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BillHare,RobertBrecha,MichielSchaefferOctober2018

Integrated AssessmentModels (IAMs)models are used to evaluate the technological andeconomic feasibilityofclimategoalssuchastheParisAgreement’s long-termtemperaturegoaltoholdglobalwarmingwellbelow2˚Candpursueeffortstolimitthiswarmingto1.5˚Cabovepre-industrial. Theresultsofthesemodelsareassessedin IntergovernmentalPanelonClimateChange(IPCC)reports,andplayacentralroleintheIPCCSpecialReport“GlobalWarmingof1.5°C”(SR1.5). IAMscoupledetailedmodelsofenergy system technologieswith simplifiedeconomicandclimate science models to evaluate different population, economic and technologicalpathways, allowing an assessment of the feasibility of achieving specific climate changemitigationgoals.Depending on the assumptions made as inputs to the models, and on the backgroundinformation used to construct different scenarios, IAMs may also take into accountadditionaltargetssuchassustainabledevelopmentconsiderations.ManyoftheIAMsusedin the SR1.5 evaluate options within a “cost-effectiveness” mode, meaning that a modeldeploysmitigationoptions inany region,any sector,atany time, so thatoverall (globally,andoverthecentury),thewarminglimitisachievedatlowestcost.TheParisAgreement’s1.5°C limit isachallenge for IAMs,pushing themto their structuralandscientific limits.Thiswasalsothecasefortheformer“holdwarmingbelow2°C”goal.TheIAMshaveevolvedtorepresentanincreasinglycomprehensivereflectionofmitigationoptionsandtechnologiestomeetthesechallenges.Theyhavetoaccountforuncertaintiesin future energy prices and technologies, and an increased complexity of the connectionsbetweenthefactorsbeingmodelled.Despite the challenges, IAM models show that limiting warming to 1.5°C abovepreindustriallevelsisstilltechnicallyandeconomicallyfeasibleunderavarietyofdifferentsocial and economic assumptions providing action begins very soon. IAMsdo, however,have limitations and an understanding of these limitations helps to infer climate-policyimplications frommitigation pathways. Belowwe set out someof the important contextandcaveatsinrelationtounderstandingIAMsandtheirresults,particularlyinrelationtotheParisAgreement’s1.5°Climit.

IntegratedAssessmentModels:whataretheyandhowdotheyarriveattheirconclusions?

ExecutiveSummary

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• Theway IAMs are set up,mitigating climate change is alwaysmore expensive thaninaction. In general, IAMs do not include avoided negative externalities, such asdamagesdue toclimatechangeor thecostofairpollution fromthecontinueduseoffossilenergy,sothenetcostsofmeetingtheParisAgreement’s1.5°ClimitmaybeevenlowerthantheIAMsproject.

• IAMsaredesignedtoconsidergradualchangestoexistingsystemsandaretherefore

notverygoodatcapturingrapidtechnologicaladvancessuchasrecentrapidadvancesindeploymentofsolarphotovoltaicsandthedecreasingcostsofrenewableenergyandenergystorage,norcantheyprojecttheapproachofsystemictippingpoints.Almostbydefinition, IAMs model established technologies and trends much better than newertechnologies,whose futurepolitical and technologicalpathwaysareharder topredict.Behaviouralchangessuchasloweredmeatconsumptionwouldhavetobeassumedasexternal inputstothemodels. IAMsoftentendtoaconservativeviewofthepotentialfortransformationalchange.

Several recent IAM scenarios do explore assumptions and novel modelling ofdramaticallyloweredenergydemandandincreasedenergyefficiencyrelatedtolifestylechoices and large-scale deployment of new technologies beyond the energy sector,includinginformationtechnology,urbandevelopment,sharingeconomiesandhealthierdiets, for example. These are also assessed in the SR1.5. Such IAM studies furtherstrengthenthealreadyavailableevidencethatenergydemandandefficiencymeasures– partly available at net economic savings – are a crucial requirement for limitingwarming to 1.5°C, reducing the challenges faced by supply-side measures alone andsubstantially enhancing the overall feasibility and sustainability of achieving the 1.5°Climit.

