Global Disappearance of Tropical Mountain Glaciers : Observations , Causes , and Challenges

This article reviews the current status of tropical glaciers in the South American Andes, East Africa, and Australasia by shedding light on past, present, and future glacier coverage in the tropics, the influence of global and regional climates on the tropical glaciers, the regional importance of these glaciers, and challenges of ongoing glacier recessions. While tropical glaciers have predominantly receded since the Little Ice Age, the rate of shrinkage has accelerated since the late 1970s as a result of climate changes. As a result, socio-ecological implications occur around ecosystem health, natural hazards, freshwater resources, agriculture, hydropower, mining, human and animal health, traditions and spirituality, and peace.


Introduction
Mountain glaciers, particularly in the tropics, are currently diminishing rapidly and are considered as good indicators of climate change [1], owing to their relatively fast response time toward perturbations in climate variables such as precipitation, air temperature, and atmospheric humidity [2].Many tropical glaciers, such as those in Peru and Bolivia, are critical buffers against reduced precipitation during the dry season [3].During the initial stages, enhanced glacier shrinkage in tropical mountains may provoke glacier-related hazards such as glacial lake outburst floods (GLOF) [4].Hydroelectric power plants in Peru [5] and Bolivia [6] often depend on glacier-fed rivers.Many glacierized tropical mountain regions are also important in tourism, for example, in the form of ski resorts.In this context, a timely assessment of tropical glaciers is important.
While numerous review articles [2,3,7,8] and research papers are available on tropical Andean glaciers, much less attention is given to tropical glaciers in Africa and Asia, partly owing to small ice coverage.Whilst tropical glaciers in the Cordillera Blanca have been studied extensively, limited studies on glacier fluctuations and the influence of climate change on the cryosphere are available from East Africa [9][10][11][12][13][14][15] and Australasia [16][17][18][19].A few recent studies using remote sensing data exist on glaciers in Australasia [20][21][22].
In this review article, we present a picture of the glacier distribution across the tropics, the influence of global and regional climates on the tropical glaciers, and their regional importance.Further, we discuss the possible impacts and challenges of continuing loss of glacier coverage throughout the tropics.For a few glaciers (Mt Kenya, Kenya; Cordillera Tres Cruces, Bolivia), we estimated glacier area changes using Landsat satellite image series and the normalized difference snow index (NDSI) method [20,22].

Glaciers in the Tropics
Tropical glaciers are found between the Tropic of Cancer and the Tropic of Capricorn (23 • 26 13.3" N; 23 • 26 13.3" S) within the Intertropical Convergence Zone (ITCZ), and wherever the diurnal temperature range equals the annual temperature range (i.e., ∆T d = ∆T a ) [1].Tropical glaciers exist in the South American Andes, East Africa, and Australasia (Figure 1).The recent Randolph Glacier Inventory (RGI; version 5.0) estimates the total area of tropical glaciers at 2344.15 km 2 (Table 1).
Geosciences 2019, 9, x FOR PEER REVIEW 2 of 26 estimated glacier area changes using Landsat satellite image series and the normalized difference snow index (NDSI) method [20,22].

Glaciers in the Tropics
Tropical glaciers are found between the Tropic of Cancer and the Tropic of Capricorn (23°26′13.3″N; 23°26′13.3″S) within the Intertropical Convergence Zone (ITCZ), and wherever the diurnal temperature range equals the annual temperature range (i.e., ΔTd = ΔTa) [1].Tropical glaciers exist in the South American Andes, East Africa, and Australasia (Figure 1).The recent Randolph Glacier Inventory (RGI; version 5.0) estimates the total area of tropical glaciers at 2344.15 km 2 (Table 1).

Tropical Andes
More than 99% of tropical glaciers are situated in the Andes of South America, including Venezuela, Colombia, Ecuador, Peru, Bolivia, Chile, and Argentina.Much of the previous research has been undertaken using remotely sensed data, as the difficult terrain presents challenges in carrying out extensive fieldwork.
The glaciated areas in Venezuela, Colombia, and Ecuador belong to the inner tropics, with a year-round precipitation pattern that lacks any seasonality.In Venezuela, the Humboldt Glacier in the Tropical glaciers in Australasia can be found on the highest summits of Puncak Jaya in the Indonesian province of Papua (formerly Irian Jaja) and include East Northwall Firn (4862 m), Carstensz Glacier (4844 m), and West Northwall Firn (4750 m).The region has a typical inner tropics climate, and glaciers are highly sensitive to variations in air temperature.The mass balance of the Puncak Jaya glaciers has been continuously negative since the early 20th century [17].

