Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science ProgramChapter 3Abrupt changes inhydroclimate thatlead to sustaineddrought can haveenormous impactson the managementof water systems.fuels, and an additional $3 billion in energycosts (Gleick and Nash, 1991).Rapid changes in climate that influence theatmospheric part of the hydrological cycle canaffect the amount, form, and delivery of precipitation,which in turn influence soil moisture,runoff, ground water, surface flows, and lakelevels, as well as atmospheric features such asclouds. Changes can take the form of shifts instate to overall wetter or drier conditions, morepersistent drought or flood-causing events, and/or a greater frequency of extreme events. All ofthese types of rapid changes can have serioussocietal impacts with far-reaching effects onwater availability, quality, and distribution(National Assessment, 2000).Shifts in the climate background state maymodulate, and either constructively or destructivelyinfluence, the “typical” hydrologicimpacts of seasonal to interannual climate variability.For example, the Southwestern UnitedStates, which tends to receive higher thanaverage winter-time precipitation during anEl Niño event, and relies on these events torefill water supply reservoirs, could benefitfrom changes that increase or enhance El Niñoevents, but suffer from increased droughts if LaNiña events, which tend to result in dry wintershere, become more frequent (Fig. 3.2).The impacts of these changes can exacerbatescarce water supplies in regions that are alreadystressed by drought, greater demand, andchanging uses. The Departmentof Interior analysis ofWestern U.S. water supplyissues (USBR, 2005) identifiesa number of potentialwater supply crises andconflicts by the year 2025based on a combination oftechnical and other factors,including population trendsand potential endangeredspecies’ needs for water,but under an assumptionof a statistically stationaryclimate (Fig. 3.1). Anytransient change in climateconditions that leads to anabrupt regime shift to morepersistent or more severe drought will onlycompound these water supply conflicts andimpact society.Abrupt changes in hydroclimate that lead tosustained drought can have enormous impactson the management of water systems, inparticular, the large managed river systems inwestern areas of the Western United States.Many of these managed systems are facingenormous challenges today, even withoutabrupt changes, due to increased demands, newuses, endangered species requirements, andtribal water-right claims. In addition, many ofthese systems have been found to be extremelyvulnerable to relatively small changes in runoff(e.g., Nemec and Schaake, 1982; Christensenand Lettenmaier, 2006).2.2 Global Context of NorthAmerican DroughtWhen drought strikes North America it isnot an isolated event. In “The Perfect Oceanfor Drought,” Hoerling and Kumar (2003)noted that the post-1998 drought that wasthen impacting North America extended fromthe western subtropical Pacific across NorthAmerica and into the Mediterranean region, theMiddle East, and central Asia. There was alsoa band of subtropical drying in the SouthernHemisphere during the same period. It haslong been known that tropical SST anomaliesgive rise to global precipitation anomalies, butthe zonal and hemispheric symmetry of ENSOimpacts has only recently been emphasized(Seager et al., 2005a).Hemispheric symmetry is expected if the forcingfor droughts comes from the tropics. Rossbywaves forced by atmospheric heating anomaliesin the tropics propagate eastward and polewardfrom the source region into the middle and highlatitudes of both hemispheres (Trenberth et al.,1998). The forced wave train will, however,be stronger in the winter hemisphere than thesummer hemisphere because the mid-latitudewesterlies are both stronger and penetratefarther equatorward, increasing the efficiencyof wave propagation from the tropics into higherlatitudes. The forcing of tropical tropospherictemperature change by the tropical SST andair-sea heat flux anomalies will also tend tocreate globally coherent hydroclimate patterns82