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Tropical glaciers are intriguing and apparently rapidly disappearing components of the cryosphere that literally crown a vast ecosystem of global significance. Half of the Earth’s surface area lies between the tropics of Capricorn and Cancer, wherein a staggering 75% of the global population resides (Thompson 2000). Tropical glaciers are highly sensitive to climate changes over different temporal and spatial scales, notably ENSO, and are important hydrological resources in tropical highlands (Francou et al. 1995; 2000; this volume; Wagnon et al. 2001; Kaser and Osmaston 2002). Moreover, resolving the complex dynamics and variability of the tropical climate over longer time periods presents important goals to the global modelling community. Compiling an accurate understanding of the timing and climate response of tropical glaciers in the past is a crucial source of palaeoclimatic information for the validation and comparison of climate models (e.g. Farrera et al. 1999; Hostetier and Clark 2000; Porter 2001; Harrison et al. 2002; Seltzer et al. 2002). Deciphering the relative strength of different climatic forcing mechanisms on tropical glacier behaviour and quantifying hydrological changes associated with glacier recession are therefore relevant to interpreting the past climate and predicting the impact of future climate changes. Much scientific, social and political attention now concerns future changes in climate, with temperature change predominant.

The IPA Circum-Polar Permafrost Map (Brown et al. 1997) shows discontinuous and sporadic permafrost in the mountains of Europe, including Scandinavia, the Alps, the Pyrenees, and further east in the Urals. In general, the lower altitudinal limit of mountain permafrost increases with decreasing latitude, from sea level in Svalbard, to around 1500 m in Southern Norway, to above 2500 m in the southern Swiss Alps. Many of these low-latitude mountain regions have permafrost temperatures that are only a few degrees below zero, so that a slight shift in energy flux at the ground surface is likely to cause a significant increase in the depth of summer thawing and, in consequence, widespread permafrost degradation. Where permafrost is ice-rich, degradation caused by global warming is likely to be associated with increased magnitude and frequency of mountain slope instability (Harris et al. 2001a). Traditional landslide hazard assessment approaches, based on forward projection of historical data on distribution and magnitude-frequency relationships (Varnes 1984), may therefore become increasingly inappropriate if climate change leads to a significant change in the thresholds of processes within the permafrost geomorphic system. In this paper, approaches to the assessment of geotechnical hazards associated with mountain permafrost in a warming climate are outlined in the context of recent European collaborative research. A critical first stage is the early detection of permafrost responses to climate change through integrated monitoring systems.

Glacier- and permafrost-related hazards represent a continuous threat to human lives and infrastructure in high mountain regions. Related disasters can kill hundreds or even thousands of people at once and cause damage with a global sum on the order of 10 Euro annually. Glacier and permafrost hazards in high mountains include:

A better understanding of the potential effects of climate change on snow cover is critical, considering the far-reaching environmental and socio-economic implications on water resources, winter tourism, ecology as well as local changes in climate. The snow coverage of the French Alps depends on weather conditions in a rather complex way. For a given winter, snow cover is the consequence of the various meteorological events encountered (frequency and intensity of snowfall events, atmospheric circulation patterns, cold and warm periods). Simple relationships between snow cover and averaged climate variables, such as mean temperature and precipitation, can therefore not adequately explain interannual variability of snow cover. Models can be used to gain a better understanding of the complex interactions between different climate variables and their effects on snow cover. In addition, models are of great interest for the assessment of potential climatic change impacts.

Glaciers are characteristic features of mountain environments but are often not recognized for their strong influence on catchment runoff quantity and distribution. Such modification occurs with glacierization of only a few percent of the total catchment area, and affects adjacent lowlands far beyond the limits of mountain ranges. The main impact occurs because glaciers temporarily store water as snow and ice on many different time scales (Jansson et al. 2003), the release from storage being controlled by both climate and internal drainage mechanisms.

