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Snow and ice melting in the Upper Indus Basin (UIB) is crucial for regional water availability for mountainous communities. We analyzed glacier changes in the Astak catchment, UIB, from 2000 to 2020 using remote sensing techniques based on optical satellite images from Landsat and ASTER digital elevation models. We used a surface feature-tracking technique to estimate glacier velocity. To assess the impact of climate variations, we examined temperature and precipitation anomalies using ERA5 Land climate data. Over the past two decades, the Astak catchment experienced a slight decrease in glacier area (−1.8 km2) and the overall specific mass balance was −0.02 ± 0.1 m w.e. a−1. The most negative mass balance of −0.09 ± 0.06 m w.e. a−1 occurred at elevations between 2810 to 3220 m a.s.l., with a lesser rate of −0.015 ± 0.12 m w.e. a−1 above 5500 m a.s.l. This variation in glacier mass balance can be attributed to temperature and precipitation gradients, as well as debris cover. Recent glacier mass loss can be linked to seasonal temperature anomalies at higher elevations during winter and autumn. Given the reliance of mountain populations on glacier melt, seasonal temperature trends can disturb water security and the well-being of dependent communities. [ABSTRACT FROM AUTHOR]

Long‐term monitoring of ice‐dammed lakes is important for understanding the process of lake storage, development, and glacial outburst flood (GLOF). Based on remote sensing observations for the period 1972–2020, we obtained what is so far the most detailed reconstruction of the filling and draining of Kyagar Lake, a typical annually ice‐dammed lake in the north Karakoram. We show that annually repeated lake fillings‐drainings were detected by satellite observations during two periods: 1996–2009 and 2015–2020, while between 2009 and 2015 no obvious cycle occurred. These two intermitted periods with regular annual revival of lake outburst were likely related to the long‐term cycle of glacier surge dynamics, namely two remarkable surge‐induced terminus accelerations during 1995–1997 and 2014–2016. Following each surge, the maximum lake areas decreased by about 33% from 1996 to 2009 and by about 88% from 2015 to 2020. Combined with climatic warming and the thinning of the ice‐dam, the storage capacity of Kyagar Lake will likely continue to decrease in the future, and the risk of lake outburst flood thus becomes lower. Repeated drainage following filling exacerbates the depletion of mass at the glacier terminal, leading to the cessation of storage and creating the conditions for the next glacial surge to occur, creating a long‐term iteration of the surge process. Considering the uncertainty and intensified climate changes, it suggests that a close monitoring of the glacial lake development and glacier dynamics would be still important. [ABSTRACT FROM AUTHOR]

Multi-temporal glacier inventories provide key information about the glaciers, their characteristics, and changes and are inevitable for glacier modelling and investigating geodetic mass changes. However, to date, no consistent multi-temporal glacier inventory for the whole of the Karakoram exists, negatively affecting the monitoring of spatio-temporal variations in glaciers' geometric parameters and their related applications. We used a semi-automatic method combining automatic segmentation and manual correction and produced a multi-temporal Karakoram glacier inventory (KGI) compiled from Landsat TM/ETM + /OLI (Thematic Mapper, Enhanced Thematic Mapper Plus, and Operational Land Imager) images for the 1990s, 2000s, 2010s, and 2020s. Our assessments using independent multiple digitisation of 37 glaciers show that the KGI is sufficiently accurate, with an overall uncertainty of ±3.68 %. We also performed uncertainty evaluation for the contiguous glacier polygons using a buffer of half a pixel, which resulted in an average mapping uncertainty of ±5.21 %. We calculated more than 20 attributes for each glacier, including coordinates, area, supraglacial debris area, date information, and topographic parameters derived from the ASTER GDEM (Advanced Spaceborne Thermal Emission and Reflection Radiometer global digital elevation model). According to KGI-2020s, approximately 10 500 alpine glaciers (>0.01 km 2 each) cover an area of 22510±828 km 2 of which 10.18±0.38 % (2290±84 km 2) is covered by supraglacial debris. Over the past 3 decades, the glaciers experienced a loss of clean ice and/or snow area but a gain in supraglacial debris. Supraglacial debris cover has increased by 17.63±1.44 % (343.30±27.95 km 2), while non-debris-covered glaciers decreased by 1.56±0.24 % (319.85±49.92 km 2). The total glacier area was relatively stable and showed only a slight insignificant increase of 23.45±28.85 km 2 (0.10±0.13 %). The glacier area has declined by 3.27±0.24 % in the eastern Karakoram, while the glacier area slightly increased in central (0.65±0.10 %) and western Karakoram (1.26±0.11 %). Supraglacial debris has increased over the whole of Karakoram, especially in areas above 4200 m a.s.l. (above sea level), showing an upward shift. The glacier area changes were characterised by strong spatial heterogeneity, influenced by surging and advancing glaciers. However, due to global warming, the glaciers are on average retreating. This is in particular true for small and debris-free glaciers. The multi-temporal KGI data are available at the National Cryosphere Desert Data Center of China: 10.12072/ncdc.glacier.db2386.2022 (F. Xie et al., 2022). [ABSTRACT FROM AUTHOR]

