Hawkins et al Unvertainty.docx
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Hawkins et al Unvertainty.docx
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HawkinsetalUnvertainty
UncertaintyintheMixed-UnitInput-OutputLifeCycleAssessmentModeloftheUSEconomy
TroyHawkinsa,ChrisHendricksonb,H.ScottMatthewsc
GreenDesignInstitute
CarnegieMellonUniversity
5000ForbesAvenue,Pittsburgh,PA15213USA
atrh@andrew.cmu.edu,bcth@andrew.cmu.edu,chsm@andrew.cmu.edu
Abstract
Bringinginput-outputbasedtechniquesforenvironmentalresearchtoabroaderaudiencerequiresbetterunderstandingandcommunicationoftheuncertaintyassociatedwiththeirresults.Herewediscussuncertaintiesininput-outputlifecycleassessmentmodelsbasedonourexperienceindevelopingtheMixed-UnitInput-OutputLifeCycleAssessment(MUIO-LCA)modelfortheUSeconomy.TheMUIO-LCAmodelextendsthe500sector1997USBenchmarkmakeandusetablesthroughtheadditionofcommoditiesandindustriestorepresenttheflowofcadmium,lead,nickel,andzincinmassunits.Thesesectorsallowexplicittrackingofmaterialflowsandforthecalculationofpollutantreleasesbasedonphysicalquantitiesratherthandollarvalues.UncertaintiesintheUSGeologicalSurveydatausedtocreatetheseaccountsarediscussed.TheeffectoflevelofaggregationontheusefulnessanduncertaintyofIO-LCAmodelsispresentedinthecontextofMUIO-LCA.GuidancerelatingtouncertaintyassociatedwiththeassumptionofaUStechnologymixforimportedmetalsisalsoprovided.UncertaintyintoxicreleasemultipliersbasedontheUSEPAToxicsReleaseInventoryispresentedaswellasadiscussionofthetreatmentofuncertaintyforasetofmaterialusemultipliersbasedonUSGeologicalSurveydata.OurexperiencewithuncertaintyinthedevelopmentoftheMUIO-LCAmodelprovidesguidancefortheinterpretationofIO-LCAmodelresultsandforimprovedtreatmentofuncertaintyinthenextgenerationofIO-LCAmodels.
Introduction
Input-outputtechniquesareincreasinglyusedforenvironmentalpolicyanalysisandenvironmentallifecycleassessment.ResearchersarerealizingthebenefitofIOmodelsinsimplifyingtheanalysisofsupplychainsandreducingthetruncationerrorassociatedwithprocess-basedanalysis.ImprovingtherobustnessoftheresultsofIObasedenvironmentalassessmentsrequiresimprovingourunderstandingofmodeluncertainty.WeofferanassessmentoftheuncertaintiesassociatedwithIOmodelsforenvironmentalassessmentbasedonourexperiencedevelopingtheMixed-UnitInput-OutputLifeCycleAssessment(MUIO-LCA)model.
LiketheEIO-LCAmodel,alsodevelopedthroughtheGreenDesignInstituteatCarnegieMellonUniversity,theMUIO-LCAmodelisbasedontheUSBenchmarkIOAccountscombinedwithadditionaldatarelatedtoreleasesofpollutants,energyconsumption,andmaterialuse.MUIO-LCAextendsthecapabilityofEIO-LCAbyaddingcommoditiesandindustriesrelatedtocadmium,lead,nickel,andzincflows.Metaloutputofthesesectorsaretrackedinmassunits.Theinclusionofadditionalsectorsallowsforexplicittrackingofmaterialflowsandcalculationofmetaluse.LikeEIO-LCA,MUIO-LCAallowsforthecalculationofpollutantreleasesandenergyusethroughoutthecompletesupplychainofanindustry.
Modelpredictionsarenevercertain.Understandinguncertaintyinamodelisimportanttointerpretingitsresults.Thisbecomesespeciallyimportantiftheoutcomestobecomparedarenearoneanotherinmagnitude.InterpretingtheresultsofanIO-LCAmodelisespeciallytrickyduetothelargeamountsofdataandmanyassumptionsonwhichtheresultsarebased.ThecommonguidancegiventothoseinterpretingresultsofEIO-LCAhasbeenthattheyshouldbeconsideredwithinanorderofmagnitudeofthetruevalues.ThroughoutdevelopmentoftheMUIO-LCAmodelwehaveattemptedtotracktheassumptions,errors,anduncertaintiesinvolvedinthemodel.HerewewillusethisexperiencetoprovideguidancerelatedtotheuncertaintyofMUIO-LCA.OurdiscussionalsohighlightsuncertaintiesinEIO-LCAandthe1997USBenchmarkAccountsonwhichMUIO-LCAisbased.
InTable1wepresentanoverviewofsourcesoferrorinIOLCAmodelspresentedinnoparticularorder.Weprovidebriefdescriptionsofthefirst7typesoferrorinthesectionthatfollows.ThefinalthreetypesoferroraredescribedinmoredetailwithspecificattentiontotheMUIO-LCAmodel.
SeveralsourcesoferrorinIOLCAmodelshavebeenillustratedinpreviouslypublishedworks.Itisnotourdesiretoprovideacomprehensivediscussionhere.Ratherwewillfocusoninstanceswhereourexperienceprovidesuniqueinsights.Lenzen('01)providesamorecomprehensivediscussionoferrorinIOLCAmodelstowhichthereadercanrefer.
