Climate change threats to environment in the tropical Andes: glaciers and water resources

Reg Environ Change DOI 10.1007/s10113-010-0177-6 ORIGINAL ARTICLE Climate change threats to environment in the tropical Andes: glaciers and water resources Pierre Chevallier Bernard Pouyaud Wilson Suarez Thomas Condom Accepted: 20 October 2010 Ó Springer-Verlag 2010 Abstract Almost all of the world s glaciers in the tropical latitudes are located in the Central Andes (Peru, Bolivia, Ecuador and Colombia). Due to their high altitude, to the high level of radiation and to the tropical climate dynamics, they all are particularly threatened by climate change, as a result of not only warming, but also of changing variability of precipitation. Many glaciers are of crucial importance for the livelihood of the local populations and even for three capitals, Lima (Peru), La Paz (Bolivia) and Quito (Ecuador), which depend on them for water and energy supplies. This paper shows that after a period of increased flow due to the glacier melt disequilibrium, the available water resource will decrease along with the rapid shrinking of the glaciers considered as water reservoirs. The case of the Cordillera Blanca (Peru) is analyzed more in detail with the mid-term P. Chevallier (&) B. Pouyaud T. Condom Joint Research Unit Hydrosciences, CNRS, IRD, Universities Montpellier 1 and 2, CC 57, 34095 Montpellier Cedex 5, France e-mail: pierre.chevallier@ird.fr P. Chevallier B. Pouyaud W. Suarez T. Condom Institut de Recherche pour le Développement, Montpellier, France W. Suarez Joint Research Unit G-Eau, Cemagref, Cirad, Engref-AgroParisTech, IRD, Montpellier SupAgro, Lima, Peru W. Suarez Servicio Nacional de Meteorología y Hydrología del Perú (SENAMHI), Lima, Peru T. Condom EGID Institute, University Michel de Montaigne, Bordeaux 3, France (20 years) and long-term (1 2 centuries) impact of the glacier shrinking on the local water resources. Associated risks for the population and consequences for the human activities (tourism, hydropower, agriculture and stockbreeding, large-scale irrigation) are described at each stage of the mountain range. Keywords Water resources Water uses High mountains Tropical glaciers Central Andes Introduction The Central Andes mountains contain more than 99% of the world glaciers located in the tropical latitudes with a total area of 2,560 km 2, distributed between Peru (70%), Bolivia (22%), Ecuador and Colombia (4% each), and finally Venezuela (approx 0.1%) (Dyurgerov and Meier 2005). Their altitude ranges between 4,000 and 6,500 m above sea level (Fig. 1). The remaining tropical glaciers are located in Africa and New Guinea, with a total area of less than 10 km 2. These glaciers play an important role not only as water resource providers, with the associated economic activities, but also in the customs and cultures of human societies that live near or are dependent upon them. Almost everywhere on Earth, climate change is generating an increase in temperature: between 0.3 and 0.5 C per decade between 1901 and 2005 (Trenberth et al. 2007). As a consequence, the observed glacier retreat is significant for all the regions of the Central Andes (Francou and Vincent 2007). This is leading to the disappearance of some of the smallest and lowest glaciers, such as the iconic Chacaltaya Glacier, which was dominating the city of La Paz, Bolivia, and which was, until 10 years ago, the site of the highest ski lift in the world.