Causes of Glacier Recession
Tropical glaciers reached their Little Ice Age (LIA) maximum extent in the 19th century and have been retreating since then [20].Tropical glaciers are widely considered as indicators of global and regional climate change [1], owing to their high sensitivity to perturbations in climate and relatively fast response time [3].Global climate change impacts on tropical glaciers may vary depending on the geographical region and its climate, as well as the topographic situation (elevation, aspect, and slope).Nevertheless, the Intergovernmental Panel on Climate Change (IPCC) [26,27] mentioned its concern about the rapid recession of tropical glaciers under global warming scenarios.
Numerous studies exist on the influence of El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on tropical glaciers in the Andes [23,[28][29][30][31][32][33][34], but are less frequent for Australasia [21].During El Niño events, a rise in air temperature and reduction in precipitation have been observed throughout the tropics [35][36][37].Since the late 1970s, the frequency of El Niño and the occurrences of warm phases of PDO have been increasing [2,38], which coincides with the general rapid shrinkage of the tropical glaciers.
By the end of the 21st century, climate change-induced temperature increase is predicted to accelerate at higher elevations of the tropical landscape, resulting from humidity conditions and water vapor feedback in the upper troposphere [39,40].In the tropical Andes, the temperature increase may reach up to 0.11 • C per decade, almost double the predicted global increase of 0.06 • C [41].This stronger warming at high altitudes results from snow albedo and surface-based feedbacks, water vapor changes and latent heat release, surface water vapor and radiative flux changes, surface heat loss and temperature change, aerosols, and elevation-dependent warming (EDW) at high altitude mountains [42].Even though the interrelationships among those factors are not fully understood, a rise in air temperature at high elevation tropical mountains will result in glacier loss.Furthermore, the daily minimum temperature will increase faster than the maximum temperature, which could be explained with different atmospheric mechanisms during the day and the night [42].When daily minimum temperatures increase faster than daily maximum temperatures, the diurnal temperature range will decrease, causing higher atmospheric water vapor contents, which in turn enhances glacier melting.Such an accelerated warming pattern at higher elevations has also been documented for the Peruvian Andes [43].However, glacier response to climatic forcing is more complex in the tropics, because glacier recession in this region is likely to be related to other components of the energy balance (net radiation, latent, sensible, and ground heat fluxes) in thermally homogeneous areas [44].
Other regional and local natural and anthropogenic phenomena like volcanic eruptions and mining activities may also influence the pace of glacier retreat [45].The presence of particulate matter such as volcanic dust or black carbon that deposits on the glacier surface reduces the ice's surface albedo, resulting in enhanced melting [46].

Historical Observations of Glacier Extent
Early observations of the glaciers in the Tropics range back to the 16th century.Initial studies were based on qualitative data such as maps and descriptions of sites, while more recently, quantitative data are increasingly being used.

Tropical Andes
Observations on the glaciers in Venezuela reach back to the second half of the 19th century, for example, at snow-covered stratovolcanoes such as the Volcano of Puracé, which are ice-free today, and continued into the early 20th century [47][48][49][50][51][52].The first more detailed map displaying the distribution of glaciers in the Sierra Nevada de Mérida dates back to the 1920s [53].
The snow-capped peaks in Ecuador and their importance as water resources have been mentioned in studies since the mid-19th century [63][64][65][66][67][68].Antisana and Cayambe had a similar snow cover extent, and a number of nearby peaks such as Cerro Saraurcu (4640 m) were glaciated [64].
Glacier observations in Peru started in the early 16th century by members of the Hernando Pizarro expedition to the Cordillera Blanca [24], although a first scientific study of the glaciers was presented much later, in the 19th century [69].In the early 20th century, expeditions and regional surveys delivered numerous glacier and glacial lake studies and presented, for example, several accurate glacier maps derived from extensive fieldwork and the interpretation of terrestrial photographs [62,[70][71][72][73][74][75][76][77][78][79]].
Peru's ice was classified into mountain glaciers (91%), rock glaciers (<9%), and Quelccaya ice cap-the second largest glaciated area in the tropics globally after Coropuna in southern Peru.
In the early 20th century, studies on the glaciers in the Dry Andes of northern Chile and Argentina mentioned the influence of westerly winds and solar radiation exposure on glacier distribution-snow accumulated on eastern slopes, which were sheltered from wind [88,89].Maps were generated by governmental agencies such as the Comisión de Límites and Carta Nacional de Chile.

Australasia
Glaciers of Australasia are located mainly on the high altitude summits of Puncak Jaya, where they were first sighted by the Dutch explorer Jan Carstensz in 1623 [107]; the name 'Carstensz Pyramid' was given to Puncak Jaya afterwards.In 1909, another Dutch explorer, Hendrik Albert Lorentz, climbed Puncak Jaya and observed its glaciers [108].

Remotely-Sensed Glacier Observations
Glacier monitoring entered a new phase in the mid-20th century with the emergence of aerial photogrammetry in the early 1950s and satellite remote sensing in the 1970s [25].

Tropical Andes
Glaciers in South America have been experiencing an unprecedented retreat since the mid-20th century, particularly since the late 1970s, although the rate of shrinkage varies with geographical location and is influenced by global and regional climate forcing [2,3,8].Detailed reviews on recent glacier changes in the tropical South America are available [2,8,109], but a brief overview is given here.
Glaciers of the inner tropics within Venezuela, Colombia, and Ecuador, where precipitation occurs throughout the year and temperature varies little throughout the year [110], showed an exceptionally high rate of shrinkage in recent decades [111][112][113][114].The glacier area in Venezuela decreased from 2.03 to 0.29 km 2 (85.7%) between 1952 and 2003 [115], and then to around 0.1 km 2 in 2011 [113], even though glaciers still covered 0.14 km 2 in 2015 [114].For Colombia, a glacier area reduction of 50% between the 1950s and 2000 was estimated [111]; the total glacial area was estimated to be 109 km 2 for the 1970s [116], 55.4 km 2 in the 2000s [111], and 36.8 km 2 in 2015 [114].At Cotopaxi, an ice-covered volcano in Ecuador, relatively stable conditions appeared between 1956 and 1976, followed by a 30% glacial area loss until 1997 [117].Cáceres et al. (2004) [118] calculated a 42% glacial area loss between 1976 and 2006 at Cotopaxi.Glaciers in the western and eastern cordilleras of Peru generally underwent a very slow retreat between the 1950s and 1970s [119], which was followed by an accelerated shrinkage between the 1970s and the late 20th century [120,121].In comparison, while the retreat rate was relatively low in the western cordilleras, some glaciers in the eastern cordilleras of Peru lost a large portion of their area, for example, the smaller Nevado Tuco in the Cordillera Blanca lost 51% from 1975 to 2015 [2,33].Between 1987 and 2010, glaciers in the entire Cordillera Blanca lost 25% in area [122].Glaciers in the Cordillera Vilcanota lost up to around 4 km 2 annually between 1988 and 2012 [123].An even greater area loss was observed in the Cordillera Carabaya, where the glacial area decreased by 86% from 1975 to 2015, resulting in the expansion and the formation of glacial lakes [124].For the Cordillera Blanca, temperature and precipitation changes alone cannot explain accelerated glacier retreat during the past few decades; in fact, local conditions including morphological (slope, aspect, altitude, etc.) and glaciological characteristics (albedo, surface velocity, etc.) seem to impact glacier mass balance under climate change conditions [44].Predicting future glacial changes in the Peruvian Andes has proven challenging.For example, while Quelccaya Ice Cap could completely lose its accumulation zone before the end of the 21st century [125,126], some other extensive ice masses such as Nevado Coropuna in the Cordillera Ampato in southern Peru might not disappear completely until the 2120s [127].

