Manifestations and mechanisms of mountain glacier-related hazards

Xin Wang,Qiao Liu,ShiYin Liu,GuangLi He

1. School of Resource Environment and Safety Engineering, Hunan University of Science and Technology, Xiangtan,Hunan 411100,China

2.Institute of Mountain Hazards and Environment,Chinese Academy of Sciences,Chengdu,Sichuan 610000,China

3.Institute of International Rivers and Eco-Security,Yunnan University,Kunming,Yunnan 650091,China

ABSTRACT Mountain glacier-related hazards occur worldwide in response to increasing glacier instability and human activity intensi‐ty in modern glacierized regions. These hazards are characterized by their spatial aggregation and temporal repeatability.Comprehensive knowledge about mountain glacier-related hazards is critical for hazard assessment, mitigation, and pre‐vention in the mountain cryosphere and downstream regions.This article systematically schematizes various mountain gla‐cier-related hazards and analyzes their inherent associations with glacier changes. Besides, the processes, manifestations,and mechanisms of each of the glacier-related hazards are summarized.In the future,more extensive and detailed system‐atic surveys, for example, considering integrated ground−air−space patterns, should be undertaken for typical glacier‐ized regions to enhance existing knowledge of such hazards. The use of coupled numerical models based on multisource data is challenging but will be essential to improve our understanding of the complex chain of processes involved in thermal−hydrogeomorphic glacier-related hazards in the mountain cryosphere.

Keywords:glacier-related hazards;mountain cryosphere;glacier changes

1 Introduction

Mountain glacier-related hazards are induced di‐rectly or indirectly by mountain glacier changes, re‐sulting in threat to human life, infrastructure, and the local environment. These hazards are typically condi‐tioned or triggered by the contemporary dynamics of mountain glaciers. Common examples of such haz‐ards include glacier surges, glacier-related floods (in‐cluding glacial meltwater floods and glacial lake out‐burst floods (GLOFs)), glacier-related debris flows, ice mass loss, ice avalanches and rockfalls, rock avalanch‐es,and slope failures due to the glacier retreat(referred to here as paraglacial destabilization) (Figure 1). Gla‐cier surges and ice avalanches are glacier instabilities characterized by the gravity-driven mass failure of glaciers. Both climatic factors (e.g., air temperature,precipitation)and non-climatic factors(e.g.,topograph‐ic features, glacier characteristics, and external forces such as earthquakes, ice-capped volcanoes, and hu‐man activity) and their interactions can induce chang‐es in glacier environments, thus triggering mountain glacier-related hazards (Haeberli and Whiteman, 2015).Glacial meltwater floods, accelerated ice mass loss,ice avalanches, and glacier surges result directly from glacier changes, both gradual and sudden. In contrast,glacier-related debris flows, GLOFs, and paraglacial destabilization are usually caused indirectly by glacier changes and can be regarded as secondary glacier-re‐lated hazards.

Figure 1 Schematic illustration of the relationships between different types of mountain glacier-related hazards

Glacier changes have intensified during recent de‐cades in the framework of global warming. Mountain glaciers are generally in a state of negative mass bal‐ance (Fujita and Nuimura, 2011; Yaoet al., 2012;Zempet al., 2019); however, glaciers in the western Kunlun Mountains of China and the Karakoram Mountains of Pakistan have been in a state of positive mass balance or have remained relatively stable over recent years (Gardelleet al., 2013; Kääbet al., 2015).The stable or thickening glaciers in the Karakoram and West Kunlun regions experience slightly acceler‐ated glacier flow (Dehecqet al., 2019); such dynamic changes would compromise glacier stability and raise the probability of glacier-related disasters. Moreover,with increasing socioeconomic development, the im‐portance of the mountain cryosphere as a resource has grown considerably; simultaneously, the intensity of human activity in the mountain cryosphere has risen.This combination of compromised stability and in‐creased human activity intensity has resulted in new manifestations of mountain glacier-related hazards.This article provides a systematic and concise summa‐ry of the processes, manifestations, and mechanisms of the key glacier-related hazards that occur in mod‐ern glacierized regions.

