IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environmentof single regions due to inconsistent or insufficient evidence.Post-AR4 studies indicate that there is medium confidence in aprojected increase in duration and intensity of droughts in someregions of the world, including southern Europe and theMediterranean region, central Europe, central North America,Central America and Mexico, northeast Brazil, and southernAfrica. Elsewhere there is overall low confidence because ofinsufficient agreement of projections of drought changes(dependent both on model and dryness index). Definitionalissues and lack of data preclude higher confidence than mediumin observations of drought changes, while these issues plus theinability of models to include all the factors likely to influencedroughts preclude stronger confidence than medium in theprojections.3.5.2. FloodsA flood is “the overflowing of the normal confines of a stream or otherbody of water, or the accumulation of water over areas that are notnormally submerged (some specific examples are discussed in Case Study9.2.6). Floods include river (fluvial) floods, flash floods, urban floods,pluvial floods, sewer floods, coastal floods, and glacial lake outburstfloods” (see Glossary). The main causes of floods are intense and/orlong-lasting precipitation, snow/ice melt, a combination of these causes,dam break (e.g., glacial lakes), reduced conveyance due to ice jams orlandslides, or by a local intense storm (Smith and Ward, 1998). Floodsare affected by various characteristics of precipitation, such as intensity,duration, amount, timing, and phase (rain or snow). They are also affectedby drainage basin conditions such as water levels in the rivers, thepresence of snow and ice, soil character and status (frozen or not, soilmoisture content and vertical distribution), rate and timing of snow/icemelt, urbanization, and the existence of dikes, dams, and reservoirs (Bateset al., 2008). Along coastal areas, flooding may be associated with stormsurge events (Section 3.5.5). A change in the climate physically changesmany of the factors affecting floods (e.g., precipitation, snow cover, soilmoisture content, sea level, glacial lake conditions, vegetation) and thusmay consequently change the characteristics of floods. Engineeringdevelopments such as dikes and reservoirs regulate flow, and land usemay also affect floods. Therefore the assessment of causes of changesin floods is complex and difficult. The focus in this section is on changesin floods that might be related to changes in climate (i.e., referred to as‘climate-driven’), rather than changes in engineering developments orland use. However, because of partial lack of documentation, these canbe difficult to distinguish in the instrumental record.Literature on the impact of climate change on pluvial floods (e.g., flashfloods and urban floods) is scarce, although the changes in heavyprecipitation discussed in Section 3.3.2 may imply changes in pluvialfloods in some regions. This chapter focuses on the spatial, temporal,and seasonal changes in high flows and peak discharge in rivers relatedto climate change, which cause changes in fluvial (river) floods. Riverdischarge simulation under a changing climate scenario requires a setof GCM or RCM outputs (e.g., precipitation and surface air temperature)and a hydrological model. A hydrological model may consist of a landsurface model of a GCM or RCM and a river routing model. Differenthydrological models may yield quantitatively different river discharge,but they may not yield different signs of the trend if the same GCM/RCM outputs are used. So the ability of models to simulate floods, inparticular regarding the signs of the past and future trends, depends onthe ability of the GCM or RCM to simulate precipitation changes. Theability of a GCM or RCM to simulate temperature is important for riverdischarge simulation in snowmelt- and glacier-fed rivers. Downscalingand/or bias-correction are frequently applied to GCM/RCM outputsbefore hydrological simulations are conducted, which becomes a sourceof uncertainty. More details on the feasibility and uncertainties inhydrological projections are described later in this section. Coastalfloods are discussed in Sections 3.5.3 and 3.5.5. Glacial lake outburstfloods are discussed in Section 3.5.6. The impact of floods on humansociety and ecosystems and related changes are discussed in Chapter 4.Case Study 9.2.6 discusses the management of floods.Worldwide instrumental records of floods at gauge stations are limitedin spatial coverage and in time, and only a limited number of gaugestations have data that span more than 50 years, and even fewer morethan 100 years (Rodier and Roche, 1984; see also Section 3.2.1). However,this can be overcome partly or substantially by using pre-instrumentalflood data from documentary records (archival reports, in Europecontinuous over the last 500 years) (Brázdil et al., 2005), and fromgeological indicators of paleofloods (sedimentary and biological recordsover centennial to millennial scales) (Kochel and Baker, 1982). Analysisof these pre-instrumental flood records suggest that (1) flood magnitudeand frequency can be sensitive to modest alterations in atmosphericcirculation, with greater sensitivity for ‘rare’ floods (e.g., 50-year flood andhigher) than for smaller and more frequent floods (e.g., 2-year floods)(Knox, 2000; Redmond et al., 2002); (2) high interannual and interdecadalvariability can be found in flood occurrences both in terms of frequencyand magnitude although in most cases, cyclic or clusters of floodoccurrence are observed in instrumental (Robson et al., 1998), historical(Vallve and Martin-Vide, 1998; Benito et al., 2003; Llasat et al., 2005),and paleoflood records (Ely et al., 1993; Benito et al., 2008); (3) pastflood records may contain analogs of unusual large floods, similar tosome recorded recently, sometimes considered to be the largest onrecord. For example, pre-instrumental flood data show that the 2002summer flood in the Elbe did not reach the highest flood levels recordedin 1118 and 1845 although it was higher than other disastrous floodsof 1432, 1805, etc. (Brázdil et al., 2006). However, the currently availablepre-instrumental flood data is also limited, particularly in spatial coverage.The AR4 and the IPCC Technical Paper VI based on the AR4 concludedthat no gauge-based evidence had been found for a climate-drivenglobally widespread change in the magnitude/frequency of floods duringthe last decades (Rosenzweig et al., 2007; Bates et al., 2008). However,the AR4 also pointed to possible changes that may imply trends in floodoccurrence with climate change. For instance, Trenberth et al. (2007)highlighted a catastrophic flood that occurred along several central175