DroughtEarlyWarning andForecasting TheoryandPractice
CHRISFUNK,PhD
UnitedStatesGeologicalSurveyEarthResources ObservationandScienceCenter(USGSEROS),Sioux Falls,SouthDakota,U.S.&ClimateHazardsCenter, DepartmentofGeography,UniversityofCalifornia, SantaBarbara,California,U.S.
SHRADDHANANDSHUKLA,PhD
ClimateHazardsCenter,DepartmentofGeography, UniversityofCalifornia,SantaBarbara,California,U.S.
Elsevier
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Notices
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Dedication
WewouldlikethankSabina,Leah,andMithileshfortheirongoingsupportandpatience.JimVerdin’sdecadesofcommitmenttoimproved droughtearlywarningprovidedcriticalinspirationforthiswork.
1.Droughts,governance,disasters,andresponsesystems1
1.1 20th-centurydroughts disastersandtheElNiño SouthernOscillation7
1.2 21st-centurydroughts developingeffectiveearlywarningsystems16 References19
2.Droughtearlywarning definitions,challenges,andopportunities23
2.1 Definitions supplyanddemand,themanyflavorsof “dry” 23
2.2 Droughts whenwaterdemandexceedswatersupply25
2.3 Slow-onsetdisasters26
2.4 Quantifyingdroughtmagnitudeinmultipledimensions27
2.5 Impact-basedsectoralordisciplinarydefinitionsofdrought30
2.6 ContrastingrecentU.S.andsouthernAfricandroughts35
2.7 Chapterreview38 References41
3.Droughtearlywarningsystems43
3.1 TheU.S.NationalIntegratedDroughtInformationSystem43
3.2 TheFamineEarlyWarningSystemsNetwork50
3.3 FamineEarlyWarningSystemsNetworkFoodSecurityOutlooks52
3.4 Multistageearlywarning anEthiopiaexample54 References56 Furtherreading58
4.Toolsofthetrade1 weatherandclimateforecasts61
4.1 Examplesofoperationaldroughtforecastingsystems61
4.2 Methodsofweatherandclimateforecasts65
4.3 Sourcesofweatherandclimateforecastskill68
4.4 Summary69 References70 Furtherreading70
5.Toolsofthetrade2 landsurfacemodels71
5.1 Introduction71
5.2 Anoverviewoflandsurfacemodels72
5.3 Operationallandsurfacemodels baseddroughtmonitors74
5.4 Limitationsofdroughtmonitoringusinglandsurfacemodels79
5.5 Summary80 References80
6.Toolsofthetrade3 mappingexposureandvulnerability83
6.1 Exposureandvulnerability83
6.2 Conclusion98 References99
7.Theory understandingatmosphericdemandinawarming world101
7.1 Background101
7.2 Referenceevapotranspirationresistanceterms106
7.3 Definingreferencecropevapotranspiration107
7.4 TheFAO56Penman Monteithformulation107
7.5 Temperaturealoneisinsufficienttoestimatereferenceevapotranspiration109
7.6 ReferenceevapotranspirationdecompositionsandMorton’ s complementaryhypothesis110
7.7 Spatiotemporalvariationsinreferenceevapotranspirationandactual evapotranspiration,andtheirrelationshiptovulnerabilityandexposure111 References115
8.Theory indicesformeasuringdroughtseverity117
8.1 Introduction117
8.2 Lengthofrecordandnonstationarysystematicerrors123
8.3 Percapitawateravailability131
8.4 Summaryanddiscussion131 References132
9.Sourcesofdroughtearlywarningskill,stagedprediction systems,andanexampleforSomalia135
9.1 Theoceanasasourceofskill137
9.2 Skillfrompersistentatmosphericconditions140
9.3 Predictiveskillfromthelandsurface142
9.4 Stagedopportunitiesforpredictionsupportdefense-in-depth145
9.5 Summary:stagedstrategiesforeffectiveearlywarning157
9.6 Conclusion158 References158
10.Practice evaluatingforecastskill161
10.1 Introduction161
10.2 Deterministicforecastskillscores162
10.3 Probabilisticforecastskillscores167
10.4 Summary174 References175
11.Practice integratingobservationsandclimateforecasts177
11.1 Approach177
11.2 Bias-correctionanddownscalingmethods179
11.3 Anexample:TheNASAHydrologicalandForecastAnalysisSystem183
11.4 Summary184 References185
12.Practice actionableinformationanddecision-makingnetworks187
12.1 Actionableinformationandthethreepillars191
12.2 Actionableinformationanddecision-makingnetworks anexample fromfamineearlywarninginEastAfrica195
12.3 Conclusion205 References206
13.Finalthoughts209 References213
Foreword
Droughtisamongthemostdamaging,andleastunderstood,ofall “natural” hazards.Althoughrecentdrought-relateddisastersfromSomalia,to CapeTown,toCalifornia,havecontributedtoasenseofurgency, droughtshavenotreceivedcommensurateattentionwithinthehazards researchandpractitionercommunities,unlikeeventssuchashurricanes, floods,andearthquakes,whichhavedirectandimmediatelyvisible impacts.Mostcountries,regions,andcommunitiescurrentlymanage droughtriskthroughreactive,crisis-drivenapproaches.Droughtthus remainsa “hidden” hazardandyetcanspantimescalesfromafewmonths andseasonstodecades,andspatialscalesfromafewsquarekilometersto entireregions,withbillionsofdollarsandthousandsoflivesandlivelihoodsaffectedorlost.Despiteprogress,therearestillimportantlimitationstoourunderstandingandabilitytopredictvariousaspectsof drought,includingonset,duration,severity,recovery,andrecurrence.