East Africa
Some researchers consider African glaciers, particularly on Mt Kilimanjaro, as an icon of global warming [13], owing to their rapid disappearance in recent decades [13,130,131].Mt Kenya's glaciers retreated dramatically between 1963 and 1987 [10] and further to 1998, when the glacial area had been reduced to 0.4 km 2 [132].Glacier area loss was 15.2% from 1987 to 1993, 31% from 1993 to 2004, and 40% from 2004 to 2015, indicating an accelerating glacier recession [133].We calculated a glacial area of 0.71 km 2 for 1986, 0.48 km 2 for 2002, and 0.11 km 2 for 2015, a reduction of 84.5% for the entire period (Figure 3).Lewis Glacier saw a drastic reduction of 2 to 10 m in thickness between 1974 and 1978 [134], and a 19.8% reduction in volume between 2004 and 2010 [135].From 2004 to 2016, its ice area shrunk by 46%, while its volume decreased by 57% [136].Hastenrath (2006) In Bolivia, the glacier shrinkage pattern showed similar trends to Peru.In the eastern cordilleras, higher retreat rates were observed on eastern slopes compared with western slopes [124].In the Cordillera Real, some glaciers lost up to 30% between 1987 and 2010, and a number of glaciers such as Chacaltaya disappeared completely [128,129].In the cordilleras Real, Apolobamba, and Tres Cruces, the glacier area loss was between 42% and 47% between 1986 and 2014 [6].Our analyses showed that in the Tres Cruces Cordillera, glaciers covered 51.5 km 2 in 1975, 31.65 km 2 in 1994, and 21.1 km 2 in 2016, which is a loss in glacial area of 59% during the entire period between 1975 and 2016 (Figure 2).

East Africa
Some researchers consider African glaciers, particularly on Mt Kilimanjaro, as an icon of global warming [13], owing to their rapid disappearance in recent decades [13,130,131].
Mt Kenya's glaciers retreated dramatically between 1963 and 1987 [10] and further to 1998, when the glacial area had been reduced to 0.4 km 2 [132].Glacier area loss was 15.2% from 1987 to 1993, 31% from 1993 to 2004, and 40% from 2004 to 2015, indicating an accelerating glacier recession [133].We calculated a glacial area of 0.71 km 2 for 1986, 0.48 km 2 for 2002, and 0.11 km 2 for 2015, a reduction of 84.5% for the entire period (Figure 3).Lewis Glacier saw a drastic reduction of 2 to 10 m in thickness between 1974 and 1978 [134], and a 19.8% reduction in volume between 2004 and 2010 [135].From 2004 to 2016, its ice area shrunk by 46%, while its volume decreased by 57% [136].Hastenrath (2006) [137] related the near disappearance of Mt Kenya's glaciers to greenhouse effects.A similar trend of glacial retreat was also observed at Mt Kilimanjaro, where the glacial area decreased from 6.7 km 2 to 3.3 km 2 between 1953 and 1989 [138], to 2.6 km 2 in 2000 [139], and to 1.7 km 2 in 2016 based on our analysis.Two different glacier systems can be identified at the mountain: (1) on the plateau above 5700 m, where the retreat of the vertical glacier wall is predominantly controlled by direct solar radiation [11,12] and temperature increase; and (2) at the slopes below 5700 m, where the rapid retreat since the early 20th century can only be explained by a change in climate characteristics such as a reduction in specific humidity and an increase in net shortwave radiation accompanied by a decrease in cloudiness and precipitation [140].Some studies predicted the complete disappearance of all ice at Mt Kilimanjaro by 2020 [139][140][141][142][143].
In the Ruwenzori Mountains, glaciers have been retreating since at least 1957 [144,145].The glacier terminus of the largest glacier, Speke Glacier, receded by 35-45 m between 1958 and 1977 and A similar trend of glacial retreat was also observed at Mt Kilimanjaro, where the glacial area decreased from 6.7 km 2 to 3.3 km 2 between 1953 and 1989 [138], to 2.6 km 2 in 2000 [139], and to 1.7 km 2 in 2016 based on our analysis.Two different glacier systems can be identified at the mountain: (1) on the plateau above 5700 m, where the retreat of the vertical glacier wall is predominantly controlled by direct solar radiation [11,12] and temperature increase; and (2) at the slopes below 5700 m, where the rapid retreat since the early 20th century can only be explained by a change in climate characteristics such as a reduction in specific humidity and an increase in net shortwave radiation accompanied by a decrease in cloudiness and precipitation [140].Some studies predicted the complete disappearance of all ice at Mt Kilimanjaro by 2020 [139][140][141][142][143].
In the Ruwenzori Mountains, glaciers have been retreating since at least 1957 [144,145].The glacier terminus of the largest glacier, Speke Glacier, receded by 35-45 m between 1958 and 1977 and by more than 150 m from 1977 to 1990 [146].The total glaciated area in the Ruwenzori Mountains declined from around 2 km 2 in 1987 to around 1 km 2 in 2003, and glaciers are predicted to disappear by 2023 [147].Mölg et al. (2003a) [11] suggested that the observed differences in glacier recession in the Ruwenzori Mountains are associated with variations in solar incidence; at the same time, Taylor et al. (2006) [147] explained the spatially uniform glacier loss at lower elevations with increased air temperature.

