 4.7 km2 from 2010 to 2018 (Table 2, Figure 3). The area of the northern slope glacier decreased by 5.95 km2 from 1972 to 2018 including a continuous decrease of 11.8 km2 from 1972 to 2010 and an increase of 5.85 km2 from 2010 to 2018. The change in the area of the southern slope glacier was relatively small. The area of the southern slope

Remote Sens. 2020, 12, 2632 6 of 16

Table 2. Glacial area and changes from 1972 to 2018 in the WKL (the area change is based on the glacier area in 1972).

North Slope South Slope Total WKL Time Area Area Change Area Area Change Area Area Change (km2) (km2) (km2) (km2) (km2) (km2) Remote Sens. 2020, 12, x FOR PEER REVIEW 6 of 16 1972/12/1 1821.9 0 1143.5 0 2965.4 0 1976/11/1 1820.54 1.4 1140.1 3.3 2960.6 4.7 glacier decreased by 2.0 km2 from 1972− to 2018, including a decrease− of 6.5 km2 from 1972− to 1991 and 1991/2/3 1816.4 5.5 1136.9 6.5 2953.3 12.0 2 − − − an increase1995 /of12 /6.231 km 1814.4 from 1991 to 20057.5 (Table 1139.5 2, Figure 3). The4.0 area of the 2953.9 glaciers on 11.5the south and − − − north slopes2000/ 12of/ 4the WKL 1813.5 decreased 8.5notably from 1141.8 1972 to 1991,1.7 resulting 2955.3 in a significant10.2 decrease in − − − the total2005 area/11 of/16 glaciers. 1811.9 From 199110.0 to 2010, the 1143.1 area of the north0.4 slope decreased 2955.0 and the10.4 area of the − − − 2010/12/8 1810.1 11.8 1142.6 0.9 2952.7 12.7 south slope increased, making the −total area approximately stable− with slight fluctuations.− From 2010 2015/12/6 1813.8 8.10 1141.8 1.7 2955.6 9.8 to 2018, the increase in the total area− of glaciers was mainly due− to the contribution from− the northern 2018/11/12 1816.0 6.0 1141.4 2.0 2957.4 8.0 slope. − − −

FigureFigure 3. 3. TheThe glacier glacier area area changes changes on on the the south south sl slope,ope, north north slope slope and for the total WKL.

2 TableThe area 2. Glacial of the area northern and changes slope from glacier 1972 decreased to 2018 in bythe 5.95WKL km (the fromarea change 1972 to is 2018based includingon the a 2 2 continuousglacier area decrease in 1972). of 11.8 km from 1972 to 2010 and an increase of 5.85 km from 2010 to 2018. The change in the area of the southern slope glacier was relatively small. The area of the southern slope North Slope South Slope Total WKL glacier decreased by 2.0 km2 from 1972 to 2018, including a decrease of 6.5 km2 from 1972 to 1991 and Time Area Area Change Area Area Change Area Area Change an increase of 6.2 km2 from 1991 to 2005 (Table2, Figure3). The area of the glaciers on the south and (km2) (km2) (km2) (km2) (km2) (km2) north slopes1972/12/1 of the WKL 1821.9 decreased notably 0 from1143.5 1972 to 1991,0 resulting2965.4 in a signiﬁcant0 decrease in the total area of1976/11/1 glaciers. From1820.54 1991 to 2010,−1.4 the area1140.1 of the north−3.3 slope decreased2960.6 and the−4.7 area of the south slope increased,1991/2/3 making 1816.4 the total area−5.5 approximately 1136.9 stable with−6.5 slight2953.3 ﬂuctuations. −12.0 From 2010 to 2018, the increase1995/12/31 in the total 1814.4 area of glaciers−7.5 was mainly1139.5 due to the−4.0 contribution 2953.9 from the−11.5 northern slope. 2000/12/4 1813.5 −8.5 1141.8 −1.7 2955.3 −10.2 3.2. Glacier2005/11/16 Surface Elevation 1811.9 Changes −10.0 1143.1 −0.4 2955.0 −10.4 2010/12/8 1810.1 −11.8 1142.6 −0.9 2952.7 −12.7 The results showed that the glaciers in the WKL were nearly balanced with a slight and not notably 2015/12/6 1813.8 −8.10 1141.8 −1.7 2955.6 −9.8 mass loss within the period from 1970–2016. The glacier surface elevation change was approximately 2018/11/12 1816.0 −6.0 1141.4 −2.0 2957.4 −8.0 1.9 1.0 m from 1970 to 2016, with a mass balance of 0.03 0.02 m w.e.a 1. However, over the − ± − ± − 3.2.entire Glacier period, Surface the trendElevation of the Changes changes in the glacier surface elevation in the WKL was not consistent. Among these changes, the glaciers thinned overall from 1970 to 2000 and the average elevation of the glacierThe surface results decreased showed bythat approximately the glaciers in 2.5 the1.0 WKL m, whilewere fromnearly 2000 balanced to 2016, with the glaciers a slight thickened and not notably mass loss within the period from 1970–2016.± The glacier surface elevation change was approximately −1.9 ± 1.0 m from 1970 to 2016, with a mass balance of −0.03 ± 0.02 m w.e.a−1. However, over the entire period, the trend of the changes in the glacier surface elevation in the WKL was not consistent. Among these changes, the glaciers thinned overall from 1970 to 2000 and the average elevation of the glacier surface decreased by approximately 2.5 ± 1.0 m, while from 2000 to 2016, the glaciers thickened slightly overall, and the average elevation of the glacier surface increased by approximately 0.6 ± 1.0 m (Table 3). In addition, there were obvious spatial differences in the changes in glacial surface elevation and ice volume in the WKL, and the change trends of the southern and northern slopes were different (Table 3). The northern slope glacier thinned by approximately 4.5 ± 1.0 m during the entire period

