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.