Australasia
Allison and Peterson (1989) [18] estimated that the total glacial area in Papua in the early 1970s was 7.5 km 2 .Veettil and Wang (2018) [22] found that glacier area decreased from 3.85 km 2 in 1988 to 0.58 km 2 in 2015, which is a decrease in glacial coverage of 85% (Figure 4).Klein and Kincaid (2006) [20] assumed that the drastic retreat of Papuan glaciers is a result of the rise in air temperature, which in the tropics is twice the global mean.A few studies hypothesized that the Puncak Jaya glaciers could disappear before 2050 [20][21][22].

Australasia
Allison and Peterson (1989) [18] estimated that the total glacial area in Papua in the early 1970s was 7.5 km 2 .Veettil and Wang (2018) [22] found that glacier area decreased from 3.85 km 2 in 1988 to 0.58 km 2 in 2015, which is a decrease in glacial coverage of 85% (Figure 4).Klein and Kincaid (2006) [20] assumed that the drastic retreat of Papuan glaciers is a result of the rise in air temperature, which in the tropics is twice the global mean.A few studies hypothesized that the Puncak Jaya glaciers

Tropical Glaciers: Relevance and Challenges
More than one-sixth of the global population relies on glaciers and seasonal snowpack for its water supply [148].Within the tropics, the importance of meltwater from glaciers varies regionally; in the tropical Andes, more people depend on it for domestic usage than in East Africa and

Tropical Glaciers: Relevance and Challenges
More than one-sixth of the global population relies on glaciers and seasonal snowpack for its water supply [148].Within the tropics, the importance of meltwater from glaciers varies regionally; in the tropical Andes, more people depend on it for domestic usage than in East Africa and Australasia, where glacial areas are small and glaciers count for less than 1% of all tropical glaciers [149].In addition, regional precipitation and melt patterns dictate the availability of water to communities.Glacier recession in tropical mountains affects erosion rates, sediment and nutrient fluxes, and the biogeochemistry of water bodies, which in turn affects water quality and ecosystems [150].Consequently, we explore the regional importance of glaciers in tropical mountain societies, as well as challenges associated with the ice loss.

Tropical Andes
Many ecosystems and communities across the tropical Andes depend on glacier meltwater.The recent trend of a widespread glacier recession and changing water availabilities represents a challenge for these coupled natural and human systems.

Water Resources
In the tropical Andes, precipitation regimes vary regionally and strongly determine the importance of glaciers and their meltwater.While two wet seasons occur annually in the inner tropics (e.g., Ecuador), the outer tropics (e.g., Bolivia and Peru) have distinct dry and wet periods [151], where catchments depend on glacial meltwater during the dry season, and any delayed or reduced precipitation-and hence melting-may have serious consequences for water availability [151,152].At the same time, during the initial stages of enhanced glacial loss, meltwater discharge initially increases and may result in annual flooding events, before the glacial area eventually is too small to produce any significant meltwater volumes.Mark and McKenzie (2007) [152] used stable isotopes and showed that the glacial discharge in the Rio Santa catchment increased by 1.6% from 2004 to 2006 as a consequence of rapid shrinkage of glaciers in the Cordillera Blanca.Bury et al. (2013) [153] concluded that many catchments in the tropical Andes already passed their peak annual discharge and are now experiencing decreased volumes, which will impact future water availability security.In contrast, the Nevado Coropuna Ice Cap, presently the most extensive ice mass in the tropics, is shrinking considerably slower than previously studies argued, and it has been predicted that it will contribute to water supply until around 2120, which differs from previously predictions by almost 100 years [127].
Numerous studies outline the importance of glaciers as a freshwater resource, and particularly for replenishing aquifers and wetlands in Colombia [154], Ecuador [155], Peru [152,153,[156][157][158][159], Bolivia [28,160,161], and the tropical Andes in general [41].Nearly 35% of the runoff in the arid regions of Peru and Chile is supplied by meltwater from glaciers and snow [162].As a result of limited rainfall at high elevations, almost all Andean cities above 2500 m depend on glacier meltwater as a freshwater resource to some degree [163].A study about water resources in the capital La Paz in Bolivia showed that between 1963 and 2006, the annual contribution of glaciers to local freshwater resources was 15%, with 14% during the wet season and 27% during the dry season, highlighting the importance of glacier meltwater, particularly during the drier times [161].A similar observation from the Cordillera Blanca revealed that the disappearance of glaciers may reduce the annual average discharge by between 2% and 30%, depending on the catchment; in seven of nine studied catchments within the Cordillera Blanca, the dry season discharge showed a trend of decrease in recent years [157].

