深圳杯D英文.docx
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深圳杯D英文.docx
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深圳杯D英文
Floodexposureisincreasingincoastalcities1,2owingtogrowingpopulationsandassets,thechangingclimate3,andsubsidence4–6.Hereweprovideaquantificationofpresentandfuturefloodlossesinthe136largestcoastalcities.Usinganewdatabaseofurbanprotectionanddifferentassumptionsonadaptation,weaccountforexistingandfutureflooddefences.Averageglobalfloodlossesin2005areestimatedtobeapproximatelyUS$6billionperyear,increasingtoUS$52billionby2050withprojectedsocio-economicchangealone.Withclimatechangeandsubsidence,presentprotectionwillneedtobeupgradedtoavoidunacceptablelossesofUS$1trillionormoreperyear.Evenifadaptationinvestmentsmaintainconstantfloodprobability,subsidenceandsea-levelrisewillincreaseglobalfloodlossestoUS$60–63billionperyearin2050.Tomaintainpresentfloodrisk,adaptationwillneedtoreducefloodprobabilitiesbelowpresentvalues.Inthiscase,themagnitudeoflosseswhenfloodsdooccurwouldincrease,oftenbymorethan50%,makingitcriticaltoalsoprepareforlargerdisastersthanweexperiencetoday.Theanalysisidentifiesthecitiesthatseemmostvulnerabletothesetrends,thatis,wherethelargestincreaseinlossescanbeexpected.
Afirstscreeningstudy1providedaglobaloverviewoffloodexposureinworldcoastalcities.Theexposuremetriccanbeviewedasaworstcasescenario,butitdoesnotestimateaverageannuallosses,whichisastandardmetricindisasterriskmanagementplanning.Todoso,itisnecessarytotakeintoaccountinfrastructure-basedadaptation(forexample,dykes)andthevulnerabilityofpopulationsandassets.Here,weassesseconomicaverageannuallosses(AAL)in136coastalportcities,usingamethoddevelopedforassessingcity-levelfloodrisk7andanewdatabaseofurbancoastalprotection(Methods).
Presentaggregatedaverageannualfloodlossesinthe136citiesareestimatedatapproximatelyUS$6billionperyear.Table1ranksthemostvulnerablecitiesin2005usingtwodifferentmetricsofvulnerability.Intheleftcolumn,thetableshowsarankingintermsofAAL,takingintoaccountallpotentialfloodsandexistingprotection.TheAALestimatescanbecomparedtomoresophisticatedapproaches.Forinstance,theannuallossesinNewOrleansareestimatedatUS$600million,closetotheUS$650millionestimatesfromtheInteragencyPerformanceEvaluationTaskforce8.Intherightcolumn,citiesarerankedaccordingtorelativevulnerability,namelytheratioofAALtothecity’sgrossdomesticproduct(GDP).Thisvaluecanbeunderstoodastheshareofthecity’seconomicoutputthatshouldbesavedannuallytopayforfuturefloodlosses.The20citiesmostvulnerableaccordingtothislastindicatorarealsopresentedinFig.1.
Therankingintermsofexposureincludesmainlyrich-countrycities(SupplementaryTableS4).Onaverage,however,richcitiesarebetterprotectedthanpoorerones,andtherankingintermsofabsolutefloodlossescontainsmorecitiesfromdevelopingcountries.Inrelativeterms,developing-countrycitiesareevenmorevulnerable,withonlythreecitiesfromdevelopedcountriesinthetop20(NewOrleans,MiamiandTampa—Saint-Petersburg).Moreovertherankinginabsoluteterms(leftcolumn)includesmainlycapitalcities,whereassecondarycitiesaremoreoftenrepresentedintherankinginrelativeterms(rightcolumn).Thisdifferencesuggeststhatriskmanagementeffortsmaybelowerinsecondarycities.
Table1showstheimportanceofexistingflooddefences:
inacitysuchasAmsterdam,exposureisextremelyhigh(US$83billionofassetsexposedtothe100-yearflood),butAALdonotexceedUS$3million,becauseestimateddefencestandardsarethehighestthatexistglobally.Ontheotherhand,acitysuchasHoChiMinh,inVietnam,hasa100yearexposureofonlyUS$18billion,butthelowerlevelofprotectionmeansthatthecityisaffectedbysmallfloodsonafrequentbasis,resultinginlargeestimatedaveragecosts.Inrelativeterms,HoChiMinhCityhasoneofthelargestvulnerabilities,withAALreaching0.74%oflocalGDP.TheratioofAALtolocalGDPexceeds1%fortwocities,GuangzhouandNewOrleans.ThevulnerabilityofNewOrleanshasbeenreducedhoweverbyrecentpost-HurricaneKatrinainvestmentsandislikelytobereducedfurtherinthenearfuture9.
AnotherconclusionfromTable1istheconcentrationoflossesinonlyafewcities.Only13citieshaveaveragelossesinexcessofUS$100million,andthreeAmericancities(Miami,NewYorkCityandNewOrleans)explain31%oftheglobalaggregatelossesinthe136cities,becauseoftheirhighwealthandlowprotectionlevel.AddingGuangzhou,thefourtopcitiesexplain43%ofgloballosses.Also,theUSseemsparticularlyvulnerable,with6Americancitiesinexposureranking,8intherankingbyabsoluteAAL,and3intherankingbyrelativeAAL.Ascoastalfloodrisksarehighlyconcentrated,floodreductionactionsinafewlocationscouldbeverycost-effective.
