Réf. Vincent & al. 2007a - A

Référence bibliographique complète
VINCENT C., LE MEUR E., SIX D., FUNK M., HOELZLE M., PREUNKERT S. Very high-elevation Mont Blanc glaciated areas not affected by the 20 th century climate change. Journal of Geophysical Research, 2007, Vol. 112, 9 p.

Abstract: This paper analyses the impact of climate change over the last 100 years on high-elevation glaciated areas of the Mont Blanc range, comprising ice fields covering the top of the Mont Blanc (4808 m) and Dôme du Goûter (4300 m) areas. Surface ablation is negligible for these high-elevation areas and the surface mass balance is mainly controlled by snow accumulation. At Dôme du Goûter, ice fluxes have been calculated through two transversal sections by two independent methods in order to assess long-term surface accumulation. A comparison between these results and recent accumulation observations, together with the strong relationship between valley precipitation and snow accumulation, suggests that surface accumulation rates did not change significantly over the entire 20th century. Moreover, the small ice thickness changes, less than 3 m on the average, observed at Mont Blanc and Dôme du Goûter between 1905 and 2005 clearly reveal that these high-elevation glaciated areas have not been significantly affected by climate change over the last 100 years.

Mont Blanc and Dôme du Goûter glaciers, surface mass balance, ice thickness, precipitation, evolution.

Organismes / Contact
Laboratoire de Glaciologie et de Géophysique de l'Environnement, CNRS, Saint Martin d'Hères, France. vincent@lgge.obs.ujf-grenoble.fr
Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie, ETH Zentrum, Zürich, Switzerland.
Department of Geography, University of Zürich-Irchel, Zürich, Switzerland.

(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
Precipitation Glaciers    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Période(s) d'observation
France Mont Blanc range Mont Blanc and Dôme du Goûter ice fields   Mont Blanc (4808 m)
Dôme du Goûter (4300 m)

(1) - Modifications des paramètres atmosphériques
The precipitation data reveal a strong annual variability but a weak decadal variability. The 5-year average shows similar temporal changes for the 3 stations. From these data, it can be seen that the decadal precipitation did not vary by more than ±10% over the 20th century.

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
The precipitation variability observed at meteorological stations in valley locations (Besse en Oisans, Bourg Saint Maurice and Chamonix) has been investigated.

(2) - Effets du changement climatique sur le milieu naturel
Long-term mass balance deduced from ice fluxes calculated at Dôme du Goûter
According to the first method, the calculation results show that the necessary surface mass balance (SMB) to compensate the output flux from the west section is 2.1 m w.e./yr on the average over the west drainage basin area, which is almost twice the value calculated for the east drainage basin area (1.1 m w.e./yr). An error calculation leads to a total uncertainty of ±0.4 m w.e./yr for the computed mean SMB for each drainage basin area.

According to the second method, total submergence is 125,900 m3 w.e./yr on WD and 57,000 m3 w.e./yr on ED resulting in respective mean rates of 2.7 and 1.2 m w.e./yr. These results agree well with previous results obtained from the cross-section ice fluxes.

Short-term surface mass balance at Dôme du Goûter
Although total accumulation varies from year to year, the patterns of spatial distribution vary little. The average accumulation over the 4 years is in good agreement with the submergence rates. Finally, the mean accumulation over these 4 years is 2.21 m w.e./yr, very close to the average submergence velocity (2.16 m w.e./yr) over the area.

The good agreement between the average submergence velocity and the 4-years-averaged observed accumulation does not prove that Dôme du Goûter is in a steady-state, as the average accumulation results from only 4 years of observations. However, this analysis shows that the recent observed snow accumulation pattern is maintained from year to year and is similar to the long-term mass balance pattern.

Correlation between surface mass balance and valley precipitation
A straightforward relationship between accumulation and valley precipitation reveals a correlation coefficient of 0.73 and 0.76 for a linear regression and a power law regression, respectively. As a first approximation, one can assume that the total accumulation over the whole area of Dôme du Goûter is roughly related to the total precipitation at Chamonix and that the accumulation variability is similar to the annual precipitation variability.

Thickness Variations
The small thickness changes observed over the 20th century are striking. For both areas, thickness variations do not exceed ±15 m. The average changes are +2.6 m at Dôme du Goûter and -0.3 m at Mont Blanc. Considering the uncertainty interval, i.e., ±5 m, it can be concluded that no significant thickness change is detectable over most of these areas. These results contrast with large changes observed on the tongues of the glaciers in the Mont Blanc area. In deed, every glacier tongue below 2000 m a.s.l. thinned by more than 100 m since the beginning of the 20th century.