• ManyIAMsstill includesomecarboncaptureandstorage(CCS)intheirassumptions,despitethefactthatthistechnologywhendeployedwithfossilfuelpowerplantisnowwidelyseenasarelativelyexpensivemitigationoptionwithanuncertainfuture.ThefactthatCCSisnotazeroemissionstechnologyhasimportantimplicationsfornaturalgasinthepowersector,wheremanyIAMsaimedatreachingclosetozeroglobalemissionsinthepower sector by 2050 still includeCCS. On-going reductions in renewable energyand storage costs alsomeans the utilisation of natural gas for power production willlikely decrease below presently expected levels – and therefore present IAMmodelsmaybeoverestimatingnaturalgasmarkets.Equally,atpresentIAMsdonotadequatelyrepresent the rapidelectrification that is takingplaceacrossmanysectors - suchas inindustrialsectorssuchassteelproductionandthetransportandbuildingsectors.

• ThechangefrominternalcombustionenginestoelectricvehiclesisanotherexampleofthedifficultyIAMshaveinkeepingupwithchangesintherealeconomy.TheuptakeofEVs and renewably hydrogen-powered fuel cell electric vehicles (FCEV) in the privatetransportandfreightsector ishappeningmuchfasterthanseenintheIAMs’transportsectordecarbonisationindicators.

• Bioenergy represents a key area of uncertainty in IAMs. Bioenergy is a renewableenergy resource.Usingbiomass to generate electricity andproducebiofuels, togetherwith the capture and storage of resulting CO2 emissions, can result in net-negativeemissions, an important option for compensating remaining emissions from other

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sectors forwhichmitigationoptionsmaybe limited, suchas inagriculture required tofeed a still growing global population. However, there is a wide range of estimatesaround limits to the sustainable potential for bioenergy use. Many IAMs alreadyconstrain climate mitigation options by considering other sustainable developmentneeds such as food production and limiting biodiversity impacts. The complexity ofbioenergy options and their implications means that IAM scenarios need to besupplemented by follow-up analyses on more specific aspects and implications ofbioenergydeployment.

• CarbonDioxideRemoval(CDR)inIAMS.ThetwomainCDRoptionsrepresentedintheIAMsunderlyingtheIPCC1.5°CSpecialReportareinthelandsector–storingcarboninforestsandsoilsviaafforestation/reforestation–and, intheenergysystem,bioenergyin combination with CCS (BECCS). IAMs differ greatly in the extent to which CDR isimplemented,againdependingonotherassumptionsmadeintheinputstothemodels.SomeIAMsincludefewconstraintsontheuseofmodernbioenergy,whileothersonlyallowforitaftersatisfyinglanddemandforagricultureandfood,andwillnottouchpre-definedbiodiversityhotspots,naturalforestsandconservationareas.IAMCDRscenariosinclude land-use changes, such as reversing current worldwide net emissions due todeforestation toward increasing reforestation and afforestation, to helpmeet climatechangemitigation targets. The capacity for “naturebased solutions” inmeetingParisAgreementtargetsisuncertain,butthesecanplayanimportantrole.The simplest summary of IAM outputs related to CDR is that the more quickly andcompletely decarbonisation and energy efficiencymeasures can be implemented andenergydemandlimited,andthemorerapidgreenhousegasemissionsarereducedinallsectors,thelessneedwilltherebeforCDR.

Overall,whileIAMsareoneoftheessentialtoolsforpolicymakerstoprovideguidanceontechnicallyandeconomicallyfeasiblepathwaystoachievespecificclimateandsustainabledevelopmentgoals,theyareonlyonecomponentofatoolboxofanalyticaltoolsandbroaderconsiderationstobeusedintheevaluationofmitigationoptionstomeetclimatetargets.