Remote Sens. 2020, 12, 2632 7 of 16 slightly overall, and the average elevation of the glacier surface increased by approximately 0.6 1.0 m ± (Table3).

Table 3. The surface elevation change from 1972 to 2016 in WKL.

North Slope North Slope South Slope South Slope Whole Mean Mass Loss Time Mean Mass Loss Mean Mass Loss Elevation Rate Intervals Elevation Rate Elevation Rate 1 1 1 Change (m) (m w.e. a− ) Change (m) (m w.e. a− ) Change (m) (m w.e. a− ) 1970–2000 4.9 0.14 1.3 0.03 2.5 0.07 − − − − 2000–2016 0.4 0.02 1.1 0.06 0.6 0.03 1970–2016 4.5 0.08 2.4 0.04 1.9 0.03 − − − −

In addition, there were obvious spatial differences in the changes in glacial surface elevation and ice volume in the WKL, and the change trends of the southern and northern slopes were different (Table3). The northern slope glacier thinned by approximately 4.5 1.0 m during the entire period ± from 1970 to 2016. Among these changes, thinning was most obvious from 1970 to 2000, and the average elevation decreased by approximately 4.9 1.0 m, while the glaciers thickened slightly from − ± 2000 to 2016, and the average elevation increased by approximately 0.4 1.0 m. The southern slope ± glacier thickened by approximately 2.4 1.0 m from 1970 to 2016. During the two periods of 1970–2000 ± and 2000–2016, the glaciers thickened by approximately 1.3 1.0 m and 1.1 1.0 m, respectively ± ± (Table3). According to the spatial distribution of the glacier thickness changes in the WKL, the glacier thickness changes in this area exhibited large spatial differences (Figure4). The pattern of the changes in the surface height of glaciers can be roughly divided into two categories. Category one, between 1970–2000 and 2000–2016, the distribution patterns of the glacial thickness changes were the same; category two, the distribution patterns of glacial changes were not consistent between the two time periods. Most glaciers exhibited almost the same thickness variation characteristics between 1970–2000 and 2000–2016. At lower elevations, there were large mass losses. As the altitude increased, glacier thinning gradually weakened. The upper part of the glacier thickened. These patterns are similar to most glacier changes around the TP, such as in the Gongga Mountains [43] or Qilian Mountains [41]. Figure5 shows the distribution of surface elevation changes of three typical glaciers and their changes along the longitudinal section. For example, in Xiyulong G., glacier thinning was at its maximum at low elevations near the terminus. As the altitude increased, the thinning of the glacier gradually weakened. At 9 km from the terminus, the surface elevation change of the glacier changed to 0; when the altitude increased again, the surface elevation change of the glacier began to increase in the periods from 1970 to 2000, 2000 to 2016 and 1970 to 2016. The same glacier surface elevation change pattern also exists in Gongxing G. and 5Y614G0038 G (Figure5). Remote Sens. 2020, 12, 2632 8 of 16 Remote Sens. 2020, 12, x FOR PEER REVIEW 8 of 16

Figure 4. Changes in glacier surface elevation over time periods: ( (aa)) 1970–2000; 1970–2000; ( (bb)) 2000–2016 2000–2016 and and ( (cc)) 1970–2016. The names of the glaciers are presented in the text.

