Glacier Changes in the Qilian Mountains, Northwest China, Between the 1960S and 2015

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Article Glacier Changes in the Qilian Mountains, Northwest China, between the 1960s and 2015

Jing He 1,2 , Ninglian Wang 1,2,3,*, An’an Chen 1,2 , Xuewen Yang 1,2 and Ting Hua 1,2

1 Shaanxi Key Laboratory of Earth Surface System and Environmental Carrying Capacity, College of Urban and Environmental Sciences, Northwest University, Xi’an 710127, China; [email protected] (J.H.); [email protected] (A.C.); [email protected] (X.Y.); [email protected] (T.H.) 2 Institute of Earth Surface System and Hazards, College of Urban and Environmental Sciences, Northwest University, Xi’an 710127, China 3 CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China * Correspondence: [email protected]

Received: 24 January 2019; Accepted: 22 March 2019; Published: 26 March 2019

Abstract: Glaciers in the Qilian Mountains are important sources of fresh-water for sustainable development in the Hexi Corridor in the arid northwest China. Over the last few decades, glaciers have generally shrunk across the globe due to climate warming. In order to understand the current state of glaciers in the Qilian Mountains, we compiled a new inventory of glaciers in the region using Landsat Operational Land Imager (OLI) images acquired in 2015, and identiﬁed 2748 glaciers that covered an area of 1539.30 ± 49.50 km2 with an ice volume of 81.69 ± 7.40 km3, among which the Shule River basin occupied the largest portion of glaciers (24.8% in number, 32.3% in area, and 35.6% in ice volume). In comparison to previous inventories, glacier area was found to shrink by 396.89 km2 (20.5%) in total, and 446 glaciers with an area of 44.79 km2 disappeared over the period from the 1960s to 2015. This situation was primarily caused by the increase in air temperature, and also related with the size of glacier and some local topographic parameters. In addition, the change of glaciers in the Qilian Mountains showed a distinct spatial pattern, i.e., their shrinking rate was large in the east and small in the west.

Keywords: glacier changes; the Qilian Mountains; climate factors

1. Introduction Glaciers are considered as key indicators of climate change [1], potentially contributing to water resources and global sea level rise [2]. Alpine glaciers are vital component of global cryosphere [3]. Global warming has signiﬁcant impacts on glaciers worldwide [4], and the majority of mountain glaciers have suffered from rapid retreat in recent decades [5]. The shrinkage rate of glacier area generally increased from the continental interior to the Himalayas and peaked in the southeastern Tibetan Plateau (TP), with an area reduction rate of −0.57% a−1 from 1970s to 2000s [6]. Glaciers and glacial meltwater are the most important fresh-water resources and play a critical role in maintaining ecological balance and social and economic sustainable development in the Qilian Mountains, located in the northeastern edge of the TP [7]. Glaciers in the Qilian Mountains are of great signiﬁcance to the runoff recharge in the Hexi Corridor, populated by over 4.5 million people [8]. Where a number of efforts has been made to monitor glacier changes here. Tian et al. (2014) [8] found that the glacier area in the Qilian Mountains has shrunk by 30% ± 8% from 1956 to 2010 and the shrinkage accelerated remarkably in the past two decades. Sun et al. (2018) [9] showed in the past half-century (1960s to 2010), the area and volume of glaciers in the Qilian Mountains have decreased by 420.81 km2 (−20.88%) and 21.63 km3 (−20.26%),

Water 2019, 11, 623; doi:10.3390/w11030623 www.mdpi.com/journal/water Water 2019, 11, 623 2 of 16 respectively. Wang et al. (2013) [10] reported an annual glacier mass loss of (534.2 ± 399.5) × 106 m3 w.e. (water equivalent) in the Qilian Mountains based on the Ice, Cloud, and Land Elevation Satellite (ICESat) altimetry data for 2003–2009, despite of the relatively higher uncertainty for small scale glaciers and sparse distribution of ICESat ground tracks, However, the results of Jacob et al. (2012) [11] indicated a positive glacier mass balance of 7 ± 7 Gt year−1 in Tibet and Qilian Shan from 2003 to 2010. These above results show a good consistency, except the last, which is due to the much larger research area, Tibet. GlacierWater area 2019, is11 FOR an importantPEER REVIEW parameter for mass balance change and model simulation2 research. Recent studies regarding the glacier area and volume loss document in the Qilian Mountains have w.e. (water equivalent) in the Qilian Mountains based on the Ice, Cloud, and Land Elevation Satellite focused primarily(ICESat) altimetry on its sub–regions data for 2003-2009, [12,13 despite] and of may the haverelatively some higher differences uncertainty even for small in the scale same region probably dueglaciers to theand differentsparse distribution data resources, of ICESat studyground period tracks, However, and interpretation the results of methods Jacob et al. based (2012) on remote sensing [14[11]]. Therefore,indicated a positive the aims glacier of mass this balance study of are 7 ± to 7 Gt (1) year retrieve−1 in Tibet the and outline Qilian Shan of glaciers from 2003 in to the Qilian 2010. These above results show a good consistency, except the last, which is due to the much larger Mountains by 2015 using remote-sensing data, thereby providing basic data for regional mass balance research area, Tibet. estimation, (2)Glacier provide area informationis an important parameter on the contemporary for mass balance distributionchange and model of glacierssimulation based research. on the First Chinese GlacierRecent Inventorystudies regarding (FCGI), the glacier the Second area and Chinese volume Glacierloss document Inventory in the Qilian (SCGI) Mountains and the have accomplished inventoryfocused of 2015, primarily and (3) on its analyze sub–regions glacier [12,13] changes and may andhave possiblesome differences climatic even drivesin the same for region glaciers in the probably due to the different data resources, study period and interpretation methods based on mountain range. remote sensing [14]. Therefore, the aims of this study are to (1) retrieve the outline of glaciers in the Qilian Mountains by 2015 using remote-sensing data, thereby providing basic data for regional mass 2. Study Areabalance estimation, (2) provide information on the contemporary distribution of glaciers based on The Qilianthe First Mountains Chinese Glacier are Inventory located in(FCGI), the northeasternthe Second Chinese margin Glacier of Inventory the Tibetan (SCGI) Plateau, and the extending accomplished inventory of 2015, and (3) analyze glacier changes and possible climatic drives for from Wushaolingglaciers in westwardthe mountain torange. Dangjin Shankou, connected to the Altun Mountains [15], and are composed of a series of northwest to southeast mountains and valleys [16]. The study area covers ~220,000 km2. Study2 and Area the elevation gradually increases from the northeast to southwest, spanning an elevation rangeThe from Qilian 4000 Mountains to 5200 are located m, with in the Mount northeaste Tuanjiern margin as the of the highest Tibetan peak Plateau, located extending in the Shule Nanshan (5826.8from Wushaoling m) [16]. westward to Dangjin Shankou, connected to the Altun Mountains [15], and are composed of a series of northwest to southeast mountains and valleys [16]. The study area covers The region~220,000 lies km in2 and a transitionalthe elevation zonegradually between increases the from arid the northwestern northeast to southwest, China and spanning the alpine an Tibetan Plateau [8elevation], and thus range is from controlled 4000 to 5200 by westerlym, with Mount winds Tuanjie in theas the west highest where peak precipitationlocated in the Shule is sparse and affected byNanshan the Asian (5826.8 summer m) [16]. monsoons in the east [9] where precipitation is relatively abundant. Precipitation isThe mainly region lies concentrated in a transitional in zone the between summer, the arid and northweste the annualrn China precipitation and the alpine and Tibetan warm season Plateau [8], and thus is controlled by westerly winds in the west where precipitation is sparse and precipitationaffected allshow by the trendsAsian summer of decreasing monsoons from in the east east to [9] west. where The precipitation drainage is systemrelativelyin abundant. the mountains is oriented NW–SE.Precipitation Figure is mainly1 shows concentrated the location in the summ maper, of and the the study annual site. precipitation and warm season Accordingprecipitation to our all glacier show trends inventory of decreasing of 2015 from for east the to Qilianwest. The Mountains, drainage system there in the were mountains 2748 glaciers is with an area oforiented 1539.30 NW–SE. km2, ranking Figure 1 shows 9th out the oflocation 14 mountains map of the study in western site. China with glaciers [17]. There are According to our glacier inventory of 2015 for the Qilian Mountains, there were 2748 glaciers with also two typesan area of of glaciers 1539.30 km in2, theranking mountains: 9th out of 14 subcontinental mountains in west glaciersern China in with the glaciers central [17]. and There eastern are region, and extremealsocontinental two types of glaciers (polar) in glaciers the mountains: in the subcontinental west [8]. glaciers in the central and eastern region, and extreme continental (polar) glaciers in the west [8].

