Réf. UNEP 2007 - R

Référence bibliographique complète
Global outlook for ice and snow. UNEP: United Nations Environment Programme. Nairobi (Kenya), Birkeland Trykkerri, Birkeland, Norway, 2007, 235 p.

Glaciers, snow cover, climate change, evolution, assessment, world.

Organismes / Contact
Division of Early Warning and Assessment (DEWA), United Nations Environment Programme
P.O. Box 30552, Nairobi 00100, Kenya
Tel: (+254) 20 7623562. dewa.director@unep.org.

Principaux rapports scientifiques sur lesquels s'est appuyé le rapport
See references below

(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
  Snow cover, Glaciers Glacial hazards, Debris flows, Mass movements, Forest fires  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Période(s) d'observation
World Alps        

(1) - Modifications des paramètres atmosphériques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(2) - Effets du changement climatique sur le milieu naturel
General warming during the transition from the Late Glacial period (between about 21 000 and 10 000 years ago) to the early Holocene (about 10 000 to 6000 years ago) led to a drastic general glacier retreat with intermittent periods of re-advances. About 11 000 to 10 000 years ago, this pronounced warming reduced the glaciers in most mountain areas to sizes comparable with conditions at the end of the 20th century (Grove, 2004). In northern Europe and western North America, which were still influenced by the remnants of the great ice sheets, this process was delayed until about 6000 to 4000 years ago. Several early-Holocene re-advances, especially those in the North Atlantic and North Pacific as well as possibly in the Alps, cluster around an event about 8000 years ago, and were likely triggered by changes in the ocean thermohaline circulation and subsequent cooling resulting from the outbursts of Lake Agassiz (Solomina et al., 2007).

On a timescale of hundreds of years there were periods of synchronous glacier advance around the world peaking in the late Holocene in the Northern Hemisphere. The moraines that were formed during the so-called Little Ice Age (from the early 14th to the mid 19th centuries) mark a Holocene maximum extent of glaciers in many regions of the world, although the time period for this maximum varies among the different regions. Glaciers in the European Alps reached their recent maximum extent around 1850 (Maisch et al., 2000; Gross, 1987; Holzhauser & Zumbühl, 2003).

There has been a general retreat of glaciers worldwide since their Holocene maximum extent towards the end of the Little Ice Age, between the 17th and the second half of the 19th century, with intermittent periods of glacier re-advance in certain regions.
Over the past hundred years a trend of dramatic shrinking is apparent over the entire globe, especially at lower elevations and latitudes.Within this general trend, strong glacier retreat is observed in the 1930s and 1940s, followed by static conditions around the 1970s and by increasing rates of glacier wasting after the mid 1980s. There are short-term regional deviations from this general trend and intermittent re-advances of glaciers in various mountain ranges occurred at different times.

Thirty reference glaciers with almost continuous mass balance measurements since 1975 show an average annual mass loss of 0.58 m water equivalent for the past decade (1996-2005), which is more than twice the loss rate of the period 1986-1995 (0.25 m), and more than four times the rate of the period 1976-1985 (0.14 m). The results from these 30 continuous mass balance series correspond well to estimates based on a larger sample of more than 300 glaciers, including short and discontinuous series (Kaser et al., 2006).

In the European Alps, the overall area loss since 1850 is estimated to be about 35 % until the 1970s, when the glaciers covered a total area of 2 909 km2, and almost 50 % by 2000. Total ice volumes in 1850, the 1970s and 2000 are estimated to be about 200 km3, 100 km3 and 75 km3, respectively (Zemp et al., 2006). Observations show intermittent glacier re-advances in the 1890s, 1920s and 1970-1980s (Pelfini & Smiraglia, 1988; Zemp et al., 2007; Patzelt, 1985). After 1985 an acceleration in glacial retreat has been observed, culminating in an annual ice loss of 5-10 % of the remaining ice volume in the extraordinarily warm year of 2003 (Zemp et al., 2005). The strong warming has made disintegration and downwasting increasingly predominant processes of glacier decline during the most recent past (Paul et al., 2004).