On 5th September 2022, a magnitude Ms-6.8 earthquake occurred nearby Mt. Gongga, western Sichuan. The stability of the glaciers in east Mt. Gongga close to the epicenter was widely concerned due to the strong shake triggered by the earthquake. Using multi-source observations (including in-situ photographs, remote sensing datasets before and after the event), we carried out a preliminary assessment of the stability and hazard risks of the Hailuogou (HLG) glacier. Triggered by the earthquake, a small block of fractured ice at the lowest part of icefall collapsed. The magnitude of the coseismic ice avalanche was relatively small, which is comparable in size to most ice avalanches over the past seven years, but much less than the previous mapped largest one (03 April 2018, runout ∼699 m). One most recent large (runout ∼608 m) ice avalanche occurred between 01 and 04 September, just before the earthquake, likely unloaded large amount of ice mass and made a larger ice avalanche avoided during the earthquake shake. Nevertheless, the momentum of collapsed snow-ice-rock mass could be safely unloaded over a wide and gentler-slope ice tongue area, limiting its mobility and the risk of a cascading hazard. Glacier-wide surface flow dynamics monitored by Sentinel-1 satellite SAR time series (12 September 2021–19 September 2022) show that HLG glacier velocity was generally consistent before and after the earthquake. The entire HLG glacier exhibited more stable than expected, with almost no abnormal features detected in its upper accumulation part, the lower ice tongue, and its lateral paraglacial slopes. Since the glacier valley has experienced remarkable downwasting and the paraglacial environment has been strongly disturbed and destabilized, we suggested that, to efficiently evaluate glacier-related cascading hazard risks, it is also necessary to systematically combine multi-source observations (e.g., high-resolution UAV survey, radar/Lidar scan, ground investigation, monitoring and warning systems) to continuously monitor the regional glacier anomalies in the post-earthquake seismic active areas. [ABSTRACT FROM AUTHOR]

Most glaciers on the Tibetan Plateau have experienced continuous mass losses in response to global warming. However, the seasonal dynamics of glaciers on the southeastern Tibetan Plateau have rarely been reported in terms of glacier surface elevation and velocity. This paper presents a first attempt to explore the seasonal dynamics of the debris-covered Dagongba Glacier within the southeastern Tibetan Plateau. We use the multitemporal unoccupied aerial vehicle images collected over the lower ablation zone on 8 June and 17 October 2018, and 13 May 2019, and then perform an analysis concerning climatic fluctuations. The results reveal that the mean surface elevation decrease of the Dagongba Glacier during the warm season ($2.81\pm 0.44$ m) was remarkably higher than the cold season ($0.72\pm 0.45$ m). Particularly notable glacier surface elevation changes were found around supraglacial lakes and ice cliffs where ice ablation rates were $\sim$ 3 times higher than the average. In addition, a larger longitudinal decline of glacier surface velocity was observed in the warm season than that in the cold season. In terms of further comparative analysis, the Dagongba Glacier experienced a decrease in surface velocity between 1982–83 and 2018–19, with a decrease in the warm season possibly twice as large as that in the cold season. [ABSTRACT FROM AUTHOR]

The Indian and East Asian summer monsoons shape the melt and accumulation patterns of glaciers in High Mountain Asia in complex ways due to the interaction of persistent cloud cover, large temperature ranges, high atmospheric water content and high precipitation rates. Glacier energy- and mass-balance modelling using in situ measurements offers insights into the ways in which surface processes are shaped by climatic regimes. In this study, we use a full energy- and mass-balance model and seven on-glacier automatic weather station datasets from different parts of the Central and Eastern Himalaya to investigate how monsoon conditions influence the glacier surface energy and mass balance. In particular, we look at how debris-covered and debris-free glaciers respond differently to monsoonal conditions. The radiation budget primarily controls the melt of clean-ice glaciers, but turbulent fluxes play an important role in modulating the melt energy on debris-covered glaciers. The sensible heat flux decreases during core monsoon, but the latent heat flux cools the surface due to evaporation of liquid water. This interplay of radiative and turbulent fluxes causes debris-covered glacier melt rates to stay almost constant through the different phases of the monsoon. Ice melt under thin debris, on the other hand, is amplified by both the dark surface and the turbulent fluxes, which intensify melt during monsoon through surface heating and condensation. Pre-monsoon snow cover can considerably delay melt onset and have a strong impact on the seasonal mass balance. Intermittent monsoon snow cover lowers the melt rates at high elevation. This work is fundamental to the understanding of the present and future Himalayan cryosphere and water budget, while informing and motivating further glacier- and catchment-scale research using process-based models. [ABSTRACT FROM AUTHOR]

Mapping the outlines of glaciers has primarily relied on the interpretation of satellite optical images. However, the accurate delineation of glaciers in complex terrain mountain regions remains challenging, mainly because the supraglacial debris-covered ablation zones and snow-covered accumulation zones often exhibit the same spectral properties as their adjacent grounds in optical images. This study presents a novel approach by exploring both the satellite synthetic aperture radar (SAR) amplitude and interferometric coherence to map mountain glaciers. This method explores the deviation of the glacier surface signal in the SAR time series to distinguish glacier ice from the surrounding stable ground. To this end, we explored the classifying capabilities of two indices from a set of SAR images, SAR interferometric coherence and amplitude deviation index (ADI), to determine glacier boundary. We found that the two indices complement each other for mapping glaciers. A ratio map based on ADI and SAR coherence (ACR) was then derived, from which the glacier outline was automatically tracked using a specified threshold, followed by manual modification. We validated this approach on two typical valley glaciers, the debris-covered Hailuogou Glacier and debris-free Mozigou Glacier, in Mount Gongga in the southeastern Tibetan Plateau. The results show that the proposed ACR criteria can significantly enhance the contrast between glaciers and their surroundings. By comparing our results with manually delineated glacier outlines from high-resolution cloud-free satellite optical imagery, we found that the misclassification rate and difference rate for our results were 2.6% and 4.2%, respectively. The approach presented in this study can be easily adapted to map the outlines of mountain glaciers worldwide efficiently and is useful for inferring glacier boundary changes in a climate warming context. [ABSTRACT FROM AUTHOR]

