Document: 3 Early Warning Systems, Monitoring, and GLOF Mitigation
Section: 3 Early Warning Systems, Monitoring, and GLOF Mitigation > Mitigation measures > Bhutan
Type: text

The earliest awareness of glacial lakes in Bhutan dates from the 1960s (Gansser 1966). After the Luggye Tsho GLOF event of 7 October 1994 in Punakha Wangdue valley, emphasis was placed on GLOF risk mitigation. The Royal Government of Bhutan sent an Indo-Bhutan team of experts to the Lunana area in 1995 to investigate any residual risk. The team recommended immediate mitigation measures for Raphstreng Tsho, including an attempt to lower the lake's water level as soon as feasible. Funding was provided by the Government of India with consulting by Water and Power Consultancy Services (India) Ltd (WAPCOS). The original plan was to reduce the lake level by 20 m in the first phase. WAPCOS found that this would actually take seven to eight years, but that lowering of the lake level by only four metres would also significantly reduce the risk of overtopping. Controlled opening of the moraine dam was carried out by manually widening the outlet channel with crowbars, pickaxes, and spades. Although a method of pumping to reduce the lake level was attempted initially, it was discarded as both ineffective and too expensive for such a remote region. Despite numerous constraints, such as having to work with manual tools, and obstructions caused by huge boulders in the channel bed, the water level in the main lake was lowered by 0.95 m, and in the two subsidiary lakes by 0.94 m and 1.5 m, by October 1995 (Häusler and Leber 1998; ICIMOD 2001b). The lowering continued until the lake level was reduced by four metres in 1998 (UNDP-ECHO 2007a).

Phase 2 of the Raphstreng Tsho Outburst Flood Mitigation Project began in 1999 supported by Austro-Bhutanese Cooperation. The main aim was to assess the geo-risks of Raphstreng Tsho and Thorthormi Tsho in the Lunana area. The activities involved fieldwork with an integrated multidisciplinary approach using remote sensing, geological, hydro-geological, and geophysical methods to interpret the subsurface characteristics of the moraine dam (Häusler et al. 2000; Mool et al. 2001b). The investigations indicated that the risk of an outburst from Raphstreng Tsho was low, but the risk from Thorthormi Tsho was high (Häusler et al. 2000).

Thus a project for 'Reducing climate change-induced risks and vulnerabilities from glacial lake outburst floods in the Punakha-Wangdue and Chamkhar valleys' was initiated by UNDP to run from 2008 to 2012 in conjunction with the Department of Geology and Mines, the Department of Energy (Ministry of Economic Affairs), and the Disaster Management Division (Ministry of Home and Cultural Affairs). The Global Environment Fund (GEF) has provided US\$ 3.5 million; the project is mainly examining the effectiveness of structural risk reduction measures. Activities cover risk reduction measures for Thorthormi glacial lake, including artificial lowering; hazard zonation mapping in


Document: 3 Early Warning Systems, Monitoring, and GLOF Mitigation
Section: 3 Early Warning Systems, Monitoring, and GLOF Mitigation > Mitigation measures > Bhutan
Type: text

Thus a project for ' Reducing climate change - induced risks and vulnerabilities from glacial lake outburst floods in the Punakha - Wangdue and Chamkhar valleys ' was initiated by UNDP to run from 2008 to 2012 in conjunction with the Department of Geology and Mines , the Department of Energy ( Ministry of Economic Affairs ) , and the Disaster Management Division ( Ministry of Home and Cultural Affairs ) . The Global Environment Fund ( GEF ) has provided US \ $ 3 . 5 million ; the project is mainly examining the effectiveness of structural risk reduction measures . Activities cover risk reduction measures for Thorthormi glacial lake , including artificial lowering ; hazard zonation mapping in

"Installing an automatic early warning system (EWS) to reduce the impact of a glacial lake outburst flood (GLOF) in the Punatshangchu basin has become urgent, according to participants of an inception workshop on the regional climate risk reduction project in Punakha. The plan to establish an automatic EWS has not taken off yet, although the government in early 2009 said that automatic sirens would be set up in the villages down from the Lunana area, which would warn people at the first hint of any impending floods in the area. The earlier plan was to install an automatic sensor a few kilometers upstream of Phochu to detect the first flood and five sirens along the area...... To make EWS comprehensive, there will now be two sensors at two locations: one at Lunana and the other 25-30 km upstream of the Phochu. United Nations Development Programme's (UNDP) Kinley Penjor said UNDP would develop a soft package to prepare or train people in the valleys to make EWS effective. He said 'after the machinery is in place, it's important for people to know what to do and how to respond to climate induced GLOF.' " (Kuensel Online 18 Jan 2010)

Chamkar chu in Bumthang; and the expansion of an early warning system along the Punakha-Wangdue valley. In addition, there are plans for improvement of national, regional, and local capacities to avert climate change-induced disaster in the Punakha-Wangdue and Chamkhar valleys. The project should demonstrate practical measures to reduce the risks associated with the Thorthormi glacial lake and facilitate replication of the lessons learned in other high-risk areas, both within and outside Bhutan (Kuensel Online, 7 December 2009; www.managingclimaterisk.org).

Proposals have also been made for Samdingkha to have a siren tower, given its high vulnerability to GLOFs. Two possible locations were identified: Siren 1 below Lorina village on the right side of Pho Chu facing Samdingha and about 60 m from the river level; and Siren 2, about 200 m above the suspension bridge. Both locations provide a good view of the vulnerable areas.

A GLOF hazard zonation plan for the Puna Tshang Chu valley from Khuruthang in Punakha to Kalikhola Lhamoyzinkha is among various risk management measures the Department of Geology and Mines (DGM) has prepared with the Netherlands Climate Assistance Programme (NCAP) (Karma et al. 2008; Kuensel Online 7 December 2009).


Document: 3 Early Warning Systems, Monitoring, and GLOF Mitigation
Section: 3 Early Warning Systems, Monitoring, and GLOF Mitigation > Mitigation measures > Nepal
Type: text

The Tsho Rolpa Mitigation and Early Warning Programme was the first glacial lake outburst flood operation to include civil engineering structures in the entire HKH region. The early warning measures are described in some detail above. The preliminary investigation of Tsho Rolpa and its downstream section of the Rolwaling valley was undertaken by WECS in 1993 (WECS, 1993, 1994; Mool, 1995). Investigations in subsequent years led to a recommendation for immediate action, as well as for long-term measures. As an immediate measure, Wavin Overseas B.V., Holland, installed siphons over part of the terminal moraine in May 1995. This was primarily to test the use of siphons at high altitude. The project was undertaken in cooperation with the Netherlands-Nepal Friendship Association. The siphon system consisted of three inlet pipes submerged in the lake and connected to a single pipe with a discharge outlet located at a stable part of the outer flank of the moraine. Though the test siphons were installed successfully, the induced outflow was far below that required to ensure reducing the lake level by the targeted three metres, and it appeared that they might never exceed inflow of additional glacier and snow melt.

In the second phase of the mitigation measures, supported by a contribution of \$2.9 million from the Dutch Government, an open channel was cut through the moraine dam and a four-metre deep artificial spillway succeeded in lowering the lake level by three metres. The spillway construction was completed by the Tsho Rolpa GLOF Risk Reduction Project of the Department of Hydrology and Meteorology in June 2000.


Document: 3 Early Warning Systems, Monitoring, and GLOF Mitigation
Section: 3 Early Warning Systems, Monitoring, and GLOF Mitigation > Mitigation measures > Pakistan
Type: text

The high altitude, fragile environment, and isolated nature of Pakistan's Northern Areas poses special constraints and challenges for mitigating natural hazards such as glacial lake outburst floods.

The glacial lakes in this area are predominantly ice-dammed lakes resulting from local glaciers that have thickened and advanced in recent decades (Hewitt 2009). They must be differentiated from the supra-glacial and moraine-dammed lakes that are the principal concern of this report. Five GLOF events were reported during the first half of 2008 in the Gojal area of the Hunza valley and substantial damage to infrastructure and arable land was reported due to GLOF events in association with the Ghulkin and Passu glaciers. There is no regular early warning system in the Hunza river basin, and the local people use traditional methods, such as lighting torches and firing weapons, to warn the villagers of flash floods.

Risk reduction measures taken in Gojal include excavation, channelling, and spillway development. (UNDP-ECHO 2007b). A local village community used a siphoning technique to drain the lake associated with the Ghulkin glacier and reduce the threat posed by a potential outburst flood. A lateral moraine was excavated to set up the siphon (Roohi et al. 2008).


Document: Characteristics and changes of glacial lakes and outburst floods
Type: figure
Figure

Image /page/0/Figure/3 description: A user interface element showing an icon next to text that reads 'Check for updates'. The icon is a square containing a circle with a color gradient from yellow to blue to red, with a red shape in the center.

data/('Characteristics and changes of glacial lakes and outburst floods', '.pdf')_extraction.md


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods
Type: text

Guoqing Zhang 1,14 □, Jonathan L. Carrivick 2,14 □, Adam Emmer3,14, Dan H. Shugar 4,14, Georg Veh5,14, Xue Wang1,6, Celeste Labedz4, Martin Mergili3, Nico Mölg7, Matthias Huss 8,9,10, Simon Allen11,12, Shin Sugiyama 13 & Natalie Lützow 5


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Abstract
Type: text

Global glacier mass loss has accelerated, producing more and larger glacial lakes. Many of these glacial lakes are a source of glacial lake outburst floods (GLOFs), which pose threats to people and infrastructure. In this Review, we synthesize global changes in glacial lakes and GLOFs. More than 110,000 glacial lakes currently exist, covering a total area of ~15,000 km2, having increased in area by ~22% dec-1 from 1990 to 2020. More than 10 million people are exposed to the impacts of GLOFs, commonly associated with dam failure or wave overtopping associated with mass movements. Although data limitations are substantial, more than 3,000 GLOFs have been recorded from 850 to 2022, particularly in Alaska (24%), High Mountain Asia (HMA; 18%) and Iceland (19%), the majority (64.8%) being from ice-dammed lakes. Recorded GLOFs have increased in most glaciated mountain regions of the world, with ongoing deglaciation and lake expansion expected to increase GLOF frequency further. In HMA, GLOF hazards are projected to triple by 2100, but changes in other regions will likely be lower given topographic constraints on lake evolution. Future research should prioritize acquiring field data on lake and dam properties, producing globally coordinated multitemporal lake mapping, and robust and efficient modelling of GLOFs for comprehensive hazard assessment and response planning.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Sections
Type: text