Snow cover
Data from satellite monitoring from 1966 to 2005 show that mean monthly snow-cover extent in the Northern Hemisphere is decreasing at a rate of 1.3 per cent per decade. For the calendar year of 2006 average snow-cover extent was 24.9 million km2, which is 0.6 million km2 less than the 37-year average. In the Northern Hemisphere, spring and summer show the strongest decreases in snow-cover extent. Satellite observations of snow-cover extent show a decreasing trend in the Northern Hemisphere for every month except November and December, with the most significant decreasing trends during May to August (Brodzik et al., 2006). The average Northern Hemisphere snow-cover extent for March and April decreased by 7.5 ± 3.5 % from 1922-2005 (Lemke et al., 2007).
Low-latitude mountain chains like the European Alps or the Southern Alps of New Zealand, where glaciers are typically medium-sized and found in quite steep mountains, will experience rapid glacier changes in adaptation to the modified climate. A modelling study shows that the European Alps would lose about 80 % of their glacier cover should summer air temperatures rise by 3°C, and that a precipitation increase of 25 % for each 1°C would be needed to offset the glacial loss (Zemp et al., 2006).

Snow cover

Climate models project significant decreases in snow cover by the end of this century, with reductions of 60 to 80 % in snow water equivalent (depth of water resulting from snow melt) in most mid-latitude regions. The largest decreases are projected over Europe, while simulated increases are seen in the Canadian Arctic and Siberia. Climate model projections indicate that the Alps and Pyrenees will experience warmer winters with possible increases in precipitation (Marinucci et al., 1995), which will raise snow lines, reduce overall snow cover, and decrease summer runoff.
According to climate scenarios for the end of the 21st century, changes in global temperature and precipitation range between +1.1 to +6.4 °C and 30 to +30 %, respectively (IPCC, 2007). Such an increase in mean air temperature will continue the already dramatic glacier changes. Cold continental-type glaciers will react in the first instance with a warming of the ice and firn temperatures, whereas glaciers with ice temperatures at the melting point will have to convert the additional energy directly into melting (Oerlemans, 2001; Kuhn, 1981).

Snow cover
It is estimated that the snow line of the European Alps will rise about 150 m for every 1.0°C increase in winter temperatures (Beniston, 2003). Snow cover in mountain regions is a critical source of freshwater; changes in snow cover could have indirect effects on ecosystems due to changes in availability of these water resources. One potential effect is increased intensity and size of wildfires because of moisture stress on mountain forests.

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(3) - Effets du changement climatique sur l'aléa
Atmospheric warming has an increasingly dramatic effect on mountain glaciers, and strongly influences the development of related hazards. For example, potentially unstable glacial lakes often form in glacier forefields dammed by frontal moraines which were left behind by retreating glaciers. Steep slopes of unconsolidated debris are a potential source for debris flows when they are no longer covered by glacier ice or cemented by ground ice. Fresh ice break-off zones may evolve in new places from glacier retreat, while existing danger zones may cease to be active.

Atmospheric warming also affects permafrost thickness and distribution. The thickness of the active layer may increase, and the magnitude and frequency of rockfalls may increase or evolve at locations where such events were historically unknown. Lateral rockwalls can be destabilized by glacier retreat as a result of the stress changes induced. In general, climate change is expected to bring about a shift of the cryospheric hazard zones. It is difficult, however, to ascertain whether the frequency and/or magnitude of events have actually increased already as a consequence of recent warming trends. Nevertheless, events with no historical precedence do already occur and must also be faced in the future (Huggel et al., 2004; Watson & Haeberli, 2004).

On slopes, vegetation and soils take decades and even centuries or sometimes millennia to follow the retreating ice and cover the newly exposed terrain (Jones &Henry, 2003). As a consequence, the zones of bare rock and loose debris will expand. Vegetation (especially forests) and ice both have a stabilizing effect on steeply inclined surfaces. During the expected long transitional period between glacier vanishing and forest immigration, erosion (including large debris flows) and instability (including large rockfalls and landslides) on slopes unprotected by ice or forest will increase substantially (Hinderer, 2001).

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

(4) - Remarques générales

(5) - Préconisations et recomandations
Destinataires et portée du rapport
The Global Outlook for Ice and Snow will serve as a reference publication in the debate, contribute to effective decision-making and ultimately the action so urgently needed.
Types de recommandations et / ou préconisations  

Références citées :

Beniston, M. (2003). Climatic change in mountain regions: a review of possible impacts. Climatic Change , 59, 5-31 - [Fiche biblio]

Brodzik, M.J., Armstrong, R.L., Weatherhead, E.C., Savoie, M.H., Knowles, K.W. and Robinson, D.A. (2006). Regional trend analysis of satellite-derived snow extent and global temperature anomalies. In: American Geophysical Union Fall 2006, San Francisco, CA, USA

Gross, G. (1987). Der Flächenverlust der Gletscher in Österreich 185019201969. Zeitschrift für Gletscherkunde und Glazialgeologie , 23(2), 131141

