IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3regions. Hence, drought is a complex phenomenon that is stronglyaffected by other extremes considered in this chapter, but that is alsoaffected by changes in mean climate features (Section 3.1.6). In addition,via land-atmosphere interactions, drought also has the potential toimpact other weather and climate elements such as temperature andprecipitation and associated extremes (Koster et al., 2004b; Seneviratneet al., 2006a; Hirschi et al., 2011; see also Section 3.1.4). Case Study 9.2.3addresses aspects related to the management of adverse consequencesof droughts; while Case Study 9.2.2 considers the possible impacts ofhigh temperatures and drought on wildfire.Observed ChangesThere are still large uncertainties regarding observed global-scale trends indroughts. The AR4 reported based on analyses using PDSI (see Box 3-3)that very dry areas had more than doubled in extent since 1970 at theglobal scale (Trenberth et al., 2007). This assessment was, however,largely based on the study by Dai et al. (2004) only. These trends in thePDSI proxy were found to be largely affected by changes in temperature,not precipitation (Dai et al., 2004). On the other hand, based on soilmoisture simulations with an observation-driven land surface model forthe time period 1950-2000, Sheffield and Wood (2008a) have inferredtrends in drought duration, intensity, and severity predominantlydecreasing, but with strong regional variation and including increases insome regions. They concluded that there was an overall moistening trendover the considered time period, but also a switch since the 1970s to adrying trend, globally and in many regions, especially in high northernlatitudes. Some regional studies are consistent with the results fromSheffield and Wood (2008a), regarding, for example, less widespreadincrease (or statistically insignificant changes or decreases) in someregions compared to the study of Dai et al. (2004) (e.g., in Europe, seebelow). More recently, Dai (2011) by extending the record did, however,find widespread increases in drought both based on various versions ofPDSI (for 1950-2008) and soil moisture output from a land surface model(for 1948-2004). Hence there are still large uncertainties with respect toglobal assessments of past changes in droughts. Nonetheless, there issome agreement between studies over the different time frames (i.e.,since 1950 versus 1970) and using different drought indicators regardingincreasing drought occurrence in some regions (e.g., southern Europe,West Africa; see below and Table 3-2), although other regions also indicateopposite trends (e.g., central North America, northwestern Australia; seebelow and Table 3-2). As mentioned in Section 3.1.6, spatially coherentshifts in drought regimes are expected with changing global circulationpatterns. Table 3-2 provides regional and continental-scale assessmentsof observed trends in dryness based on different indices (Box 3-3). Thefollowing paragraphs provide more details by continent.From a paleoclimate perspective recent droughts are not unprecedented,with severe ‘megadroughts’ reported in the paleoclimatic record forEurope, North America, and Australia (Jansen et al., 2007). Recent studiesextend this observation to African and Indian droughts (Sinha et al.,2007; Shanahan et al., 2009): much more severe and longer droughtsoccurred in the past centuries with widespread ecological, political, andsocioeconomic consequences. Overall, these studies confirm that in thelast millennium several extreme droughts have occurred (Breda andBadeau, 2008; Kallis, 2008; Büntgen et al., 2010).In North America, there is medium confidence that there has been anoverall slight tendency toward less dryness (wetting trend with more soilmoisture and runoff; Table 3-2), although analyses for some subregionsalso indicate tendencies toward increasing dryness. This assessment isbased on several lines of evidence, including simulations with differenthydrological models as well as PDSI and CDD estimates (Alexander etal., 2006; Andreadis and Lettenmaier, 2006; van der Schrier et al., 2006a;Kunkel et al., 2008; Sheffield and Wood, 2008a; Dai, 2011). The mostsevere droughts in the 20th century have occurred in the 1930s and1950s, where the 1930s Dust Bowl was most intense and the 1950sdrought most persistent (Andreadis et al., 2005) in the United States,while in Mexico the 1950s and late 1990s were the driest periods.Recent regional trends toward more severe drought conditions wereidentified over southern and western Canada, Alaska, and Mexico, withsubregional exceptions (Dai, 2011).In Europe, there is medium confidence regarding increases in drynessbased on some indices in the southern part of the continent, but largeinconsistencies between indices in this region, and inconsistent orstatistically insignificant trends in the rest of the continent (Table 3-2).Although Dai et al. (2004) found an increase in dryness for most of theEuropean continent based on PDSI, Lloyd-Hughes and Saunders (2002)and van der Schrier et al. (2006b) concluded, based on the analysis ofSPI and self-calibrating PDSI for the 20th century (for 1901-1999 and1901-2002, respectively), that no statistically significant changes wereobserved in extreme and moderate drought conditions in Europe [withthe exception of the Mediterranean region in van der Schrier et al.(2006b)]. Sheffield and Wood (2008a) also found contrasting drynesstrends in Europe, with increases in the southern and eastern part of thecontinent, but decreases elsewhere. Beniston (2009b) reported a strongincrease in warm-dry conditions over all central-southern (includingmaritime) Europe via a quartile analysis from the middle to the end ofthe 20th century. Alexander et al. (2006) found trends toward increasingCDD mostly in the southern and central part of the continent. Trends ofdecreasing precipitation and discharge are consistent with increasingsalinity in the Mediterranean Sea, indicating a trend toward freshwaterdeficits (Mariotti et al., 2008), but this could also be partly caused byincreased human water use. In France, an analysis based on a variationof the PDSI model also reported a significant increasing trend in droughtconditions, in particular from the 1990s onward (Corti et al., 2009).Stahl et al. (2010) investigated streamflow data across Europe andfound negative trends (lower streamflow) in southern and easternregions, and generally positive trends (higher streamflow) elsewhere(especially in northern latitudes). Low flows have decreased in mostregions where the lowest mean monthly flow occurs in summer, butvary for catchments that have flow minima in winter and secondary lowflows in summer. The exceptional 2003 summer heat wave on theEuropean continent (see Section 3.3.1) was also associated with a170