UncertaintyinIO-LCA
SourceDataUncertainty
Sourcedatauncertaintyreferstouncertaintyintheunderlyingdataonwhichthemakeandusetablesarebased.Inthecaseofthe1997BenchmarkAccountstatisticaltechniquesareappliedtoalargeamountofdatafromtheEconomicCensus,ForeignTradeDatabase,andCommodityFlowSurveytoestimatetheentriesinthemakeandusetables.ResponsestotheEconomicCensusarenotalwaysaccurate.Althoughadjustmentsaremadetoaccountforthis,someamountofuncertaintypropagatesthroughthemodel.Uncertaintyisalsointroducedbysampling,estimations,anddatamanipulation.
EstimationofTransactions
Estimationoftransactionsreferstouncertaintyintroducedbytheestimationofmakeandusetableentries.Thisuncertaintyisstronglyrelatedtosourcedata.Incaseswheresourcedataisverylimited,simplifyingassumptionsmustbemadetoallowtheestimationofinter-industrytransactions.Entriesinthe1997USBenchmarkmakeandusetablesarealsoadjustedtoreallocateproductionofsomesecondaryproductstotheirprimaryindustryandtobalancethetotaloutputsofthemakeandusetables.Commodityproductionandconsumptionarereallocatedfromtoreducetheamountofsecondaryproductsproducedbyindustries.Productionofcertaincommoditiesismovedtotheprimaryindustryandtheconsumptionmixisadjustedaccordingly.Tablesarebalancedbyadjustingtheentriesuntilthetotalindustryoutputandtotalcommodityoutputcalculatedasthesumsofrowsandcolumnsofthemakeandusetablesbalance.Thesequantitiesoftendonotmatchinitiallyduetomisreported,erroneous,ormissingdataaswellasthetimelagbetweenthepurchaseofinputsandtheproductionofgoods.BalancingwasperformedbytheBEAbasedonexpertopinionandcomparisontothe1992account.Remainingdifferencesarecorrectedbyadjustingentriesinothervalueadded(Lawson'02).
ProportionalityAssumption
IOmodelsestimatesupplychainaffectsunderanassumptionofproportionality.Large-scalechangeswhicheffectavailabilityofsupply,augmentationofinfrastructure,orpricesarenotwellrepresentedintypicalIOmodelsdescribedhere.Generallytheimpactoflarge-scalechangesisunderestimatedbyIO-LCAmodels.
Cradle-to-GateTruncation
IO-LCAmodelscaptureonlycradletogateimpactsofaproduct.Thatistheimpactoccurringfrommaterialextractionthroughmanufacturingtothepointofsale.Additionalinformationisneededtoestimatetheuseandend-of-lifephasesoftheproductlifecycle.Thisshouldnotintroduceuncertaintyintoresultsaslongastheuserunderstandstheproperuseofthemodel.Oftenhowever,IOmodelresultsaremisrepresentedastheentireimpactofaproduct.
ChangesinTechnologyorProductionMixOverTime
ChangesintechnologyorproductionmixovertimeareoftennotwellcharacterizedbyIO-LCAaccountswhichrepresentasnapshotofaneconomy.Allofthedatausedarefromaspecificpointintime,1997inthecaseofthe1997USBenchmarkAccounts.Changesaffectingthetechnologystructureoccurevenoveraoneyeartimeperiod.Beyondthis,theresultsofIOmodelsareoftenextrapolatedtorepresentfutureyears.TheUSEconomicCensusisperformedevery5years.TheUSBEArequiresanother5yearstoconstructthemakeandusetables.Thusthemostrecentmodelavailableisoftenbasedondatafrom5to10yearsearlier.Properlyinterpretingmodelpredictionsoftheconsequencesofcurrentdecisionsshouldinvolveconsiderationoftheinfluenceofchangesintheeconomyoverthepast5-10yearsonmodelpredictions.
ModelInputUncertainty
UsersoftheEIO-LCAmodelareofteninterestedintheproductionofacertainamountofagoodsuchasabarrelofoil,alead-acidbattery,oranautomobile.Usingthemodelrequirestransformingthefunctionalunittoadollaramountoffinaldemandinthemostcloselyrelatedsector.Inputsmustalsobeadjustedtoreflectproducer’spricesforgoods(UNDESA'99).Marginsanddeliverycostsshouldbeinputtothemodelasfinaldemandsforretailtrade(4A0000),wholesaletrade(420000),trucktransportation(484000),railtransportation(482000),watertransportation(483000),airtransportation(481000),etc.Allfinaldemandinputsmustalsobeinflatedordeflatedtoreflect1997dollars.
Generallymodelusersaremorefamiliarwiththevaluesofgoodsincurrentpurchaser’sprices.DevelopersofIOLCAmodelsshouldtakethisintoconsiderationwhendesigningtheiruserinterfaceanddocumentation.Ideallyuserswouldbepromptedwithinformationabouthowthemodelinputshouldbedetermined.Consumerpriceindices(CPI)areavailableforinflating/deflatingpricesto1997dollars,howeverthecalculationofCPIitselfintroduceserror.Adjustingafinaldemandinpurchaserpricetoreflectproducerprice,margins,anddeliverycanbedonewiththeuseofatransformationmatrixbasedontheaveragemarginsforacommodity.Thepurchaser-producerpricetransformationmatrixcanbecalculatedusinginformationprovidedintheUSBenchmarkAccountsbasedonintermediateorf
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