Réf. Casassa& al. 2009 - A

Référence bibliographique complète

CASASSA, G., LÓPEZ, P., POUYAUD, B., ESCOBAR, F. 2009. Detection of changes in glacial run-off in alpine basins: examples from North America, the Alps, central Asia and the Andes. Hydrological Processes, Vol. 23, 31–41.

Abstract: Atmospheric warming and enhanced melting of glaciers is already resulting in changes in the glacial contribution to run-off in mountain basins around the world. The enhanced melting of glaciers leads at first to increased run-off and discharge peaks and an increased melt season, while in the longer time frame glacier wasting can be so severe that it results in decreased run-off. Glacier basins with a decreasing run-off trend have been observed in south-central British Columbia, at low elevations in the Swiss Alps and in the central Andes of Chile, which is probably a combined effect of reduced melt from seasonal snow cover as the snow line rises, and relevant glacier area losses. In contrast, significant run-off increases are reported in Alberta, north-western British Columbia and Yukon in Canada, in highly glacierized basins in the Swiss and Austrian Alps, the Tianshan Mountains and Tibet in central Asia and in the tropical Andes of Peru. The run-off increase within these basins is closely related to observed temperature rise, indicating that there is an unequivocal signal of enhanced glacier melting under the present warming trends. In future warming scenarios, glacier run-off should start to decrease even in high-altitude basins, affecting water availability.

Mots-clés

 

 

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

 

 

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 

 

 

 

 

 

 

(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

Alps:
Streamflow records for 48 stations in Switzerland with undisturbed run-off regimes in the period 1931–2000 have been studied by Birsan et al. (2005). Almost all basins with more than 10% glacier cover were found to have an increasing run-off trend in summer flow, whereas decreasing trends or non-significant trends were detected in basins with less than 10% glacierization. At the Porte du Scex station in the lower Rhone basin, a run-off reduction was measured within the period 1916–1968, which was explained as a result of a decreased ice-covered area (Kasser, 1973). Run-off decrease due to glacier area reduction was also reported for the period 1910/1919–1968/1972 by Chen and Ohmura (1990) for Porte du Scex and for Massa, a station located in the upper Rhone in a highly glacierized basin. Run-off in four glacierized basins of the upper Rhone catchment, central Swiss Alps, has been studied by Collins (2005, 2007, 2008). At Massa, the highest glacierized basin of those studied by Collins, run-off closely followed the temperature trend, with a rise in the warm 1940s, a decrease in the cool 1970s, and a clear increase since the 1990s which have been unusually warm. The firn line at Massa has not yet reached the highest point of the basin, therefore allowing for enhanced glacier melt as the snow line rises due to decreased albedo of bare ice. In basins with intermediate glacierization (35–60%), run-off increased until the 1980s but started decreasing in the 1990s.

In the highly glacierized (72%) basin above the gauging station of Vernagtferner Glacier, Oetztal Alps, Austria, diurnal amplitude of the run-off significantly increased in the period 1974–2000, consistent with an increase in the bare ice area (Hock et al., 2005). Owing to positive mass balance years, in the 1970s the bare ice area covered 10–30% of the total glacier, increasing in the 1990s to values of 90% due to strongly negative mass balances.

In summary, the available run-off records for the Alps show a run-off increase in recent decades in highly glacierized basins due to warmer temperatures that result in snow line rise, glacier wastage and enhanced melt. Basins with intermediate glacierization already show a declining run-off trend, indicating that areal reduction of glaciers prevails over enhanced melt due to thinning. Finally, basins with low glacierization also show a declining run-off trend that is driven both by the area reduction of glaciers and by the precipitation decrease in recent decades.

World:

A collection of fifteen published data series and one unpublished data series (Li et al., submitted) from glacierized basins are reviewed. Data include basins from Canada, the Alps (Switzerland and Austria), central Asia (Tianshan Mountains and central Tibet, China) and the Andes of Peru and Chile, with glacierization fractions ranging from 0.015 to 75%. Strong glacier retreat has been occurring at all basins in recent decades, mainly in response to atmospheric warming. Inspection of the data shows a geographical pattern of increasing trends in runoff in central Asia and Peru, decreasing trends in Chile and both increasing and decreasing trends in Canada and the Alps. New analysis of run-off data is also performed for mountain basins in the Andes of Chile, finding no significant trends in the period 1950–2007.

Decreasing run-off trends tend to occur in low elevation basins and/or basins with low glacierization, whereas increasing trends are found in high elevation basins and/or basins with high glacierization. In glacierized basins that are experiencing relevant deglaciation, the glacier contribution to run-off occurs in two distinctive stages. At first, the enhanced melting of glaciers which occurs mainly by thinning leads to increased run-off and discharge peaks and an increased melt season. This process is accompanied by snow line/equilibrium line rise, exposing a larger area of bare ice that in turn reduces the albedo and acts as a positive feedback mechanism, further enhancing glacier melt. A critical stage is reached when glacier area reduction prevails over glacier thinning. At the same time, the firn line/equilibrium line can reach the highest part of the glacier, exposing all glacier area as bare ice at the end of the summer. After this critical point of severe glacier wasting is reached, the glacier run-off in the basin starts to decrease.

The critical stage associated with decrease of glacier run-off has already been attained in southern and central British Columbia, Alberta, and in low elevation/low glacierization basins in the Swiss Alps.

Modélisations

 

Hypothèses

The critical stage associated with decrease of glacier run-off has already been attained in southern and central British Columbia, Alberta, and in low elevation/low glacierization basins in the Swiss Alps. In future warming scenarios glacier run-off decrease should become more predominant as glaciers experience severe wastage, affecting water availability of vast populations who rely on water resources that originate largely from glacier melt, particularly at the end of the summer and in dry years. More data and analyses are clearly needed to assess current run-off trends and relate them to glacier changes and to climate trends, which is essential for improving the forecast of future water availability from mountain regions.

 

Sensibilité du milieu à des paramètres climatiques

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

 

A collection of fifteen published data series and one unpublished data series from glacierized basins are reviewed. Data include basins from Canada, the Alps (Switzerland and Austria), central Asia (Tianshan Mountains and central Tibet, China) and the Andes of Peru and Chile.

 

(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

 

Références citées :

Birsan et al. (2005

Chen and Ohmura (1990

Collins (2005, 2007, 2008

Hock et al., 2005

Kasser, 1973

Li et al., submitted