Réf. Gardent & al 2014 - A

Référence bibliographique complète

GARDENT, M., RABATEL, A., DEDIEU, J.-P., DELINE, P. 2014. Multitemporal glacier inventory of the French Alps from the late 1960s to the late 2000s. Global and Planetary Change, Vol. 120, 24–37. DOI PDF

Abstract: The most recent and complete French glacier inventory was previously the Vivian database, dating from the end of the 1960s but incorporated in the World Glacier Inventory database at the end of the 1990s. Because of the important changes in glacier extent over recent decades an update of the inventory of glaciers of the French Alps was made in a digital vector format (with the associated database) for several dates covering the last 40 years. Such a multitemporal glacier inventory matches a key demand of the Global Terrestrial Network for Glaciers and the Global Land Ice Measurements from Space initiative (GLIMS). Topographical maps, aerial photographs and satellite images were used to map the extent of glaciers using both manual and automatic methods; and the database was generated considering the design of the GLIMS database. Glaciers in the French Alps covered 369 km2 in 1967/71, 340 km2 in 1985/86, 300 km2 in 2003, and 275 km2 in 2006/09. This represents a decrease in surface area of about 25% over the entire study period. Acceleration in glacier shrinkage during the study period was revealed, probably linked to the increase in average air temperature in the 20th century, which has been particularly pronounced since the 1970s. The behaviour of glaciers of the French Alps is in agreement with that of glaciers observed by other studies across the European Alps. We also report the distribution of the morpho-topographic variables (aspect, elevation, etc.) of glaciers of the French Alps for the period 2006/09, and analyse changes of these variables in the last four decades.

Mots-clés

Glacier inventory - GIS - Satellite images - Aerial photographs - French Alps

 

Organismes / Contact

• Université de Savoie, Laboratoire EDYTEM, F-73376 Le Bourget du Lac cedex - E-mail: marie.gardent@univ-savoie.fr (M. Gardent)
Univ. Grenoble Alpes, LGGE, F-38000 Grenoble, France
CNRS, LGGE, F-38000 Grenoble, France
Univ. Grenoble Alpes, LTHE, F-38000 Grenoble, France
CNRS, LTHE, F-38000 Grenoble, France
IRD, LTHE, F-38000 Grenoble, France

The current work was made within the framework of the GLIMS Regional Centre #33 (French Alps) and the French observatory of glaciers: GLACIOCLIM which manages the glaciological measurements conducted on the French glaciers.

 

(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

Air temperature

Glaciers

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

French Alps

Aiguilles Rouges – Ruan

Mont Blanc (FR)

Vanoise

Belledonne

Thabor - Aiguilles d'Arves

Grandes Rousses

Ecrins

Ubaye

 

All

1429-4810 m

1960s to the late 2000s

 

(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

Characteristics of the glaciers of the French Alps for the 2006/09 period

Distribution of glaciers according to size classes

Glaciers of the French Alps covered 275 ± 1 km². The Mont-Blanc (102 km²), Vanoise (93 km²) and Ecrins massifs (69 km²) accounted for 96% of the glacierized area and 89% of the glacier number. The size of the glaciers ranged from 0.01 km² (minimum size considered in the inventories) to 30.4 km² (Mer de Glace)—33 glacierets or perennial snowpatches in the range 0.001–0.01 km² were not taken into account. At the scale of the French Alps: mean glacier size was 0.5 km² (1 σ = 1.7 km²); glaciers < 0.5 km² represented 80% of all glaciers and 19% of the total glacierized area; glaciers > 5 km² accounted for 2% of all glaciers (10% for the Mont-Blanc massif) and 30% of the glacierized area. Glaciers > 1 km² accounted for 88%, 57% and 52% of the glacierized area in the Mont-Blanc, Vanoise and Ecrins massifs respectively.

Distribution of glaciers according to aspect

For the whole French Alps, 60% of the glaciers (73% of the glacierized area) were facing NW, N and NE, while those facing SE, S and SW were about 16%, and 10% of the glacierized area. A similar pattern emerged according to class sizes, or at the scale of the massifs—except in the Mont-Blanc massif, where glaciers facing NE to S are almost entirely missing since our study only concerns its French side, so that in this case the interpretation of the results is biased by the sample.

