ABSTRACTS 'Extreme Discharges' - CHR-KHR

Climate Change and Hydrological Extremes
Christoph Schär
Atmospheric and Climate Science, ETH Zürich
Winterthurerstr.190, CH-8057 Zürich, Switzerland
schaer@env.ethz.ch
The estimation of flood and drought frequencies is a delicate and difficult task, even in a stationary climate.
Modern estimation methods utilize some probabilistic methodology and thus provide a probability density function
(PDF), for instance of the frequency of a certain event category. Climate change will further complicate the
task of estimation, as the assumption of a stationary climate system is rejected. The most immediate effect of
climate change in this context is thus to increase the uncertainty in flood and drought estimation (corresponding
to a widening of the PDF). Such an increase in uncertainty in essence implies an increase in safety margins and
thus costlier protection for the same level of security. It is important to realize that this kind of effect takes
place, irrespective of whether climate change will actually increase or decrease the frequency of the respective
event category (corresponding to a shift of the PDF).
In the long run, however, appropriate estimation methodologies should rely on a combination of past observations
and climate change scenarios. Climate models have made rapid progress in recent years. Yet the projection
of regional climate changes – and in particular of catchment-scale extreme events – is associated with large
uncertainties.
Here results of climate change scenarios will be discussed with reference to European-scale summer droughts.
Anthropogenic climate change is usually perceived as a comparatively slow trend towards higher temperatures,
with attendent shifts in precipitation distribution and hydrological conditions. Associated changes in interannual
(year-to-year) variability are usually considered small. In a recent paper (Schär et al. 2004, see also Vidale et al.
2005), we have proposed that climate change may give rise to a pronounced increase in interannual variability
during the extratropical summer season. Such an increase in variability would have important repercussions for
the frequency of extreme summer heat waves and droughts, and would also represent a challenge to adaptive
response strategies designed to cope with climate change.
We utilize regional climate models (RCMs) from the European project PRUDENCE and provide a detailed
analysis of physical processes implicating on the frequency of heat waves and droughts over Europe. RCM
simulations suggest that interannual summer temperature variability may increase considerably, by up to a factor
two towards the end of the current century. The main region of enhanced variability is the temperate climate
zone in Central and Eastern Europe. Analysis of the simulated water cycle shows that the increase in variability
is at least partly related to land-surface processes, and associated with an increasing frequency of summer
droughts. The representation of the associated non-linearities requires a realistic description of the continentalscale
water cycle in the coupled atmosphere / land-surface system. Intercomparison of model simulations suggests
that there is considerable agreement between different models regarding the occurrence of the effect, but
there are large differences regarding its geographical distribution, seasonal evolution and amplitude. These differences
are related to a wide range of model characteristics, among them the representation of the soil hydrology
and the simulation of large-scale circulation anomalies. A particularly important uncertainty in climate
models relates to the seasonal cycle of terrestrial water storage.
The study was motivated by the extreme European summer 2003. In much of Central Europe, mean summer
temperatures exceeded their long-term mean by more than 4 standard deviations, leading to an extreme heat
wave. A brief analysis of this event will also be presented. It will be argued that the frequency of extreme heat
waves and droughts is more sensitive to an increase in temperature variability (i.e. a widening of the statistical
distribution), than to a mere warming (i.e. a shift of the distribution towards warmer and dryer conditions).
References:
Frei, C., H.C. Davies, A. Gurtz and C. Schär, 2000. Climate Dynamics and Extreme Precipitation and Flood
Events in Central Europe. Integrated Assessment, 1, 281-299
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