5.3.3. Cryosphere, Hydrology, Water Resources, and Water Management
5.3.3.1. Snow and Ice
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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