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Secular glacier mass balances derived from cumulative
glacier length changes
M. Hoelzlea,b,*, W. Haeberlia, M. Dischla, W. Peschkeb
aDepartment of Geography, Glaciology and Geomorphodynamics Group, University of Zurich, Winterthurerstr. 190,
CH-8057 Zurich, SwitzerlandbLaboratory of Hydraulics, Hydrology and Glaciology, Federal Institute of Technology, Gloriastr. 37/39, CH-8092 Zurich, Switzerland
Received 5 April 2002; accepted 17 September 2002
Abstract
Glacier mass changes are considered to represent natural key variables with respect to strategies for early detection of
enhanced greenhouse effects on climate. The main problem, however, with interpreting worldwide glacier mass balance
evolution concerns the question of representativity. One important key to deal with such uncertainties and to assess the spatio-
temporal representativity of the few available measurements is the long-term change in cumulative glacier length. The mean
specific mass balance determined from glacier length change data since 1900 shows considerable regional variability but centers
around a mean value of about � 0.25 m year� 1 water equivalent.
D 2003 Elsevier Science B.V. All rights reserved.
Keywords: Glacier fluctuations; Glacier length changes; Glacier mass changes; Climate change
1. Introduction
Observation of worldwide glacier changes as com-
piled by the World Glacier Monitoring Service
(WGMS) are presently being built into Global Climate
Observing Systems (GCOS, WMO, 1997; Haeberli et
al., 2000). Especially glacier mass changes are consid-
ered to represent natural key variables with respect to
strategies for early detection of enhanced greenhouse
effects on climate (Kuhn, 1980; Haeberli et al., 1999).
The latent heat required to cause the measured glacier
wastage can be compared with the estimated excess
radiation income and with changes in sensible heat as
calculated by numerical climate models. Several
attempts have recently been undertaken to regionally
or globally summarize the available data using various
approaches such as area-weighting with glacier inven-
tory data, spatial interpolation based on global ice
extent and correlations between mass balance time
series, comparison with integrated geometric changes
as determined by laser altimetry flights and GPS
surveys on selected flowlines, or cumulative length
changes as combined with glacier inventory data (Cog-
ley and Adams, 1998; Dyurgerov and Meier, 1997a,b,
2000; Dyurgerov, 2002; Echelmeyer et al., 1996;
Gregory and Oerlemans, 1998; IAHS, 1999; Kuhn,
0921-8181/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0921-8181(02)00223-0
* Corresponding author. Department of Geography, Glaciology
and Geomorphodynamics Group, University of Zurich, Winter-
thurerstr. 190, CH-8057 Zurich, Switzerland. Tel.: +41-16355139;
fax: +41-16356848.
E-mail address: [email protected] (M. Hoelzle).
www.elsevier.com/locate/gloplacha
Global and Planetary Change 36 (2003) 295–306
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1993, 1995; Haeberli and Hoelzle, 1995, Letreguilly
and Reynaud, 1989, 1990; Oerlemans, 1994; Rabus
and Echelmeyer, 1998; Zuo and Oerlemans, 1997a).
The results all confirm the order of magnitude (a few
decimeters per year) characterizing worldwide annual
ice thickness loss during recent decades. Presently
observed rates of change in glacier mass and corre-
sponding acceleration trends could well contain man-
induced effects on greenhouse forcing. The anthropo-
genic influences on the atmosphere could now and for
the first time represent a major contributing factor to
ongoing glacier shrinkage at a global scale (Haeberli et
al., 1999).
