While many of the extreme events discussed here have been, and will increasingly be, costly in social, economic, and environmental terms, it needs to be emphasized once again that strong impacts are not necessarily or exclusively related to extreme weather events. Indeed, many of the geomorphologic hazards in the Alps are the result of long-term climatic and geological forcings that at some stage result in a particular threshold exceedance leading to sudden and intense slope instabilities.
Impacts of climate change in the Alps can be placed into five broad categories: hydrology, snow, and ice; plants, forest ecosystems, and mountain biodiversity; human health (this aspect will not be discussed further here); socioeconomic sectors such as tourism, agriculture, and hydropower; and financial services such as insurance. While extreme events may have a significant impact on one or more of these sectors, they can also be viewed as short-term and rapid ''pulses of energy'' into systems that are already experiencing decadal-scale climate change.
Snow and ice are key components of the hydrological cycle in the Alps, and the seasonal character and amount of runoff is closely linked to cryospheric processes, such as the timing of the spring snowmelt and the water that is added to rivers by seasonal glacier melt towards the end of the summer. Because of the sensitivity of mountain glaciers to temperature and precipitation, changes in climate have been shown to result in shifts in seasonal snowpack (Beniston et al., 2003). In temperate mountain regions, glacier ice is often close to its melting point, so it may respond rapidly to apparently minor changes in temperature. The persistence and intensity of the 2003 heat wave had rapid and adverse effects on glacier mass balance, and the projected increases in the numbers of heat waves in the future will lead to an acceleration of glacier retreat. The consequences of changing snow patterns and reduced glacier volume for river runoff are likely to affect not only the watersheds within the mountains themselves, but also lowland regions in Germany, Italy, and France that are heavily dependent on the Alps for their water supply.
Biodiversity in mountain areas encompasses both natural and cultivated species; these systems are sensitive to climatic factors and are likely to have different vulnerability thresholds according to the species, the amplitude, and the rate of climate change. Plant life at high elevations is constrained primarily by direct and indirect effects of low temperatures, radiation, wind and stormi-ness, or lack of water (Korner and Larcher, 1988). Plants respond to these climatological influences through a number of morphological and physiological adjustments. Adaptation to environmental change includes the progressive replacement of currently dominant species by more thermophilic species. Observations in the Alps (Grabherr et al., 1994; Keller et al, 2000) suggest that certain plants have already begun to respond in this manner to observed twentieth-century warming. A further mechanism is that the dominant species may be replaced by pioneer species of the same community that have enhanced adaptation capabilities (Pauli et al., 1998). A third possibility is that environmental change may favor less-dominant species, which then replace the dominant ones through competition (Street and Semenov, 1990). Extreme events, especially heavy precipitation and also rare but severe winter windstorms, severely damage trees in particular and thereby reduce their anchoring capacity for soils on steep slopes, thus opening the way to enhanced slope erosion. Increasing loss of vegetation and forest cover under the long-term influence of climate change and the shock effects of repeated extreme weather would be detrimental to the environmental health of the Alps, which is strongly related to the quality of vegetation preservation in natural, seminatural, agricultural, and forest ecosystems. Maintaining Alpine biodiversity would represent an optimal strategy for averting natural hazards such as landslides, and for maintaining water quality in the numerous Alpine watersheds.
In the latter decades of the twentieth century, tourism and recreation was one of the fastest growing industries worldwide (Perry, 2000). Tourism has economic benefits for, and potential adverse effects on, mountain environments and local mountain communities. Changing climates may alter the seasonal patterns of tourism (for example, skiing in winter) and thus the environmental pressures associated with different forms of leisure activities. Lack of snow during some recent winters in the Alps has translated into serious economic shortfalls for many mountain resorts that have few alternate solutions for attracting tourists in winter. While the Alps may be seen as a cool haven during heat waves in the lowland regions, summer tourism rarely compensates for winter sports in terms of the income that skiing generates for many mountain communities. Extreme events, particularly floods and windstorms, can damage infrastructure such as housing, communication routes, or cable cars.
In the Alps, hydropower is the main source of energy for electricity production. Changes in the seasonal character of precipitation, and also the timing of the melting of the Alpine snowpack, will substantially change the periods during which reservoirs will be filled; they may occur towards the late spring and early summer, compared with autumn today. In this case, new water management techniques will be required to assess whether to produce more electricity during the off-peak summer months or wait until the peak-demand winter season before doing so, which would imply economic shortfalls as production is suspended or reduced. Hydropower infrastructure can partially buffer the effects of strong convective precipitation, by holding back flood-waters that would otherwise impact communities downstream in the valleys. This has not always been successful, however, as was seen during some of the flood events in the autumns of the early 1990s. In the central and southern Alps, where the dams were already full in preparation for the winter peak-demand period, they could not cope with a sudden and massive influx of water.
While long-term climate change will certainly contribute to environmental and economic adversity to many sectors of Alpine life, the additional effects of sudden and unexpected extreme events may compound these detrimental effects, sometimes beyond the threshold of economic viability. In the past, the negative impacts of an extreme weather event have been financially absorbed over time; if the return periods of floods, heat waves, storms, or drought were to be reduced, however, then even the robust economies of the Alpine countries would be stressed, and government subsidies and public and private insurance could have difficulties in coping with the increasing and recurrent events that many models are projecting for coming decades.
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