IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentChapter 3as being affected by more severe droughts, consistent with availableglobal projections (Table 3-3; see also Giorgi, 2006; Rowell and Jones,2006; Beniston et al., 2007; Mariotti et al., 2008; Planton et al., 2008).Mediterranean (summer) droughts are projected to start earlier in theyear and last longer. Also, increased variability during the dry and warmseason is projected (Giorgi, 2006). One GCM-based study projected oneto three weeks of additional dry days for the Mediterranean region bythe end of the century (Giannakopoulos et al., 2009). For North America,intense and heavy episodic rainfall events with high runoff amountsare interspersed with longer relatively dry periods with increasedevapotranspiration, particularly in the subtropics. There is a consensus ofmost climate model projections for a reduction in cool season precipitationacross the US southwest and northwest Mexico (Christensen et al., 2007),with more frequent multi-year drought in the US southwest (Seager et al.,2007; Cayan et al., 2010). Reduced cool season precipitation promotesdrier summer conditions by reducing the amount of soil water availablefor evapotranspiration in summer. For Australia, Alexander and Arblaster(2009) project increases in consecutive dry days, although consensusbetween models is only found in the interior of the continent. Africanstudies indicate the possibility of relatively small-scale (500-km)heterogeneity of changes in precipitation and drought, based on climatemodel simulations (Funk et al., 2008; Shongwe et al., 2009). Regionalclimate simulations of South America project spatially coherent increasesin CDD, particularly large over the Brazilian Plateau, and northern Chileand the Altiplano (Kitoh et al., 2011).Available global and regional studies of hydrological drought (Hirabayashiet al., 2008b; Feyen and Dankers, 2009) project a higher likelihood ofhydrological drought by the end of this century, with a substantialincrease in the number of drought days (defined as streamflow below aspecific threshold) during the last 30 years of the 21st century overNorth and South America, central and southern Africa, the Middle East,southern Asia from Indochina to southern China, and central and westernAustralia. Some regions, including eastern Europe to central Eurasia,inland China, and northern North America, project increases in drought.In contrast, wide areas over eastern Russia project a decrease in droughtdays. At least in Europe, hydrological drought is primarily projected tooccur in the frost-free season.Increased confidence in modeling drought stems from consistencybetween models and satisfactory simulation of drought indices duringthe past century (Sheffield and Wood, 2008a; Sillmann and Roeckner,2008). Inter-model agreement is stronger for long-term droughts andlarger spatial scales (in some regions, see above discussion), while local toregional and short-term precipitation deficits are highly spatially variableand much less consistent between models (Blenkinsop and Fowler,2007b). Insufficient knowledge of the physical causes of meteorologicaldroughts, and of the links to the large-scale atmospheric and oceancirculation, is still a source of uncertainty in drought simulations andprojections. For example, plausible explanations have been proposed forprojections of both a worsening drought and a substantial increase inrainfall in the Sahara (Biasutti et al., 2009; Burke et al., 2010). Anotherexample is illustrated with the relationship of rainfall in southernAustralia with SSTs around northern Australia. On annual time scales,low rainfall is associated with cooler than normal SSTs. Yet the warmingobserved in SST over the past few decades has not been associated withincreased rainfall, but with a trend toward more drought-like conditions(N. Nicholls, 2010).There are still further sources of uncertainties affecting the projectionsof trends in meteorological drought for the coming century. The twomost important may be uncertainties in the development of the oceancirculation and feedbacks between land surface and atmosphericprocesses. These latter processes are related to the effects of drought onvegetation physiology and dynamics (e.g., affecting canopy conductance,albedo, and roughness), with resulting (positive or negative) feedbacksto precipitation formation (Findell and Eltahir, 2003a,b; Koster et al.,2004b; Cook et al., 2006; Hohenegger et al., 2009; Seneviratne et al.,2010; van den Hurk and van Meijgaard, 2010), and possibly – as onlyrecently highlighted – also feedbacks between droughts, fires, andaerosols (Bevan et al., 2009). Furthermore, the development of soilmoisture that results from complex interactions among precipitation,water storage as soil moisture (and snow), and evapotranspiration byvegetation is still associated with large uncertainties, in particularbecause of lack of observations of soil moisture and evapotranspiration(Section 3.2.1), and issues in the representation of soil moistureevapotranspirationcoupling in current climate models (Dirmeyer et al.,2006; Seneviratne et al., 2010). Uncertainties regarding soil moistureclimateinteractions are also due to uncertainties regarding the behaviorof plant transpiration, growth, and water use efficiency under enhancedatmospheric CO 2 concentrations, which could potentially have impactson the hydrological cycle (Betts et al., 2007), but are not well understoodyet (Hungate et al., 2003; Piao et al., 2007; Bonan, 2008; Teuling et al.,2009; see also above discussion on the causes of observed changes).The space-time development of hydrological drought as a response to ameteorological drought and the associated soil moisture drought(drought propagation, e.g., Peters et al., 2003) also needs more attention.There is some understanding of these issues at the catchment scale(e.g., Tallaksen et al., 2009), but these need to be extended to theregional and continental scales. This would lead to better understandingof the projections of hydrological droughts, which would contribute toa better identification and attribution of droughts and help to improveglobal hydrological models and land surface models.In summary, there is medium confidence that since the 1950ssome regions of the world have experienced trends toward moreintense and longer droughts, in particular in southern Europeand West Africa, but in some regions droughts have become lessfrequent, less intense, or shorter, for example, central NorthAmerica and northwestern Australia. There is medium confidencethat anthropogenic influence has contributed to some changesin the drought patterns observed in the second half of the 20thcentury, based on its attributed impact on precipitation andtemperature changes (though temperature can only be indirectlyrelated to drought trends; see Box 3-3). However there is lowconfidence in the attribution of changes in droughts at the level174