The main problem with interpreting worldwide
glacier mass balance evolution concerns the question
of representativity, i.e. the possibilities of comparing
the small sample of values measured during a few
decades with the evolution in unmeasured areas and
during previous time periods. One important key to
deal with such uncertainties and to assess the spatio-
temporal representativity of the few available meas-
urements is long-term changes in cumulative glacier
length. Corresponding possibilities had long remained
unexploited because of two main reasons: (1) the time
necessary for glacier adjustment to changed climatic
conditions had been overestimated by earlier theoret-
ical approaches (Nye, 1960) and (2) the straightfor-
ward averaging of annual length changes as
percentages of advancing/retreating glaciers or as
mean annual length changes (Paterson, 1981) had left
undetected the important information contained in the
observed data. Theoretical developments by Johan-
nesson et al., (1989a,b) and the potential of numeri-
cally modelling time-dependent glacier evolution
(Oerlemans, 1988, 1997, 1998; Oerlemans et al.,
1998; Zuo and Oerlemans, 1997b) helped to over-
come such problems. Today, the following three
approaches can be applied for the interpretation of
data concerning cumulative glacier length change:
(a) intercomparison between curves from geometri-
cally similar glaciers;
(b) application of continuity concepts for assumed
step changes between steady-state conditions
reached after the dynamic response time; and
(c) dynamic fitting of time-dependent flow models to
present-day geometries and observed long-term
length change.
The advantage of the first two approaches is the
simplicity of the procedure and its applicability to
glaciers with strongly limited data (for instance,
glacier inventory information). The third approach
(cf. especially Oerlemans et al., 1998 for coordinated
model experiments) requires a thorough parameter-
ization involving a number of uncertainties but allows
for better time resolution, gives information on varia-
tions in equilibrium line altitude, and helps testing the
simpler first two approaches. An ideal concept, there-
fore, consists in combining all three approaches and
comparing the results—as far as possible—with meas-
ured data and observed evidence. The present article
deals with the first two approaches and attempts to
establish a baseline of global/long-term information as
available from the database of the World Glacier
Monitoring Service.
2. Scientific background
The curves of cumulative glacier advance and
retreat are converted into time series of temporally
averaged mass balance by applying a continuity
model originally proposed by Nye (1960). This
approach considers step changes after full dynamic
response and new equilibrium of the glacier. Thereby,
mass balance disturbance (yb) leading to a corre-
sponding glacier length change (yL) depends on the
original length (Lo) and the annual mass balance
(ablation) at the glacier terminus (bt):
yb ¼ btyL=Lo:
The dynamic response time (sr) is hmax/bt (Johan-
nesson et al. 1989a,b), where h is a characteristic ice
thickness, usually taken at the equilibrium line where
ice depths are near maximum. Assuming a linear
adjustment of the mass balance b to zero during the
dynamic response, the average mass balance < b> is
taken as yb/2. The so-obtained value < b> is given in
annual ice thickness change (meters of water equiv-
alent per year) averaged over the entire glacier sur-
face, and can be directly compared with values
measured in the field. The method is simple and the
results compare very well with long-term observations
(Herren et al., 1999). The main limitation is the
resolution in time: with a characteristic value for bt
M. Hoelzle et al. / Global and Planetary Change 36 (2003) 295–306296
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at the snout of Grosser Aletschgletscher of 12 m
year� 1 and a maximum thickness of about 900 m,
the response time is somewhere in between 50 and
100 years. The calculated mass balance values are
therefore half-secular to secular averages. These mass
balance values are therefore calculated for the glaciers
according to their individual characteristic response
time or multiples thereof.
3. Data compilation and processing
Data compilation was performed in two steps.
Swiss data on glacier length change was compiled
by Peschke (1998) and corresponding worldwide data
by Dischl (1999). Both data sets were combined to
enable intercomparison of cumulative length changes
(see Intercomparison of regional glacier length evo-
lution) and estimates of regional mass balances for
secular time periods (see Derived secular mass balan-
ces). The map in Fig. 1 shows the mountain regions
where length change data were compiled.