Remote Sens. 2020, 12, 2632 9 of 16 Remote Sens. 2020, 12, x FOR PEER REVIEW 9 of 16

Figure 5.5.Changes Changes in glacierin glacier surface surface elevation elevation over timeover periods time andperiods their and changes their along changes the longitudinal along the sectionlongitudinal in the section Xiyulong in the Glacier Xiyulong (abbreviated Glacier (abbreviated as Xiyulong as G.,Xiyulong the same G., the below), same Gongxingbelow), Gongxing G. and 5Y614G0038G. and 5Y614G0038 G. The locationsG. The locations of glaciers of glaciers are shown are inshown Figure in4 Figurein black 4 in font. black font.

However, for some glaciers, the melting pattern of glaciers was reversed in certain periods of time. For example, in some glaciers, such as Zhongfeng G., Xikunlun G. and 5Y641H0067 G., from 1970 to 2000, the glaciers’ ablation pattern was normal. As the altitude decreased, the thinning of the glaciers increased. However, from 2000 to 2016, in areas with low glacier elevations, there was strong thickening, and the areas with the strongest thickening could reach 150 m (Figure6). From 2000 to 2016, some glaciers thinned normally, but from 1970 to 2000, the phenomenon of thickening in low-altitude areas appeared, such as for the Congce G., Yulong G. and Kunlun G. (Figure7).

Remote Sens. 2020, 12, 2632 10 of 16 Remote Sens. 2020, 12, x FOR PEER REVIEW 10 of 16

Figure 6.6.Changes Changes in glacierin glacier surface surface elevation elevation over timeover periods time andperiods their and changes their along changes the longitudinal along the sectionlongitudinal in the section Zhongfeng in the G., Zhon Xikunlungfeng G., G. andXikunlun 5Y641H0067 G. and G.5Y641H0067 The locations G. The of glaciers locations are of shown glaciers in Figureare shown4 in brownin Figure font. 4 in brown font.

Remote Sens. 2020, 12, 2632 11 of 16 Remote Sens. 2020, 12, x FOR PEER REVIEW 11 of 16

Figure 7.7.Changes Changes in glacierin glacier surface surface elevation elevation over timeover periods time andperiods their and changes their along changes the longitudinal along the sectionlongitudinal in Congce section G., in Yulong Congce G. G., and Yulong Kunlun G. and G. The Kunlun locations G. The of locations glaciers are of glaciers shown inare Figure shown4 in in greenFigure font. 4 in green font.

4. Discussion

4.1. Glacial Changes in WKL Compared with Others Many scholarsscholars have have studied studied the the area area of glaciersof glaciers in the in WKL the WKL [9,11– [9,11–14].14]. However, However, most of most the studies of the havestudies coarse have temporal coarse temporal resolutions, resolutions, with only with one only or twoone periodsor two periods of area changesof area changes on average. on average. In this study,In this westudy, obtained we obtained a total ofa eighttotal of epoch eight area epoch changes area changes from 1972 from to 2018.1972 to In 2018. this way, In this more way, detailed more characteristicsdetailed characteristics of the changes of the in changes the area in of the glaciers area areof glaciers obtained. are From obtained. 1970 toFrom 1990, 1970 the resultsto 1990, of the all scholarsresults of showed all scholars that theshowed glaciers that were the glaciers retreating. were From retreating. 1990 to From 2000, 1990 Wang to et 2000, al. [9 Wang], Han et [13 al.] and[9], BaoHan et[13] al. and [11] Baobelieved et al. [11] that believed the WKL that glaciers the WKL were glaciers in a state were of in retreat, a state whileof retreat, Shangguan while Shangguan et al. [12] et al. [12] thought they were in a slightly expanding state, and this study also showed that they were in a overall slight expansion, but with spatial variability. From 2000 to 2010, Wang et al. [9] and Han

