Transport electrification: A key element for energy systemtransformation and climate stabilization

David McCollum & Volker Krey & Peter Kolp &Yu Nagai & Keywan Riahi

Received: 28 October 2012 /Accepted: 25 September 2013# Springer Science+Business Media Dordrecht 2013

Abstract This paper analyzes the role of transport electrification in the broader context ofenergy system transformation and climate stabilization. As part of the EMF27 model inter-comparison exercise, we employ the MESSAGE integrated assessment modeling frameworkto conduct a systematic variation of availability, cost, and performance of particular energysupply technologies, thereby deriving implications for feasibility of climate stabilizationgoals and the associated costs of mitigation. In addition, we explore a wide range ofassumptions regarding the potential degree of electrification of the transportation sector.These analyses allow us to (i) test the extent to which the feasible attainment of stringentclimate policy targets depends on transport electrification, and (ii) assess the far-reachingimpacts that transport electrification could have throughout the rest of the energy system. Adetailed analysis of the transition to electricity within the transport sector is not conducted.Our results indicate that while a low-carbon transport system built upon conventional liquid-based fuel delivery infrastructures is destined to become increasingly reliant on biofuels andsynthetic liquids, electrification opens up a door through which nuclear energy and non-biomass renewables can flow. The latter has important implications for mitigation costs.

1 Introduction and motivation

The worlds current stock of cars, trucks, buses, two-wheelers, trains, ships, and airplanesaccount for approximately one-fifth of all primary energy consumed by mankind; andbecause transport is almost completely dependent on fossil fuels (93 % oil), the sectorscontribution to energy-related carbon dioxide emissions is an even greater share (25 %) (IEA2012). Meanwhile, the demand for mobility is growing virtually everywhere, with no

Climatic ChangeDOI 10.1007/s10584-013-0969-z

David McCollum and Volker Krey contributed equally to this work.

This article is part of the Special Issue on The EMF27 Study on Global Technology and Climate PolicyStrategies edited by John Weyant, Elmar Kriegler, Geoffrey Blanford, Volker Krey, Jae Edmonds, KeywanRiahi, Richard Richels, and Massimo Tavoni.

Electronic supplementary material The online version of this article (doi:10.1007/s10584-013-0969-z)contains supplementary material, which is available to authorized users.

D. McCollum (*) : V. Krey : P. Kolp :Y. Nagai : K. RiahiInternational Institute for Applied Systems Analysis, Laxenburg 2361, Austriae-mail: mccollum@iiasa.ac.at

immediate signs of slowing down. While such trends are both an indicator and driver ofincreasing affluence, the great challenge for transport in the twenty-first century is to ensurethat its development is sustainable: meeting the mobility demands of billions whilst mini-mizing the consequent environmental and social impacts (WBCSD 2004).

Over the past decade, countless scenario studies have envisioned a myriad of divergingpaths for the energy system at varying scales (e.g., local, national, or global; and for differentsectors). Many of these studies focus in particular on low-carbon futureshow to get there,what mitigation strategies are needed, what are the costs and benefits. And while no twostudies are ever the same, a common result among all of them tends to be that the transportsector must be to some extent decarbonized. For example, transport figures prominently inthe climate stabilization framework of Pacala and Socolow (2004): several of the proposedmitigation strategies relate to vehicle efficiency improvement, alternative fuel production,and travel demand management. Yet, here as elsewhere (e.g., see also Bosetti and Longden(2013), Grahn et al. (2007), Gl et al. (2009), Hedenus et al. (2010), and Kyle and Kim(2011)), there seems to be little agreement on the exact role that transport should play inachieving deep economy-wide reductions in greenhouse gas (GHG) emissions (by 2050 forinstance), given the unique challenges faced in this sector, be they technological, economic,behavioral, or political in nature. At the heart of the issuein fact one of the key uncer-tainties going forwardis the extent to which electric-drive technologies (e.g., plug-inhybrid-electric, battery-electric, and hydrogen fuel cell vehicles; electrified rail, etc.) canpenetrate the global market in sufficient numbers over the next several decades. Outside ofthe obvious technical hurdles on the end-use side, the advent of such technologies wouldnecessitate wholesale changes to todays liquid-based fuel delivery infrastructure, an ar-rangement more conducive to fossil fuels (conventional or synthetic) and biofuels.

