IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Changes in Impacts of Climate Extremes: Human Systems and EcosystemsChapter 4these two opposing drivers, which also interact. Flood or watermanagement measures may reduce vulnerability in the short term, butincreased security may generate more development and ultimately leadto increased exposure and vulnerability.Extreme events considered in this section can threaten the ability of thewater supply ‘system’ (from highly managed systems with multiplesources to a single rural well) to supply water to users. This may bebecause a surplus of water affects the operation of systems, but moretypically results from a shortage of water relative to demands – adrought. Water supply shortages may be triggered by a shortage of riverflows and groundwater, deterioration in water quality, an increase indemand, or an increase in vulnerability to water shortage. There ismedium confidence that since the 1950s some regions of the worldhave experienced more intense and longer droughts, in particular insouthern Europe and West Africa (see Section 3.5.1), but it is not possibleto attribute trends in the human impact of drought directly or just tothese climatic trends because of the simultaneous change in the otherdrivers of drought impact.There is medium confidence that the projected duration and intensity ofhydrological drought will increase in some regions with climate change(Section 3.5.1), but other factors leading to a reduction in river flows orgroundwater recharge are changes in agricultural land cover andupstream interventions. A deterioration in water quality may be drivenby climate change (as shown for example by Delpla et al., 2009;Whitehead et al., 2009; Park et al., 2010), change in land cover, orupstream human interventions. An increase in demand may be driven bydemographic, economic, technological, or cultural drivers as well as byclimate change (see Section 2.5). An increase in vulnerability to watershortage may be caused, for example, by increasing reliance on specificsources or volumes of supply, or changes in the availability of alternatives.Indicators of hydrological and water resources drought impact includelost production (of irrigated crops, industrial products, and energy), thecost of alternative or replacement water sources, and altered humanwell-being, alongside consequences for freshwater ecosystems (impactsof meteorological and agricultural droughts on production of rain-fedcrops are summarized in Section 4.3.4).Few studies have so far been published on the effect of climate changeon the impacts of drought in water resources terms at the local catchmentscale. Virtually all of these have looked at water system supply reliabilityduring a drought, or the change in the yield expected with a givenreliability, rather than indicators such as lost production, cost, or wellbeing.Changes in the reliability of a given yield, or yield with a givenreliability, of course vary with local hydrological and water managementcircumstances, the details of the climate scenarios used, and otherdrivers of drought risk. Some studies show large potential reductions insupply reliability due to climate change that challenge existing watermanagement systems (e.g., Fowler et al., 2003; Kim et al., 2009; Takaraet al., 2009; Vanham et al., 2009); some show relatively small reductionsthat can be managed – albeit at increased cost – by existing systems(e.g., Fowler et al., 2007), and some show that under some scenarios thereliability of supply increases (e.g., Kim and Kaluarachchi, 2009; Li et al.,2010). While it is not currently possible to reliably project specificchanges at the catchment scale, there is high confidence that changesin climate have the potential to seriously affect water managementsystems. However, climate change is in many instances only one of thedrivers of future changes in supply reliability, and is not necessarily themost important driver at the local scale. MacDonald et al. (2009), forexample, demonstrate that the future reliability of small-scale rural watersources in Africa is largely determined by local demands, biologicalaspects of water quality, or access constraints, rather than changes inregional recharge, because domestic supply requires only 3-10 mm ofrecharge per year. However, they noted that up to 90 million people inlow rainfall areas (200-500 mm) would be at risk if rainfall reduces tothe point at which groundwater resources become nonrenewable.There have been several continental- or global-scale assessments ofpotential change in hydrometeorological drought indicators (seeSection 3.5.1), but relatively few on measures of water resourcesdrought or drought impacts. This is because these impacts are verydependent on context. One published large-scale assessment (Lehner etal., 2006) used a generalized drought deficit volume indicator, calculatedby comparing simulated river flows with estimated withdrawals formunicipal, industrial, and agricultural uses. The indicator was calculatedacross Europe, using climate change projections from two climatemodels and assuming changes in withdrawals over time. They showedsubstantial changes in the return period of the drought deficit volume,comparing the 100-year return period for the 1961-1990 period withprojections for the 2070s (Figure 4-3). Across large parts of Europe, the1961-1990 100-year drought deficit volume is projected to have areturn period of less than 10 years by the 2070s. Lehner et al. (2006)also demonstrated that this projected pattern of change was generallydriven by changes in climate, rather than the projected changes inwithdrawals of water (Figure 4-3). In southern and western Europe,changing withdrawals alone only are projected to increase deficitvolumes by less than 5%, whereas the combined effect of changingwithdrawals and climate change is projected to increase deficit volumesby at least 10%, and frequently by more than 25%. In eastern Europe,increasing withdrawals are projected to increase drought deficitvolumes by over 5%, and more than 10% across large areas, but this isoffset under both climate scenarios by increasing runoff.Climate change has the potential to change river flood characteristicsthrough changing the volume and timing of precipitation, by altering theproportions of precipitation falling as snow and rain, and to a lesserextent, by changing evaporation and hence accumulated soil moisturedeficits. However, there is considerable uncertainty in the magnitude,frequency, and direction of change in flood characteristics (Section3.5.2). Changes in catchment surface characteristics (such as landcover), floodplain storage, and the river network can also lead tochanges in the physical characteristics of river floods (e.g., along theRhine: Bronstert et al., 2007). The impacts of extreme flood eventsinclude direct effects on livelihoods, property, health, production, andcommunication, together with indirect effects of these consequences242