Georg Kaser

Tropical Glaciology Group, Institute of Geography, Innrain 52, 6020 Innsbruck, Austria

53.1 Introduction

A glacier forms when the accumulation of ice exceeds its loss over a time span longer than a few years. The appropriate climatic conditions are driven by the general regional free atmosphere conditions but are also affected by the local topography (e.g. Oerlemans, 2001). A glacier's extent is determined not only by climate: glacier-bed conditions also influence the glacier's geometry. Whereas in the highest latitudes climate allows glaciers to cover land up to continental scale and to reach sea level, in lower latitudes glaciers occur only on mountains. The warmer the climate, the higher, generally, are the lowest limits of glaciers. A very rough approximation is that glaciers rise by approximately 1000 m in elevation for each 1000 km in horizontal distance from polar and subpolar regions toward the Equator. Consequently, mountain glaciers cover considerably less area than land ice masses at high latitudes. Because of their small extent, mountain glaciers are rather sensitive to climate variations and changes. They are also 'closest' to human activities and, therefore, attract public and scientific interest:

1 Since human beings first started to extend their activities into the high mountain areas, glaciers have been in their sphere of interest. Since the finding of the Ice Man in the Otztal Alps we know that this has occurred for more than 5000 yr (Bortenschlager & Oggl, 2000).

2 The variation of mountain glaciers affects the appearance of a landscape in a way that is recognizable within a human's lifetime. Tourism in high mountains is, to a certain extent, also encouraged by glacier landscapes. Recent years of strong glacier melt in the European Alps have affected glacier ski resorts that now seek strategies to protect glaciers from too much loss.

3 In many mountain regions, glaciers substantially control the availability of fresh water. Their growth and shrinkage can have a dramatic impact on the economic, social and cultural activities close to mountains.

4 In conjunction with tectonic and geomorphological conditions of mountain environments, glacier variations can cause hazards. It was, actually, the threat from an ice-dammed lake to which we owe the very first painting of a glacier, the Vernagtferner, damming the runoff from Hintereisferner and Hochjochferner in the Austrian Otztal Alps in 1601 (Nicolussi, 1990).

5 Since the end of the Little Ice Age, mountain glaciers have lost a considerable amount of mass, which is considered to have contributed significantly to the observed 20th century sea-level rise (e.g. Warrick et al., 1996).

The interest in mountain glaciers and in their fluctuations focuses partly on their impact on the world at lower elevations, but also on the driving forces behind observed changes. Mountain glaciers appear to be sensitive indicators of climate and are comparatively well distributed over the globe. Among climate proxies, glaciers are the only ones that exclusively follow physical laws and these laws are the same, no matter where the glaciers are. Furthermore, these laws are well known and understood, although their application for analytical or numerical solutions in order to describe glacier-climate interaction is complex and calls for a variety of simplifications. Among the advantages of using glaciers as climate indicators is the narrow band of climate conditions in which glaciers exist. Several mechanical and thermodynamic properties, such as glacier ice being impermeable to water and the upper limit of temperature at the pressure melting point, are helpful. Others, such as being partially transparent to solar radiation and the particular way in which glaciers create their own atmospheric boundary layer, are unhelpful to simple solutions for the description of the behaviour of glaciers. In the following, the potential of understanding mountain glaciers as products of a complex climate and of their impacts on human interests, mainly for water supply, is outlined.

Most of today's mountain glaciers are considered to be the products of the Little Ice Age climate. They have grown within a few centuries and they may again diminish or even vanish within similar time periods (e.g. Cogley & Adams, 1998). On geological time-scales they are short-lived phenomena and they have grown and diminished several times throughout the Holocene (e.g. Hormes et al., 2001). Still, as already mentioned, their short-term variation is evident on a human scale, especially as mountain regions occupy about one-fifth of the Earth's surface and provide goods and services to about half of humanity (Messerli & Ives, 1997).

53.2 Mass balance on mountain glaciers

Climate and, in many cases, mechanical processes drive the mass balance of a glacier. The resulting and permanently changing storage of ice, snow and water can be seen at different time-scales (Jansson et al., 2003). The long-term storage of ice and firn on time-scales of 100-102yr and longer determines the extent of glaciers: their volume, length and geometry. This relates to the general character of the hydrological regime of the corresponding catchment areas, and affects global sea-level changes. The seasonal variations of snow and water masses can be seen as intermediate-term storage. Glacier mass balance studies usually investigate the intermediate-term storage and provide net variations over monthly to 1-yr time-steps in order to relate them to seasonal climate. Seasonal runoff from glacierized basins is characterized by intermediate-term storage changes. Short-term storage concerns diurnal processes of melt-water production and drainage. Individual storage releases such as drainage from glacier surges and drainage of glacier-dammed water are catastrophic events in many cases.