P. Chevallier et al. Fig. 1 Tropical glaciers in the Central Andes (Ecuador, Peru and Bolivia): relief (left) and precipitation maps (right). Sources: ESRI (1992); Legates and Willmott (1990) Since the beginning of the 1990s, numerous studies have been undertaken in this regions, many of them initiated by the French Institut de Recherche pour le Développement (IRD), in partnership with regional research groups and monitoring agencies in Bolivia, Peru and Ecuador, on the one hand, and with scientists from Europe and North America, on the other hand (Coudrain et al. 2005). Tropical glaciers, water resources and climate change Among these research efforts, three recent PhD theses have analyzed the data, the processes and the consequences of climate change for tropical glaciers and related water resources flowing from the high mountains in Peru (Suarez 2007; Suarez et al. 2008), Ecuador (Villacis 2008) and Bolivia (Soruco 2008; Soruco et al. 2009). Figure 1 shows the location of the glaciers in these three countries. All are positioned on the boundary between the dry side of the Pacific Ocean and the humid side of the Amazonian basin under the influence of the Atlantic Ocean. A common point of the three countries is that their capitals La Paz, Lima and Quito are close to the glaciers and depend on mountain water flows both for their water supply and for a large part of their energy, supplied by hydropower plants. While this paper will examine in detail the case of Peru, it is interesting here to first highlight concisely the main results and conclusions obtained in Bolivia and Ecuador from this recent work. Study cases in Bolivia and Ecuador In Bolivia, Soruco (2008) used a photogrammetric approach to study the evolution of 21 glaciers located in the Cordillera Real, from which water flows are supplying the capital, La Paz. After a period of stability between 1956 and 1975, the glaciers were observed to retreat rapidly, in particular between 1975 and 1983, and again between 1997 and 2006. Although the trends are similar for all glaciers, confirming a regional climate origin to their retreat, their mass balance varies strongly from one to the other, due to the strong influence of exposure and altitude. Those with the least retreat are the highest and oriented to the south or the east. Soruco (2008) also showed that the loss of mass of the glaciers is mainly due to a decrease in ice depth. As a consequence, the shrinkage of the glacier area and the retreat of the snout of the glaciers were found not to be the best indicators for rapid melting. Finally, it was estimated that in the four basins used for the water supply of La Paz,

Climate change threats to environment in the tropical Andes the loss of glacier volume was above 50% between 1975 and 2006 (Soruco 2008). In the dry season, 27% of the discharge is already used by the water distribution company and the demand is continuously increasing. In Ecuador, Villacis (2008) focused his work on a local scale, studying and modeling the glacial discharge from basins located on the Antizana volcano. He showed that the observed discharge is strongly linked with temperature, and also that the glacier melting process depends directly on the glacier s albedo. As a consequence, the impact of both extremes of the ENSO phenomenon is high in this region, with low temperature, high precipitation, high wind speed, high albedo and low discharge during La Niña events, and, by contrast, high temperature, low precipitation, low wind speed, low albedo and high discharge during El Niño events (Favier 2004; Favier et al. 2004). The modeling approach confirms the conclusion obtained by the study in Peru and will be detailed further in this paper: for the glaciated basins of the Antizana, volcano warming will generate a rapid increase in water discharge, until a peak, which could appear in 60 140 years, following the IPCC SRES A2 and B1 emission scenarios (Nakicenovic and Swart 2000), respectively. How do tropical glaciers work and how they respond to global warming? Compared to glaciers in temperate or polar regions, tropical glaciers have two special features (Kaser and Ostmaston 2002): (1) they are subject to considerably higher levels of energy forcing since they are located in low tropical latitudes, but higher altitudes; (2) the period of maximum precipitation coincides with the summer period, accumulating snow in the highest sector, when the ice is melting rapidly in the lowest sector under relatively high temperature. This combination of processes reveals that tropical glaciers are particularly sensitive to climate variations, providing an excellent indicator for these. Indeed, the mass balance of the glaciers, the sum of the ablation and accumulation processes, is driven by climate. Figure 2 shows an example of a tropical glaciated hydrological basin. Part