Réf. Paul & al. 2007 - A

Référence bibliographique complète
PAUL F., KAAB A., HAEBERLI W. Recent glacier changes in the Alps observed by satellite: Consequences for future monitoring strategies. Global and Planetary Change, 2007, Vol. 56, p. 111-122.

Abstract: The new satellite-derived Swiss glacier inventory revealed that mean glacier area loss per decade from 1985 to 1998/99 has accelerated by a factor of seven compared to the period 1850–1973. Moreover, the satellite data display much evidence that downwasting (i.e. stationary thinning) has become a major source of glacier mass loss, an observation that is confirmed by in situ mass balance measurements. Many of the observed changes (growing rock outcrops, tongue separation, formation of pro-glacial lakes, albedo lowering, collapse structures) are related to positive feedbacks which accelerate further glacier disintegration once they are initiated. As such, it is unlikely that the recent trend of glacier wastage will stop (or reverse) in the near future. In view of the rapid non-uniform geometry changes, special challenges emerged for the recently established tiered glacier monitoring strategy.

Mots-clés
Alpine glacier change, multispectral satellite data, glacier monitoring.

Organismes / Contact
Department of Geography, Glaciology and Geomorphodynamics Group, University of Zurich, Winterthurer Strasse 190, CH- 8057 Zurich, Switzerland.
Tel.: +41 1 635 5175 ; fpaul@geo.unizh.ch
Department of Geosciences, University of Oslo, 316 Oslo, Norway.

(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
Europe Alps Gran Paradiso mountain range, Bernina group and Ötztaler Alps.
Taelli, Cavagnoli and Caresèr glaciers.
    1985-2004

(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
Fields observations
Specific results of glacier changes in Switzerland from 1973 to 1985 to 2000 as well as an extrapolation to the entire Alps have been reported in Paul (2004) and Paul et al. (2004). In Switzerland, glaciers lost about 18% of their area from 1985 to 1998/99 (from 1973 to 1985 the change is only -1%). This corresponds to an average relative area loss of 14% per decade, which is about seven times higher than the decadal loss rate between 1850 and 1973 (-2.2%). There is an even higher relative loss of area towards smaller glaciers, but the scatter among values increases as well, indicating a very specific behaviour of individual glaciers that are smaller than 1 km2. Such small glaciers account also for a major part (44%) of the total area loss since 1973, although they cover only 18% of the total area in 1973. Such data also reveal that non-uniform geometry changes (i.e. not related to active glacier retreat) can occur everywhere on a glacier.

According to the mass balance data from ten Alpine glaciers (IUGG et al., 2005) the mean cumulative specific mass loss was about 17 m water equivalent (we) between 1981 and 2003, corresponding to about -0.8 m we per year. This is about three times the long-term mean value for the 20th century of -0.27 m we (Haeberli and Hoelzle, 1995; Hoelzle et al., 2003). Apart from 3 years (1984, 1995 and 2001) with small mass gains, all years since 1981 exhibit mass losses. A linear trend line on the data points suggests an increasing speed of glacier mass loss, indicating that glaciers were not able to adjust by a dynamic retreat towards higher elevations. The reduction in driving stress and flow facilitates down-wasting. The continuous mass loss has also diminished or even eliminated most of the firn reserves from previous years. Thus, the decreasing mass flux from the accumulation area has also steadily lowered the ice flow velocity (e.g. Herren et al., 2002) which in turn led to many of the observed disintegration features (hollows within a glacier, caves and deep tunnels at the glacier front).

Observations from satellite imagery
The recent analysis of satellite data revealed a strong acceleration of glacier shrinkage in the Alps since 1985, with a mean decadal rate of area reduction seven times higher than during the 1850-1973 period (Paul et al., 2004). The strong acceleration of glacier shrinkage (in size and thickness) has also been observed in several other places around the world. Although changes in glacier thickness cannot be measured directly from optical satellite data, the analysis of image time series gives indirect evidence that down-wasting (i.e. stationary thinning) has become a major source of Alpine glacier mass loss during the past 20 years. This was also confirmed by the mainly negative mean mass balances of ten Alpine glaciers since 1980 (Frauenfelder et al., 2005).

The major indicators of down-wasting that have been observed on Landsat images are: growing rock outcrops, separation from tributaries, formation of pro-glacial lakes, non-uniform geometry changes, e.g. disintegration and shrinkage along the entire perimeter. Such changes can be observed throughout the entire Alps, independent of the precipitation regime, glacier size or exposition. In some regions nearly all of these changes could be observed at the same time. However, it should be noted that individual glaciers with little or no change can often be found in the same region or even adjacent to a disintegrating glacier. The reason for this high-variability over short distances has not been determined yet.