Natural Hazards
Glacier shrinkage since the late 20th century has resulted in the formation and expansion of numerous glacial lakes throughout the tropical Andes [6,124,164,165], which might pose a potential threat in the form of glacial lake outburst floods (GLOFs)-a flooding of the lake waters down-valley after the breaching of the debris/ice dam [165].In the tropical Andes, GLOF potential has been identified for several locations in the cordilleras Blanca and Carabaya of Peru [124,164,165], and in the cordilleras Apolobamba, Real, and Tres Cruces of Bolivia [6].
The threat from landsliding in recently deglaciated areas is widespread throughout the tropical Andes [166,167].An extreme rainfall in 1998 that followed a prolonged glacier retreat at Nevado Salcantay in the Eastern Cordillera of Peru triggered a landslide of mainly glacial sediment [168].The landslide reached the Vilcanota River, where it created a 70 m high debris dam and caused 100 million dollars in damage at a hydroelectric power plant.Vilímek et al. (2015) [169] discussed the impacts of a GLOF resulting from a mass movement of about 500,000 m 3 of ice and rock at Mt Hualcan in the Chucchun Valley of the Cordillera Blanca in Peru in 2010 (Figure 5).Despite no fatalities being reported, some houses, roads, bridges, and a water treatment plant were destroyed, affecting the local population for some years.Similar conclusions were presented for a GLOF that had occurred in the Chucchun Valley in 2010 [170].The threat from landsliding in recently deglaciated areas is widespread throughout the tropical Andes [166,167].An extreme rainfall in 1998 that followed a prolonged glacier retreat at Nevado Salcantay in the Eastern Cordillera of Peru triggered a landslide of mainly glacial sediment [168].The landslide reached the Vilcanota River, where it created a 70 m high debris dam and caused 100 million dollars in damage at a hydroelectric power plant.Vilímek et al. (2015) [169] discussed the impacts of a GLOF resulting from a mass movement of about 500,000 m 3 of ice and rock at Mt Hualcan in the Chucchun Valley of the Cordillera Blanca in Peru in 2010 (Figure 5).Despite no fatalities being reported, some houses, roads, bridges, and a water treatment plant were destroyed, affecting the local population for some years.Similar conclusions were presented for a GLOF that had occurred in the Chucchun Valley in 2010 [170].

Ecosystems
Many ecosystems in the tropical Andes are influenced by the presence of glaciers.The wetlands or páramos found between the Cordillera Merida in Venezuela and the Huancabamba Depression in northern Peru represent a complex and fragile ecosystem that is fed also by glacier meltwater and provides water to other ecosystems, as well as to human societies along the Pacific coast [151].Many rivers here are either glacier-or páramos-fed [171].Furthermore, the wider páramos-ecosystem consists of a collection of smaller neotropical alpine grassland ecosystems that all play a key role in the hydrology of the northern Andes and the Amazon Basin.The water yield of a smaller páramos basin (<3 km 2 ) can reach 67% of the total rainfall [172].However, the páramos are becoming increasingly vulnerable to a warming climate and loss of glacier area [172][173][174].The effect of glacier shrinkage on páramos is still unclear, but a decrease in glacier runoff might accelerate the transition from a wet páramos to a dry páramos [172,175].The biodiversity of the dry páramos in Ecuador is threatened by a changing hydrology [176].When assessing the consequences of a decreased glacier meltwater contribution for freshwater resources, agriculture and hydropower production in the Cordillera Blanca of Peru between 1987 and 1995, a change in wetland status was observed: (1) glacier shrinkage Many rivers here are either glacier-or páramos-fed [171].Furthermore, the wider páramos-ecosystem consists of a collection of smaller neotropical alpine grassland ecosystems that all play a key role in the hydrology of the northern Andes and the Amazon Basin.The water yield of a smaller páramos basin (<3 km 2 ) can reach 67% of the total rainfall [172].However, the páramos are becoming increasingly vulnerable to a warming climate and loss of glacier area [172][173][174].The effect of glacier shrinkage on páramos is still unclear, but a decrease in glacier runoff might accelerate the transition from a wet páramos to a dry páramos [172,175].The biodiversity of the dry páramos in Ecuador is threatened by a changing hydrology [176].When assessing the consequences of a decreased glacier meltwater contribution for freshwater resources, agriculture and hydropower production in the Cordillera Blanca of Peru between 1987 and 1995, a change in wetland status was observed: (1) glacier shrinkage was associated with increased wetland area; and (2) an increase in mean annual stream discharge in the previous twelve months increased the wetland area [177].High elevation wetlands in the Bolivian Cordillera Real revealed a similar transition: perennial and complex wetlands experienced strong drying processes, whereas a number of smaller temporary wetlands were formed owing to the rapid melting of glaciers [178].
The disappearance of the alpine glaciers is predicted to cause migration of plant species (e.g., Pleurodema, Telmatobius, Liolaemus, Diuca speculifera) to higher elevations, and this colonizing of new high elevation habitats would introduce competition for and displacement of existing species [171].The upward migration of 54 vascular plants and 39 lichen and bryophytes species colonizing rocks in recently deglaciated areas above 5400 m was observed in the Cordillera Vilcanota in Peru [179].However, the dynamics of primary succession is slow compared with the decline in glacier coverage resulting from accelerated warming [180].As a consequence of glacial recession in the Cordillera Blanca of Peru, the entire treeline migrated to higher elevations [181].In many tropical mountains, the glacial recession also resulted in land use changes [182].More research on biodiversity and ecosystem consequences of successional changes is needed, particularly on direct effects of glacier retreat in higher elevations, and of indirect effects from socio-ecological drivers at lower elevations [182].
Animals may also relocate to higher elevations; for example, some amphibians such as anurans (frogs and toads) have been found in recently deglaciated terrain in Peru [179].Some of these new anurans species have been associated with the fungal pathogen Batrachochytrium dendrobatidis, which has previously been related to the disappearance of various amphibians throughout South America [179].As such, the invasive species can possess a threat from carried pathogens that might infect the native species.Tropical glaciers are nesting grounds for some bird species (e.g., Idiospar speculifera), and the ongoing loss of tropical glaciers may have a negative effect on biodiversity with regard to the loss of habitat [183].
Increased meltwater volumes during the initial stage of a glacier recession can impact aquatic ecosystems.A decrease in diversity of the benthic fauna has been observed in several equatorial high-elevation Andean streams [184].In some lakes of Ecuador, algal blooms and eutrophication from toxin-producing microorganisms have been noticed [185], and populations of the planktonic thalassiosiroid diatom Discostella stelligera have increased [186].Such changes can influence the nutrient cycle and food web structure in these aquatic ecosystems [186].