Physical erosion and chemical weathering rates beneath glaciers are expected to increase in a warming climate with enhanced melting but are poorly constrained. We present a global dataset of cations in meltwaters of 77 glaciers, including new data from 19 Asian glaciers. Our study shows that contemporary cation denudation rates (CDRs) beneath glaciers (2174 ± 977 Σ*meq+ m−2 year−1) are ~3 times higher than two decades ago, up to 10 times higher than ice sheet catchments (~150-2000 Σ*meq+ m−2 year−1), up to 50 times higher than whole ice sheet means (~30-45 Σ*meq+ m−2 year−1) and ~4 times higher than major non-glacial riverine means (~500 Σ*meq+ m−2 year−1). Glacial CDRs are positively correlated with air temperature, suggesting glacial chemical weathering yields are likely to increase in future. Our findings highlight that chemical weathering beneath glaciers is more intense than many other terrestrial systems and may become increasingly important for regional biogeochemical cycles. Global glacial chemical denudation is one of the largest contributors to global elemental cycles and, amplified by climate warming, will significantly impact nutrient loads in downstream ecosystems. [ABSTRACT FROM AUTHOR]

Topographic development via paraglacial slope failure (PSF) represents a complex interplay between geological structure, climate, and glacial denudation. Southeastern Tibet has experienced amongst the highest rates of ice mass loss in High Mountain Asia in recent decades, but few studies have focused on the implications of this mass loss on the stability of paraglacial slopes. We used repeat satellite- and unpiloted aerial vehicle (UAV)-derived imagery between 1990 and 2020 as the basis for mapping PSFs from slopes adjacent to Hailuogou Glacier (HLG), a 5 km long monsoon temperate valley glacier in the Mt. Gongga region. We observed recent lowering of the glacier tongue surface at rates of up to 0.88 m a -1 in the period 2000 to 2016, whilst overall paraglacial bare ground area (PBGA) on glacier-adjacent slopes increased from 0.31 ± 0.27 km 2 in 1990 to 1.38 ± 0.06 km 2 in 2020. Decadal PBGA expansion rates were ∼ 0.01 km 2 a -1 , 0.02 km 2 a -1 , and 0.08 km 2 in the periods 1990–2000, 2000–2011, and 2011–2020 respectively, indicating an increasing rate of expansion of PBGA. Three types of PSFs, including rockfalls, sediment-mantled slope slides, and headward gully erosion, were mapped, with a total area of 0.75 ± 0.03 km 2 in 2020. South-facing valley slopes (true left of the glacier) exhibited more destabilization (56 % of the total PSF area) than north-facing (true right) valley slopes (44 % of the total PSF area). Deformation of sediment-mantled moraine slopes (mean 1.65–2.63 ± 0.04 cm d -1) and an increase in erosion activity in ice-marginal tributary valleys caused by a drop in local base level (gully headward erosion rates are 0.76–3.39 cm d -1) have occurred in tandem with recent glacier downwasting. We also observe deformation of glacier ice, possibly driven by destabilization of lateral moraine, as has been reported in other deglaciating mountain glacier catchments. The formation, evolution, and future trajectory of PSFs at HLG (as well as other monsoon-dominated deglaciating mountain areas) are related to glacial history, including recent rapid downwasting leading to the exposure of steep, unstable bedrock and moraine slopes, and climatic conditions that promote slope instability, such as very high seasonal precipitation and seasonal temperature fluctuations that are conducive to freeze–thaw and ice segregation processes. [ABSTRACT FROM AUTHOR]

Detachments of large parts of low-angle mountain glaciers in recent years have raised great attention due to their threats to lives and properties downstream. While current studies have mainly focused on post-event analysis, a few opportunities have presented themselves to assess the potential hazards of a glacier prone to detachment. Here we present a comprehensive analysis of the dynamics and runout hazard of a low-angle (∼20 ∘) valley glacier, close to the Qinghai–Tibet railway and highway, in the East Kunlun Mountains on the Qinghai–Tibet Plateau. The changes in morphology, terminus position, and surface elevation of the glacier between 1975 and 2021 were characterized with a stereo-image pair from the historical KH-9 spy satellite, six digital elevation models (DEMs), and 11 high-resolution images from Planet Labs. The surface flow velocities of the glacier tongue between 2009 and 2020 were also tracked based on cross-correlation of Planet images. Our observations show that the glacier snout has been progressively advancing in the past 4 decades, with a stepwise increase in advance velocity from 4.55±0.46ma-1 between 1975 and 2009 to 30.88±2.36ma-1 between 2015 and 2020. DEM differencing confirms the glacial advance, with surface thinning in the source region and thickening in the tongue. The net volume loss over the glacier tongue was about 11.21±2.66×105 m3 during 1975–2018. Image cross-correlation reveals that the surface flow velocity of the glacier tongue has been increasing in recent years, with the mean velocity below 4800 m more than tripling from 6.3±1.8ma-1 during 2009–2010 to 22.3±3.2ma-1 during 2019–2020. With a combined analysis of the geomorphic, climatic, and hydrologic conditions of the glacier, we suggest that the flow of the glacier tongue is mainly controlled by the glacier geometry, while the presence of an ice-dammed lake and a supraglacial pond implies a hydrological influence as well. Taking the whole glacier and glacier tongue as two endmember avalanche sources, we assessed the potential runout distances of these two scenarios using the angle of reach and the Voellmy–Salm avalanche model. The assessments show that the avalanche of the whole glacier would easily travel a distance that would threaten the safety of the railway. In contrast, the detachment of the glacier tongue would threaten the railway only with a small angle of reach or when employing a low-friction parameter in the Voellmy–Salm modeling. [ABSTRACT FROM AUTHOR]