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Introduction

Formorethan20years,energy,economyandclimatemodelshaveplayedacentralroleinevaluatingthetechnicalandeconomicfeasibilityofclimatetargets.TheseareIntegratedAssessmentModels(IAMs):theyintegrateknowledgefromdifferentdomainsintoasingleassessment.InthecontextoftheIntergovernmentalPanelonClimateChange(IPCC)Assessments,IAMshaveplayedanimportantroleinframingquestionsaroundwhatisfeasible,andwhatisnot,inclimateandenergypolicyterms.Researchgroupsinvolvedinintegratedassessmentmodellinghaveplayedasignificantrolein definingwhat policy scenarios can - and should - bemodelled, and have responded toquestionsfromgovernmentandotherstakeholdersaboutthefeasibilityoflimitingwarmingtorelatively lowlevels. ThenewRCPP1.9scenario,which limitswarmingtoabout1.5oC1,wasdevelopedaftertheadoptionoftheParisAgreement.IAMshavethebenefitofbringingthewholesocialeconomicsystemtogetherandevaluatingdifferent population, economic and technological development pathways within aninternally consistent, science-based frameworkwith a view to evaluating the feasibility ofdifferent climate and/or sustainable development goals. Many IAMs have developed insophistication and now include considerable technological detail and coupling betweendifferent systems, including land use, forestry and agriculture. Nevertheless, as with allmodels there are still significant limitations and caveats, some structural and otherscontingentuponscientificandtechnologicaldevelopment.The IPCC Special Report on 1.5°C, more than any previous IPCC report, is evaluating thetechnologicalandeconomic feasibilityof limitingwarming to1.5°C. Theenergymodellingcommunityfindsthislevelofambition–whilealreadyquiteriskyfortheglobalearthsystem-tobeachallengefortheirmodels.ItpushesIAMmodelstotheirlimitsinbothstructuralandscientificterms.Undersuchastringentclimatetargetvirtuallyeveryknownoptionformitigationhas tobedeployed. Models arepressed to - in effect - “choose”between thetiming of deployment of options based on (future) energy price, and technologicalinformation that is inherently uncertain. And as with any mitigation pathway, there areimportant connections and synergies, both positive and negative, with sustainabledevelopmentobjectives.Evaluating the implications of mitigation pathways for sustainable development metricsoften relies upon post-processing of a model’s output. This includes, for example, co-benefits of mitigation such as reduced air pollution and improving human health andagriculture outcomes, which are not normally included in themodel assessments, nor inestimationofthecostsofmitigation.

1Rogelj,J.etal.(2018)Scenariostowardslimitingglobalmeantemperatureincreasebelow1.5C,NatureClimateChange,doi:10.1038/s41558-018-0091-32Wherecountriescommittedto“Holdingtheincreaseintheglobalaveragetemperaturetowellbelow2°Cabovepre-industriallevelsandtopursueeffortstolimitthetemperatureincreaseto1.5°Cabovepre-industrial

Fullbriefing

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Theoverall findingof the recent scientific literatureon1.5°C compatiblepathways is thatthisgoalisstilltechnologicallywithinreachunderarangeofdifferentsocial,economicandpopulationassumptions.Notsurprisingly,modelsfindthatthecostsoflimitingwarmingtothislevelarehigherthanthecostforlimitingwarmingto2°C–asreportedintheIPCCAR5.On the other hand, the IAMs considered here do not take into account the costs climatechange impacts,whichwill be higher for 2°C, nor economic benefits of reduced pollutionfromfossilfueluse.Thisbackgroundpaper isdesigned to layout important contextandcaveats in relation tounderstandingIAMsandtheirresults-particularlyinrelationtothe1.5°CissueastheywillbepresentedintheIPCC1.5oCSR.

