Climate change, impacts and vulnerability in Europe ... - MemoFin.fr

Climate impacts on environmental systems3.3.6 Lake and river iceRelevanceLake ice reduces underwater light (Leppäranta et al.,2003) and vertical mixing, that is, the exchange ofwater from different depth layers (Livingstone, 1993;Melles et al., 2007). The existence of lake ice, andthe timing of lake ice break-up, is thus of criticalecological importance, influencing for instancethe production and biodiversity of phytoplankton(Weyhenmeyer et al., 1999) and the occurrence ofwinter fish kills (Stefan et al., 2001; Jackson et al.,2007).Past trendsThe duration of ice cover in the northern hemispherehas shortened at a mean rate of 12 days per centuryover the last 150–200 years, resulting from a 5.8 daylater ice cover and a 6.5 day earlier ice break-up onaverage (Magnuson et al., 2000; EEA, 2008).ProjectionsOne recent study has simulated changes in lakeice cover throughout the Northern Hemisphere(40–75 °N) based on one global climate model drivenby the SRES A2 emission scenario. The resultsindicate an overall decrease in the duration of lakeice cover of 15–50 days across regions by 2040–2079,compared to the baseline period 1960–1999 (Dibikeet al., 2011). The ice cover of lakes in regionswhere the ice season is already short or whereice cover only occurs in cold winters is generallymore strongly affected by increasing temperaturethan that of lakes in colder regions (Weyhenmeyeret al., 2011). However, the ecological consequencesof reduced ice cover are expected to be lower inareas where ice cover normally is less frequent andecosystems thereby are adapted to less ice or ice-freeconditions.There are, however, large differences across regions.At the Hungarian section of the River Danube, thedate of first ice appearance has shifted 19–29 dayslater over the 1876–2011 period, while the date offinal ice disappearance shifted 18–23 days earlier(Takács, 2011) (see Figure 3.11a). In Lake Kallavesi,eastern Finland the freezing date has shifted 15 dayslater in 1833–2011, while the break-up date hasshifted 12 days earlier in 1822–2011 (SYKE, 2011)(see Figure 3.11b).Key messages: 3.3.6 Lake and river ice• The existence of ice cover and the timing of ice break-up influence the vertical mixing of lakes and aretherefore of critical ecological importance.• The duration of ice cover on European lakes and rivers has shortened at a mean rate of 12 days percentury over the last 150–200 years.• A further decrease in the duration of lake ice cover is projected with projected climate change.124 Climate change, impacts and vulnerability in Europe 2012