Todeveloppossiblefuturepatternsofdriversofriskto2070,ouranalysisintroducesthreescenariosforsocio-economicchangesandsixforenvironmentalchange.Fromthere,weretainfourmainscenarios:
SECassumesonlysocio-economicchanges,derivedfromOECDandUNscenarios;SEC-SaddssubsidencetoscenarioSEC(40cmin2050inthecitiessubjectedtosubsidence);andSLR-1andSLR-2addoptimisticandpessimisticsea-levelrisescenariostoSEC-S,respectively(with20cmand40cmin2050).Here,wereportresultsfor2050,butresultsfor2030and2070areavailableintheSupplementaryInformation.
Withnoadaptation,theprojectedincreaseinaveragelossesby2050ishuge,withaggregatelossesincreasingtomorethanUS$1trillionperyearinscenariosSLR-1andSLR-2(SupplementaryTableS6).Allcitiesexperienceasimilarincreaseinrisk.Intheabsenceofadaptation,theimpactofenvironmentalchangeismuchlargerthantheeffectofsocioeconomicchange.Thesenumbersshouldnotbeconsideredaspredictions,buttheydemonstratetheneedforadaptation,becauseinactionwouldresultinunacceptablyhighlosses.
Wethenconsideradaptationandhowitwillalterlosses.Weassumefirstthatadaptationactionincreasescoastalflooddefencestomaintainaconstantprobabilityofflooding(adaptationoption:
presentdesign,PD).TheincreaseinaggregateAALismuchlowerinthiscase.Owingtosocio-economicchange,thereisstillaninefoldincreaseinaggregatelosses,fromUS$6toUS$52billionperyear,butthisismademoremanageablebythefactthatthesecitiesarealsomuchricher.However,risingwaterlevelsstillincreaseAAL:
subsidenceby12%andsea-levelrisebyanadditional2–8%,reachingbetweenUS$60and63billionperyear.
Table2showsthetop20citiesintermsofAALin2050inthescenariowithsubsidenceandoptimisticsea-levelrise(SLR-1),withadaptationtomaintainpresentfloodprobability.Guangzhoure-mainsthemostvulnerablecity,withAALexceedingUS$13billion.Withsocio-economicchangealone,AALinGuangzhouwouldbearoundUS$12billionperyearin2050(a17-foldincreaseinabso-luteterms).Subsidenceandsea-levelrisearethusresponsibleforanadditional10%increase,thatis,a10%increaseintheAAL-to-GDPratio.Indeed,eveniftheprobabilityofcoastalfloodingisun-changedthankstoupgradedcoastaldefenceinfrastructure,thefactthatalargershareofexistingassetsisprotectedbythesedefencesmeansthatannuallosseswillriserelativetolocalGDP.Forinstance,sea-levelriseandsubsidenceincreasetheAAL-to-GDPratioby54%inJakartaandby24%inAbidjanevenifpresentfloodprobabilitiesaremaintainedthankstobetterdefences.Inotherwords,theworldseesnomorefloods,buteachfloodismoredestructiveowingtosea-levelriseandsubsidence,evenwithbetterdefences.Thiseffectreinforcesatrendthatcanbeexpectedfromsocio-economicchangealone,evenintheabsenceofenvironmentalchange.10
Figure2showsthe20citieswheretheincreaseinaverageannuallossesbetween2005and2050isgreatestinrelativeterms;detailednumericalvaluesareprovidedinSupplementaryTableS7.InAlexandria,forinstance,maintainingfloodprobabilityleadstoanincreaseby154%inAAL.Thesemostvulnerablecitiesaredistributedallovertheworld,withaconcentrationintheMediterraneanBasin,theGulfofMexicoandEastAsia.Eventhoughabsolutelevelsofriskaresometimeslowinthesecities,theycanbeconsideredasadaptationhotspotsbecausethisiswherefloodrisksarelikelytoincreasethemostinrelativeterms.
Toavoidanyincreaseinrisk,anadaptationpolicyneedstodomorethanmaintainpresentfloodprobability.Rather,maintainingpresentlevelsofrisk(relativetolocalGDP)inthecontextofrisingsealevels,subsidenceandsocio-economicchangesrequiresadaptationpolicythatreducesfloodprobabilityovertime.Intheadaptationoptiontermedpresentlosses,anupgradeindefenceisthuscalibratedtocanceltheimpactofenvironmentalchangesandtomaintainpresentlossesonaveragerelativetolocalwealth,keepingaggregatelossesatUS$52billion.
Foreachcity,weestimatetheincreaseindefencestandardthatwouldmaintaintherelativerisklevel(thatis,keepconstanttheratioofaverageannuallossestolocalGDP).Therequiredincreaseinprotectionislargerthanlocalsea-levelrise.Forinstance,inAlexandria,protectionneedstoberaisedby67cm,fora60cmriseinlocalsealevel;thiscorrespondstomovingfroma100-yeardesignstandardtoa270-yeardesignstandard(thatis,adivisionby2.7oftheprobabilityofflooding).Inothercities(SupplementaryTableS7)theincreaseindykeheightisbetween2and8cmlargerthanse
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