*2.2. Datasets* 2.2.1.ICESat-1Data

ICESat-1 is part of NASA's earth observation system and the first satellite with a lidar sensor to monitor the earth's elevation information. The GLAS onboard ICESat-1 satellite emits 40 Hz laser pulses vertically along the orbit and estimates the surface elevation from the round-trip time of the laser pulses. The vertical detection accuracy of the GLAS is less than 34 cm [44]. Its main scientific purpose is to observe changes in the glacier mass balance, cloud layer and vegetation features from 2003–2009. GLAS14 data is GLAS Level 2 altimetry data that includes the geographical location of the laser beam footprint and the correction parameters for elevation measurement. In this study, GLAS14 data was used to extract glacier elevation information at laser points, including elevation, longitude, latitude, ellipsoid correction parameters, etc. The data were downloaded from the National Snow & Ice Data Center (NSIDC) (https://nsidc.org/data/icesat, accessed on 1 1 September 2020).

### 2.2.2. ICESat-2 Data

ICESat-2 uses ATLAS to monitor the elevation information of the earth's surface [45]. Compared to the ICESat-1 GLAS, the working technology of the ICESat-2 ATLAS is greatly improved. The ATLAS emits six laser beams at a time, divided into three groups. Each group consists of a strong laser beam and a weak laser beam. The energy ratio of the strong laser beam to the weak laser beam is about 4:1. The smaller spatial sampling interval (~0.7 m) and higher frequency (10 kHz) [27] of the ATLAS allow it to collect denser data points for more effective monitoring of glacier elevation changes. For example, the data points observed by ATLAS in 2020 can occupy about 30,000 glaciers in the HMA region, which is about nine times as many as all data points observed by GLAS in 2003–2008 (Figure 2). In addition, the ATLAS has higher detection accuracy; Zhang et al. [46] found that the ICESat-2 data can extract glacier elevation in the Qilian Mountains with an accuracy of 0.08 m compared with the data of Unmanned Aerial Vehicle (UAV). In the Antarctic area, the accuracy can reach 1–2 cm [47]. Brunt et al. [48] used the Global Navigation Satellite Systems (GNSS) to verify that the accuracy of ICESat-2 data in extracting the elevation change of Antarctic ice sheet was better than 3 cm. In this study, the ICESat-2 ATL06 data were used, including elevation, time, latitude, longitude, confidence parameters, etc. The data were downloaded from the NSIDC (https://nsidc.org/data/icesat-2, accessed on 1 June 2021).

**Figure 2.** Comparison of observation capabilities by ICESat-1 and ICESat-2: the number of glaciers and the total area of glaciers in the HMA region observed by ICESat-1 data in 2003–2008 and by ICESat-2 data in 2020.

### 2.2.3. SRTM DEM

SRTM uses Synthetic Aperture Radar (SAR) technology to collect the earth's surface elevation data. It uses C-band Synthetic Aperture Radar (C-SAR) and X-band Synthetic Aperture Radar (X-SAR) to collect data of the earth's environment. The earth's surface elevation information from these data is converted to the height information specified in

the DEM and used to create an accurate earth map. The revisit period of the SRTM mission is 11 days. Since February 2000, the SRTM has used the phase difference between two SAR images to calculate the DEM information from 60◦N to 56◦S [23]. It obtains the topographic information of 80% of the earth's surface (except the Arctic and Antarctic) and 95% of the residential areas. In this study, the SRTM DEM from C-SAR data were used as the reference elevation to obtain the glacier surface elevation information of the HMA in 2000. The data were downloaded from the EARTHDATA platform (https://search.earthdata.nasa.gov/, accessed on 1 September 2020).