International glacier data collection has been coor-
dinated since 1894. At that time, the Swiss limnologist
F.A. Forel started periodical publishing of the ‘Rap-
ports sur les variations periodiques des glaciers’ on
behalf of the then established ‘Commission Internatio-
nale des Glaciers’ (Forel, 1895). Up to 1961, the data
compilations constituting the main source of length
change data worldwide were published in French,
Italian, German, and English. Since 1967, the publica-
tions are all written in English. The first reports contain
mainly qualitative observations with the exception of
the glaciers in the Alps and Scandinavia, which are
quite well documented by quantitative measurements
from the very beginning (Bruckner and Muret, 1908,
1909, 1910, 1911; Hamberg and Mercanton, 1914;
Finsterwalder and Muret, 1901, 1902, 1903; Forel
and Du Pasquier, 1896; 1897; Rabot and Muret,
1911, 1912, 1913; Rabot and Mercanton, 1913;
Richter, 1898, 1899, 1900; Reid and Muret, 1904,
1905, 1906). After the First World War, Mercanton
(1930, 1934, 1936, 1948, 1952, 1954, 1958, 1961)
edited the more rarely appearing publications since
1933 on behalf of the International Commission on
Snow and Ice (ICSI) of the International Association of
Hydrological Sciences (IAHS). Starting with 1967, the
data are published in five yearly intervals under the title
‘Fluctuations of Glaciers’, first by the Permanent
Service on the Fluctuations of Glaciers (PSFG, Kasser,
Fig. 1. Mountain regions with long-term glacier length change data. Selected glaciers in the regions with names are presented in Fig. 3.
M. Hoelzle et al. / Global and Planetary Change 36 (2003) 295–306 297
Page 4
1970) and—after merger of PSFG with the Temporary
Technical Secretariat for the World Glacier Inventory
(TTS/WGI) in 1986—by theWorld Glacier Monitoring
Service (WGMS). The corresponding publications are
IAHS (ICSI)/UNESCO (1967, 1973, 1977, 1985) and
IAHS (ICSI)/UNEP/UNESCO (1988, 1993a, 1998). In
addition to this data collection, information in other
sources such as the Journal and the Annals of Glaciol-
ogy or other scientific publications was collected and
integrated in the database (cf. Baird and Field, 1952;
Bouverot, 1958; Casassa et al., 1998; Desio, 1967;
Ding and Haeberli, 1998; Hofmann, 1958; Johannes-
son and Sigurdsson, 1998; Johnson, 1954; Kaser, 1996,
1999; Matthes, 1934; Ommanney et al., 1998; Sigurds-
son, 1998; Tsvetkov et al., 1998; Vanni, 1954).
The field method of data collection for the frontal
glacier-tongue variations in most cases consists of
simple tape readings and sometimes of geodetic/pho-
togrammetric surveys using reference points marked
in the glacier forefield. The accuracy of annual
measurements is in a range of about F 1–2 m. Such
an accuracy is by far good enough for the order-of-
magnitude estimates presented here. Possibilities of
intercomparison between the documented times ser-
ies, however, are sometimes reduced due to intermit-
tent interruptions and methodological heterogeneities
within the recorded time series. Complete time series
are available in the European Mountain ranges, but
large gaps exist in most other mountain regions.
Additional problems with data compilation and inter-
pretation relate to information sometimes presented in
text form rather than tables, to different languages
used, or to glaciers having changed their names (for
instance, Belengi–Bezengi in CIS) or political state
(for instance, Furkele-Austria to Forcolo-Italy). Espe-
cially old records also contain numerous typing errors
(for instance, 1930/31 instead of 1931/32 in a table
given by Mercanton, 1936). As far as possible, such
uncertainties were eliminated by careful examination
of the situation.
In addition to data on annual front variations for
each glacier, the following variables were collected:
the glacier code of the WGMS database, the political
unit (country abbreviation), the general and specific
location, latitude and longitude, highest, median or
mean and lowest elevation, and length of the glacier
around 1960 to 1975 as based on inventory data and
aspect (Hoelzle and Trindler, 1998).