Introduction

Global and regional changes of lake size and abundance

Historical glacial lake outburst floods

Future glacial lakes and outburst floods

Summary and future perspectives

A full list of affiliations appears at the end of the paper. ⊠e-mail: guoqing.zhang@itpcas.ac.cn; J.L.Carrivick@leeds.ac.uk


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Introduction
Type: text

Glaciers have been losing mass since the end of the Little Ice Age1-3, and at high – and accelerating – rates since the 1960s (refs. 4,5). Indeed, overall mass loss from glaciers and ice caps (excluding the Greenland and Antarctic ice sheets) totalled  $267 \pm 16$  Gt year-1 from 2000 to 2019 (ref. 4), much of which can be attributed to anthropogenic warming6-8. When the resultant meltwater is naturally dammed (by ice, a moraine or bedrock), glacial lakes can form on (supraglacial), within (englacial), under (subglacial), at the edge of (ice-marginal) or beyond the front of (proglacial) glaciers. There are an estimated 14,394 of these lakes (>0.05 km2) globally9, the majority of which are in Greenland, the Southern Andes, Antarctica and the Subantarctic, Alaska, Canada and High Mountain Asia (HMA) (Fig. 1a). The number and size of glacial lakes is generally increasing, especially for proglacial lakes, owing to close connections with contemporary glacier retreat and meltwater production at local10,11, regional12-15 and global9 scales. However, there is considerable regional variability9,16–19, including notable expansion in Alaska, Greenland periphery, Patagonia and HMA. Nevertheless, the overall growth pattern is expected to continue20-24, in line with a projected 26-41% decrease in glacier mass by 2100 under temperature increases of 1.5-4 °C25.

The stability and longevity of glacial lakes are highly variable. Glacial lakes can drain suddenly and very quickly (hours to days), releasing a glacial lake outburst flood (GLOF). More than 3,000 GLOFs have been reported worldwide from 850 to 2022 (ref. 26) (Fig. 1b), each with characteristic rapid rise to peak discharge and often very large flood volumes. For example, the 1996 volcanic eruption beneath the Vatnajökull ice cap, Iceland, produced an outburst flood with an estimated peak discharge of ~55,000 m³ s⁻¹, thereby temporarily becoming the second largest river in the world after the Amazon²7,28. Indeed, ice-dammed lakes have produced some of the largest peak discharges recorded in human history²9-3¹, including peak discharges of >10⁵ m³ s⁻¹ in HMA³² and 10° m³ s⁻¹ in the Andes and Alaska³0,33,3⁴.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Introduction
Type: text

Fig . 1b ) , each with characteristic rapid rise to peak discharge and often very large flood volumes . For example , the 1996 volcanic eruption beneath the Vatnajökull ice cap , Iceland , produced an outburst flood with an estimated peak discharge of ~ 55 , 000 m³ s ⁻ ¹ , thereby temporarily becoming the second largest river in the world after the Amazon²7 , 28 . Indeed , ice - dammed lakes have produced some of the largest peak discharges recorded in human history²9 - 3¹ , including peak discharges of > 10⁵ m³ s ⁻ ¹ in HMA³² and 10 ° m³ s ⁻ ¹ in the Andes and Alaska³0 , 33 , 3⁴ .

Accordingly, GLOFs rank among the most prominent glacier hazards, killing thousands of people over the past century35 and causing extensive damage tens to hundreds of kilometres downstream. For instance, the 1941 GLOF in Huaraz, Peru, destroyed a third of the city and killed about 5,000 people36, and the 2013 GLOF in Kedarnath, India, killed more than 6,000 people37. Given that millions of people are potentially at risk from GLOFs worldwide38, especially in a warming climate, understanding the location, timing of formation, evolution and physical characteristics of glacial lakes is critical to mitigating the downstream impacts to communities and infrastructure.

In this Review, we synthesize glacial lake and GLOF hazard data sets from most glaciated regions of the world to assess their ongoing and projected changes. We begin by documenting the global and regional distribution of glacial lakes. We follow by assessing the characteristics and historical trends of GLOFs. We subsequently discuss future projections in glacial lakes and the corresponding GLOF hazard and risk, before ending with research priorities. Throughout, focus is on the physical aspects of glacial lakes and GLOFs in a warming world.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance
Type: text

Understanding glacial lakes is the first step in estimating the GLOF hazard and risk (Fig. 2). Methods used to map glacial lakes are now discussed, followed by quantification of their historical changes at global and regional scales.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Mapping glacial lakes
Type: text

A growing archive of satellite products and enhanced computing power39 have improved mapping of glacial lakes (Fig. 2). In most cases, mapping relies on the use of the normalized difference water index (NDWI)40 – a satellite image-derived calculation combining green and near-infrared wavelength bands to differentiate water bodies from all other landcover. Automated and semi-automated methods are used for lake mapping from an NDWI classification, although lake outlines have also been delineated manually with enhanced accuracy12,41. Landsat satellite imagery is often used as the basis for glacial lake mapping by the NDWI owing to its temporal coverage and a medium-resolution sensor 9,17,42. Nevertheless, Landsat imagery has limitations for NDWI-based lake mapping, including poor image contrast due to seasonal snow and  $ice\,cover\,on\,lakes, as\,well\,as\,confusion\,introduced\,by\,mountain\,shadows$ and cloud cover43. Cloud-free synthetic aperture radar (SAR) can overcome this limitation, but is of limited length and hence, generally, only suitable for validation of mapping algorithms or case studies  $^{44,45}\!$  . Cloud computing, artificial intelligence and machine learning are rapidly improving the ability to efficiently manage and automatically analyse remote sensing data46,47. However, interactive visual inspection and manual editing combined with original imagery are still essential to ensure a consistent data quality with time.

In addition to mapping lake outlines, knowledge of glacial lake volume is important for quantifying hazards and for estimating downstream impacts of GLOFs48,49 (Fig. 2). Empirical volume–area relationships for glacial lakes have been established in many instances (see Supplementary Table 1). Global compilations of lake depths, areas and volumes suggest that lake area and depths are moderately correlated, and lake area and volumes are well correlated50. These correlations allow for the estimation of lake depths and volumes from area, especially in regions where direct measurements are rare and difficult (see Supplementary Table 1). Bathymetric measurements of ice-marginal lakes have also been carried out in some locations, including the Southern Alps of New Zealand51,52 and on the Tibetan Plateau53,54.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Global glacial lake characteristics
Type: text

After mapping the outline of glacial lakes, it is possible to determine their overarching characteristics, including size, area and volume. Various such inventories exist (see Supplementary Table 2), each compiled from satellite imagery but differing in maximum distance between glaciers and lakes (buffers of 1, 3 or 5 km) and the smallest lake size included  $(0.0002, 0.005 \, \text{or} \, 0.05 \, \text{km}^2)$ , introducing slight discrepancies at the regional and global scales. Here, several regional inventories  $^{9,12,18,19}$  are merged to produce a global contemporary glacial lake compilation; a size threshold of  $0.05 \, \text{km}^2$  is set for all Randolph Glacier Inventory (RGI) regions  $^{55}$  except Svalbard and Jan Mayen, Southern Andes and New Zealand (see Supplementary Table 2).

This global compilation provides much insight into glacial lake characteristics. A total of 22,133 glacial lakes were revealed in 2015–2019, collectively covering -14,438 km² (see Supplementary Table 2). These totals are more than other reports of 14,394 lakes (>0.05 km²) situated within 1 km of glacier margins, covering a total area of -8,950 km² in 2015–2018 (ref. 9). Regional differences in the number, area and volume of global glacial lakes are evident (Fig. 1a). Glacial lakes are predominantly distributed in Greenland (31% of the total area), the Southern Andes (16%), Alaska and western Canada (16%), the eastern Canadian Arctic (15%) and HMA (6%). Glacial lakes in HMA are widespread, but with a smaller signal size (average area of 0.22 km² and volume of 0.002 km³ per lake). The remaining water mass storage in


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Global glacial lake characteristics
Type: figure
Figure

Image /page/2/Figure/1 description: This figure, titled "Global distribution of glacial lakes and historical GLOFs," is divided into two parts, 'a' and 'b'.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Global glacial lake characteristics
Type: text

Part 'a', labeled "Glacial lake characteristics," displays a world map with red dots indicating the distribution and size of glacial lakes. Superimposed on the map are several multi-ringed circular charts for different regions, showing the total volume of glacial lakes in Gigatons (Gt). The volumes for each region are as follows:
- Alaska: 22.0 Gt
- Western Canada and USA: 8.4 Gt
- Arctic Canada, north: 1.9 Gt
- Arctic Canada, south: 24.9 Gt
- Greenland periphery: 43.6 Gt
- Scandinavia: 5.6 Gt
- Russian Arctic: 7.2 Gt
- Iceland: 2.4 Gt
- Central Europe: 0.2 Gt
- Caucasus and Middle East: 0.03 Gt
- Asia, North: 0.4 Gt
- Asia, Central: 1.7 Gt
- Asia, Southwest: 0.7 Gt
- Asia, Southeast: 2.1 Gt
- Low-latitude Andes: 0.7 Gt
- Southern Andes: 38.3 Gt
- New Zealand: 0.2 Gt
A large chart at the bottom indicates a global total volume of 160.3 Gt. A legend explains that the concentric rings on these charts represent Lake size (km²), Volume ratio, Area ratio, and Number ratio.

Part 'b', labeled "GLOF characteristics," shows a similar world map illustrating the locations of historical Glacial Lake Outburst Floods (GLOFs). Pie charts for various regions depict the proportion of different lake (dam) types that caused these floods. The legend for the dam types includes: Bedrock, Combined, Ice, Moraine, Supraglacial, Englacial, and Others. The pie charts show that moraine-dammed lakes (orange) and bedrock-dammed lakes (light beige) are common causes of GLOFs in many regions. A legend also provides a scale for the number of GLOFs.