Distribution of glaciers according to elevation variables (minimum, mean and maximum elevation)

The average minimum/maximum elevation of the glaciers in the French Alps was 2826 m (1σ = 286 m)/3213 m (1σ = 293 m). The lowest elevations were reached by the largest glaciers of the Mont-Blanc massif: Glacier des Bossons (1429 m), Mer de Glace (1531 m), and Glacier d’Argentière (1590 m). In the Ecrins and Vanoise massifs, the lowest elevations were reached by Glacier Noir (2174 m) and Glacier de Pramort (2325 m). Most of the glaciers reaching the lowest elevations are debris-covered in their lower reaches, partly a consequence of the reduced ablation on debris-covered ice in comparison to clean-ice areas. This is the case for the above mentioned glaciers except for Glacier des Bossons, which is interestingly, the French glacier reaching the lowest elevation. This has to be related to the fact that Glacier des Bossons is also the one with the highest maximum elevation (4810 m at the Mont-Blanc summit) resulting in the most widely extended accumulation zone in terms of elevation range (difference in elevation N 1800 m), combined with an important average slope (approx. 30°, about 10° more important than the average slope of glacier larger than1 km²) resulting in an important mass transfer from the accumulation to the ablation zone.

The mean elevation of a glacier is probably the most glaciologically relevant elevation variable since it can be considered as a proxy of the equilibrium-line altitude, ELA (Braithwaite and Raper, 2010; Rabatel et al., 2013b). The average mean elevation of the glaciers in the French Alps was 3019m (1σ=242 m), ~50m lower in the Mont-Blanc massif (2963 m), and respectively ~60 and ~35 m higher in the Vanoise and Ecrins massifs. This difference in average mean elevation of about 100 m between the glaciers in the northern French Alps (Mont-Blanc massif) and the southern Vanoise and Ecrins massifs is in close agreement with the difference of ~150 m in the measured ELAs using field data from the GLACIOCLIM Observatory (Rabatel et al., 2005, 2008) or remote sensing data (Rabatel et al., 2013b).

Glacier size and average slope

Large glaciers generally had a lower average slope than small glaciers: 22° (1σ = 6°) for glaciers > 1 km², generally valley glaciers with an extended gently sloped ablation tongue; 32° (1σ = 8°) for glaciers < 0.1 km². Fig. 6 shows that the variability in mean slope values increases towards smaller glaciers, which implies that glaciers of small size can have nearly any slope which potentially results in very different mean thickness values.

Supraglacial debris covers

A debris cover is considered to be continuous in a given zone when more than 90% of its surface is covered with debris. In 2006/09, >11% (30.7 km²) of the total glacierized area had a continuous debris cover, and about 25% of glaciers had a continuous debris cover over > 10% of their total area. These debris-covered areas were generally found on the lower reaches of the gentle glacier tongues (average slope of ~26°).

Characteristics of the glacier inventories in 1967/71, 1985/86, and 2003

The glaciers of the French Alps covered 369±2km² at the end of the 1960s, 340± 8 km² in 1985/86, and 300 ± 8 km² in 2003. At the three dates, glaciers < 0.5 km² represented 71%, 73%, and 78% of all glaciers, respectively, and ~16% of the total glacierized area, while glaciers > 5 km² accounted for 2% of all glaciers, and ~30% of the glacierized area.

Note that the uncertainty associated with the total surface area computed for 1985/86 and 2003 is higher than for 2006/09 and 1967/71 because of the lower spatial resolution of the used satellite images in comparison with the aerial photographs and topographic maps.


Glacier changes during the successive periods

Glacier changes have been computed for each period defined by the date of the inventories. P represents the whole period from 1967/71 to 2006/09; P1 extends from 1967/71 to 1985/86; P2 from 1985/86 to 2003; and P3 from 2003 to 2006/09. Note that the duration of P3 is only three, five and six years for the Vanoise, Mont-Blanc and Ecrins massifs respectively. As a consequence, the glacier changes for this period have to be considered with caution because the length of P3 is short.