FigureFigure 1. The 1. The glaciers glaciers distribution distribution in in the the Qilian Qilian Mountains. Mountains.

Water 2019, 11, 623 3 of 16

3. Data and Methods

3.1. Data

3.1.1. Landsat Imagery We used 18 Landsat 8 Operational Land Imager (OLI) images with a 30 m spatial resolution from 2013 to 2016. The Landsat scenes with minimal cloud and snow cover were selected from paths 132–137 and rows 033–034 in the Worldwide Reference System 2. The Landsat data was available from USGS Earth Explorer (https://glovis.usgs.gov/), and the series of Landsat scenes were preprocessed with Standard Terrain Correction (Level 1T) by the United States Geological Survey (USGS). The months of these imagery were mainly concentrated in summertime. Before glacier delineation, the image sharpening operation was processed in all images to a 15 m resolution, which would effectively reduce errors caused by pixel resolution.

3.1.2. First Chinese Glacier Inventory (FCGI) and Second Chinese Glacier Inventory (SCGI) The compilation of the FCGI dataset was based on topographic maps and aerial photographs acquired from the 1950s to the 1980s by Chinese glaciologists [16]. In addition, most of the aerial photographs used in the interpretation of the glacier in Qilian Mountains were taken in 1956, and the rest in 1963. Therefore, 1960s was selected as the median date for the mountains. The SCGI dataset released in December 2014 was compiled using geographic information system (GIS) and remote-sensing techniques, after validations with in situ ﬁeld investigations and detailed monitor of selected glacier changes, covering 86% of the glacierized area of China compared to the digitized FCGI. In total, 218 Landsat scenes were used in SCGI, with ~92% of them being taken during 2006–10 [18].

3.1.3. Digital Elevation Models We employed the Shuttle Radar Topography Mission (SRTM) V4.1 to acquire the elevation attributes of glaciers as it shows less error throughout the central Tibetan Plateau than the Advanced Spaceborne Thermal Emission and Reﬂection Radiometer Global Digital Elevation Model 2 (ASTER GDEM 2) when compared with ICESat GLA 14 [19]. The SRTM was provided by the Consultative Group on International Agricultural Research (CGIAR, http://srtm.csi.cgiar.org/).

3.1.4. Climate Data Monthly temperature and precipitation data for the period 1960–2015 at seven meteorological stations (Table1) around glaciers, were obtained from the China Meteorological Data Service Centre (http://data.cma.cn/). Moreover, the CRU (Climatic Research Unit) TS v.4.01 land precipitation and air temperature datasets were used in this study to analyse the spatial distribution and change pattern of glaciers, which are based on monthly observational data from 1901 to 2015 with a high resolution (0.5 × 0.5 degree).

Table 1. The location of seven meteorological stations in the Qilian Mountains.

Station Number Name Latitude Longitude Elevation 52633 Tuole 38.48 98.25 3820 52645 Yeniugou 38.25 99.35 4429 52657 Qilian 38.11 100.15 3597 52679 Wuwei 37.55 102.4 3272 52713 Dachaidan 37.51 95.22 3425 52765 Menyuan 37.23 101.37 3502 52787 Wushaoling 37.12 102.52 3339 Water 2019, 11, 623 4 of 16

3.2. Methods

3.2.1. Glacier Outline Delineation We applied a semi-automated method using the Red/Shortwave Infrared (SWIR) band ratio with an appropriate threshold to extract glacier outlines. This method is most applicable and effective for glacier classiﬁcation according to the Global Land Ice Measurement Form Space (GLIMS) protocol [14,20]. However, the automated mapping of glaciers might be inaccurate or misclassiﬁed when the quality of image is affected by the presence of clouds, seasonal snow, glacier lakes, mountain shadow regions and debris-covered parts, which makes it essential to improve the automatic delineation results manually [20]. We used different band combinations to solve the main problems of the automated method. True color composite images were used for the general situation in glacier delineation, while the false color images were utilized to distinguish ice and snow from cloud owing to the weak absorption of clouds in the SWIR. In addition, the high-resolution Google EarthTM imagery was used to verify glaciers with large shadow or termini ending in lakes and some regions where image quality was unsatisfactory. The accurate discrimination of a glacier from cloud and snow depends more on operational experience rather than a clear standard, and therefore one operator made all manual improvements so as to ensure consistency and comparability across the entire region [21]. The accuracy of manually digitized glacier boundary is within half pixel. Attributes of glaciers such as aspect, elevation, etc., were calculated using methods that Paul suggested [14].

3.2.2. Calculation of Glacier Area Variation Changes in glacier area were expressed as the difference between each of three glacier inventories. The normalized percentage (AP, %) and rates of glacier change (AR, % a−1) were used to measure the area variation to avoid the interference of differing size of glaciers and time interval between three inventories, which are calculated as follows [22]:

AP = 100(S 1 − S0)/S0 (1)

AR = AP/∆t = AP/(t 1 − t0) (2) where S (km2) represents glacier area and t represents year, subscript 0 and 1 denotes the earlier and later time of any two inventories, respectively. The comparable AR is a ‘simple-interest’ calculation for being divided by the initial area S0 [23].