Fig. 1 | Global distribution of glacial lakes and historical GLOFs. a, Area and volume of present-day (-2018) glacial lakes within 1 km of glaciers, aggregated over 220 km hexagon grid cells (red shades), and regional statistics of lake volume, area and number according to Global Terrestrial Network for Glaciers (GTN-G) glacier regions of the Randolph Glacier Inventory (RGI) (doughnut plots); outer, middle and inner circles indicate the total volume, area and number percentage of glacial lakes in a given size interval, respectively, with the total lake area in each region in the centre of the circle. Two RGI subregions (Svalbard and Jan Mayen, Antarctic and Subantarctic) are not included as they are not part of

the underlying global glacial lake data set\*. **b**, Global distribution of historical glacial lake outburst floods (GLOFs) aggregated over 220 km hexagon grid cells (red shading), and regional breakdown of the GLOF number by glacial lake (dam) types²6 (pie charts). Some RGI subregions are not presented as they are not covered by the global GLOF inventory²6. More than 80% of global glacial lakes are distributed predominantly in Greenland, the Southern Andes, Alaska and western Canada, the eastern Canadian Arctic and High Mountain Asia (HMA); >60% of historical GLOFs occurred in Alaska, HMA and Iceland.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Global glacial lake characteristics
Type: figure
Figure

Image /page/3/Figure/1 description: A diagram illustrating the components of assessing glacial lake outburst flood (GLOF) risk. The diagram features three main circular sections arranged in a triangle around a central concept of 'Risk'.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Global glacial lake characteristics
Type: text

The top red section, 'Mapping of glacial lake', is divided into 'Manual' and 'Automatic or semi-automatic' methods. These methods utilize 'High-resolution satellite data', 'Cloud-free SAR imagery', 'Optical multispectral imagery', 'Cloud computing', 'Artificial intelligence', and 'Machine learning'.

The bottom-left yellow section is labeled 'Estimates of volume and discharge'. It shows a process flow from 'Area → volume' to 'Volume → discharge', based on an 'Empirical relationship'.

The bottom-right blue section is titled 'GLOF susceptibility and simulation'. The inner circle is split between 'GLOF susceptibility' and 'GLOF simulation'. The outer ring, labeled 'Hydrodynamic modelling', lists factors such as 'External triggers (landslide, ice/snow avalanche, extreme rainfall)', 'Lake expansion', 'Ice dam flotation', 'Dam stability', 'Downstream sediment transport and inundation', 'Moraine erosion and failure', 'Wave generation and overtopping', 'Hydrodynamics of lake', and 'Ice or rock avalanche into a lake'.

Arrows show that all three sections feed into the central concept of 'Risk', which is defined as 'Socio-economic exposure and vulnerability'. There is also an arrow from the 'GLOF susceptibility and simulation' section to the 'Estimates of volume and discharge' section.

Fig. 2 | Methodology for quantifying glacial lake changes and impacts. Methods to map glacial lakes (red), estimate lake volume and peak (yellow) and assess glacial lake outburst flood (GLOF) susceptibility and simulation (blue). Glacial lake mapping, estimates of GLOF susceptibility, volume and peak discharge, and hydrodynamic simulations provide important baseline integrated data for GLOF hazard assessment. SAR, synthetic aperture radar.

the world's glacial lakes in 2015–2018 was about 160 Gt, equivalent to about 0.43 mm of sea-level rise9.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Glacial lake changes
Type: text

Glacial lakes are naturally transient features. Each tends to follow a common trajectory, as documented in many mountainous regions around the world16,56-59. Specifically, glacial lakes showed a relatively stable trend from 1850 to 1970, followed by a gradual expansion in most regions of the world from 1970 to 1990 and a robust expansion from 1990 to 2020 (see Supplementary Fig. 1). Various processes can lead to such changes in glacial lake evolution, including shifts in inflow or outflow; gradual infilling by sediment, particularly in deglaciating environments23,60,61; abrupt infilling following mass movements62,63; coalescence of several lakes; detachment of an ice-marginal lake from the parent glacier as it retreats; or partial or complete dam failure resulting in a GLOF23. Although global statistics are unavailable, evidence from the tropical Andes suggests that, on average, only one in ten GLOFs has resulted in complete lake drainage62. Indeed, most moraine-dammed and bedrock-dammed glacial lakes persist after a GLOF has occurred62. Accordingly, lakes can exist for vastly different timeframes: ice-dammed lakes can form and fail repeatedly as long as the ice dam is thick enough to impound a lake  $^{33,64}$ ; bedrock-dammed lakes within overdeepenings can persist for tens of thousands of years65-67; and moraine-dammed lakes can survive over centennial or millennial timescales68.

The aforementioned inventories provide an opportunity to track these glacial lakes over time (Fig. 3a; see Supplementary Fig. 1). Overall, the area of global glacial lakes expanded steadily from 1990 to 2018. Specifically, the glacial lake (>0.05 km²) number and area increased by -53% (from 9,414 to 14,379) and 51% (from  $5.93 \times 10^3$  km² to

$8.95 \times 10^3$  km2) from 1990 to 2018 (ref. 9). However, this global increase is subject to uncertainties due to missing lakes when compared with regional multi-temporal lake inventories (see Supplementary Table 2).


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Glacial lake changes
Type: text

, the area of global glacial lakes expanded steadily from 1990 to 2018 . Specifically , the glacial lake ( > 0 . 05 km² ) number and area increased by - 53 % ( from 9 , 414 to 14 , 379 ) and 51 % ( from $ 5 . 93 \ times 10 ^ 3 $ km² to $ 8 . 95 \ times 10 ^ 3 $ km  2  ) from 1990 to 2018 ( ref . 9 ) . However , this global increase is subject to uncertainties due to missing lakes when compared with regional multi - temporal lake inventories ( see Supplementary Table 2 ) .

Regional data sets, which undergo extensive quality control during compilation, also indicate a general increase in glacial lake area. Between ~1990 and ~2020, glacial lake area increased by an average of ~22% dec-1 (Fig. 3a), higher than the 17% dec-1 suggested by the global data set9. Of course, much regional and temporal variability is embedded. For instance, overarching changes of <10% dec-1 from ~1990 to ~2020 were observed in the Southern Andes (from 4.463.9 km2 to 4,789.7 km2), Central Europe (from 30.9 km2 to 31.5 km2) and Svalbard (from 110.0 km2 to 111.0 km2) (Fig. 3a), potentially highlighting the importance of short-term temporal variability; lake area temporarily decreased at the edge of the Northern Patagonian Icefield from 1945 to 1976 (ref. 69) and in Greenland from 1987 to 1992 and from 2005 to 2010 (ref. 16), perhaps contributing to the smaller, yet still positive, long-term trends16,69. In contrast, much larger changes of >40% dec-1 were observed in Iceland (from 54.8 km2 to 132.4 km2), Scandinavia (from 258.2 km2 to 596.9 km2) and the Russian Arctic (from 189.9 km2 to 478.4 km2) (Fig. 3a).

In HMA, local variability is observed, as evidenced by sub-regional records of glacial lake growth (see Supplementary Fig. 1). In the greater Himalaya, glacial lake area increased by ~3.6 km² year $^{-1}$  (0.6% year $^{-1}$ , from 666.4 km² to 774.7 km²) from 1990 to 2020. Yet changes in the Chinese Himalaya and Nepal Himalaya were ~1.28 km² year $^{-1}$  (0.8% year $^{-1}$ , from 166.5 km² to 215.3 km²) from 1970 to 2008 (ref. 70) and 0.6 km² year $^{-1}$  (1.1% year $^{-1}$ , from 55.5 km² to 81.0 km²) from 1977 to 2017 (ref. 71), respectively.

In addition to such substantial local and regional changes, there is also evidence of changes by glacial lake type. Ice-marginal proglacial


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Glacial lake changes
Type: text

. 28 km² year $ ^ { - 1 } $ ( 0 . 8 % year $ ^ { - 1 } $ , from 166 . 5 km² to 215 . 3 km² ) from 1970 to 2008 ( ref . 70 ) and 0 . 6 km² year $ ^ { - 1 } $ ( 1 . 1 % year $ ^ { - 1 } $ , from 55 . 5 km² to 81 . 0 km² ) from 1977 to 2017 ( ref . 71 ) , respectively . In addition to such substantial local and regional changes , there is also evidence of changes by glacial lake type . Ice - marginal proglacial

lakes are in direct contact with their parent glaciers and have strong interactions between them, which is mainly dammed by moraines, with more GLOFs recorded by lake type26,68. The imbalance between ice-marginal and glacier-detached proglacial lakes has been recorded worldwide (Fig. 3b-e). For example, in Southern Pamir, the number of proglacial lakes remained largely constant from 1968 to 1993 (the lower number in 1977 might be related to uncertainties due to poor image quality), increased to a peak of 44 in 2008 and then decreased to 26 by 2015 (ref. 72) (Fig. 3b).

In the Andes, the number and area of proglacial lakes have progressively increased. From 1948 to 2017, three to four new proglacial lakes formed each year, with an average expansion rate of ~0.1 km² year¹ (ref. 73). Of these total lakes, the number of proglacial lakes was stable from 1948 to 1990 (71–74 lakes, with some fluctuations in between) and then decreased to 35 by 2018 (ref. 73). For the Peruvian Andes, an almost constant gain of detached proglacial lake area was observed throughout the study period (Fig. 3c). This phenomenon, together with the increasing average elevation of proglacial lakes, is a strong indication that large lakes dammed by Little Ice Age terminal moraines have lost contact with their parent glaciers³3.

In the Southern Alps of New Zealand, an almost linear trend in proglacial lake area change has also been reported, with an average of -0.5 km² year⁻¹ in four time steps from 1990 to 2020 (ref. 74). Tasman Glacier, New Zealand's largest glacier, is retreating rapidly, and a proglacial lake formed in 1990 (ref. 52). The number of ice-marginal proglacial lakes in New Zealand was greatest in 2011, and thereafter has decreased slightly as lakes coalesced and as some became disconnected from the glaciers 74 (Fig. 3d).


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Global and regional changes of lake size and abundance > Glacial lake changes
Type: text

the Southern Alps of New Zealand , an almost linear trend in proglacial lake area change has also been reported , with an average of - 0 . 5 km² year ⁻ ¹ in four time steps from 1990 to 2020 ( ref . 74 ) . Tasman Glacier , New Zealand ' s largest glacier , is retreating rapidly , and a proglacial lake formed in 1990 ( ref . 52 ) . The number of ice - marginal proglacial lakes in New Zealand was greatest in 2011 , and thereafter has decreased slightly as lakes coalesced and as some became disconnected from the glaciers < sup > 74 < / sup > ( Fig . 3d ) .