A parameterization scheme earlier developed for
analyzing glacier inventory data (cf. Haeberli, 1991;
Haeberli and Hoelzle, 1995; Hoelzle and Haeberli,
1995 for more detailed discussion) was used. This
scheme builds on measured data about total length
(L0) as well as maximum, mean or median and
minimum altitude (Hmax, Hmean or median, Hmin) of
the investigated glaciers. From these basic parameters,
mean altitude was calculated from Hmax and Hmin
wherenot available.Vertical extent (DH =Hmax�Hmin)
and average surface slope (a = arctan {DH/L0}) were
then derived as a first step. Average ice depth along
the central flowline (hf) was estimated from a and a
mean basal shear stress along the central flowline
(sf = fqghf sina, with q = density and g = acceleration
due to gravity), whereby sf depends in a nonlinear
way on DH as a function of mass turnover (cf.
Driedger and Kennard, 1986; Haeberli, 1985; Hae-
berli and Hoelzle, 1995). The shape factor f was
chosen as 0.8 for simplicity in all cases. Glacier long
profiles along the central flowlines are generalized as
two simple wedges pointing up-slope in the accumu-
lation area, down-slope in the ablation area and
having in common the side representing the maximum
thickness (hmax) at the equilibrium line. The value for
hmax is very roughly determined at 2.5hf—instead of
2hf—as estimated from known ice thickness measure-
ments on various Alpine glaciers (Muller et al., 1976
and unpublished radio-echo soundings/hot water drill-
ings by VAW/ETH Zurich) in order to account for
some longitudinal variations in a.Mean altitude is taken as an approximation for
equilibrium-line altitude ELA (cf. Braithwaite and
Muller, 1980), and the mass balance (annual ablation)
at the glacier tongue is computed as bt = db/dH
(Hmean�Hmin) where the mass balance gradient db/
dH receives values of 0.3 to 1.2 m water equivalent per
100 m and year for the ablation area. The determination
of the mass balance gradient may represent the most
delicate point in the parameterization. Realistic values
for the gradients were sought by taking direct measure-
ments from various mountain areas as a guide where
available. The gradients used are based on IAHS
(ICSI)/UNEP/UNESCO (1991, 1993b, 1994, 1996,
1999, 2001), Oerlemans and Fortuin (1992) and Oerle-
mans and Hoogendoorn (1989).
Disturbances in mass balance (yb) were calculated
from cumulative glacier length changes (see Scientific
M. Hoelzle et al. / Global and Planetary Change 36 (2003) 295–306298
Page 5
background, cf. Paterson, 1994; Haeberli, 1991) in the
sense of step functions between assumed steady-state
conditions with respect to time periods corresponding
to the characteristic dynamic response time sr = hmax/bt,
(cf. Johannesson et al., 1989b) of the involved glaciers.
Average mass balance over the considered time interval
is then taken as half the disturbance, assuming linear
adjustment to new equilibrium conditions.
4. Intercomparison of regional glacier length
evolution
Length change measurements of more than 1000
glaciers worldwide were compiled. The here-pre-
sented intercomparison is based on 68 glaciers from
the Swiss glacier network and 90 selected glaciers
worldwide. The Swiss glaciers were treated separately
because of the large sample size with highly variable
glacier characteristics and exceptionally complete
long-term records.
The dynamic response to climatic forcing of gla-
ciers with variable geometry involves striking differ-
ences in the recorded curves (Haeberli, 1994). Such
differences reflect strong effects of size-dependent
filtering, smoothing, and enhancing of the delayed
tongue response with respect to the undelayed input
(mass balance) signal. As a consequence, the some-
times still popular straight averaging of annual length
change data (annual percentage of advancing/retreat-
ing glaciers, average annual length change) destroys
essential aspects of the observed signal and must be
avoided. The sample of Swiss glaciers shows that
length and slope of a glacier constitute the predom-
inant factor controlling glacier tongue reaction (see
Fig. 2).
Fig. 2. Total cumulative length in the Swiss Alps classified after the total length of the glaciers.
