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

IV.B.5 PHYSICS
Thermal characteristics
In northern Europe, four regional climate simulations were made within the
PRUDENCE project (Räisänen et al., 2004). In Southern and Central Europe, the
winter-summer contrast in warming is reversed from that in the north. A large
increase in summer temperatures occurs especially in the southwestern parts of the
European continent, where the warming locally exceeds 10ºC. Such a large warming
may have adverse consequences, even without the accompanying decrease in
precipitation discussed elsewhere. The most pronounced increases in the air
temperatures will be recorded during the summer with the more extreme conditions
being recorded in the southeast of the UK and in Ireland. The responses of the lakes
to the projected increases will vary from site to site and season to season.
Long distance climatic forcing affects several thermal parameters such as the onset,
timing and duration of thermal stratification, heat content (Ambrosetti and Brabanti,
2002a,b), extent of mixing, or duration and break-up date of ice-cover, to name a
few. Changes in the freeze-thaw cycle of lakes at high latitudes and altitudes are
often used as proxy indicators of regional changes in the weather (Magnuson et al.,
2000). The European Topic Centre on Air and Climate Change selected changes in
duration of lake ice and lake temperature from among a list of climate change state
indicators (Erhard et al., 2002). Both indicators fulfil most of the criteria on
indicativeness, sensitivity, representativeness, comparability, accessibility, and data
quality.
Ice duration
A long-term trend towards shorter periods of ice cover due to a later freezing and an
earlier ice break-up has been reported for lakes around the Northern Hemisphere
(Palecki and Barry, 1986; Kuusisto, 1987; Assel and Robertson, 1995; Livingstone,
1997; Magnuson et al., 2000; Assel et al., 2003). Additionally, year-to-year variability
in ice break-up dates in Northern Europe are related to climatic (NAO and regional
atmospheric indices) variation (Weyhenmeyer et al., 1999; Blenckner and Chen
2003) in a complex manner. The trend toan earlier ice-out increases the ice-free
period and lake temperatures in spring (Blenckner et al. 2002). A further increase in
climatic warming could imply that dimictic lakes may become warm monomictic
(Blenckner et al., 2002).
In the arctic, sub arctic, and alpine regions freshwater systems are particularly
sensitive to climate change, and most climate change scenarios indicate that the
highest and most rapid temperature increases will occur in these regions (McCarthy
et al., 2001; Arctic Council, 2004). The biggest changes will occur in lakes, which
previously were permanently ice-covered become temporarily ice-free (Psenner
2003), and also in lakes which totally loose their winter ice-cover. Ohlendorf et al.
(2000) concluded from their observations on a remote high alpine lake that the mere
occurrence of ice cover is more important than the duration to preserve climate
signals. Historical observations from a high alpine lake in Switzerland indicate that
the date of ice break-up occurred 12 days earlier in 1990 than 150 years ago. As a
result of reduced ice duration, effects of UV radiation became more pronounced in
some high alpine lakes in the last century (Psenner and Schmidt, 1992).
Very few records of ice-cover are available for the Atlantic lakes. There are, however,
records of a progressive reduction in ice cover at Loch Leven in Scotland and
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