Collapses of large parts of low-angle mountain glaciers in recent years have raised great attention due to their threats to lives and properties downstream. While current studies have mainly focused on post-event analysis, assessing the potential hazard of glaciers prone to collapse is rare. Here we presented a comprehensive analysis of the dynamics and runout hazard of a low-angle (~ 20°) valley glacier, close to the Qinghai-Tibet railway and highway, in the Kunlun Pass of East Kunlun Mountains on the Qinghai-Tibet Plateau. The changes in morphology, terminus position, and surface elevation of the glacier between 1975 and 2019 were characterized with multi-sensor remote sensing data including a stereo-image pair from the historical KH-9 spy satellite, six Digital Elevation Models (DEMs), and nine high-resolution images from Planet Labs. The surface flow velocities of the glacier tongue between 2009 and 2019 were also tracked based on cross-correlation of Planet images. Our observations show that the glacier snout has been progressively advancing in recent four decades, with a stepwise increase of advance velocity from 4.25 ± 0.28 m a-1 between 1975 and 2009 to 32.53 ± 4.45 m a-1 between 2015 and 2019. DEM differencing confirms the glacial advance, with surface thinning in the source region and thickening in the tongue region. The net volume loss over the glacier tongue was about 11.21 ± 2.66 × 105 m3 during 1975-2019. Image cross-correlation reveals that the surface flow velocity of the glacier tongue has been increasing in recent years, with the mean velocity below 4800 m almost trebled from 22 ± 4 cm a-1 during 2009-2012 to 61 ± 5 cm a-1 during 2016-2019. Piecing these observations together, we suggest that the flow of the glacier tongue is mainly controlled by the geometry of the glacier, while the presence of an ice-dammed lake and a supraglacial pond implies a hydrological influence as well. Taking the glacier tongue as an avalanche source, we quantitively simulated the potential runout distance using the Voellmy-Salm avalanche model. The simulations predict that the avalanche of the glacier tongue will result in a maximum runout distance of about 1.3 km with moderate friction parameters, unlikely to threaten the safety of the Qinghai-Tibet railway. [ABSTRACT FROM AUTHOR]

Influenced by the Indian monsoon, the Kangri Karpo Mountains (KKM) of the southeastern Tibetan Plateau is the most humid part of the plateau, and one of the most important regions with numerous monsoon temperate glaciers. Glacier mass balance estimates have been strongly negative in the KKM over recent decades, but the spatiotemporal characteristics of surface velocity are poorly understood. Using phase-correlation feature tracking on Landsat images, this study estimates spatiotemporal variabilities of monsoon temperate glaciers for the period of 1988–2019. Results show that a significant slowdown was observed below an elevation of 4900 m, while an accelerated ice flow was found at an elevation of 4900–5800 m over the past 30 years. The trend of slowdown was −0.1 m a−1 dec−1 during 1988–2000, and then it increased to −0.5 m a−1 dec−1 during 2001–2019. [ABSTRACT FROM AUTHOR]

Meltwater habitats are found on glacier surfaces worldwide, but much of the current understanding of these habitats comes from clean glaciers sprinkled with cryoconite. Cryoconite is windblown dust particles covered with biological material that form small meltwater holes due to difference in the albedo of cryoconite compared to clean ice. However, no information is available on the physical, chemical, and biological characteristics of supraglacial pools on debris-covered glaciers. We measured physical and chemical variables and sampled macroinvertebrates from forty-six supraglacial pools during the summers of 2018 and 2019 on the debris-covered Hailuogou Glacier in southeastern Tibet. Our physical and chemical results indicated that the sampled supraglacial pools exhibited a greater diversity of shapes, were larger, contained larger substrate sizes, and had greater substrate diversity than cryoconite holes. The sampled supraglacial pools frequently contained Chironomidae (Diptera) and Isotomidae (Collembola), which are macroinvertebrate taxa that are uncommon in cryoconite holes. Chironomidae occurrence and abundance was not correlated with any measured environmental variable. The best predictors of Isotomidae and macroinvertebrate occurrence and abundance were conductivity, ice to water surface depth, and number of supraglacial pools within 5 m. Our results highlight the uniqueness of supraglacial pools on a debris-covered glacier in southeastern Tibet. [ABSTRACT FROM AUTHOR]

Systematic differences in glacier mass balance response to climate warming are apparent in the Tianshan Mountains, which are primarily caused by different climatic regimes and glacier hypsography. Combined mass balance data of nine monitored glaciers in the Tianshan Mountains shows that most glaciers accelerated their mass losing rate since 1970s (averaged from −24.6 mm w.e. a in 1957-1970 to −444.6 mm w.e. a in 1971-2009), but also exhibiting discrepancy and consistency during the second half of the twentieth century. To see their climatic-mass balance relationships, we employ a simple temperature index mass balance model on five well monitored glaciers in Tianshan. The model is calibrated by the observed annual, summer and winter mass balance data over the period of 1957-1980 and validated over 1981-2002. A comparison of modeled and measured annual mass balance yields an overall standard deviation of 0.465 m w.e. during the period of model runs. The calibrated mass balance model is also used to perform sensitivity experiments, which indicates the significant differences of individual glaciers in response to climate changes. This study, for the first time, tests a temperature index mass balance model on the selected observed glaciers in the Tianshan Mountains. Although there exists considerable uncertainties, we propose its potential possibility of improvement and applicability for regional glacier mass balance reconstructions and future predictions. [ABSTRACT FROM AUTHOR]