More than half of the accessible freshwater is used directly or indirectly by humankind, and much of this precious resource has its origin in mountainous regions, ultimately in the form of orographic precipitation. In many areas, mountains function as “water towers” for the surrounding regions. Melt from snow cover and glaciers represents an important contribution to runoff in the surrounding areas, especially during seasons when precipitation is sparse or completely absent. Mountain freshwater resources are heavily utilized for agricultural purposes (e.g. irrigation) and for the generation of hydropower, thus being of great socio-economic importance. Yet, heavy orographic precipitation events also represent a potential hazard, as they may lead to floods, avalanches and mudslides that often cause countless loss of life and tremendous damage. The potential consequences of such events may be extreme. For instance, a single catastrophic mudslide event that took place in Venezuela on December 15, 1999, is estimated to have caused more than 20,000 casualties according to re-insurance estimates.

Mountain ecosystems of the western United States are complex, and include cold desert biomes, such as those found in Nevada, subpolar biomes found in the upper treeline zone, and tundra ecosystems, occurring above timberline. Many studies (e.g. Thompson 2000) suggest that high elevation environments, comprising glaciers, snow, permafrost, water, and the uppermost limits of vegetation and other complex life forms are among the most sensitive to climatic changes occurring on a global scale. The stratified, elevationally-controlled vegetation belts found on mountain slopes represent an analogue for the different latitudinally-controlled climatic zones, but these condensed vertical gradients are capable of producing unique hotspots of biodiversity, such as those that serve as habitat for a variety of species ranging from butterflies, frogs and toads, to species of birds, trout and salmon. High relief and high gradients make mountain ecosystems very vulnerable to slight changes of temperatures and to extreme precipitation events (Parmesan 1999; Pounds et al. 1999).

In most alpine regions, the presence of snow controls the hydro-climatic situation over a great part of the year. The delayed and long-lasting process of snowmelt guarantees a relatively well-balanced discharge regime of rivers in the spring and summer melting season, even if only a small part of their catchment includes high mountain areas. For the typical alpine weather conditions, this results in high melt water runoff during dry conditions when net radiation and air temperature are high, while, during cooler periods, rainfall compensates for reduced or discontinued melt rates and sustains streamflow at a balanced level. Furthermore, because of the relatively high albedo of snow, changes in alpine snowcover are associated with a feedback to climate, a process that has not yet been very well investigated. For example, a climate-induced decrease in snowcover will reduce surface albedo, which leads to an amplification of the initial warming.

Runoff generation in mountain catchments is one of the most complex hydrological processes. It is highly variable in space and time, depending on the combination of three main controlling factors: (1) climate, (2) soil and geology, and (3) vegetation. The different combinations of these three factors determine the water balance of landscape units, including soil moisture dynamics, evapotranspiration and runoff generation. When assessing runoff generation, not only the runoff amounts need to be considered, but also the relative streamflow contributions of surface and subsurface runoff, which may differ considerably between areas (Buttle 1998). An overview of runoff mechanisms and components in different environments is given in Uhlenbrook and Leibundgut (1997) and Bonell (1998). The main focus of this paper is on subsurface stormflow, the least understood flow component.

Global change will influence hillslope hydrological processes for a variety of reasons. On the one hand, climate change might alter the hydrological input, i.e. precipitation and snow melt, which might cause an increase or decrease in the intensity of specific hillslope processes. For instance, overland flow might be amplified by increased rain intensities (Horton 1933) or by reduced infiltration due to surface crusts (Yair 1990) or increased hydrophobicity (Doerr et al. 2002), triggered by longer and more pronounced drought periods. However, overland flow could also be significantly influenced by antecedent moisture conditions of the substrate that were either altered due to wetter climate and reduced evapotranspiration at a site or due to different snow and snow melt regimes, changing the hydrological input for a specific precipitation event. On the other hand, global change in the form of land use changes will play a key role in defining the dominant runoff generation processes on hillslopes (cf. summary given in DVWK 1999).