All three glaciers (Taelli, Cavagnoli and Caresèr glaciers) are placed at about the same geographical latitude (46.5° N) and clearly demonstrate how fast disintegration has proceeded in the last 20 years. While Taelli Glacier has already disintegrated into several small patches of ice remnants, Cavagnoli Glacier will likely follow next and the somewhat larger Caresèr Glacier shows rapidly growing regions with rock outcrops. A common characteristic of all three glaciers is that they are comparably flat and not protected much by rock walls from direct solar radiation during summer.

Somewhat larger regions, located in the Gran Paradiso mountain range in the southwestern part of the Alps (FR/I), the Bernina group in the central-southern part (CH/I) and in the Ötztaler Alps in the central-northern part (A/I), have been studied too. In all three regions several processes resulting from the overall glacier down-wasting or shrinkage are visible: formation or growing of proglacial lakes, new rock outcrops, tongue separation, strong retreat, and disintegration. Again, it is obvious that the observed changes took place on an Alpine-wide scale, but nearly unchanged glaciers can often be found within the same region.

Positive feedback processes
Most of the observed changes are related to positive feedbacks, i.e. once started they have the tendency to intensify further. The formation of pro-glacial lakes that are in contact with a glacier tongue often leads to rapid further growth, as the water can get warmer than 0°C and cause additional ice melt (so called thermokarst). A thermally driven internal circulation erodes the ice at the waterline and leads to the formation of ice cliffs with the related calving events (Kääb and Haeberli, 2001).

Due to their lower albedo and thermal inertia, new rock outcrops heat up more quickly than the surrounding ice (or snow) and emit this heat also after local sunset and during night. This process can very efficiently create a small gap between the rock and the ice, which further grows by turbulent heat fluxes. As such, rock outcrops that appear somewhere within a glacier (depending on the bedrock topography) are very efficient in separating a glacier into smaller parts. Once several rock outcrops have separated a part of a glacier from the accumulation area, the dead ice body will melt down quickly (at least if not protected by a thick debris cover).

Another important aspect that could be observed is the gradual lowering of glacier albedo (in the ablation zone) in the course of the past 20 years, reaching values as low as 0.15 in 2003 (Paul et al., 2005). It seems that albedo decreased steadily as a result of the mainly negative mass balances since 1981. In the Alps, glacier albedo exerts a major influence on the energy balance (e.g. Klok and Oerlemans, 2002; Paul et al., 2005) and thus on the summer ablation, which governs the variability of the annual balance for most glaciers (Oerlemans and Reichert, 2000). The decreasing glacier albedo is also part of a positive feedback that enhances glacier melt even more.
Modélisations
 
Hypothèses
A common characteristic of all three glaciers (Taelli, Cavagnoli and Caresèr glaciers) is that they are comparably flat and not protected much by rock walls from direct solar radiation during summer. As such, their disintegration will most-likely continue in the following years as positive feedbacks can accelerate the down-wasting even further.

In total, all the processes observed here act together and in the same direction, leading to a self-acceleration of glacier decline. It can be assumed that it will be very difficult to stop this process for several reasons: (1) Most glaciers have lost all of their firn reserves from the 1970s and would need several years with large amounts of snow in winter (and little ablation in summer) to gain some mass that could then be redistributed by increased flow velocity to the glacier front. Although changes in precipitation are difficult to predict, it seems unlikely that the required increase of more than 50% (e.g. Kuhn, 1989) will take place. (2) There is a general trend of increasing temperatures in the future as predicted by nearly all climate models (e.g. Räisänen et al., 2004). This would further enhance the observed changes and also makes the required snowfall in summer less probable. (3) Even the still flowing and fast-reacting steeper mountain glaciers have response times of several years and their actual shape is not yet in balance with current climatic conditions. As such, they would continue to retreat for several more years even if temperatures are not increasing any further.

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
Observations made by Landsat Thematic Mapper (TM) and ASTER satellite data throughout the Alps will be presented. The examples discussed cover various climatic regions and include glaciers of different exposition and size. However, for better visibility of the changes, some of the more prominent examples have been selected. In principle, the changes can be observed in every region of the Alps, but not necessarily for all in the same region.

