Water temperatures in four selected European rivers and lakes in the 20th century

Note: Annual average water temperature in River Rhine (1909-2006), River Danube (1901-1998), Lake Võrtsjärv (1947-2006), and average water temperature in August in Lake Saimaa, Finland (1924-2000).

Data source:

Source 1) Trend in River Rhine water temperature 1910-2006. Rijkswaterstaat, measurements Rhine River at Lobith period 1908-2006 (non published), 2007. Timo Kroon, RIZA; t.kroon@riza.rws.minvenw.nl Source 2) Trend in River Danube water temperature 1901-1990, 2006. Severin Hohensinner; severin.hohensinner@boku.ac.at Source 3) Trend in August water temperature in Lake Saimaa 1924-2000, 2006. Korhonen Johanna; johanna.korhonen@ymparisto.fi Source 4) Trend in water temperature in Lake Võrtsjärv (1947-2006), 2007. Peeter Noges & Tiina Nõges; peeter.noges@jrc.it tiina.noges@emu.ee

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Past trends

Long time-series, covering the past 100 years, show that the surface water temperatures of some of the major rivers in Europe have increased by 1-3 ^oC over the last century (Figure 1). The temperature of the river Rhine increased by 3 ^oC between 1910 and 2006. Two-thirds of this is estimated to be due to the increased use of cooling water in Germany and one-third to the increase in temperature as a result of climate change (MNP, 2006). In the river Danube the annual average temperature increased by around by 1 ^oC during the last century. A similar temperature increase was found in some large lakes: Lake Vortsjarv in Estonia had a 0.7 ^oC increase between 1947 and 2006 and the summer (August) water temperature of Lake Saimaa, Finland increased more than 1 ^oC over the last century. There are many shorter time-series of water temperature covering the past 30-50 years and the general trend has been for   temperatures in European freshwater systems to increase, generally by from 0.05 to 0.8 ^oC/decade.
George and Hurley (2004) found that the temperature of Lake Windermere (England) and Lough Feeagh (Ireland) increased by 0.7-1.4 ^oC between 1960 and 2000. The water temperature of Lake Veluwe (the Netherlands) has increased by more than 1 ^oC since 1960 (MNP, 2006).
Marked increases in water temperature were found in eight Lithuanian lakes (Pernaraviciute, 2004) and six Polish lakes (Dabrowski et al., 2004). Since 1950, water temperatures in rivers and lake surface waters in Switzerland have in some cases increased by more than 2 ^oC (BUWAL, 2004; Hari et al., 2006). In the large lakes in the Alps the water temperature has generally increased by 0.1-0.3 ^oC per decade: Lake Maggiore and other large Italian lakes (Ambrosetti and Barbanti, 1999), Lake Zurich (Livingstone, 2003), Lake Constance and Lake Geneva (Anneville et al., 2005).
Dokulil et al. (2006) studied the trend in hypolimnion (bottom water) temperature in 12 deep European lakes and found generally a temperature increase of 0.1-0.2 ^oC per decade. This may have significant effects on thermal stratification and mixing of water in lakes, which in turn affects deep water oxygen conditions and nutrient cycling.

Projections

As water temperature is closely linked to change in air temperature, the predicted increase in air temperature due to climate change will be reflected in increased surface water temperature. This is in addition to temperature changes caused by other factors, such as changes in cooling water releases. Projected increases in surface water temperatures are often 50 to 70 % of the projected increases in air temperature. In line with the projected increases in air temperature, lake surface water temperatures may be around 2 ^oC higher by 2070, but with a clear seasonal dependency and depending on lake properties (Malmaeus et al., 2006; George et al.,  2007).