Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 41

larger than those observed during the 20th century. The best estimate of the
global average surface warming, expressed as the temperature change from
1980-99 to 2090-99, is projected to range from 1.8°C for the low emissions
scenario, B1, to 4.0°C for the high scenario, A1F1. Even for the case of a constant
radiative forcing, if greenhouse gases and aerosols were kept constant at year
2000 levels, models give a temperature increase of a further 0.6°C by the end of
the century.
For Europe, warming is projected to be greater than the global mean. Regional
climate model simulations run under the PRUDENCE project (http://prudence.
dmi.dk) indicate that warming is likely (i.e. >66% probability) to be greatest in
winter in Northern Europe and in summer in the Mediterranean area. Similarly,
the increase in minimum winter temperatures is likely to be higher than average
in Northern Europe, while maximum summer temperatures are likely to rise
more than average in Southern and Central Europe (Räisänen et al . 2004;
Christensen et al . 2007a, b) (Fig. 3.3a).
Geographical patterns of changes in annual precipitation are projected to be
similar to those already observed over the last few decades, with a high probability
of an increase across most of Northern Europe and a decrease in the Mediterranean.
In Central Europe, precipitation is likely to increase in winter and decrease in
summer (Fig. 3.3b). However, changes in precipitation may vary considerably at
local scales, particularly in areas of complex topography, such as the Alps, where
there are strong orographic effects, and there is still considerable uncertainty in
the projection of future precipitation.
Climate change is often perceived through the occurrence of extreme events,
although extreme weather events occur, and will occur, in most regions even
with an unchanged climate. Extremes are infrequent events at the low and high
ends of a probability distribution of any variable. Assuming the probability of
occurrence of values is shaped like a Gaussian (or bell shaped) curve, a small shift
in the average or the centre of the distribution also moves the extremes accordingly.
The higher frequency of hot days in a warmer climate, for instance, may be
accompanied by a decreasing number of cold days or frosts (Fig. 3.4a). A changing
probability of extremes may be caused not only by a shift in the mean value of a
variable, but also by a change in its variance, and most likely by the interaction
of changes in both mean and variance (Fig. 3.4b and c). The IPCC Fourth
Assessment Report (2007) states that although more models, ensembles and
statistical techniques have been used in the simulation and projection of extreme
events, some assessments still rely on simple reasoning about how climate
warming might change extremes and others rely on the qualitative similarity
between observed and simulated changes.
Heat waves, i.e. episodes of several consecutive high temperature days with
maxima exceeding the 90th percentile of the daily distribution in the 1961-90
IPCC reference period, are projected to show an increase in frequency, intensity
and duration (IPCC 2007). Global and regional climate models project a likely
increase in the inter-annual variability of summer mean temperature, in particular,
in Central Europe (e.g. Schär et al . 2004; Vidale et al . 2007). The heat wave in
Europe in summer 2003 may be taken as an example of what might become more
common in a future warmer climate. For instance, the average summer temperature