Remote Sens. 2020, 12, 2632 12 of 16 thought they were in a slightly expanding state, and this study also showed that they were in a overall slight expansion, but with spatial variability. From 2000 to 2010, Wang et al. [9] and Han [13] showed that the glaciers were in a state of retreat, while Bao et al. [11] and this study showed that they were in a state of slight retreat. From 2010 to 2018, Wang et al. [9] presented that the glaciers showed a trend of retreat, while Ma [14] and this study showed a state of expansion. Scholars have also performed additional research on the mass balance of the WKL, mainly including the material balance from 1970 to 2000 based on the geodetic method [9,10] and the mass balance from 2003 to 2008 based on ICESat [8,11,44]. Zhou et al. [10] obtained a mass balance of 0.02 0.1 m w.e. a 1 ± − from the mid-1970s to 2000 based on KH-9 and SRTM. Wang et al. [9] obtained a mass balance of 0.06 0.13 m w.e. a 1 from the 1970s to 2000 based on topographic maps and SRTM. In addition, − ± − according to Bao et al. [11], Neckel et al. [44] and Ke et al. [8], from 2003 to 2008, WKL’s mass balance 1 was approximately 0.03 to 0.23 w.e. a− based on ICESat. In this article, we obtained the mass balance of glaciers in the WKL region from 1970 to 2016, a longer and continuous time series.

4.2. Influencing Factors of Glacier Changes According to the two categories of thickness changes for the different glaciers in the WKL, we believe that the category one of glaciers are normal glaciers, which conform to the general glacier change patterns. The thickness of the ablation zone is thin, and the accumulation zone is thick. The thickness change pattern of the category two of the glacier should be caused by glacier surging. It is clearly seen that surge events occurred over the Congce G., Yulong G. and Kunlun G. from the 1970s to 2000 and the Zhongfeng G., Xikunlun G. and 5Y641H0067 G. from 2000 to 2016. Glacial surging will have a certain effect on the change in glaciers. When glacier surging occurs, the rapid movement transports the mass in the accumulation zone to the ablation zone and causes the glacier to advance and increase in area. At this time, the accumulation zone is thinned, and the ablation zone is thickened; after glacier surging, a large amount of mass is exposed to the ablation zone, causing subsequent rapid melting [45,46]. Overall, from 1970 to 2018, the glaciers in the WKL retreated slightly with fluctuations. Before 2000, especially before 1990, the glaciers showed a retreating trend; after 2000, especially after 2010, they showed an expansion. Before 2000, the glaciers showed a slight thinning phenomenon, and after 2000, they showed a slight thickening phenomenon. In 1985, the end-of-summer transient snowlines ranged from 5700 to 6120 m a.s.l., and the average equilibrium line altitude (ELA) was 5930 m a.s.l. [9,47]. The snowlines from 2003 to 2013 varied from 5929 to 6061 m, with a mean of approximately 5990 m [11]. This result means that the ELA of glaciers has shown a slight upward trend in the past few decades, but the increase is not obvious. The results of the ELA change also confirm that the glaciers of the WKL were in a slight retreat. Climate change is generally considered to be the main factor causing glacier change [41,48]. To understand the relationship between climate and glacier change here, we analyzed meteorological data from six stations (Pishan, Hotan, Yutian, Mingfeng, Shijiehe and Geze). Meteorological data can be downloaded from the China Meteorological Data Sharing Service system (http://cdc.cma.gov.cn/). Using annual precipitation and summer (June, July and August) average air temperature data from 1973 to 2019, we present the climate change trend of WKL (Figure8). The results show that from 1973 to 2019, the temperature showed a large upward trend, while the annual precipitation during the period increased slightly. Mass accumulation caused by increased precipitation is not enough to compensate for the mass loss caused by increased temperature. In detail, the expansion of glaciers during 2010–2018 was probably caused by increased precipitation and decreased temperatures during this period (Figure8). Climate change has di fficulty fully explaining the multiple fluctuations in glaciers from 1970 to 2018, which is most likely caused by glacial surging. Remote Sens. 2020, 12, 2632 13 of 16

Remote Sens. 2020, 12, x FOR PEER REVIEW 13 of 16

FigureFigure 8. 8.Changes Changes inin meanmean annualannual precipitation and and mean mean summer summer air air temperature temperature (June, (June, July July and and August)August) based based on on thethe sixsix nearestnearest weatherweather stations around around the the WKL WKL as as anomalies anomalies from from the the mean mean betweenbetween 1973 1973 and and 2019. 2019. TheThe red and blue dashed dashed line liness are are the the linear linear trend trend lines lines of ofair air temperature temperature andand precipitation. precipitation.

