Climate change impacts and vulnerability in Europe 2016

Climate change impacts on environmental systems
cooling water and one-third is attributed to the increase
in air temperature as a result of climate change (CBS
et al., 2014). The annual average temperature of the
Danube increased by around by 1 °C during the 20th
century (Webb and Nobilis, 1994).
Increases in surface water temperature were also
found in lakes. Lake Võrtsjärv in Estonia had a 0.7 °C
increase between 1947 and 2014, and the summer
(August) water temperature of Lake Saimaa, Finland,
increased by more than 1 °C over the last century
(Figure 4.14). Many other time series indicate a general
trend of increasing water temperature in European
rivers and lakes in the range of 0.05 to 0.8 °C per
decade, with some water bodies warming by more than
1 °C per decade (Dokulil, 2013; CBS et al., 2014; FOEN,
2015; Orr et al., 2015; Sharma et al., 2015).
Projections
Inland surface water temperatures are projected to
increase further, in parallel with the projected increases
in air temperature. The exact amount of warming
depends on the magnitude of air temperature increase,
on the region, on the season and on lake properties
and river catchment.
An average increase in summer surface water
temperature of 2 °C (1.2–2.9 °C) by 2050 has been
estimated for 15 Austrian lakes, depending on the
present thermal regime and geographical region
(Dokulil, 2013). A global study estimated that mean
river water temperatures of major European rivers
will increase by 1.6–2.1 °C during the 21st century
(2071–2100 relative to 1971–2000, mean of a GCM
ensemble driven by SRES A2 and B1 scenarios) (van
Vliet et al., 2013). A detailed assessment for the Rhine
gives an estimated increase in mean annual and August
temperature in the range of 3.0–3.5 °C during the
21st century, due to climate change. The number of
days with water temperatures above 25 °C, which is the
threshold for significant stress to river fauna and flora,
would increase at least five-fold (e.g. from 2–15 days
in the reference period 2001–2010 to 32–75 days in
2071–2100) (ICPR, 2014).
Further effects of increased lake and river water
temperature are described in Box 4.5.
Box 4.5
Lake and river ice cover
Water temperature affects the ice cover and the timing of ice break-up in lakes and rivers. Ice cover in turn influences the
vertical mixing and the light conditions in lakes, which are important factors for phytoplankton production.
The duration of ice cover in the northern hemisphere has shortened at a rate of 7 to 17 days per century over the last
100–150 years, resulting from ice cover beginning between 3 and 11 days later and ice break up beginning between
5 and 9 days earlier, on average, than previously (Benson et al., 2011). At the Hungarian section of the river Danube, the
date of the first ice appearance has shifted 19–29 days later over the 1876–2011 period, while the date of the final ice
disappearance has shifted 18–23 days earlier (Takács, 2011). In Lake Kallavesi (eastern Finland), the freezing date has
shifted to 15 days later in the period 1833–2011, while the break-up date has shifted to 12 days earlier in the period
1822–2011 (SYKE, 2011). The ice break-up date in the Lake St. Moritz (Swiss Alps) has shifted to 15–20 days earlier since
1832 (Naturwissenschaften Schweiz, 2016). In the two large Estonian lakes, Peipsi and Võrtsjärv, trends in ice phenology
were weak or absent, despite a marked increase in water temperature, in particular since 1961. These findings imply that
the processes governing ice phenology are more complex than those governing lake surface water temperature (Nõges
and Nõges, 2013).
Simulated changes in lake ice cover throughout the northern hemisphere (40–75 °N) based on one global climate model
driven by the SRES A2 emissions scenario indicate an overall decrease in the duration of lake ice cover of 15–50 days across
regions by 2040–2079, compared with the baseline period 1960–1999 (Dibike et al., 2011). In the two Estonian lakes, an
increase of the average winter air temperature of 2 °C would presumably halve the ice cover duration in Peipsi but shorten
it by only about 20 % in Võrtsjärv (Nõges and Nõges, 2013). The ice cover of lakes in regions where the ice season is already
short or where ice cover occurs only in cold winters is generally more strongly affected by increasing temperature than that
of lakes in colder regions (Weyhenmeyer et al., 2011).
150 Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report