Réf. Huss & al. 2010b - A

Référence bibliographique complète

HUSS, M., USSELMANN, S., FARINOTTI, D., BAUDER, A. 2010b. Glacier mass balance in the south-eastern Swiss Alps since 1900 and perspectives for the future. Erdkunde, Vol. 64, N°2, 119–140. [PDF]

Abstract: In this study, we analyzed the 20th century ice volume changes for 20 glaciers in the south-eastern Swiss Alps. Our sample included different glacier geometries, sizes and exposures and allowed us to investigate glacier response to climate change. Using a distributed accumulation and temperature-index melt model, we derived mass balance time series in seasonal resolution from 1900. The model was calibrated using ice volume changes obtained from differentiating digital elevation models based on (i) terrestrial topographic surveys, (ii) the Shuttle Radar Topographic Mission (SRTM), and (iii) aerial photogrammetry. In-situ point measurements of annual mass balance and winter accumulation were available for some glaciers, and long-term discharge records were used for model validation. The rate of mass loss between 1900 to 2008 strongly differed between adjacent glaciers. Whereas large valley glaciers (e.g. Vadrec del Forno) showed average mass balances of up to -0.60 m w.e. a-1, smaller and steeper glaciers (e.g. Vadret da Palü) exhibited slower mass loss in the order of -0.20 m w.e. a-1. Over the last century, the regional ice volume decreased by 47%, with strong differences between individual glaciers (30–75%). Using a combined model for 3D glacier evolution and stream-flow runoff driven by regional climate scenarios, we generated perspectives for the 21st century. We determined a decrease in glacier area of 63% until 2050 and an increase in annual discharge over the next three decades for catchments with high glacierization. By 2100, the model results indicated a shift in the hydrological regime and a 23% decrease in annual runoff attributed to increased evapotranspiration and strongly reduced glacier melt contribution.

Mots-clés

Glacier retreat - Ice volume change - Mass balance modelling - Glacier runoff - Vadret da Morteratsch

 

Organismes / Contact

 

 

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

 

Glaciers

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

South-eastern Swiss Alps

Engadine, Val Poschiavo, Val Bregaglia

 

 

 

1900-2008

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

(2) - Effets du changement climatique sur le milieu naturel

Reconstitutions

 

Observations

Glacier mass balance in the 20th century

Mass balance time series are presented in seasonal resolution for all investigated glaciers over the period 1900 to 2008. Annual mass balance is calculated for the hydrological year (Oct. 1–Sept. 30). The winter balance refers to the period Oct. 1 to April 30 and the summer balance is determined for May 1–Sept. 30. ‘Conventional’ specific mass balances are evaluated in meter water equivalent (w.e.), defined as the mass change over one year divided by that year’s glacier surface area (Harrison et al. 2005). Glacier surface elevation and area is updated annually based on linear interpolation between successive DEMs (Huss et al. 2008a).

The method provides spatially distributed mass balance maps on a 25 m grid for each glacier and every year. The calculated mass balance reasonably reproduces in-situ point measurements of mass balance. This highly resolved spatial mass balance distribution provides the required input for 3D ice flow models (e.g. Jouvet et al. 2009), but also allows the direct evaluation of other important variables for impact studies, such as the Equilibrium Line Altitude (ELA), the Accumulation Area Ratio (AAR) and altitudinal mass balance gradients.

Cumulative mass balance time series of the 20 investigated glaciers over the last century are shown. The significant differences in the rate of glacier mass loss are evident. The mean annual balances over the last century differ by a factor of more than four. Vadrec del Forno shows the most negative cumulative mass balance since 1900 (-66 m w.e.). Vadret da Morteratsch, the largest glacier in the region, also has strongly negative mass balance (-49 m w.e.), whereas Vadret da Palü yields a cumulative balance of -16 m w.e. over the same period. Some small glaciers only experienced insignificant mass loss (e.g. Vadrec dal Cengal, -10 m w.e.).

In general, large and flat glaciers exhibit faster mass loss than small and steep ice masses. The arithmetic 20-glacier average mass balance (-0.29 m w.e. a-1) is less negative than the area-weighted average (-0.39 m w.e. a-1), which is dominated by the valley glaciers. For the set of 20 glaciers we obtained a total ice volume change of -3.5 km³ since 1900.

