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Permafrost degradation increases the likelihood of glacial lake outburst floods on the Tibetan Plateau. Analyses of the freeze–thaw conditions in moraine dams and associated impacts on dam stability contribute toward reducing natural hazard risks. We used the heat transfer module of COMSOL Multiphysics to simulate the soil temperature field in the Longbasaba moraine dam, which is on the northern slope of central Himalaya. There is close agreement between the simulated and observed soil temperature values. Root mean squared errors (the square root of the mean of the square of all of the error) are below 1.30°C and mean bias errors vary from −0.10 to −0.52°C for the different soil layers. Between 1959 and 2020, active layer thickness increased at a mean annual rate of 0.024 m a−1. Under the Coupled Model Intercomparison Project Phase 6 scenarios, the buried ice inside the moraine dam is clearly melting. The maximum‐buried ice thaw depth is currently 3.4 m and at the end of the 21st century is projected to be 9.53, 16.69, and 21.83 m under SSP1‐2.6, SSP2‐4.5, and SSP5‐8.5, which indicates a continuous decrease in moraine dam stability. Plain Language Summary: Glacial Lake Outburst Floods have frequently occurred on the Tibetan Plateau in recent years and have caused many casualties and substantial property damage. Longbasaba lake is located at the end of Longbasaba glacier and is dammed by a terminal moraine complex. We used the heat transfer module of COMSOL Multiphysics simulation software to simulate the freeze–thaw process and melting of buried ice inside the Longbasaba moraine dam between 1959 and 2100 and examined dam stability. The maximum thawing depth of the dam in summer has increased over the past decades and buried ice has been melting since the mid‐1970s. Under future warming scenarios, the depth of melting inside the dam is projected to exceed the depth of freezing and the melting rate of buried ice will accelerate. This indicates that seepage and piping will increase and dam stability will continue to decrease. We must remain alert about the possibility of future dam failure. Key Points: Soil temperatures at different depths of the Longbasaba moraine dam were well simulated by COMSOL Multiphysics softwareBetween 1959 and 2020, annual active layer depth increased at a mean rate of 0.024 m a−1Under Coupled Model Intercomparison Project Phase 6 scenarios, melting of buried ice is projected to accelerate and reduce dam stability [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]

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]

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]

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]