2 Glacier surge

Surging glaciers are those that move rapidly and periodically. Their movement oscillates between peri‐ods of brief rapid flow and lengthy slow flow or con‐ditions approximating stagnation; these are referred to as the"active"(or"surge")and"quiescent"phases,re‐spectively. During its active or surge phase, a glacier can move at velocities that are up to hundreds of times faster than their typical movement. The time that elapses between two glacier surges is known as the surge cycle; each cycle includes two surge phases and a quiescent phase, and cycle durations can range from ten years to hundreds of years (Figure 2). The duration of an individual surge phase is variable.Each surge phase can take several months to several years,with some glaciers exhibiting surge phases that are longer than a decade. Glacier surges are characterized predominantly by bottom sliding that is tens to hun‐dreds of times faster than conventional glaciers and by severe fragmentation of the glacier surface.Associ‐ated disasters can occur as a result of a glacial mass moving quickly from its upper reservoir area to its lower receiving area, or as a result of flooding caused by the rapid release from glacial lakes (Roundet al.,2017;Kääbet al.,2018).The quiescent phase of surg‐ing glaciers can last for periods ranging from decades to hundreds of years and is characterized predomi‐nantly by creep and conditions of slow movement or near stagnation. The reservoir area of surging glaciers is typically thickened gradually by the slow accumula‐tion of glacial mass during the quiescent phase, ac‐companied by fragmentation of the receiving area due to enhanced melting(Charles and Raymond,1987).

Figure 2 Characteristic volumes and return periods of different mountain glacier-related hazards(revised from Huggel et al.,2012)

It is generally assumed that glacial surges occur owing to periodic turbulence exerted on the glacier basement by its internal characteristics. In particular,changes in a glacier's mass budget and internal ther‐mal/hydraulic conditions are known to be controlling factors for the triggering, acceleration, and termina‐tion of glacier surges (Bennet al., 2019; Bhambriet al., 2020). Meltwater also plays an important role in the development and evolution of surging glaciers be‐cause subglacial water lubricates their basement,caus‐ing rapid sliding during the surge phase. Surging gla‐ciers have been classified into Svalbard-type and Alas‐ka-type based on their climatic setting and source of subglacial water (Bhambriet al., 2020). Svalbardtype surging glaciers are controlled by the constant ac‐cumulation of ice mass under a cold climate. Under such conditions, the accumulating ice mass boosts the subglacial pressure and induces pressure melting; the resulting subglacial meltwater lubricates the glacier and causes glacier surge(Murrayet al.,2003).In con‐trast,Alaska-type surging glaciers are typically domi‐nated by the increasing hydrostatic pressure of the subglacial water from glacial meltwater as it flows downward along fissures under warm conditions, lu‐bricating the ice bed and inducing surges (Kambet al.,1985;Sharp,1988;Björnsson,1998).As previous‐ly reported, both types of surging glaciers can coexist in the same area or adjacent areas, despite differences in their formation mechanisms (Sevestre and Benn,2015). Some researchers consider that the existence of glacial till and the occurrence of ice deformation and its interaction with sub-ice hydraulic processes are essential factors that control the formation and de‐velopment of glacier surges (Roundet al., 2017).However, this hypothesis has yet to be confirmed ow‐ing to the difficulty of observing these processes di‐rectly. Recent modeling studies using the theory of heat function equilibrium and considering the coupled thermal and hydraulic parameters of glaciers suggest that the enthalpy budget could cause glacier surges(Bennet al.,2019).Additionally,the tendency of surg‐ing glaciers to occur on particular substrates,especial‐ly in weak sedimentary rocks or fault zones, has also been reported (Björnssonet al., 2003). In general, the formation mechanisms of surging glaciers are diverse and depends on the characteristics of individual gla‐ciers. Accordingly, many gaps in knowledge remain regarding the formation and mechanisms of glacier surges.