Changes in glacierized area and number of glaciers

In 2006/09 the glacierized area of the French Alps was 8% lower than in 2003, 19% lower than in 1985/86, and 25% lower than in 1967/71. At the glacier, the massif or the class size scale, glacier shrinkage is significant over the entire time period because the difference in surface exceed the uncertainty. It is also the case for the shorter periods P1, P2 and P3, but not systematically, in particular for the glaciers >10 km² during P2. Indeed in this last case, although the shrinkage is visually apparent when looking at the outlines of the glaciers > 10km² in 1985/86 and 2003, because the uncertainties related to the use of satellite images are more important than for the maps or aerial photos, the uncertainties associated to the surfaces of these dates overlap and the shrinkage is thus not statistically significant.

Rate of glacier shrinkage was about three times higher: (i) during the 2000s than in the three previous decades; and (ii) in the Ecrins massif compared to the Mont-Blanc massif (French side) during the last 40 years. Except for glaciers < 0.1 km² the glacier surface area decreased within each size class. In spite of a marked scatter, the relative change in surface area of the glaciers decreased inversely to their original size. In agreement with Tennant et al. (2012) this can be explained by: (i) the high area-to-volume ratio of small glaciers (Granshaw and Fountain, 2006): at the same ablation rate, small glaciers shrink faster than large glaciers; and (ii) the small perimeter-to-area ratio of small glaciers (Demuth et al., 2008; Jiskoot and Mueller, 2012) makes them more sensitive to radiation from the surrounding terrain. On the other hand, some small glaciers are located in favourable conditions (topography, debris cover, snow supply, wind, high elevation, etc.) and have suffered little or no shrinkage has occurred contributing to the high variability in the shrinkage of small glaciers (ranging from almost no change to a complete disappearance).

The number of glaciers was 528 in 1967/71, 522 in 1985/86, 553 in 2003, and 548 in 2006/09. This variability partly results from the fragmentation of several glaciers during each of the three periods; considering each set of fragments as a single glacier, the number of glaciers was 502, 499 and 421 in 1985/86, 2003 and 2006/09 respectively.

Changes in hypsography

The glacier hypsography is shown for each date of the multitemporal inventory and the relative surface change per elevation range for the entire study periods P (1967/71–2006/09). The following points are worth emphasizing:

- The mode of the hypsometric distribution has risen by 50–100m between 1967/71 and 2006/09.

- The surface changes per elevation range are higher at lower elevations (<1700 m) than around the mean elevation of glacier (~3000 m); with a complete disappearance of ice below 1400 m, and almost no change above 3500 m. The latter point is in agreement with results from Vincent et al. (2007) showing that high-elevation glacierized areas have not been significantly reduced in the last 100 years.

- The elevation ranges between 1700 and 2200 m show smaller relative surface changes than the elevation ranges between 2200 and 3300m. This has to be related to the facts that glaciers reaching these elevations are: (i) mostly debris-covered, with lower shrinkage; and/or (ii) the biggest glaciers where the surface loss on the glacier banks for a considered elevation range can be partly counter-balanced by the surface elevation lowering introducing an apparent shift of the contour lines.

Changes in the other topographic variables

The mean elevation for all the studied glaciers has increased by about 50 m from 1967/71 to 2006/09, with 80% of this increase since the mid-1980s. This increase is of the same magnitude in the different massifs.

NW, N and NE remain the dominant orientations of the glaciers for all the inventoried dates. Over the entire period, area loss of glaciers facing E, SE and S was 38%, 37% and 63% respectively, while it was in the range 23%–31% for SW, W, NW and NE facing glaciers, and b 20% for north facing glaciers.

The glacier slope slightly decreased in the average (between 0.7° to 3° depending of the size class). It should be noted that the surface area loss increased with the glacier slope up to 40°: 22% for glaciers with a slope b 25°, 30%–34% between 25° and 35°, and ~46% between 35° and 40°. For glaciers with a slope N 40°, the surface area decreased by ~36%. This is in agreement with the link between glacier slope and glacier size, already mentioned. The exception for glaciers with a slope N 40° has to be related with the fact that such glaciers are small glaciers, located on high-altitude mountain faces, probably partly comprised of cold ice, i.e. located in more favourable conditions.