Ashasbeennoted,muchisassumedaboutthevulnerabilityandcapacityofthoseaffectedbyenvironmentalhazards.Inthecontextofachangingclimate,muchisalsoassumedaboutthereliabilityofprojectionsand, asimportantly,aboutourunderstandingofpresent-dayvariabilityand howfeaturessuchasENSOmightbechanging.Theassumptionsthatare beingmadebybothresearchersandpractitionerswarrantamorecareful andupdatedexplicationofthestateofknowledge,uncertainties,challenges,andbenefits.Thisvolumeisamuchneededandtimelycomplementtothelong-standingchallengeofunderstandingthesocio-ecological conditioningfactorsofdrought-relatedrisk,framedundertheactionorientedconceptof “earlywarning.” Aselucidatedinthetext,fasterrates ofenvironmentalchange,includingcompoundingeffectsofheatstress/ evaporativedemand,andtheincreasinglycomplexdevelopmentandecologicalpathwaysthroughwhichdroughtimpactsfilter,maydrivesurprises andrapidtransitionsinwhichearlywarningsofemergingthresholdsare increasinglycritical.Improvingpredictionsofthefulllifecycleof droughtsrequiresabetterunderstandingofhowwater,vegetation,and energysignalspropagatethroughtheocean atmosphere landsystem, sheddinglightonthepredictabilityofthevariousphysicalfacetsof drought,includingprecipitation,temperature,soilmoisture,snow,and runoff.
Aslongrecognized,andfullyexploredinthisvolume,aforecastby itselfisnotanearlywarningsystem.Inaproactiveframinganearlywarningsysteminvolvesmuchmorethandevelopmentanddisseminationofa forecast;itincludesthesystematiccollectionandanalysisofrelevantinformationabout,andcomingfrom,areasofimpendingriskthat(1)informs thedevelopmentofstrategicresponsestoanticipatecrisesandcrisisevolution;(2)providescapabilitiesforgeneratingproblem-specificriskassessmentsandscenarios;and(3)effectivelyworkswithandcommunicates optionstocriticalactorsforthepurposesofdecision-making,preparedness,andmitigation.Successfuldroughtinformationsystemshavemultiple subsystemssupportedbyresearchinintegratedriskassessment,communication,anddecisionsupport,ofwhichearlywarningisacomponentand output.Thisvolumebringstobeartheexperienceoftheauthors,who havebeenengagedinearlywarningsystemdevelopmentsincetheEast Africandroughtsofthemid-1980s,andsystematicallyscrutinizesthe “ pros ” and “ cons ” ofexistingandproposedsystems,distillinglessonsfrom pastpractices,landmarkdroughtevents,andadvancesinthefield.
Theauthorscapturekeyaspectsofearlywarningdesign,includingthe importanceofconvergenceofevidence,placingmultipleindicatorswithin consistenttriggeringframeworks,andtheconfoundingfactorsofpopulation,technology,andenvironmentalchange.Sourcesofphysicalinformationasnotedinthetextarederivedfromsatellitedata,insitu observations,landsurfacesimulation,andmodel-basedforecastskill.Key totheproductiveuseofsuchinformationwithinearlywarningsisclearer delineationofsourcesofuncertainty,theirreliabilityatdifferenttimesof theyear,andtheintegrationofdiverseenvironmentaldatasetsinto coherentdatabasessuchastheCHIRPS.
Astheauthorsnote,thevolumeaimsatpragmaticgoals toprovidea readable,accessibleresourcethatisusefulinbothaclassroomandin national,meteorological,andhydrologicalagencies.Severalstudieshave identifiedthecharacteristicsofpredecisionalpracticesthatleadtoeffective communicationoverthelongterm.Keyamongthesecharacteristicsisthe needtobringthedeliverypersons(e.g.,extensionpersonnelwithinlocal communitiesandtheresearchcommunity)toanunderstandingofwhat hastobedonetotranslatecurrentinformation,reliably,intolocalcontexts,aswellastheneedtodevelop,support,andtrainacadreofprofessionalswhoviewtheroleoflinkingscience,policy,andpracticesasa coregoaloverthelongterm.Thechaptersonpracticeonactionable informationanddecision-makingnetworksprovideanexcellentgroundingforthetrainingofsuchprofessionals.