5.5. Conclusions Conclusions InIn this this study, study, we we analyzedanalyzed thethe changeschanges in in glacier glacier area area and and mass mass in in the the WKL WKL region region using using topographictopographic maps, maps, Landsat Landsat seriesseries data,data, SRTM and and TanDEM-x TanDEM-x data data from from the the 1970s 1970s to to2018. 2018. We We found found thatthat from from 1972 1972 to to 2018, 2018, the the total total areaarea ofof glaciers in in the the WKL WKL decreased decreased by by 7.99 7.99 km km2. The2. The area area decreased decreased byby 12.02 12.02 km km2 2from from 1972 1972 to to 19911991 andand increased by by 4.70 4.70 km km2 2fromfrom 2010 2010 to to 2018. 2018. Glacial Glacial surface surface elevation elevation andand ice ice volume volume change change results results in the in WKLthe WKL showed showed that thethat overall the overall glacier glacier thickness thickness slightly slightly decreased fromdecreased 1970 to from 2016, 1970 with to an 2016, average with ofan average1.9 1.0 of m −1.9 and ± a1.0 mass m and balance a mass of balance0.03 of0.02 −0.03 m w.e.± 0.02 The m w.e. glacier − ± − ± thinningThe glacier was thinning approximately was approximately 2.5 m from 1970 2.5 m to from 2000, 1970 while to from2000, 2000 while to from 2016, 2000 the glacierto 2016, was the thickenedglacier was thickened by approximately 0.6 m. Overall, the glaciers in the WKL showed a very slight retreat. by approximately 0.6 m. Overall, the glaciers in the WKL showed a very slight retreat. Our conclusion Our conclusion also proves that there is indeed the existent of the “Karakoram Anomaly” also proves that there is indeed the existent of the “Karakoram Anomaly” phenomenon in the WKL in phenomenon in the WKL in the past few decades. In addition, the mass changes of glaciers are largely the past few decades. In addition, the mass changes of glaciers are largely affected by glacial surging. affected by glacial surging. Author Contributions: B.C. and B.P. led the project; W.G. and K.L. processed the data; B.C. and W.G. wrote theAuthor paper. Contributions: Validation, Z.W.; B.C. Dataand B.P. curation, led the H.H. project; All W.G. authors and haveK.L. processed read and the agreed data; to B.C. the and published W.G. wrote version the of thepaper. manuscript. Validation, Z.W.; Data curation, H.H. All authors have read and agreed to the published version of the manuscript. Funding: This study was funded by the Second Tibetan Plateau Scientific Expedition and Research Program (STEP, grantFunding: No. 2019QZKK0205), This study was funded National by Natural the Second Science Tibetan Foundation Plateau Scientific of China (grantExpedition No. 41701057),and Research the Program Ministry of Science(STEP, and grant Technology No. 2019QZKK0205), of the People’s National Republic Natural of China Science (grant Foundation No. 2013FY111400). of China (grant And No. the APC41701057), was fundedthe by the Second Tibetan Plateau Scientific Expedition and Research Program (STEP, grant No. 2019QZKK0205). Ministry of Science and Technology of the People’s Republic of China (grant No. 2013FY111400). And the APC Conflictswas funded of Interest: by the TheSecond authors Tibetan declare Plateau no conflict Scientific of interest.Expedition and Research Program (STEP, grant No. 2019QZKK0205).