Glacial lake outburst flood (GLOF) is a serious hazard in high, mountainous regions. In the Himalayas, catastrophic risks of GLOFs have increased in recent years because most Himalayan glaciers have experienced remarkable downwasting under a warming climate. However, current knowledge about the distribution and recent changes in glacial lakes within the central Himalaya mountain range is still limited. Here, we conducted a systematic investigation of the glacial lakes within the entire central Himalaya range by using an object-oriented image processing method based on the Landsat Thematic Mapper (TM) or Enhanced Thematic Mapper (ETM) images from 1990 to 2010. We extracted the lake boundaries for four time points (1990, 2000, 2005 and 2010) and used a time series inspection method combined with a consistent spatial resolution of Landsat images that consistently revealed lake expansion. Our results show that the glacial lakes expanded rapidly by 17.11% from 1990 to 2010. The pre-existing, larger glacial lakes, rather than the newly formed lakes, contributed most to the areal expansion. The greatest expansions occurred at the altitudinal zones between 4800 m and 5600 m at the north side of the main Himalayan range and between 4500 m and 5600 m at the south side, respectively. Based on the expansion rate, area and type of glacial lakes, we identified 67 rapidly expanding glacial lakes in the central Himalayan region that need to be closely monitored in the future. The warming and increasing amounts of light-absorbing constituents of snow and ice could have accelerated the melting that directly affected the glacial lake expansion. Across the main central Himalayas, glacial lakes at the north side show more remarkable expansion than those at the south side. An effective monitoring and warning system for critical glacial lakes is urgently needed. [ABSTRACT FROM AUTHOR]

The article presents a study which showed the decrease in the area and the resulting increase in river water runoff into local water catchments by the monsoon temperate Hailuogou glacier. Located on the east slope of Mount Gogga, China, the glacier had shown a decrease of 3.5 percent in surface area between the years of 1966 and 2007. However, in spite of the increased glacier water runoff, a monthly study of catchment water-balance calculated for the period between 1994 and 2005 showed a loss in glacier water storage especially in a non-glacierized but forested subcatchment. Both the loss in glacier mass and river runoff during the period in which the study was conducted were attributed to air-temperature rise between 1988 and 2005.

Climate warming and concomitant glacier recession in the High Mountain Asia (HMA) have led to widespread development and expansion of glacial lakes, which reserved the freshwater resource, but also may increase risks of glacial lake outburst floods (GLOFs) or debris floods. Using 46 moderate- and high-resolution satellite images, including declassified Keyhole and Landsat missions between 1964 and 2020, we provide a comprehensive area mapping of glaciers and glacial lakes in the Tama Koshi (Rongxer) basin, a highly glacierized China-Nepal transnational catchment in the central Himalayas with high potential risks of glacier-related hazards. Results show that the 329.2 ± 1.9 km2 total area of 271 glaciers in the region has decreased by 26.2 ± 3.2 km2 in the past 56 years. During 2000–2016, remarkable ice mass loss caused the mean glacier surface elevation to decrease with a rate of −0.63 m a−1, and the mean glacier surface velocity slowed by ~25% between 1999 and 2015. The total area of glacial lakes increased by 9.2 ± 0.4 km2 (~180%) from 5.1 ± 0.1 km2 in 1964 to 14.4 ± 0.3 km2 in 2020, while ice-contacted proglacial lakes have a much higher expansion rate (~204%). Large-scale glacial lakes are developed preferentially and experienced rapid expansion on the east side of the basin, suggesting that in addition to climate warming, the glacial geomorphological characters (aspect and slope) are also key controlling factors of the lake growing process. We hypothesize that lake expansion will continue in some cases until critical local topography (i.e., steepening icefall) is reached, but the lake number may not necessarily increase. Further monitoring should be focused on eight rapidly expanding proglacial lakes due to their high potential risks of failure and relatively high lake volumes. [ABSTRACT FROM AUTHOR]

• Periodic accelerations of a lake-terminating glacier are linked to rapid frontal retreats. • Proglacial lake-ice interactions can greatly modify the ice flow regime. • Recent terminus acceleration has likely induced glacier upstream dynamic thinning. • In contrast, a neighboring land-terminating glacier shows on-going deceleration. Most lake-terminating glaciers in the Himalaya retreat rapidly due to periodic frontal ice loss at their terminus, but long-term observations are still limited regarding their flow dynamics, which is crucial for understanding the processes of ice mass loss and proglacial lake growth. We present multi-decadal surface velocity dynamics of the Longbasaba Glacier, a rapid retreating lake-terminating glacier in the Chinese Himalaya, using an image feature tracking method applied on optical satellite images between 1989 and 2018. We show that, in companion with rapid retreat (−51.7 m a−1), its lower 5 km tongue experienced high interannual fluctuations in velocity, comprising periodic acceleration and slowdown in 1989-1995 and 2001-2010 and a recent remarkable acceleration since 2012. The temporal variation of longitudinal velocity distribution indicates an upward propagation of the lake-ward acceleration (namely a downglacier inversion of strain from compression to extension). This propagation is coupled to the retreat of the glacier front and occurs along the lowermost 1∼1.5 km lake-adjacent section as the proglacial lake expands. The most recent acceleration of the near-lake section since 2012 has likely facilitated a dynamic thinning on its upper sections, where flow acceleration started two years later in 2014. This pattern contrasts markedly with a nearby decelerating land-terminating glacier, which has experienced a much slower retreat rate (−7.8 m a−1) and the same magnitude of mean thinning rate at its lower part since 2000. Our results confirm the strong influence of the proglacial lake on ice flow dynamics and suggest that lake-ice interactions are important to consider when analyzing, interpreting or modeling dynamics of rapidly retreating lake-terminating glaciers in the Himalayas as well as around the world. [ABSTRACT FROM AUTHOR]