Theauthorsareabundantlyclearthatthechaptersdonotpresumeto coverallaspectsofdroughtriskandresiliencemanagement.Theychoose amoremodestgoal,highlightingthescienceandobservationsthatunderpinactionableimpactsassessmentsandearlywarning.Thedepthtowhich theseissuesarepursuedinthevolume,whilemaintainingreadabilityand accessibility,isimpressiveandverymuchwelcomed.Thisbookisan importantcontributiontothechallengeofdrawinginformationalongthe weather climatecontinuum,frominternalatmosphericvariabilityto modesofclimate.Thechaptersillustratetheneedtonotonlyunderstand anddesigninformationsystems “for” changebutalsotodesignrobust science-basedsystemsthathelpusnavigate “through” change.Morethan usefulorevenusableinformationisneeded;whatisrequiredarepathways toimproveddecisionsthatthreadthroughdisasterriskreduction,adaptation,sustainability.Droughtinformationsystemsalongthiscontinuumare investmentsratherthan “costs.” Suchaninformeddroughtearlywarning systemwouldnotbesimplytranslationalbut,bydesign,transformative. Thisbookprovidesrobustandneededguidanceonbuildingsuchapath. Thechaptersinthisvolumewillbevitaltoolsinthequiverofeffective earlywarningpractitionersandresearchersandforprovidingaspacein whichthesecommunitiescanworktogether.
RogerS.Pulwarty
NOAAPhysicalSciencesLaboratory,Boulder,CO,U.S.
Preface
Wearepleasedtosharewithyou DroughtEarlyWarningandForecasting: TheoryandPractice.Thisbookisintendedtoassistbothstudentsand droughtearlywarningpractitionersinunderstanding,developing,and applyingdroughtearlywarningsystems.Everyyeardroughtsimpactmillionsofpeopleandcauselossestotalingbillionsofdollars.Asourpopulationandeconomiesexpand,sogrowsourexposuretodroughtrisk. Increasingwaterdemandsandtheimpactsofclimatechangekeepacceleratingtheneedforeffectivedroughtmanagementanddroughtearlywarning.Focusingonthissecondchallenge,thismanuscriptgatherstogether manyofthedifferentcomponentsofdroughtearlywarningsystems.The introductorychapters(Chapters1 3)describethehistoricnatureof droughtswhileintroducingdroughtearlywarningandearlywarningsystems.Droughtsarecomplexslow-onsetmultiscaledisastersthatimpact manydifferentsectors,anddroughtearlywarningsystemsmustbecorrespondinglysophisticated.Tothisend,thenextthreechapters(Chapters 4 6)describekey “toolsofthetrade” (weatherandclimateforecasts, landsurfacemodels,andmapsdescribingexposureandvulnerability).We thendescribe(Chapters7 8)twotheoreticalframeworkscentralto understandingandmonitoringdroughts:atmosphericwaterdemandand droughtindices.Theremaining “practice” chapters(Chapters9 12)of thebookaddressvariousaspectsrelatedtodevelopingintegratedsystems. Whilesomeofthismaterialhasbeendrawnfromourmanyyearsof experiencewiththeFamineEarlyWarningSystemsNetwork,thetechnicaldescriptions,generalstrategies,andlessonslearnedshouldbeapplicable inmanydroughtearlywarningsettings.
Droughtearlywarningpractitionershaveauniqueopportunitytouse theirskillstobenefitsociety,guardingthelivesandlivelihoodsofhundredsofthousandsorevenmillionsofpeople.Theseskillswillbeputto thetestinthecomingcentury,andwehopethatthisaccessible “ onestop-shop” discussionofdroughtearlywarningsciencewillprove valuable.
CHAPTER1
Droughts,governance,disasters, andresponsesystems
Historyissomethingthatveryfewpeoplehavebeendoingwhileeveryoneelse wasplowingfieldsandcarryingwaterbuckets.
Sapiens,YuvalHarari,p.101
HistoryforuswaitssilentlyinthebasementoftheNationalMuseum ofEthiopia,inAddisAbaba(Fig.1.1).There,Lucy,akaAL288-1,rests,as shehasrestedforsome3.2millionyears.Amemberof Australopithecus afarensis,diminutiveLucywalkedacrossthehot,lowAfarTrianglein northeasternEthiopiaduringthePlioceneEpoch,ormorespecifically, duringthemid-Pliocenewarmperiod.TheEarthatthistimewas extremelywarm,withcarbondioxidelevelsonparwiththosetoday (Raymoetal.,1996).High-resolutionpollendatafromHadar,Ethiopia,
Figure1.1 SkullandpelvisofLucy,takenattheNationalMuseumofEthiopia,Addis Ababa. ChrisFunk.
whereLucywasfound,showsalargebiomeshift,withupto5°Cof coolingandalargeincreaseinannualrainfalloccurringbetween3.4and 2.9millionyearsago(Bonnefilleetal.,2004).Thisclimateshiftmayhave helpedstimulatetheevolutionofbipedalproto-humans.Lucy’sbones representaspeciesintransition.Theshapeofherkneesandpelvisindicate bipedallocomotion,andthelengthofherarmswasrelativelyshortand herlegsrelativelylong,comparedtochimpanzees.Herbrain,however, wasquitesmall.