Human and Animal Health
Natural hazards, such as avalanches, GLOFs, and landsliding, have caused fatalities and injuries throughout the tropical Andes.For example, two avalanches in 1962 and 1970 at Nevado Huascaran (Cordillera Blanca, Peru) caused the deaths of more than 25,000 people [24].Glacier recession has also been related to the spreading of vector-borne diseases such as Malaria [187,188] and Zika [189], which was explained with the formation of small meltwater ponds that offer ideal conditions for breading insects.Furthermore, it can be argued that the particulate materials found in glacier ice, now released into rivers and streams, can be potentially harmful to human health, if they are not filtered out.

Agriculture and Mining
The glacier disappearance throughout the tropical Andes affects hydrological regimes and water availability for agricultural and industrial purposes.The decline in agricultural productivity in the Cotacachi region in Ecuador has been related to glacier recession [190], and in the Cordillera Blanca in Peru, a reduction in agricultural production could be disastrous for food security and economy [162].In Peru, the majority of the human population-and economic activity-is concentrated on the western dry side of the Andes, where water resources are highly dependent on glacier meltwater [162].Here, with increasing demand for food for a growing population, agricultural land is expanding.In the Department of Ancash, which includes the Parc Nacional Huascaran, agricultural land area has increased from 46,000 hectares in 1995 to 353,000 hectares in 2000 [191].Mark et al. (2010) [192] discussed the influence of climate change and glacier shrinkage on agriculture and livestock along the length of the Rio Santa and called for adaptation strategies in the future; their surveys revealed that a large majority of households were aware of the rapid glacier recession and climate change impacts on freshwater resources and agro-pastoral production.
Mining activities throughout the tropical Andes require increasing water supplies, which includes meltwater from glaciers [193,194].Additionally, these mining activities produce air pollutants such as black carbon that result in a darkening of glaciers, enhancing the general glacier shrinkage, but increasing meltwater runoff [8,132].The contamination from mining activities resulted in health risks for ecosystems and communities.

Hydropower
Most of the countries in the tropical Andes depend on meltwater from ice and snow to meet the demand for electricity production; around 50% in Ecuador, and almost 80% in Peru [162].In Ecuador, hydropower generation largely depends on the water originating in the páramos, which depend themselves on glacier meltwater influx [195].In the fast growing La Paz-El Alto region in Bolivia, 75% of the electric power is generated by eight power plants in the Zongo Valley [196], fed by the Zongo and Chacaltaya glaciers, which showed high negative mass balances for many years [197]; by 2009, Chacaltaya Glacier, one of the highest glaciers in the Andes, had totally disappeared.
It is very likely that the continuing decrease in glacier coverage will result in significant changes in the economy of affected countries [148], particularly if precipitation alone cannot provide sufficient stream flow.Kronenberg et al. (2016) [5] investigated the glacier-climate interactions in the Cordillera Vilcanota in Peru and warned about serious consequences of a future reduction in precipitation, glacier shrinkage, and unstable runoff for the hydropower sector.Nevertheless, an increase of around 300% in planned hydroelectric dam projects throughout the region was noticed [151]-a surprise in the light of the fact that many of the new reservoirs would also depend on glacier meltwater.

Tourism
Impacts of glacier recession on the tourism sector have often been underestimated in discussions about climate change and glacier loss throughout the tropics [198], despite that many recreational activities are associated with the snow-covered mountains.The cordilleras of Peru have received increasing attention from mountain climbers from all over the world for the last 30 years, and the associated infrastructure in the form of hotels, resorts, and transportation boomed during this period [165,199].Every year, more than 100,000 tourists visit Huascarán National Park in the Cordillera Blanca and the city of Huaraz [200].At Chacaltaya Glacier in Bolivia, the first ski lift in South America and also the highest in the world was built in 1939 [165,201].However, after 70 years, the glacier had disappeared by 2009 and the resort had to close.It was suggested that the Bolivian authorities could maintain this location as a 'national memorial site' or as an iconic place [201].The disappearance of the Cotacachi Glacier in Ecuador lowered water levels in the crater lake Cuicocha, which affected boat tour tourism, a major attraction [190].Last, but not least, potential GLOF risk at some glacial lakes that formed as a consequence of the recent recession may influence tourists in their decision to visit these areas.
On the other hand, the tourism industry itself affects glacial ecosystems and local communities through human disturbances such as littering and pollution [202].For Bolivia, it was suggested that future tourism activities should focus on cultural destinations and colorful festivals rather than tour packages to disappearing glaciers [203].On the other hand, formerly glaciated landscapes are still beautiful and attractive to tourists [201] and would allow for an educational tourism.

Traditions and Spirituality
Numerous mountain glaciers throughout the tropical Andes are associated with religious beliefs and festivals.In the Sinacara Valley of the Peruvian Andes, at the bottom of the Qulqipunku Glacier at 4500 m, the annual 'snow festival' (known as Qoyllur Rit'i) combines Catholic, Incan, and other indigenous beliefs.By 2016, the glacier had receded so far that a relocation of the festival to even higher elevations was considered; however, the authorities decided for an event date later in the year, when snowfall would reach the elevation of the traditional location [204].The highest known ceremonial site in the world is on the summit of Mt Llullaillaco in northern Argentina, where three well-preserved frozen mummies of Inca children were discovered at 6700 m [205].In the ancient Inca culture, the disappearance of snow-covered peaks signified the end of the world.Eventually, the ongoing glacier recession throughout the tropical Andes may result in the loss of traditions at the local and regional scale affecting cultural identities.

Conflicts and Migrations
Glacier disappearances and recessions resulting in reduced water resources have previously been related to conflicts [206,207] and migrations of human populations [198,208].Conflicts may arise at different levels and scales, for example, between countries, among communities, or between a community and a hydroelectric company, particularly when a dependency on glacier meltwater in a region with highly seasonal precipitation exists [209].
Once the water resources become too vulnerable, local communities previously dependent on snowmelt will be forced to migrate under a changing climate, seeking more available water resources, agricultural land, and lower exposure to vector-borne diseases that may arise with a warming climate [142,210,211].It is believed that the glacier melting in the tropical Andes will have a larger negative impact on small-scale societies, such as the indigenous people (e.g., Quechua people in the Peruvian Andes), who may be forced to migrate [211,212].