The current ice volume of the investigated glaciers inferred with ITEM was used to put the calculated 1900–2008 ice volume changes into context. Based on the mass balance time series, we obtain the initial ice volume in 1900 for each glacier individually and calculated the relative volume change over the 20th century. On average, the investigated glaciers lost almost 50% of their ice volume over the 20th century, ranging from a 75% loss for small glaciers (e.g. Vadrettin da Misaun) to a 30% decrease only for some ice fields (e.g. Vadret da Palü). Surprisingly, the relative change in ice volume is larger for Vadret da Morteratsch (-38%) than for some small, but well-protected glaciers (e.g. Vadrec da la Bondasca). The observed area changes are in line with these figures. Whereas small glaciers lost up to half of their area between the mid-1930s and 2003, large glaciers show area changes of 15–20%, although the terminus position of their tongues has changed dramatically.

The highly variable mass balance response of glaciers within a single mountain group to climatic warming is intriguing and indicates that the extrapolation of individual mass balance time series to large glacierized regions (e.g. Raper and Braithwaite 2006; Ipcc 2007) is difficult. Understanding the processes that cause strongly different glacier mass balances under similar variations in climatic forcing is a prerequisite to calculate glacier mass changes for entire mountain ranges, and, thus, to make reliable projections of the contribution of mountain glaciers to future sea level rise. Response times of individual glaciers and their climate sensitivity must be taken into account when analyzing their reaction to climate change (e.g. Chinn 1996). Changes in glacier length illustrate the differing reactions of the glacier termini to climate warming being strongly related to the glacier response time. Several studies proposed methods to infer long-term glacier mass balance from length change measurements (e.g. Oerlemans 1994; Hoelzle and Haeberli 1995; Hoelzle et al. 2003). The length change measurements available for six of the investigated glaciers were exploited to derive independent five-decadal mass balance estimates that can be compared to the results based on field data and modelling.

The authors apply a simple scheme based on a continuity formulation (Nye 1960) that considers step changes in climate and a transition of the glacier from one to another steady-state (Hoelzle et al. 2003). (…) The mass balance calculation [see equation in the study] was applied to two 50-year periods in the 20th century for all glaciers with measurements of glacier length and compared the length change derived from mean mass balance to model results. Mass balance obtained from this simple method from length change data corresponds to some degree to the five decadal mass balances given by modelling and ice volume changes, but yield slightly lower rates of mass loss in general. Discrepancies can be explained by geometrical effects that are neglected in the equation, as well as non-equilibrium conditions. The length change data are useful for roughly estimating the rate of mass loss for different glaciers in low temporal resolution and allow a first distinction of glaciers with high mass loss from glaciers close to equilibrium.

Oerlemans (2007) derived response times of 33 years for Vadret da Morteratsch and only 4 years for Vadret da Palü, both descending from the same mountain. This result is based on ‘backward modelling’ using the length record and the assumption that the changes in ELA over the last century were the same for both glaciers (Oerlemans 2007). The difference in mass balance inferred from our data is in line with this finding: the long response time of Vadret da Morteratsch results in a low-lying glacier tongue that is too large for the current climate. Thus, this glacier exhibits disproportionately negative mass balance at low elevations. Vadret da Palü, in contrast, has a flat high-elevation accumulation area and a steep and shallow glacier tongue. Hence, the glacier reacts quickly to climate warming by getting rid of its tongue and retreating to higher elevations. The comparison of glacier dynamics (Oerlemans 2007) and mass balance for these neighbouring glaciers illustrates the impact of glacier shape on its reaction to climate warming. Treating glaciers as indicators for climate change is, thus, not straightforward, but is complicated by the geometrical characteristics of each individual glacier. The concept described above theoretically leads to high rates of mass loss for large and flat glaciers and to less negative mass balances for small and steep glaciers for the same anomalies in climatic forcing.

