High Mountain Areas

Past changes of surface air temperature and precipitation in high mountain areas have been documented by in situ observations and regional reanalyses (Table SM2.2 and Table SM2.4). However, mountain observation networks do not always follow standard measurement procedures (Oyler et al., 2015⁵; Nitu et al., 2018⁶) and are often insufficiently dense to capture fine-scale changes (Lawrimore et al., 2011⁷) and the underlying larger scale patterns. Future changes are projected using General Circulation Models (GCMs) or Regional Climate Models (RCMs) or simplified versions thereof (e.g., Gutmann et al., 2016), used to represent processes at play in a dynamically consistent manner, and to relate mountain changes to larger-scale atmospheric forcing based on physical principles. Existing mountain-specific model studies typically cover individual mountain ranges, and there is currently no initiative found, such as model intercomparisons or coordinated model experiments, which specifically and comprehensively addresses high mountain meteorology and climate globally. This makes it difficult to provide a globally uniform assessment.

Snow on the ground is an essential and widespread component of the mountain cryosphere. It affects mountain ecosystems and plays a major role for mass movement and floods in the mountains. It plays a key role in nourishing glaciers and provides an insulating and reflective cover at their surface. It influences the thermal regime of the underlying ground, including permafrost, with implications for ecosystems. Climate change modifies key variables driving the onset and development of the snow cover (e.g., solid precipitation), and those responsible for its ablation (e.g., air temperature, radiation). The snow cover, especially in low-lying and mid-elevation areas of mountain regions, has long been identified to be particularly sensitive to climate change. 

The mountain snow cover is characterised by a very strong interannual and decadal variability, similar to its main driving force solid precipitation (Lafaysse et al., 2014⁵²; Mankin and Diffenbaugh, 2015⁵³). Observations spanning several decades are required to quantify trends. Long-term in situ records are scarce in some regions of the world, particularly in High Mountain Asia, Northern Asia and South America (Rohrer et al., 2013⁵⁴). Satellite remote sensing provides new capabilities for monitoring mountain snow cover on regional scales. The satellite record length is often insufficient to assess trends (Bormann et al., 2018⁵⁵). Evidence of past changes from regional studies is provided in Table SM2.6. At lower elevation, there is high confidence that the mountain snow cover has generally declined in duration (on average by 5 snow cover days per decade, with a likely range from 0 to 10 days per decade), mean snow depth and accumulated mass (snow water equivalent) since the middle of the 20th century, with regional variations. At higher elevation, snow cover trends are generally insignificant (medium confidence) or unknown.

Most of the snow cover changes can be attributed, at lower elevation, to more precipitation falling as liquid precipitation (rain) and to increases in melt at all elevations, mostly due to changes in atmospheric forcings, especially increased air temperature (Kapnick and Hall, 2012⁵⁶; Marty et al., 2017⁵⁷) which in turn are attributed to anthropogenic forcings at a larger scale (Section 2.2.1). Formal anthropogenic attribution studies provide similar conclusions in Western North America (Pierce et al., 2008⁵⁸; Najafi et al., 2017⁵⁹).

Assessing the impact of the deposition of short-lived climate forcers on snow cover changes is an emerging issue (Skiles et al., 2018⁶⁰ and references therein). This concerns light absorbing particles, in particular, which include deposited aerosols such as black carbon, organic carbon and mineral dust, or microbial growth (Qian et al., 2015⁶¹), although the role of the latter has not been specifically quantified. Due to their seasonally variable deposition flux and impact, and mostly episodic nature in case of dust deposition (Kaspari et al., 2014⁶²; Di Mauro et al., 2015⁶³), light absorbing particles contribute to interannual fluctuations of seasonal snowmelt rate (Painter et al., 2018) (medium evidence, high agreement). There is limited evidence (medium agreement) that increases in black carbon deposition from anthropogenic and biomass burning sources have contributed to snow cover decline in High Mountain Asia (Li et al., 2016⁶⁴; Zhang et al., 2018⁶⁵) and South America (Molina et al., 2015⁶⁶). 

Projected changes of mountain snow cover are studied based on climate model experiments, either directly from GCM or RCM output, or following downscaling and the use of snowpack models. These projections generally do not specifically account for future changes in the deposition rate of light absorbing particles on snow (or, if so, simple approaches have been used hitherto; e.g., Deems et al., 2013), so that future changes in snow conditions are mostly driven by changes in meteorological drivers assessed in Section 2.2.1. Evidence from regional studies is provided in Table SM2.7. Although existing studies in mountain regions do not use homogenous reference periods and model configurations, common future trends can be summarised as follows. At lower elevation in many regions such as the European Alps, Western North America, Himalaya and subtropical Andes, the snow depth or mass is projected to decline by 25% (likely range between 10 and 40%), between the recent past period (1986–2005) and the near future (2031–2050), regardless of the greenhouse gas emission scenario (Cross-Chapter Box 1 in Chapter 1). This corresponds to a continuation of the ongoing decrease in annual snow cover duration (on average 5 days per decade, with a likely range from 0 to 10). By the end of the century (2081–2100), reductions of up to 80% (likely range from 50 to 90%) are expected under RCP8.5, 50% (likely range from 30 to 70%) under RCP4.5 and 30% (likely range from 10 to 40%) under RCP2.6. At higher elevations, projected reductions are smaller (high confidence), as temperature increases at higher elevations affect the ablation component of snow mass evolution, rather than both the onset and accumulation components. The projected increase in winter snow accumulation may result in a net increase in winter snow mass (medium confidence). All elevation levels and mountain regions are projected to exhibit sustained interannual variability of snow conditions throughout the 21st century (high confidence). Figure 2.3 provides projections of temperature and snow cover in mountain areas in Europe, High Mountain Asia (Hindu Kush, Karakoram and Himalaya), North America (Rocky Mountains) and South America (sub-tropical Central Andes), illustrating how changes vary with elevation, season, region, future time period and climate scenario.

The high mountain areas considered in this chapter (Figure 2.1), including all glacier regions in the world except those in Antarctica, Greenland, the Canadian and Russian Arctic, and Svalbard (which are covered in Chapter 3) include ~170,000 glaciers covering an area of ~250,000 km2 (RGI Consortium, 2017) with a total ice volume of 87 ± 15 mm sea level equivalent (Farinotti et al., 2019⁷⁰). These glaciers span an elevation range from sea level, for example in south-east Alaska, to >8,000 m a.s.l. in the Himalaya and Karakoram, and occupy diverse climatic regions. Their mass budget is determined largely by the balance between snow accumulation and melt at the glacier surface, driven primarily by atmospheric conditions. Rapid changes in mountain glaciers have multiple impacts for social-ecological systems, affecting not only biophysical properties such as runoff volume and sediment fluxes in glacier-fed rivers, glacier related hazards, and global sea level (Chapter 4) but also ecosystems and human livelihoods, socioeconomic activities and sectors such as agriculture and tourism, as well as other intrinsic assets such as cultural values. While glaciers worldwide have experienced considerable fluctuations throughout the Holocene driven by multidecadal variations of solar and volcanic activity, and changes in atmospheric circulation (Solomina et al., 2016⁷¹), this section focuses on observed glacier changes during recent decades and changes projected for the 21st century (Cross-Chapter Box 6 in Chapter 2).

Satellite and in situ observations of changes in glacier area, length and mass show a globally largely coherent picture of mountain glacier recession in the last decades (Zemp et al., 2015⁷²), although annual variability and regional differences are large (Figure 2.4; very high confidence). The global trend is statistically significant despite considerable interannual and regional variations (Medwedeff and Roe, 2017⁷³). Since AR5’s global 2003–2009 estimate based on Gardner et al. (2013)⁷⁴, several new estimates of global-scale glacier mass budgets have emerged using largely improved data coverage and methods (Bamber et al., 2018⁷⁵; Wouters et al., 2019⁷⁶; Zemp et al., 2019⁷⁷).