M. Hoelzle et al. / Global and Planetary Change 36 (2003) 295–306 299
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For intercomparison purposes, therefore, values of
cumulative length change are presented with respect
to size categories chosen in a way to optimally reflect
common characteristics of the tongue-reaction signal.
Glaciers with heavy debris cover, periodical surge
activity, or calving instability in deep water were
excluded from the analysis because of the strong
non-climatic effects influencing them. Small, some-
what static, low-shear stress glaciers (cirque glaciers)
have altitudinal extents comparable with the interan-
nual variability of equilibrium-line altitude and hence
reflect yearly changes in mass balance practically
Fig. 3. Cumulative length change in different mountain regions of the earth.
M. Hoelzle et al. / Global and Planetary Change 36 (2003) 295–306300
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without any delay (Fig. 2a). Larger, dynamic, high-
stress glaciers (mountain glaciers) react with enhanced
amplitudes but a delay of several years to decadal
fluctuations in climatic and mass-balance forcing (Fig.
2b and c). Large valley glaciers in the Alps give—
with a delay of several decades—strong and most
efficiently smoothed signals of tongue reactions to
secular trends (see Fig. 2d). In fact, long glaciers such
as Grosser Aletschgletscher never had an advancing
period since the 19th century in contrast to smaller
mountain glaciers such as Trient or Oberer Grind-
elwald which show two marked advancing periods in
the 1920s and in the 1970–1980s. The smallest
glaciers like Pizol directly respond to annual mass
balance and snow line variability through deposition/
melting of snow/firn at the glacier margin. Consider-
ing all different types of glacier response obviously
gives the best information on secular, decadal, and
annual developments.
Fig. 3 clearly shows the well-known fact that glacier
retreat in the 20th century is a worldwide phenomenon.
Large glaciers have suffered from the largest absolute
length change measured since 1894. Long glaciers
(>10 km) retreated continuously or remained stationary
except in western Iceland. Glaciers in the size category
of 2 to 10 km show clear decadal reactions. Advance
periods in the 1970–1980s could not only be observed
in the European Alps, but also in the Pamir-Alai, Tien-
Shan, Olympic, and Coast Mountains. Advance ten-
dencies continued into the 1990s for glaciers near the
Norwegian West Coast and in Iceland. This develop-
ment in the North Atlantic appears to parallel a similar
development in the New Zealand Alps and forms a
strong contrast to the European Alps, Rocky Moun-
tains, Coast Mountains, and Cordillera Central where
general retreat in the 1980–1990s is pronounced.
Consideration of the cumulative length change curves
in more detail reveals distinct differences between
evolutions in various mountain ranges at decadal time.
The worldwide glacier signal of climate change seems
to be more or less homogenous at multi-decadal to
secular time scales only.
5. Derived secular mass balances
Reconstructed mass-balance values can be com-
pared much easier than length change because the
complex size effects on flow dynamics are removed to
a certain degree: the direct response to climate forcing
can be considered in a standard format, the mean
annual mass change expressed as an average thickness
change in meters of water equivalent. The following
presents average mass-balance values reconstructed
from multi-decadal to secular length change data of 68
Swiss glaciers and, correspondingly, calculated secu-
lar mass balances of 50 selected glaciers in different
countries of the world.
5.1. Swiss glaciers
Sixty-eight glaciers with their overall length and
mean slope were subdivided into five classes as
follows:
� Class 1 (long and flat valley glaciers, sample: 4
glaciers): glaciers longer than 10 km with a mean
slope of < 15j; glaciers in this class reveal constantretreat since the beginning of the measurements.