Differences in the rate of mass loss between the glaciers can also be explained by processes that affect either accumulation or melt for individual glaciers unequally. Although the investigated mountain group is relatively small in size, not all weather conditions favour snow accumulation for the entire glacier cluster in the same way. Large-scale atmospheric flow paths over the Alps showed strong variations over the 20th century leading to significant differences in the amount and the distribution of snow accumulation in Switzerland (Beniston 1997). Especially the glaciers of the Val Bregaglia, which strongly rely on high amounts of winter precipitation as well as avalanche-deposited snow, might be more sensitive to changes in the circulation pattern and thus wind direction than other ice masses. Positive and negative backcoupling mechanisms are expected with climate change, thus affecting individual glaciers differently in magnitude. Oerlemans et al. (2009) reported on increased dust-concentration in the ablation area of Vadret da Morteratsch leading to enhanced melt rates due to lower ice albedo. Conversely, many glaciers build up increasingly debris-covered tongues with glacier retreat, resulting in strongly reduced ice melt (Kayastha et al. 2000; Kellerer-Pirklbauer et al. 2008). The debris covered portion of Vadrec da l’Albigna, for example, increased from 16% in 1942 to 26% in 2003. Additionally, the supraglacial debris layer probably thickened, further reducing ice melt locally (Nakawo and Rana 1999). The authors expect this trend to persist over the next decades contributing to less negative mass balances and a slower reaction of Vadrec da l’Albigna to climate warming than for non-debris covered glaciers. Correlations of decadal mean mass balance for large glacier samples with variables describing glacier geometry are generally weak; Paul and Haeberli (2008) obtained some dependency of the magnitude of glacier surface elevation change with glacier area and potential solar radiation. We divided the glacier areas of our 20-glacier sample into five groups corresponding to 20%-quantiles and evaluated the mean mass balance between 1900 and 2008 for each group. The relation between glacier area and mass balance is not straightforward; the group of the largest glaciers, however, shows by far the fastest mass loss. This corresponds to long response times and consequently slow adjustment of the glacier extent to changed climate conditions.

The authors define the volume ratio Rv that characterizes the distribution of the glacier’s ice volume along its flowline. Rv is calculated by dividing the ice volume stored below the median glacier surface elevation by the volume stored above. The ice thickness distribution is obtained from ITEM (Farinotti et al. 2009a). We find a clear relation between Rv and secular mass balance. Glaciers with high Rv, i.e. concentration of the ice volume in the ablation area, exhibit high rates of mass loss (e.g. Vadrec del Forno). Glaciers with low Rv, i.e. a steep and shallow glacier tongue and a flat and wide accumulation area, are characterized by less negative mass balances (e.g. Vadret da Palü). The volume ratio Rv seems to be a good indicator for estimating the mass balance reaction of glaciers to changed climate conditions.

The glaciers in the study area underwent short periods of mass gain in the 1910s and the late 1970s and showed rapid mass loss in the 1940s and since the mid-1980s. This indicates that glacier retreat in the investigated region is driven by spatially coherent changes in meteorological variables to which the glaciers respond differently. For the investigation of long-term fluctuations and trends in the seasonal mass balance components, we analyzed the arithmetic 20-glacier average (Fig. 10). In general, long-term variations in glacier mass balance are due to changes in the melting conditions, well represented by the summer balance (Fig. 10). Long-term changes in the winter balance are small, although the year-to-year variability is considerable. However, periods of negative anomalies in summer balance often coincide with low winter balance, indicating that accumulation changes were at least partly responsible for rapid glacier mass loss in the 1940s and in the last decades. Reduced accumulation in warm periods is related to the state of precipitation. For Swiss glaciers, Huss et al. (2009b) reported on a decrease in the fraction of solid precipitation of 12% since the 1970s. Furthermore, low winter accumulation enhances summer melting over an albedo feedback mechanism.

Extreme years – both with mass gain or loss – have strong impacts on glacier evolution. Our time series reveal some of these years, interestingly clustering in the first years of the 21st century. The year with the most negative summer balance is 2003, however, above average winter accumulation in south-eastern Switzerland has prevented the glaciers from undergoing very high mass losses. According to our evaluation, the mass balance year with the highest mass loss is 2006, characterized by low winter balance. In 1977 and 2001, the investigated glaciers showed very positive mass balances of +1.5 m w.e., due to winter accumulation being more than twice the long-term average.

In comparison to other studies addressing the glacier mass balance in the study region, we find similar results. Nemec et al. (2009) reported a cumulative mass balance for Vadret da Morteratsch of -46 m w.e. for the period 1865–2005. This result refers to an interpolation of mass balances calculated over the 1850 glacier extent and the current glacier geometry, and is thus difficult to directly compare to our cumulative mass balances calculated over annually updated glacier surfaces. The average glacier-wide mass balance of Vadret da Morteratsch for the period 1982–2002, which was determined by Klok and Oerlemans (2004) and based on an energy balance model, is -0.78 m w.e. a-1. For the same time period, we obtained slightly less negative mass balances (-0.60 m w.e. a-1). Hoelzle and Haeberli (1995) estimated the average mass balance for 7 glaciers contained in our sample between 1850 and 1973 as in the range of -0.27 to -0.52 m w.e. a-1. For the same glaciers, we found a mean 1900–2008 mass balance of -0.33 m w.e. a-1.