of the catchment area is not covered by ice. On the glacier itself, there are two areas: (1) the highest, or accumulation area where the snow accumulates before being turned under pressure into firn, then into ice, the major source of glacier build-up; (2) the lowest, or ablation area, which is the major source of melt waters at the glacier snout. These two zones are separated by the equilibrium line where the net balance is equilibrated, accumulation offsetting ablation. The equilibrium line altitude (ELA) varies according to the season and the year. If precipitation remains equal, a Fig. 2 Working scheme of a tropical glacial hydrological basin (Pouyaud 2005). This example corresponds to the Zongo Glacier (Cordillera Real, Bolivia) decrease or increase in temperature leads to an advance or retreat of the glacier snout and equilibrium line as a result of a variation in the accumulation/ablation ratio. Consequently, the limits of glaciated and non-glaciated areas gradually change in response to climate variations. General empirical relations between the annual precipitation and the temperature on the one hand, and the annual ELA on the other hand can be found in Greene et al. (2002), Condom et al. (2007) and Suarez et al. (2008). To interpret the relation between climate variations and glacier response, changes in the ELA are not sufficient. Other characteristics should also be studied such as the area and volume of ice. The use of a degree-day model helps to understand the sensitivity of glacier mass balance to climate change (Braithwaite and Zhang 1999). Climate variations lead to a significant change in the volume and pattern of runoff. Warming, without change of precipitation, results in: (1) the glacier snout retreating and the equilibrium line moving up; (2) the accumulation area shrinking and the ablation area increasing; and (3) decreasing of the total surface area of the glacier, while the proportion of non-glaciated area increases relative to the glaciated area in the whole catchment. What are the consequences for river discharge? The scenario described earlier changes the pattern of water volume in the rivers fed by the glacier, in accordance with changes in the percentage of glaciated area within the basin. Considering the annual water balance, the resultant water flow is a combination of two simultaneous processes:

P. Chevallier et al. (1) an acceleration of glacier melt, which will increase the average volume of glacier outflows by accelerating the water imbalance: the volume discharged will be higher than the volume precipitated as snowfall or rainfall; (2) the water reservoir represented by the glacier will gradually empty and the non-glaciated area increase, while the contribution from melt water will decrease and will not be offset by precipitation: discharges from the whole basin will decrease (Barnett et al. 2005). Initially, the first mechanism will compensate for the second one leading to an increase in water flow, but finally the second one will dominate, and hence, after a peak of glacial melt water discharge, there will be a decline in water flow. There are two main questions that then arise for all water users: when will the peak discharge occur, and what is the minimum discharge reached after the extinction of the glacier resource and when it will occur? The case of the Cordillera Blanca (Peru) Located in a tropical zone (between the Equator and 18 S) (Fig. 1), Peru, with 1,285,000 km 2, is the third largest country in South America, after Brazil and Argentina. There are basically three main geographic regions: the Pacific Coast (costa), the Cordillera of the Andes (sierra), which runs the length of the country from north to south, and the Amazonian Forest (selva) to the east. Peru s climate is strongly defined by the mountain barrier of the Andes, as shown by the precipitation gradients visible in Fig. 1. To the east, the Atlantic air masses are responsible for the warm and humid climate of the Amazonian Forest. To the west, along the whole coastal strip, the proximity of the South Pacific anti-cyclone and its accompanying subsidence, reinforced by the cold Humboldt current, which flows parallel to the Pacific coast, generates a climate, which becomes increasingly dry the further south one goes (Aceituno 1988). The average precipitation in Lima is only some 25 mm yr -1. In the mountains, the annual oscillation of the intertropical convergence zone gives rise to alternate dry and wet seasons, the latter occurring over the southern hemisphere summer (December April), which vary depending on the altitude and the orientation of the mountain slopes. Broadly speaking, the length of the rainy season diminishes from north to south and the precipitation amount from east to west. Given this climate, water resources are very unevenly spread. Roughly half of the country