East Africa
Despite being on a much smaller scale than in the tropical Andes, the glaciers of East Africa also contribute to stream flow, and hence are an important water resource.Therefore, glacier recession threatens ecosystems and human activities.

Water Resources
A constant decrease in water availability has been observed in villages located in the foothills of glaciated mountains in East Africa [213].Around Mt Kilimanjaro, many canals in the foothills have dried up and water levels of streams are generally lower, causing conflicts over access to water [214].However, regionally, the negative impact of reduced glacier meltwater volumes is considered to be less of a problem in the Rwenzori Mountains, because here, glaciers contribute less than 2% to the total discharge of the principal rivers during both the dry and wet season [149,215].

Ecosystems
Previously glaciated mountains in East Africa witnessed vegetation succession after the loss of ice coverage.The slopes of Tyndall Glacier on Mt Kenya, which were still glacier-covered in the 1970s, have been invaded by pioneer species such as Senecio keniophytum, Arabis alpine, and Agrostis trachyphylla since the late 2000s [216].The rate of upslope migration of the vegetation was 2.1 to 4.6 m/year between 1958 and 1997, similar to the glacier retreat rate of 2.9 m/year.Between 1997 and 2002, vegetation advanced at an accelerated rate of 6.4 to 12.2 m/year; over the same period, the glacier retreat rate had increased to 9.8 m/year [216].Zawierucha et al. (2018) [217] expressed concerns that, as a result of the glacier recession, the presence of a large number of rotifers on glaciers in Uganda and their extinction in the near future is inevitable.

Human Health
The ongoing glacial recession at the East African mountains led to the formation of meltwater ponds, which can be breeding grounds for disease-carrying insect species such as the mosquito, particularly when occurring alongside increasing air temperature.In 1997/98, a strong El Nino had been associated with the retreat of tropical glaciers worldwide; during this period, there was an increase in cases of cholera, malaria, and other diseases in the East African mountains [218].It is unclear, however, if the former caused the latter.

Agriculture
Agriculture is a major contributor to the economy of East African countries.A reduced snow cover and extensive melting of ice were identified as being one of the reasons for variations in runoff and soil moisture content, which would eventually affect crops [219].For example, coffee production around Mt Kilimanjaro suffered from low rainfall as well as reduced glacial runoff [220].

Tourism
The glaciated mountains of East Africa attract high numbers of tourists every year [221] and consequently, tourism contributes significantly to local economies.However, it has been predicted that the plateau ice at Mt Kilimanjaro, which is of exceptionally high tourist potential in Tanzania, would disappear in the mid-21st century, resulting in a considerably reduced touristic appeal of the mountain [12].In Uganda, the recession of the Rwenzori glaciers was expected to negatively impact the eco-tourism sector [222].

Australasia
It has been predicted that the Australasian glaciers will disappear in the 2020s under the current climate conditions [20,22].While their small extent contributes an insignificant amount of water to river discharge [149], negative impacts on local ecosystems, such as natural habitat loss or emergence of disease-causing agents, must be expected.
The aerial deposition of black carbon on glacier surfaces and their related darkening absorbs more solar radiation, which results in enhanced melting.This is a concern for the survival of the glaciers of Papua, particularly after the expansion of mining activities throughout the region [22].Other sources for dust particles on these glaciers might be wood burning in Kalimantan, Sumatra, and even northern Australia.

Future "Darkened Peaks"?
Many studies have documented the ongoing disappearance of tropical glaciers worldwide [223,224].While the retreat in in some regions of South America is still relatively slow [2], the glaciers in East Africa [15] and Australasia [20,22] are close to extinction.While a few authors assume regional factors behind this ice loss [225], many others affirmed global climate change, particularly temperature rise, as the main culprit [223].
Even though a reduction in glacier area was observed since the Little Ice Age (LIA) throughout the tropics, the rate of shrinkage generally increased in the late 1970s [2,3,8], particularly in lower elevations [1].In the process of this glacier recession, the additional darker land surface area that absorbs higher rates of radiation now accelerates further glacier loss-positive feedback [41].On the other side, ice-core records supported the view that temperatures on and near glacier surfaces increase more at higher elevations [223].As most of the tropical glaciers are located above 4800 m, the future of these snow-covered peaks could be their darkening.Without a doubt, the warming at higher rates in mountain regions in general is concerning [41,42,226], and even more so in the tropics where glaciers seem to be more sensitive to temperature variations.Even though a hiatus in global warming was proposed in some studies [227], it does not seem to have influenced tropical mountain glaciers at high altitudes [2] and these glaciers continued to shrink.Furthermore, smaller glaciers respond to a warming climate faster than larger sized ones [223], which is evident from the near disappearance of glaciers in Papua [20,22].