ScientificbackgroundIntegrated Assessment Models (IAMs) represent the coming together of three differentworlds of scientific thinking in relation to energy and climate science: energysystem/technological progress models, economic system models, and climate sciencemodels.Energysystemmodelswerehistoricallyfocusedonmakingprognosesaboutfossilfuelandnuclear power systems, and how issues of resource supply and societal demand wouldbalance out over time periods of a few decades, to several decades. In the context ofclimate change scenarios, these models have been adapted to provide more detailedmodelingoflonger-termdevelopmentsinregionalandglobalenergysystems.Economic system modeling is based on fundamental notions of how a market-basedeconomy functions,oftenstartingwithassumptions thatworldmarketsare inequilibriumand functioning efficiently,with perfect information available to all equally-placed actors.Thesemodelscantakedifferentassumptionsonfutureforesight,fromperfectoveralltimeforesighttowhatiscalledmyopic,orshort-termforesight.Climate science models, as used in IAMs, integrate the physics and chemistry of theatmosphere, carbon cycle and the ocean, in reduced complexity models designed totranslate greenhouse gas emissions and air pollutants to GHG concentrations, radiativeforcingandtemperature,andotherlarge-scalephysicalchangesintheclimatesystem.Thecoreideaof IAMsistocapturetheinteractionsbetweensocioeconomic,technologicaland natural systems by combining simplified economic and climate models with moredetailedmodelingofregionalandglobalenergysystemstomakerelevantprojectionsoftheconsequencesofclimatepolicychoices.Their scenarios describe an internally consistent and calibrated way for regional energysystems to get from current developments tomeeting long-term global climate goals like1.5°C.Scenariosreflectanumberoffactors:regionaldiversity,(future)GDPandpopulationdevelopments, supply and demand of fossil fuel and renewable energy carriers,technological potentials and developments, structural change, autonomous and price-inducedenergyefficiencychanges,energy resourcesand reserves, inter-fuel substitutions,economiesofscale, imports,exports,etc.,allaffectingpricesandultimatelyallowingustofindan“optimal”prioritisationofmitigationoptions.

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Fromitsstartingpointtoend-of-century,amodeldeploysmitigationoptionsinanyregion,anysector,atanytime,sothatoverall(globally,andoverthecentury),thetemperaturegoalisachievedcost-effectively.Thisiswhytheoutputsofthesemodelsareoftenreferredtoas“least-cost” pathways; as opposed to trying to compare costs and benefits of mitigationaction,theIAMsconsideredheretakenatemperaturelimitasagivenexternallydeterminedparameter. However, the models also allow the assessment of a large variety of explicitconstraintsthatmaketheirresultsmorerealisticordesirableinsomeway,forexamplebyscaling up ambitious climate action in some regions with a delay compared to the cost-effectivesolution,orbyputtinglimitsontheuseofmodernbioenergy.

IAMsandtheParisAgreementSomebackground is needed to set the stage forplacing IAM results in the contextof theParisAgreement(PA)2.ThespecificPAlimitonglobaltemperatureincreasecanbeassessedby IAMs intermsofhowtoachievethis inacost-effectivenessmode. Interestingly,manyIAMsof a previousmodeling generation (from, say, 5-10 years ago) could not findwithintheirownmodelingstructureeconomicallyviablesolutionstolimitwarmingto2or1.5°C.Asbothscientificunderstandingoftheimpactsofdifferenttemperaturechangeshavebeenrecognised, and the sophistication of IAMs increased,more stringent limits have become“feasible.”IAMsarenowabletotraceoutenergysystemtransformationpathwaysthatleadto 1.5°C-compatible scenarios. In addition, real-world developments such as decreasingcosts of renewable energy technologies have increased thepalette of economically viablemitigationoptions.Thesechangesintheabilityofmodelstofindcost-effectivesolutionstoachieveclimategoalsrepresentaninterestingandcautionarydynamic.Modelsofanykindareonlyasgoodastheirinputassumptions,fromthoseaboutup-to-datetechnologicalandmarket developments to the more fundamental questions of how “rational” people andgovernmentalpolicieswillbe.Given this introduction, what do IAMs now tell us inmore detail about the potential forachievingthePAgoals?WeillustratehowIAMresultscanbeunderstoodbydiscussingfivepertinenttopics.

1.EconomiccostsofmitigationStartingwiththe“rational”assumptionthattheeconomicsystemisalreadyinanoptimumstate, at least for the sake of argument, IAMs first determine the “business-as-usual”pathwayfordifferentfuturescenariosofsocioeconomicdevelopment.Then, by changing to a lower “allowed” total carbon budget, lower greenhouseconcentrations reachedbyendof century, orby setting a carbonpriceon carbondioxideemissions,themodelcalculatestheoptimal,least-costpathwayfromthebusiness-as-usual,orreferencecase,underthesenewconstraints.By definition, thismethodwith thesemodelswillalways find that the new situation (e.g.Paris Agreement target) costs more than “business-as-usual.” We know that economicrationalityisnotuniversallythecase–forexamplewhileitisnoteconomicallyefficientfor

2Wherecountriescommittedto“Holdingtheincreaseintheglobalaveragetemperaturetowellbelow2°Cabovepre-industriallevelsandtopursueeffortstolimitthetemperatureincreaseto1.5°Cabovepre-industriallevels,recognisingthatthiswouldsignificantlyreducetherisksandimpactsofclimatechange.”