Therearenumeroustheoriesastowhyhumansevolvedbipedal motion.Bipedallocomotionismoreefficient,andshiftingforestsand fragmentedlandscapesmayhaverewardedthisinnovation.Bipedalism mayhavemadeiteasiertogivebirthtobig-brainedbabies(Falketal., 2012).Bipedalcreatureshavetheirhandsfree,enablingthemtobetter produceandmanipulatetools.This,inturn,mayhaveledtoamore protein-richdiet,potentiallyleadingtoanincreaseinbrainsize(Johanson andEdgar,1996).
By2.5millionyearsago,ourhumanancestorswereusingsimpletools. Ourbrainshadgrownprodigiously,necessitatingtheearlybirthofchildrenandthuslongerperiodsofchildhooddependency.Thismayhave necessitatedstrongsocialnetworksandlanguagedevelopment.Asingle motherwouldhavehadalotoftroubleforagingforfoodandtakingcare oflittlechildren.Theearlybirthofchildren,inturn,mayhavecreated uniqueopportunitiesforeducation,innovation,andculturalevolution. Tooluseandcommunicationmayhavehelpedmovehumansintoatop predatorpositionby400,000yearsago.Itwasthenthatsomehuman bandswerehuntinglargegameonaregularbasis.By300,000yearsago, wewereusingfireonaregularbasis.Firekilledgermsandparasitesand madefoodeasiertochewanddigest,allowingourancestorstoeata widervarietyoffood.Easierdigestionmayhaveledtoshorterintestinal tracksinSapiensandNeanderthals(Gibbons,2007),reducingenergyconsumption,helpingusfuelourmassivebrains.Thesehugebrains,combinedwithoursocialproclivities,helpedleadtoevolutionarysuccess,and out-migrationfromAfricaabout70,000yearsago(Harari,2015).
HumanitymovedoutofAfrica(Fig.1.2)andspreadacrossAsiaand Europe,with Homosapiens supplantingNeanderthals,perhapsduetotheir advantageouscommunicationskillsandsocietalcoordination.These advantageshelpedtriggerthecognitiverevolution,whichallowedusto formlargecommunitiesandlearn,teach,andengageincomplexbehaviors.Slowly,thesecoordinationskillsgrew,untilearlyhumanswereable
Figure1.2 Expansionof Homosapiens outofAfrica. ChrisFunk,afterMap1in SapiensABriefHistoryofHumankind,YuvalNoahHarari.
Figure1.3 Sophisticatedhunter gatherersbuiltatemplebeforeagriculture. Wikimedia.
toassemblecomplextempleslikeGöbekliTepe,whichisincurrent-day southernTurkeyneartheSyrianborder(Fig.1.3).Builtaround10,000 BCE,thisNeolithic(StoneAge)templepredatedagriculture.Immense6m,20-tonT-shapedstonemegalithswerefittedintosocketshewninthe bedrocktoformcircles.Intricatecarvingsofbulls,foxes,cranes,andpeopleadornpillarsandtotems.GöbekliTepesupportstheearlyimportance ofreligionandsocialstructureinhumandevelopment.Beforeagriculture, pottery,writing,metallurgy,ortheinventionofthewheel,Neolithic hunterandgatherersbuiltcomplexsocietiesandbuildings.
Soon,however,agriculturewouldchangetheworld(Fig.1.4).Inthe MiddleEast’sFertileCrescent,wheatandgoatsweredomesticatedby about9000BCE.c.7000BCE,independentagricultureinnovationsled tothedomesticationofmilletincurrent-dayChina.By4000BCEearly varietiesofmaize,beans,andsquashwerebeingraisedinCentral America.Insub-SaharanAfrica,agricultureemergedintheEthiopian highlands,WestAfrica,andtheSahel(Diamond,1997).Taro,bananas, andsugarcaneweredomesticatedinNewGuineawhileSouthAmericans begantoharvestpotatoesandmanioc.InthesoutheasternUnitedStates, NativeAmericansraisedsunflowers,sumpweed,andgoosefoot.
Withtheriseofagriculturecameanincreasedsocietalsensitivity todrought.Withhighmobilityandlowpopulationdensities,earlier hunter gathererpopulationswereprobablyrelativelyresilienttomost
Figure1.4 Sitesofearlyagriculturaldevelopment. ChrisFunk,aftermapinSapiensA BriefHistoryofHumankind,YuvalNoahHarari.
climaticextremes.Theycouldmovetowardmoreabundantrains,unlike thefarmersofmorerecenttimes.Thespatialcompassofearlyagriculturalistsextendedtothefewmilesaroundtheirhabitations.Villagesandcitieslivedordiedbasedonthestrengthoftheirharvests.Forexample,a studyofdroughtstressvariabilityinancientNearEasternagriculturalsystemsfinds “Theemergenceanddeclineofearlycivilizationsisintrinsically tiedtoagriculturalsurplusproduction,eitherenablingafocusontechnologicalprogressandtheaccumulationofwealthor,inthecaseofinsufficientyield,leadingtohunger,violence,andwar(Riehletal.,2014).”
Oneofthemostseverecrisesinancienttimeswasthe4220BP(2200 BCE)Holoceneclimateevent,whichwasassociatedwithseverearidityin NorthAfrica,Egypt,theMiddleEast,theIndianContinent,andNorth America.Thisaridificationeventmayhavehelpedtriggerthedemiseof theOldKingdominEgypt,theIndusValleyCivilizationinIndia,the AkkadianEmpireinMesopotamia,andtheLiangzhucultureinpresentdayChina.
