Secular glacier mass balances derived from cumulative ...hoelzle/hoelzleetal2003a.pdf · Secular...

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

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 is

taken as yb/2. The so-obtained value 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

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

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


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.


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.


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

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 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 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 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 m year� 1 in different

mountain regions (since ca. 1900) calculated from length change

data.

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 m year� 1 classified after

different climate types (since ca. 1900).

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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