Remote Sens. 2020, 12, 2632 14 of 16

References

1. Bolch, T.; Kulkarni, A.; Kääb, A.; Huggel, C.; Paul, F.; Cogley, J.G.; Frey, H.; Kargel, J.S.; Fujita, K.; Scheel, M.; et al. The State and Fate of Himalayan Glaciers. Science 2012, 336, 310–314. [CrossRef][PubMed] 2. Gardelle, J.; Berthier, E.; Arnaud, Y. Slight mass gain of Karakoram glaciers in the early twenty-first century. Nat. Geosci. 2012, 5, 322–325. [CrossRef] 3. Gardelle, J.; Berthier, E.; Arnaud, Y.; Kääb, A. Region-wide glacier mass balances over the Pamir-Karakoram-Himalaya during 1999–2011. Cryosphere 2013, 7, 1263–1286. [CrossRef] 4. Gardner, A.S.; Moholdt, G.; Cogley, J.G.; Wouters, B.; Arendt, A.A.; Wahr, J.; Berthier, E.; Hock, R.; Pfeffer, W.T.; Kaser, G.; et al. A Reconciled Estimate of Glacier Contributions to Sea Level Rise: 2003 to 2009. Science 2013, 340, 852–857. [CrossRef][PubMed] 5. Yao, T.; Thompson, L.; Yang, W.; Yu, W.; Gao, Y.; Guo, X.; Yang, X.; Duan, K.; Zhao, H.; Xu, B.; et al. Different glacier status with atmospheric circulations in Tibetan Plateau and surroundings. Nat. Clim. Chang. 2012, 2, 663–667. [CrossRef] 6. Guo, W.; Liu, S.; Xu, J.; Wu, L.; Shangguan, D.; Yao, X.; Wei, J.; Bao, W.; Yu, P.; Liu, Q.; et al. The second Chinese glacier inventory: Data, methods and results. J. Glaciol. 2015, 61, 357–372. [CrossRef] 7. Hewitt, K. The Karakoram Anomaly? Glacier Expansion and the Elevation Effect, Karakoram Himalaya. Mt. Res. Dev. 2005, 25, 332–340. [CrossRef] 8. Ke, L.; Ding, X.; Song, C. Heterogeneous changes of glaciers over the western Kunlun Mountains based on ICESat and Landsat-8 derived glacier inventory. Remote. Sens. Environ. 2005, 168, 13–23. [CrossRef] 9. Wang, Y.; Hou, S.; Huai, B.; An, W.; Pang, H.; Liu, Y. Glacier anomaly over the western Kunlun Mountains, Northwestern Tibetan Plateau, since the 1970s. J. Glaciol. 2018, 64, 624–636. [CrossRef] 10. Zhou, Y.; Li, Z.; Li, J.; Zhao, R.; Ding, X. Glacier mass balance in the Qinghai–Tibet Plateau and its surroundings from the mid-1970s to 2000 based on Hexagon KH-9 and SRTM DEMs. Remote. Sens. Environ. 2018, 210, 96–112. [CrossRef] 11. Bao, W.; Liu, S.; Wei, J.; Guo, W. Glacier changes during the past 40 years in the West Kunlun Shan. J. Mt. Sci. 2005, 12, 344–357. [CrossRef] 12. Shangguan, D.; Liu, S.; Ding, Y.; Li, J.; Zhang, Y.; Ding, L.; Wang, X.; Xie, C.; Li, G. Glacier changes in the west Kunlun Shan from 1970 to 2001 derived from Landsat TM/ETM+ and Chinese glacier inventory data. Ann. Glaciol. 2007, 46, 204–208. [CrossRef] 13. Han, Y. Research on Glacier Change in the West Kunlun Mountains and Flow Velocity Estimation Based on Landsat Images (1977–2013); Nanjing University: Nanjing, China, 2015. (In Chinese) 14. Ma, Q. Monitoring Glacier Change on West Kunlun Shan Based on Multi-Source Remote Sensing Data; Nanjing University: Nanjing, China, 2018. (In Chinese) 15. Scherler, D.; Bookhagen, B.; Strecker, M.R. Spatially variable response of Himalayan glaciers to climate change affected by debris cover. Nat. Geosci. 2011, 4, 156–159. [CrossRef] 16. Yasuda, T.; Furuya, M. Dynamics of surge-type glaciers in West Kunlun Shan, Northwestern Tibet. J. Geophys. Res. Earth Surf. 2015, 120, 2393–2405. [CrossRef] 17. Chen, F.; Yu, Z.; Yang, M.; Ito, E.; Wang, S.; Madsen, D.B.; Huang, X.; Zhao, Y.; Sato, T.; Birks, H.J.B.; et al. Holocene moisture evolution in arid central Asia and its out-of-phase relationship with Asian monsoon history. Quat. Sci. Rev. 2008, 27, 351–364. [CrossRef] 18. Shi, Y.; Liu, S. Estimation on the response of glaciers in China to the global warming in the 21st century. Chin. Sci. Bull. 2000, 45, 668–672. [CrossRef] 19. Rabus, B.; Eineder, M.; Roth, A.; Bamler, R. The shuttle radar topography mission—A new class of digital elevation models acquired by spaceborne radar. ISPRS J. Photogramm. Remote. Sens. 2003, 57, 241–262. [CrossRef] 20. Pieczonka, T.; Bolch, T.; Wei, J.; Liu, S. Heterogeneous mass loss of glaciers in the Aksu-Tarim Catchment (Central Tien Shan) revealed by 1976 KH-9 Hexagon and 2009 SPOT-5 stereo imagery. Remote. Sens. Environ. 2013, 130, 233–244. [CrossRef] 21. Shangguan, D.H.; Bolch, T.; Ding, Y.J.; Kröhnert, M.; Pieczonka, T.; Wetzel, H.U.; Liu, S.Y. Mass changes of Southern and Northern Inylchek Glacier, Central Tian Shan, Kyrgyzstan, during—1975 and 2007 derived from remote sensing data. Cryosphere 2015, 9, 703–717. [CrossRef] Remote Sens. 2020, 12, 2632 15 of 16