s 27.1 million inhabitants (INED 2004) live on the coastal strip, one-third in the mountainous regions of the Cordillera of the Andes and one-sixth in the vast Amazonian plain (Consejo Nacional del Ambiente 2001). This distribution of the population results in extreme differences in levels of access to water resources across the country. In the barren coastal region, nearly all the available water comes today from rivers flowing down the western slopes of the Cordillera of the Andes and, therefore, it is partly of glacial origin in the dry season. Nevertheless, it is in this region that most of the large cities are located including the capital Lima, with more than 8 million inhabitants in total and where the great majority of economic activity is concentrated (Vergara et al. 2007). During the rainy season, the rivers flowing into the Pacific Ocean are fed by rainfall in the mountains, glacier melt and a contribution from groundwater. During the dry season, the rivers are fed by melt water from the glaciers, which cover the mountain tops higher than 5,000 m, and by groundwater. At catchment scale, groundwater plays a significant role in the river s flow regime and roughly half of the dry season discharge is provided by aquifers (Kaser et al. 2003; Mark et al. 2005; Barear et al. 2008). Peru contains some 70% of the planet s tropical glaciers. Records kept for the last 50 years or so show that these glaciers are in general retreat. This trend has been accelerating since the mid 1970s (Francou and Vincent 2007). For the 18 main Peruvian ranges of the Cordillera of the Andes with glaciers, it is estimated that between the 1960s and the end of the 1990s, there has been a loss of more than 20% in surface area and volume (Leavell and Portocarrero 2003). Since the end of the 1990s, in partnership with the National Meteorology and Hydrology Service (SENAMHI) and the National Natural Resources Institute (INRENA) of Peru (in particular its Glaciology and Water Resources Unit (UGRH) in Huaraz), IRD has been carrying out a research program aimed precisely at evaluating the impact of climate change on water resources, and the resulting consequences for human activity (Pouyaud 2003). In its first phase, the study focused on the Rio Santa valley, which drains the Cordillera Blanca, the highest and most ice-bound cordillera in the country (Fig. 3). The Rio Santa finds its source at the Laguna Conococha in the southern part of the watershed and is fed partly by the glaciers of the Cordillera Blanca. Studies have been made of the variations in surface areas of glaciers using remote sensing images (photographic and satellite). Glacial area and the changes in this were calculated for the Cordillera Blanca by different authors over a range of time periods: Total area reduced from 800-850 km 2 in 1930, to 660-680 km 2 in 1970, and 620 km 2 in 1990 (Georges 2004); area reduced from 643 ± 63 km 2 to 600 ± 61 km 2 between 1987 and 1996 (Silverio and Jaquet 2005); and in another estimate, from 728 to 536 km 2 between 1960 and 2003 (Hindrandina

Climate change threats to environment in the tropical Andes an increase in temperature at regional scale but not to a decrease in precipitation. Mid-term (20 years) forecast for the glacier water resources in the Cordillera Blanca Fig. 3 Location of the Rio Santa Basin in Peru and the glaciers, which are supplying it (Suarez 2003) 1989; Zapata et al. 2008). The most recent study concerning the glacier area changes was done by Racoviteanu et al. (2008), giving a loss in glaciated area of 22.4% from 1970 to 2003. One of their conclusions is that the shrinkage, which occurred during the last 30 years, was related to The Llanganuco River has been chosen as representative of the headwater catchments of the Cordillera Blanca. It has the advantage of a relatively long time series of observed discharge from 1953 onwards, a significant percentage of glaciated area (39%) and no change in water flow processes due to the construction of dams or other hydraulic works. This is not the case for the Parón River (Suarez 2007; Suarez et al. 2008), which was modified several times since the 1970s for civil protection against lake outbursts and for water control in order to produce hydroelectricity (see below). An excellent correlation has been found between the runoff from highly glaciated drainage basins (above 40% glaciated area) and the air temperatures obtained from the NCEP-NCAR Reanalysis data for the last 50 years (Pouyaud 2005) (Fig. 4). Based on this empirical relationship, forecasts or projections over mid and long terms were carried out. Considering only the basins with a relatively high proportion of glaciated area ([40%), where glaciers begin to flow at a very high altitude (above 5,500 m), the expected shrinking of glaciated area during the next 20 years would not be sufficient