In the European Alps, particularly the Austrian Alps, the number and area of proglacial lakes increased from 1850 to 2015, with the mean change in area doubling from -0.038 km² year  $^{-1}$  during 1998–2006 to -0.079 km² year  $^{-1}$  during 2006–2015 (ref. 75). Similar changes have occurred in Switzerland. There, the number of proglacial lakes increased from 21 in 1900 to 47 in 2006, and from 47 in 2006 to 82 in 2016 (ref. 59) (Fig. 3e). The increase in the total area of proglacial lakes in the Swiss Alps has accelerated from -0.01 km² year  $^{-1}$  in 1850–1900 up to -0.15 km² year  $^{-1}$  in 2006–2016 (ref. 59). However, part of this variability is due to the artificial impoundment of some proglacial lakes, mainly for hydropower generation purposes.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Historical glacial lake outburst floods
Type: text

With the presence of glacial lakes comes the risk of a GLOF. GLOFs arise when the lake dam is breached (by various triggers depending on the dam type) and meltwater is suddenly and rapidly released from the lake, subsequently flowing downstream where it can cause substantial societal and environmental impacts (Fig. 4). Together, data from river gauges, satellite images, sediment stratigraphy, and eyewitness and media reports suggest that at least 3,151 GLOFs have occurred globally since 850 (ref. 26) (Fig. 1b), with most recorded in Alaska (24%, n = 768), HMA (18%, n = 569) and Iceland (19%, n = 590).

However, understanding of GLOFs is severely limited by inadequate data. Triggers are often unknown or speculated owing to absent field data, necessitating reliance on climatic or geological-geomorphological evidence35,76-78. Numerical modelling is similarly challenged by absent knowledge of parameter values such as lake water temperature or time-varying lake level, limiting insight58,79,80. Moreover, GLOF frequency calculations suffer from selective reporting and changes in instrumentation and satellite coverage33,81,82. Nevertheless, it is possible to glean evidence of GLOF characteristics, as now

discussed for ice-dammed, moraine-dammed and bedrock-dammed GLOFs. Resulting hazards and changes are also assessed.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Historical glacial lake outburst floods > Ice-dammed GLOFs
Type: text

GLOFs from ice-dammed lakes are the most common type. Their activity is largely controlled by the activity of the damming glaciers and resulting fracture or flexure of the ice dam (Fig. 4a). Flotation of the ice dam once a critical lake level is reached also acts as a potential breach mechanism33,83,84. When lake water suddenly drains through an ice dam, thermal and mechanical erosion (usually progressively enlarging a subglacial tunnel) leads to an exponentially rising discharge54,85,86.

GLOFs from ice-dammed lakes constitute roughly two-thirds (2,039 of 3,151) of all reported GLOFs worldwide26,81. Their frequent occurrence is linked to the fact that they can drain repeatedly as an ice dam will 're-heal' temporarily once energetic egress of lake water has subsided. At the regional scale, Alaska is a hot spot of GLOFs from ice-dammed lakes (n = 764; 99% within Alaska, 37% of the global total in this category)19,26,33 (Fig. 1b). Indeed, 750 ice-dammed lakes have formed in Alaska over the twentieth century87,88, with some lakes having a volume >109 m3 when impounded by a thick outlet glacier87. Yet their GLOFs rarely cause damage, owing to low exposure of assets along river channels. Ice-dammed lakes are also the dominant source of reported GLOFs in Greenland periphery (n = 153; 100% of all GLOFs within Greenland periphery, 7.5% of global ice-dammed GLOFs)89,90 the Karakoram (n = 190; 33.4% within HMA, 9.3% of global ice-dammed GLOFs) $^{29,91}$ , Scandinavia (n = 183; 96.8% within Scandinavia, 9.0% of globalice-dammed GLOFs) $^{92}$ , Central Europe (n = 258; 58.2% within Central Europe, 12.7% of global ice-dammed GLOFs) and Iceland (n = 237; 40.2% within Iceland, 11.6% of global ice-dammed GLOFs)86 (Fig. 1b). In Central Europe, fatalities and damage from these occasional ice-dam failures were particularly high before the twentieth century, possibly related to limited warning and flood control measures93. In Scandinavia, and particularly in Norway, although the percentage of ice-dammed GLOFs was high in the mid-twentieth century, the total number and size of outbursts are small compared with other regions26.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Historical glacial lake outburst floods > Moraine-dammed GLOFs
Type: text

GLOFs from moraine-dammed lakes are the second most common type globally after ice-dammed lakes (Fig. 1b). In these situations, dam breaching can be caused by seepage of lake water through unconsolidated material, degradation of an ice core, or overtopping due to calving, avalanches or landslides into the lake76,83 (Fig. 4). Moraine-dammed lakes in the ice-marginal phase of evolution are the most likely to produce GLOFs due to calving processes and proximity to ice avalanche release zones. In contrast, glacial lakes detached from glaciers become disconnected from the most frequent GLOF-triggering processes with increasing distance from glaciers56. The almost instantaneous, and sometimes complete, failure of a moraine dam produces a linearly rising discharge94. Furthermore, complete failure of the (moraine) dam restricts any future ability to impound meltwater95,96. Hence, moraine-dammed lakes usually fail only once.

GLOFs from moraine-dammed lakes make up at least -13% (398 of 3,151) of the global total number of recorded GLOFs from 850 to 2022 (Fig. 1b). However, their occurrence is highly regionally variable. The majority of those reported are found in the low-latitude Andes (n=100; 61.3% of GLOFs within the low-latitude Andes, 25.1% of global moraine-dammed GLOFs), Central Asia (n=110; 46.2% within Central Asia, 27.6% of global moraine-dammed GLOFs) and the Himalaya (n=71; 77.2% within the Himalaya, 17.8% of global moraine-dammed GLOFs) (Fig. 1b).


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Historical glacial lake outburst floods > Moraine-dammed GLOFs
Type: figure
Figure

Image /page/5/Figure/1 description: A multi-part figure from a scientific publication detailing changes in glacial lakes. The figure is divided into five sections labeled 'a' through 'e'.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Historical glacial lake outburst floods > Moraine-dammed GLOFs
Type: text

Part 'a', titled 'Glacial lake area changes', displays a world map with bar charts showing the increase in glacial lake area (in km²) for various regions between two different years. Red dots on the map indicate the locations and the magnitude of area change per decade, with a legend showing dot sizes for percentage changes from 0-5% to 60-70%. The bar charts show the following approximate data:
- Alaska: from ~650 km² in 1984 to ~1,300 km² in 2019.
- Western Canada and USA: from ~250 km² in 1990 to ~400 km² in 2015.
- Low-latitude Andes: from ~40 km² in 1990 to ~70 km² in 2015.
- Arctic Canada, north: from ~110 km² in 1990 to ~230 km² in 2015.
- Arctic Canada, south: from ~700 km² in 1990 to ~1,300 km² in 2015.
- Greenland periphery: from ~1,800 km² in 1990 to ~3,000 km² in 2015.
- Iceland: from ~40 km² in 1990 to ~140 km² in 2015.
- Svalbard: from ~80 km² in 1990 to ~180 km² in 2020.
- Scandinavia: from ~200 km² in 1990 to ~700 km² in 2015.
- Russian Arctic: from ~150 km² in 1990 to ~550 km² in 2015.
- Central Europe: from ~30 km² in 1990 to ~32 km² in 2015.
- Caucasus and Middle East: from ~0.4 km² in 1990 to ~0.7 km² in 2015.
- Asia, Southwest: from ~120 km² in 1990 to ~200 km² in 2018.
- Asia, Southeast: from ~270 km² in 1990 to ~430 km² in 2018.
- Asia, Central: from ~450 km² in 1990 to ~730 km² in 2018.
- Asia, North: from ~10 km² in 1990 to ~19 km² in 2018.
- Southern Andes: from ~3,500 km² in 1986 to ~5,100 km² in 2016.
- New Zealand: from ~7 km² in 1990 to ~32 km² in 2020.
- Global: from ~10,000 km² in 1990 to ~15,000 km² in 2020.

Parts 'b' through 'e' are titled 'Proglacial lake changes' and show detailed charts for specific regions. These charts plot the number of new lakes (blue, above the x-axis) and detached lakes (green, below the x-axis) per year, as well as the area of these lakes. The total area (A) is noted for each time period.

- Part 'b' (Southern Pamir): Data for 1968 (A=0.68 km²), 1977 (A=0.83 km²), 1993 (A=1.09 km²), 2002 (A=1.45 km²), 2008 (A=1.46 km²), and 2015 (A=0.90 km²).
- Part 'c' (Peruvian Andes): Data for 1948 (A=6.27 km²), 1962 (A=5.84 km²), 1970 (A=0.55 km²), 1990 (A=4.98 km²), 2000 (A=4.26 km²), 2010 (A=3.16 km²), and 2018 (A=1.65 km²).
- Part 'd' (New Zealand): Data for 1990 (A=9.60 km²), 2000 (A=1.26 km²), 2011 (A=10.34 km²), and 2020 (A=20.55 km²).
- Part 'e' (Switzerland): Data for 1900 (A=0.20 km²), 1946 (A=0.49 km²), 1973 (A=11.13 km²), 1982 (A=0.40 km²), 2006 (A=0.52 km²), and 2016 (A=0.93 km²).


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Historical glacial lake outburst floods > Moraine-dammed GLOFs
Type: text

( New Zealand ) : Data for 1990 ( A = 9 . 60 km² ) , 2000 ( A = 1 . 26 km² ) , 2011 ( A = 10 . 34 km² ) , and 2020 ( A = 20 . 55 km² ) . - Part ' e ' ( Switzerland ) : Data for 1900 ( A = 0 . 20 km² ) , 1946 ( A = 0 . 49 km² ) , 1973 ( A = 11 . 13 km² ) , 1982 ( A = 0 . 40 km² ) , 2006 ( A = 0 . 52 km² ) , and 2016 ( A = 0 . 93 km² ) .

**Fig. 3** | **Global and regional changes in glacial lake area. a**, Absolute regional lake area in -1990 and -2020 (bars) and the percentage changes from 1990 to 2020 (red circles) (see Supplementary Table 2); owing to data availability, years were selected as close to 1990 and 2020 as possible. **b-e**, Changes in the number (dark blue line) and area (light blue bar) of new lakes, and the number (dark green line) and area (light green bar) of detached proglacial lakes in Southern

Pamir $^{72}$  (panel **b**), Peruvian Andes $^{73}$  (panel **c**), New Zealand $^{74}$  (panel **d**) and Switzerland $^{59}$  (panel **e**) (see panel **a** for locations); time blocks differ owing to contrasting data availability. Error bars indicate uncertainties and inaccuracies due to low image resolution or quality, or the use of images from different years for the same time step. Glacial lakes have expanded worldwide, especially for proglacial lakes.