Finally, it is worth emphasizing that area loss is related to all these parameters due to their correlation with glacier size (i.e. larger glaciers are longer, have a higher maximum and a lower minimum elevation, and smaller mean slope).


Discussion – Potential causes of glacier shrinkage in the French Alps

Conversely to changes in surface mass balance and equilibrium-line altitude, morpho-topographic changes of glaciers (i.e. surface, length, elevation variables …) are not directly linked to climate conditions’ changes. Morpho-topographic changes are mostly a function of: (i) the surface mass balance and its sensitivity to climate variables; (ii) the hypsometry; and (iii) the ice thickness distribution and the slope which control the mass flux. Morpho-topographic changes of glaciers are consequently a delayed response to climate forcing, and the response time of each glacier is different. That said, glacier shrinkage is a consequence of negative mass balances and reduced mass flux, and can be used as a proxy of climate conditions' changes at multidecadal or longer time scales.

It is beyond the scope of the current study to address the question of mass balance changes and the link between mass balance and climate conditions’ changes over the last decades. However, it is interesting to present a short summary of the climate conditions’ changes in the French Alps over the last century to put into context the observed changes in the glacier morpho-topographic variables. As in the rest of the Alps in the last decades (Böhm et al., 2009), the MAAT in the French Alps increased by ~1.5 °C between 1900 and 2007; the increase was c. 2.5 times more between 1970 and 2007 than between 1900 and 1970 (Auer et al., 2007). On the other hand, no clear trend in precipitation in the French Alps has been observed since the mid-20th century (Vincent, 2002; Auer et al., 2007; Durand et al., 2009; Rabatel et al., 2013b).

Regarding the glacier ELA, Rabatel et al. (2013b) showed that the average ELA at the scale of the French Alps was 3035 ± 120 m for the 1984–2010 period; with a difference of about 100mbetween the northern Mont-Blanc massif (2981 ± 120 m) and the southern Ecrins massif (3080 ± 90). 23% of the total area of the Mont-Blanc massif is located above 3000 m, whereas it is only 9% and 5% of the area in the Vanoise and Ecrins massifs respectively. Hence, even if the extension of the accumulation zone also depends on the topography; glaciers in the Mont-Blanc massif have, based on the elevation criteria, a potentially more extended accumulation zone, which is in good agreement with the more important size of the glaciers in the Mont-Blanc massif. We observed that glacier size and glacier shrinkage are linked, and consequently, the lower glacier shrinkage in the Mont-Blanc massif mainly results from the difference in glacier size. However, Rabatel et al. (2013b) have also shown that over the 1984–2010 period, the ELA has increased by about 170 m at the scale of the French Alps, with a more pronounced rate for the Vanoise and Ecrins massifs (6.6 m yr−1) than for the Mont-Blanc massif (5.6 m yr−1). The slightly lower rate quantified in the Mont-Blanc massif had probably also partly contributed to the lower glacier shrinkage in this massif.

Concerning the acceleration of glacier shrinkage in the last decade, and although the period P3 is short, which implies that this acceleration needs to be confirmed by the glacier changes in the coming years, it is interesting to note that:

- At the massif scale, the accelerated glacier shrinkage can be observed everywhere. For instance, glacier shrinkage in the Ecrins massif increased more than fourfold between the end of the 1970s and the 2000s. However, differences can be observed at the scale of the whole French Alps, with stronger glacier shrinkage in the southern massifs. For instance, the acceleration of glacier shrinkage in the Ecrins massif was more than three times higher than that in the Mont-Blanc massif.