Glacier surface temperature (GST) is influenced by both the energy flux from the atmosphere above and the thermal dynamics at the ice–water–debris interfaces. However, previous studies on GST are inadequate in time series research and mountain glacier surface temperature retrieval. We evaluate the GST variability at Hailuogou glacier, a temperate glacier located in Southeastern Tibetan Plateau, from 1990 to 2018. We utilized a modified mono-window algorithm to calculate the GST using the Landsat 8 thermal infrared sensor (TIRS) band 10 data and Landsat 5 thematic mapper (TM) band 6 data. Three essential parameters, including the emissivity of ice and snow, atmospheric transmittance, and effective mean atmospheric temperature, were employed in the GST algorithm. The remotely-sensed temperatures were compared with two other single-channel algorithms to validate GST algorithm's accuracy. Results from different algorithms showed a good agreement, with a mean difference of about 0.6 ℃. Our results showed that the GST of the Hailuogou glacier, both in the upper debris-free part and the lower debris-covered tongue, has experienced a slightly increasing trend at a rate of 0.054 ℃ a−1 during the past decades. Atmospheric warming, expanding debris cover in the lower part, and a darkening debris-free accumulation area are the main causes of the warming of the glacier surface. [ABSTRACT FROM AUTHOR]

Runoff from high-elevation, debris-covered glaciers is a crucial water supply in the Tibetan Plateau (TP) and its surroundings, where insufficient debris thickness data make it difficult to analyze its influence. Here, we investigated the role of debris cover in runoff formation of the Hailuogou catchment in the south-eastern Tibetan Plateau for the 1988–2017 period, based on long-term observations combined with a physically based glacio-hydrological model. The catchment is characterized by extensive thin debris on the ablation zones of three debris-covered glaciers. An increasing trend in catchment runoff has been observed in the past three decades, more than 50% of which is attributed to glacier runoff in the last decade. With the exception of the influence of temperature rising and precipitation decreasing, our results underline the importance of debris cover and its spatial features in the glaciological and hydrological processes of the catchment, in which the acceleration effect of debris cover is dominant in the catchment. An experimental analysis indicated that the extraordinary excess meltwater in the catchment is generated from the debris-covered surface, especially the lower elevation region below 3600 m a.s.l. [ABSTRACT FROM AUTHOR]

Seasonal evolution of the subglacial drainage system in the lower ablation area of the Hailuogou glacier ice tongue were revealed by repeated dye tracer (Rhodamine WT) experiments during the 2009 ablation season. Between April and October, 18 dye tracer experiments were conducted by injecting the tracer at one location of the lower ablation area of the Hailuogou Glacier to diagnose the seasonal variation of the subglacial drainage system of this section of glacier ice tongue. Using a simple advection-dispersion model (ADM), the flow velocity, hydrodynamic dispersion coefficient, and degree of tracer spreading were deduced. Tracer transit velocity through the tested subglacial channel varied from 0.148 to 0.555 m s−1 during the 2009 ablation season. Dispersivity showed a relatively high value than that found at other glaciers, which varied between 27.05 and 287.49 m2 s−1. Seasonal changes of these indexes indicated that the subglacial drainage system of the lower ablation area of the Hailougou Glacier is a relatively stable existing system in the case of its longitudinal shape, whereas its hydraulic efficiency is low in the early and late ablation seasons and high during the middle of summer due to subglacial channel expansion. [ABSTRACT FROM AUTHOR]

Himalayan glacier changes in the context of global climate change have attracted worldwide attention due to their profound cryo-hydrological ramifications. However, an integrated understanding of the debris-free and debris-covered glacier evolution and its interaction with glacial lake is still lacking. Using one case study in the Gyirong River Basin located in the central Himalayas, this paper applied archival Landsat imagery and an automated mapping method to understand how glaciers and glacial lakes interactively evolved between 1988 and 2015. Our analyses identified 467 glaciers in 1988, containing 435 debris-free and 32 debris-covered glaciers, with a total area of 614.09 ± 36.69 km2. These glaciers decreased by 16.45% in area from 1988 to 2015, with an accelerated retreat rate after 1994. Debris-free glaciers retreated faster than debris-covered glaciers. As a result of glacial downwasting, supraglacial debris coverage expanded upward by 17.79 km2 (24.44%). Concurrent with glacial retreat, glacial lakes increased in both number (+41) and area (+54.11%). Glacier-connected lakes likely accelerated the glacial retreat via thermal energy transmission and contributed to over 15% of the area loss in their connected glaciers. On the other hand, significant glacial retreats led to disconnections from their proglacial lakes, which appeared to stabilize the lake areas. Continuous expansions in the lakes connected with debris-covered glaciers, therefore, need additional attention due to their potential outbursts. In comparison with precipitation variation, temperature increase was the primary driver of such glacier and glacial lake changes. In addition, debris coverage, size, altitude, and connectivity with glacial lakes also affected the degree of glacial changes and resulted in the spatial heterogeneity of glacial wastage across the Gyirong River Basin. [ABSTRACT FROM AUTHOR]