Indeed, 52% and 32% of glacial lakes in the Central Andes and Himalaya are moraine-dammed, respectively  $^{17,97}$ , and many others have associated geomorphological features suggesting possible past dam failures  $^{62,98,99}$ . The exact timing of these dam failures has yet to be determined, as the geomorphological evidence is largely limited to that visible in satellite images that were first available in the 1960s.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Historical glacial lake outburst floods > Bedrock-dammed and subglacial GLOFs
Type: text

Bedrock dams are usually stable and therefore these glacial lakes produce fewer GLOFs than ice-dammed and moraine-dammed lakes, with dam overtopping being the only mechanism for GLOF occurrence 100,101. GLOFs from bedrock-dammed lakes make up only ~1% (30 of 3,151) of the total number of GLOFs from 850 to 2022 (Fig. 1b). Indeed, overtopping of bedrock barriers has only rarely been reported81, in contrast to the high regional abundance of bedrock-dammed lakes62, especially in the Himalaya and Andes 17,97. Their limited occurrence might arise from the fact that unfractured bedrock dams can withstand increases in hydrostatic pressure, and hence are only overtopped by displacement waves from a mass movement entering the lake62. GLOFs from bedrockdammed lakes are only reported after the 1960s and those occurred mainly in the low-latitude Andes (n = 27; 16.6% within the low-latitude Andes, 90% of global bedrock-dammed GLOFs) (Fig. 1b). Although evidence is limited, GLOF counts from bedrock-dammed lakes might be underestimated given the short preservation time of any geomorphic evidence, especially compared with moraine dam failures102.

Similarly, there are relatively few GLOFs attributed to englacial or subglacial lakes induced by geothermal or volcanic activity compared with other GLOF triggers and mechanisms103. GLOFs from englacial lakes produce poorly preserved geomorphological evidence and are difficult to detect with optical remote sensing. They are mainly reported in Central Europe (n = 112, 25.3% within Central Europe, 69.1% of global englacial GLOFs) (Fig. 1b). Floods caused by rapidly draining englacial lakes have also occasionally been reported in New Zealand, western Canada and the United States 104-106 (Fig. 1b). The majority of GLOFs from subglacial lakes are also found in Central Europe (n = 5; 1.13% within Central Europe, 71.4% of global subglacial GLOFs), but further evidence is found in Southeast Asia (n = 1; 1.18% within Southeast Asia, 14.3% of global subglacial GLOFs) and the low-latitude Andes (n = 1; 0.61%) within the low-latitude Andes, 14.3% of global subglacial GLOFs) (Fig. 1b). In Iceland, for example, at least 264 outbursts from a small number of subglacial lakes31, which repeatedly form and drain owing to geothermal activity beneath the ice caps, have been observed, with further evidence of glacier-volcano interactions both in historical and prehistoric times 107,108.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > GLOF hazard assessment
Type: text

Understanding the characteristics and changes of glacial lakes and GLOF triggers and mechanisms is the basis for GLOF hazard assessment, which is critical for disaster prevention and damage mitigation. Globally, more than 10 million people are potentially exposed to the impacts of GLOFs, mainly in HMA38,109. Potentially dangerous glacial lakes have

been identified using a range of assessment approaches, ranging from large-scale automated methods considering key determinants such as lake size, catchment area, ice/rock avalanche potential and dam steepness110,111, through to detailed catchment or lake-specific assessments including field investigations112-114. Identifying the dam type of the glacial lake is a critical first step in GLOF hazard assessments, as it determines the stability of the lake, triggering processes, the potential magnitude of the GLOF and the mechanisms of drainage115.

For GLOF hazard assessment a distinction is typically made between those factors that are critical to the inherent stability of the lake dam (geometry and structure of the dam) and those that influence the potential for an external triggering event (a rock or ice avalanche)100. Additionally, the hydro-geomorphic characteristics of the lake catchment area, which can influence the susceptibility to precipitation or melt-triggered outburst events, have also been highlighted37,116. With high-resolution optical imagery (such as available from Google Earth) and corresponding high-quality digital terrain models, it has become possible to quantify various physical characteristics of the dam and catchment area remotely over large spatial scales111,117. However, precise geometric measurements (such as dam freeboard or dam height) and in situ characteristics (such as ice core, lithology) are lacking, and can only be confirmed through local site investigations.

Typically, large glacial lakes are emphasized owing to the potential high-magnitude GLOFs that could originate from them, yet outbursts from small or even seasonal lakes have proven capable of eroding huge volumes of sediment and producing catastrophic downstream process chains 118. GLOFs can be both the trigger of a process chain or initiated as part of a larger process chain, as in the case of a mass movement causing a displacement wave, overtopping and a downstream flood 119. Several hydrodynamic numerical models 80,120-123 have been used to numerically propagate flood waves downstream, modelling not only a GLOF but also entire process chains (see Supplementary Table 3). However, in the absence of robust field data from past events, modelling is often undertaken with poorly constrained conditions and parameters. Some morphodynamic models that include sediment transport and bed elevation changes during a simulation have also been applied where independent pre-GLOF and post-GLOF topographic data are available 124,125. These models can estimate sediment entrainment and transport rates 126, and more generally this numerical modelling has the potential to interpolate between observations along a river reach, or to extend beyond them downstream.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > GLOF hazard assessment > Trends in GLOF frequency and magnitude
Type: text

Although suffering from known biases and limitations 62,102, remote sensing observations and geomorphological evidence offer an opportunity to roughly estimate whether GLOFs have changed in frequency. The number of reported GLOFs increased from 1 in 850 to 3,151 in 2022, peaking from 2000 to 2020 (refs. 26,35). However, poor instrumentation and limited available satellite imagery might be responsible for fewer records of historical GLOF events before -1980, suggesting a bias in the estimation of the historical GLOF trend26,81,127. At the regional


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > GLOF hazard assessment > Trends in GLOF frequency and magnitude
Type: figure
Figure

Image /page/7/Figure/1 description: A flowchart titled 'a GLOF initiation' illustrates the causes, processes, and impacts of a Glacial Lake Outburst Flood (GLOF). The chart is color-coded with a legend indicating that blue boxes represent 'Trigger or mechanism', yellow boxes represent 'Flood modelling', and red boxes represent 'Downstream impact'.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > GLOF hazard assessment > Trends in GLOF frequency and magnitude
Type: text

The 'Trigger or mechanism' section shows several pathways. A 'Moraine-dammed' lake leads to a 'Moraine', which can be breached by an 'Ice or rock avalanche, landslide, overtopping, piping and dam breach'. Separately, 'Ice-dammed' and 'Supraglacial' lakes lead to 'Ice', which can be breached by an 'Ice avalanche, subaerial breaching, terminal retreat and dam breach'. Other triggers like 'Earthquake', 'Temperature, snowfall or rainfall', and 'Ice avalanche or collapse' also contribute to the breach mechanisms.

These triggers lead to a 'GLOF', which is the start of the 'Flood modelling' section. The GLOF event influences its 'Discharge' and 'Extent'. The GLOF initiates the 'Process of flood flow downstream'.

This downstream flow leads to 'Erosion and hazard', which is the start of the 'Downstream impact' section. The impacts listed are on 'Road', 'Bridge', 'Hydropower station', 'Electrical facility', 'Building', 'Livelihood', and 'Culture heritage'.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > GLOF hazard assessment > b GLOF hazard chain
Type: figure
Figure

Image /page/7/Figure/3 description: An infographic titled "Hazard chain" illustrating the causes and effects of natural disasters in a mountainous, glacial environment. The top portion is a diagram showing a sequence of events. At the highest elevation, "Extreme snowfall and rainfall" occurs over snow-capped mountains. A "Glacier" flows down a valley, where an "Ice avalanche" (labeled 2) falls into a "Glacial lake" (1). A "Landslide" (3) is also shown falling into the lake. An "Earthquake" is indicated as another potential trigger. Below the lake, an "Ice core" (4) is depicted. The outflow from the lake creates a "Debris fan" (5), leading to "Erosion" (6) and eventually "Downstream damage and deposition" (7 and 8) in a village. An "Ice-dammed lake" is also shown in a separate valley. The bottom portion of the image consists of eight numbered photographs that correspond to the numbered locations in the diagram, providing real-world examples: 1. A muddy glacial lake. 2. An ice avalanche calving into a lake. 3. The scar of a landslide on a mountainside. 4. A glacier with a terminal lake. 5. A large debris fan in a valley. 6. A river valley showing significant erosion. 7. A damaged bridge and debris-filled riverbed. 8. A building damaged by large rocks and flood debris.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > GLOF hazard assessment > b GLOF hazard chain
Type: text

**Fig. 4** | **The triggers and mechanisms of GLOFs. a**, The process of a glacial lake outburst flood (GLOF), from trigger of mechanisms (blue) and flood modelling (yellow) to downstream impact (red). **b**, The hazard chain, from triggering factors

to glacial lake outburst, downstream impacts and damage, and corresponding photographs (1–8). Understanding the triggers and mechanisms of GLOFs and flood modelling practices can improve hazard mitigation.

scale, GLOFs from ice-dammed lakes in Alaska revealed an unchanged frequency and decreasing magnitude from 1985 to 2020 (ref. 82). Furthermore, time-series analysis of satellite images in the Himalaya suggest an unchanged frequency of moraine-dammed GLOFs from 1988 to 2017 (ref. 128). Therefore, the increasing number of moraine-dammed and bedrock-dammed lakes in mountainous regions worldwide does not necessarily imply more GLOFs 912.