- A causality can be evocated between this acceleration in glacier shrinkage over the 2000s and: (i) the increasing trend of the ELA reported by Rabatel et al. (2013b) for the glaciers in the French Alps over the last decades, with since the early 2000s an ELA permanently higher than its average position over the last three decades; and (ii) the almost permanently negative annual mass balances measured since the 1990s on the glaciers in the French Alps within the GLACIOCLIM observatory, with annual values all the more negative for the 2000s (Thibert et al., 2013; Vincent et al., 2014). Whether for the ELA or the mass balance, the authors mention the primary role of the increasing temperature as the driving factor of the observed changes.

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

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

 

This multitemporal inventory is based on the following data sources:

- Topographic maps (scale 1:25,000) published by the IGN (first edition). As dates of the aerial photographs used for mapping glaciers were not recorded by the IGN, we had to compare aerial photographs taken in the 1960s and 1970s with the position of the glaciers on the topographic maps to conclude that the extent of the glaciers on these maps dates from 1967 to 1971, depending on the massifs.

- Landsat 5 TM images (30-m and 15-m resolutions) dating from 1985/86.

- Landsat 5 TM and Landsat 7 ETM + images (30-m and 15-m resolutions) dating from 2003.

- IGN 50-cm pixel orthophotographs from 2006 for the Vanoise massif, 2008 for the Aiguilles Rouges-Ruan and Mont-Blanc massifs, and 2009 for the Belledonne, Grandes Rousses, Thabor-Aiguilles d’Arves and Ecrins massifs. Images taken in 2004 were used for the three small glaciers in the Ubaye massif because cloud cover or fresh snow hides their boundaries on the subsequent orthophotos.

The topographic variables of the glaciers were extracted from the 1979 IGN DEM (25-m resolution, vertical accuracy of 10 m; IGN, 2011) in the case of the topographic maps and 1985/86 Landsat images; and from the mid-2000 ASTER GDEM V2 (27-m resolution, vertical accuracy of about 10 m; Tachikawa et al., 2011) in the case of the 2003 Landsat images and the 2006/09 orthophotos.

The methods (delineation of glacier outlines; quantification of uncertainties) are described in the paper.

 

(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

Discussion – Comparison with other glacier inventories in the Alps

Data from the 2006/09 glacier inventory of the French Alps were compared with data obtained in the 2000s in other regions of the Alps: (1) Lombardy, Italy, in 2003 (272 glaciers, 92 km²); (2) Ötztal, Austria, in 2006 (81 glaciers, 116 km²); (3) the entire Alps, in 2003 (3769 glaciers, 2050 km²); and (4) Aosta Valley, Italy, in 2005 (174 glaciers, 119 km²). In the figure, the different regions are compared considering the same size classes as otherwise the results would be biased for regions dominated by a specific size class. In that way, the last two size classes of our dataset have been merged into one. Despite the differences in sample size (number and surface area), the French dataset is in good agreement with the others, with many small glaciers covering a small total area and few large glaciers covering a large total area. Numbers vary greatly from region to region at the small-size end of the size distribution, while areas vary greatly at the large-size end. This is likely due to the fact that the 2006 Ötztal Alps inventory did not consider many glaciers less than 0.1 km² in size, which explains smaller number of glaciers of this region in this size class.

Glacier shrinkage was compared in all the datasets available for the Alps. Despite time intervals differing between regions, similar patterns of accelerating shrinkage can be observed for the Alps as a whole and for each region. Indeed, for each decade, the mean relative area change per year has been computed for each region, and then averaged for all the regions. This computation shows that the mean relative area change was −0.64% ± 0.5% yr−1 in the 1970s, −0.77% ± 0.7% yr−1 in the 1980s, −1.04% ± 0.7% yr−1 in the 1990s, and −1.5% ± 1.8% yr−1 during the 2000s.