Themostsuccessfulearlycivilizationssupporteddensepopulationsby enablingthemtodevoteresourcestoreligious,administrative,military, artistic,andindustrialactivities.Droughtsposedanexistentialthreatto thesesocieties.Inresponse,variouswaterandagriculturalmanagement practicesweredeveloped.InancientAssyria,granariesandartificialirrigationhelpedovercomefoodshortages(Sołtysiak,2016).Inancient Mesopotamia,whereHammurabiestablishedhisfamouscodein1754 BCE,theredevelopedasystemofcommunalcanalsandirrigationworks andalegalframeworktogoverntheseworks(Kornfeld,2009).Inancient Egypt,pharaohseffectivelycontrolledequitablewaterdistributionsystems (primarilydrivenbymanpower)thatpromotedsocialstabilitywhileprovidingsuppliestocitiesandtowns(Driaux,2016).InancientIndia, “deficienciesofrainfallwereovercomebymeansofoneortheotherformof irrigation rivers,canals,lakes,tanks,wells,artificialreservoirs,pondsand pools” (Date,2008).
Giventheacknowledgedwaterdependenciesofancientagricultural societies,itcomesasnosurprisethattheyalsoshowedakeeninterestin droughtearlywarning.Earlyagrarianpopulationsgrewrapidlyandcould onlybesustainedwithextensiveeffortandstableaccesstowater.Inhistorictimes,agriculturallandsmadeupasmallfractionoftheEarth’sland surface,2%inCE1400(Marks,2006).Theselandsneededtobe defended,andwhendroughtstruck,therewaslittlemobility,andthe poormustmakedo.
AsdescribedbyYuvalHarari:
Fromtheveryadventofagriculture,worriesaboutthefuturebecamemajor playersinthetheatreofthehumanmind.Wherefarmersdependedonrainsto watertheirfields,theonsetoftherainyseasonmeantthateachmorningfarmersgazedtowardsthehorizon,sniffingthewindandstrainingtheireyes.Isthat acloud?Wouldtherainscomeontime?Wouldtherebeenough?Wouldviolatestormswashtheseedsfromthefieldsandbatterdowntheseedlings? Meanwhile,inthevalleysoftheEuphrates,Indus,andYellowRivers,otherpeasantsmonitored,withnolesstrepidation,theheightsofthewater.
Sapiens,YuvalHarari,p.101
Ouragriculturalfoundationshaveguaranteedastrongdependenceon adequateprecipitationthatremainswithustoday.InCE1400theEarth supportedabout360millionpeople,whereasnowitsupportsabout7.6 billion a21-foldincrease.Todaywefarmabout12%oftheEarth’sicefreesurface(Ramankuttyetal.,2008).Inmanypartsoftheworld,technologyhasdramaticallyincreasedyieldsandalteredfarmingpractices.In others,poorfarmersstillsowandreapmostlybyhand.Inallofthese regions,however,cropsstillrequirewater.Technologycandolittleto overcomecropwaterdeficits,and80%ofourcroplandsarestillunirrigated.Watercanbemoved,butthisisoftenexpensiveandenergyintensive.Inmanypartsofthedevelopingworld,furthermore,hydropower hasemergedasacriticalsourceofenergy,avaluablebutpotentiallyfragile alternativetofossilfuels.Commoditymarkets,bothlocalandglobal,can exposemillionsofpoorpeopletopriceshocks,sometimesoriginating fromdroughtsthousandsofmilesaway.Thepoorestofthesepoor,often subsistingon100or200USDayear,typicallyspendmorethanhalftheir incomesonfood;whenpricesjumpup,theymustoftendowithout food.In2018thecityofCapeTowninSouthAfricafaced “DayZero,” orthedaywhenthecitywasexpectedtorunoutofwater.Suchcrises arebecomingincreasinglycommon.In2015and2016,Harare, Zimbabwe,grapplingwiththeimpactofanElNiño-relateddrought, facedsimilarconditions,asdidSaoPaulo,Brazil.In2018Mexico’s21 millionresidentsalsofacedshortagesofrunningwater.
Inancienttimes,settlementsandcitiesgrewalongriversandgaverise togreatcivilizations,andtoday,accordingtoarecentWorldBankreport, weremainhighlydependentonwateravailability(Damaniaetal.,2017). Thisreportfindsthat “throughoutmuchoftheworld,evenmoderate deviationsfromnormalrainfalllevelscancauselargechangesincrop yields.” Rainfallshocksinduceacascadeofeffectsthatincludelower
agriculturalproductionanddeforestation.Ironically,inmanyareas,irrigationcanresultinthe paradoxofsupply,whenwaterissuppliedtoocheaply andconsumedrecklessly.Droughtscanleadtosevereundernutritionof childrenandmothers,producingchildhoodstunting,andanincreaseof low-birthweightchildren(Davenportetal.,2017;Graceetal.,2012, 2015),allofwhichcanincreasepovertyandreinforceintergenerational healthproblems.