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governments to subsidize the extraction anduse of fossil fuels, in reality, they do. At thesame time, it would be economically rational for governments, or more generally, idealmarket-based mechanisms, to add the negative external costs of fossil fuel consumption(e.g. air pollution costs, environmental damages) to the price consumers pay for energyfromthesesources;again,forthemostpart,theydonot.IAMsdonotusuallytakeeitherofthesefactorssufficientlyintoaccount.Evenso,IAMsfindthecoststogetontoaParisAgreementpathwayarequitelow-afractionofapercentagepoint of GDP per year. In the case of 2°C targets reported in AR5, the average was0.06%/year–barelydistinguishablefromthenoiseofreal-worldGDPfluctuations.RecentIAMpathways(Rogeljetal2018)findtime-averageddiscountedreductionsinglobalGDP associated with mitigation measures for 1.5°C (compared to a no-climate-policybaseline)tobethreetimeshigherthanfor2°Cinthelongterm.GDPlosseswouldbethreeto fourtimeshigher in thenext fewdecadesduetothegreaterneedforrapid,near-termreductions in 1.5°C pathways. These GDP losses would still be very small compared toprojectedoverallGDPgrowth.Also, theydonot account for larger andearlierbenefitsof1.5°Ccomparedto2°Cintermsofthereducedclimatechangedamages–orthelargerco-benefitsfromlowerpollutionrelatedtofossilfuelextractionanduse.

2.1.5°CpathwaysundertherecentandcontinuingdropinthepriceofrenewablesIAMshaveadifficulttimeprojectingemissionspathwaysforafutureofdramaticchanges.Thisisnotsurprising,assuchchangesarealmostbydefinitionsurprisingandrapid,andareoftenrecognisedonlyinhindsightashavingbeenatruetransition.Ifgovernmentpoliciesorpersonal preferences nudge behavior toward a “tipping point”, a rapid change in energyconsumption could be possible, one that would be difficult to replicate in IAMs that arepredicatedonslow,smooth,marginalchanges.For example, IAMs have been “surprised” in recent years by the dramatic costimprovementsof renewableenergy technologiescompared to fossil fuels. A largepartofthis cost development has been due to behavior by consumers, and by governmentsdeciding thatmore solar panels, wind turbines and,more recently, batteries and electricvehicles, would be a good long-term policy to support – not because those actionswereinherently the most rational and optimal decisions. The resulting sharp increase ininstallationsandpurchases inducedboth learningandscaleeffectsthatdrovedowncosts,makingfurtherinstallationsevenmoreattractive–a“virtuouscycle”.In fact, IAMs are beginning to find solutions that are optimal under the conditions of themodel over the long-term and that move toward much higher shares of renewables inelectricitythanwasthecaseevenafewyearsago,withresults inthemostrecentmodelsreaching60-80%non-biomassrenewableelectricityby2050.Whereas2°CscenariosatthetimeofIPCC’sFifthAssessmentReport(2014)projectedthatabout 5-17% of global electricity supply would be covered by solar PV by mid-century,Creutzig et al (2017) explicitly updated an IAM with the very recent drop in solarphotovoltaics(PV)prices.TheyshowedthattheupdatedprojectionsleadsolarPVmeeting30-50% of global electricity generation by mid-century –assuming further advances insystem integration of renewables. Bottom-up sectoral studies still generally find even

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stronger emissions-reduction potential at the same or lower cost for the near term thanrealisedinIAMs(UNEP2017).