These estimates combined with available regional estimates (Table 2.A.1) that the glacier mass budget of all mountain regions (excluding Antarctica, Greenland, the Canadian and Russian Arctic, and Svalbard) was very likely -490 ± 100 kg m-2 yr-1 (-123 ± 24 Gt yr-1) during the period 2006–2015 with most negative averages (less than -850 kg m-2 yr-1) in the Southern Andes, Caucasus/Middle East, European Alps and Pyrenees. High Mountain Asia shows the least negative mass budget (-150 ± 110 kg m-2 yr-1, Figure 2.4), but variations within the region are large with most negative regional balance estimates in Nyainqentanglha, Tibet (-620 ± 230 kg m-2 yr-1) and slightly positive balances in the Kunlun Mountains for the period 2000–2016 (Brun et al., 2017⁷⁸). Due to large ice extent, the total mass loss and corresponding contribution to sea level 2006–2015 is largest in Alaska, followed by the Southern Andes and High Mountain Asia (Table 2.A.1). Zemp et al. (2019) estimated an increase in mean global-scale glacier mass loss by ~30% between 1986–2005 and 2006–2015.

It is very likely that atmospheric warming is the primary driver for the global glacier recession (Marzeion et al., 2014⁷⁹; Vuille et al., 2018⁸⁰). There is limited evidence (high agreement) that human-induced increases in greenhouse gases have contributed to the observed mass changes (Hirabayashi et al., 2016⁸¹). It was estimated that the anthropogenic fraction of mass loss of all glaciers outside Greenland and Antarctica increased from 25 ± 35% during 1851–2010 to 69 ± 24% during 1991–2010 (Marzeion et al., 2014⁸²). 

Other factors, such as changes in meteorological variables other than air temperature or internal glacier dynamics, have modified the temperature-induced glacier response in some regions (high confidence). For example, glacier mass loss over the last seven decades on a glacier in the European Alps was intensified by higher air moisture leading to increased longwave irradiance and reduced sublimation (Thibert et al., 2018⁸³). Changes in air moisture have also been found to play a significant role in past glacier mass changes in eastern Africa (Prinz et al., 2016⁸⁴), while an increase in shortwave radiation due to reduced cloud cover contributed to an acceleration in glacier recession in the Caucasus (Toropov et al., 2019⁸⁵). In the Tien Shan mountains changes in atmospheric circulation in the North Atlantic and North Pacific in the 1970s resulted in an abrupt reduction in precipitation and thus snow accumulation, amplifying temperature-induced glacier mass loss (Duethmann et al., 2015⁸⁶). Deposition of light absorbing particles, growth of algae and bacteria and local amplification phenomena such as the enhancement of particles concentration due to surface snow and ice melt, and cryoconite holes, have been shown to enhance ice melt (e.g., Ginot et al., 2014; Zhang et al., 2017⁸⁷; Williamson et al., 2019⁸⁸) but there is limited evidence and low agreement that long-term changes in glacier mass are linked to light absorbing particles (Painter et al., 2013⁸⁹; Sigl et al., 2018⁹⁰). Debris cover can  modulate glacier melt but there is limited evidence on its role in recent glacier changes (Gardelle et al., 2012⁹¹; Pellicciotti et al., 2015⁹²). Rapid retreat of calving outlet glaciers in Patagonia was attributed to changes in glacier dynamics (Sakakibara and Sugiyama, 2014⁹³).

Departing from this global trend of glacier recession, a small fraction of glaciers have gained mass or advanced in some regions mostly due to internal glacier dynamics or, in some cases, locally restricted climatic causes. For example, in Alaska 36 marine-terminating glaciers exhibited a complex pattern of periods of significant retreat and advance during 1948–2012, highly variable in time and lacking coherent regional behaviour (McNabb and Hock, 2014⁹⁴). These fluctuations can be explained by internal retreat-advance cycles typical of tidewater glaciers that are largely independent of climate (Brinkerhoff et al., 2017⁹⁵). Irregular and spatially inconsistent glacier advances, for example, in Alaska, Iceland and Karakoram, have been associated with surge-type flow instabilities largely independent of changes in climate (Sevestre and Benn, 2015⁹⁶; Bhambri et al., 2017⁹⁷; Section 2.3.2). Regional scale glacier mass gain and advances in Norway in the 1990s and in New Zealand between 1983–2008 have been linked to local increases in snow precipitation (Andreassen et al., 2005⁹⁸) and lower air temperatures (Mackintosh et al., 2017⁹⁹), respectively, caused by changes in atmospheric circulation. Advances of some glaciers in Alaska, the Andes, Kamchatka and the Caucasus were attributed to volcanic activity causing flow acceleration through enhanced melt water at the ice-bed interface (Barr et al., 2018¹⁰⁰).

Region averaged glacier mass budgets have been nearly balanced in the Karakoram since at least the 1970s (Bolch et al., 2017¹⁰¹; Zhou et al., 2017¹⁰²; Azam et al., 2018¹⁰³), while slightly positive balances since 2000 have been reported in the western Kunlun Shan, eastern Pamir, and the central and northern Karakoram mountains (Gardelle et al., 2013¹⁰⁴; Brun et al., 2017¹⁰⁵; Lin et al., 2017¹⁰⁶; Berthier and Brun, 2019¹⁰⁷). This anomalous behavior has been related to specific mechanisms countering the effects of atmospheric warming, for example, an increase in cloudiness (Bashir et al., 2017¹⁰⁸) and snowfall (Kapnick et al., 2014¹⁰⁹) spatially heterogeneous glacier mass balance sensitivity (Sakai and Fujita, 2017¹¹⁰), feedbacks due to intensified lowland irrigation (de Kok et al., 2018), and changes in summer atmospheric circulation (Forsythe et al., 2017¹¹¹).

There is medium evidence (high agreement) that recent glacier mass changes have modified glacier flow. A study covering all glaciers in High Mountain Asia showed glacier slowdown for regions with negative mass budgets since the 1970s and slightly accelerated glacier flow for Karakoram and West Kunlun regions where mass budgets were close to balance (Dehecq et al., 2019¹¹²). Waechter et al. (2015)¹¹³ report reduced flow velocities in the St. Elias Mountains in North America, especially in areas of rapid ice thinning near glacier termini. In contrast Mouginot and Rignot (2015) found complex ice flow patterns with simultaneous acceleration and deceleration for glaciers of the Patagonian Icefield as well as large interannual variability during the last three decades concurrent with general thinning of the ice field.

This section assesses permafrost, but not seasonally frozen ground, in high mountain areas. As mountains also exist in polar areas, some overlap exists between this section and Chapter 3. Observations of permafrost are scarce (Tables 2.1 and 2.2, PERMOS, 2016; Bolch et al., 2018¹⁴⁶) and unevenly distributed among and within mountain regions. Unlike glaciers and snow, permafrost is a subsurface phenomenon that cannot easily be observed remotely. As a consequence, its distribution and change are less understood than for glaciers or snow, and in many mountain regions it can only be inferred (Gruber et al., 2017¹⁴⁷). Permafrost thaw and degradation impact people via runoff and water quality (Section 2.3.1), hazards and infrastructure (Section 2.3.2) and greenhouse gas emissions (Box 2.2).

AR5 and IPCC’s Special Report on ‘Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation’ (SREX) assessed permafrost change globally, but not separately for mountains. AR5 concluded that permafrost temperatures had increased in most regions since the early 1980s (high confidence), although warming rates varied regionally, and attributed this warming to increased air temperature and changes in snow cover (high confidence). The temperature increase for colder permafrost was generally greater than for warmer permafrost (high confidence). SREX found a likely warming of permafrost in recent decades and expressed high confidence that its temperatures will continue to increase. AR5 found decreases of northern high-latitude near surface permafrost for 2016–2035 to be very likely and a general retreat of permafrost extent for the end of the 21st century and beyond to be virtually certain. While some permafrost phenomena, methods of observation and scale issues in scenario simulations are specific to mountainous terrain, the basic mechanisms connecting climate and permafrost are the same in mountains and polar regions.