Adaptation Strategies
The negative impacts of the glacier loss on ecological and socio-economic systems require the development of adaptation strategies ranging from large-scale general management plans to small-scale specialized solutions.
Emmer and Vilímek (2013, 2014) [228,229] explained that before adaptation strategies related to the GLOF hazard mainly in the Andes of Peru and Bolivia could be developed and implemented, a proper assessment of the hazard is necessary.The authors recommended a two-phase GLOF assessment: (1) estimate the probability of water release from a given glacial lake based on a lake and breach hazard assessment; and (2) identify the areas at risk in a downstream hazard assessment.It has been demonstrated that implementations of adaptation strategies related to GLOFs must involve affected communities and risk hazard awareness.For example, after a rock-ice avalanche had initiated a GLOF at Lake 513 near Nevado Hualcan in the Cordillera Blanca of Peru, Carey et al. (2012b) [170] discussed an integrated socio-environmental framework for climate change adaptation in glacial lake environments and concluded that only the participation of stakeholders from multiple levels-international, national, and local-promises success.The authors [170] proposed "a holistic approach for integrating disaster disk reduction and climate change adaptation." In the light of decreasing glacier meltwater volumes, alternative water supply resources gain importance.However, these can be very expensive or impractical to implement, owing to an increased demand and high per-capita consumption [41].On the other hand, relatively inexpensive methods, such as rain harvesting, may not be able to meet the additional demands.The World Bank (2014b) [230] presented recommendations to tackle the impacts of climate change and glacier shrinkage on water resources in Bolivia, Ecuador, and Peru: (1) strengthen agricultural practices (including sustainable irrigation systems); (2) improve irrigation infrastructure; (3) reduce water losses for the water distribution system of large urban areas (e.g., La Paz and El Alto in Bolivia); (4) increase the resilience of key glaciered basins with ecosystem restoration and conservation at high-altitude, and increase irrigation efficiency at low altitude; (5) install glacier monitoring stations for the better understanding of glacier retreat; and (6) develop investment plans, integrated watershed management plans, and strategic development plans for changing climate conditions.However, expected demographic changes are likely to outpace the impact of climate change on water resources and need to be considered in policy making for the future [231].

Conclusions
Tropical mountain glaciers are shrinking globally.Currently, more than 99% of the tropical glaciers worldwide are found in the South American Andes, with the remaining ice being located in East Africa and Australasia.Global, regional, and local climates influence the stability of these tropical glaciers.Additionally, non-climate factors such as volcanic dust and industrial black carbon drive the glacial loss.
The importance of tropical glaciers may vary regionally or locally.Glacier meltwater contributes to water supply, agriculture, and hydropower generation throughout the tropical Andes, and some glaciers represent important cultural sites or are magnets in outdoor tourism.Their social-economic significance is particularly visible in the South American Andes.The ongoing glacier recession across the tropics has many direct and indirect effects on coupled human and natural systems in the form of ecosystem changes, draughts, and floods, as well as water shortages, hazards, and economic loss.In some regions such as Bolivia and Peru, the glacier shrinkage has already resulted in conflicts and migration.The increasing darkening of ice-covered tropical mountains only enhances recession, and thus the impact of resulting challenges.
While the increase in water availability resulting from initial glacier melting might enhance economic opportunities and productivity, it needs to be understood that once the glacier recession reaches a threshold, meltwater runoff will decrease dramatically.Hence, the challenges related to glacier loss call for rigorous mitigation and adaptation strategies that include stakeholders at multiple levels and consider short-, medium-, and long-term solutions.Therefore, glaciers in the tropical Andes must be systematically monitored, observed changes must be assessed, and these data must be considered in planning and management strategies.Such strategies must merge the conservation of tropical glacier water reservoirs ('Water Towers') with the preservation of ecosystems and traditions; for example, by implementing new or extending existing protected areas.Glaciers are not only moving ice masses; instead, individual glaciers or wider glaciated landscapes can be seen as geomorphosites-"geomorphological landforms that have acquired a scientific, cultural/historical, aesthetic and/or social/economic value due to human perception or exploitation" [232].

Figure 2 .
Figure 2. Glacier recession in the Cordillera Tres Cruces in the Bolivian Andes between 1975 and 2016.

Figure 2 .
Figure 2. Glacier recession in the Cordillera Tres Cruces in the Bolivian Andes between 1975 and 2016.

26 Figure 3 .
Figure 3. Glacier recession at Mt Kenya in East Africa between 1986 and 2015.

Figure 3 .
Figure 3. Glacier recession at Mt Kenya in East Africa between 1986 and 2015.

Figure 4 .
Figure 4. Glacier recession of glaciers in Papua in Australasia between 1988 and 2015.

Figure 4 .
Figure 4. Glacier recession of glaciers in Papua in Australasia between 1988 and 2015.

Figure 5 .
Figure 5.The Chucchun Valley in the Cordillera Blanca, Peru, before (left) and after (right) the 2010 GLOF at Lake 513.Movement of sediments and the direction of ice avalanches are highlighted.

Figure 5 .
Figure 5.The Chucchun Valley in the Cordillera Blanca, Peru, before (left) and after (right) the 2010 GLOF at Lake 513.Movement of sediments and the direction of ice avalanches are highlighted.6.1.3.Ecosystems Many ecosystems in the tropical Andes are influenced by the presence of glaciers.The wetlands or páramos found between the Cordillera Merida in Venezuela and the Huancabamba Depression in northern Peru represent a complex and fragile ecosystem that is fed also by glacier meltwater and provides water to other ecosystems, as well as to human societies along the Pacific coast [151].Many rivers here are either glacier-or páramos-fed[171].Furthermore, the wider páramos-ecosystem consists of a collection of smaller neotropical alpine grassland ecosystems that all play a key role in the hydrology of the northern Andes and the Amazon Basin.The water yield of a smaller páramos basin (<3 km 2 ) can reach 67% of the total rainfall[172].However, the páramos are becoming increasingly vulnerable to a warming climate and loss of glacier area[172][173][174].The effect of glacier shrinkage on páramos is still unclear, but a decrease in glacier runoff might accelerate the transition from a wet páramos to a dry páramos[172,175].The biodiversity of the dry páramos in Ecuador is threatened by a changing hydrology[176].When assessing the consequences of a decreased glacier meltwater

Table 1 .
Tropical glacier areas by region and country using data from the Randolph Glacier Inventory.
Figure 1.Distribution of tropical glaciers globally.

Table 1 .
Tropical glacier areas by region and country using data from the Randolph Glacier Inventory.