3.FossilfuelphaseoutandCCSUnsurprisingly, thedifferencebetween2°Cand1.5°C scenarios is that the1.5˚C scenariosrequire an even more rapid decarbonisation of the energy system, with global zero CO2emissions being achieved around the 2070s with 2˚C, but around the 2050s for 1.5°Ccompatible pathways, with coal phased out even earlier (before 2050) than other fossilfuels.In IAMs, a new technology is scaled up most easily if it is very similar to older, knowntechnologies and, in this context, to many in the modeling community adding carboncaptureandstorage(CCS)tofossilfuelpowerplantappearedtobeamorelogicalandcost-effectivestepforthemodelthanachangetoanewtechnology,suchasutilityscalePVorevenmassivedeploymentofdistributedrooftopsolarphotovoltaics(PV).In this context, many IAMs produced results that indicated CCS from fossil fuels powerplantswereaverypromisingoption.Inpractice,however,thecostsofthistechnologywerehigher than estimated and its feasibility proved more difficult: progress towardcommercialisingthetechnologyhasbeenmuchslowerthanprojectedtenyearsago. Asaconsequence,fossilCCSisnowwidelyseenasarelativelyexpensivemitigationoptionwithanuncertainfuture.However,manyIAMmodelshavecontinuedtodeploythistechnologyfor fossil fuel power plants at large scale over the 21st century in both 1.5°C and 2°Cpathways. Model deployment of fossil CCS is lower in 1.5°C pathways as it is not a zeroemissions technology and is estimated to achieve only 85-95% reduction when 100%reductionsareneededby2050.Thishasimportantimplicationsforpolicy,forexamplewithnaturalgasinthepowersectorand undermines the commonly-advanced argument that gas is a bridging fuel. It is quitecommontoseenaturalgasdeployedintheIAMmodelassessmentsaimedatreachingclosetozeroglobalemissionsfromthepowersectorby2050;however,insuchpathwaysnaturalgas is often accompanied by deploying CCS. As renewable sources of power increase inscale,theutilisationrateofnaturalgaspowerplantsdecreases,makingtheadditionofCCSanevenmoreexpensiveproposition.Given the recent developments in renewable energy and storage technologies, some ofwhichhaveyettobeincludedinstate-of-the-artIAMs,itwouldseemunlikelythatCCScouldbeeconomicallydeployedfornaturalgaspowerturbines,withtheimplicationthatpresentIAMmodelsmaybeoverestimatingfuturenaturalgasmarketsforthepowersector.Another important factor is that at present IAMs do not adequately represent the rapidelectrification across many sectors that seems increasingly plausible to many analysts,particularlyinindustrialsectorssuchassteelproduction,inthetransportsector(seebelow)and inthebuildingssector(Kriegleretal2018;Grubleretal2018). Implementationoftheassociated technologies would allow for an even more rapid phase-out of fossil fuelemissions,withoutrelyingonCCS.

4.Uptakeofelectricvehicles–electrificationoftransportThe change from internal combustion engines to electric vehicles are another example ofthedifficulty IAMshave in keepingupwith likely changes in the structureof the complex

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worldenergysystem. Inaparallelwith renewables inelectricitygeneration, theuptakeofEVs and renewably hydrogen-powered fuel cell electric vehicles (FCEV) in the privatetransport and freight sector has recently been faster than expected, and is expected tospeed up in the coming decades. This is all happening much faster than in the IAMs’transportsectordecarbonisationindicators.IAMs try to capture a change in the transportation systemwhere low-carbon alternativessuch as biofuels, hydrogen and electric vehicles displace internal combustion enginevehicles.The technological change is reflected inagradualdecrease inconsumptionofoil(thus decreasing CO2 emissions), coupled with an increase in the use of biomass andelectricity. However,what themodelsdon’tusually capturewell - ifatall -wouldbe theanswers toquestionssuchas“whatwill theshareofelectricvehiclesalesbe in2030?”or“howmanyEVchargingstationsmustChinabuildby2030tosupporttheelectrificationofpersonalvehicletransportation?”Findingdetailsofpathwayswithinindividualsectorsthatmightstillbecompatiblewiththeoverall global, Paris Agreement-compatible total emissions profile requires an additionaleffort of downscaling the high-level results of IAMs, while also taking into accountdevelopmentsthathavemadefasterprogressthaninitiallyanticipated.