Grove, J.M. (2004). Little Ice Ages: Ancient and modern . Routledge, London and New York

Hinderer, M., (2001). Late quaternary denudation of the Alps, valley and lake fillings and modern river loads. Geodinamica Acta , 14, 231263

Holzhauser, H.P. and Zumbühl, H.J. (2003). Nacheiszeitliche Gletscherschwankungen. Sonderdruck zum 54. Geographentag Bern, Hydrologischer Atlas der Schweiz, Tafel 3.8

Huggel, C., Haeberli, W., Kääb, A., Bieri, D. and Richardson, S. (2004a). Assessment procedures for glacial hazards in the Swiss Alps. Canadian Geotechnical Journal , 41(6), 10681083

IPCC (2007). Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (eds. S. Solomon, D. Qin, M. Manning, Z. Chen, M.C. Marquis, K. Averyt, M. Tignor and H.L. Miller). Intergovernmental Panel on Climate Change, Cambridge and New York

Jones, G. A. and Henry, G.H.R. (2003). Primary plant succession on recently deglaciated terrain in the Canadian High Arctic. Journal of Biogeography , 30(2), 277296

Kaser, G., Cogley, J.G., Dyurgerov, M.B., Meier, M.F. and Ohmura, A. (2006). Mass balance of glaciers and ice caps: Consensus estimates for 19612004. Geophysical Research Letters , 33(L19501), doi. 10.1029/2006GL027511

Kuhn, M. (1981). Climate and glaciers. IAHS , 131, 320

Lemke, P., Ren, J., Alley, R., Allison, I., Carrasco, J., Flato, G., Fujii, Y., Kaser, G., Mote, P., Thomas, R. and Zhang, T. (2007). Chapter 4: Observations: Changes in Snow, Ice and Frozen Ground. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (eds. S. Solomon, D. Qin, M. Manning, Z. Chen, M.C. Marquis, K. Averyt, M. Tignor and H.L. Miller). Intergovernmental Panel on Climate Change, Cambridge and New York

Maisch, M., Wipf, A., Denneler, B., Battaglia, J. and Benz, C. (2000). Die Gletscher der Schweizer Alpen. Gletscherhochstand 1850, Aktuelle Vergletscherung, Gletscherschwund Szenarien . Schlussbericht NFP31. 2. Auflage, VdF Hochschulverlag, Zürich

Marinucci, M.R., Giorgi, F., Beniston, M., Wild, M., Tschuck, P., Ohmura, A. and Bernasconi, A. (1995). High-resolution simulations of January and July climate over the western alpine region with a nested regional modeling system. Theoretical and Applied Climatology , 51, 119-138

Oerlemans, J. (2001). Glaciers and climate change . A.A. Balkema Publishers, Lisse

Paul, F., Kääb, A., Maisch, M., Kellenberger, T. and Haeberli, W. (2004). Rapid disintegration of Alpine glaciers observed with satellite data. Geophysical Research Letters , 31(L21402), doi:10.1029/ 2004GL020816

Patzelt, G. (1985). The period of glacier advances in the Alps, 1965 to 1980. Zeitschrift für Gletscherkunde und Glazialgeologie , 21, 403407

Pelfini, M. and Smiraglia, C. (1988). L'evoluzione recente del glacialismo sulle Alpi Italiani: strumenti e temi di ricerca. Bollettino della Societa Geografica Italiana , 13, 127 154

Solomina, O., Haeberli, W., Kull, C. and Wiles, G. (2007). Historical and Holocene glacier-climate variations: General concepts and overview. In print

Watson, R.T. and Haeberli, W. (2004). Environmental threats, mitigation strategies and high-mountain areas. Royal Colloquium: Mountain Areas a Global Resource Ambio Special Report , 13, 210

Zemp, M., Frauenfelder, R., Haeberli, W. and Hoelzle, M. (2005). Worldwide glacier mass balance measurements: General trends and first results of the extraordinary year 2003 in Central Europe. Data of Glaciological Studies , 99, 312

Zemp, M., Haeberli, W., Hoelzle, M. and Paul, F. (2006), Alpine glaciers to disappear within decades? Geophysical Research Letters , 33(L13504), doi:10.1029/2006GL026319 - [Fiche biblio]

Zemp, M., Paul, F., Hoelzle, M. and Haeberli, W. (2007). Glacier fluctuations in the European Alps 18502000: An overview and spatio- temporal analysis of available data. In The darkening peaks: Glacial retreat in scientific and social context (eds. B. Orlove, E. Wiegandt and B. Luckman). In print