22. Rodríguez, E.; Morris, C.S.; Belz, Z.E. A global assessment of the SRTM performance. Photogramm. Eng. Remote. Sens. 2006, 72, 249–260. [CrossRef] 23. Rossi, C.; Rodriguez Gonzalez, F.; Fritz, T.; Yague-Martinez, N.; Eineder, M. TanDEM-X calibrated Raw DEM generation. ISPRS J. Photogramm. Remote. Sens. 2012, 73, 12–20. [CrossRef] 24. Gruber, A.; Wessel, B.; Huber, M.; Roth, A. Operational TanDEM-X DEM calibration and ﬁrst validation results. ISPRS J. Photogramm. Remote. Sens. 2012, 73, 39–49. [CrossRef] 25. Huang, L.; Li, Z.; Han, H.; Tian, B.; Zhou, J. Analysis of thickness changes and the associated driving factors on a debris-covered glacier in the Tienshan Mountain. Remote. Sens. Environ. 2018, 206, 63–71. [CrossRef] 26. Ke, L.; Ding, X.; Li, W.; Qiu, B. Remote Sensing of Glacier Change in the Central Qinghai-Tibet Plateau and the Relationship with Changing Climate. Remote. Sens. 2017, 9, 114. [CrossRef] 27. Paul, F.; Bolch, T.; Kääb, A.; Nagler, T.; Nuth, C.; Scharrer, K.; Shepherd, A.; Strozzi, T.; Ticconi, F.; Bhambri, R.; et al. The glaciers climate change initiative: Methods for creating glacier area, elevation change and velocity products. Remote. Sens. Environ. 2015, 162, 408–426. [CrossRef] 28. Bolch, T.; Menounos, B.; Wheate, R. Landsat-based inventory of glaciers in western Canada, 1985–2005. Remote. Sens. Environ. 2010, 114, 127–137. [CrossRef] 29. Paul, F.; Barrand, N.E.; Baumann, S.; Berthier, E.; Bolch, T.; Casey, K.; Frey, H.; Joshi, S.P.; Konovalov, V.; Le Bris, R.; et al. On the accuracy of glacier outlines derived from remote-sensing data. Ann. Glaciol. 2013, 54, 171–182. [CrossRef] 30. Wei, J.; Liu, S.; Guo, W.; Yao, X.; Xu, J.; Weijia, B.; Zongli, J. Surface-area changes of glaciers in the Tibetan Plateau interior area since the 1970s using recent Landsat images and historical maps. Ann. Glaciol. 2014, 55, 213–222. 31. Zhang, Z.; Liu, S.; Wei, J.; Xu, J.; Guo, W.; Bao, W.; Jiang, Z. Mass Change of Glaciers in Muztag Ata–Kongur Tagh, Eastern Pamir, China from 1971/76 to 2013/14 as Derived from Remote Sensing Data. PLoS ONE 2016, 11, e0147327. [CrossRef] 32. Nuth, C.; Kääb, A. Co-registration and bias corrections of satellite elevation data sets for quantifying glacier thickness change. Cryosphere 2011, 5, 271–290. [CrossRef] 33. Berthier, E.; Schiefer, E.; Clarke, G.K.C.; Menounos, B.; Remy, F. Contribution of Alaskan glaciers to sea-level rise derived from satellite imagery. Nat. Geosci. 2010, 3, 92–95. [CrossRef] 34. Wei, J.; Liu, S.; Guo, W.; Xu, J.; Bao, W.; Donghui, S. Changes in Glacier Volume in the North Bank of the Bangong Co Basin from 1968 to 2007 Based on Historical Topographic Maps, SRTM, and ASTER Stereo Images. Arct. Antarct. Alp. Res. 2015, 47, 301–311. 