It has been reported that approximately 1% of glaciers globally have been observed to surge (Jis‐kootet al., 1998; Sevestre and Benn, 2015). Evi‐dence of surge glaciers can be found in every region with extensive glacier distribution worldwide, in‐cluding the cryosphere of the Karakoram and Pamir plateaus, the Tianshan Mountains, the Svalbard Is‐lands, Iceland,Greenland,Alaska and the Yukon Ter‐ritory, the Canadian Arctic, the Andes, and even parts of the southwest polar ice sheet.Based on glob‐al statistics from the most recent Randolph Glacier Inventory (RGI 6.0), there are 1,343 known surging glaciers in the world, including 511 possible ones,384 probable ones, and 448 confirmed ones. More‐over, there are another 42,515 glaciers worldwide that can be classified as potential surging glaciers,although insufficient evidence is available yet (RGI Consortium, 2017). Within China, there are 1,659 glaciers with various probabilities of surging, which are distributed primarily in the Karakoram Moun‐tains, Pamir Plateau, western Kunlun Mountains, and Qinghai-Tibet Plateau (Zhanget al., 2016; Bhambriet al., 2017; Kääbet al., 2018). However, only 11 of these have been rated as possible, probable, or con‐firmed.The remaining 1,648 in these regions are clas‐sified as potential surging glaciers (RGI Consortium,2017). At least 210 surge glaciers are present in the Karakoram Mountains region, occupying a total area of 7,836 km2, or nearly half of the Karakoram Gla‐cier area (Bhambriet al., 2017).The average veloci‐ties of glaciers in the Karakoram Mountains and the western Kunlun Mountains accelerated slightly during 2000−2017; the numbers of advancing and surging glaciers also rose during this period (Dehecqet al.,2019).A recent inventory in the Pamir Plateau iden‐tified 206 spatially distinct surges within 186 gla‐cier bodies, with most surges clustered in the north‐ern and central parts of the plateau during 1988 −2018(Goerlichet al.,2020).

3 Glacier-related floods

3.1 Glacial meltwater-induced floods

Glacier meltwater-induced floods are seasonal and related closely to seasonal changes in air temperature.Ice and snow melt to form liquid water when the tem‐perature rises above 0 °C. Generally, glacier melting intensifies with increasing solar radiation and glacier area; in particular, the generation of runoff from gla‐cier melting is synchronous with changes in air tem‐perature.Air temperature remains above 0°C continu‐ously in many regions during late spring and summer,resulting in upward shifts in the snowline. As part of this process, the melting of ice intensifies gradually,with the lower bare ice or thin debris-covered ice in glacier ablation zones melting first, followed by an upward expansion of melting (Wanget al., 2008).Meltwater on the surface of the glacier increases due to the lower albedo of glacier melting surface com‐pared with that of ice and snow. And glacial meltwa‐ter-induced floods begin to occur as meltwater flow rises (Sakaiet al., 2000; Cuffey and Paterson, 2010).The production of glacial meltwater at temperatures above 0 °C further promotes glacier melting, both on the surface of and within the ice, which possibly re‐sults in flooding due to the blockage of channels with‐in the ice by thermal cracking (Benn and Evans,2010). Under similar terrain conditions, the peak dis‐charge,volume,and flow pattern of glacial meltwaterinduced floods depend primarily on the area of a gla‐cier's ablation zone. Broadly, however, the hydro‐graphs of glacial meltwater floods rise and fall more gradually than those of flash floods found in other geographic settings(Figure 3).

Figure 3 Representative types of flood hydrographs in mountain cryosphere regions

Ice and snow floods occur widely in mid- to high-latitude areas and at high altitudes in alpine re‐gions globally, including the Russian Caucasus, Cen‐tral Asia,the European Alps,and the west coast moun‐tain ranges of North America. In China, glacial melt‐water floods occur primarily in the Manasi River Ba‐sin on the northern slope of the central Tianshan area,the Muzhalte River and Tailanhe River on the south‐ern slope of the western Tianshan area, the Karakash River in the western Kunlun Mountains, the Yarkant River in the Karakorum Mountains,the Changma Riv‐er and Danghe River in the western Qilian Mountains,and the tributaries of the Yalu Tsangpo River on the northern slope of the Himalayas. Both the intensity and frequency of glacial meltwater floods have been increasing in recent decades in conjunction with cli‐mate warming.In addition,in areas with frequent vol‐canic activity (such as Iceland), glacier-related floods commonly result from rapid subglacial melting caused by glacier-covered volcanoes (Haeberli and Whiteman,2015).