Droughtsstillcontributetoseverehunger.In2017approximately815 millionpeople,roughly1outofevery10peopleonourplanet,were deemedbytheUnitedNationsFoodandAgriculturalOrganizationtobe sufferingfromchronichunger.Contemporaneousassessmentsof extremelyfood-insecurepopulations thosefacingarealthreatoffamine withoutimmediateemergencyassistance totaled81million,roughly oneofeveryhundredhumanbeings.Despiteourgreattechnological achievements,hungeranddroughtstillplaguetheadvanceofhumanity.
Aspopulationsexpandandourclimatebecomeswarmerandmore variable,wewillneedimproveddroughtearlywarningsystems(DEWS). Thisbookdescribesboththetheoryandpracticalmethodsrequiredto createandeffectivelyusethesesystems,helpingtopotentiallysavelives andlivelihoodswhilemitigatingsomeoftheimpactsofclimatechange. Overthenext20years,weknowthatincreasingpopulation,economic growth,andrisingairtemperatureswillincreasedemandsforwater,while rainfedandglacier-basedwatersuppliesarelikelytobecomemoreerratic. Wealsoknow,however,thatourabilitytoobservetheEarthusingsatellitesisrapidlyincreasing.Witheachpassingyear,wecanbetterobserve, explain,andpredictweatherandclimateextremes.Richersocialand computernetworksaresupportingenhancedcommunicationanddecision support.Thisinformationisbeingusedsuccessfullytohelpusanticipate, preparefor,andrespondto21st-centurydroughts.Atthebeginningof the21stcentury,weareachievingwhatcouldonlybedreamedofatthe outsetofthe20thcentury.
1.120th-centurydroughts
The20thcenturybeganwithseverefaminesthatkilledtensofmillionsof people,helpingtomotivatethemodernscienceofdroughtearlywarning. Between1896and1902,inIndiaandChina,themonsoonrainsfailed, bringingdestructiveepidemicsofmalaria,bubonicplague,dysentery,
—disastersandtheEl Niño SouthernOscillation
smallpox,andcholera(Davis,2002).MortalityestimatesforIndiaindicate 19millionpeoplemayhaveperished,whileinChina,itwasestimatedto be10million.TheBombayGovernment’ s “ReportontheFamineinthe BombayPresidency” foundthat1899 1900harvestsintheBombay Deccan,Karnatak,andGujaratprovinceswereonly4% 16%ofnormal. Indianauthorities,heldunderrigidinflexibleideologicalBritishrule, failedtorespondadequatelytotheextremeconditions.GeorgeNathaniel Curzon,firstMarquessCurzonofKedleston,servedasviceroy.Inhiszeal tosuppressHomeRule,Curzontightenedpresscensorship,clamped downoneducation,andpittedHinduagainstMuslim.ForCurzon, financingtheBoerWarinSouthAfricawasmuchmoreimportantthan relievingthedistressofthefamine-strickenpeopleofIndia.Writingatthe time,andquotingdatafromtheLancet,WilliamDigbywrote “This statementbywhatisprobablytheforemostmedicaljournalintheworld meansthatthelossoflifethusrecordedrepresentedthe ‘disappearance’ of fullyone-halfapopulationaslargeasthatoftheUnitedKingdom” (Davis,2002).
ForIndiathesuccessivedroughtsof1876 79,1896 97,and 1899 1900hadahugenegativeimpactonproductivity,livestock,and development. “Almostalltheprogressmadeinagriculturaldevelopment since1880wasnullifiedduringthefamines” (Davis,2002).
India’sterribledroughts,however,didhelppromptmajorintellectual advancesthateventuallyhelpedusunderstand,andsometimesanticipate, futureclimateextremes.
GilbertWalker’sappointmentasSpecialAssistanttotheDirector GeneraloftheIndianObservatoriesin1903cameasasurprise(Walker, 1997).Beforebeingselectedforthejob,Gilbert “Boomerang” Walkerhad recentlypublishedanoriginalandimaginativepaperontheseAustralian spinningdevices(boomerangs)inthePhilosophicalTransactionsofthe RoyalSociety(1897,190,pp.23 42).Walkerhadbeenselectedbecause ofhisexceptionallystrongmathematicalcapabilities.In1904hetook chargeoftheIndiaMeteorologicalDepartment.WhileorganizingthevariousIndianweatherobservatoriesandservicestookupmuchofhistime,he quicklyturnedtoanalyzingtheaccuracyofmonsoonforecasts.Realizing thathecouldnotpredictthemonsoondroughtsanalytically,heturnedto theanalysisoflaggedcorrelations.Interestingly,thisworkledtofundamentaladvancesinourunderstandingofoneimportantfeatureofthemean globalclimate(theWalkerCirculation),themostimportantquasiperiodic climatevariation[theElNiño SouthernOscillation(ENSO)],anda
mathematicaldescriptionoftheexpectedautocorrelationstructureofan autoregressiveprocess(theYule Walkerequations)(Katz,2002).