The differences in the 20th century mean ELA are considerable (Tab. 5) and range from 2250 m a.s.l. (Vadrec dal Cengal) to 3240 m a.s.l. (Vadrettin da Tschierva). This indicates that even within a small glacier cluster, accumulation and melting conditions can strongly differ. Low ELAs can either be explained by enhanced accumulation or by reduced melt, e.g. in shaded, north-exposed cirques. Analysis of the regional distribution of ELA over the 20th century shows that exposure is not dominant, but strong regional gradients in precipitation and snow redistribution processes are responsible for the relatively low-lying glacierization in the Val Bregaglia and the high ELAs around Piz Bernina.

The calculated 100-year trend in ELA rise is 15 m per decade and is significant according to the Mann-Kendall Test (Mann 1945; Kendall 1975). According to previous studies in the Alps (Greene et al. 1999; Maisch 2000) the ELA sensitivity to temperature rise is in the order of 150 m °C-1. Our results are consistent with this number. The long-term fluctuations in the ELA are important and reached maxima around 1950 and over the last 20 years, and a local minimum between 1960 and 1980. ELA0 is defined as the ELA required to yield a balanced mass budget of the glacier. ELA0 was determined for each year based on glacier extent and the Accumulation Area Ratio providing a balanced mass budget. ELA0 increased steadily over the 20th century related to the retreat of the glacier towards higher elevations corresponding to a linear trend of 8 m per decade. The comparison of time series of ELA and ELA0 showed that the glaciers were almost always out of equilibrium conditions throughout the 20th century, except for two short periods in the 1910s and the 1960/70s. Differences in the long-term trend between ELA and ELA0 indicate that the glaciers tend to go increasingly further from equilibrium conditions, i.e. they are not able to adjust their size as fast as the climate is changing.

Modélisations

Future perspectives for glacier extent and runoff

In order to provide perspectives for the future glacier changes in south-eastern Switzerland and to estimate potential impacts of glacier wastage on the hydrological cycle, we performed model runs using GERM for the 21st century based on climate scenarios. We assumed seasonal changes in air temperature and precipitation corresponding to the median of 16 Regional Climate Models (RCMs) within the PRUDENCE project (Christensen and Christensen 2007; Frei 2007). The models anticipated a significant increase in air temperature, especially pronounced in summer. Annual precipitation sums showed no large changes, however, more precipitation is expected in winter and less in summer. Uncertainties in projections of future climate were high. This is both due to unknown future greenhouse gas emissions and backcoupling effects in the climate system that are insufficiently understood and implemented in the climate models. By using the median of a large model ensemble for the impact study, our scenario refers to a consensus about future climate evolution.

For each of the four analyzed drainage basins, we performed 20 transient calculations for the period 2009–2100 based on a random weather variability superimposed on the projected linear trends in climate change (see Huss et al., 2008b for details). GERM yielded both the annual change in 3D glacier geometry and ice-covered area and calculated all components of the water balance in daily resolution. The simulated retreat of the glaciers in the south-eastern Swiss Alps over the 21st century is strong. By 2050, we expect a decrease in total glacier area by 63% compared to 2003. Our results showed that very small glaciers will disappear within the next three decades (e.g. Vadret Boval Dadour, Vadrettin da Misaun), and glaciers with an area of currently 1–2 km² around 2050 (e.g. Vadret da Fedoz, Vadret dal Tremoggia). Some small glaciers in the Val Bregaglia, situated in north-exposed cirques, might last until the end of the 21st century (Vadrec da la Bondasca, Vadrec da la Trubinasca). This is intriguing, as their ELAs are partly as low as 2300 m a.s.l., an elevation, where one would not expect glaciers in a rapidly warming climate. We explain their resistivity to climate change with the strong dependency of their mass budget on winter accumulation and avalanches. Medium-sized glaciers are projected to withstand climate warming until about 2070 (e.g. Vadret da Roseg, Vadrec da l’Albigna).