3.2.3. Uncertainty Assessment The sources of glacier delineation uncertainties can be divided into three types [24,25]: technical uncertainties, interpretation uncertainties, and methodological uncertainties. Technical uncertainties can be ignored for the images we applied are orthorectiﬁed. Interpretation uncertainties are mostly decided for the purposes of inventory compilation and are difﬁcult to evaluate. Methodological uncertainties depend on the resolution of the Landsat images and the skills of the interpretation operator [26], which can be assessed in this study. The area error assessed by glacier buffers [4,27] is feasible because it depends strongly on the glacier size, and therefore, the uncertainty of the outline position (EA) is expressed as half of an image pixel [28] multiplied with the length of glacier boundary. We also represent the area uncertainties as percentages of the total glacier area. In addition, the errors from the interior ice divides are also ignored at watershed or larger scales [18]. The uncertainty of glacier area changes (EAC) is calculated by adding the area uncertainties in quadrature [22,29]. EA and EAC can be expressed as follows:

2 EA= N·λ /2 (3) Water 2019, 11, 623 5 of 16

s 2 2 E = (E + (E (4) AC A0 A1 where EA represents area uncertainties, N is the number of pixels along the boundary of glaciers, and λ is the spatial resolution (15 m for processed Landsat 8 imagery). The uncertainty of our inventory due to image spatial resolution was ±49.50 km2 (±3.21%).

4. Results

4.1. The Glacier Distribution in 2015 In total, there were 2748 glaciers with an area of 1539.30 ± 49.50 km2 in 2015, unevenly distributed within different basins. Generally, only a small number of glaciers accounts for most of the total area, while the rest of the smaller glaciers that are larger in number occupy a small proportion [30,31], which still holds in the Qilian Mountains. As shown in Figure2, glaciers with areas ranging from 2 to 5 km 2 covered a larger area (23.8%) of the total glacier area in the Qilian Mountains (Figure2a), occupying merely 4.4% of the total number, whereas the higher number of glaciers can be found in the size class <0.1 km2 and 0.1–0.5 km2 (Figure2b), accounting for 37.4% and 38.0% of the whole number, but 3.1% and 16.7% of entire area, respectively. The biggest glacier named Laohugou No.12 Glacier covered an area of 20.22 km2, located in the Shule River basin. There are some different distribution of the glaciers within different area classes in drainage basin scale and the overall regional trends. The statistics of glaciers in different watersheds are listed in Table2. The Shule River (5Y44) basin had the largest number (24.8%) and area (32.3%) of the glaciers, while the number of glaciers was the second largest (21.3%) in the Beida River (5Y43) basin, but its area occupied 13.5%, which was only less than half that of Shule River. Almost all basins had its majority number (>70%) of glaciers with an area of less than 0.5 km2 (Figure2d). The basin with least glaciers resources was the Bayan Gol River, with only 10 glaciers covering an area of 2.11 km2. The Bayan Gol River basin (5Y59) and Heihe River basin (5Y42) had the largest area proportions (>80%) of glaciers smaller than 1 km2 (Figure2c), indicating that more glaciers in these basins are small.

Table 2. Glacier distributions in different drainage basins from three inventories.

Drainage Basin FCGI SCGI Inventory in 2015 Area Area Area Area Name Code Number Number Number (km2) (km2) (km2) (km2) Datong River 5J42 9765 40.97 108 20.82 68 18.91 71 Shiyang River 5Y41 10,950 64.84 141 39.93 97 35.34 100 Heihe River 5Y42 15,490 129.79 428 78.30 375 71.27 384 Beida River 5Y43 9990 290.76 650 215.22 577 207.75 586 Shule River 5Y44 17,630 589.68 639 509.87 660 497.34 682 Danghe River 5Y45 17,050 232.66 308 203.73 318 198.02 325 Buh River-Qinghai Lake 5Y51 14,370 14.71 21 10.28 24 9.44 24 Haltang River 5Y56 20,630 310.18 239 283.48 268 273.52 276 Har Lake 5Y57 4870 89.27 106 78.75 108 76.83 108 Iqe-Tatalin Gol River 5Y58 9890 168.89 168 155.12 179 148.74 182 Bayan Gol River 5Y59 5520 4.42 16 2.19 10 2.11 10 Total 1936.17 2824 1597.69 2684 1539.28 2748 Notes: FGGI: First Chinese Glacier Inventory. SCGI: Second Chinese Glacier Inventory.

Water 2019, 11, 623 6 of 16 glaciers in the western Qilian Mountains are located at an elevation of >4700 m while glaciers in the eastWater developed 2019, 11 FOR below PEER 4500REVIEW m. 6

Figure 2. Total glaciers areas (a) and numbers (b) of the 2015 inventory and their respective proportions (c, d) within different area classes and different drainage basins.

The statistical analysis with a 200 m elevation increment shows that the hypsography of the glacier area in the Qilian Mountains presented a spatial difference and quasi-normal distribution (Figure 3a). The elevation of glaciers ranged from 4017 to 5764 m and increased from northeast to southwest along the parallel mountains and valleys with NE trend. Glaciers located between 4700– 5300 m accounted for 76.3% of the total area of glaciers, while the least proportion of glaciers area (only about 0.8%) was found in those over 5500 m. The spatial pattern of glacier distribution by altitude shows that most glaciers in the western Qilian Mountains are located at an elevation of >4700 m while glaciers in the east developed below 4500 m. Among all eight azimuth angles, the largest proportion of glacier aspects was north, which was approximately 40%, in terms of both glacier numbers and areas. Glaciers with NE aspect roughly FigureFigure 3. 3.Glacier Glacier distribution distribution byby altitudealtitude andand aspect. ( (aa)) Hypsography Hypsography of of glaciers glaciers area area in inthe the Qilian Qilian equaled to those with NW aspect in number, but were larger in glacier◦ area, hence the NE facing Mountains;Mountains; (b ()b proportion) proportion of of glacier glacier numbernumber andand areaarea by mean mean aspect aspect (45° (45 sectors).sectors). glaciers were generally larger. There were few glaciers in the remaining five directions, accounting for 24.1% of number and 21.4% of area in total. The Qilian Mountains have one of the greatest north– 4.2.Among The Characteristics all eight azimuth of Glacier angles, Changes the largest proportion of glacier aspects was north, which was approximatelysouth local glaciation 40%, in termsasymmetries of both in glacier the world. numbers Figure and 3b areas.shows a Glaciers strong northward with NE aspect tendency roughly in equaledboth area to those and number with NW of aspectglaciers, in with number, slightly but more were in larger the northeast in glacier than area, northwest: hence the in NE strong facing glaciersagreement were with generally Evans’s larger. results There [32], were especially few glaciers for the innumber the remaining of glaciers. ﬁve directions, accounting for 24.1% of number and 21.4% of area in total. The Qilian Mountains have one of the greatest north–south local glaciation asymmetries in the world. Figure3b shows a strong northward tendency in both area and number of glaciers, with slightly more in the northeast than northwest: in strong agreement with Evans’s results [32], especially for the number of glaciers.