In high-mountain topography exact orthorectification of satellite data is required if glacier outlines are combined with other sources of georeferenced information (e.g. other satellite sensors or digitized outlines of former glacier extent). This requires a high-resolution digital elevation model (DEM) of appropriate accuracy as well as accurate topographic maps for collection of GCPs (Paul, 2004).

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

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

(4) - Remarques générales
There is a strong bias towards larger glaciers in the length measurement sample, due to the remote location of most small glaciers. As such, the changes of the latter are less well documented and the retreat signal is dominated by large valley and mountain glaciers. While the valley glaciers reflect the secular trend, mountain glaciers reveal decadal oscillations in the climate signal (Hoelzle et al., 2003), i.e. the advance period of the 1920s and 1970s.

(5) - Syntèses et préconisations
In view of the rapid non-uniform geometry changes, special challenges emerged for the recently established tiered glacier monitoring strategy within the framework of the Global Climate/Terrestrial Observing System (GCOS/GTOS). The challenges include: (1) loss of mass balance series due to disintegrating glaciers, (2) problematic extrapolation of index stake measurements from a calibration period under different climate conditions, (3) critical evaluation of measured length changes, (4) establishment of an operational glacier inventorying strategy using satellite data and (5) the calculation of new topographic parameters after glacier split up that can be compared to previous parameters.

Références citées :

Frauenfelder, R., Zemp, M., Haeberli, W., Hoelzle, M., 2005. Worldwide glacier mass balance measurements: trends and first results of an extraordinary year in Central Europe. Ice and Climate News 6, 9-10.

Haeberli, W., Hoelzle, M., 1995. Application of inventory data for estimating characteristics of and regional climate-change effects on mountain glaciers: a pilot study with the European Alps. Annals of Glaciology 21, 206-212. - [Fiche biblio]

Herren, E., Bauder, A., Hoelzle, M., Maisch, M. (Eds.), 2002. The Swiss glaciers 1999/2000 and 2000/2001. Glaciological Commission of the Swiss Academy of Sciences and Laboratory of Hydraulics, Hydrology and Glaciology at the Federal Institute of Technology, Zurich. Glaciological Report, vol. 121/122.

Hoelzle, M., Haeberli, W., Dischl, M., Peschke, W., 2003. Secular glacier mass balances derived from cumulative glacier length changes. Global and Planetary Change 36 (4), 295306. - [Fiche biblio]

IUGG(CCS)/UNEP/UNESCO/WMO, 2005. In: Haeberli,W., Noetzli, J., Zemp, M., Baumann, S., Frauenfelder, R., Hoelzle, M. (Eds.), Glacier Mass Balance Bulletin, vol. 8. World Glacier Monitoring Service, Zurich. 100pp.

Kääb, A., Haeberli, W., 2001. Evolution of a high-mountain thermokarst lake in the Swiss Alps. Arctic, Antarctic, and Alpine Research 33 (4), 385390.

Klok, E.J., Oerlemans, J., 2002. Model study of the spatial distribution of the energy and mass balance of Morteratschgletscher, Switzerland. Journal of Glaciology 48 (163), 505518.

Kuhn,M., 1989. The response of the equilibrium line altitude to climatic fluctuations: theory and observations. In:Oerlemans, J. (Ed.),Glacier Fluctuations and Climatic Change. Kluwer, Dodrecht, pp. 407417.

Oerlemans, J., Reichert, B.K., 2000. Relating glacier mass balance to meteorological data using a Seasonal Sensitivity Characteristic (SSC). Journal of Glaciology 46 (152), 16.

Paul, F., 2004. The new Swiss glacier inventory 2000 - Application of remote sensing and GIS. PhD thesis, Department of Geography, University of Zurich. - [Fiche biblio]

Paul, F., Kääb, A., Maisch, M., Kellenberger, T.W., Haeberli, W., 2004. Rapid disintegration of Alpine glaciers observed with satellite data. Geophysical Research Letters 31, L21402.

Paul, F., Machguth, H., Kääb, A., 2005. On the impact of glacier albedo under conditions of extreme glacier melt: the summer of 2003 in the Alps. EARSeL eProceedings 4 (2), 139149 (CD-ROM).

Räisänen, J., Hansson, U., Ullerstig, A., Döscher, R., Graham, L.P., Jones, C., Meier, H.E.M., Samuelsson, P., Willèn, U., 2004. European climate in the late twenty-first century: regional simulations with two driving global models and two forcing scenarios. Climate Dynamics 22, 1331.