Debris-covered glaciers (DCGs) are globally distributed and thought to contain greater microbial diversity than clean surface continental glaciers, but the ecological characteristics of microbial communities on the surface of DCGs have remained underexplored. Here, we investigated bacterial and fungal diversity and co-occurrence networks on the supraglacial debris habitat of two DCGs (Hailuogou and Dagongba Glaciers) in the southeastern Tibetan Plateau. We found that the supraglacial debris harbored abundant microbes with Proteobacteria occupying more than half (51.5%) of the total bacteria operational taxonomic units. The composition, diversity, and co-occurrence networks of both bacterial and fungal communities in the debris were significantly different between Hailuogou Glacier and Dagongba Glacier even though the glaciers are geographically adjacent within the same mountain range. Bacteria were more diverse in the debris of the Dagongba Glacier, where a lower surface velocity and thicker debris layer allowed the supraglacial debris to continuously weather and accumulate nutrients. Fungi were more diverse in the debris of the Hailuogou Glacier, which experiences a wetter monsoonal climate, is richer in calcium, has greater debris instability, and greater ice velocity than the Dagongba Glacier. These factors may provide ideal conditions for the dispersal and propagation of fungi spores on the Hailuogou Glacier. In addition, we found an obvious gradient of bacterial diversity along the supraglacial debris transect on the Hailuogou Glacier. Bacterial diversity was lower where debris cover was thin and scattered and became more diverse near the glacial terminus in thick, slow-moving debris. No such increasing bacterial pattern was detected on the Dagongba Glacier, which implies a positive relationship of debris age, thickness, and weathering on bacterial diversity. Additionally, a highly connected bacterial co-occurrence network with low modularity was found in the debris of the Hailuogou Glacier. In contrast, debris from the Dagongba Glacier exhibited less connected but more modularized co-occurrence networks of both bacterial and fungal communities. These findings indicate that less disturbed supraglacial debris conditions are crucial for microbes to form stable communities on DCGs. • Debris-covered glaciers harbor diverse microbial communities. • A chronosequence exists on the surface of typical debris-covered glaciers • Microbial co-occurrence network were more connected but less modularized in glaciated habitats that are more extreme and less productive. [ABSTRACT FROM AUTHOR]

Glacial lake outburst floods (GLOF) are one of the most destructive natural disasters. Understanding GLOF evolution, and their impacts, plays a fundamental role in GLOF hazard assessment and risk management. Reconstructing historical GLOFs is an important exercise because detailed case studies of such glacial hazards are rare, which hinders the capacity of glacial hazard practitioners to learn from these events. In this study, we reconstruct a historical GLOF from moraine-dammed Chongbaxia Tsho (89.745°E, 28.211°N) in the Eastern Himalaya, which is a unique case study because the outburst flood cascaded into two further lakes downstream. We employ a combination of i) multi-source and multi-temporal satellite imagery, ii) field investigation (including an unmanned aerial vehicle survey), iii) numerical dam breach and hydrodynamic modelling and, iv) qualitative and quantitative cryospheric and meteorological analysis, to investigate the evolution of the GLOF hazard, simulate moraine dam failure and GLOF propagation, and explore the role that long- and short-term climate trends played in providing the conditioning factors for the outburst. Chongbaxia Tsho expanded rapidly between 1987 until 2001 in response to glacier recession most likely caused by a regional warming trend of +0.37 °C per decade. Based on satellite image analysis we refine the outburst date to be 6 August 2001, instead of 6 August 2000, as previously reported, and attribute an ice avalanche into the glacial lake originating from the receding parent glacier as the most likely trigger for moraine dam failure. Through DEM differencing and lake level decrease, we estimate that a total water volume of 27.1 ± 1.6 × 106 m3 was released from the lake during the event, and using dam breach modelling we estimate that the peak discharge at the breach was >6600 m3 s−1. The GLOF flowed through downstream Chongbamang Tsho and Chongbayong Tsho, both of which served to attenuate the GLOF and reduce downstream losses; the latter stored an estimated 96% of the flood volume. Precipitation totals in the weeks preceding the GLOF exceeded the historical mean by up to 40%, and may have contributed to instability of the parent glacier, and generation of an ice avalanche with enough impact energy to cause lake water to overtop the moraine dam and initiate breach development. A future GLOF from Chongbaxia Tsho cannot be ruled out, but more field data, including detailed lake bathymetry, and information pertaining to the sedimentological and geotechnical characteristics of the moraine dam, are required for a more robust parameterization of a predictive GLOF model and quantification of the hazard posed by a future GLOF. • Chongbaxia Tsho GLOF occurred on 6 August 2001, undergoing a rapid lake expansion prior to outburst. • It is most likely that the event was triggered by an ice avalanche. • A total water volume of 27.1 ± 1.6 × 106 m3 was released, caused a severe damage to downstream. • Downstream Chongbamang Tsho and Chongbayong Tsho greatly buffered the GLOF discharge. [ABSTRACT FROM AUTHOR]

Hailuogou (HLG) Glacier, a rapidly receding temperate glacier in the southeastern Tibetan Plateau, has been observed to lose mass partly through ice frontal mechanical ablation (i.e., ice collapse). These events are difficult to monitor and quantify due to their small scale and frequent nature. However, recent developments in Uncrewed Aerial Vehicles (UAV) have provided a possible approach to track their spatiotemporal variation and their impact on the geomorphological evolution of the glacier terminus area. Here, we present analysis from UAV surveys conducted over eight field campaigns to the HLG Glacier, providing evidence of glacier change between October 2017 and November 2020. Structure from Motion with Multi-View Stereo (SfM-MVS) was applied to produce multi-temporal Digital Surface Models (DSMs) and orthophoto mosaics, from which geomorphological maps and DEMs of Difference (DoDs) were derived to quantify glacier changes. These analyses reveal that at the margins of the glacier terminus retreated 132.1 m over the period of analysis, and that in the area specifically affected by collapsing (i.e., the glacier collapsed terminus), it retreated 236.4 m. Overall the volume lost in the terminal area was of the order of 184.61 ± 10.32 × 104 m3, within which the volume change due to observed collapsing events comprises approximately 28%. We show that ice volume changes at the terminus due to a single ice collapse event may exceed the interannual level of volume change, and the daily volume of ice loss due to ice calving exceeds the seasonal and interannual level by a factor of ~2.5 and 4. Our results suggest that the evolution of the HLG Glacier terminus is dominantly controlled by the frontal ice-water interactions. If the future evolution of glaciers such as HLG Glacier is to be robustly predicted, the contribution of mechanical ablation should be accounted for by numerical models. • Recent changes of Hailuogou Glacier terminus are analyzed using SfM-MVS workflow with UAV images from 2017 to 2020 • The terminal area not affected by collapse events retreated 132.1 m, whereas the area subject to collapse events retreated 236.4 m. • The ice volume reduction for the observed areas is 184.61 ± 10.32 × 104 m3, and ~28% was attributed to ice collapse. • The evolution of the HLG Glacier terminus will continue to be dominated by the frontal ice collapse [ABSTRACT FROM AUTHOR]