According to the calculations, the largest glacier in the Region, Vadret da Morteratsch, will still be present, however, in strongly reduced size by 2100. By 2070, the ELA is expected to rise to 3500M a.s.l. and the ice volume to have dropped to 10% relative to 2008. The AAR of Vadret da Morteratsch is around 40% throughout the entire 21st century indicating a strong and persistent imbalance of the glacier. An AAR of 58% would be required to yield a balanced mass budget. The simple and widely applicable method used to simulate future glacier geometry and extent does not assume the glacier to be in equilibrium with warming climate, but transiently keeps track of the ice volume. Taking into account these effects in future runoff projections is crucial for the management of water resources in alpine drainage basins.

In the glacierized drainage basins, strong changes are projected in the components of the water balance. Although no significant trends in annual precipitation are expected, the calculated annual runoff over the 21st century shows considerable long-term variations. Compared to the climatic normal period 1961–1990 a small increase is found in annual runoff (+2%) in 2030 which is due to the release of water from long-term glacier storage. For catchments with little vegetation and substantial glacierization the runoff maximum due to glacier melt is more pronounced. The subsequent runoff decrease is explained by higher evapotranspiration and the glaciers, strongly diminished in size, providing only small quantities of melt water. Annual runoff volume from the four drainage basins is expected to decrease by -23% by 2100. In catchments with a small portion of ice-covered and a high percentage of vegetated surfaces the changes in the water balance are dominated by the increase in evapotranspiration. The modelling of this variable in high alpine environments and for future conditions is, however, relatively uncertain.

Changes in the runoff regime are expected to pose the major challenge for the water resource management in the 21st century. Our results show the gradual transition from a glacier-melt dominated to a snow-melt dominated hydrograph. Whereas in a first phase until about 2050, an increase in summer runoff is expected due to strong ice melt and a fast reduction of glacier volume, we project a significant decrease in runoff in the months July and August for the second half of the century. In a warmer climate, snow melt runoff starts earlier in the year, is more concentrated and has already terminated in summer. The glaciers – strongly reduced in size – can no longer provide ice melt runoff in the summer months, when there is the greatest need for water supply in dry alpine valleys. Higher evapotranspiration losses further increase the water shortage in summertime.

The results indicate a 24% decrease in summer runoff (July–August) compared to 1961–1990 for the year 2050. By 2090, July–August runoff is expected to be diminished by even 63%. Due to an earlier onset of the melting season, a significant increase in the spring runoff (April–May) by 54% is anticipated for 2050.

Hypothèses

In the near future, a substantial retreat of Alpine glaciers is expected (e.g. Maisch 2000; Zemp et al. 2006; Jouvet et al. 2009), leading to major impacts on water resource management (Zierl and Bugmann 2005; Horton et al. 2006; Stahl et al. 2008), tourism (Bürki et al. 2003) and natural hazards (Richardson and Reynolds 2000). Providing realistic scenarios for future impacts of climate warming on the environment is highly important for alpine communities in order to adapt to the rapid changes currently occurring.

 

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Since the 1850s, the end of the Little Ice Age, glaciers in the European Alps have suffered major ice volume losses (Vincent 2002; Kaser et al. 2006; Steiner et al. 2008). The effect of changes in climate forcing on glaciers is most clearly reflected in their surface mass budget. Long-term glacier mass balance observations are indispensable to investigate how climate acts on glaciers (Vincent et al. 2004; Ohmura et al. 2007). However, mass balance time series in the Alps are typically short and only cover a few relatively small glaciers, leaving entire glacierized regions unobserved (Zemp et al. 2009). Furthermore, the difference in the rate of mass loss of individual glaciers is large (e.g. Kuhn et al. 1985; Huss et al. 2008a; Paul and Haeberli 2008), resulting in a high uncertainty in extrapolating glacier mass balance to unmeasured glaciers.

The south-eastern Swiss Alps have a substantial glacier cover that clusters around the 4000 m high summit of Piz Bernina. In total, there are about two dozen individual glaciers in the region, several of them prominent valley glaciers. So far, glaciological research has mainly addressed Vadret da Morteratsch. The local surface energy balance (Oerlemans 2000; Oerlemans and Klok 2002; Oerlemans et al. 2009), the glacier mass budget (Klok and Oerlemans 2002; Klok and Oerlemans 2004; Machguth et al. 2008; Nemec et al. 2009) and glacier dynamics (Oerlemans 2007) were investigated. Hoelzle and Haeberli (1995) estimated the ice volume and the mean mass balance for several glaciers in the study region based on glacier inventory data and observed length change between 1850 and 1973.

