Open Session - SWISS GEOSCIENCE MEETINGs

1 6
Symposium 6: Apply! Snow, ice and Permafrost Science + Geomorphology
6.21
Spatial variability of glacier elevation changes in the Swiss Alps obtained
from differencing two DEMs
Paul Frank* & Haeberli Wilfried*
* Department of Geography, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich (frank.paul@geo.uzh.ch)
During the past 25 years massive glacier thinning has been observed in the Alps. This is documented by direct mass balance
measurements on nine regularly observed glaciers as well as field evidence (e.g. disintegration of glacier tongues, increased
areas of rock outcrops, collapsing rock walls, revised hiking trails) at several sites (Paul et al., 2007). However, it remained
uncertain how representative the nine directly measured glaciers are for the volume change of the entire Alps. A possible
way of assessing the representativity is to subtract two digital elevation models (DEMs) from two points in time which are of
sufficient accuracy with respect to the expected changes.
For the Swiss Alps, a DEM with 25 m spatial resolution (DEM25) was generated by swisstopo around 1985. In February 2000
a near-global DEM was acquired by interferometric techniques during the Shuttle Radar Topography Mission (SRTM). This
DEM is available for free from an ftp-server at 90 m (or 3") spatial resolution. It offers now the unique opportunity to assess
glacier elevation changes over the entire Alps by subtracting it from an earlier DEM (like the DEM25 in Switzerland). It has
to be noted that both DEMs could have larger errors in the (snow-covered) accumulation area of glaciers. For the DEM25, the
stereo matching of optical aerial photography suffers from poor contrast over snow and the SRTM DEM has some uncertainty
due to a variable snow penetration depth of the C-band radar beam.
For mainly two reasons we have not corrected the elevation dependent bias of the SRTM elevations which was reported in
earlier studies. One reason is that a similar bias results as an artefact when DEMs of different cell size are subtracted (Paul,
in press), the other is that a huge portion of glaciers would get positive elevation changes (mass gains) in the accumulation
area, which is highly unlikely during the period 1985 to 1999.
For this study we have calculated glacier specific elevation changes from 1985 to 1999 for about 1050 glaciers larger than 0.1
km 2 in the Swiss Alps (Paul and Haeberli, 2008). The changes refer to the glacier geometry of 1973 which is close to the 1985
glacier extent (Paul et al., 2004) and digitally available for all glaciers in Switzerland. In order to compare the results with
the direct measurements, two mean volume change values (in meter water equivalent, m w.e.) are calculated. One is (method
A) the arithmetic mean of the individual mean changes of all glaciers and another one is (method B) the mean value for the
entire glacierized area that does not consider individual glaciers (all glaciers together are taken as one glacier entity).
The analysis reveals strong thickness losses (partly >80 m) for flat / low-lying glacier tongues and a strong overall surface lowering
also for glacier tongues under a thick debris cover. The difference image (Fig. 1) displays fine spatial details of the
change.
Apart from the obvious massive downwasting of the tongue of Triftglacier, it clearly reveals that some glaciers (e.g. Stein,
Kehlen, Damma) have been larger in 1985 than in 1973 as the zone of thinning is outside the 1973 extent. The mean cumulative
mass balance of the nine glaciers with direct measurements (-10.8 m w.e.) agrees well with the mean change (method
B) for the Swiss Alps from DEM differencing (-11 m w.e.) and can thus be considered to be representative for the Alps. Mean
thickness change of individual glaciers is correlated with their size, elevation, and exposure to solar irradiation. This implies
that mass losses of large glaciers can be underestimated when they are directly inferred from values measured at the generally
much smaller glaciers in the mass balance network. Indeed, the mean change with method (A) is only -7.0 m w.e. and
thus much less negative than the field derived value. In particular the large regions at low elevations from the largest ice
masses contribute to the stronger loss.
REFERENCES
Paul, F. In press: Calculation of glacier elevation changes with SRTM: Is there an elevation dependent bias? Journal of
Glaciology.
Paul, F. & Haeberli, W. 2008: Spatial variability of glacier elevation changes in the Swiss Alps obtained from two digital
elevation models. Geophysical Research Letters, doi:10.1029/2008GL034718.
Paul, F., Kääb, A. & Haeberli, W. 2007: Recent glacier changes in the Alps observed from satellite: Consequences for future
monitoring strategies. Global and Planetary Change, 56, 111-122.
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, doi:10.1029/2004GL020816.