River floods (CLIM 017) - Assessment published Sep 2008
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Climate change (Primary topic)
Typology: Descriptive indicator (Type A – What is happening to the environment and to humans?)
- CLIM 017
Key policy question: What is the trend in river floods across Europe?
- Although a significant trend in extreme river flows has not yet been observed, twice as many river flow maxima occurred in Europe between 1981 and 2000 than between 1961 and 1980.
- Since 1990, 259 major river floods have been reported in Europe, of which 165 have been reported since 2000. The rise in the reported number of flood events over recent decades results mainly from better reporting and land-use changes.
- Nevertheless, global warming is projected to intensify the hydrological cycle and increase the occurrence and frequency of flood events in large parts of Europe, although estimates of changes in flood frequency and magnitude remain highly uncertain.
- Projections suggest that warming will result in less snow accumulation during winter and therefore a lower risk of early spring flooding.
Occurrence of flood events in Europe 1998-2008
Note: The map shows the number of flood events in Europe from 1998 to 2008
Based on data from Dartmouth Flood Observatory (http://www.dartmouth.edu/~floods/).
Projected change in 100-year return level of river discharge between 2071-2100 and the reference period 1961-1990
Note: Simulations with LISFLOOD driven by HIRHAM HadAM3H/HadCM3 based on IPCC SRES scenario A2.
Dankers, R.; Feyen, L., 2008b. Climate change impact on flood hazard in Europe: An assessment based on high resolution climate simulations. Manuscript submitted to Journal of Geophysical Research Atmospheres.
Despite the considerable rise in the number of reported major flood events and economic losses caused by floods in Europe over recent decades, no significant general climate-related trend in extreme high river flows that induce floods has yet been detected (Becker and Grunewald, 2003; Glaser and Stangl, 2003; Mudelsee et al., 2003; Kundzewicz et al., 2005; Pinter et al., 2006; Hisdal et al., 2007; Macklin and Rumsby, 2007).
Some changes, however, have been reported that may be linked to climate change. For example, in Europe twice as many river flow maxima occurred between 1981 and 2000 than between 1961 and 1980 (Kundzewicz et al., 2005), whereas globally there has probably been an increase in the frequency of extreme flood events in very large catchments (Milly et al., 2002). On the other hand, the frequency and severity of snowmelt and ice-jam floods in central Europe has decreased over recent decades because of the warming of European winters combined with less abundant snow cover (e.g. Mudelsee et al., 2003; Brázdil et al., 2006; Cyberski et al., 2006). In the Nordic countries, snowmelt floods have occurred earlier because of warmer winters (Hisdal et al., 2007). In Portugal, changed precipitation patterns have resulted in larger and more frequent floods during autumn but a decline in the number of floods in winter and spring (Ramos and Reis, 2002). In the United Kingdom, positive trends in high flows have been observed over the past 30-50 years (Robson, 2002; Dixon et al., 2006), some of which are consistent with observed changes in the North Atlantic Oscillation. Comparisons of historic climate variability with flood records suggest, however, that many of the changes observed in recent decades could have resulted from natural climatic variation. Changes in the terrestrial system, such as urbanisation, deforestation, loss of natural floodplain storage, as well as river and flood management have also strongly affected flood occurrence (Barnolas and Llasat, 2007).
Although there is as yet no proof that the extreme flood events of recent years are a direct consequence of climate change, they may give an indication of what can be expected: the frequency and intensity of floods in large parts of Europe is projected to increase (Lehner et al., 2006; Dankers and Feyen, 2008b). In particular, flash and urban floods, triggered by local intense precipitation events, are likely to be more frequent throughout Europe (Christensen and Christensen, 2003; Kundzewicz et al., 2006). Flood hazard will also probably increase during wetter and warmer winters, with more frequent rain and less frequent snow (Palmer and Raisanen, 2002). Even in regions where mean river flows will drop significantly, as in the Iberian Peninsula, the projected increase in precipitation intensity and variability may cause more floods. In snow-dominated regions such as the Alps, the Carpathian Mountains and northern parts of Europe, spring snowmelt floods are projected to decrease due to a shorter snow season and less snow accumulation in warmer winters (Kay et al., 2006; Dankers and Feyen, 2008b).
provided by Dartmouth Flood Observatory
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