Between 3.6–5.2 million km2 are underlain by permafrost in the eleven high mountain regions outlined in Figure 2.1 (medium confidence) based on data from two modelling studies (Gruber, 2012¹⁴⁸; Obu et al., 2019¹⁴⁹). For comparison, this is 14–21 times the area of glaciers (Section 2.2.3) in these regions (Figure 2.1) or 27–29% of the global permafrost area. The distribution of permafrost in mountains is spatially highly heterogeneous, as shown in detailed regional modelling studies (Boeckli et al., 2012¹⁵⁰; Bonnaventure et al., 2012¹⁵¹; Westermann et al., 2015¹⁵²; Azócar et al., 2017¹⁵³; Zou et al., 2017¹⁵⁴).

Permafrost in the European Alps, Scandinavia, Canada, Mongolia, the Tien Shan and the Tibetan Plateau has warmed during recent decades and some observations reveal ground-ice loss and permafrost degradation (high confidence). The heterogeneity of mountain environments and scarcity of long-term observations challenge the quantification of representative regional or global warming rates. A recent analysis finds that permafrost at 28 mountain locations in the European Alps, Scandinavia, Canada as well as High Mountain Asia and North Asia, warmed on average by 0.19 ± 0.05 °C per decade between 2007–2016 (Biskaborn et al., 2019¹⁵⁵). Over longer periods, observations in the European Alps, Scandinavia, Mongolia, the Tien Shan and the Tibetan Plateau (see also Cao et al., 2018) show general warming (Table 2.1, Figure 2.5) and degradation of permafrost at individual sites (e.g., Phillips et al., 2009). Permafrost close to 0ºC warms at a lower rate than colder permafrost because ground-ice melt slows warming. Similarly, bedrock warms faster than debris or soil because of low ice content. For example, several European bedrock sites (Table 2.1) have warmed rapidly, by up to 1ºC per decade, during the past two decades. By contrast, total warming of 0.5ºC–0.8ºC has been inferred for the second half of the 20th century based on thermal gradients at depth in an ensemble of European bedrock sites (Isaksen et al., 2001¹⁵⁶; Harris et al., 2003¹⁵⁷). Warming has been shown to accelerate at sites in Scandinavia (Isaksen et al., 2007¹⁵⁸) and in mountains globally within the past decade (Biskaborn et al., 2019¹⁵⁹). During recent decades, rates of permafrost warming in the European Alps and Scandinavia exceeded values of the late 20th century (limited evidence, high agreement).

The observed thickness of the active layer (see Annex I: Glossary), the layer of ground above permafrost subject to annual thawing and freezing, increased in the European Alps, Scandinavia (Christiansen et al., 2010¹⁶⁰), and on the Tibetan Plateau during the past few decades (Table 2.2), indicating permafrost degradation. Geophysical monitoring in the European Alps during approximately the past 15 years revealed increasing subsurface liquid water content (Hilbich et al., 2008¹⁶¹; Bodin et al., 2009¹⁶²; PERMOS, 2016¹⁶³), indicating gradual ground-ice loss. 

During recent decades, the velocity of rock glaciers in the European Alps exceeded values of the late 20th century (limited evidence, high agreement). Some rock glaciers, that is, masses of ice-rich debris that show evidence of past or present movement, show increasing velocity as a transient response to warming and water input, although continued permafrost degradation would eventually inactivate them (Ikeda and Matsuoka, 2002¹⁶⁴). Rock glacier velocities observed in the European Alps in the 1990s were on the order of a few decimetres per year and during approximately the past 15 years they often were about 2–10 times higher (Bodin et al., 2009¹⁶⁵; Lugon and Stoffel, 2010¹⁶⁶; PERMOS, 2016¹⁶⁷). Destabilisation, including collapse and rapid acceleration, has been documented (Delaloye et al., 2010¹⁶⁸; Buchli et al., 2013¹⁶⁹; Bodin et al., 2016¹⁷⁰). One particularly long time series shows velocities around 1960 just slightly lower than during recent years (Hartl et al., 2016¹⁷¹). In contrast to nearby glaciers, no clear change in rock glacier velocity or elevation was detected at a site in the Andes between 1955–1996 (Bodin et al., 2010¹⁷²). The majority of similar landforms investigated in the Alaska Brooks Range increased their velocity since the 1950s, while few others slowed down (Darrow et al., 2016¹⁷³). 

Decadal-scale permafrost warming and degradation are driven by air temperature increase and additionally affected by changes in snow cover, vegetation and soil moisture. Bedrock locations, especially when steep and free of snow, produce the most direct signal of climate change on the ground thermal regime (Smith and Riseborough, 1996¹⁷⁴), increasing the confidence in attribution. Periods of cooling, one or few years long, have been observed and attributed to extraordinary low-snow conditions (PERMOS, 2016¹⁷⁵). Extreme increases of active-layer thickness often correspond with summer heat waves (PERMOS, 2016¹⁷⁶) and permafrost degradation can be accelerated by water percolation (Luethi et al., 2017¹⁷⁷). Similarity and synchronicity of interannual to decadal velocity changes of rock glaciers within the European Alps (Bodin et al., 2009¹⁷⁸; Delaloye et al., 2010¹⁷⁹) and the Tien Shan (Sorg et al., 2015¹⁸⁰), suggest common regional forcing such as summer air temperature or snow cover.

Because air temperature is the major driver of permafrost change, permafrost in high mountain regions is expected to undergo increasing thaw and degradation during the 21st century, with stronger consequences expected for higher greenhouse gas emission scenarios (very high confidence). Scenario simulations for the Tibetan Plateau until 2100 estimate permafrost area to be strongly reduced, for example by 22–64% for RCP2.6 and RCP8.5 and a spatial resolution of 0.5º (Lu et al., 2017¹⁸¹). Such coarse-scale studies (Guo et al., 2012¹⁸²; Slater and Lawrence, 2013¹⁸³; Guo and Wang, 2016¹⁸⁴), however, are of limited use in quantifying changes and informing impact studies in steep terrain due to inadequate representation of topography (Fiddes and Gruber, 2012¹⁸⁵). Fine-scale simulations, on the other hand, are local or regional, limited in areal extent and differ widely in their representation of climate change and permafrost. They reveal regional and elevational differences of warming and degradation (Bonnaventure and Lewkowicz, 2011¹⁸⁶; Hipp et al., 2012¹⁸⁷; Farbrot et al., 2013¹⁸⁸) as well as warming rates that differ between locations (Marmy et al., 2016¹⁸⁹) and seasons (Marmy et al., 2013¹⁹⁰). While structural differences in simulations preclude a quantitative summary, these studies agree on increasing warming and thaw of permafrost for the 21st century and reveal increased loss of permafrost under stronger atmospheric warming (Chadburn et al., 2017¹⁹¹). Permafrost thaw at depth is slow but can be accelerated by mountain peaks warming from multiple sides (Noetzli and Gruber, 2009¹⁹²) and deep percolation of water (Hasler et al., 2011¹⁹³). Near Mont Blanc in the European Alps, narrow peaks below 3,850 m a.s.l. may lose permafrost entirely under RCP8.5 by the end of the 21st century (Magnin et al., 2017¹⁹⁴). As ground-ice from permafrost usually melts slower than glacier ice, some mountain regions will transition from having abundant glaciers to having few and small glaciers but large areas of permafrost that is thawing (Haeberli et al., 2017¹⁹⁵).