In a long to intermediate-term, climatological, view, only the storage of firn and ice is of concern. It increases mainly from solid precipitation, which, to a varying extent, is redistributed by drift and avalanches. Mass loss is most effective from melt-water runoff but can be also due to sublimation, which increases with dryness of the atmospheric conditions. Calving and avalanches can contribute to the ablation as well. When mechanical processes such as avalanches and calving are of minor importance for a given mountain glacier, the mass balance can be related directly to weather and climate. In most monitoring and research programmes, attention is focused on the mass balance on the surface of the glacier. Mass changes at the base of a glacier are comparatively small and the refreezing of melt water is usually limited to parts of a glacier and to subseasonal time periods. Basal mass change is only of concern in areas of extraordinarily high geo-thermal heat. For example, the growth and sudden outburst of subglacial water storages as jokullhlaups in Iceland are the result of marked basal melt (e.g. Bjornsson, 2002). Internal accumulation occurs when melt water from the surface penetrates into the firn and refreezes in layers older than from the ongoing year. It is difficult to measure but, particularly at high latitudes where the winter cold wave penetrates into lower and older firn layers, internal accumulation is thought to contribute considerably to the mass balance of glaciers (e.g.Trabant & March, 1999).When snow starts to melt on a glacier tongue during the early ablation season, melt water can also refreeze on the previous summer's glacier-ice surface. Still, this superimposed ice is usually removed during the summer and only in positive mass balance years may some survive in a small area on which winter snow is not removed from the previous year's ice surface.

The surface mass balance of a mountain glacier is different in intensity, timing and effect on different parts of a glacier, but the variation of mass balance with elevation clearly dominates (e.g. Klok & Oerlemans, 2002). Annual net accumulation is highest in the higher parts of a glacier and decreases toward lower elevations until net ablation occurs and increases down-glacier. Drift removal of snow usually leads to little accumulation on the uppermost flanks of a mountain glacier, whereas shading and drift snow deposition may cause smaller ablation amounts on the lowest tips of the tongues. In many cases, glacier tongues are covered by debris that can increase or decrease ablation depending on the type of debris and its thickness. Ablation underneath a debris cover is difficult to measure and monitor and model approaches are used to determine ablation (e.g. Nakawo et al., 2000). The so-called vertical balance profile, VBP, is the mass balance per unit area or specific mass balance (in kgm-2 or metres water equiva

Figure 53.1 Spatial distribution of annual specific mass balance (kgm 2) on Hintereisferner, Austrian Alps in 1984 as an example. (Kindly provided by G. Markl and M. Kuhn, Institute of Meteorology and Geophysics, University of Innsbruck.)

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Figure 53.1 Spatial distribution of annual specific mass balance (kgm 2) on Hintereisferner, Austrian Alps in 1984 as an example. (Kindly provided by G. Markl and M. Kuhn, Institute of Meteorology and Geophysics, University of Innsbruck.)

lent, m we) changing with elevation. It is best obtained from direct glaciological mass balance measurements carried out with a well-distributed network of ablation stakes and snow pits (e.g. Fountain et al., 1999) (Fig. 53.1). Traditional techniques as well as more recent high-resolution geodetic techniques also yield spatial distributions of glacier changes by subtracting one terrain model from another. With this method, however, densities of snow and ice must be assumed and the spatial distribution of changes shows not only the climatologically induced variations of mass balance but also variations associated with ice-flow properties. Careful glaciological measurements show that the mass balance not only varies with altitude but also has horizontal differences (see Fig. 53.1). However, they are by far less pronounced and a VBP, if derived as mentioned above, averages the horizontal differences for each altitude step (Fig. 53.2). In an inverse way of application, the dominance of the vertical variation of the specific mass balance is often used to derive a total mass balance (in kg or m3 we) of a glacier from a series of measuring points lined up, for example, along the central flow line. This, however, ignores all possible horizontal gradients, which cannot be disregarded when looking in detail at an individual glacier's reaction to climate variations (e.g. Oerlemans & Hoogendoorn, 1989). It is obvious that the VBP is the result of a combination of climate parameters and their vertical gradients. This is, in most cases, the basis for glacier mass-balance models of different complexity. Such models are used for calculating glacier mass balance from measured or prognostically modelled climate input, for deriving climate from reconstructed volume and mass changes from former glacier extents (e.g. Oerlemans, 2001), and for modelling glacier runoff (e.g. Hock, 2003; Juen et al., 2003).

The VBP is, in a first step, the result of the vertical distribution of snow accumulation from precipitation and, possibly, redistribution by wind. The latter is of concern under cold and strong wind conditions, namely in the higher latitudes and in very high altitudes. It decreases toward warmer environments where drift is

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Figure 53.2 The vertical balance profile as well as the area altitude distribution of Hintereisferner (Fig. 53.1) in the hydrological year 1984. (Kindly provided by G. Markl and M. Kuhn, Institute of Meteorology and Geophysics, University of Innsbruck.)

more and more reduced toward a limited small-scale importance on crests and saddles. During the ablation period, climate impact on a glacier and its VBP is much more complex. Different parameters drive the fluxes of energy that are finally used for ablation. Out of all fluxes that determine the processes at the atmosphere-glacier interface, extraterrestrial solar radiation is the only independent one. Its portion reaching the glacier surface is affected by topography, atmospheric moisture content and related cloudiness. The reflection of shortwave radiation and, finally, the available net shortwave radiation is determined by the glacier surface albedo, which is, in turn, the result of previous accumulation and ablation processes. Profiles of wind, temperature and air humidity in the atmospheric boundary above the glacier surface determine the turbulent fluxes of sensible and latent heat.