4.2. The Characteristics of Glacier Changes

4.2.1. The Overall Glacier Area Variations From the 1960s to 2015, the area of glaciers in the Qilian Mountains shrank by 20.5% ± 6.0% ± 2 (396.89 Figure116.90 3. Glacier km ) baseddistribution on the by FCGI altitude and and our aspect. data. (a Figure) Hypsography4 shows of changes glaciers inarea glacier in the numberQilian and combinedMountains; area according (b) proportion to area of sizeglacier class. number From and the area perspective by mean aspect of area (45° change,sectors). almost all size classes

4.2. The Characteristics of Glacier Changes

Water 2019, 11 FOR PEER REVIEW 7 Water 2019, 11 FOR PEER REVIEW 7 4.2.1. The Overall Glacier Area Variations 4.2.1. The Overall Glacier Area Variations From the 1960s to 2015, the area of glaciers in the Qilian Mountains shrank by 20.5% ± 6.0% (396.89From ± 116.90 the 1960s km2 )to based 2015, on the the area FCGI of andglaciers our data. in the Figure Qilian 4 showsMountains changes shrank in glacier by 20.5% number ± 6.0% and Water 2019, 11, 623 7 of 16 (396.89combined ± 116.90 area km according2) based onto thearea FCGI size andclass. our From data. the Figure perspective 4 shows changesof area change,in glacier almost number all and size combinedclasses of areaglaciers according experienced to area a desizecreasing class. Fromtrend, the except perspective for <0.1 ofkm area2 and change, 5–10 km almost2 over allthe sizepast classesofdecades. glaciers of Theglaciers experienced number experienced of a decreasingglaciers a de withcreasing trend, an area except trend, of <0.1for except <0.1km2 for kmincreased 2<0.1and km 5–10 from2 and km 589 25–10over to 1027km the2 over pastwith the decades.the pasttotal decades.Thearea number from The 39.72 ofnumber glaciers km2 toof with 47.23glaciers an km area with2, while of an <0.1 areathat km inof2 the<0.1increased area km2 rangeincreased from of 589 0.1–0.5 from to 1027 589km with 2to had 1027 the diminished totalwith area the total fromfrom area39.721387 from kmto 21044, 39.72to 47.23 andkm km 2 theto2 ,47.23 whilearea kmcorrespondingly that2, while in the areathat in range decreased the ofarea 0.1–0.5 range from km of354.602 0.1–0.5had diminishedto km256.322 had km fromdiminished2. One 1387 possible to from 1044, 1387andexplanation theto area1044, correspondingly forand the the opposite area correspondingly decreased change trends from 354.60decreasedbetween to 256.32 twofrom adjacent km354.602. One toscale possible256.32 classes km explanation 2.could One possiblebe for those the explanationoppositerelatively change larger for the glaciers trends opposite between (0.1–0.5 change twokm2) adjacent trendsmight havebetw scale disintegratedeen classes two couldadjacent or be get thosescale shrunk relativelyclasses as a resultcould larger ofbe glaciersmelting, those relatively(0.1–0.5which kmwould larger2) might glacierscontribute have (0.1–0.5 disintegrated to the km 2rise) might or in get thehave shrunk smaller disintegrated as a resultglaciers of or melting, get(<0.1 shrunk km which2). as The woulda result above contribute of melting,analysis to whichthedemonstrates rise would in the smaller thatcontribute the glaciers number to (<0.1the of glaciers kmrise2 ).in The in the the above Qiliansmaller analysis Mountains glaciers demonstrates (<0.1was dominated km that2). theThe numberby above smaller ofanalysis glaciersglaciers demonstratesinwith the an Qilian area Mountainsofthat less the than number was1 km dominatedof2 (~85%), glaciers while in by the smaller the Qilian area glaciers Mountainswas dominated with was an area dominatedby glaciers of less thanbyin thesmaller 1 >1 km km2 glaciers(~85%),2 range withwhile(~65%). an the area area of wasless dominatedthan 1 km2 by(~85%), glaciers while in thethe >1area km was2 range dominated (~65%). by glaciers in the >1 km2 range (~65%).

Figure 4. Size and count distributions of glacier in different area classes from three inventories. FigureFigure 4. 4. SizeSize and and count count distributions distributions of of glacier glacier in in di differentfferent area area classes classes from from three three inventories. inventories. Glacier changes in the Qilian Mountains varied by size, small glaciers had higher area change Glacier changes in the Qilian Mountains varied by size, small glaciers had higher area change ratesGlacier than large changes glaciers in the from Qilian the Mountains 1960s to 2015. varied Based by size,on the small relative glaciers changes had higher in the areaarea changeof each rates than large glaciers from the 1960s to 2015. Based on the relative changes in the area of each ratesglacier than (Figure large 5),glaciers the glaciers from thein the 1960s Qilian to 2015.mountains Based with on the an arearelative of <5.0 changes km2 hadin the a larger area ofrange each of glacier (Figure5), the glaciers in the Qilian mountains with an area of <5.0 km 2 had a larger range of glacierchange, (Figure while 5),almost the glaciers all >5.0 in km the2 glaciersQilian mountains decreased with by less an areathan of 20%. <5.0 Thekm2 glaciershad a larger in the range <1 km of 2 change, while almost all >5.0 km2 glaciers decreased by less than 20%. The glaciers in the <1 km2 range change,range decreased while almost by 63.6% all >5.0 of thekm 2total glaciers area decreasedloss, and most by less glaciers than whose20%. The area glaciers shrinkage in the percentage <1 km2 decreased by 63.6% of the total area loss, and most glaciers whose area shrinkage percentage was more rangewas more decreased than 50%by 63.6% were oflocated the total in the area range loss, of an <1d kmmost2, accountingglaciers whose for 39.5% area shrinkageof the total percentage number of than 50% were located in the range of <1 km2, accounting for 39.5% of the total number of glaciers. wasglaciers. more Thesethan 50% analyses were locatedrevealed in thatthe range<1 km of2 glaciers<1 km2, accountingdominated forboth 39.5% absolute of the and total relative number area of These analyses revealed that <1 km2 glaciers dominated both absolute and relative area changes. glaciers.changes. These analyses revealed that <1 km2 glaciers dominated both absolute and relative area changes.

Figure 5. The area change percentage in glaciers in the Qilian Mountains from the 1960s to 2015.

The relationship of glacier area in the FCGI and the area change percent with longitude is shown in Figure6. Overall, the glacier area in the eastern part was generally smaller than that in the western WaterWater 20192019,, 1111 FORFOR PEERPEER REVIEWREVIEW 88

FigureFigure 5.5. TheThe areaarea changechange percentagepercentage inin glaciersglaciers inin thethe QilianQilian MountainMountainss fromfrom thethe 1960s1960s toto 2015.2015.

Water 2019, 11, 623 8 of 16 TheThe relationshiprelationship ofof glacierglacier areaarea inin thethe FCGIFCGI andand thethe areaarea changechange percentpercent withwith longitudelongitude isis shownshown inin FigureFigure 6.6. Overall,Overall, thethe glacierglacier areaarea inin thethe easterneastern partpart waswas generallygenerally smallersmaller thanthan thatthat inin thethe region,westernwestern whereas region,region, their whereaswhereas area their changetheir areaarea percentages changechange percentagespercentages were just the werewere opposite. justjust thethe However, opposite.opposite. there However,However, was no therethere deﬁnite waswas patternnono definitedefinite of changes patternpattern in ofof glacier changeschanges area inin glacierglacier from west areaarea to fromfrom east westwest with toto longitude. easteast withwith longitude. Thislongitude. can be ThisThis explained cancan bebe explainedexplained by lower elevationbyby lowerlower inelevationelevation the eastern inin thethe Qilian easterneastern Mountains. QilianQilian Mountains.Mountains.