3.2 Glacial lake outburst floods

GLOFs are characterized by the sudden cata‐strophic release of water reservoirs from glacial lakes that have formed alongside, in front of, within, be‐neath, or on the surface of glaciers. The hydrographs of GLOFs are characterized by more rapid rising and descending, with higher peak flows, than hydrographs for other types of flooding that occur in the mountain cryosphere (Figure 3). The most common types of GLOFs are moraine-dammed and ice-dammed out‐burst floods.Ice-dammed lake outburst floods typical‐ly occur by one of two following mechanisms. (1) A glacial lake can drain owing to floating of the associ‐ated ice dam under the high buoyancy when the depth of the lake reaches 90% of the dam height.(2)Glacial lakes often drain along intersecting drainage channels,crevasses, or fractures on the surface of, inside, and under the ice as the glacier moves and melts. The drainage can occur in conjunction with the action of lake water hydrostatic pressure and thermal erosion.Moraine-dammed lake outburst floods are typically in‐duced by four mechanisms.(1)Lake water static pres‐sure on a moraine dam can rise as increasing amounts of water are produced by ice/snow melting, precipita‐tion, and the release of water from the parent glacier.(2)A sudden surge toward the moraine dam can hap‐pen as ice/rock avalanche mass pours into the lake.(3) The melting of dead ice in moraine dams, the ex‐pansion of dam piping, and the melting and sliding of steep dam slopes can all take place under a warming climate.(4)Additionally,an earthquake or other exter‐nal forces damage the moraine dam. Thus, in general,GLOFs can develop and eventually be triggered by both external factors (such as ice/snow avalanches,rainfall, temperature increases, changes in the parent glacier, and earthquakes) and internal factors (such as melting of dead ice in moraine dams, expansion of dam piping,and cracking or thermal channeling of ice dams). Typically, the changes of external factors break the previous balances of water and heat in the glacial lake itself,resulting in further changes in its in‐ternal factors and ultimately periodic dam failure. In summry, many controlling factors collectively lead to GLOFs(Wanget al.,2016).

GLOFs have occurred worldwide and are known to be particularly frequent in mountainous regions, in‐cluding the Himalayas, the Andes, Central Asia, the Alps,Iceland,and North America.Carrivick and Tweed(2016)reported that 1,348 GLOFs have occurred in ap‐proximately 20 countries during the past ten centuries.These floods originated from 332 sites, among which 70% originated from ice-dammed lakes, and 36% re‐sulted in documented societal impacts. Notably, ap‐proximately 2%−10% of all glacial lakes in the Hima‐layas and the Tianshan area have been identified as being potentially dangerous (Wanget al., 2011; Wanget al., 2012; Nieet al., 2017). Similarly, 16% of all glacial lakes in the Tibetan Plateau area are consid‐ered a threat to human settlements (Allenet al.,2019). The average annual frequency of GLOFs has been estimated to be 1.3 in the region of Hindu Kush−Karakoram −Himalaya −Nyainqentanglha (Vehet al.,2019). However, there is little credible evidence for any temporal trend in the frequency distribution, al‐though anecdotal reports suggest that temporal increas‐es in frequency have occurred in glacial lake areas in the Himalayas(Vehet al.,2020).In contrast,the global trend appears to indicate temporal decreases in frequen‐cy since the mid-1990s (Carrivick and Tweed, 2016).In the future, the GLOF hazard may surge in regions that currently have large glaciers (e.g., the Himalayas)if the projected future ice loss generates more unstable glacial lakes than are present today (Vehet al., 2020).In fact, from an assessment of the timing of climate forcing, lag times in glacier recession, lake formation and moraine-dam failure, Harrisonet al.(2018) pre‐dicted increased GLOF frequencies during the next decades and into the 22nd century.

