Book 2.indb - US Climate Change Science Program

Abrupt Climate Changemodels forced by these anomalies can reproducesome aspects of these droughts indicates thatthe ocean is an important driver. In additionto the ocean influence, some modeling andobservational studies estimate that soil moisturefeedbacks also influence precipitation variability(Oglesby and Erickson, 1989; Namias,1991; Oglesby, 1991). Koster et al. (2004) usedobservations to show that on the time scaleof weeks, precipitation in the Great Plainsis significantly correlated with antecedentprecipitation. Schubert et al. (2004b) comparedmodels run with average SSTs, with and withoutvariations in evaporation efficiency, and showedthat multiyear North American hydroclimatevariability was significantly reduced if evaporationefficiency was not taken into account.Indeed, their model without SST variability wascapable of producing multiyear droughts fromthe interaction of the atmosphere and deep soilmoisture. This result needs to be interpretedwith caution since Koster et al. (2004) also showthat the soil moisture feedback in models seemsto exceed that deduced from observations. Ina detailed analysis of models, observationsand reanalyses, Ruiz-Barradas and Nigam(2005) and Nigam and Ruiz-Barradas (2006)conclude that interannual variability of GreatPlains hydroclimate is dominated by transportvariability of atmospheric moisture and that thelocal precipitation recycling, which depends onsoil moisture, is overestimated in models andprovides a spuriously strong coupling betweensoil moisture and precipitation.Past droughts have also caused changes invegetation. For example, during the Dust Bowldrought there was widespread failure of nondrought-resistantcrops that led to exposureof bare soil. Also, during the Medieval megadroughtsthere is evidence of dune activity inthe Great Plains (Forman et al., 2001), whichimplies a reduction in vegetation cover. Conversionsof croplands and natural grasses to baresoil could also impact the local hydroclimatethrough changes in surface energy balanceand hydrology. Further, it has been argued onthe basis of experiments with an atmospheremodel with interactive dust aerosols that thedust storms of the 1930s worsened the drought,and moved it northward, by altering the radiationbalance over the affected area (Cook et al.,2008). The widespread devegetation causedby crop failure in the 1930s could also haveimpacted the local climate. These aspects ofland-surface feedbacks on drought over NorthAmerica need to be examined further with othermodels, and efforts need to be made to betterquantify the land-surface changes and dustemissions during the Dust Bowl.2.1.3 Historical Droughts over NorthAmerica and their ImpactsAccording to the National Oceanic and AtmosphericAdministration (NOAA; see http://www.ncdc.noaa.gov/oa/reports/billionz.htmlfor periodically updated economic informationregarding U.S. weather disasters), overthe period from 1980 to 2006, droughts andheat waves are among the most expensivenatural disasters in the United States along withtropical storms (including the devastating 2005hurricane season) and widespread or regionalflooding episodes. The annual cost of droughtto the United States is estimated to be in thetens of billions of dollars.The above describes the regular year-in, yearoutcosts of drought. In addition, persistentmultiyear droughts have had important consequencesin national affairs. The icon of droughtimpacts in North America is the Dust Bowl ofthe 1930s. In the early 20th century, settlerstransferred large areas of the Great Plains fromnatural prairie grasses, used to some extent forranching, to wheat farms. After World War I,food demand in Europe encouraged increasedconversion of prairie to crops. This was allpossible because these decades were unusuallywet in the Great Plains. When drought struck inthe early 1930s, the non-drought-resistant wheatdied, thus exposing bare soil. Faced with a lossof income, farmers responded by planting evenOver the periodfrom 1980 to 2006,droughts and heatwaves are amongthe most expensivenatural disasters inthe United Statesalong with tropicalstorms (includingthe devastating 2005hurricane season)and widespread orregional floodingepisodes.79