Figure 6. Glacier area proportion in FCGI and area changes with longitude from 1960s to 2015. FigureFigure 6.6. GlacierGlacier areaarea proportionproportion inin FCGIFCGI andand areaarea changeschanges withwith longitudelongitude fromfrom 1960s1960s toto 2015.2015. Figure7 displays the results from our hypsometric analysis from the SCGI and our inventory. The QilianFigureFigure Mountains 77 displaysdisplays had thethe a resultsresults unimodal fromfrom distribution, ourour hypsomethypsomet withricric ice analysisanalysis coverage fromfrom above thethe 5500SCGISCGI m andand remaining ourour inventory.inventory. almost unchangedTheThe QilianQilian whileMountainsMountains the highest hadhad aa absolute unimodalunimodal ice distribution,distribution, loss occurred withwith between iceice coveragecoverage 4700 and aboveabove 4900 5500 m,5500 accounting mm remainingremaining for 24.5%almostalmost of unchangedunchanged the total area whilewhile loss. thethe In highesthighest addition, absoluteabsolute the area iceice recession lossloss occurredoccurred of glaciers betweenbetween below 47004700 5100 andand m 49004900 occupied m,m, accountingaccounting 87.5% of generalforfor 24.5%24.5% area ofof change. thethe totaltotal There areaarea isloss.loss. a clear InIn addition,addition, result that thethe area areaarea losses recessionrecession increase ofof asglaciersglaciers elevation belowbelow decreases, 51005100 mm meaningoccupiedoccupied that87.5%87.5% equilibrium-line ofof generalgeneral areaarea altitude change.change. (ELA) ThereThere in isis balance aa clearclear with resultresult glacier thatthat areaarea geometry losseslosses has increaseincrease risen. as Thisas elevationelevation is consistent decreases,decreases, with themeaningmeaning study bythatthat Wang equilibrium-lineequilibrium-line et al [33]. The altitudealtitude ELA of the(ELA)(ELA) Qiyi inin Glacier balancebalance in thewithwith mountains glacierglacier geometrygeometry has generally hashas risen.risen. increased ThisThis by isis aboutconsistentconsistent 230 m withwith from thethe 1958 studystudy to 2008. byby Wang DifferentWang etet recessionalal [33].[33]. TheThe rates ELAELA were ofof highlighted thethe QiyiQiyi GlacierGlacier when glaciersinin thethe mountainsmountains classiﬁed into hashas sevengenerallygenerally elevation increasedincreased intervals, byby aboutabout as the 230230 percentage mm fromfrom of 19581958 glacier toto 2008.2008. area changeDifferentDifferent decreased recessionrecession with ratesrates increasing werewere highlightedhighlighted elevation, withwhenwhen the glaciersglaciers greatest classifiedclassified shrinkage intointo percent sevenseven of elevationelevation 14.9% for intervals,intervals, those lying asas <4500 thethe percentagepercentage m. The reason ofof glacierglacier of which areaarea might changechange be thatdecreaseddecreased glaciers withwith at lower increasingincreasing elevation elevation,elevation, without withwith sufﬁcient thethe grgr accumulationeatesteatest shrinkageshrinkage may percent havepercent a faster ofof 14.9%14.9% response forfor thosethose to climate lyinglying change.<4500<4500 m.m. Hence, TheThe thereasonreason glaciers ofof which belowwhich 5100mightmight m werebebe thatthat overwhelmingly glaciersglaciers atat lowerlower predominant elevationelevation in bothwithoutwithout absolute sufficientsufficient and relativeaccumulationaccumulation area changes. maymay havehave aa fasterfaster responseresponse toto climateclimate change.change. Hence,Hence, thethe glaciersglaciers belowbelow 51005100 mm werewere overwhelminglyoverwhelmingly predominantpredominant inin bothboth absoluteabsolute andand relativerelative areaarea changes.changes.

FigureFigureFigure 7. 7.7.Area AreaArea changes changeschanges in inin glacier glacierglacier hypsometry hypsometryhypsometry based basedbased on onon the thethe SCGI SCGISCGI and andand our ourour inventory. inventory.inventory. All AllAll hypsometry hypsometryhypsometry waswaswas calculated calculatedcalculated using usingusing SRTM SRTMSRTM (Shuttle (Shuttle(Shuttle Radar RadarRadar Topography TopographyTopography Mission). Mission).Mission).

Water 2019, 11, 623 9 of 16

Water 2019, 11 FOR PEER REVIEW 9 4.2.2. Comparisons of Glacier Area Variations between Different Inventories 4.2.2. Comparisons of Glacier Area Variations between Different Inventories Statistical analysis indicates that glacier recession of both glacier area and area change rate have acceleratedStatistical in all analysis orientations indicates over thethat past glacier ﬁve recession years. The of highest both glacier absolute area area and changes area change were observed rate have onaccelerated glaciers oriented in all orientations northward over (165.92 the kmpast2), five followed years. by The those highest with absolute northeast area (91.93 changes km2) andwere northwestobserved (62.28on glaciers km2) orientationsoriented northward (Figure8 b),(165.92 and therekm2), were followed similar by area those changes with northeast of glaciers (91.93 oriented km2) south,and northwest southeast (62.28 and southwest km2) orientations from the (Figure 1960s to 8b), 2015. and It there should were be notedsimilar that area the changes absolute of glacierglaciers areaoriented changes south, were southeast consistent and with southwest the scale from of total the 1960s glaciers to by2015. orientations. It should be From noted the that perspective the absolute of areaglacier change area percentage, changes were the largest consistent shrinkage with of the glaciers scale orientedof total southeastglaciers by (8.7%/10a), orientations. while From glaciers the facingperspective northwest of showedarea change the least percentage, decrease (6.0%/10a) the largest from shrinkage 2010 to 2015.of glaciers The reason oriented may be southeast that the smaller(8.7%/10a), glaciers while generally glaciers have facing NW–N–NE northwest orientations. showed the least decrease (6.0%/10a) from 2010 to 2015. The reason may be that the smaller glaciers generally have NW–N–NE orientations.

FigureFigure 8. 8.Orientation Orientation characteristics characteristics of of glacier glacier area area changes changes in in the the Qilian Qilian Mountains: Mountains: ( a()a) glacier glacier area area 2 decreasedecrease (%/10a), (%/10a), andand ( b(b)) area area changes changes (km (/10a)./10a).