4 Glacier-related debris flows

Glacier-related debris flows are, usually a conse‐quence of general glacier retreat and exposure of large quantities of unconsolidated, unvegetated and sometimes ice-cored glacial sediments transported by meltwater, GLOFs, ice/snow avalanches or rainfall(Evans and Clague, 1994; Chiarleet al., 2007). Just like all other types of debris flow, the glacier-related debris flows also typically comprise solid−liquid twophase fluid motion, which can be considered some‐where between mountain torrents and solid mass flows(such as landslides). Yet, they are rather unique be‐cause of the unpredictability(may start in fair weather)and potential volumes (debris availability in the gla‐cial/proglacial system). Glacier-related debris flows are commonly initiated by the outburst of glacial lakes, the flow of glacial meltwater, ice/snow ava‐lanches, or rainfall in glacial basins. The solid debris typically originates from steep moraines, talus slopes at the feet of eroding rockwalls, destabilized rock gla‐cier tongues, and fluvioglacial deposits within steep stream channels (Haeberli and Whiteman, 2015).Their common triggering mechanism is high summer temperatures and melting of snow and ice (Chiarleet al.,2007;Jomelliet al.,2007).Generally,the develop‐ment of these debris flows is favored by the presence of steep terrain, plentiful moraines and fluvial depos‐its, and abundant meltwater. Four categories (meltwa‐ter-induced, GLOF-induced, ice/snow avalanche-in‐duced, and rainfall or rainfall mixed with ice/snow melt induced) and eight subcategories of glacier-relat‐ed debris flows have been defined based on the domi‐nant water source,as shown in Table 1.

Table 1 Types of glacier-related debris flow,categorized by their causes of formation(revised from Deng et al.,1988;Lü et al.,1999;Shi et al.,2000;Evans and Delaney,2015;GAPHAZ,2017)

Glacier-related debris flows often occur in sum‐mer and autumn, as the mountain cryosphere warms globally. The mechanisms that trigger melt-related processes or convective precipitation are more likely to emerge during the warmer months; moreover, sedi‐ments are less likely to be frozen at these times,result‐ing in the greater availability of material for erosion.For example,a strong seasonality to debris flow activ‐ity has been identified, with events occurring more frequently in summer and autumn in the European Alps (Rebetezet al., 1997; Stoffelet al., 2011), and during summer in the Russian Caucasus (Perovet al.,2017). Recent studies have demonstrated that the en‐hanced debris flow activity associated with recent rap‐id deglaciation and the exposure of morainic deposits are usually characterized by thermokarst features in Southern Russia (Seinovaet al., 2011). In Tibet, gla‐cier-related debris flows are often induced by abun‐dant summer meltwater,plentiful unfrozen debris,and the typically steep topography(Cuiet al.,2010).Melt‐water-induced and GLOF-induced debris flows often have large magnitude (i.e., total debris mass exceed‐ing 10×104m3per event) or extremely large magni‐tude (i.e., total debris mass exceeding 100×104m3per event); thus, they are extremely destructive and often cause disasters in Tibet (Cuiet al., 2010).Additional‐ly, the occurrence of glacier-related debris flows is not only controlled by the abovementioned climatic conditions, but is also closely related to the attributes,glaciation characteristics, and topographic conditions of modern glaciers. Generally, glacier-related debris flows in Tibet exhibit lower frequency and magnitude in areas of greater continentality due to the glaciers'relatively lower activities and the lower frequency of GLOFs under continental climate conditions than those in areas under maritime climate conditions (Lüet al.,1999;Wanget al.,2016).

5 Ice avalanches

Ice avalanches occur when ice falls from a steep part of a glacier or the edge of an ice shelf under grav‐ity.There is a general distinction between ice falls and ice avalanches, based on their rheological behavior,which in turn depends mainly on the volume of the de‐tached mass. Typically, ice avalanches originate from the steep frontal section of a glacier or a sloping gla‐cier bed and occur at the end of a mountain glacier or the edge of an ice shelf in polar regions (Alean,1985).These avalanches frequently arise in the icefall areas of mountain glaciers. Ice avalanches occur pri‐marily in suspended glaciers, and accordingly, they can be divided into ramp-type and cliff-type based on the terrain of their original area (Figure 4). Usually,cliff-type ice avalanches (or ice falls) are associated with smaller, more frequent and repetitive events, and less destructive effects on downslope areas, whereas ramp-type avalanches tend to produce less frequent avalanches with larger volumes and more destructive impact on downslope sites. Ramp-type ice avalanches can be further subdivided based on ice temperature,including cold-based glaciers, polythermal glaciers,and steep temperate glaciers. Cold ice is more diffi‐cult to move than warm ice because it freezes into the ice bed; accordingly, cold ice can remain on steeper slopes longer before ice slides occur, even though small-scale cliff frontal ice slab detachment is a kind of "ablation" that occurs on a regular basis. Cliff-type ice avalanches (also known as wedge failures and frontal block failures) take place in response to sud‐den changes in the ice bed of suspended glaciers(e.g.,sudden increases in slope)or at the edges of ice cliffs.

