1. Introduction
Glacier mass balance plays a critical role in changes in the surrounding water resources, sea level rise, atmospheric circulation, etc., and is closely related to regional economic development and social stability [
1,
2,
3,
4]. The glaciers of the southeastern Tibetan Plateau (SETP) are located in the southeastern part of the Qinghai–Tibet Plateau, known as the “roof of the world”, and are an important water source for the Yarlung Zangbo River, Nujiang River, Ganges River, and Lancangjiang River. In recent decades, with the increase in global temperatures, coupled with the influence of the hot and humid Indian Ocean monsoon, SETP glaciers have been rapidly melting which has resulted in large-scale glacier mass loss [
3,
4,
5,
6]. Due to the lack of large-scale, high-resolution, and high-precision observations, the knowledge on the spatial distribution of the glacier mass balance and the response to climate change in this region is limited.
Glacier mass balance in the SETP has been a hotspot of glaciological research. The methods for glacier mass balance monitoring in the published literature include direct field surveys, modeling (e.g., energy balance), indirect estimations (e.g., from length and area surveys), and estimations using satellite altimetry or gravimetry. For the field surveys and modeling method, Yao et al. and Yang et al. used the energy mass balance method to obtain the mass balance of several glaciers, including the 24K glacier and Parlung No. 94 glacier in this region; the results showed that the glaciers were in a state of rapid melting [
7,
8]. For the indirect estimation method, measuring changes in glacier area and/or length can indicate changes in its mass balance. Pan et al., Liu et al., Wu et al., Ke et al., Fu et al., and Liu et al. monitored the glacier area changes of Hailuogou glacier, Buga Gangri Icefield, and Yalong glacier in the SETP using optical remote sensing data. According to their results, most glaciers have been in retreat in the recent decades [
9,
10,
11,
12,
13,
14]. However, all of the aforementioned methods, either through modeling or observations, can only indirectly indicate the changes in glacier mass balance. For the method of estimations using satellite altimetry, Gardner et al., Neckel et al., and Kääb et al. directly estimated glacier mass balance in the SETP by using ICESat satellite altimetry data from 2003 to 2009, and found that the rate of glacier surface elevation change was −0.30 ± 0.13 m/yr, −0.81 ± 0.32 m/yr, and −1.34 ± 0.29 m/yr, respectively [
15,
16,
17]. For the method of estimations using optical remote sensing, Gardelle et al. estimated the mass balance of glaciers in the SETP using SPOT-5 optical remote sensing stereo pairs from 2000 to 2011, yielding a glacier surface elevation change rate of −0.39 ± 0.16 m/yr [
5]. Brun et al. used ASTER optical stereo pairs data from 2000 to 2016 to estimate the glacier mass balance, obtaining a value of −0.73 ± 0.23 m/yr [
18]. Recently, Zhao et al. used ASTER stereo image pairs to obtain the mass balance of glaciers in the SETP during 2000–2020, yielding a glacier surface elevation change rate of −0.66 ± 0.02 m/yr [
19]. For the method of estimations using DEM products, Ke et al. used the 90 m resolution TanDEM-X DEM product released by the German Aerospace Center (DLR) to obtain glacier mass balance results from 2000 to 2013 for the central and eastern Nyainqentanglha, located in the SETP, and found a glacier mass balance rate of −4.11 ± 1.52 Gt/yr [
20]. Although satellite altimetry provides direct measurements about the changes in ice volume, several approximations are necessary to translate these observations into changes in glacier mass balance. Gravimetric surveys, on the other hand, measure changes in mass directly. Thus, after filtering for confounding factors, they can provide direct information on glacier mass balance. Song et al. estimated that the mass loss over the entire SETP was −5.99 ± 2.78 Gt/yr using GRACE satellite gravity data from 2003 to 2009 [
3].
The published results show that glaciers in the SETP have been in a state of strong ablation and retreat in recent decades. However, there are large differences between different satellite geodetic observations. Gardner et al., Neckel et al., and Kääb et al. all used ICESat satellite altimetry data, and their glacier mass balance estimates were significantly different [
15,
16,
17]. Although Gardelle et al. [
5] and Brun et al. [
18] both used optical remote sensing stereo image pairs, their estimated glacier mass balance results also exhibit some discrepancies. The glacier mass balance obtained by Ke et al. [
20] using the 90 m resolution TanDEM-X DEM product is also different from the above results. Due to the limitations of the traditional glaciological method and the large spread of estimations obtained by satellite geodetic monitoring technology, the results of the existing glacier mass balance estimations are highly controversial and uncertain.