TheThe spatial spatial pattern pattern of of glacier glacier shrinkage shrinkage across across the the Qilian Qilian Mountains Mountains isis illustratedillustrated inin FigureFigure9 .9. TheThe area area changes changes of of glaciers glaciers at at drainage drainage scale scale had had significant significant differences differences in in the the period period from from the the 1960s 1960s toto 2015, 2015, although although they they all experiencedall experienced an accelerated an accelerated shrink shrink in both in absolute both absolute and relative and arearelative change area rateschange in recent rates in years. recent Among years. Among all the drainage, all the drainage, the Shule the River Shule basin River (5Y44) basin suffered(5Y44) suffered from the from most the 2 2 rapidmost change rapid change in glacier in areaglacier with area losses with of losses 15.94 of km 15.94/10a km and2/10a 25.04 and km 25.04/10a km in2/10a the periodin the period of the 1960s of the to1960s 2010 to and 2010 2010–2015, and 2010–2015, respectively. respectively. Next rapid Next decrease rapid decrease losses werelosses found were infound the Beida in the River Beida basin River (5Y43)basin and(5Y43) Heihe and River Heihe basin River (5Y42). basin The (5Y42). Bayan GolThe RiverBayan basin Gol (5Y59) River hadbasin the (5 lowestY59) absolutehad the losseslowest 2 ofabsolute glacier arealosses (0.42 of glacier km /10a), area possibly (0.42 km due2/10a), to itspossibly smallest due glacier to its area smallest among glacier all the area basins. among all the basins.The relative change rate of glacier area showed an accelerated trend from west to east, while the area changesThe relative of the change western rate region of glacier were area much showed larger an than accelerated eastern Qilian trend Mountains.from west to From east, 1960swhile tothe 2015,area thechanges fastest of ratethe inwestern the relative region decrease were much (9.79%/10a) larger than occurred eastern inQilian the Datong Mountains. River From basin 1960s (5J42), to followed2015, the by fastest the Shiyang rate in the River relative basin decrease (5Y41) and (9.79%/10a) Heihe River occurred basin (5Y42)in the Datong with the River relative basin rates (5J42), of 8.27%/10afollowed by and the 8.20%/10a, Shiyang River respectively. basin (5Y41) By comparison, and Heih thee River relative basin decrease (5Y42) rateswith ofthe five relative basins rates in the of west8.27%/10a of Har and Lake 8.20%/10a, were lower, respectively. with values By below comparison, 2.85%/10a. the Itrelative was worth decrease noting rates that of the five glaciers basins in in thethe Shiyang west of RiverHar Lake basin were have lower, changed with fastest values ( −below2.30%/a) 2.85%/10a. in the It past was five worth years. noting The spatialthat the pattern glaciers ofin shrinkage the Shiyang rates River in the basin Qilian have Mountains changed fastest is broadly (−2.30%/a) consistent in the with past patterns five years. of area The changes spatial pattern in the Tibetanof shrinkage Plateau rates and in smaller the Qilian samples Mountains reported is earlierbroadly by consistent Yao and otherswith patterns [6], and of Liu area and changes others [ 34in ].the Tibetan Plateau and smaller samples reported earlier by Yao and others [6], and Liu and others [34].

Water 2019, 11, 623 10 of 16 Water 2019, 11 FOR PEER REVIEW 10

FigureFigure 9. Area 9. Area changes changes of of glaciers glaciers at at drainagedrainage scale scale in in the the Qilian Qilian Mountains Mountains from from three three inventories. inventories.

4.2.3.4.2.3. The DisappearanceThe Disappearance of of Glaciers Glaciers ComparisonsComparisons of the of threethe three inventories inventories show show that 109that glaciers 109 glaciers with anwith area an of area 8.94 kmof 28.94disappeared km2 over 1960sdisappeared to 2015, over while 1960s only to 2015, 7 glaciers while (0.17 only km7 glaciers2) disappeared (0.17 km2) from disappeared 2010 to 2015.from 2010 Due to to 2015. the limitation Due of topographicto the limitation map scaleof topographic and imagery map resolution,scale and imagery glaciers resolution, with an glaciers area of lesswith thanan area 0.01 of kmless2 thanwere not 2 considered0.01 km for were the not disappearance considered for situation. the disappearance In addition, situation. all disappeared In addition, glaciersall disappeared have been glaciers veriﬁed have been verified in Google Earth. The statistical analysis demonstrated that disappeared glaciers in Google Earth. The statistical2 analysis demonstrated that disappeared glaciers were dominated by were2 dominated by <0.1 km glaciers (52.0% of total area) and mostly in the elevation of <4900 m <0.1 km(86.0%),glaciers while (52.0% glaciers of totalfacing area) northeast andmostly (28.3%), in north the elevation (24.4%) and of <4900 northwest m (86.0%), (23.3%) while tend glaciersto facingdisappeared. northeast (28.3%),This is consistent north (24.4%) with andrelevant northwest attributes (23.3%) of disappeared tend to disappeared. glaciers from This2010 isto consistent 2015 withlisted relevant in Table attributes 3. Figure of 10 disappeared shows that the glaciers disappearance from 2010 of glaciers to 2015 was listed mainly in Table found3. Figurein the eastern 10 shows that thepart disappearance of the Qilian Mountains, of glaciers with was lower mainly elevatio foundn, and in the the eastern most serious part ofdisappearance the Qilian Mountains, occurred in with lowerthe elevation, Heihe River and basin the most(5Y42) serious and Datong disappearance River basin (5J42), occurred with in an the area Heihe of 2.56 River km2 and basin 1.71 (5Y42) km2, and Datongrespectively. River basin (5J42), with an area of 2.56 km2 and 1.71 km2, respectively.

TableTable 3. Some 3. Some attributes attributes of ofdisappeared disappeared glaciers glaciers from from 2010 2010 to to2015. 2015. Longitude Latitude Area FCGI Area SCGI Mean Elevation Aspect FCGI_ID SCGI_ID Longitude(°) Latitude(°) Area(km2 FCGI) (kmArea2) SCGI Mean(m) Elevation(°) Aspect FCGI_ID SCGI_ID ◦ ◦ 2 2 ◦ 5Y417G0004 G101456E37746N ( 101.46) 37.75( ) 0.2(km ) 0.049(km ) 4319.10 (m) 23.40 ( ) 5Y417G00045Y427H0005G101456E37746N G099131E38980N 101.46 99.13 37.75 38.98 0.07 0.2 0.026 0.049 4896.70 4319.10 144.60 23.40 5Y427H00055Y429B0006G099131E38980N G099079E38955N 99.13 99.08 38.98 38.96 0.09 0.07 0.010 0.026 4629.70 4896.70 32.70 144.60 5Y429B00065Y435G0001G099079E38955N G098587E38539N 99.08 98.59 38.96 38.54 0.05 0.09 0.010 0.010 4684.30 4629.70 12.80 32.70 5Y435G00015Y435K0005G098587E38539N G098432E38595N 98.59 98.43 38.54 38.60 0.09 0.05 0.015 0.010 4686.00 4684.30 15.20 12.80 5Y435K00055Y571C0003G098432E38595N G097356E38597N 98.43 97.36 38.60 38.60 0.05 0.09 0.023 0.015 4972.80 4686.00 44.30 15.20 5Y571C00035Y581H0004G097356E38597N G095607E37998N 97.36 95.61 38.60 38.00 0.08 0.05 0.032 0.023 5126.30 4972.80 24.50 44.30 5Y581H0004 G095607E37998N 95.61 38.00 0.08 0.032 5126.30 24.50

Water 2019, 11, 623 11 of 16

Water 2019, 11 FOR PEER REVIEW 11

FigureFigure 10. 10.The The location location of of disappeared disappeared glaciers glaciers from from the the 1960s 1960s to 2015. to 2015.

