Melting ice – a hot topic for Europe’s water, too
This article supplements the UNEP report with additional information on some of the observed and foreseeable impacts of climate change on river flow, and rivers and lake ice cover in Europe.
Reduced water storage during winter
to as the ‘water towers’ of Europe, the Alps are essentially a huge water
reservoir where precipitation that falls in winter is retained and stored in
the form of snow fields and glaciers. Similarly, in other mountainous regions
and in Northern Europe winter storage in the form
of snow determines the seasonality of river flow. The snow fields and glaciers
melt in the spring and summer months and provide a huge volume of high quality
water that replenishes downstream surface and groundwaters. This natural
storage mechanism benefits many river systems throughout Europe, including the
Rhine and Danube in Germany,
the river Po in Italy, and
the Rhône in France.
This often provides water when it is most needed, for instance, in the dry,
precipitation-poor months of late summer.
Higher temperatures push the snow limit in mountainous regions upwards and reduce the fraction of total precipitation falling as snow. This results in both a marked drop in winter retention and higher winter river flows in northern European and Alpine rivers. Moreover, earlier spring melts will lead to a shift in peak flow levels. As a result of the declining snow reservoir, the earlier snow melt and the general decrease in summer precipitation, longer periods of low river-flow rates may be observed in summer in many parts of Europe.
Changes in the seasonal flow regime may change the periods with enhanced flood risk and drought risk. Beniston (2006), for example, predicts a 90 % increase in winter run-off and a 45 % decrease in summer run-off in the central Alps, which may result in periods of enhanced flood and drought risks, respectively (Figure 1).
Figure 1. Change in seasonal run-off in the central Alps in 2071–2100 compared with 1961–1990 (HIRHAM RCM; A-2 Scenario)
Source: Beniston, 2006.
Periods of low water flow and droughts can have severe consequences for several economic sectors, particularly agriculture, navigation, energy production and drinking water provision. Activities that depend on high water abstraction and use, such as irrigated agriculture, hydropower generation and the use of cooling water, will be affected by changed flow regimes and reduced water availability. Moreover, wetlands and aquatic ecosystems will be threatened which will affect the sectors that depend on the goods and services they provide.
Higher water temperature means less river and lake ice-cover
Higher air temperature leads to higher water temperatures. During the last century the water temperature of European rivers and lakes increased by 1–3 °C (Figure 2). The River Rhine had a 3 °C increase in temperature from 1910 to 2006. Two-thirds of this temperature rise is due to the increased use of cooling water in Germany and one-third of the increase in temperature is the result of climate change (MNP, 2006).
Figure 2. Trend in annual water temperature in river Rhine (1909–2006), Danube (1901–1998) and average water temperature in August in Lake Saimaa, Finland (1924–2000)
Source: River Rhine: Rijkswaterstaat; River Danube: Hohensinner, 2006; and Lake Saimaa: Korhonen, 2002.
Climate change-related temperature changes are projected to result in less ice formation. For example, studies have shown that ice break-up in rivers now occurs 15–20 days earlier than in the 1950s, and a shift towards a longer annual ice-free period and earlier ice break-up have been observed in many northern lakes and rivers in recent decades (Korhonen 2005; Magnusson et al., 2000). The timing of lake ice break-up is of ecological importance because the disappearance of ice cover affects the production and the composition of the phytoplankton community and the occurrence of winter fish kills (Weyhenmeyer, 2006).
The ice cover period for the Tornio River in Finland has been recorded since 1693 (Figure 3). Freeze-up dates have been delayed and break-up occurs earlier. The period of river-ice has been shortened by up to a month. Many rivers within the temperate regions could become ice-free or develop only intermittent or partial ice coverage.
Figure 3. Trend in ice-breaking date of river Tornio, on the border of Finland and Sweden (1693–2003)