Satellite InSAR (Interferometric Synthetic Aperture Radar) is one of the most effective earth observation technologies for large-scale topographic mapping, with high-resolution, all-day, and all-weather earth observation capabilities and large imaging area [
21]. The German Aerospace Center (DLR) and Astrium GmbH jointly implemented the TanDEM-X science program, and on 21 June 2010, DLR successfully launched the TanDEM-X (TDX) satellite, which operates in a two-satellite constellation mode with the previous TerraSAR-X (TSX) satellite, constituting a bi-static interferometry system [
22]. Because the bi-static observation model can effectively eliminate the effects of atmospheric disturbance and temporal incoherence, this technology has great advantages and broad application prospects in the field of high-resolution and high-precision DEM mapping on a global scale [
23,
24]. The bi-static InSAR technique has successfully obtained glacier mass balance data in the Tien Shan, Nyainqentanglha Mountains, Karakoram, Himalayas, Puruogangri Icefield, western Qilian Mountains, Geladandong Glacier, Patagonia Icefield, and Alpine Mountain Range, demonstrating its effectiveness in monitoring valley glaciers [
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37].
In this paper, we investigate the mass balance of glaciers in the SETP, using bi-static InSAR detection and inversion methods. We also investigate non-local mean filter and the multi-source systematic error correction methods to enhance the accuracy of our observations. Using TanDEM-X bi-static InSAR data and historical topographic data, we obtained glacier mass balance results for the SETP. The spatial distribution of the glacier mass balance in relation to climatic data was also investigated to reveal the response of glaciers to climate change in this region.
4. Discussion
The study area is mainly affected by the Indian Ocean monsoon, and most of the glaciers belong to marine-type glaciers. Compared with continental-type glaciers, these glaciers are characterized by high accumulation and high melting, and are very sensitive to climate change. In recent decades, due to global warming, the glaciers in this region have rapidly retreated. However, the precise interaction of the glaciers and the changing monsoon patterns is still unclear.
Figure 11 shows the spatial distribution of near-surface temperature changes in the SETP from 2000 to 2014; the annual rate of temperature change in the southeastern part of the SETP showed a strong increasing trend, while the northwestern part of the SETP had a smaller increasing trend. Some lower-lying regions even exhibited a cooling trend. This pattern is consistent with the spatial distribution of the glacier mass loss rate in the study area which increased towards the SE. In addition, we selected three watersheds, 5O282B, 5O282A, and 5O281B, in the southeast–northwest direction with glacier surface elevation change rates of −0.841 ± 0.005 m/yr, −0.577 ± 0.005 m/yr, and −0.556 ± 0.005 m/yr, respectively, for quantitative comparison. Through the above comparison and analysis, we found that there was a positive correlation between temperature change and glacier mass loss rate in the SETP region.
The spatial distribution of surface precipitation changes shows that precipitation in the southeastern part of the SETP was increasing, while precipitation in the northwestern part of SETP was decreasing (
Figure 12). This is contrary to the observed spatial distribution of glacier mass balance and suggests that the key climatic factor behind glacier retreat in this region is the changing temperature.
According to the spatial distribution of temperature and precipitation changes, glaciers of the SETP are melting more rapidly as a seasonal trend, i.e., strong summer ablation due to the monsoon influence, which, in normal (i.e., equilibrium) circumstances, is counterbalanced by heavy accumulation in the winter. This seasonal equilibrium is broken and glaciers lose mass all over the region. This appears to be more pronounced in the SE, which means that ablation was increasing and/or accumulation was decreasing more here compared to other regions of the SETP.
Kääb et al., Neckel et al., and Gardner et al. estimated the glacier mass balance in the SETP using satellite altimetry ICESat; their estimated annual glacier elevation change rates were −1.34 ± 0.29 m/yr, −0.81 ± 0.32 m/yr, and −0.30 ± 0.13 m/yr, respectively [
15,
16,
17] (
Table 7). The estimate in this paper was −0.505 ± 0.005 m/yr, which is consistent with the results of Neckel et al. [
16] and Gardner et al. [
17] Some of this discrepancy might be attributed to the different observational periods, i.e., ICESat covers the period between 2003 and 2009, while our results were obtained for 2000–2014. We also used a different methodology from these authors. Although the accuracy of satellite altimetry is high (±0.2 m in flat areas), these measurements are point-like, with a low resolution for a few or single glaciers.