4.2.4.4.2.4. The VolumeThe Volume Changes Changes of Glaciersof Glaciers We estimatedWe estimated the volumethe volume of glaciersof glaciers using using volume-area volume-area (V-A) (V-A) scaling scaling methodmethod based on three three sets of empiricalsets of empirical parameters parameters (Table4 (Table). In the 4). presentIn the present study, study, the V-A the V-A scaling scaling was was described described as as [35 [35]:]: V = cAγ (5) V = cAγ (5) where V and A are volume and area of a single glacier, while c and γ are scaling parameters. From 1960s to 2015, the average volume decrease of glaciers using different constants was 25.74 where V and A are volume and area of a single glacier, while c and γ are scaling parameters. km3 in the Qilian Mountains, and the change rate of glacier ice volume loss was −0.47 km3/a in the mountains. Table 4. Glacier volume changes based on three series of empirical parameters. Table 4. Glacier volume changes based on three series of empirical parameters. Ice Volume of Ice Volume of Glacier Volume Change Calculation Reference GlaciersIce in Volume the FCGI of IceGlaciers Volume in of 2015 3 3 Method Glacier kmVolume Changekm /a Calculation Glaciers Glaciers Reference 109.405 83.005 −26.4 −0.48 MethodV = 0.04A 1.35 Liu et al. [36] in the FCGI in 2015 km3 km3/a 103.383 78.001 −25.382 −0.46 V = 0.0365A1.375 Radic and Hock [37] 1.35 109.508109.405 83.005 84.078 −26.4− 25.43 −0.48− 0.46 V = 0.04AV = 0.0433A 1.29 Liu et al.Grinsted [36] [38] 103.383 78.001 −25.382 −0.46 V = 0.0365A1.375 Radic and Hock [37] 109.508 84.078 −25.43 −0.46 V = 0.0433A1.29 Grinsted [38] From 1960s to 2015, the average volume decrease of glaciers using different constants was 25.74 km3Inin addition the Qilian to Mountains,the data above, and thewe changealso em rateployed of glacier previously ice volume published loss results was − 0.47from km the3/a in the mountains.literature (Table 5) to obtain temporally comprehensive information for the glacier mass balance or Insurface addition elevation to the changes. data above, Some wedata also of elevatio employedn changes previously were transformed published resultsfrom the from mass the balance literature results taking 850 kg·m−3 as the glacier density [39]. The surface elevation changes of the eastern (Table5) to obtain temporally comprehensive information for the glacier mass balance or surface mountain were obviously larger while the changes in glacier volume were precisely opposite, and elevationthis was changes. due to the Some differences data of in elevation total glacier changes area scale. were Additionally, transformed the from glacier the volume mass balancechanges resultsin −3 takingthe 850 Qilian kg·m Mountainsas the glacierbased on density empirical [39 ].formula The surface showed elevation a good consistency changes of with the easternthat of in mountain situ weremeasurements. obviously larger while the changes in glacier volume were precisely opposite, and this was due to the differences in total glacier area scale. Additionally, the glacier volume changes in the Qilian Mountains based on empiricalTable formula 5. Glacier showedvolume changes a good results consistency used in this with study. that of in situ measurements. Volume Changes Segment Location Period dh/dt (m/a) Reference Table 5. Glacier volume changes results used(km in3/a) this study. Laohugou West region 1957–2007 −0.37 ± 0.11 −0.26Volume Changes Zhang et al. [40] SegmentGlacier Location No.12 Period dh/dt (m/a) Reference (km3/a) Central region Qiyi Glacier 1975–2010 −0.212 −0.18 Yao et al. [6] West region Laohugou Glacier No.12 1957–2007 −0.37 ± 0.11 −0.26 Zhang et al. [40] Central region Qiyi Glacier 1975–2010 −0.212 −0.18 Yao et al. [6] East region Lenglongling 1972–2007 −0.64 ± 0.29 −0.04 Cao [41] Water 2019, 11 FOR PEER REVIEW 12 Water 2019, 11, 623 12 of 16 East region Lenglongling 1972–2007 −0.64 ± 0.29 −0.04 Cao [41] Figure 11 shows a high consistency in the spatial pattern in the proportion of area and of volume Figure 11 shows a high consistency in the spatial pattern in the proportion of area and of changes between different basins. The glacier volume changes of all basins had an increased rate volume changes between different basins. The glacier volume changes of all basins had an increased in recent years, and were more pronounced in the eastern Qilian Mountains than in other regions. rate in recent years, and were more pronounced in the eastern Qilian Mountains than in other In addition, the Datong River basin (5J42) experienced the fastest decrease (10.32%/10a). regions. In addition, the Datong River basin (5J42) experienced the fastest decrease (10.32%/10a).

Figure 11. Volume change percent of glaciers in different basins from three inventories. Figure 11. Volume change percent of glaciers in different basins from three inventories. 4.3. Climatic Considerations 4.3. Climatic Considerations Water (precipitation), heat (air temperature), and their combination are the main climatic factorsWater affecting (precipitation), glacier development, heat (air temperature), and thus their and interannual their combination changes will are influence the main the climatic nature, factorsdevelopment affecting and glacier evolution development, of glaciers and [42 ].thus Generally, their interannual the glacier changes mass balance will influence is inversely the nature, related developmentto temperature and and evolution positively of correlatedglaciers [42]. with Generally, precipitation the glacier [43]. Studies mass balance of mass is balance inversely modeling related tohave temperature shown that and a 25%positively increase correlated in annual with precipitation precipitation is typically[43]. Studies needed of mass to compensate balance modeling for the haveglacier shown melting that caused a 25% by increase a 1 K warming in annual [44 precipitation]. In addition, is the typically response needed of glacial to compensate changes to climatefor the variationsglacier melting are not caused always by immediate,a 1 K warming and [44]. the time In addition, lag is generally the response several of decades glacial changes on average to climate [28]. variationsIn this are study, not always we discussed immediate, glacier and the changes time lag due is togenerally regional several climate decades using on the average monthly [28]. air temperatureIn this study, and precipitation we discussed data glacier from thechanges CRU due and meteorologicalto regional climate stations. using Over the themonthly past fiveair temperaturedecades, the and mean precipitation summer temperature data from inthe the CRU Qilian and Mountains meteorological has significantly stations. Over increased the past by five an decades,average rate the ofmean 0.36 ◦summerC/10a, whiletemperature annual totalin the precipitation Qilian Mountains also shows has an significantly upward trend increased in fluctuation by an averageby 9.61 mm/10a rate of (Figure0.36 °C/10a, 12). Both while temperature annual andtotal precipitation precipitation have also regionally shows varied,an upward and thetrend spatial in fluctuationarea change by distributions 9.61 mm/10a of (Figure these two 12). factors Both temperature are often similar and (Figureprecipitation 13). For have summer regionally temperature varied, andchanges, the spatial the rise area rate decreaseschange distributions gradually from of northwestthese two tofactors southeast, are whileoften thesimilar northern (Figure region 13). has For a summerfaster rate temperature of precipitation changes, change the in rise the rate Qilian decrease Mountains.s gradually from northwest to southeast, while the northernGlacier distributionregion has a andfaster shrinkage rate of pr overecipitation the past change decades in the had Qilian significant Mountains. regional differences. Overall, considering the increasing trend of temperature and precipitation with different extent in the Qilian Mountains, these reductions are probably because the increased precipitation may not be sufficient to compensate for the melting effect of increasing temperature. The increasing temperature rate after 2000 may be the main reason for the accelerating retreat of glaciers during 2010–2015. Climatic differences in altitude involve important issues relating to warming [6]. There are larger temperature increases at higher elevation over the Tibetan Plateau [45]; however, such an increase tendency disappears above 5000 m [46]. Our study also shows few glacier changes in regions above 5500 m, corresponding well with these results. In addition to these climatic factors, the distribution of the elevation of glacier terminus was also one of the key factors in influencing the glacial area

Water 2019, 11, 623 13 of 16 shrinkage, as the average minimum elevation increased from east (4413.41 m) to the west (4826.53 m).