5.Demand-sideandenergyefficiencymeasuresTraditionally, supply-sidemanagementof theenergy system is representedat thehighestlevel of detail in IAMs and, alongside technological energy efficiency improvements thatlowerenergydemand, is their strongestaspect. Lesswell coveredby IAMs,and thereforeless represented in the scenarios they produce, are fundamental interventions on thedemand side, which were usually explored through simplified variations in underlyingassumptions.Thisisstartingtochange,however,withsomeoftherecentpublicationscoveredintheIPCCSpecial Report on 1.5°C. These explore elements and synergies related to, for example,behavioralchangesandlifestylechoices, leadingtofundamentalshifts intransportmodes,indietstowardslowmeatconsumption,or lowpopulationgrowth(vanVuurenetal2018,Grubleretal2018).These variations generally facilitate a more rapid near-term decarbonisation, enhancesustainability synergies of 1.5°C pathways and may lower the need for Carbon DioxideRemoval(seenextsection),withthecaveatthattheauthorscitedabovedonotnecessarilydeemitplausiblethatallsuchchangeswouldmaterializesimultaneouslytotheextentthemostfar-reachingofthesescenariosassume.

6.CarbonDioxideRemovalAsafinalexampleofIAMoutputwelookatthequestionofCarbonDioxideRemoval(CDR),ornegativeemissions.TheneedfornegativeCO2emissionsarisesforseveralreasons.First,today’shighCO2concentrationsfromhistoricalemissionsmeanthatweneedextremeratesof CO2 emission reductions to halt the rise in CO2 concentration. To reduce CO2concentrations inorder tomeet theParisAgreement’s temperature limit,wewill have totakesomeCO2outoftheatmosphere.

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Second,therearesignificantnon-CO2greenhousegasemissionsthataredifficulttoreduce,notably from agriculture and some industrial sources. For example, a growing populationmeansitwillbequiteachallengetojuststabilizenon-CO2emissionsfromagricultureinthenext few decades. Reducing more non-CO2 greenhouse gas emissions will require fewernegativeCO2emissions,andviceversa.Thirdly,wecannotjustswitchenergysystemsovernight,andsignificanttimewillbeneededtomovetoalowcarbonenergysystem.Thefasterwecandothis,themorewecanreducetheneedfornegativeCO2emissions,andconversely,theslowerouraction,thegreatertheneedfornegativeCO2emissions.Takentogether,itisverydifficulttoseehowCDRcanbecompletelyeliminatedfromthemixofclimatechangemitigationoptionsandstillachievethePAtarget.ThetwoCDRoptionsrepresentedintheIAMsunderlyingtheIPCC1.5°CSpecialReportarein the land sector – storing carbon in forests and soils via afforestation/reforestation,includingecosystemrestoration–and,intheenergysystem,bioenergyincombinationwithCCS(BECCS).Intypicalscenarios,theseareroughlydeployedinequalmeasure,exceptwithscenarios that reach very high levels of CDR, exhausting the limited potential ofafforestation/reforestation.IAMsdiffer greatly in theextent towhichCDR is implemented, againdependingonotherassumptionsmadeintheinputstothemodels(suchaspopulationsizeandsocioeconomicconditions), and in the expected speed of transformation in socioeconomic andtechnologicaltrendsrepresentedinthemodels.For example, some IAMs include few constraints on the use of modern bioenergy, whileothersonlyallow for it after satisfying landdemand foragricultureand food,andwillnottouchpre-definedbiodiversityhotspots,naturalforestsandconservationareas.Also available are 1.5°C pathways that set hard limits on the total amount of modernbioenergy deployed, restricting these to levels considered likely to be met by the moresustainable bioenergy options available, such as a limit of around 100 EJ/year, about twotimes the present-day use of bioenergy in the global energy mix. These are importantconsiderationsgiventheongoingdiscussiononthesustainabilityoflarge-scaledeploymentofbioenergyandconcernsrelatedtofoodsecurity.BecauseIAMshavealimitedlevelofdetailinbioenergyoptions,theidentifieddemandforitinIAMscenariosisbestsupplementedbyfollow-upanalysesthatassessawideportfolioofbioenergyfeedstockstomeetthetotaldemand-inlightoftheirvariouscharacteristicsandsustainabilityimplications.Suchaportfoliocouldinclude,forexample,bioenergygrownonmarginal agricultural land, the use of agriculture and forest residues and maximisedsynergieswithsoilrestoration,andwouldexcludeasmuchaspossiblededicatedbioenergycropsonproductiveagriculturalland,aswellasoptionsthatdecreaseforestarea.The simplest summary of IAM outputs related to CDR is that the more quickly andcompletelydecarbonisationandenergyefficiencymeasurescanbeimplementedandenergydemand limited, and themore rapid greenhousegas emissions are reduced in all sectors,the lessneedwill therebe forCDR. Asimpleway tobalancenear-termreduction targetswith the desirability of limiting future reliance on CDR is to base such targets on 1.5°Cpathways with limited overshoot: even if the level of peak warming in the 21st century