Based on limited evidence, AR5 reported shorter seasonal ice cover duration during the past decades (low confidence), however, did not specifically address changes in mountain lakes and rivers. Observations of extent, timing, duration and thickness of lake and river ice rely mostly on in situ measurements (e.g., Sharma et al., 2019) and, increasingly on remote sensing (Duguay et al., 2014²¹⁸). Lake and river ice studies focusing specifically on mountain regions are rare but observations from lakes in the European Alps, Scandinavia and the Tibetan Plateau show highly variable trends in ice cover duration during the past decades.

For example, Cai et al. (2019) reported shorter ice cover duration for 40 lakes and longer duration for 18 lakes on the Tibetan Plateau during the period 2000–2017. Similarly, using microwave remote sensing, Du et al. (2017) found shorter ice cover duration for 43 out of 71 lakes >50 km2 including lakes on the Tibetan Plateau during 2002–2015, but only five of these had statistically significant trends (p  <  0.05), due to large interannual variability. The variable trends in the duration of lake ice cover on the Tibetan Plateau between 2002–2015 corresponded to variable trends in surface water temperatures. Of 52 study lakes in this region, 31 lakes showed a mean warming rate of 0.055 ± 0.033 °C yr-1, and 21 lakes showed a mean cooling rate of -0.053 ± 0.038 °C yr-1 during 2001–2012 (Zhang et al., 2014²¹⁹). Kainz et al. (2017)²²⁰ reported a significant (p  <  0.05) increase in the interannual variability in ice cover duration for a subalpine lake in Austria during 1921–2015 in addition to a significant trend in later freeze on, earlier ice break up and shorter ice cover duration. A significant (p < 0.05) trend towards shorter ice cover duration was found for another Austrian alpine lake during 1972–2015 (Niedrist et al., 2018²⁰⁹).

Highly variable trends were also found in the timing and magnitude of river ice jams during 1903–2015, as reported by Rokaya et al. (2018) for Canadian rivers, including rivers in the mountains. Most of the variability in river ice trends could be explained by variable water flow, in particular due to flow regulation.

There is high confidence that air temperature and solar radiation are the most important drivers to explain observed changes of lake ice dynamics (Sharma et al., 2019²¹⁰). In mountainous regions where the interannual variability in ice cover duration is high, additional drivers become important, for example, morphometry, wind exposure, salinity, and hydrology, in particular hydrological processes driven by glaciers (Kropácek et al., 2013²¹¹; Song et al., 2014²¹²; Yao et al., 2016²¹³; Gou et al., 2017²¹⁴). Despite high spatial and temporal variability in lake and river ice cover dynamics in mountain regions there is limited evidence (high agreement) that further air temperature increases will result in a general trend towards later freezing, earlier break-up, and shorter ice cover duration in the future (Gebre et al., 2014²¹⁵; Du et al., 2017²¹⁶).

Overall, there is only limited evidence on changes in lake and river ice specifically in the mountains, indicating a trend, but not universally, towards shorter lake ice cover duration consistent with increased water temperature.

The mountain cryosphere is an important source of freshwater in the mountains themselves and in downstream regions. The runoff per unit area generated in mountains is on average approximately twice as high as in lowlands (Viviroli et al., 2011²³⁷) making mountains a significant source of fresh water in sustaining ecosystem and supporting livelihoods in and far beyond the mountain ranges themselves. The presence of snow, glaciers, and permafrost generally exert a strong control on the amount, timing and biogeochemical properties of runoff (FAQ 2.1). Changes to the cryosphere due to climate change can alter freshwater availability with direct consequences for human populations and ecosystems

High mountains are particularly prone to hazards related to snow, ice and permafrost as these elements exert key controls on mountain slope stability (Haeberli and Whiteman, 2015⁴⁴⁵). This section assesses knowledge gained since previous IPCC reports, in particular SREX (e.g., Seneviratne et al., 2012), and AR5 Working Group II (Cramer et al., 2014⁴⁴⁶). In this section, observed and projected changes in hazards are covered first, followed by exposure, vulnerability and resulting impacts and risks, and finally disaster risk reduction and adaptation. Cryospheric hazards that constitute tipping points are also listed in Table 6.1 in Chapter 6. 

Hazards assessed in this section range from localised effects on mountain slopes and adjacent valley floors (distance reach of up to several kilometres) to events reaching far into major valleys and even surrounding lowlands (reach of tens to hundreds of kilometres), and include cascading events. Changes in the cryosphere due to climate change influence the frequency and magnitude of hazards, the processes involved, and the locations exposed to the hazards (Figure 2.7). Natural hazards and associated disasters are sporadic by nature, and vulnerability and exposure exhibit strong geographic variations. Assessments of change are based not only on direct evidence, but also on laboratory experiments, theoretical considerations and calculations, and numerical modelling.

Widespread climate driven ecological changes have occurred in high mountain ecosystems over the past century. Those impacts were assessed in a dedicated manner only in earlier IPCC assessments (Beniston and Fox, 1996⁶⁸²; Gitay et al., 2001⁶⁸³; Fischlin et al., 2007⁶⁸⁴) but not in AR5 (Settele et al., 2014⁶⁸⁵). Two of the most evident changes include range shifts of plants and animals in Central Europe and the Himalaya but also for other mountain regions (e.g., Morueta-Holme et al., 2015; Evangelista et al., 2016⁶⁸⁶; Freeman et al., 2018⁶⁸⁷; Liang et al., 2018⁶⁸⁸; You et al., 2018⁶⁸⁹; He et al., 2019⁶⁹⁰), and increases in species richness on mountain summits (Khamis et al., 2016⁶⁹¹; Fell et al., 2017⁶⁹²; Steinbauer et al., 2018⁶⁹³) of which some have accelerated during recent decades (e.g., Steinbauer et al., 2018), though slowing over the past ten years in Austria (e.g., Lamprecht et al., 2018). While many changes in freshwater communities have been directly attributed to changes in the cryosphere (Jacobsen et al., 2012⁶⁹⁴; Milner et al., 2017⁶⁹⁵), separating the direct influence of atmospheric warming from the influence of concomitant cryospheric change and independent biotic processes has been often challenging for terrestrial ecosystems (Grytnes et al., 2014⁶⁹⁶; Lesica and Crone, 2016⁶⁹⁷; Frei et al., 2018⁶⁹⁸; Lamprecht et al., 2018⁶⁹⁹). Changing climate in high mountains places further stress on biota, which are already impacted by land use and its change, direct exploitation, and pollutants (Díaz et al., 2019⁷⁰⁰; Wester et al., 2019⁷⁰¹). Species are required to shift their behaviours, including seasonal aspects, and distributional ranges to track suitable climate conditions (Settele et al., 2014⁷⁰²). In the Special Report on Global Warming of 1.5°C (SR15), climate change scenarios exceeding mean global warming of 1.5° C relative to preindustrial levels have been estimated to lead to major impacts on species abundances, community structure, and ecosystem functioning in high mountain areas (Hoegh-Guldberg et al., 2018⁷⁰³). The size and isolation of mountain habitats (Steinbauer et al., 2016⁷⁰⁴; Cotto et al., 2017⁷⁰⁵), which may vary strongly with the topography of mountain ridges (Elsen and Tingley, 2015⁷⁰⁶; Graae et al., 2018⁷⁰⁷), affects critically the survival of species as they migrate across mountain ranges, increasing in general the risks for many species from climate change (Settele et al., 2014⁷⁰⁸; Dobrowski and Parks, 2016⁷⁰⁹).