The hypsometry calculated for glaciers in Switzerland (Paul et al., 2004b), Austria (Lambrecht and Kuhn, 2007), the Ötztal Alps (Abermann et al., 2009), South Tyrol (Knoll and Kerschner, 2009), and the Ortles-Cevedale massif (Carturan et al., 2013) was compared with our data. In the 2000s, the elevation band with the most glacierized surface area was higher in France (3000–3100 m) than in Austria and Switzerland (2900–3000 m), but lower than in the Ortles-Cevedale (3200–3300 m) and the Ötztal Alps (3100–3200 m). Bigger losses in glacierized area occurred at lower elevations in France and in the Ötztal Alps than in the other regions. This difference can be explained by: (i) the different time periods considered (1967/71–2006/09 in France and 1969–2006 in the Ötztal Alps, 1969–1998 in Austria, and 1973–1998 in Switzerland); and (ii) the different climate of the eastern and western Alps, with more humid conditions along the northern rim of the Alps, over Lake Maggiore and in north-eastern Italy (Frei and Schär, 1998).

Limitations of this study

This multitemporal inventory of the French Alps was based on four different data sources (maps, aerial photos, satellite images) and different methodologies which may impact the results. As shown previously, the uncertainty with respect to the size of the glaciers is mainly linked with differences in spatial resolution, i.e. differences in pixel size between the sources of data. Paul et al. (2013) mention the importance to quantify the accuracy in the delineation of glaciers because area changes should be larger than the accuracy to be significant. As seen, even though in our case the uncertainties associated with each data source differ, losses at a glacier scale or at the scale of the whole French Alps are in a large majority bigger than the uncertainties and are consequently significant.

The combination of manual and automatic methods, as well as different sources of data is a good way to minimize uncertainties in delineation. The very detailed 2006/09 glacier inventory were used as an aid for manual corrections of glacier boundaries based on the satellite images, specifically for the debris-covered areas. Interestingly, when comparing the inventory made within GlobGlacier by Paul et al. (2011) using an automatic delineation and manual corrections of 2003 Landsat images with our own data; several discrepancies emerge: total glacierized area in 2003 was 300 km² [in the present study], whereas Paul et al. (2011) found 270 km², which is about 10% lower than the present estimate. By overlaying our contour lines on those of Paul et al. (2011), the authors of the present study were able to show that this difference was due to: (i) some debris-covered glaciers not inventoried by Paul et al. (2011), e.g. the debris-covered part of the Mer de Glace and Glacier de Gébroulaz, the Glacier de Pramort, the Glacier Inférieur des Balmes. This is probably mostly due to the coarse resolution of the Landsat images, precluding an accurate visual checking and manual adjusting of such landforms; (ii) some glaciers located at the border between France and Italy, and erroneously counted in Italy, e.g. the Glacier de l’Invernet, the Glacier du Grand; and (iii) small glaciers (but larger than 0.01 km²) that were probably misclassified as perennial snowfields in the GlobGlacier inventory.

 

(5) - Syntèses et préconisations

Conclusion

This study presented the results of a multitemporal inventory of the glaciers of the French Alps for four time periods covering the last 40 years. The use of automatic and manual delineation, and different data sources proved to be an effective way to minimize errors in delineation. Glaciers in the French Alps are mainly distributed in the Mont-Blanc, Vanoise and Ecrins massifs and covered 369 km² in 1967/71, 340 km² in 1985/86, 300 km² in 2003, and 275 km² in 2006/09. Acceleration in glacier shrinkage was observed over the study period, mainly in the 2000s: the rate of change increased from 0.52% yr−1 in the period 1967/71–1985/86, to 0.65% yr−1 in the period 1985/86–2003, and to 1.70% yr−1 in the period 2003–2006/09. Glacier shrinkage was accompanied by changes in a number of topographic variables (e.g. surface area, length, mean elevation, slope). Between 1967/71 and 2006/09, the glaciers that shrank the most were the smallest glaciers, the glaciers that faced E, SE and S, the steepest glaciers in the class with an average slope of less than 40°, and glaciers with a high minimum elevation.

We also showed that the acceleration in glacier shrinkage could be linked with the increasing trend in ELA and the almost permanently negative mass balances documented for the last two decades, both being mainly a consequence of the increase in average annual air temperature observed during the 20th century, which has been particularly pronounced since the 1970s. Finally, our data on the number and size of glaciers, as well as changes in glacierized area over time, are in agreement with data obtained in other regions of the Alps revealing similar glacier behaviour throughout the European Alps, and acceleration in glacier shrinkage in the last 40 years.

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