Little is also known about how average or extreme GLOF flood discharges and volumes have changed, but some general themes have emerged. For instance, flood volumes of the largest reported GLOFs from ice-dammed lakes worldwide have decreased by an order of magnitude since 1900 (ref. 33). In contrast, the average peak discharge and volume of ice-dammed GLOFs remain unchanged 33. These changes align with the thinning of ice dams with atmospheric warming and glacier ablation, such that maximum lake levels gradually fall and drainage of ice-dammed lakes occurs earlier in the melt season 33,83. Moreover, as glaciers retreat, many glacier dams can no longer impound lakes at their termini at lower elevations, or the glaciers are lost altogether 82. Where ice-dammed lakes can form at higher elevations they tend to be smaller because the topography is steeper, and thus the water storage capacity is limited.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods
Type: text

Whether the increasing number and size of glacial lakes will lead to higher-magnitude GLOF discharges and greater GLOF volumes in the future remains a controversial hypothesis. With such apparent historical changes in glacial lake number and corresponding GLOFs, predicting their changes in a warming climate is important for understanding future natural hazards 129,130. The future of glacial lakes and potential hazard hot spots are now discussed.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > Future glacial lake evolution
Type: text

The life cycle of glacial lakes is known to shift in a warming climate (Fig. 5). Supraglacial lakes can grow, coalesce and, eventually, form proglacial lakes. Simultaneously, existing proglacial lakes can expand in area and in volume (Fig. 5). Formation of new lakes and the growth of existing proglacial lakes can accelerate glacier terminus retreat, via increased frontal ablation 131,132. Future increases in the volume of proglacial lakes will generally increase the downstream damage of any GLOFs 38. In addition, some ice-dammed lakes can form following glacier surges, and episodic failures of ice-dammed lakes are highly destructive 133,134 (Fig. 5). Ice-dammed lakes will continue to breach in the near future, even as glaciers thin and retreat 33,64.

Beyond these conceptual changes, estimating subglacial topography and identifying potential overdeepenings are critical to project the location and size of future glacial lakes. As glaciers retreat, depressions can be exposed that have the potential to fill partly or fully with meltwater to develop into glacial lakes129. Identification of these overdeepenings requires knowledge (or an estimate) of subglacial bedrock morphology, which can be modelled by subtracting ice thickness20,135 from a digital elevation model136, and a hydrological filling algorithm applied to fill the overdeepenings137. The difference between the filled and the original subglacial bed topography provides a bathymetric grid suggesting the extent and volume of the overdeepenings and, hence, of potential future glacial lakes24,136. Glacier evolution models20,138 can also predict future rates of glacier retreat so as to estimate when glacial lakes might begin to form21,24,136. However, these approaches cannot currently determine the dam type nor confidently suggest the height of the future lake outlet24. Nonetheless, despite the high and as yet unassessed uncertainty, these ice thickness–overdeepening approaches have been used to project regional changes in glacial lakes, including  $HMA^{21,24}$  (Fig. 6; see Supplementary Table 4).


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > Future glacial lake evolution
Type: text

< sup > 20 , 138 < / sup > can also predict future rates of glacier retreat so as to estimate when glacial lakes might begin to form < sup > 21 , 24 , 136 < / sup > . However , these approaches cannot currently determine the dam type nor confidently suggest the height of the future lake outlet < sup > 24 < / sup > . Nonetheless , despite the high and as yet unassessed uncertainty , these ice thickness – overdeepening approaches have been used to project regional changes in glacial lakes , including $ HMA ^ { 21 , 24 } $ ( Fig . 6 ; see Supplementary Table 4 ) .

In HMA, glacial lake area and volume are generally expected to increase. These changes arise owing to high projected glacier volume losses (60–75% by 2100 under unabated  $\rm CO_2$  emissions  $^{25,139}$ ), exposing subglacial overdeepenings and, thus, expanding existing lakes and developing new glacial lakes  $^{21,24}$ . For example, with complete glacier disappearance in HMA, -13,000 new glacial lakes (each larger than 0.01 km²) could potentially form, with a total area of -1,510 km² and volume of -52.3 km³ (ref. 24). Similar estimates of a maximum number of 10,068 [6,981, 13,538] new ice-dammed lakes are projected by 2100 under SSP585, with an area of 290 [204, 639] km² and volume of 2.9 [1.9, 4.9] km³ (ref. 64). However, these quantifications are uncertain. Indeed, a different ice thickness model and, hence, subglacial bed topography suggests -25,285 lakes (larger than 0.01 km²) could form by complete deglaciation, having an area of 2,683 ± 773.8 km² and volume of 99.1 ± 28.6 km³ (ref. 136).


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > Future glacial lake evolution
Type: text

, 13 , 538 ] new ice - dammed lakes are projected by 2100 under SSP585 , with an area of 290 [ 204 , 639 ] km² and volume of 2 . 9 [ 1 . 9 , 4 . 9 ] km³ ( ref . 64 ) . However , these quantifications are uncertain . Indeed , a different ice thickness model and , hence , subglacial bed topography suggests - 25 , 285 lakes ( larger than 0 . 01 km² ) could form by complete deglaciation , having an area of 2 , 683 ± 773 . 8 km² and volume of 99 . 1 ± 28 . 6 km³ ( ref . 136 ) .

The magnitude of the projected glacial lake changes in HMA is regionally variable as well as strongly dependent on the emission scenario (hence, temperature rise). Generally, greater changes are expected in western regions (Fig. 6a-c). The total area of glacial lakes, for example, is expected to increase by  $7.2 \times 10^2$  km2 in Central Asia compared with  $5.6 \times 10^2$  km2 in Southwest Asia and  $2.3 \times 10^2$  km2 in Southeast Asia (Fig. 6a,c). Glacial lake volume also reflects similar shifts (Fig. 6b,c). Future lake area and volume estimates show an increase of about 771.4 km2 and 37.0 km3, respectively, from 2030 to 2100 under a high-emission scenario; generally, these have a continuous increase until about 2090 and a decrease thereafter (Fig. 6e,f). However, there are sub-regional differences, with larger increases in the Karakoram (82.5 km2 and 4.2 km3) and Himalaya (187.0 km2 and 10.4 km3) compared with smaller increases in the Hissar Alay (1.7 km2) and 0.03 km3) and the Hindu Kush (1.4 km2 and 0.02 km3) over the same time period and emission scenario (Fig. 6e,f).


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > Future glacial lake evolution
Type: text

sup > 3  ) and Himalaya ( 187 . 0 km  2  and 10 . 4 km  3  ) compared with smaller increases in the Hissar Alay ( 1 . 7 km  2  ) and 0 . 03 km  3  ) and the Hindu Kush ( 1 . 4 km  2  and 0 . 02 km  3  ) over the same time period and emission scenario ( Fig . 6e , f ) .

Glacial lake area and volume are also expected to increase somewhat in the European Alps. At the bigger regional scale under complete deglaciation, lake area and volume are expected to increase by about 477.0 km2 and 1.7 km3, respectively (Fig. 6a,b). However, at a national level, different climate scenarios and glacier evolution models are often used that make comparisons difficult. In the Swiss Alps, the number and area of lakes are 987 and  $6.22 \pm 0.25 \,\mathrm{km^2}$  in 2016, respectively (ref. 59). If all glaciers were to disappear, ~500 new glacial lakes (of at least 0.01 km2) could form, with a total area of about 50 km2 and volume of 1.6 km3 (ref. 129). These estimates are broadly in keeping with ice-free scenarios based on current ice thickness measurements; they suggest that 683 glacial lakes (larger than 0.005 km²) with a total area and volume of  $45.2 \pm 9.3 \text{ km}^2$  and  $1.16 [1.05, 1.32] \text{ km}^3$  (mean with 95% confidence interval), respectively, could develop23. Looking at specific emission scenarios rather than total ice elimination, glacial lake volumes of 0.35 [0.12, 0.49] km3 and 0.94 [0.51, 1.04] km3 could be realized by 2100 for SSP1-2.6 and SSP5-8.5, respectively23. In the upper Rhône catchment of the south-western Swiss Alps alone, 100 sites are predicted to have a high potential for future glacial lake formation140.

Smaller changes are anticipated in other regions of the European Alps. In Austria, 42 potential new glacial lakes with a total area of about 2 km² could form with complete deglaciation 141, an increase from the current 1,410 lakes with a total area of 17.1 km² in 2015 (ref. 75). A similar number of new lakes are expected under complete ice loss


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > Future glacial lake evolution
Type: figure
Figure

Image /page/9/Figure/1 description: A multi-panel figure illustrating the formation and growth of glacial lakes and the associated hazard of Glacial Lake Outburst Floods (GLOF). The figure is divided into four sections labeled a, b, c, and d. Sections a, b, and c each contain two diagrams comparing a 'Current climate' scenario with a 'Future climate' scenario. Section a, titled 'Supraglacial lake', shows small pools of water on a glacier's surface in the current climate, which increase in number and size in the future climate, a process labeled 'Supraglacial lake increase and proglacial lake formation'. Section b, 'Proglacial lake', depicts a lake at the glacier's terminus that grows larger as the glacier recedes in the future climate, labeled 'Proglacial lake growth'. Section c, 'Ice-dammed lake', shows the formation of a lake in a tributary valley dammed by the main glacier in the future climate, a process called 'Ice-dammed lake formation'. Section d, titled 'Increasing GLOF hazard potential', illustrates a GLOF event where a landslide falls into a glacial lake, causing a flood that damages a populated area downstream. This section also includes a Venn diagram where three overlapping circles for 'Hazard', 'Vulnerability', and 'Exposure' intersect to create 'Risk'. Arrows labeled 'Adaptation' and 'Countermeasure' are shown, linking the GLOF illustration to the risk concept.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > Future glacial lake evolution
Type: text

Fig. 5 | Future formation and growth of glacial lakes and GLOF hazard.  $\mathbf{a}$ - $\mathbf{c}$ , Evolution of supraglacial lakes (panel  $\mathbf{a}$ ), proglacial lakes (panel  $\mathbf{b}$ ) and ice-dammed lakes (panel  $\mathbf{c}$ ) from the current climate (top panel) to a future climate (middle panel).  $\mathbf{d}$ , The overarching change in glacial lake outburst flood

(GLOF) hazard in a future climate ('+' and '-' indicate increases and decreases that influence the GLOF hazard), feeding into GLOF risk. The life cycle of different types of glacial lakes shifts in a warming climate and can contribute to increased GLOF risk, with more and larger lakes and greater exposure.

in the Aosta Valley region, Italy; 46 potential new lakes larger than  $0.01\,\mathrm{km^2}$  might develop, with a total area of  $3.1\pm0.9\,\mathrm{km^2}$  and a volume of  $0.06\pm0.02\,\mathrm{km^3}$  (ref. 142), expanding the current 186 lakes with a total area of  $1.4\,\mathrm{km^2}$  (ref. 143). Likewise, 80 (48 with a high level of confidence) potential new lakes could form under full deglaciation in the Mont Blanc massif of the French Alps 144.