Figure 4 Types of Ice avalanche(left:ramp-type,right:cliff-type)

The dominant factor controlling ice avalanches is the stability of the ice body. Several broad factors de‐termine the occurrence and magnitude of catastrophic break-offs. These include the shear strength at the base of the glacier ice(which is related to thermal and hydrological conditions), the inclination and shape of the basal slope, and the tensile strength of the glacier body itself (Haeberli and Whiteman, 2015). Studies have shown that warm glaciers become unstable at slopes of approximately 30°,whereas cold glaciers be‐come unstable at about 45° (Faillettazet al., 2015).Of course, the location and development of ice fis‐sures and the speed of flow of the ice body may also affect ice avalanches. Generally speaking, once an ice avalanche occurs, it will happen again according to the same mechanism. Kääbet al.(2018) reported that global warming in recent decades was a primary cause of ice avalanche disasters western Tibet. How‐ever, in some situations, glacier thinning due to cli‐mate warming may be associated with a reduced risk of ice falls,including both reduced frequency and vol‐ume. Climatic warming in areas where modern gla‐ciers survive usually leads to increases in both temper‐ature and humidity. Rising temperatures also result in the melting of glaciers and frozen soil, promoting fractures on the glacier surface.In contrast,increasing humidity favors glacier mass accumulation, thus ac‐celerating the ice flow. Seismic activity may also in‐duce ice avalanches. For example, the ice avalanche event that occurred at the Sedongpu glacier in the Galapagal Barrier of southern Tibet in 2018 and blocked the Yalu Tsangpo River, was likely related to preseismic activity in this region (Tonget al., 2018).Broadly, ice avalanches can be driven by climate and weather forcing and earthquakes, whose effects de‐pend on specific topographic conditions and the prop‐erties of polythermal and soft-bed glaciers (Fujitaet al.,2017;Kääbet al.,2018).

Ice avalanches frequently take place worldwide.Large-volume ice avalanches(>106m3)are a rare phe‐nomenon but have been reported from the European Alps, parts of North America, the Andes, the Himala‐yas, and Tibet (Evans and Delaney, 2015). In 1965 and 2000, the Allalin glacier avalanche in Switzerland involved the detachment of more than 106m3from the toe region of the glacier, and the burst of fragmented ice into the glacier foreland marked by Little Ice Age lateral moraines (Faillettazet al., 2012; Evans and Delaney,2015).In 2016,two large-magnitude low-an‐gle ice avalanches with volumes of (68±2)×106m3and (83±2)×106m3occurred simultaneously in western Tibet (Kääbet al., 2018). In such large-scale ava‐lanches,the ice body typically slides downslope rapid‐ly, inflicting destruction along its path. The previous 2016 events in western Tibet killed nine people and hundreds of animals. Another example is the 2002 Kolka glacier rock/ice avalanche, which travelled 18 km down the valley and claimed 120 lives (Haeberliet al.,2004).Furthermore,ice avalanches are often ac‐companied by rock avalanches or glacial lake out‐bursts, thus forming glacier-related disaster chains.For instance, based on historical data, Wanget al.(2016) reported that more than 60% of GLOFs world‐wide were caused by ice avalanches.