These fluxes can be positive or negative toward the surface. The latent heat flux is the only term that makes up part of both the energy and, as sublimation or resublimation, the mass balance. Air temperature and atmospheric moisture content also determine the atmospheric longwave emission and stand for the occurrence of solid precipitation which, in turn, interferes via the albedo with the shortwave radiation balance. The resulting energy heats the glacier surface and the layers below whereas the consequent surface temperature determines the emission of longwave radiation from the glacier. If, at the very end, a surplus of energy results, this is used for melting which, together with sublimation, makes up ablation.

In many cases, air temperature and its variation with elevation have a very high statistical correlation with ablation and also with the mass balance (e.g. Ohmura, 2001). Air temperature influences the sensible heat flux, the condensation of moisture in the atmosphere, thus cloudiness and precipitation, the ratio between liquid and solid precipitation, thus the albedo, and the emission oflong-wave radiation from the atmosphere. Air temperature is itself influenced by solar radiation, surface temperature and sublimation. The so-called temperature index or positive degree-day models (e.g. Hock, 2003) are based on these relations and give good results in many studies on mid- and high-latitude mountain glaciers where temperature follows a pronounced seasonality separating an accumulation from an ablation season (e.g. Braithwaite & Zhang, 2000) and mean hygric conditions vary comparatively little. If applied inversely to retrieve climate scenarios from reconstructed glacier extents, they exclude possible changes in atmospheric moisture content. However, the exclusive use of air temperature and, possibly, precipitation for climate-mass-balance studies becomes insufficient if looking at low-latitude, high-mountain glaciers. In the tropics and subtropics, seasonal variation in air temperature is subdued but the seasonal variation of atmospheric moisture content and all related climate variables is dominant. This fact makes low-latitude glaciers highly sensitive not only to trends in air temperature but also to the fluctuation of moisture-related parameters and their seasonal and interan-nual variation (e.g. Wagnon et al., 1999). The occurrence of strong sublimation under dry conditions or the time of onset of accumulation at the beginning of the wet season with its effect on albedo are crucial for the mass balance of low-latitude glaciers (Francou et al., 2003). These variations change the VBP markedly and have implications on the equilibrium line altitude, its position in relation to the 0°C level, a possible steady-state extent of a glacier, the ratio of ice loss and, finally, the glacier runoff.

53.3 The runoff from mountain glaciers

From a mid- and high-latitude viewpoint, runoff from mountainous catchment areas basically follows the seasonal variation of air temperature and is dominated by the storage of precipitation as snow cover during the cold season and its progressive release during spring and summer. The higher the ratio of glaci-erization, the higher and later the summer maximum of runoff (Fig. 53.3). During periods of strong mass loss and consequent retreat of glaciers, runoff is generally increased until the effect is compensated by reduced glacier surface area. When a glacier dis

Vent - 41% glacie

Oct Dec Feb Apr Jun Aug

Tumpen - 2% glaci

Tumpen - 2% glaci

Oct Dec Feb Apr Jun Aug

Llanganuco - 33.6% glacier P '

Llanganuco - 33.6% glacier P '

Oct Dec Feb Apr Jun Aug Querocoba - 3.2% glacier

Oct Dec Feb Apr Jun Aug

Figure 53.3 Seasonal variations of precipitation, P, and runoff, R (normalized), for differently glacierized catchment areas in the Austrian Alps (left) and the Peruvian Cordillera Blanca (right). (Modified from Kaser et al., 2003.)

appears, runoff follows the melt of the seasonal snow cover and, during the warm periods, liquid precipitation (e.g. Kuhn, 2003). Owing to the reduced or non-existent thermal seasonality, the situation looks different in lower latitudes. There, no seasonal snow cover occurs outside the glaciers and, thus, glacier runoff is almost the only freshwater supply during dry seasons (Fig. 53.3). Glacier runoff is, as in higher latitudes, increased when glaciers retreat but when they vanish, runoff only follows the occurrence of precipitation. It is important to note also that renewed steady-state conditions or glacier advances would reduce the availability of water. Major attention must be paid to this fact in terms of near-future water management not only in the tropical South American Andes, but also in parts of the Himalayas which are, climatically speaking, intermediate between the mid-latitudes and the tropics and seasonality is characterized by both temperature and hygric variations in a complex way (e.g. Ageta & Higuchi, 1984).

Oct Dec Feb Apr Jun Aug Querocoba - 3.2% glacier

Oct Dec Feb Apr Jun Aug

Figure 53.3 Seasonal variations of precipitation, P, and runoff, R (normalized), for differently glacierized catchment areas in the Austrian Alps (left) and the Peruvian Cordillera Blanca (right). (Modified from Kaser et al., 2003.)

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