Smaller changes are generally expected in glacial regions of the Southern Hemisphere, including the New Zealand Alps. Here, lake area is expected to increase by 42.3 km2 by 2100 under RCP4.5 (Fig. 6a,b). More specifically, ice-marginal lakes across the Southern Alps could

expand from 20.9 km² in 2020 to 40.4 km² in 2050 (ref. 74). However, from 2050 onwards, the rate of increase in total area is projected to decrease from 0.65 km² year $^{-1}$  in 2020–2050 to 0.04 km² year $^{-1}$  in 2050–2100 as lakes detach from glaciers and/or fill with sediment $^{74}$ .

In the Peruvian Andes, glacial lake area is projected to increase by 65.5 km² and volume by 0.7 km³ by 2100 under RCP8.5 (refs. 73,145) (Fig. 6a,b). In the still glaciated Cordillera Blanca, 38 or 50 new lakes could emerge by 2100 under RCP2.6 or RCP8.5, respectively  $^{73}$ , increasing from the current 870 lakes; accordingly, the glacial lake area is expected to increase by -10% compared with 2018 values, from 35.2 km²


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > a Projected glacial lake area
Type: figure
Figure

Image /page/10/Figure/2 description: A world map with several rectangular regions outlined. Some of these regions are color-coded according to a legend at the bottom. The legend is a color scale from dark blue to light blue, to pale yellow, to orange, corresponding to a numerical scale from 0 to 8, with the unit labeled as "10² km²". A large region in South America is colored dark blue. A region in Europe is colored pale yellow. A region in Central Asia is colored with orange, pale yellow, and light blue sections. An inset map of New Zealand in the bottom right is also colored dark blue. Other outlined regions in North America and northern Asia are not colored.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > b Projected glacial lake volume
Type: figure
Figure

Image /page/10/Figure/4 description: A world map showing continents in light gray. Several rectangular regions are outlined across the globe. Some of these regions are color-coded based on a legend at the bottom. The legend is a color bar with a scale from 0 to 3 in units of 10 km³. The colors range from dark blue (0-1), to a lighter blue-gray (1-2), to a pale yellow (2-3). On the map, a large portion of South America is colored dark blue. The Mediterranean region is colored light blue-gray. A region in East Asia is colored pale yellow, with its southern coastal area colored dark blue.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > C Projected glacial lake area and volume in HMA
Type: figure
Figure

Image /page/10/Figure/6 description: A map titled 'Projected glacial lake area and volume in HMA' shows a geographical region, likely High Mountain Asia, with data points represented by circles of varying sizes and colors. There are two legends. The first legend explains the size of the circles, which corresponds to the glacial lake area in square kilometers (km²). The categories are: a small dot for <3, and progressively larger circles for 3-10, 10-30, 30-60, and >60 km². The second legend is a color bar indicating the volume in 10⁻¹ km³, where colors range from blue (0.001) to red (1.5). The map shows a high concentration of large, red circles (indicating large area and high volume) in the central-western part of the region. Smaller circles, colored yellow and dark blue, are scattered more widely.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > d Projected GLOF hazard and risk in HMA
Type: figure
Figure

Image /page/10/Figure/8 description: A map showing projected GLOF (Glacial Lake Outburst Flood) hazard and risk in HMA (High Mountain Asia). The map displays a mountainous region divided into several sub-regions. Overlaid on these regions are groups of three vertical bar charts. A legend in the bottom-left corner explains that the bars represent 'Risk' (yellow), 'Exposure' (light orange), and 'Hazard' (darker orange). The legend also includes a scale bar labeled '600' corresponding to the height of the example 'Hazard' bar. The heights of the bars vary across the different regions on the map, indicating different levels of projected hazard, exposure, and risk.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > e Scenario dependence of HMA glacial lake area
Type: figure
Figure

Image /page/10/Figure/10 description: A set of four stacked area charts showing projections of glacial lake area from the year 2040 to 2100 under four different climate scenarios: SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5. The vertical axis is labeled "Glacial lake area (×10² km²)" and ranges from 0 to 8. The horizontal axis shows the years 2040, 2060, 2080, and 2100. Each chart shows a progressive increase in glacial lake area over time, with the area represented by stacked layers of different shades of blue. The total area increases with the severity of the climate scenario. For SSP1-2.6, the area reaches approximately 4.5 ×10² km² by 2100. For SSP2-4.5, it reaches about 5.8 ×10² km². For SSP3-7.0, it reaches about 6.8 ×10² km². For SSP5-8.5, the area increases most sharply, exceeding 8 ×10² km² by 2100.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > f Scenario dependence of HMA glacial lake volume
Type: figure
Figure

Image /page/10/Figure/12 description: A set of four stacked area charts illustrating the scenario dependence of HMA (High-Mountain Asia) glacial lake volume. The vertical axis represents "Glacial lake volume (×10 km³)" and ranges from 0 to 4. The horizontal axis for each chart shows the years from 2040 to 2100, with labels at 20-year intervals. Each chart depicts a different scenario, showing a projected increase in total glacial lake volume over time. The total volume is broken down by region, with each region represented by a different shade of blue or green in the stacked area chart. A legend at the bottom identifies the regions: Tibet, Himalaya, Karakoram, Kun Lun, Tien Shan, Qilian Shan, Pamir, Hissar Alay, Hindu Kush, and Hengduan Shan. In all four scenarios, the total volume increases from near zero in 2040. By 2100, the projected total volumes for the four scenarios are approximately 2.1, 2.8, 3.0, and 3.8 (×10 km³), respectively, from left to right.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > f Scenario dependence of HMA glacial lake volume
Type: text

**Fig. 6 | Global potential future glacial lakes and GLOF hazard. a, b,** Published estimates of potential future glacial lake area (panel **a**) and potential future glacial lake volume (panel **b**) with complete deglaciation (see Supplementary Table 4). **c**, Potential future glacial lake area (circles) and volume (shading) in High Mountain Asia (HMA) by 2100 under RCP8.5 (ref. 24). Background shading depicts drainage basins using the colours in panels **e** and **f. d**, Future glacial lake outburst flood (GLOF) risk (light shading), exposure (medium shading) and

hazard (dark shading) in HMA by 2100 under RCP8.5 (ref. 24). Physical or social vulnerability is not considered. **e,f**, Time series of potential future glacial lake area (panel **e**) and lake volume (panel **f**) in HMA under different SSP values, with shading representing different subregions21. Projected global glacial lakes will increase globally by 2100 under different emission scenarios, posing a potential hazard threat to downstream communities.

to 37.0 km². In the deglaciating Vilcanota–Urubamba basin, the lake area in 2100 is projected to be 3.2% higher than 2016 values under RCP2.6 (from 26.9 km² to 27.8 km²) and 6.0% higher under RCP8.5 (from 26.9 km² to 28.5 km²)  $^{145}$ . The topographic setting means that the rate of increase is higher for 2016–2050 compared with 2050–2100 as glaciers are already retreating to very steep slopes by 2050 (ref. 145). In deglaciated areas of the Peruvian Andes, 201 sites (of at least 0.01 km²) with a total volume of 0.26 km³ have been compiled for a future glacial lake inventory  $^{146}$ .


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > f Scenario dependence of HMA glacial lake volume
Type: text

26 . 9 km² to 27 . 8 km² ) and 6 . 0 % higher under RCP8 . 5 ( from 26 . 9 km² to 28 . 5 km² ) $ ^ { 145 } $ . The topographic setting means that the rate of increase is higher for 2016 – 2050 compared with 2050 – 2100 as glaciers are already retreating to very steep slopes by 2050 ( ref . 145 ) . In deglaciated areas of the Peruvian Andes , 201 sites ( of at least 0 . 01 km² ) with a total volume of 0 . 26 km³ have been compiled for a future glacial lake inventory $ ^ { 146 } $ .

Where lakes $ receive \ , meltwater \ , from \ , GLOFs \ , as \ , well \ , as \ , from \ , ablation \ text { - } fed \ , meltwater \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant $ flows , and where GLOF - induced bank erosion widens river channels hundreds of kilometres downstream < sup > 23 , 78 < / sup > , the sedimentation rates within future lakes will also be very high .


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > f Scenario dependence of HMA glacial lake volume
Type: text

, constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant \ , and \ , constant $ flows , and where GLOF - induced bank erosion widens river channels hundreds of kilometres downstream  23 , 78  , the sedimentation rates within future lakes will also be very high .

Often not included in these assessments is sediment deposition within newly exposed overdeepenings, infilling potential future lakes and complicating specific projections. Such infilling is particularly likely in tectonically active, high-relief regions with abundant sediment supply such as HMA and the Southern Alps61,147. Indeed, erosion rates and sediment transport in glacierized basins are generally expected to increase owing to cryosphere degradation61,147, but there has been almost no quantitative analysis of this process. One exception is in the Swiss Alps, where approximately half of newly formed glacial lakes and 11-25% of the total overdeepening volume are expected to disappear by 2100 owing to sediment infilling23. If sediment delivery is not considered, the potential future glacial lakes are likely to be overestimated in number and size.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Future glacial lakes and outburst floods > Future GLOF hazard and risk
Type: text

With projected changes in glacial lakes come potential changes in GLOFs. Based on physical understanding and supported by models, increasing GLOFs are assumed for the future 24,64,136 (Fig. 5), albeit with much regional variability. In HMA, future GLOF hazards are expected to triple from current conditions due to lake development (Fig. 6d) owing to glacial and glacial lake changes, alongside a large and exposed downstream population. Even greater increases (greater than fivefold) are projected in the Karakoram and Pamir regions24, which are expected to become a hot spot of potential GLOF hazard136, although the timing of lake formation is uncertain 148. The characteristics of GLOFs in HMA are also expected to shift. Scenario modelling in the Poiqu River basin of China and Nepal, for example, suggests that permafrost degradation will enhance large mass movements (of tens of millions of cubic metres) into glacial lakes, resulting in larger flood magnitudes and, thereby, longer distances downstream of flood impact than historically observed149.