WaterTherefore, 2019, 11 theFOR difference PEER REVIEW of glacier shrinkage rate in the eastern and western regions is likely to be13 relatedWater 2019 to, 11 the FOR difference PEER REVIEW of altitude. 13

Figure 12. Anomalies analysis of mean summer temperature (a) and annual total precipitation (b) in Figure 12. Anomalies analysis of mean summer temperature (a) and annualannual totaltotal precipitationprecipitation (b)) inin the Qilian Mountains. the Qilian Mountains.Mountains.

Figure 13. Changes in summer temperature (a) and total change percent in annual precipitation (b) Figure 13.13. Changes in summer temperature ( a) and total change percent in annual precipitation ( b) from 1960 to 2015. from 19601960 toto 2015.2015.

GlacierFurthermore, distribution others factorsand shrinkage are also importantover the past for glacier decades status, had includingsignificant size, regional type, condensation differences. Glacier distribution and shrinkage over the past decades had significant regional differences. Overall,levels, sublimation,considering the topography increasing of trend glaciers of temper or theirature combination and precipitation effect with [6], whichdifferent prevent extent usin Overall, considering the increasing trend of temperature and precipitation with different extent in thefrom Qilian undertaking Mountains, a quantitative these reductions analysis are probably of the relative because importance the increased of specific precipitation climate may factors not be on the Qilian Mountains, these reductions are probably because the increased precipitation may not be sufficientglacier changes. to compensate for the melting effect of increasing temperature. The increasing temperature sufficient to compensate for the melting effect of increasing temperature. The increasing temperature rate after 2000 may be the main reason for the accelerating retreat of glaciers during 2010–2015. rate after 2000 may be the main reason for the accelerating retreat of glaciers during 2010–2015. Climatic5. Conclusions differences in altitude involve important issues relating to warming [6]. There are larger Climatic differences in altitude involve important issues relating to warming [6]. There are larger temperature increases at higher elevation over the Tibetan 2Plateau [45]; however, such an increase3 temperatureThere were increases 2748 glaciers at higher covering elevation 1539.30 over ±the49.50 Tibetan km Plateauwith an [4 ice5]; volume however, of 81.69such ±an7.40 increase km tendency disappears above 5000 m [46]. Our study2 also shows few glacier changes in regions above intendency 2015, and disappears glaciers above in the 5000 range m of [46]. 2–5 Our km studycovered also theshows largest few areaglacier (23.8%). changes Glaciers in regions here above have 5500 m, corresponding well with these results. In addition to these climatic factors, the distribution experienced5500 m, corresponding continuous well and with accelerated these results. retreat In during addition this to period. these climatic The ice-covered factors, the area distribution decreased of the elevation2 of glacier terminus was also one of the key− factors1 in influencing the glacial area byof the 396.89 elevation km (20.5%) of glacier with terminus a mean shrinkage was also rateone ofof 0.47%the key a factors, and the in averageinfluencing volume the glacial decrease area of − shrinkage,glaciers was as 25.74the average km3 (0.47 minimum km3 a 1elevation); 446 glaciers increased (44.79 from km 2eastin area)(4413.41 disappeared, m) to the withwest the(4826.53 most shrinkage, as the average minimum elevation increased from east (4413.41 m) to the west (4826.53 m). Therefore, the difference of glacier shrinkage rate in the eastern and western regions−1 is likely to dramaticm). Therefore, glacier the shrinkage difference in of the glacier Shule shrinkage River basin rate (5J42) in the (change easternrates and western of 0.98% regions a ). In is addition, likely to be related to the difference of altitude. therebe related was ato spatial the difference gradient of across altitude. the mountains of glacier shrinkage rate, decreasing from the east Furthermore, others factors are also important for glacier status, including size, type, to west.Furthermore, others factors are also important for glacier status, including size, type, condensation levels, sublimation, topography of glaciers or their combination effect [6], which condensationThe highest levels, absolute sublimation, ice loss was topography observed between of glaciers 4700 or and their 4900 combination m (24.50%), and effect the percentage[6], which prevent us from undertaking a quantitative analysis of the relative importance of specific climate ofprevent glacier us area from change undertaking gradually a decreasedquantitative with analysis the increasing of the relative elevation. importance The absolute of specific area shrinkage climate factors on glacier changes. offactors glaciers on glacier with N changes. and NE aspects exceeded that of others. The long-term glacier wastage is mainly attributed to the significant increase in air temperature during the wet season, although precipitation, 5. Conclusions glacier5. Conclusions size, and local topographic parameters may have also influenced the nature of glacier retreat. WithThere the given were temperature2748 glaciers increase, covering the1539.30 ongoing ± 49.50 trend km of2 with accelerated an ice volume glacier of recession 81.69 ± 7.40 is likely km3 in to There were 2748 glaciers covering 1539.30 ± 49.50 km2 with an ice volume of 81.69 ± 7.40 km3 in 2015,continue. and Theglaciers inventory in the presented range of here2–5 km will2 allowcovered future the researchlargest area to focus (23.8%). on the Glaciers assessment here ofhave the 2015, and glaciers in the range of 2–5 km2 covered the largest area (23.8%). Glaciers here have experiencedtotal glacier volumecontinuous and and hydrologic accelerated modelling. retreat during this period. The ice-covered area decreased experienced continuous and accelerated retreat during this period. The ice-covered area decreased by 396.89 km2 (20.5%) with a mean shrinkage rate of 0.47% a−1, and the average volume decrease of by 396.89 km2 (20.5%) with a mean shrinkage rate of 0.47% a−1, and the average volume decrease of glaciers was 25.74 km3 (0.47 km3 a−1); 446 glaciers (44.79 km2 in area) disappeared, with the most glaciers was 25.74 km3 (0.47 km3 a−1); 446 glaciers (44.79 km2 in area) disappeared, with the most dramatic glacier shrinkage in the Shule River basin (5J42) (change rates of 0.98% a−1). In addition, dramatic glacier shrinkage in the Shule River basin (5J42) (change rates of 0.98% a−1). In addition, there was a spatial gradient across the mountains of glacier shrinkage rate, decreasing from the east there was a spatial gradient across the mountains of glacier shrinkage rate, decreasing from the east to west. to west.

Water 2019, 11, 623 14 of 16

Author Contributions: Conceptualization, J.H. and N.W.; methodology, J.H. and A.C.; software, J.H.; validation, X.Y.; formal analysis, J.H. and A.C.; investigation, X.Y.; resources, A.C.; data curation, J.H.; writing—original draft preparation, J.H.; writing—review and editing, N.W., X.Y. and T.H.; visualization, J.H. and T.H.; funding acquisition, N.W. and A.C. Funding: This research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (grant numbers XDA20060201, XDA19070302, and XDA20060700), and the National Natural Science Foundation of China (grant numbers 41801035, 41601080, and 41571076). Acknowledgments: We gratefully acknowledge NASA for the provision of remote-sensing data, as well as the Climatic Research Unit and the China Meteorological Administration for climate data. We thank the editors and anonymous reviewers for their constructive comments on this manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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