6 Ice mass loss

Ice and snow resources are referred to as 'water towers' owing to their reliable supply of large volume of meltwater for downstream areas in summer. These water resources are used widely for agricultural irriga‐tion, industrial purposes, hydroelectric power, and household usage in cold and dry regions (Pritchard,2019). Globally, more than 1.6 billion people live in areas that receive water from the mountain cryo‐sphere, and approximately 800 million people depend in part on meltwater from the thousands of glaciers in the high mountains of Asia (Immerzeelet al., 2010;Immerzeelet al., 2020). The depletion of long-term ice storage is unlikely to be replenished by precipita‐tion in the coming decades. Thus, ice mass loss is ex‐pected to continue and even accelerate, increasing the potential for glacier-related hazards that may be diffi‐cult to predict. For example, ice mass loss threatens established water supplies for people living down‐stream and alpine hydropower plants that rely directly on the water released by glacier mass loss (Gaoet al.,2019; Schaefliet al., 2019). Conversely, ice mass loss can enhance available water resources, although these effects are likely to be dispersed temporally and spa‐tially.By 2100,one-third of ice-covered basins world‐wide might experience runoff declines greater than 10% due to glacier mass loss in at least one month of the melt season, with the largest in central Asia and the Andes (Huss and Hock, 2018). Given the large and growing populations currently living downstream of thinning glaciers,the importance of glacier meltwa‐ter is expected to rise.In fact,this water resource is al‐ready considered to be under stress and will become more so during future drought(Pritchard,2019).

Recent climate change has threatened the vast fro‐zen reservoir of mountain glaciers. Glaciers are shrinking worldwide: their rate of shrinking has been increasing since the turn of the 21st century (Zempet al., 2015), with a mean mass balance of −0.42 m w.e.per year(Gardneret al.,2013).Under four representa‐tive concentration pathways (RCPs), 22 general circu‐lation models (GCMs) predict that glaciers in High Mountain Asia will lose between 29%±12% (RCP 2.6)and 67%±10% (RCP 8.5) of their total mass relative to 2015 by the end of this century (Rounceet al.,2019). Such extensive shrinkage and continuous mass loss are exacerbating the deficit in total glacier water resources. This increases the variability of meltwater resources and reduces the efficiency of meltwater uti‐lization and can become a potential but as yet unquan‐tified disaster in the mountain cryosphere. In recent decades, the meltwater resources in the Indus River,Tarim River, and Amu Darya River basins have be‐come particularly vulnerable compared with other drainage basins of the global cryosphere (Pritchard,2019). Accelerating and atypical rates of ice loss can exert serious irreversible effects on local water re‐sources and hydrological buffers vital for ecology,agriculture, and hydropower. In many cases, these high rates of ice loss can also cause secondary glacial meltwater flooding and GLOFs (Bolchet al., 2012;Pritchard, 2019).Thus, ice mass loss boosts the threat to water resources, biodiversity, and associated eco‐system services in mountain regions.

7 Paraglacial destabilization

Paraglacial destabilization related to glacier chang‐es or associated permafrost and snowpack changes in‐cludes rockfalls, rock avalanches, and slope failures/slides in periglacial mountain regions. A number of glacial mechanisms can trigger these processes. (1)Glacier retreat produces large quantities of poorly con‐solidated moraine prone to paraglacial destabilization(Evans and Delaney, 2015). (2) Glacier retreat leaves the original ice-covered bedrock exposed, promoting slope weathering, fragmentation, and destabilization(Evans and Delaney, 2015). (3) Ice mass loss can also change the temperature and stress regimes in the un‐derlying bedrock due to infiltration of meltwater, re‐sulting in the thawing of frozen bedrock and hence in‐stability (Huggel, 2009). (4) Glacier erosion both steepens and lengthens valley sides and increases shear and self-weight stresses, leading to instabilities within the valley side slopes (Mccoll, 2012). (5) Gla‐cier de-buttressing (i.e., the removal of the glacier load and the subsequent release of stress through gla‐cier downwasting/retreat) can reduce slope stability and hasten slope collapse (Delineet al., 2015). (6)Ice meltwater may exert hydrostatic pressure (if not flowing) and seepage pressure (if flowing) on the walls of joints/clefts, which may compromise slope instability and trigger paraglacial destabilization(Mccoll,2012).

