IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3European rivers in 2002, although neither flood nor mean precipitationtrends could be identified in this region; however, there was a trendtoward increasing precipitation variability during the last century whichitself could imply an enhanced probability of flood occurrence.Kundzewicz et al. (2007) argued that climate change (i.e., observedincrease in precipitation intensity and other observed climate changes)might already have had an impact on floods. Regarding the spring peakflows, the AR4 concluded with high confidence that abundant evidencewas found for an earlier occurrence in snowmelt- and glacier-fed rivers(Rosenzweig et al., 2007; Bates et al., 2008), though we expressly notehere that a change in the timing of peak flows does not necessarilyimply nor preclude changes in flood magnitude or frequency in theaffected regions.Although changes in flood magnitude/frequency might be expected inregions where temperature change affects precipitation type (i.e., rain/snow separation), snowmelt, or ice cover (in particular northern highlatitudeand polar regions), widespread evidence of such climate-drivenchanges in floods is not available. For example, there is no evidence ofwidespread common trends in the magnitude of floods based on thedaily river discharge of 139 Russian gauge stations for the last few toseveral decades, though a significant shift in spring discharge to earlierdates has been found (Shiklomanov et al., 2007). Lindström andBergström (2004) noted that it is difficult to conclude that flood levelsare increasing from an analysis of runoff trends in Sweden for 1807 to2002.In the United States and Canada during the 20th century and in theearly 21st century, there is no compelling evidence for climate-drivenchanges in the magnitude or frequency of floods (Lins and Slack, 1999;Douglas et al., 2000; McCabe and Wolock, 2002; Cunderlik and Ouarda,2009; Villarini et al., 2009). There are relatively abundant studies on thechanges and trends for rivers in Europe such as rivers in Germany andits neighboring regions (Mudelsee et al., 2003; Tu et al., 2005; Yiou etal., 2006; Petrow and Merz, 2009), in the Swiss Alps (Allamano et al.,2009), in France (Renard et al., 2008), in Spain (Benito et al., 2005), andin the United Kingdom (Robson et al., 1998; Hannaford and Marsh, 2008),but a continental-scale assessment of climate-driven changes in theflood magnitude and frequency for Europe is difficult to providebecause geographically organized patterns are not seen in the reportedchanges.Available (limited) analyses for Asia suggest the following changes: theannual flood maxima of the lower Yangtze region show an upwardtrend over the last 40 years (Jiang et al., 2008), the likelihood forextreme floods in the Mekong River has increased during the secondhalf of the 20th century although the probability of an average floodhas decreased (Delgado et al., 2009), and both upward and downwardtrends are identified over the last four decades in four selected riverbasins of the northwestern Himalaya (Bhutiyani et al., 2008). In theAmazon region in South America, the 2009 flood set record highs in the106 years of data for the Rio Negro at the Manaus gauge site in July2009 (Marengo et al., 2011). Recent increases have also been reportedin flood frequency in some other river basins in South America(Camilloni and Barros, 2003; Barros et al., 2004). Conway et al. (2009)concluded that robust identification of hydrological change was severelylimited by data limitations and other issues for sub-Saharan Africa.Di Baldassarre et al. (2010) found no evidence that the magnitude ofAfrican floods has increased during the 20th century. However, suchanalyses cover only limited parts of the world. Evidence in the scientificliterature from the other parts of the world, and for other river basins,appears to be very limited.Many river systems are not in their natural state anymore, making itdifficult to separate changes in the streamflow data that are caused bythe changes in climate from those caused by human regulation of theriver systems. River engineering and land use may have altered floodprobability. Many dams are designed to reduce flooding. Large damshave resulted in large-scale land use change and may have changed theeffective rainfall in some regions (Hossain et al., 2009).The above analysis indicates that research subsequent to the AR4 stilldoes not show clear and widespread evidence of climate-drivenobserved changes in the magnitude or frequency of floods at the globallevel based on instrumental records, and there is thus low confidenceregarding the magnitude and frequency and even the sign of thesechanges. The main reason for this lack of confidence is due to limitedevidence in many regions, since available instrumental records of floodsat gauge stations are limited in space and time, which limits the numberof analyses. Moreover, the confounding effects of changes in land useand engineering mentioned above also make the identification ofclimate-driven trends difficult. There are limited regions with mediumevidence, where no ubiquitous change is apparent (low agreement).Pre-instrumental flood data can provide information for longer periods,but current availability of these data is even scarcer particularly in spatialcoverage. There is abundant evidence for an earlier occurrence of springpeak flows in snowmelt- and glacier-fed rivers (high confidence), thoughthis feature may not necessarily be linked with changes in the magnitudeof spring peak flows in the concerned regions.The possible causes for changes in floods were discussed in the AR4 andBates et al. (2008), but cause-and-effect between external forcing andchanges in floods was not explicitly assessed. A rare example consideredin Rosenzweig et al. (2007) and Bates et al. (2008) was a study by Millyet al. (2002) which, based on monthly river discharge, reported animpact of anthropogenic climate change on changes (mostly increases)in ‘large’ floods during the 20th century in selected extratropical riverbasins larger than 20,000 km 2 , but they did not endorse the studybecause of the lack of widespread observed evidence of such trends inother studies. More recent literature has detected the influence ofanthropogenically induced climate change in variables that affectfloods, such as aspects of the hydrological cycle (see Section 3.2.2.2)including mean precipitation (X. Zhang et al., 2007), heavy precipitation(see Section 3.3.2), and snowpack (Barnett et al., 2008), though a directstatistical link between anthropogenic climate change and trends in themagnitude and frequency of floods is still not established.176