Likepreviousmeteorologistsbeforehim,Walkerbeganthedaunting prospectoflookingforpredictiveanomaliesthatcouldbeusedtoforecast Indiandroughts.Thisledhimtoanalyzevariationsinworldwideweather. FollowingthenewworkofthestatisticianKarlPearson,hebecameapioneerintheuseofcorrelationinmeteorology.By1908hewasusingmultipleregressiontopredictmonsoonrainfall.Examiningtherelativelyrich setofglobalweatherdataavailabletoaBritishmeteorologist,Walkeralso lookedfor “CentersofAction” byexaminingextensivetablesofautocorrelationsandcross-correlationsinsea-levelpressureatmultiplelocations.
Fromthisanalysis(Katz,2002),hedetermined, “thereisaswayingof pressureonabigscalebackwardsandforwardsbetweenthePacificOcean andtheIndianOcean,thereareswayings,onamuchsmallerscale, betweentheAzoresandIceland,andbetweentheareasofhighandlow pressureintheN.Pacific” (Walker,1923).Theswayingofpressure betweenthePacificandIndianOceansisnowreferredtoasthe “SouthernOscillation” (SO).TheseesawbetweentheAzoresandIceland isknownastheNorthAtlanticOscillation,andtheoscillationinthe PacificisreferredtoastheNorthPacificOscillation.Allthreeofthese patternsofclimatevariabilityhaveturnedouttobecriticaldriversofclimateanddrought(Fig.1.5).
Walker,however,pointedoutthattheinfluenceoftheSOseemed muchgreaterandmorepersistentthantheothertwooscillatorypatterns. ThingshavechangedalotsinceWalkerflunghisboomerangsandpainfullyworkedouthiscross-correlationtablesbyhand.Technology,the Internet,computers,andcarefullycompileddatasetsnowmakeiteasyto analyzetheimportanceofclimatepatternsliketheSO.Forexample,with afewwell-chosenclicks,wecancompileamapofthecorrelation betweenlocalsea-levelpressurevaluesandthe “SOIndex”—anindex
measuringthedifferenceinsea-levelpressurebetweenTahitiintheSouth PacificandDarwin,Australia.Thispattern,firstidentifiedbyWalker, identifiesaquasi-globalseesaw1 inpressurebetweenthewestPacific/ IndianOceanregionandtheeastPacific.
Asownersofavastinternationalnavalempire,theBritishwerekeen totrackandanalyzesea-levelpressuredata,likethedataanalyzedby GilbertWalker,becauseitwasstronglyrelatedtochangesinsurface winds.Understandingthemean(long-termaverage)structureofatmosphericcirculationsturnsouttobeacriticalcomponentofeffective droughtearlywarning.Understandingour “normal” climateprovidesan importantfoundationforunderstandingextremes.Onekeyadvanceon thisfrontwasmadebyGeorgeHadley(1685 1786).Hadleywasan Englishlawyerandamateurmeteorologistwhoproposedtheatmospheric mechanismbywhichthetradewindsaresustained.Asakeyfactorin ensuringthatEuropeansailingvesselsreachedNorthAmericanshores, understandingthetradewindswasamatterofgreatimportanceatthe time.Hadleywasintriguedbythefactthatwinds,whichshouldbyall rightshaveblownstraighttowardtheequator,hadapronouncedwesterly flow. Hadley(1735) beganconsideringhowthedifferentialheatingofthe equatorproducedlowatmosphericpressure,whichdrewinthelow-level winds: “ForletussupposetheAirineveryParttokeepanequalPace withtheEarthinitsdiurnalMotion;inwhichcasetherewillbenorelativeMotionoftheSurfaceoftheEarthandAir,andconsequentlyno Wind;thenbytheActionoftheSunonthepartsabouttheEquator,and theRarefactionoftheAirproceedingtherefrom,lettheAirbedrawn thitherfromtheN.andS.parts.” Hadleythenwentontointroducethe ideaoftheconservationofangularmomentum: “Fromwhichitfollows, thattheAir,asitmovesfromtheTropicstowardstheEquator,havinga lessVelocitythanthePartsoftheEarthitarrivedat,willhavearelative MotioncontrarytothatofthediurnalMotionoftheEarthinthoseParts, whichbeingcombinedwiththeMotiontowardstheEquator,aN.E. windbeproducedonthisSideoftheEquator,andS.E.ontheother.”
Hadley’stheoryturnedouttobecorrect,andthestructurehe describedisnowknownastheHadleyCirculation(Fig.1.6).TheHadley CirculationdescribedtheNorth Southandup downmotionsofthe
1 Inwritingthefirstdraftofthissentence,wemadeaprescienttypographicerror describingseesawpatterns.ButitwouldbedecadesbeforescientistslinkedtheatmosphericSouthernOscillationwiththeoceanicphenomenonknownasElNiño.
atmosphereaveragedacrosseverylongitudeband.Thedifferentialheating oftheequatorcombinedwiththeconservationofangularmomentum producesnear-surfaceatmosphericconvergenceneartheequator.This bandofconvergenceisalsoassociatedwithheavyrainfallandascending atmosphericmotions(redbandin Fig.1.6).Airparcelsriseandmove towardthepoles(andtheeast).Theconservationofangularmotionagain turnsthemintoastronglyeastwardwindfield(thesub-TropicalEasterly Jet).Theserelativelywarmparcelsofaircool,radiatingtheirextrawarmth backouttospace.Thiscoolingmakestheatmosphereverystable,andwe findatendencyforairtosubside(sink)inthesubtropics,atlatitudesof about30-degreenorthandsouth.Thislatitudeiswherewetendtofind mostoftheworld’sdesertsandaridlands:theSahara,Kalahari,Arabian Peninsula,SouthwestandCentralAsia,AtacamaDesert,Australia,andthe southwesternUnitedStatesareassociatedwiththesinkingbranchofthe HadleyCirculation.Manyseveredroughtsoccurwithinorontheedges ofthesedryregimes,whereconditionscanslipfromtentativetodisastrous,sometimesforyearsonend.