to significantly modify the melting process and consequently the glacial runoff (Juen et al. 2007). The correlation between temperatures (from the reanalysis data) and the observed discharges can be applied to the air temperature projections from GCM simulations under different emissions scenarios. Figure 4 presents the results obtained for the discharges on the Llanganuco River in the Cordillera Blanca, under the IPCC SRES B2 scenario (Nakicenovic and Swart 2000) with the RAMS model Fig. 4 Middle-term forecast of the monthly discharges of the Llanganuco River for the next 20 years (scenario B2). Three time intervals are presented: (1) the observed period 1953 1997 (thin continuous line); (2) the calibration period 1998 2004 (thin dashed line) and (3) the forecast period 2005 2020 (thin dotted line). The bold continuous line represents a 13 months- moving average, giving a smoothed trend of the monthly discharge for the whole time period

P. Chevallier et al. (Regional Atmospheric Modeling System, see: http://rams. atmos.colostate.edu/ and http://www.atmet.com/html/rams_ research.shtml used by SENAMHI). The time series of air temperature of the period (1953 2004) is extracted from the NCEP-NCAR reanalysis data base at an elevation of 500 hpa. The period 1998 2004 was used to calibrate the forecast temperatures given by the GCM and the observed temperatures of reanalysis for the next 20 years, when the same correlation is applied in order to obtain simulated discharges at Llanganuco. It needs to be noted that this correlation will not be applicable after some time, when the retreat of the glaciers will significantly modify the relationship between water discharge and air temperature. Thus, this empirically based projection needs to be complemented by taking into account the continuous decrease in glacier area. Long-term (1 2 centuries) forecast for glacier water resources in the Cordillera Blanca In the tropical Andes, air temperature is projected to increase by about 0.1 C/decade; however, precipitation projections are much more uncertain and differences exist between the inner and the outer tropics (Vuille et al. 2008). Furthermore, climatic change at high elevation sites during the last century is characterized by a high degree of complexity (e.g., orographic effects) and uncertainty (e.g., lack of observations) (Beniston et al. 1997). Under a climate change scenario with continuous warming, but with constant snow and rain precipitation rates at present levels, hydrological processes in the non-glaciated area will not be modified (Fig. 2). By contrast, the hydrological processes of the glaciated areas will be strongly modified by both the increase in the ice melting and the ongoing decrease in the glacier area accompanying glacier retreat (see the above section on tropical glacier functioning). This observation leads to another modeling approach, combining both effects, which is detailed by Great Ice IRD and partners (2007). Examples of results under the IPCC SRES B2 scenario are presented in Figs. 5 and 6 (Pouyaud 2005). This example at annual time steps demonstrates that for all the basins, a slight increase in the water resource (flow) is still expected for the next 20 50 years, under current glaciated conditions. However, if climate change continues, or even accelerates, there will be a dramatic impoverishment of this resource. The main water flow regime within the basin will change from glacier dominated to rain and snow dominated, beginning with the smallest and lowest elevation glaciers. What are the associated risks for the population? Looking at a map or satellite picture, one sees that the cordilleras of Peru are dotted with innumerable glacial Fig. 5 Long-term projections of the specific annual discharges (in m) for the Llanganuco basin (39% glaciated area in 2000) under three scenarios of warming during the twenty-first century: 1 C(continuous line), 2 C (large dotted line) and 4 C (small dotted line). The permanent flow obtained when the glacier has disappeared corresponds to that due to rainfall, showing that the amount of precipitation is slightly increasing with temperature Fig. 6 Long-term projections of the annual discharges (in m) for basins with different glacier covered areas under a warming scenario of 2 C during the twenty-first century (SRES B2 scenario). Llanganuco basin (39% glaciated in 2000, thin continuous line), Paron basin (48% glaciated, thin dashed line), Yanamarey basin (73% glaciated, bold continuous line) and Artesoncocha basin (79% glaciated, bold dashed line). The differences in the permanent flow after the disappearance of the glaciers are linked to the average altitude of the basin. In this scenario, the average precipitation and hence the permanent flow is higher at higher elevations lakes. Some of these were formed recently (Ames 1998). These lakes constitute a major hazard for people living in the valley. Since these glacial lakes could burst their banks at any time, the resulting flood waters, surging down the slopes, would lay waste to everything in their path. Such catastrophes, known as Glacial Lake Outburst Floods (GLOFs), can occur spontaneously or be linked to three potential causes: 1. With the increase in runoff due to glacier melt, the volume of water retained in the lakes increases, increasing the hydrostatic pressure on the retaining dam, which is usually morainic material. Excess volume is likely to produce overflow and the breaking of the moraine dam, which will release the lake waters. In some places, new lakes have appeared, the banks of