The comparison between observed accumulation and valley meteorological data suggests a low decadal variability of snow accumulation and no SMB trend over the 20th century. Given that (1) the long term accumulation rate, calculated from ice fluxes or submergence velocities, is very close to the average accumulation rate observed over the period 1993-1995 and 1997-1999, (2) there is a good relationship between the accumulation change at Dôme du Goûter and the valley precipitation change, and (3) the 1993-1995 and 1997-1999 mean valley precipitation rate (1.35 m w.e./yr) is close to the 20th century mean precipitation rate (1.26 m w.e./yr), we can conclude that the Dôme du Goûter SMB did not change significantly over the whole 20th century. Although thickness changes are not highly sensitive to accumulation changes, these results concerning accumulation could explain the very small changes observed at high altitudes on these ice fields. In any case, this study reveals that the very high-elevation ice fields in the Mont Blanc area have not been affected by the climate warming. The 20th century climate warming affected the atmospheric temperature in the Alps by +1°C [Böhm et al., 2001]. However this change did not significantly affect the ice deformation rate in the high-altitude ice fields since the ice temperature remains far below the melting point and therefore keeping the glacier frozen to its bed.
Over the next 100 years, according to climate warming scenarios, a significant part of precipitation could become rain above 4300 m a.s.l. which could warm up the deep firn and ice. Some studies show that substantial warming of the firn temperature at shallow depths has taken place over the last few decades [Lüthi and Funk, 2001; Suter et al., 2001]. Should this warming reach the bottom ice, the ice dynamics would be greatly modified.

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
The surface mass balance is dominated by accumulation (thus precipitation) and sublimation at high altitude.
Long-term mass balance deduced from ice fluxes calculated at Dôme du Goûter
The ice fluxes have been calculated through two sections using thickness and surface ice flow velocity measurements. Using available accumulation, velocity, and glacier bed data, two drainage basin areas have been outlined. The ice flux from a western, WD, and an eastern drainage basin, ED, through respective cross sections has been calculated. The glacier is cold and the sliding velocity is assumed to be zero [Paterson, 1994]. Consequently, the mean horizontal ice velocity through the cross section is derived from the analytical formulation proposed by Lliboutry [1981].

The mean surface mass balance (SMB) can also be determined using the vertical velocity observations. The submergence velocities obtained from repeatedly taken stake observations between 1993 and 2004 have been used. These values have been integrated over the drainage basin areas using krigging interpolation to obtain the total submerging ice flux.

Short-term surface mass balance
at Dôme du Goûter
Snow accumulation measurements are not available at every site for each year. However, accumulation data are available in the western part of Col du Dôme for the years 1993-1994,1994-1995, 1997-1998, and 1998-1999. From mass balance observations, average accumulations within this area have been calculated.

Correlation between surface mass balance and valley precipitation
The closest meteorological station is Chamonix, at an elevation of 1000 m a.s.l., and 8 km from Dôme du Goûter. To compare high elevation accumulation and valley precipitation, a site with numerous accumulation observations has been selected (130 m north of the Col du Dôme). 22 observations are available for time periods ranging from 33 to 221 days between 1994 and 2004. These observations have been divided by the number of days to obtain mean daily accumulation.

The next step consists of investigating the precipitation variability observed at meteorological stations in valley locations. For this purpose, the longest precipitation data series in the French Alps have been used, i.e., those of Besse en Oisans and Bourg Saint Maurice, 30 km and 95 km from Chamonix, respectively.

Thickness Variations
Thickness variations have been deduced by comparing two digital elevation models computed from recent geodetic measurements and an old map dating back to the beginning of the 20th century. Measurements using differential GPS were carried out in 2005 and are accurate to within a few centimeters. The old map [Vallot et al., 1948] was established in 1905.

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

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(4) - Remarques générales

(5) - Syntèses et préconisations

Références citées :

Böhm, R., I. Auer, M. Brunetti, M. Maugeri, T. Nanni, and W. Schöner (2001), Regional temperature variability in the European Alps 1760 1998, Int. J. Climatol., 21, 1779 1801.

Lliboutry, L. (1981), A critical review of analytical approximate solutions for steady state velocities and temperatures in cold ice sheets, Z. Gletscherkd. Glazialgeol., 15, 135148.

Lüthi, M., and M. Funk (2001), Modelling heat flow in a cold, high-altitude glacier: Interpretation of measurements from Colle Gnifetti, Swiss Alps, J. Glaciol., 47, 314 324.

Paterson, W. S. B. (1994), The Physics of Glaciers, 3rd ed., Elsevier, New York.

Suter, S., M. Laternser, W. Haeberli, M. Hoelzle, and R. Frauenfelder (2001), Cold firn and ice of high-altitude glaciers in the Alps: Measurements and distribution modeling, J. Glaciol., 47, 8596.

Vallot, H., J. Vallot, and C. Vallot (1948), Cartes du Massif du Mont Blanc à l'échelle du 1:20 000, Edition Girard, Barrère et Thomas, France.

Vincent, C., M. Vallon, J. F. Pinglot, M. Funk, and L. Reynaud (1997), Snow accumulation and ice flow at Dôme du Goûter (4300 m), Mont Blanc, French Alps, J. Glaciol., 43, 513521, (Erratum, J. Glaciol., 44, 194, 1998.)