Future GLOF hazards in other regions of the world are unlikely to increase as in HMA. For example, there is relatively low potential future for GLOFs in the Mont Blanc massif (Western European Alps) owing to the flat glacier bed topography144. In the Cordillera Blanca (Peru), glacier retreat and lake formation are already at an advanced stage, and new lakes still to form will be predominantly dammed by stable bedrock73. However, in Iceland, mega GLOFs induced by volcanic activity can occur in the future, irrespective and independent of climate change89.

Of course, although GLOF hazard might increase, overall risk also necessitates consideration of exposure and vulnerability. To date, comprehensive risk assessments considering the changing flood hazard and societal vulnerability and exposure have been lacking. However, the expansion of communities and infrastructure, including hydropower facilities, has been and will continue to be a major driver

of GLOF risk150–152. Thus, infrastructure expansion in glacial valleys, coupled with expanded glacial lakes, will likely result in enhanced GLOF risk, threatening downstream populations and infrastructure130,153–155. Accordingly, there are new challenges for warning systems or other disaster risk reduction measures156.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Summary and future perspectives
Type: text

Global glacier melt has accelerated owing to atmospheric warming, feeding rapid growth of glacial lakes in most world regions from the 1960s onwards. Although uncertain, it is estimated that the glacial lake number and area have increased -50% from 1990 to 2018 (ref. 9), such that more than 110,000 glacial lakes with a total area of -15,000 km² in 2015–2019 have been identified. This trend in glacial lakes is associated with a potential increase in GLOF hazards and risks. More than 3,000 GLOFs have occurred worldwide from 850 to 2022, with ~88% recorded since 1900. Indeed, recorded GLOFs have increased in frequency and magnitude in most glaciated mountain regions, especially after 2000 (refs. 26,35), a trend that is likely to continue with ongoing deglaciation, increasing GLOF risk and hazard when coupled with enhanced development in downstream river valleys. In HMA, in particular, GLOF risk is projected to increase threefold with complete deglaciation24,127.

Although GLOF knowledge has advanced considerably, key challenges remain, particularly surrounding data. As a starting point, homogeneous and standardized multi-temporal glacial lake inventories are needed, so far limited by mapping efficiency. Visual inspection and manual editing of lake outlines have been the main methods used to date — time-consuming approaches that rely heavily on individual expertise. The use of cloud computing, machine learning and artificial intelligence technologies should overcome manual limitations. However, access to regional high-quality lake inventories to train machine learning classifiers is vitally important. With any such inventories, consistent thresholds for minimum lake area and distance from glaciers are needed, as well as consistent classifications. An updated multi-period inventory of global glacial lakes with a single criterion is expected to fill the gaps of missing lakes and regional deficiencies.

Hydrodynamic modelling and reconstruction of GLOFs is also hindered by absent data. Field data of lake properties (including bathymetry, temperature and lake outflow peak discharge) and GLOF mechanisms are desperately needed in different glaciated environments and for different lake types to refine and test model parameters, as well as parametrize dam breach models. Yet such data are extremely difficult to acquire given accessibility and safety that make successful recovery of instrumentation and sensors challenging, especially in ice-proximal locations. Accordingly, local, regional and even global understanding of GLOF flow dynamics is limited, feeding into hazard assessment, early warning systems, infrastructure design and emergency response. In addition to the use of modern field observation instruments and data transmission, broader international cooperation and the establishment of data opening and sharing are important ways forward. Future research should focus on comparing model results with field observations, historical flood records and other independent data sets to improve model credibility.

Assessments of future glacier lakes under-report the common problem of uncertainty about the location, size and timing of potential future lakes. The average error in measured ice thickness of  $10\pm24\%^{157}$  masks considerably higher uncertainties for specific glaciers, including an uncertainty range of  $\pm30\%$  suggested by the validation of ground-penetrating radar measurements in the Swiss Alps129. Thus, although


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Summary and future perspectives
Type: text

sharing are important ways forward . Future research should focus on comparing model results with field observations , historical flood records and other independent data sets to improve model credibility . Assessments of future glacier lakes under - report the common problem of uncertainty about the location , size and timing of potential future lakes . The average error in measured ice thickness of $ 10 \ pm24 \ % ^ { 157 } $ masks considerably higher uncertainties for specific glaciers , including an uncertainty range of $ \ pm30 \ % $ suggested by the validation of ground - penetrating radar measurements in the Swiss Alps  129  . Thus , although

common ice thickness models can estimate the total ice volume of all glaciers globally20,135 or regionally158,159, differences between ice thickness models can lead to substantial local differences. Crucially, the potential future lake area is highly sensitive to small errors in, for example, the height of a subglacial rock barrier or the depth of an overdeepening23. Obtaining accurate glacier bed topography by ice thickness modelling remains a challenge, but it can be improved by increasing the accuracy of the underlying input data, adding measured thickness data and multi-model synthesis20,157,160,161. In the future, with the launch of P-band and L-band radar satellites, obtaining ice thickness using tomographic SAR technology will provide new opportunities and possibilities to improve the accuracy of future glacial lake prediction162,163.

These improved data and models are vital to enhance GLOF hazard assessment, particularly in a warming world associated with enhanced GLOF risk. However, GLOF hazard assessments suffer from inconsistent approaches (indicators and their thresholds) and the lack of a common assessment framework (filed survey, hazard and risk assessment and management action)110,117. Technical guidelines have been established at the international level, but have yet to be adapted or widely consolidated into regional or national approaches, as the level and development of guidelines vary widely across regions115. Where existing assessments provide conflicting results, efforts should be undertaken to consolidate findings to determine which lakes should be prioritized for monitoring and other response actions. In particular, the need to consider small lakes is emphasized, as such lakes have often been overlooked but can have devastating downstream impacts77,118. Importantly, this knowledge needs to be used to anticipate the hazards of future GLOF events, including worst-case scenarios. Given the tendency for GLOFs to be triggered by or associated with other mass movements and cascading processes, multi-hazard perspectives are encouraged.


Document: Characteristics and changes of glacial lakes and outburst floods
Section: Characteristics and changes of glacial lakes and outburst floods > Summary and future perspectives
Type: text

> 115  . Where existing assessments provide conflicting results , efforts should be undertaken to consolidate findings to determine which lakes should be prioritized for monitoring and other response actions . In particular , the need to consider small lakes is emphasized , as such lakes have often been overlooked but can have devastating downstream impacts  77 , 118  . Importantly , this knowledge needs to be used to anticipate the hazards of future GLOF events , including worst - case scenarios . Given the tendency for GLOFs to be triggered by or associated with other mass movements and cascading processes , multi - hazard perspectives are encouraged .

Future efforts to move beyond hazards to comprehensive GLOF risk assessment must also involve a wider network of local experts and broader disciplines. Failure to adequately consider local vulnerabilities, needs and perceptions can ultimately limit the success of disaster risk reduction measures, such as early warning systems 164. As a transboundary phenomenon, GLOFs pose particular challenges for disaster risk reduction where cooperation and sharing across political boundaries are required, and where societal and institutional capacities to manage GLOF risks can vary widely between upstream and downstream regions. Scientists, policymakers and stakeholders should work together towards broader international and regional transboundary cooperation to reduce the risk of GLOFs.


Document: Climate change and the global pattern of moraine-dammed
Type: text

Abstract. Despite recent research identifying a clear anthropogenic impact on glacier recession, the effect of recent climate change on glacier-related hazards is at present unclear. Here we present the first global spatio-temporal assessment of glacial lake outburst floods (GLOFs) focusing explicitly on lake drainage following moraine dam failure. These floods occur as mountain glaciers recede and downwaste. GLOFs can have an enormous impact on downstream communities and infrastructure. Our assessment of GLOFs associated with the rapid drainage of moraine-dammed lakes provides insights into the historical trends of GLOFs and their distributions under current and future global climate change. We observe a clear global increase in GLOF frequency and their regularity around 1930, which likely represents a lagged response to post-Little Ice Age warming. Notably, we also

show that GLOF frequency and regularity – rather unexpectedly – have declined in recent decades even during a time of rapid glacier recession. Although previous studies have suggested that GLOFs will increase in response to climate warming and glacier recession, our global results demonstrate that this has not yet clearly happened. From an assessment of the timing of climate forcing, lag times in glacier recession, lake formation and moraine-dam failure, we predict increased GLOF frequencies during the next decades and into the 22nd century.

<sup>1College of Life and Environmental Sciences, Exeter University, Exeter, UK

<sup>2Planetary Science Institute, Tucson, AZ 85719, USA

<sup>3Department of Hydrology & Atmospheric Science, University of Arizona, Tucson, AZ 85742, USA

<sup>4Department of Geography, University of Zurich, 8057 Zurich, Switzerland

<sup>5Reynolds International Ltd, Suite 2, Broncoed House, Broncoed Business Park, Wrexham Road, Mold, Flintshire, UK

<sup>6Water, Sediment, Hazards, and Earth-surface Dynamics Laboratory, University of Washington Tacoma, WA, 98402, USA

<sup>7Met Office Hadley Centre, FitzRoy Road, Exeter, Devon, UK

<sup>8Department of Physical Geography and Geoecology, Charles University in Prague, Faculty of Science, Albertov 6, 128 43 Prague, Czech Republic

<sup>9Centre for Glaciology, Department of Geography and Earth Sciences, Aberystwyth University, Wales, SY23 3DB, Aberystwyth, UK

<sup>10Department of Geology, University of Dayton, 300 College Park, Dayton, OH 45469-2364, USA

<sup>11Department of Human Dimensions of Global Change, Global Change Research Institute, Czech Academy of Sciences, Bělidla 986/4a, 60300 Brno, Czech Republic

data/('Climate change and the global pattern of moraine-dammed', '.pdf')_extraction.md


Document: Climate change and the global pattern of moraine-dammed
Type: text

sup > &  lt ; sup > 9  Centre for Glaciology , Department of Geography and Earth Sciences , Aberystwyth University , Wales , SY23 3DB , Aberystwyth , UK  &  lt ; sup > 10  Department of Geology , University of Dayton , 300 College Park , Dayton , OH 45469 - 2364 , USA  &  lt ; sup > 11  Department of Human Dimensions of Global Change , Global Change Research Institute , Czech Academy of Sciences , Bělidla 986 / 4a , 60300 Brno , Czech Republic

<sup>12Department of Engineering Geology, Institute of Rock Structure and Mechanics, Czech Academy of Sciences,

V Holešovičkách 41, 182 09 Prague 8, Czech Republic

<sup>13Himalayan Research Center, Lainchaur, Kathmandu, Nepal