Climate change threats to environment in the tropical Andes which are not consolidated and are particularly unstable. 2. Global warming increases the instability of the ice and avalanches of seracs (blocks or columns of ice formed by intersecting crevasses on a glacier) are more frequent. These avalanches can launch heavy materials into the lakes, creating shock waves that can initiate a sudden overflow and breaking of lake banks. 3. Glacier retreat can give rise to major rock-slides. In addition to these factors, the Cordilleras of the Andes is a region of intense seismic activity. Any earthquake is likely to amplify the processes described earlier. This is what happened in Yungay at the foot of Huascaran in the Cordillera Blanca in May 1970, leading to the destruction of the town, with 20,000 victims within a few minutes. To prevent accidents, the Peruvian authorities introduced, as early as the 1940s, an important monitoring program for mountain lakes. For some of these, an overflow tunnel has been created, making it possible to drain the lake to some extent and keep it at a level significantly below overflow. This approach was extremely useful in 2002, for example, when an avalanche of several million cubic meters of rocks fell into the Laguna Safuna (northeast of the Cordillera Blanca). Although this caused a wave estimated at nearly 70 m high, it did not give rise to any human victims, merely sweeping away a few cows and llamas from a mountain pasture (Reynolds et al. 2003). Mountain water resource and use for human activities In the Rio Santa valley (the focus of the study by IRD and its Peruvian partners), the harnessing and use of glacier water is of vital social and economic importance, not only for the region but for the country as a whole: 1. Above 5,000 m, the glaciers themselves and the mountain summits above them are a tourist asset which have attracted mountaineers from the whole world for several decades; 2. Between 2,000 and 4,000 m, irrigated slope agriculture has been practiced for centuries by the Quechua peasants with the help of complex systems of small channels following contour lines, the acequias. 3. Below 2,000 m, taking advantage of the extraordinary natural site of the Cañón del Pato and its differences in altitude, the waters of the Rio Santa drive turbines to generate electricity. 4. Below 800 m, at the foot of the Andes, water from the Rio Santa is used to irrigate huge agricultural areas recently created in the barren coastal zone, thus considerably increasing the traditional irrigation areas of the main deltas of small coastal rivers. Tourist attraction of the high mountains For some 30 years already, the cordilleras of Peru have maintained their attraction to mountain climbers and walkers of all levels, and more recently for other mountain sports practitioners of, for example, mountain biking, canoeing and rafting, the last two of which directly depend on the state of the water recourses. This influx of foreigners has led to the development of an important local tourism and recreation industry (hotels, supplies, guide agencies, services and transport, shops of specialized equipment, etc.). The fragility of this economic system confronted by sudden changes to the glacier environment can be illustrated by an event, which occurred in 2003 in the region of Huaraz (Kaser and Georges 2003). On 2 April 2003, NASA announced, on the basis of a picture taken by the ASTER satellite, the imminent fall of a large ice-cap into a lake above the town of Huaraz (100,0000 inhabitants), the heart of high mountain tourism in Peru. The anxiety provoked by this announcement ruined the tourist season, which was just beginning, causing damage estimated in millions of dollars. It later transpired that the picture had been wrongly interpreted and no particular new risk was identified. Hydropower The Huallanca hydroelectric plant (270 MW) accounting for some 5% of Peru s electricity production capacity harnesses the waters of the Rio Santa. At full capacity, a discharge of 60 m 3 s -1 is needed. The Rio Santa cannot provide such volume at its minimum flow. The plant