In this study, the temporal and spatial changes in 20 glaciers in south-eastern Switzerland between 1900 and 2008 were analyzed. The glaciers investigated represent all of the ice masses in the region and cover different glacier geometries, sizes and exposures. The authors compiled a comprehensive field data basis incorporating all available measurements from the 20th century originating from various sources: repeated topographical information, long-term runoff records, in-situ seasonal mass balance measurements and observations of glacier length change. These data were used to constrain a distributed glacier mass balance model (Hock 1999; Huss et al. 2008a) driven by daily meteorological variables. Seasonal glacier mass balance time series since 1900 that allow to study the response of a 20-glacier sample to current climate warming are presented. A glacio-hydrological model (Huss et al. 2008b) is run into the future based on regional climate scenarios, providing estimates of the impact of climate change on glacier extent and the hydrological cycle in the south-eastern Swiss Alps.

The 20 investigated glaciers, which are located in the south-eastern Swiss Alps, are representative for all of the significant ice masses in this mountain range. The glaciers are located in the head watersheds of three valleys, which drain to the north-east (Engadine), to the south (Val Poschiavo) and to the south-west (Val Bregaglia). The size of the studied glaciers ranges from 0.3 km² to 16 km² and they occupied a total area of 62 km² in 2003. Theses glaciers represent a wide range of different glacier types. The long-term equilibrium line altitude (ELA) is at around 3000 m a.s.l. for the eastern glaciers of the study area and drops to 2600 m a.s.l. towards the west, indicating a regional precipitation gradient. The study area contains two typical valley glaciers (Vadret da Morteratsch, Vadrec del Forno) and several other prominent ice masses (e.g. Vadret da Palü, Vadret da Roseg, Vadret da Tschierva). The glaciers in the Val Bregaglia are often situated in cirques and are fed by avalanches and wind-deposited snow. The tongue of Vadrec da l’Albigna is debris-covered. Its retreat rate accelerated after the late 1950s due to a proglacial reservoir.

[See details in the study]

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

 

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

Conclusion

This study compiles the first set of long-term mass balance time series for all items in a glacier cluster. This allows the analysis of the differences in the response of adjacent ice masses to climatic warming. Our results are based on a simple mass balance model driven by daily meteorological data and constrained by various types of field measurements covering large parts of the 20th century. The authors provide seasonal glacier mass balance time series for 20 glaciers, different in size and geometry, in south-eastern Switzerland for the period 1900–2008. Strongly differing rates of glacier mass loss are mainly explained by the dynamic adjustment of the glacier to the current climate. The analysis of glacier change is complemented with estimates of the total ice volume, which is required for projections of glacier change into the future. Based on regional climate scenarios in seasonal resolution, model runs until 2100 were performed in order to provide perspectives for the change in glacier extent and the impact of glacier retreat on the runoff regime of glacierized drainage basins in south-eastern Switzerland . Glaciers in the study area are expected to retreat significantly over the next decades. Most of them will disappear in the second half of the 21st century. The hydrological cycle will be strongly affected by glacier retreat. According to our results, an initial increase in runoff due to strong reduction in glacier ice volume will be followed by a water shortage, particularly important during the summer months. Our analysis of glacier change over the last 100 years shows that understanding the response of glaciers to current atmospheric warming is complicated by many interactions of climate, glacier surface processes, ice dynamics and glacier geometry. Field data are the key to understanding these processes. However, for many regions, field measurements documenting long-term glacier changes are too sparse and thus do not clearly reveal the climate change impacts. Modelling is an essential tool for homogenizing and unifying these measurements and, hence, for increasing their value. This study does not present the results from a long-term glacier monitoring program, but is based on a compilation and re-analysis of existing data sets. Similar information is potentially available for a large number of glaciers in different mountain ranges and provides – in combination with short series of in-situ field measurements and modelling – an extended view of the response of mountain glaciers to 20th century climate warming. The authors recommend a strategy of combining various data sets to constrain models for the past and future change in mountain glacier extent, in order to estimate their contribution to global sea level rise, or for hydrological applications.

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