� Class 2 (intermediate valley and mountain gla-
ciers, sample: 11 glaciers): glaciers with a length
between 5 and 10 km and a mean slope between
10j and 25j; such glaciers show strong fluctua-
tions with large amplitudes and up to three advance
and retreat periods since 1880.� Class 3 (steep mountain glaciers, sample: 19
glaciers): glaciers with a length between 1 and 5
km and a mean slope ranging from 15j to 25j;these glaciers show moderate fluctuations and
amplitudes but exhibit quite large variability and
strongly individual reaction.� Class 4 (flat mountain glaciers, sample: 14
glaciers): glaciers with a length between 1 and
10 km and a mean slope < 15j; glaciers of this
type underwent weak fluctuations with small
amplitudes but a clear overall retreat.� Class 5: (extremely small and extremely steep
glaciers, sample: 20 glaciers): glaciers shorter than
1 km with a mean slope larger than 15j or with a
length between 1 and 5 km and a mean slope larger
than 25j; glaciers at the extremes of size and slope
show a pronounced high-frequency variability with
moderate to large amplitude.
For all glaciers, individual response times were
calculated and mean specific mass balances for two
M. Hoelzle et al. / Global and Planetary Change 36 (2003) 295–306 301
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different time periods (1850–1996 and around 1880–
1996) were determined according to the above-
described parameterization scheme. The whole proce-
dure was verified by comparing directly measured
mean specific mass balances at the four glaciers
Grosser Aletsch, Rhone, Silvretta, and Gries with
those calculated on the basis of the length change
measurements. The results displayed in Table 1 con-
firm the method and prove that at least reliable order-
of-magnitude estimates can be performed in this way.
Information on the first time period is based on data
from the 1850 glacier inventory as reconstructed and
compiled by Maisch et al. (1999). The second time
period (around 1890) is covered by the direct obser-
vations of the Swiss Glaciological Commission. Table
2 shows that average mass losses of long and flat
glaciers have exceeded those of smaller glaciers:
typical values center around � 0.25 m year� 1 for
larger glaciers and around � 0.11 m year� 1 for the
smaller ones. The main reason for large/flat glaciers to
have higher mass losses may probably be that the
larger thickness limits long-term ice losses to a lesser
degree than in small glaciers where the bed is reached
relatively soon. This result confirms that—other fac-
tors being equal—length and slope exert a predom-
inant influence not only on flow dynamics but also on
overall mass losses of glaciers—an interesting feed-
back between mass balance and flow dynamics over
decadal to secular time scales.
5.2. Glaciers worldwide
On the worldwide database, similar calculations as
for the Swiss glaciers were carried out. Determination
of realistic mass balance gradients in each mountain
region constitutes the most uncertain step in the
procedure. The gradients applied for the 50 glaciers
selected worldwide were estimated by using directly
measured data relating to glaciers in the vicinity or in
the same mountain range. Where such information
Table 1
Comparison of direct measured and from the length change
calculated mean mass balances < b> m year� 1 for time intervals
z the response time of the glaciers
Glaciers Time
period
Mean specific
mass balance
(m year� 1)
Reference
Rhone 1881–1987 � 0.25 Chen and Funk (1990)
� 0.28 calculated from yLGries 1962–1996 � 0.27 direct measurement
� 0.22 calculated from yLSilvretta 1960–1996 � 0.05 direct measurement
� 0.02 calculated from yLGrosser 1920–1996 � 0.22 direct measurement
Aletsch � 0.22 calculated from yL
Table 2
Different mean specific mass balances < b> m year� 1 for the five
classes and for the periods (a) 1850 to 1996 and (b) ca. 1880 to ca.
1996
Table 3
Mean mass balance < b> m year� 1 sorted after four length classes
(since ca. 1900)
Variables Class 1 Class 2 Class 3 Class 4
Total length (km) V 2.5 2.5– V 4.0 4.0– V 8.0 >8.0
Mean specific mass
balance (m year� 1)
� 0.14 � 0.19 � 0.25 � 0.25
Fig. 4. Mean specific mass balance < b> m year� 1 in different
mountain regions (since ca. 1900) calculated from length change
data.