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exceeds1.5°Cbeforedroppingdownagain,thispeakwarmingshouldbeverycloseto1.5°C.If one applies a selection criterion of a maximum “overshoot” below 0.1°C (i.e. peakwarmingbelow1.6°C),onlypathwayswithlimitedCDRwillqualify.

A keypointof anyeconomicmodel is balancing trade-offsbetween choices. Economics isthestudyof scarcity, including thatofatmospheric space tosafelyabsorbgreenhousegasemissions, natural resources, and other factors. By accepting the Paris Agreement and itsstringenttargets,thereisarecognitionthatnetcarbondioxideandotheremissionsmustbereducedtozero,orevenbelow,withinageneration.Thinkingintermsofatotalcarbonbudgetforglobalindustrialcivilisationoverthepasttwohundredyears, thereareonlya fewchoicesavailable to limit temperature increases. Wecan choose to strictly remain within that budget, eliminating emissions from all of ouractivities, including not only the energy systems discussed so far, but also agriculture,aviationandindustrialprocesses(e.g.cementproduction),andturningthelandsectorintoanetCO2sink.Alternatively,wecanreduceemissionstotheextentpossible,continuewithrelativelysmallamounts of emissions from sectors that are difficult to decarbonize (meat and riceproduction), use land management options for CDR as much as possible, and then usetechnological means to capture and store carbon dioxide (CCS) in stable undergroundreservoirs.ThegoalofIAMsistoexplorethesealternativepathwaysandtopresentoptionsfortransformationunderdifferentscenarios.IAMsdoprovideuseful informationforpolicymakersandhelpguidethethinkingofotherinstitutions and groups. In an integrated view of sustainability, the economy forms onesubsetofsociety,andglobalsocietiesmustfunctionwithintheplanetaryboundariessetbynaturalsystems.Followingthatlineofthought,andashasbeendiscussedinthisBrief,IAMsprovideonetool,withstrengthsandweaknesses,inwhatshouldbeatoolboxofmeasures,policies andbroader considerationsofhow to shape the futureof theenergy systemandclimatechangemitigation,andofsustainabledevelopmentpolicies.

Conclusion

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ReferencesCreutzig et al. (2017) The underestimated potential of solar energy to mitigate climatechangeNatureEnergy217140DOI:10.1038/nenergy.2017.140Grubler et al. (2018) A low energy demand scenario for meeting the 1.5 °C target andsustainable development goals without negative emission technologies Nature Energy 3517-525.DOI:10.1038/s41560-018-0172-6Kriegler et al. (2018) Pathways limiting warming to 1.5°C: a tale of turning around in notime?Phil.Trans.R.Soc.A37620160457DOI:10.1098/rsta.2016.0457Rogeljetal.(2018)Scenariostowardslimitingglobalmeantemperatureincreasebelow1.5°CNatureClimateChange8325–332(2018)DOI:10.1038/s41558-018-0119-8UNEP (2017) The Emissions Gap Report 2017 United Nations Environment Programme(UNEP),Nairobivan Vuuren et al. (2018) Alternative pathways to the 1.5 °C target reduce the need fornegative emission technologies Nature Climate Change 8 391–397, DOI:10.1038/s41558-018-0119-8