As contributors to hydrological systems, snow and ice and their potential changes in a warmer global climate will have profound impacts on European streams and rivers. Mountains-in particular the Alps-are the source of most of Europe's major rivers; the timing and amount of flow in rivers such as the Rhine, the Rhone, and the Danube (via the Inn River) are strongly dependent on the seasonal accumulation and melting of snow and, during the summer and fall, on meltwater from mountain glaciers. Changes in the mountain cryosphere in the Alps and the Fennoscandian mountains would have significant consequences for the flow regimes of rivers originating in these mountains. Changes in hydrological regimes would particularly affect populations living downstream of the mountains that depend on the water the mountains provide for freshwater supply, industrial and energy usage, irrigation, and, in some cases, transportation-as on the Rhine and the Danube, both of which are particularly sensitive to flow changes. As the extensive flooding in Poland, Germany, and the Czech Republic in 1997 has demonstrated, many flood defense systems have a limited capacity, and any changes in hydrological regimes could have major impacts in floodplain areas.
Higher temperatures will push the snowline upwards by about 150 m for every 1�C rise; the seasonal patterns of snowfall are likely to change, with the snow season beginning later and ending earlier. The timing and amount of seasonal flow patterns currently experienced by European rivers also would change as a result of snowpack conditions in the mountains; peak flow would occur earlier in the season, and there could be shortfalls as a result of drier summer conditions over much of Europe and reduced river flow in the summer. The fact that winter runoff is likely to increase and spring runoff probably will decrease could benefit the hydropower industry, however, because it would be in a more favorable position to generate electricity at the peak demand period of the year.
Mountain glaciers generally have been shrinking and are projected to lose about 25% of their mass worldwide by the middle of the 21st century. In the European Alps, about half of the original ice volume has been lost since 1850; as much as 95% of the existing glacier mass could disappear over the next 100 years with anticipated warming, and many of the small glaciers could disappear altogether within decades (Haeberli and Hoezle, 1995). Large reductions in glacier mass would impact most hydrological basins, with significant reductions in flow regimes during the summer and early fall. Accelerated glacier shrinkage and permafrost degradation also is likely to cause increasing slope stability problems in areas above the present timberline (Haeberli et al., 1997).
In terms of river and lake ice-which are particularly relevant to northern Sweden, Finland, and Russia, as well as high-altitude Alpine lakes-a warmer climate will lead to delayed freezing at the beginning of the winter season, while break-up will begin earlier. In Russia and Fennoscandia, the river-ice season could be shortened by up to one month. Many rivers in the temperate regions of central Europe will become ice-free or develop only intermittent or partial ice coverage.
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