M. Hoelzle et al. / Global and Planetary Change 36 (2003) 295–306302
Page 9
was unavailable, climatic data was used to estimate
characteristic values for dry, continental-type (e.g.
Altai) climatic conditions with gradients between 0.3
and 0.5 m year� 1 per 100 m altitude, transitional
climates (e.g. Caucasus) with gradients between 0.6
and 0.8 m year� 1 per 100 m altitude, and humid,
maritime-type conditions (e.g. Western Norway) with
gradients between 0.9 and 1.2 m year� 1 per 100 m
altitude (cf. IAHS (ICSI)/UNEP/UNESCO, 1991,
1993a,b, 1994, 1996, 1999, 2001; Oerlemans and
Fortuin, 1992; Oerlemans and Hoogendoorn, 1989).
The results of the parameterization confirm the
trend observed in the sample from the Swiss Alps
for smaller glaciers to have lost mass at a slower rate
than larger ones (Table 3). On average of the world-
wide sample, larger glaciers have lost around � 0.25
m year� 1, a value which is identical to the value
calculated for the larger Swiss glaciers. The recon-
structed rates of secular mass losses strongly differ
between humid-maritime-type glaciers such as those
of western Scandinavia and dry-continental type gla-
ciers in the Altai area, for instance (Figs. 4 and 5).
This primarily results from the choice of the mass
balance gradient in the calculation. The climatic
dependence of the chosen gradients, however, is a
well-established fact (Oerlemans and Hoogendoorn,
1989; Oerlemans, 2001) and must certainly be con-
sidered to be realistic, even though absolute values are
somewhat uncertain. The sensitivity with respect to
secular trends in global warming of maritime-type
glaciers is much higher than the one of continental-
type glaciers.
6. Discussion
The study presented here clearly shows that for
direct intercomparisons of cumulative glacier length
changes, shorter time scales and high temporal meas-
urements are necessary. Especially, to derive the
annual (very small glaciers) or decadal (medium-
sized glaciers) fluctuations, such measurements have
to be done. The high temporal measurements in the
Alps and in Scandinavia are good examples. New
technologies like satellites offer new possibilities to
derive in the future long-term length changes, espe-
cially for deriving secular trends in mean mass
balances. The concept of Johannesson et al. (1989a,b)
presents the possibility to roughly estimate secular
mass balance changes by using length change measure-
ments. This means that length change measurements
are, for the future, one of the most important key
variables in global glacier monitoring strategies. The
secular mass loss is a worldwide phenomenon in the
period since 1850. Future changes will affect firstly the
maritime ones and then, with a certain delay, the
continental ones, which are mostly of polythermal or
cold stage.
7. Conclusions and recommendations
In addition to mass balance, this study shows that
length observations of a representative subset of the
world glaciers are and will be, in the future, a very
valuable key factor, among others, for assessing
climate change effects at regional or worldwide scale
(Haeberli, 1998). In the strategy of the Global Terres-
trial Network for Glaciers (GTNet-G) within the
Global Climate Observing System (GCOS)/Global
Terrestrial Observing System (GTOS), long-term
observations of glacier length change data at a mini-
mum of about 10 sites within each mountain range are
attributed highest priority. These glaciers should be
selected according to size and dynamic response from
the existing set of sites where glacier length is
monitored. At this level, spatial representativeness is
very important. Today, approximately 800 glaciers
where only length is measured are compatible with
Tier 4 of the GTNet-G-strategy. Because access is
infrequent, they can be located wherever necessary to
ensure representativeness.Fig. 5. Mean specific mass balance < b> m year� 1 classified after
different climate types (since ca. 1900).
M. Hoelzle et al. / Global and Planetary Change 36 (2003) 295–306 303
Page 10
Acknowledgements
We would like to thank M. Maisch for providing us
with the length data of 1850 and G.H. Gudmundsson
for many interesting discussions about this topic. Very
much appreciated and helpful were the very con-
structive comments of L. Braun and J.O. Hagen.
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