Glacier changes can have considerable effects on the surrounding rock slope over centuries and even de‐cades. There is often a significant lag time between glacier retreat and the peak of paraglacial destabiliza‐tion activity (Mccoll, 2012). Most bedrock instabili‐ties are located in areas where surface ice has disap‐peared recently, and the failure zones are frequently correlated spatially to areas left open by glacier with‐drawal or permafrost degradation (Mccoll, 2012).Ac‐cordingly,failure zones often develop from lower alti‐tudes and move higher progressively as the glacier re‐treats upward(Fischeret al.,2013).It has been report‐ed that 19 of 20 rock avalanches recorded in the cen‐tral Southern Alps of New Zealand over the last 100 years initiated from locations less than 300 m (in the vertical direction) from glaciers (Allenet al., 2011).Moreover, many massive slope failures that occurred in the 1990s and 2000s in the high-mountain regions of the Alps, Alaska, western Canada, and New Zea‐land were preceded by periods of extremely high max‐imum temperatures in the days to weeks before slope failure (Huggelet al., 2012). Therefore, further cli‐mate warming and extensive glacier retreat would likely be paralleled by escalations in the frequency and magnitude of paraglacial destabilization in many high-mountain regions worldwide. For instance, a large rock slope failure occurred on the eastern face of Monte Rosa in the Italian Alps,which is partly cov‐ered by steep glaciers that have thinned and retreated over the past 30 years. This slope failure involved about 0.3×106m3of rock at about 4,000 m a.s.l. that likely detached from a dip slope, which experienced a large amount of ice loss and subsequent alteration of the local temperature and stress fields in recent years(Fischeret al.,2011).

8 Remarks and outlook

Mountain glacier-related hazards are systematic processes that are inherently associated with the heat and water flow related to glacier changes. They are usually characterized by their spatial aggregation (i.e.,adjacent to glaciers) and temporal repeatability (with different return periods). They frequently result in a glacier-related hazard chain, where clusters of differ‐ent hazards are spatially and temporally linked. The manifestations of such hazards are widespread in the mountain cryosphere. Recent ice meltwater and ther‐mal perturbations caused by climatic changes may in‐crease the likelihood of glacier-related hazards occur‐ring in glacierized mountain regions (e.g., recently in‐creasing glacier hazards was reported on the Tibetan Plateau and its surrounding alpines (Wuet al.,2019)).Therefore, the spatio-temporal distribution of glacierrelated hazards and their characteristics (including magnitude and frequency) are changing and require particularly adequate attention in the future.

Remote sensing data are essential to inform moni‐toring, characterization, mapping, and modeling ef‐forts for glacier-related hazard processes and their in‐teractions and effects.In the future,systematic glacierrelated hazard surveys should be enhanced by consid‐ering integrated ground−air−space patterns for typical glacierized regions where glacier-related hazards oc‐cur frequently. The production of inventories of past catastrophic events associated with glacier-related hazards should be a fundamental prerequisite for as‐sessing hazards and risks and should consider differ‐ent hazard types on a global scale. For example, in‐ventories could take the form of a global dataset of glacier outburst floods (Carrivick and Tweed, 2016).It will also be crucial to consider synthetic glacier-re‐lated hazards on a regional scale.

Recent advances in the modeling of glacier-relat‐ed hazards have included the following: robust proba‐bilistic estimation of average GLOF return periods(Vehet al., 2020); application of the enthalpy balance theory of surging glaciers and a vertically integrated thermomechanical ice dynamics model (Bennet al.,2019; Clarke and Hambrey, 2019); development of a coupled catastrophic mass flow model (incorporating ice/rock avalanches, GLOFs, and flooding) (Sch‐neideret al., 2014); and development of an r.avaflow computational framework to process chains of multilake outburst floods incorporating two-phase mass flow (Mergiliet al., 2018). However, the simulation of chains of hazardous processes in the mountain cryosphere is likely to remain challenging in the near future. We emphasize the importance of developing new numerical models based on multi-source data. In particular, we suggest prioritizing the collection of in‐tensivein situsurvey data to describe the complex thermal−hydrogeomorphic dynamics of glacier-relat‐ed hazards and associated process chains in the moun‐tain cryosphere.

Acknowledgments:

The study was funded by the Ministry of Science and Technology (2018YFE010010002) and the National Natural Science Foundation of China (No. 41771075 and No. 41701061).We thank Lynsey MacLeary, PhD,from Liwen Bianji,Edanz Editing China(www.liwen‐bianji.cn/ac), for editing the English text of a draft of this manuscript.