35. Berthier, E.; Arnaud, Y.; Vincent, C.; Rémy, F. Biases of SRTM in high-mountain areas: Implications for the monitoring of glacier volume changes. Geophys. Res. Lett. 2006, 33, L08502. [CrossRef] 36. Paul, F. Calculation of glacier elevation changes with SRTM: Is there an elevation-dependent bias? J. Glaciol. 2008, 55, 945–946. [CrossRef] 37. Gardelle, J.; Berthier, E.; Arnaud, Y. Impact of resolution and radar penetration on glacier elevation changes computed from DEM diﬀerencing. J. Glaciol. 2012, 58, 419–422. [CrossRef] 38. Fischer, M.; Huss, M.; Hoelzle, M. Surface elevation and mass changes of all Swiss glaciers 1980–2010. Cryosphere 2015, 9, 525–540. [CrossRef] 39. Barandun, M.; Huss, M.; Sold, L.; Farinotti, D.; Azisov, E.; Salzmann, N.; Usubaliev, R.; Merkushkin, A.; Hoelzle, M. Re-analysis of seasonal mass balance at Abramov glacier 1968–2014. J. Glaciol. 2015, 61, 1103. [CrossRef] 40. Zhang, Z.; Liu, S.; Zhang, Y.; Wei, J.; Jiang, Z.; Wu, K. Glacier variations at Aru Co in western Tibet from 1971 to 2016 derived from remote-sensing data. J. Glaciol. 2018, 64, 397–406. [CrossRef] 41. Cao, B.; Pan, B.; Wen, Z.; Guan, W.; Li, K. Changes in glacier mass in the Lenglongling Mountains from 1972 to 2016 based on remote sensing data and modeling. J. Hydrol. 2019, 578, 124010. [CrossRef] 42. Huss, M. Density assumptions for converting geodetic glacier volume change to mass change. Cryosphere 2013, 7, 877–887. [CrossRef] 43. Cao, B.; Pan, B.; Guan, W.; Wen, Z.; Wang, J. Changes in glacier volume on Mt. Gongga, southeastern Tibetan Plateau, based on the analysis of multi-temporal DEMs from 1966 to 2015. J. Glaciol. 2019, 65, 366–375. [CrossRef] 44. Neckel, N.; Kropáˇcek, J.; Bolch, T.; Hochschild, V. Glacier Mass Changes on the Tibetan Plateau 2003–2009 Derived from ICESat Laser Altimetry Measurements. Environ. Res. Lett. 2014, 9, 014009. [CrossRef] Remote Sens. 2020, 12, 2632 16 of 16

45. Cuﬀey, K.M.; Paterson, W.S.B. The Physics of Glaciers, 4th ed.; Academic Press: Cambridge, MA, USA, 2010. 46. Cogley, J.G.; Hock, R.; Rasmussen, L.A.; Arendt, A.A.; Bauder, A.; Braithwaite, R.J.; Jansson, P.; Kaser, G.; Möller, M.; Nicholson, L.; et al. Glossary of glacier mass balance and related terms. In IHP-VII Technical Documents in Hydrology No 86 IACS Contribution No 2; UNESCO-IHP: Paris, France, 2011. 47. Zhang, Z.; Jiao, K. Modern glaciers on the south slope of West Kunlun Mountains (in Aksayqin Lake and Guozha Co Lake drainage areas). Bull. Glacier Res. 1987, 5, 85–91. 48. Pritchard, H.D. Asia’s shrinking glaciers protect large populations from drought stress. Nature 2019, 569, 649–654. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).