ateam - Potsdam Institute for Climate Impact Research
ateam - Potsdam Institute for Climate Impact Research
ateam - Potsdam Institute for Climate Impact Research
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ATEAM final report Section 5 and 6 (2001-2004) 34<br />
observations; and third, it was applied to project future development in the case studies. The model was<br />
spun up to a long-term equilibrium using temporally and spatially downscaled data (1900-1950,<br />
recycled) from the ATEAM climate scenarios and from local land use maps (sources: Arealstatistik,<br />
Bundesamt für Statistik, Switzerland; Nationalpark Berchtesgaden, Germany). RHESSys was then<br />
initialised with these spin-up results <strong>for</strong> the scenario simulations.<br />
Seven different scenarios of climate and land use change (1950-2100, ATEAM, A1f HadCM3, A2<br />
HadCM3, B1 HadCM3, B2 HadCM3, A2 CGCM2, A2 CSIRO2, A2 PCM) were downscaled to the case<br />
study areas and used to simulate daily stream flow, snow water equivalent and carbon cycling <strong>for</strong> the<br />
time period from 1950 to 2100. Finally, the simulated data were aggregated and evaluated to obtain<br />
indicators of mountain ecosystem services: annual and summer stream flow, and maximum monthly<br />
peak flow, carbon storage in the vegetation, reliability of the snow cover, and number of heat and rainy<br />
days. Furthermore, the results were interpreted with respect to natural hazards and tourism.<br />
Assessments of the potential impacts of climate and land use change on fresh water supply, carbon<br />
storage, natural hazards (floods) and tourism were conducted <strong>for</strong> five Alpine catchments. It showed that<br />
one of the most sensitive processes in the Alpine region is the temporary storage of water during the<br />
winter season in <strong>for</strong>m of snow and ice that (1) brings about a delay in stream flow, (2) controls the flood<br />
regime, and (3) constitutes an indispensable prerequisite <strong>for</strong> winter tourism.<br />
Simulation results show that under a warming climate, the snow cover will diminish substantially. As a<br />
consequence, (1) summer discharge of Alpine catchments will significantly decrease, affecting the<br />
irrigation and hydropower sectors, (2) winter floods will become more frequent, influencing protection<br />
strategies, and (3) the snow line will rise, impacting winter tourism.<br />
Finally, mountain regions and their ecosystems are potential carbon sinks, because highland agriculture<br />
and grazing land are being abandoned and become re<strong>for</strong>ested by natural regeneration or tree planting<br />
and because <strong>for</strong>ests at high elevations profit from the warming climate. However, the sink capacity is<br />
expected to decline after approximately the first quarter of the 21 st century.<br />
Fresh water supply in mountain catchments<br />
At present, the typical Alpine stream flow pattern is characterized by a single-peak regime with<br />
maximum flow occurring between May and June. All scenarios produce a change in this regime. The<br />
annual pattern of discharge becomes flatter with rising temperature, i.e. summer discharge decreases<br />
and winter discharge increases (Figure 26).<br />
Summer discharge in mountain catchments is most sensitive to a warming climate. It strongly depends<br />
on snow accumulation and snowmelt. During winter, water is stored as snow or ice at high elevations. In<br />
spring this water is released and strongly contributes to summer stream flow. Under a warming climate,<br />
this storage function of the snow cover is reduced. Furthermore, enhanced evapotranspiration in<br />
summer further reduces soil moisture content and consequently summer stream flow.<br />
The simulations show a decrease of summer discharge between 5% and 50% over the next century<br />
(Figure 27). At the same time, monthly peak flows in summer decrease and shift to earlier dates by up<br />
to one month within the 21 st century in most catchments and <strong>for</strong> most scenarios as a result of the<br />
diminished snow cover in winter. In contrast, none of the simulations that were per<strong>for</strong>med revealed<br />
significant changes in annual stream flow.<br />
Floods in the Alpine area<br />
Currently, most of the major floods in the Alpine area occur in summer. Model results indicate that this<br />
summertime characteristic will shift towards or even into the winter half year as all scenarios produce a<br />
decrease in summer stream flow, an increase in winter stream flow and a shift of monthly peak flow to<br />
earlier dates (Figure 28). As a consequence, winter floods will become more frequent, and at lower lying<br />
Alpine catchments they might even become more common than summer floods under a warming<br />
climate. Regarding summer floods, the reduction of the winter snow cover will substantially reduce the