GilbertWalker’sSO,however,focusedonEast Westvariations betweentheeasternandwesternPacific.Eventually,thiswouldturnout tobeacriticaladvanceleadingtoeffectivedroughtforecasts.Akeystep towardthatpoint,though,wasdescribingthemean(long-termaverage) structureoftheeast westatmosphericmotionsoverthePacific.ThispioneeringworkwascarriedoutbyJacobBjerknesinthe1960s.Hecoined theterm “WalkerCirculation” andwasthefirstpersontodescribethe
Figure1.6 SchematicrepresentationoftheHadleyCirculation. ChrisFunk.
linkbetweenElNiñoandtheSouthernOscillation(Bjerknes,1969). Thiscoupledair-seaclimatevariationisnowcommonlyreferredtoasthe ENSO.BjerknesstartedbynotingthattheeasternPacificseasurfacetemperatureswereexceptionallycold(Fig.1.7),whilethewesternPacific/ IndianOceanseasurfacetemperaturesweremuchwarmer.Becauseatthe equator,wedonotneedtoworryabouttheconservationofmotion, Bjerknestheorizedthatthistemperaturegradientwouldsetupathermally directcirculation.Coldwatersproducecoldairwitharelativelyhigh density.Warmwatersproducewarmairwithlowdensities.Windsatthe equatorwillmovetowardthelowerpressure.Thiswindpatternproduces manypositivefeedbacksthathelpcreatetheIndo-PacificWarmPool (Clementetal.,2005),whilealsoreinforcingthestrongeast westtropicaltemperaturegradientbetweentheWarmPoolandtheequatorialeast Pacific.
The “WarmPool” isatermusedtodescribetheworld’swarmest oceanwatersinanareaoftheequatorialeasternIndianandWestern PacificOceans(Fig.1.8).InthisrainiestregionontheEarth,seasurface temperaturesroutinelyexceed29°C(84°F)andannualprecipitationtotals oftenexceed3m(129in.)ofrainfall.TotheeastoftheWarmPoolthe rapidpersistentwestwardtradewindsblowacrosstheeasternPacific.In thePacificOcean,thesewindstransportheatfromtheeastPacificinto
Figure1.7 Seasurfacetemperaturerepresentedasdeviationfromtheaverageat eachlatitude. FromBjerknes,J.,1969.Atmosphericteleconnectionsfromtheequatorial Pacific.97(3),163–172 Figure7.
theWarmPoolregion.ThisactstocooltheeastPacificandwarmthe westPacific.Thiswarmingpushesdownthe “thermocline” (thedepthat whichtheoceanrapidlytransitionstomuchcoolertemperatures).This hastheeffectofproducingverypersistentwarmoceanconditions.Above theWarmPool,wefindasimilarconvergenceofwarm,moistair.The buildupofthiswarm,moistairincreasesthedepthoftheplanetary boundarylayer(thelowestwell-mixedlayeroftheatmosphere,which tendstobehumidovertheoceans).Thisincreaseinwet,warmairhas severalimportantfeedbacksthathelpwarmtheWarmPoolandmaintain theWalkerCirculation.ThethickboundarylayerovertheWarmPool holdsalotofwater(about50kgm 2),andthiswatervaporisavery activegreenhousegas.Whenthereisabundantwatervaporoverthe WarmPool,thismoistureactsaspositivefeedback,warmingtheWarm Poolbyincreasingtheamountoflongwave(infrared)radiationabsorbed bytheocean.
Alessdirectbutequallyimportantfeedinvolvestheatmospheric responsetotheconditionallyunstablewarmmoistairabovetheWarm Pool.Intenseprecipitationoccurseasilyundersuchconditions. Condensingwatervaporreleasesagreatdealofenergy,andthisheating producesrisingairneartheintersectionoftheIndianandWesternPacific Oceans(Fig.1.9).Thisairrisesandcoolsandthentendstomovetoward theeasternPacificandwesternIndianOceans,whereitsinksproducinga tendencyforhotdrysurfaceconditions.Thissubsidenceproduceshigh surfacepressures,whichresultsinasurfacepressuregradientbetweenthe eastPacificandthewestPacific.Thisgradientsupportstheeasterlytrade winds,reinforcingheatandwatervaportransportsintotheWarmPool andoutoftheeasternequatorialregion,completingtheWalker
Figure1.8 SchematicdiagramdescribingtheIndo-PacificWarmPool. ChrisFunk.