Gardelle et al. estimated a mass balance of −0.39 ± 0.16 m/yr for the glaciers in the SETP between 2000 and 2011 using SPOT-5 optical stereo pairs data and a C-band SRTM DEM [
5]. This estimation aligns with our calculations fairly well, though a slight discrepancy still persists. We propose that the different spatial coverage of the Gardelle et al. [
5], which only covered about 1/3 of the SETP, could explain this. Brun et al. estimated an average mass balance of −0.73 ± 0.23 m/yr for the SETP using ASTER optical stereo image pair data from 2000 and 2016 [
18]. Recently, Zhao et al. also used ASTER stereo image pairs and satellite altimetry data to obtain a similar estimation: −0.66 ± 0.02 m/yr [
19]. ASTER optical data are affected by “light saturation”, which prevents us from obtaining glacier elevation change data within the accumulation area, which could contribute to the higher mass loss estimations of these studies.
Neckel et al. used TanDEM-X bi-static InSAR and a C-band SRTM DEM to obtain the surface elevation changes from 2000 to 2014 for five glaciers in Unknown I, Zepu, Unknown II, Maguolong, and Guanxing in the SETP, which were −0.50 ± 0.57 m/yr, −0.58 ± 0.57 m/yr, −0.67 ± 0.57 m/yr, −1.19 ± 0.57 m/yr, and −1.40 ± 0.57 m/yr, respectively [
44]. The results for the first three glaciers are similar to our regional average mass balance estimations, while the last two results are relatively large. However, Neckel et al. provided data for individual glacier which might deviate significantly from the regional average. Wu et al. and Li et al. also utilized bi-static InSAR data to obtain glacier mass balance results for certain sub-regions within the SETP such as the Kangri Karpo Range and the Nyainqentanglha [
27,
35,
36,
45].
5. Conclusions
In this paper, a TanDEM-X bi-static InSAR algorithm was proposed to calculate glacier surface elevation changes, and we used the algorithm of glacier mass balance estimation to obtain the glacier mass loss in the SETP. We added the non-local mean filtering method and phase difference strategy to the TanDEM-X bi-static InSAR algorithm to facilitate the suppression of phase noise and to preserve texture details, and to solve the problems of unwrapping phase jump and discontinuity, and the presence of dense interferometric fringes which all hinder the traditional InSAR method. The approach followed by this study improves the accuracy and reliability of glacier topography and glacier surface elevation change data. The correction of systematic errors such as seasonality and radar penetration differences were carried out, which improved the accuracy of the glacier mass balance estimation. In addition, the uncertainty of the glacier surface elevation change, the uncertainty of the seasonal systematic error, and the uncertainty of the radar penetration difference error were evaluated.
We used 13 pairs of TanDEM-X bi-static InSAR images to estimate the glacier surface elevation change for the entire SETP, where two pairs of bi-static InSAR images covering the Parlung No. 4 glacier were used for seasonal systematic error correction, and the X-band and C-band SRTM DEMs covering the study area were used for radar penetration difference systematic error correction; the values were −2.295 m and 1.908 m, respectively. A regional average glacier surface elevation change of −0.505 ± 0.005 m/yr was obtained. Based on this, the regional average net mass balance was −454.5 ± 13.1 mm w.eq. during the observational period. Compared with published results obtained by satellite altimetry, optical stereo image pairing, and bi-static InSAR, the glacier mass balance estimation reported in this paper is moderate. We also analyzed the spatial distribution of the glacier mass balance within the SETP using the sub-watershed analysis method. The results showed that the mass loss rate of glaciers in the SETP was larger in the southeast than in the northwest. We used temperature and precipitation data to analyze the climatic factors affecting the glacier mass balance. The results showed that the spatial distribution of glacier mass balance was consistent with the spatial distribution of temperature, i.e., increased temperature in the southeastern part of the study area, and a slight decrease in temperature in the northwestern part of the study area. The results from this study contribute to our understanding of glacier dynamics in this region and can also provide scientific support for major national goals such as the rational utilization of water resources and construction of important infrastructure projects.