There is high confidence that permafrost thaw has had negative impacts on the integrity of infrastructure in high mountain areas. Like in polar regions (Section 3.4.3.3.4), the local effects of infrastructure together with climate change degraded permafrost beneath and around structures (Dall’Amico et al., 2011⁸⁵⁷; Doré et al., 2016⁸⁵⁸) Infrastructure on permafrost in the European Alps, mostly found near mountain summits but not in major valleys, has been destabilised by permafrost thaw, including mountain stations in France and Austria (Ravanel et al., 2013⁸⁵⁹; Keuschnig et al., 2015⁸⁶⁰; Duvillard et al., 2019⁸⁶¹) as well as avalanche defence structures (Phillips and Margreth, 2008⁸⁶²) and a ski lift (Phillips et al., 2007⁸⁶³) in Switzerland. On the Tibetan Plateau, deformation or damage has been found on roads (Yu et al., 2013⁸⁶⁴; Chai et al., 2018⁸⁶⁵), power transmission infrastructure (Guo et al., 2016⁸⁶⁶) and around an oil pipeline (Yu et al., 2016⁸⁶⁷). For infrastructure on permafrost, engineering practices suitable for polar and high mountain environments (Doré et al., 2016⁸⁶⁸) as well as specific for steep terrain (Bommer et al., 2010⁸⁶⁹) have been developed to support adaptation.

In some mountain regions, glacier retreat and related processes of change in the cryosphere have afforded greater accessibility for extractive industries and related activities to mine minerals and metals (medium confidence). Accelerated glacier shrinkage and retreat have been reported to facilitate mining activities in Chile, Argentina and Peru (Brenning, 2008⁸⁷⁰; Brenning and Azócar, 2010⁸⁷¹; Anacona et al., 2018⁸⁷²), and Kyrgyzstan (Kronenberg, 2013⁸⁷³; Petrakov et al., 2016⁸⁷⁴), which also interact with and have consequences for other social, cultural, economic, political and legal measures, where climate change impacts also play a role (Brenning and Azócar, 2010⁸⁷⁵; Evans et al., 2016⁸⁷⁶; Khadim, 2016⁸⁷⁷; Anacona et al., 2018⁸⁷⁸). However, negative impacts due to cryosphere changes may also occur. One study projects that reductions in glacier melt water and snowmelt in the watershed in the Chilean Andes will lead to a reduction of water supply to a copper mine by 2075–2100 of 28% under scenario A2 and of 6% under B2; construction of infrastructure to draw water from other sources will cost between 16–137 million USD (Correa-Ibanez et al., 2018⁸⁷⁹).

Conversely, there is also evidence suggesting that some of these mining activities affect glaciers locally, and the mountain environment around them, further altering glacier dynamics, glacier structure and permafrost degradation. This is due mainly to excavation, extraction, and use of explosives (Brenning, 2008⁸⁸⁰; Brenning and Azócar, 2010⁸⁸¹; Kronenberg, 2013⁸⁸²), and deposition of dust and other mine waste material close to or top of glaciers during extraction and transportation (Brenning, 2008⁸⁸³; Torgoev and Omorov, 2014⁸⁸⁴; Arenson et al., 2015b⁸⁸⁵; Jamieson et al., 2015⁸⁸⁶). These activities have reportedly generated slope instabilities (Brenning, 2008⁸⁸⁷; Brenning and Azócar, 2010⁸⁸⁸; Torgoev and Omorov, 2014⁸⁸⁹), glacier mass loss due to enhanced surface melt from dust and debris deposition (Torgoev and Omorov, 2014⁸⁹⁰; Arenson et al., 2015b⁸⁹¹; Petrakov et al., 2016⁸⁹²), and even glacier advance by several kilometres (Jamieson et al., 2015⁸⁹³), although their impact is considered less than that reported for changes in glaciers due to climatic change (limited evidence, medium agreement). Glacier Protection Laws and similar measures have been introduced in countries such as Chile and Argentina to address these impacts (Khadim, 2016⁸⁹⁴; Anacona et al., 2018⁸⁹⁵; Navarro et al., 2018⁸⁹⁶). In addition, the United Nations Human Rights Council passed a declaration in 2018 to “protect and restore water-related ecosystems” in mountain areas as elsewhere from contamination by mining (UNHRC, 2018⁸⁹⁷); however, evidence on the effectiveness of these measures remains inconclusive.

The mountain cryosphere provides important aesthetic, cultural, and recreational services to society (Xiao et al., 2015⁸⁹⁸). These services support tourism, providing economic contributions and livelihood options to mountain communities and beyond. The relevant changes in the cryosphere affecting mountain tourism and recreation include shorter seasons of snow cover, more winter precipitation falling as rain instead of snow, and declining glaciers and permafrost (Sections 2.2.1, 2.2.2, 2.2.3 and 2.2.4). Downhill skiing, the most popular form of snow recreation, occurs in 67 countries (Vanat, 2018⁸⁹⁹). The Alps in Europe support the largest ski industry (Vanat, 2018⁹⁰⁰). In Europe, the growth of alpine skiing and winter tourism after 1930 brought major economic growth to alpine regions and transformed winter sports into a multi-billion USD industry (Denning, 2014⁹⁰¹). Sixteen percent of skier visits occur in the USA, where expenditures from all recreational snow sports generated more than 695,000 jobs and 72.7 billion USD in trip-related spending in 2016 (Outdoor Industry Association, 2017⁹⁰²). While the number of ski resorts in the USA has been decreasing since the 1980s, China added 57 new ski resorts in 2017 (Vanat, 2018⁹⁰³). Although the bulk of economic activity is held within mountain communities, supply chains for production of ski equipment and apparel span the globe. Steiger et al. (2017)⁹⁰⁴ point out that Asia, Africa and South America are underrepresented in the ski tourism literature, and Africa and the Middle East are not significant markets from a ski tourism perspective.

Skiing’s reliance on favourable atmospheric and snow conditions make it particularly vulnerable to climate change (Arent et al., 2014⁹⁰⁵; Hoegh-Guldberg et al., 2018⁹⁰⁶). Snow reliability, although not universally defined, quantifies whether the snow cover is sufficient for ski resorts operations. Depending on the context, it focuses on specific periods of the winter season, and may account for interannual variability and/or for snow management (Steiger et al., 2017⁹⁰⁷). The effects of less snow, due to strong correlation between snow cover and skier visits, cost the USA economy 1 billion USD and 17,400 jobs per year between 2001–2016 in years of less seasonal snow (Hagenstad et al., 2018⁹⁰⁸). Efforts to reduce climate change impacts and risks to economic losses focus on increased snowmaking, such as artificial production of snow (Steiger et al., 2017⁹⁰⁹), summertime slope preparation (Pintaldi et al., 2017⁹¹⁺), grooming (Steiger et al., 2017⁹¹¹), and snow farming, that is, storage of snow (Grünewald et al., 2018⁹¹²). The effectiveness of snow management methods as adaptation to long-term climate change depends on sufficiently low air temperature conditions needed for snowmaking, water and energy availability, compliance with environmental regulations (de Jong, 2015), and ability to pay for investment and operating costs. When these requirements are met, evidence over the past decades shows that snow management methods have generally proven efficient in reducing the impact of reduced natural snow cover duration for many resorts (Dawson and Scott, 2013⁹¹³; Hopkins and Maclean, 2014⁹¹⁴; Steiger et al., 2017⁹¹⁵; Spandre et al., 2019a⁹¹⁶). The number of skier visits was found to be 39% less sensitive to natural snow variations in Swiss ski resorts with 30% areal snowmaking coverage (representing the national average), compared to resorts without snowmaking (Gonseth, 2013⁹¹⁷). In some regions, many resorts (mostly smaller, low-elevation resorts) have closed due to unfavourable snow conditions brought on by climate change and/or the associated need for large capital investments for snowmaking capacities (e.g., in northeast USA; Beaudin and Huang, 2014⁹¹⁸)). To offset loss in ski tourism revenue, a key adaptation strategy is diversification, offering other non-snow recreation options such as mountain biking, mountain coasters and alpine slides, indoor climbing walls and water parks, festivals and other special events (Figure 2.9; Hagenstad et al., 2018⁹¹⁹; Da Silva et al., 2019).

