Glacier signals are very confident temperature indicators since they filter out the large year-to-year fluctuations of the regional climate. Changes of the glaciers reflect predominantly the slowly varying signals of long-term trends. Glacier signals are a valuable element of early detection of man-induced climate change since the time scales are similar.

The volume of ice in a glacier - and correspondingly its surface area, thickness, and length - is determined by the balance between inputs (accumulation of snow and ice) and outputs (melting and calving). As climate changes, the balance between inputs and outputs may change, resulting in a change in thickness and advancement or retreat of the glacier. Temperature, precipitation, humidity, wind speed, and other factors such as slope and the reflectivity of the glacier surface all affect the balance between inputs and outputs. Most glaciers in the world, however, are more sensitive to temperature than to other climatic factors.

Glacial ice can range in age from several thousands to millions of years, making it an important climate archive. To see long-term climate records, ice cores have been extracted from glaciers around the world, including Peru, Canada, Greenland, Antarctica, Europe, and Asia. Scientists analyze various components of cores, particularly trapped air bubbles, which reveal past atmospheric composition (e.g. of greenhouse gases), temperature variations, and types of vegetation.

Glaciers are retreating in many mountain areas of the world. Since 1850 the glaciers of the European Alps have lost about 30 to 40% of their surface area and about half of their volume. However, the range of extremes observed on individual glaciers is roughly one order of magnitude higher than the mean value of length or mass changes. The 1991 discovery of the 5,000 year-old "ice man," preserved in a glacier in the European Alps, fascinated the world. But this also points to the fact that this glacier is retreating farther now than it has in 5,000 years.

Fig. 1.: Mean mass balance (m/a) in different mountain regions (since ca. 1900) calculated from length change data (Source: Hölzle, M., Dischl, M., & Frauenfelder, R. (2000). Weltweite Gletscherbeobachtung als Indikator der globalen Klimaänderung. Vierteljahresschrift der Naturforschenden Gesellschaft in Zürich 145/1: 5-12.)

Glacier shrinkage seem to accelerate during the last years. Continuous mass balance records for the period 1980-1997 are now available for 32 glaciers. The corresponding results of this sample from glaciers in North America and Eurasia can be summarized as follows:

Year	1980-1995	1995/96	1996/97
mean (annual) net balance:	- 258 mm	- 378 mm	- 478 mm

Taking the two last years together, the mean mass balance was negative by 428 mm. The calculated mean is almost two-thirds higher than the average 1980-1995. The rate of glacier melt in the northern hemisphere, thus, clearly accelerated during the two years reported. The mean specific net balance (-306 mm) for the seven years 1991-1997 is markedly higher than the decadal mean of 1980-1990 (-259 mm). The difference corresponds to an increase in additional energy flux of about 0.05 W/m2 per year.

The mean of all 32 glaciers is strongly influenced by the great number of Alpine and Scandinavian glaciers. However, considering the mean balance of the 10 mountain ranges involved the result is similar: -350 mm for the seven-year period of 1991-1997 are clearly higher than the mean of 1980-1990 (-317 mm).

Fig. 2.:Mean cummulative specific net balance for 32 glaciers of 10 mountain ranges in the northern hemisphere. The mean mass change is around 0.3 m/a. (Source: Hölzle, M., Dischl, M., & Frauenfelder, R. (2000). Weltweite Gletscherbeobachtung als Indikator der globalen Klimaänderung. Vierteljahresschrift der Naturforschenden Gesellschaft in Zürich 145/1: 5-12.)

The shrinkage of mountain glaciers during the 20th century reflects the rapid secular change in the energy balance of the earth's surface taking place on a global scale. The characteristic rate of this change (a few decimeters ice depth per year) as deduced from glacier mass losses reflects an additional energy flux which is broadly consistent with the estimated anthropogenic greenhouse forcing. The beginning of this rapid glacier retreat tendency was probably little affected by human activity. The observed evolution may, however, include an increasing part of anthropogenic influence. The glaciers appear to evolve at a high rate of change towards or even beyond the "warm" limit of natural holocene (=actual warm period after last ice age) glacier variability.

The shrinking of glaciers will likely have a significant socioeconomic impact in some mountain regions, though the exact local impacts remain uncertain and will vary.

• Regions that lose major parts of their glacier cover will experience alterations in hydrology.
• The glaciers will initially provide extra runoff from melting; but as the ice diminishes, the runoff will wane.
• Because revegetation of terrain is slower at high altitudes, deglaciated areas will be subject to erosion and decreased stability, heightening the need to protect buildings, roads, communication links, and other structures.
• For areas dependent on tourism, uncertain snow cover during peak winter sports seasons, natural hazards such as rock and ice falls, or loss of scenic beauty (e.g. the Bosson glacier in the Mont-Blanc region, which is a characteristic feature in the panoramic view of Chamonix) are of particular concern.

An example of the type of scenario that could become more frequent occurred during the warm summer of 1998, when a ski area in the Tyrolian Alps was forced to close a lift after melting ice dislodged rocks and soil, destabilizing the peak.

Additional informations on future impacts of glacier shrinkage can be found on: Climate Change Impacts - an interactive informationsystem on the regional impacts of climate change

Chamonix - Mont Blanc, Besson Glacier 1818


Chamonix - Mont Blanc, Besson Glacier 1928


Chamonix - Mont Blanc, Besson Glacier today
Image copyright by: History of Mont-Blanc Bosson glacier

Sources:
World Glacial Monitoring Service
Glacier Mass Balance Bulletin, No. 5, 1999. Haeberli W., M. Hoelzle and R Frauenfelder (ed.), Department of Geography, University of Zurich - IAHS (ICSI) / UNEP / UNESCO
Haeberli, W. and M.Beniston, 1998: Climate change and its impacts on glaciers and permafrost in the Alps. Ambio 27/4, 258-265.
GECR, 1998. Melting glacier destabilizes Austrian peak. Global Environmental Change Report, vol. X, no. 22, p. 6.
Union of Concerned Scientists, Climatehotmap
U.S. National Snow and Ice Data Center (NSIDC)
Cebon, P., U. Dahinden, H.C. Davies, D. Imboden, and C. Jaeger (ed.), 1998: Views from the Alps - Regional Perspectives on Climate Change. MIT Press, Cambridge, Massachussetts, 515 p.
Intergovernmental Panel on Climate Change, IPCC, Second Assessment Report, 1995, The Global Climate System Review, December 1993-May 1996, WMO 856