Climate Change and the European Water Dimension - Agri ...

Chapter V.A. Climate Change and Extreme Events: Floods 2
V.A.1. Introduction
The aim of this document is to provide a synthesis of what is currently known about
possible climate changes in Europe and how these will affect the occurrence of
extreme flood and drought events. Furthermore, known gaps and questions are
defined. The document is intended as a basis to evaluate the link of climate change
with water policies. Other aspects, such as adaptation and reduction, also are
approached in the document.
Main drivers for Flood and Drought Events
The main driver for flood and drought events is extreme precipitation. Furthermore,
increasing ambient air temperatures will increase heat stress and potential
evaporation, which will change soil moisture availability and therefore directly
influence the occurrence of droughts. Also, changing soil moisture conditions will
affect the initial conditions for flood events: precipitation on drier soils can be buffered
because of the increased soil water storage potential. A further driver for flood and
drought events is the increased vulnerability to natural disasters due to growing
urban population, environmental degradation and a lack of planning, land
management and preparedness.
Current Knowledge of Expected Climate Changes in Europe
Schnur (2002) states that “changes in extreme climate, such as hot spells, droughts
or floods, potentially have a much greater impact on society than changes in mean
climate, such as summertime temperature averaged over several decades”. The
Third Assessment Report on Climate Change (IPCC, 2001) states that “it is very
likely (a 90-99% chance) that precipitation has increased by 0.5 to 1.0 % per decade
in the 20 th century over most mid- and high latitudes of the Northern Hemisphere
continents” and that “in the mid- and high latitudes of the Northern Hemisphere over
the latter half of the 20 th century, it is likely (a 66-90% chance) that there has been a
2-4% increase in the frequency of heavy precipitation events” As a consequence, it
has been concluded (IPCC-McCarthy et al, 2001) that: “flood magnitude and
frequency are likely (a 66-90% probability) to increase in most regions, and low flows
are likely to decrease in many regions.”
Palmer & Räisänen (2002) and Milly et al. (2002) concluded that the risk of extreme
precipitation and flooding would increase in the future because of rising levels of
atmospheric carbon dioxide. These conclusions are based on climate models that
are run using a rate of net greenhouse gas increase that is twice the current
observed value. Palmer and Räisänen (2002) find that in the winter over Europe, the
probability of extremely high seasonal precipitation increases by about two to five
times over the course of the next 50 to 100 years (Figure V.A.1). These changes
appear because, in general, the climate models produce more precipitation in those
regions when the greenhouse effect is enhanced. According to the authors, the
"increase in CO2 [used in their models] is somewhat faster than is anticipated for the
21st century, but can be justified from the neglect of other anthropogenic greenhouse
gases."
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Acknowledgements:
The following persons assisted in the establishment of this document by sending papers or through
discussions: Prof. Axel Bronstert (Univ. Potsdam), Prof. Nigel Arnell (Tyndall), Prof. Paolo Burlando
(ETH Zurich) and Dr. Lucas Menzel (PIK Potsdam). Their help is greatly appreciated.
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