In the near term (2031–2050) and regardless of the greenhouse gas emission scenario, risks to snow reliability exist for many resorts, especially at lower elevation, although snow reliability is projected to be maintained at many resorts in North America (Wobus et al., 2017⁹²⁰) and in the European Alps, Pyrenees and  Scandinavia (Marke et al., 2015⁹²¹; Steiger et al., 2017⁹²²; Scott et al., 2019⁹²³; Spandre et al., 2019a⁹²⁴; Spandre et al., 2019b⁹²⁵). At the end of the century (2081–2100), under RCP8.5, snow reliability is projected to be unviable for most ski resorts under current operating practices in North America, the European Alps and Pyrenees, Scandinavia and Japan, with some exceptions at high elevation or high-latitudes (Steiger et al., 2017⁹²⁶; Wobus et al., 2017⁹²⁷; Suzuki-Parker et al., 2018⁹²⁸; Scott et al., 2019⁹²⁹; Spandre et al., 2019a⁹³⁰; Spandre et al., 2019b⁹³¹). Only few studies have used RCP2.6 in the context of ski tourism, and results indicate that the risks at the end of the century (2081–2100) are expected to be similar to the near term impacts (2031–2050) for RCP8.5 (Scott et al., 2019⁹³²; Spandre et al., 2019a⁹³³).

The projected economic losses reported in the literature include an annual loss in hotel revenues of EUR 560 million  (2012 value) in Europe, compared to the period 1971–2000 under a 2°C global warming scenario (Damm et al., 2017⁹³⁵). This estimate includes population projections but does not account for snow management. In the USA, Wobus et al. (2017) estimate annual revenue losses from tickets (skiing) and day fees (cross country skiing and snowmobiling) due to reduced snow season length, will range from 340–780 million USD in 2050 for RCP4.5 and RCP8.5, respectively, and from 130 million to 2 billion USD in 2090 for RCP4.5 and RCP8.5 respectively, taking into account snow management and population projections. Total economic losses from these studies would be much higher if all costs were included (costs for tickets, transport, lodging, food and equipment). Regardless of the climate scenario, as risk of financial unviability increases, there are reported expectations that companies would need to forecast when their assets may become stranded assets and require devaluation or conversion to liabilities, and report this on their balance sheets (Caldecott et al., 2016⁹³⁶). Economic impacts are projected to occur in other snow-based winter activities including events (e.g., ski races) and other recreation activities such as cross-country skiing, snowshoeing, backcountry skiing, ice climbing, sledding, snowmobiling and snow tubing. By 2050, 13 (out of 21) prior Olympic Winter Games locations are projected to exhibit adequate snow reliability under RCP2.6, and 10 under RCP8.5. By 2080, the number decreases to 12 and 8, respectively (Scott et al., 2018⁹³⁷). Even for cities remaining cold enough to host ski competitions, costs are projected to rise for making and stockpiling snow, as was the case in Sochi, Russia in 2014 and Vancouver, Canada in 2010 (Scott et al., 2018⁹³⁸), and preserving race courses through salting (Hagenstad et al., 2018⁹³⁹).

In summer, cryosphere changes are impacting glacier-related activities (hiking, sightseeing, skiing, climbing and mountaineering) (Figure 2.8). In recent years, several ski resorts operating on glaciers have ceased summer operations due to unfavourable snow conditions and excessive operating costs (e.g., Falk, 2016). Snow management and snowmaking are increasingly used on glaciers (Fischer et al., 2016⁹⁴⁰). Glacier retreat has led to increased moraine instability which can compromise hiker and climber safety along established trails and common access routes, for example, in Iceland (Welling et al., 2019⁹⁴¹), though it has made some areas in the Peruvian Andes more accessible to trekkers (Vuille et al., 2018⁹⁴²). In response, some hiking routes have been adjusted and ladders and fixed anchors installed (Duvillard et al., 2015⁹⁴³; Mourey and Ravanel, 2017⁹⁴⁴). As permafrost thaws, rock falls on and off glaciers are increasingly observed, threatening the safety of hikers and mountaineers, for example, in Switzerland (Temme, 2015⁹⁴⁵) and New Zealand (Purdie et al., 2015⁹⁴⁶). Glacier retreat and permafrost thaw have induced major changes to iconic mountaineering routes in the Mont Blanc area with impacts on mountaineering practices, such as shifts in suitable climbing seasons, and reduced route safety (Mourey and Ravanel, 2017⁹⁴⁷; Mourey et al., 2019⁹⁴⁸). Cryosphere decline has also reduced opportunities for ice climbing and reduced attractions for summer trekking in the Cascade Mountains, USA (Orlove et al., 2019⁹⁴⁹). In response to these impacts, tour companies have shifted to new sites, diversified to offer other activities or simply reduced their activities (Furunes and Mykletun, 2012⁹⁵⁰) (Figure 2.9). Steps to improve consultation and participatory approaches to understand risk perception and design joint action between affected communities, authorities and operators, are evident, for example, in Iceland (Welling et al., 2019⁹⁵¹). In some cases, new opportunities are presented such as marketing ‘climate change tourism’ where visitors are attracted by ‘last chance’ opportunities to view a glacier; for example, in New Zealand (Stewart et al., 2016⁹⁵²), in China (Wang et al., 2010⁹⁵³) or through changing landscapes such as new lakes, for instance in Iceland (Þórhallsdóttir and Ólafsson, 2017⁹⁵⁴), or to view the loss of a glacier, for example, in the Bolivian Andes (Kaenzig et al., 2016⁹⁵⁵). The opening of a trekking route promoting this opportunity created tensions between a National Park and a local indigenous community in the Peruvian Andes over the management and allocation of revenue from the route (Rasmussen, 2019⁹⁵⁶). The consequences of ongoing and future glacier retreat are projected to negatively impact trekking and mountaineering in the Himalaya (Watson and King, 2018⁹⁵⁷). Reduced snow cover has also negatively impacted trekking in the Himalaya, since tourists find the mountains less attractive as a destination, and the reduced water availability affects the ability of hotels and campsites to serve visitors (Becken et al., 2013⁹⁵⁸).

In summary, financial risks to mountain communities that depend on tourism for income, are high and include losses to revenues generated from recreation primarily in the winter season. Adaptation to cryosphere change for ski tourism focuses on snowmaking and is expected to be moderately effective for many locations in the near term (2031–2050), but it is unlikely to substantially reduce the risks in most locations in the longer term (end of century) (high confidence). Determining the extent to which glacier retreat and permafrost thaw impact upon overall visitor numbers in summer tourism, and how any losses or increased costs are offset by opportunities, is inconclusive. Furthermore, tourism is also impacted by cryospheric change that impacts on water resources availability, increasing competition for its use (Section 2.3.1.3).

Cryosphere changes also impact cultural values, which are held by populations in high mountains and other regions around the world; these impacts often harm human well-being (Tschakert et al., 2019⁹⁵⁹) (medium evidence, high agreement). Cultural values were covered extensively in AR5, with particular emphasis on small island states and the Arctic; the research on cultural values in high mountain regions is relatively new. Out of a total of 247 UNESCO World Heritage natural sites recognised for their outstanding universal value, 46 sites include glaciers within their boundaries, where the presence of glaciers is stated among the principal reason (5 sites), or secondary reason (28 sites), for World Heritage inscription; complete glacier extinction is projected by 2100 in 8 to 21 of these sites, under RCP2.6 and RCP8.5 scenarios, respectively, compromising the outstanding universal value placed on these sites, which have been inscribed at least partly for their exceptional glaciers (Bosson et al., 2019⁹⁶⁰). UNESCO defines “outstanding universal value” as “cultural and/or natural significance which is so exceptional as to transcend national boundaries and to be of common importance for present and future generations of all humanity” (UNESCO, 2012⁹⁶¹). Furthermore, in recognising the importance of the cultural and intangible value placed by communities on aspects of their surrounding environment, such as those afforded by cryosphere elements in the high mountains, are mentioned under the workplan of the Warsaw International Mechanism as a specific work area under ‘Non-economic loss and damage’ (UNFCCC Secretariat, 2014⁹⁶²; Serdeczny, 2019⁹⁶³).