• Glaciers in the UIBKK experienced a slight negative mass budget from 2000 to 2019. • Mass balances in the UIBKK shifted from negative to positive from West to East. • An accelerated mass loss was found in the West UIBKK from 2000–2013 to 2013–2019. • Mass budgets in the East UIBKK shifted from negative to positive from 2000 to 2019. • Climate warming may play more important role in glacier changes in the study area. Water supply contributed by glacier and snow melt is of particular significance and play a key role for social stability in the Karakoram region of Upper Indus Basin (UIBKK). Based on the SRTM DEM, TerraSAR-X/TanDEM-X images and ZY3-02 stereo images, the spatiotemporal pattern of glacier mass budgets in the UIBKK have been estimated for the early 21st century (2000–2019) period in this study. A comparison of the total area in 2000 with that in 2019 shows that glacier area remained stable in the early 21st century, with a slight expansion of 56.3 km2 (0.4%). Glaciers in the UIBKK experienced a balanced or slight negative mass budget of −0.08 ± 0.07 m w.e. a−1 in the early 21st century. The spatial variability of mass budget in the UIBKK showed that glacier mass budgets shifted from negative to positive from West to East, respectively, along longitude. Glacier mass loss decelerated from −0.12 ± 0.16 m w.e. a−1 in 2000–2013 to −0.03 ± 0.21 m w.e. a−1 in 2013–2019. No significant differences were observed in thinning rates on debris-covered glaciers (≥19% area covered by debris) and clean-ice glaciers (<19% area covered by debris) in the early 21st century (−0.06 m a−1 vs −0.04 m a−1). The overall homogeneous mass budgets for surge and non-surge glaciers indicated that glacier surges seem not have a notable impact on glacier-wide mass budget over short time scales. The spatiotemporal variability of glacier mass budgets in the UIBKK are consistent with the tendencies of winter precipitation and summer temperature. Climate warming may play more important role in glacier changes in the UIBKK. [ABSTRACT FROM AUTHOR]

Abstract: Large changes have occurred at Hailuogou glacier, Mt. Gongga, China, during the Holocene. Various data indicate that it retreated by >1.88km during the 20th century, and mass balance records show that it suffered a sustained mass loss of snow and ice in the period 1959/60–2003/04, with an accumulated mass balance of −10.83m water equivalent. Hydrological data demonstrate that runoff from the glacier has been increasing both seasonally and annually, the principal cause being an increase of ice and snow melt water. As the mean temperature in the Hailuogou basin rose by 0.1°C between 1994 and 2004, the runoff increased by 2.6m3 s−1. Changes of glacier morphology which occurred during this period include a decrease of thickness, enlargement of glacial caves, and reduction of the size of seracs. The Hailuogou glacier advanced three times during the Little Ice Age in response to three cold periods in the Mt. Hengduan region. There also were three noticeable advances in the Neoglacial. There was a very clear relationship between glacier change, climate change and forest line change in the period 3000aBP–100aBP. Overall, it is clear that changes of the Hailuogou glacier in the Holocene have been responded to climatic fluctuations, but quantitative evaluation of the relationship is difficult owing to the sparse spatial coverage of reliable evidence and the limited available longer term data. [Copyright &y& Elsevier]

• An accelerated ice flow was found above 4 600 m in southeastern TP. • The contribution of the basal sliding velocities is ~32% for maritime glacier. • Ice below 4 600 m in the SETP would likely disappear in 2100. • More than 55% of glacier volume is expected to be decreased in 2100. Influenced by climate warming, glacial hazards become increasingly frequent after glacier mass loss in the southeastern Tibetan Plateau. Investigations of mountain glacier surface velocities and volume are of great significance. However, no information is given regarding the specific changes in ice flow to climate warming, the spatiotemporal variations of glacier surface velocity and ice thickness are still unclear. In this study, offset tracking was used to measure glacier surface velocity in the southeastern Tibetan Plateau using Landsat TM/ETM+/OLI imagery from 1990 to 2018. And then spatially-distributed glacier ice thickness was estimated from mass-balance distribution data, glacier surface velocities and the principles of ice-flow. Results show that all maritime glaciers velocity in study area experienced a clear regional slowdown from 1991 to 2018, with robust overall trends of −5.26 m a−1 dec−1 (-19.8% dec−1) and 7.16 m a−1 dec−1 (25.2% dec−1) at elevations below and above 4 600 m a.s.l., respectively. Total volume for glaciers with an area of 954.34 km2 was found to be ~ 86.64 ± 11.09 km3 in 2013, corresponding to an average ice thickness of ~ 91 ± 12 m. The average ice thickness thinned by 0.62% a−1 during 2000–2013, and an accelerated melting can be expected in the long term and then glacier volume decreases more rapidly than in the last decades. [ABSTRACT FROM AUTHOR]