operator, EGENOR, has constructed small side dams immediately upstream from where the water is taken, where water is stored during the day to be released each evening, at the time of peak consumption. But in order to regulate the water resources of the Rio Santa, EGENOR relies above all on managing natural lakes, the main one of which is Lake Parón, with a regulation capacity of 60 10 6 m 3. If done properly, managing these lakes can offer advantages in terms of security against flood risks. This is the case for Lake Parón, which now constitutes a lower level of risk for the valley below, but the planned raising of the water level in other lakes may be dangerous given the fragility of their banks of morainic material. Traditional agriculture and stock-breeding In any event, damming water for hydroelectric purposes can jeopardize traditional irrigation on the high mountain slopes, if the period over which such irrigation is needed does not match. The increase in the peasant population in the Andes area and the fact that, with global warming, it will become possible to cultivate, therefore irrigate, at ever

P. Chevallier et al. higher altitudes, means that the resources required by traditional irrigation are set to grow and will increasingly compete with hydroelectric needs. Thus, even in the highest regions, there will be greater and more intense conflicts between different users. Large-scale irrigated agriculture The Proyecto Especial Chavimochic, north of the Rio Santa, covers 135,000 ha of various crops destined mainly for export (fruit, asparagus, etc.) and attracts agricultural workers who migrate from mountain areas to settle on the coastal plain. The economy of the city of Trujillo (one million inhabitants) depends to a considerable extent on these facilities, which also provide its drinking water. Other hydro-agricultural projects are being studied, such as the one at Chineca, south of the Rio Santa s mouth. At the driest period of the year, the river flow will be generated mainly by the direct runoff from the precipitation and by the groundwater, the glacial contribution sending strongly compromise with the glacier s disappearance. These minimum flows will inevitably be reduced further, and the topography of the valley does not allow for an increase in the number of reservoirs that would be needed to compensate for the loss of glacial flow. The approximate 20 m 3 s -1, to which the Rio Santa has been known to fall, will not be enough to satisfy irrigation projects, which would need, over the same period, over 100 m 3 s -1. Conclusion Tropical glaciers are particularly sensitive to variations in climate. Warming will very likely reduce their area, and with this reduction, there will be deep and long-lasting adverse changes to the river flow regimes, hence water availability, on the western side of the Andes and along the adjacent Pacific Coast. The simulations presented in this paper under scenarios of regional warming suggest that glacial melt water discharge in the Peruvian Andes will be significantly reduced and could possibly cease by the end of the twenty-first century. This would have adverse effects on regional water resource availability, particularly during the dry seasons. The results presented here are on the one hand based on observation of the present state of the environment, and, on the other hand, the output of modeling tools, based on consistent and very likely hypotheses. But uncertainties remain on the true state of the glaciers and on the water flow regimes in a middle-term or long-term future. In order to make the hypotheses and results more precise, observations, modeling scenarios and forecast tools need to be improved. A priority should be given to interdisciplinary research, including social sciences that were superficially covered in this paper. Acknowledgments A preliminary version of this paper has been presented in November 2004 at the OECD Global Forum on Sustainable Development and Climate Change in Paris, France, with the title: Climate Change Impact on the Water Resources from the Mountains in Peru. OECD Publication Service gave its kind permission for the current actualization and publication. The PhD thesis of Wilson Suarez (2007), Alvaro Soruco (2008) and Marcos Villacis (2008) were funded by IRD DSF. The data have been kindly provided by the following organizations: SENAMHI, INRENA, EGENOR, Duke Energy and Proyecto Especial Chavimocic from Peru; University of Innsbruck from Austria; and IRD from France with its antennas in Bolivia, Ecuador and Peru. The authors thank the anonymous reviewer, who helped to improve significantly the quality of the text, as well as the scientific editors, who corrected the English expression and remained confident during the successive versions of this paper. 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