Cultural values include spiritual, intrinsic and existence values, as well as aesthetic dimensions, which are also an element of tourism and recreation (Section 2.3.5), though they focus more directly on ties to sacred beings or to inherent rights of entities to exist. However, these values overlap, since the visual appeal of natural landscapes links with a sense of the immensity of mountain landscapes, glaciers and fresh snow (Paden et al., 2013⁹⁶⁴; Gagné et al., 2014⁹⁶⁵). Moreover, different stakeholders, such as local communities, tourists and policymakers, may place different emphasis on specific cultural values (Schirpke et al., 2016⁹⁶⁶). For the indigenous Manangi community of the Annapurna Conservation Area of Nepal, the loss of glaciers which they have observed threatens their ethnic identity (Konchar et al., 2015⁹⁶⁷). Villagers in the Italian Alps also report that glacier retreat weakens their identity (Jurt et al., 2015⁹⁶⁸).

Spiritual and intrinsic values in high mountain regions often, but not exclusively, rest on deeply held religious beliefs and other local customs (medium evidence, high agreement). Some communities understand mountains through a religious framework (Bernbaum, 2006⁹⁶⁹). In settings as diverse as the Peruvian Andes, the Nepal Himalaya, the Alps, the North Cascades (USA), Mount Kilimanjaro and the Hengduan Mountains of southwest China, local populations view glacier retreat as the product of their failure to show respect to sacred beings or to follow proper conduct. Experiencing deep concern that they have disturbed cosmic order, they seek to behave in closer accord with established traditions; they anticipate that the retreat will continue, leading to further environmental degradation and to the decline of natural and social orders—a prospect which causes them distress (Becken et al., 2013⁹⁷⁰; Gagné et al., 2014⁹⁷¹; Allison, 2015⁹⁷²). In the USA, the snow covered peaks of the Cascades have also evoked a deep sense of awe and majesty, and an obligation to protect them (Carroll, 2012⁹⁷³; Duntley, 2015⁹⁷⁴). Similar views are found in the Italian Alps, where villagers speak of treating glacier peaks with “respect,” and state that glacier retreat is due, at least in part, to humans “disturbing” the glaciers (Brugger et al., 2013⁹⁷⁵), resulting in an emotion which Albrecht et al. (2007)⁹⁷⁶ termed solastalgia, a kind of deep environmental distress or ecological grief (Cunsolo and Ellis, 2018⁹⁷⁷).

Glacier retreat threatens the Indigenous knowledge and local knowledge of populations in mountain regions; this knowledge constitutes a cultural service to wider society by contributing to scientific understanding of glaciers (Cross-Chapter Box 4 in Chapter 1). Though this knowledge is dynamic, and records previous states of glaciers, it has been undermined by the complete disappearance of glaciers in a local area (Rhoades et al., 2008⁹⁷⁸). This knowledge of glaciers is often tied to religious beliefs and practices. It is based on direct observation, stories passed down from one generation to another within community, placenames, locations of structures and other sources (Gagné et al., 2014⁹⁷⁹). Residents of mountain areas can provide dates for previous locations of glacier fronts, sometimes documenting these locations through the presence of structures (Brugger et al., 2013⁹⁸⁰). Much like other cases of data from citizen science (Theobald et al., 2015⁹⁸¹), their observations often overlap with the record of instrumental observations (Deng et al., 2012⁹⁸²), and can significantly extend this record (Mark et al., 2010⁹⁸³).

An additional cultural value is the contribution of glaciers to the understanding of human history. Glacier retreat has supported the increase of knowledge of past societies by providing access to archaeological materials and other cultural resources that had previously been covered by ice. The discovery of Oetzi, a mummified Bronze Age man whose remains were discovered in 1991 in the Alps near the Italian-Austrian border, marked the beginning of scientific research with such materials (Putzer and Festi, 2014⁹⁸⁴). Subsequent papers described objects that were uncovered in retreating glaciers and shrinking ice patches in the Wrangell-Saint Elias Range (Dixon et al., 2005⁹⁸⁵), the Rocky Mountains (Lee, 2012) and Norway (Bjørgo et al., 2016⁹⁸⁶). This field provides new insight into human cultural history and contributes to global awareness of climate change (Dixon et al., 2014⁹⁸⁷). Though climate change permits the discovery of new artefacts and sites, it also threatens these objects and places, since they become newly exposed to harsh weather (Callanan, 2016⁹⁸⁸).

High mountain communities have historically included mobility in their sets of livelihood strategies, as a means to gain access to production zones at different elevations within mountain zones and in lowland areas, and as a response to the strong seasonality of agricultural and pastoral livelihoods. Cryosphere changes in high mountain areas have influenced human mobility and migration during this century by altering water availability and increasing exposure to mass movements and floods and other cryospheric induced disasters (Figure 2.7) (Barnett et al., 2005⁹⁸⁹; Carey et al., 2017⁹⁹⁰; Rasul and Molden, 2019⁹⁹¹). These changes affect three forms of human mobility: transhumant pastoralism, temporary or permanent wage labour migration and displacement, in which entire communities resettle in new areas.

Transhumant pastoralism, involving movements between summer and winter pastures, is a centuries old practice in high mountain areas (Lozny, 2013⁹⁹²). In High Mountain Asia and other regions, it is declining due to both climatic factors, including changes in snow distribution and glaciers, and to non-climatic factors, and is projected to continue declining, at least in the short term (medium evidence, high agreement). The changes in snow and glaciers adversely affect herders at their summer residences and winter camps in the Himalaya (Namgay et al., 2014⁹⁹³) and in Scandinavian mountains (Mallory and Boyce, 2018⁹⁹⁴). Reduced winter snowfall has led to poorer pasture quality in Nepal (Gentle and Maraseni, 2012⁹⁹⁵) and India (Ingty, 2017⁹⁹⁶). Other climate change impacts, including erratic snowfall patterns and a decrease in rainfall, are perceived by herders in Afghanistan, Nepal and Pakistan to have resulted in vegetation of lower quality and quantity (Shaoliang et al., 2012⁹⁹⁷; Joshi et al., 2013⁹⁹⁸; Gentle and Thwaites, 2016⁹⁹⁹). Heavy snowfall incidents in winter caused deaths of a large number of livestock in northern Pakistan in 2009 (Shaoliang et al., 2012¹⁰⁰⁰). Herders in Nepal reported of water scarcity in traditional water sources along migration routes (Gentle and Thwaites, 2016¹⁰⁰¹). Increased glacier melt water has caused lakes on the Tibetan Plateau to increase in size, covering pasture areas and leading pastoralists to alter their patterns of seasonal movement (Nyima and Hopping, 2019¹⁰⁰²). However, rising temperatures, with associated effects on snow cover, have some positive impacts. Seasonal migration from winter to summer pastures start earlier in Northern Pakistan, and residence in summer pasture lasts longer (Joshi et al., 2013¹⁰⁰³), as it does in Afghanistan (Shaoliang et al., 2012¹⁰⁰⁴).

Wage labour migration is also a centuries old practice in the Himalaya, the Andes and the European Alps (Macfarlane, 1976¹⁰⁰⁵; Cole, 1985¹⁰⁰⁶; Viazzo, 1989¹⁰⁰⁷). Studies show that migration is a second-order effect of cryosphere changes, since the first-order effects, a decrease in agricultural production (Section 2.3.1.3.2), have led to increased wage labour migration to provide supplementary income in a number of regions (medium evidence, high agreement). Wage labour migration linked to cryosphere changes occurs on several time scales, including short-term, long-term and permanent migration, and on different spatial scales. Though migration usually takes place within the country of origin, and sometimes within the region, cases of international migration have also been recorded (Merrey et al., 2018¹⁰⁰⁸). The studies since AR5 on migration driven by cryosphere changes are concentrated in High Mountain Asia and the Andes, supporting the finding, reported in AR5 Working Group II (Section 12.7), that stress on livelihoods is an important driver of climate change induced migration. The research on such migration also supports the finding in SR15 (Section 4.3.5.6) that migration can have mixed outcomes on reducing socioeconomic vulnerability, since cases of increase and of reduction of vulnerability are both found in migration from high mountain regions that is driven by cryosphere changes. 

Changing water availability, mass movements and floods are cryosphere processes which drive wage labour migration (medium evidence, high agreement). A debris flow in central Nepal in 2014, in a region where landslides have increased in recent decades, led more than half the households to migrate for months (van der Geest and Schindler, 2016¹⁰⁰⁹). In the Santa River drainage, Peru, rural populations have declined 10% between 1970–2000, and the area of several major subsistence crops also declined (Bury et al., 2013¹⁰¹⁰). Research in this region suggests that seasonal wage labour migration from small basins within the main Santa basin is largest in the small drainages in which glacier retreat has reduced melt water flow most significantly; where this process is not as acute, and streamflow is less reduced, migration rates are lower  (Wrathall et al., 2014¹⁰¹¹). A study from a region in the central Peruvian Andes shows that the residents of the villages that have the highest dependence on glacier melt water travel further and stay away longer than the residents of the villages where glacier melt water forms a smaller portion of stream flow (Milan and Ho, 2014¹⁰¹²). However, the inverse relation between reliance on cryosphere-related water sources and migration was noted in a case in the Naryn River drainage in Kyrgyzstan, where the villages that are more dependent on glacier melt water had lower, rather than higher, rates of wage labour migration than the villages which were less dependent on it; the villages with lower rates of such migration also had more efficient water management institutions than the others (Hill et al., 2017¹⁰¹³). Several studies, which project cryosphere-related emigration to continue in the short term, emphasise decreased water availability, due to glacier retreat as a driver in Kyrgyzstan (Chandonnet et al., 2016¹⁰¹⁴) and Peru (Oliver-Smith, 2014¹⁰¹⁵), and to reduced snow cover in Nepal (Prasain, 2018¹⁰¹⁶). In most cases, climate is only one of several drivers (employment opportunities and better educational and health services in lowland areas are others).

Several studies show that wage labour migration is more frequent among young adults than among other age groups, supporting the observation in AR5 that climate change migrants worldwide are concentrated in this age (limited evidence, high agreement). This age-specific pattern is found in a valley in Northern Pakistan in which agriculture relies on glacier melt water for irrigation; as river flow decreases, the returns to agricultural labour have declined, and emigration has increased, particularly among the youth, who are assigned, by local cultural practices, to carry out the heaviest work (Parveen et al., 2015¹⁰¹⁷). Emigration has increased in recent decades from two valleys in highland Bolivia which rely on glacier melt water, as water supplies have declined, though other factors also contribute to emigration, including land fragmentation, increasing household needs for income, the lack of local wage-labour opportunities and an interest among the young in educational opportunities located in cities (Brandt et al., 2016¹⁰¹⁸). In Nepal, young members of high-elevation pastoral households impacted by cryosphere change have been increasingly engaged in tourism and labour migration since 2000 (Shaoliang et al., 2012¹⁰¹⁹); similar responses are reported for Sikkim in the Indian Himalaya (Ingty, 2017¹⁰²⁰). A recent study documents the inter-generational dynamics of emigration from a livestock raising community in the Peruvian Andes, where glacier retreat has led to reduced streamflow that supports crucial dry season pasture (Alata et al., 2018¹⁰²¹). Though people 50 years old or older in this community are accustomed to living in the high pasture zones, younger people use livestock raising as a means of accumulating capital. They sell off their animals and move to towns at lower elevations. This loss of young adults has reduced the capacity of households to undertake the most demanding tasks, particularly in periods of inclement weather, accelerating the decline of herding. As a result, the human and animal populations of the communities are shrinking.

Recent research on cryosphere driven migration shows some cases of complex livelihood interactions or feedback loops, in which migration is not merely a result of changes in agricultural livelihoods, but also has impacts, either positive or negative, on these livelihoods (medium confidence). In some instances, the different livelihood strategies complement each other to support income and well-being. A review of migration in the Himalaya and Hindu Kush found that households that participated in labour migration and received remittances had improved adaptive capacity, and lowered exposure to natural hazards (Banerjee et al., 2018¹⁰²²). In other cases, the households and communities, which undertake wage labour migration, encounter conflicts or incompatibilities between migration and agricultural livelihoods. Sustainable management of land, water and other resources is highly labour intensive, and hence labour mobility constrains and limits the adoption of sustainable practices (Gilles et al., 2013¹⁰²³). Moreover, the labour available to a household is differentiated by age. In Northern Pakistan, where cryosphere changes are reducing streamflow the emigration of young people has led to a decline not only in the labour in fields and orchards, but also a decline in the maintenance of irrigation infrastructure, leading to an overall reduction of the agricultural livelihoods in the community (Parveen et al., 2015¹⁰²⁴).

In addition to affecting pastoral transhumance and increasing wage labour migration, cryosphere changes impact human mobility by creating cases of displacement. These cases differ from wage labour migration because they involve entire communities. As a result, they are irreversible, unlike cases in which individuals undertake long-term or permanent migration from their communities but retain the possibility of returning, because, for example, some relatives or former neighbours have remained in place. In this way, these cases of displacement represent cryosphere driven challenges to habitability. Though natural hazards have historically led some communities to relocate, cryosphere changes have contributed to instances of displacement. Unreliable water availability and increased risks of natural hazards are responsible for resettlement of villages in certain high mountain areas (McDonald, 1989¹⁰²⁵; Parveen et al., 2015¹⁰²⁶). A village in Western Nepal moved to lower elevation after decreasing snowfall reduced the flow of water in the river on which their pastoralism and agriculture depended (Barnett et al., 2005¹⁰²⁷). Three villages in Nepal faced severe declines in agricultural and pastoral livelihoods because decreased snow cover led to reduced soil moisture and to the drying up of springs, which were the historical source of irrigation water; in conjunction with an international non-governmental organisation (INGO), the residents planned a move to a lower area (Prasain, 2018¹⁰²⁸). 

The issue of habitability arises in the cases, mentioned above, of communities that relocate after floods or debris flows destroy houses and irrigation infrastructure, or damage fields and pastures. It occurs as well in the cases of households with extensive long-term migration, where agricultural and pastoral livelihoods are undermined by reduced water supply caused by cryospheric change (Barnett et al., 2005¹⁰²⁹). In addition, the loss of cultural values, including spiritual and intrinsic values (Section 2.3.6), can contribute to decisions to migrate (Kaenzig, 2015¹⁰³⁰). Combined with the patterns of permanent emigration, this issue of habitability raises the issue of limits to adaptation in mountain areas (Huggel et al., 2019¹⁰³¹). Projections of decreased streamflow by 2100 in watersheds with strong glacier melt water components in Asia, Europe, and North and South America (Section 2.3.1.1) indicate that threats to habitability may continue through this period and affect the endeavours of achieving the SDGs in developing countries (Rasul et al., 2019¹⁰³²).