This paper analyzes transports role in the decarbonization of the global energy system,considering the complex interplay between this sector and all other energy producing andconsuming components of the system. Because each of these sectors faces its own uniquechallenges and will be forced to compete simultaneously for a limited pool of resources andfinancial capital, there are very real constraints to what can be achieved system-wide. Toaccount for these inter-dependencies, we employ the MESSAGE integrated assessmentmodeling framework, which combines a global (multi-region, multi-sector) systems engi-neering, inter-temporal optimization model (Riahi et al. 2007; van Vliet et al. 2012), anaggregated macro-economic model, and a simple climate model. (Further information onMESSAGE can be found in the Supplementary Material, SM.) MESSAGE is rich intechnological detail on the supply side of the energy system (e.g., resource extraction,secondary fuel conversion, and fuel delivery and transport); however, the version we employhere lacks the kind of demand-side detail (e.g., representation of technologies in thetransport sector) found in other models. In short, our stylized transport module only capturesfuel switching and price-elastic demand response (via linkage with the macro-economicmodel) at the aggregate level of the entire transport sector; individual transport modes arenot explicitly modeled. Despite these shortcomings, the strength of MESSAGE lies in itsability to embed most of the key components of the global energy-environment-economicsystem within a common framework, thereby allowing systems-analytical questions to beadequately addressed, such as the transport-related ones focused upon in this paper.Although transport was not one of the emphases of the Energy Modeling Forum 27(EMF27) model inter-comparison exercise, we believe the study designwith its emphasison energy system transitions under alternative technology and policy regimes (Kriegler et al.2013)provides a nice platform for conducting such an analysis, principally because itallows us to (i) test the extent to which the feasible attainment of stringent climate policy

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targets (e.g., a 450 ppm CO2-equivalent stabilization pathway) depends on electrification ofthe transport sector, and (ii) assess the far-reaching impacts that transport electrificationcould have throughout the rest of the energy system.

2 The MESSAGE interpretation of the EMF27 scenarios

The main aim of the EMF27 model inter-comparison exercise is to explore the role oftechnology for limiting dangerous climate change. The study protocol therefore combinesdifferent levels of climate stabilization550 and 450 ppm CO2-equivalent (CO2-eq)concentrationwith varying assumptions about the availability, cost, and performance ofcertain energy-supply technologies and thus the extent to which these technologies might beable to contribute to a future energy system transformation. The less stringent 550 ppm targetis specified in such a way that CO2-eq concentrations never exceed the target levelthroughout the 21st century, whereas temporarily overshooting the 450 ppm target istolerated if concentrations can be returned to this level by 2100. Table 1 lists the names ofthe technology portfolio variations in EMF27 along with a brief description of the scenariospecifications. A more detailed overview of the study design can be found in the EMF27overview by Kriegler et al. (2013).

Table 1 also provides an overview of the different scenarios that were analyzed with theMESSAGE model in the EMF27 exercise. Scenario feasibility, based on the variouscombinations of climate targets and technology and socio-economic assumptions, is

Table 1 Overview of names and specifications of the EMF27 scenarios and feasibility matrix for theMESSAGE model. The 450 ppm sensitivity cases shown in the lower part of the table are not part of theEMF27 study protocol, but constitute additional analysis with MESSAGE that explores the role of thetransportation sector in reaching low climate stabilization targets. The percentages refer to the maximumtransport electricity shares in the scenarios that were found to be feasible (Section 3)

Scenario name Scenario specification

FullTech All technologies included and final energy intensity improvements compatible with historically observed improvements of about 1.2% per year globally

LowEI Low energy intensity scenario with 20-30% lower final energy demand in 2050 and 35-45% in 2100 compared to the FullTech case

NoCCS Carbon Capture and Storage (CCS) excluded from technology portfolio in all sectors NucOff Phase-out of nuclear energy with no new nuclear power plants built beyond those that are already under

construction. Existing plants can be operated until end of their lifetime

LimSW Electricity generation from intermittent solar and wind technologies (on-/offshore wind, solar PV and CSP) limited to 20%; conservative techno-economic assumptions for solar and wind LimBio Global primary bio-energy supply (excluding traditional biomass) limited to 100 EJ/yr Conv Conventional energy system (combination of LimSW and LimBio) EERE Renewable energy system (combination of LowEI, NoCCS and NucOff) LimTech Constrained technology case (combination of NoCCS, NucOff, LimSW and LimBio)

FullTech LowEI NoCCS NucOff LimSW LimBio Conv EERE LimTech FroTech

Baseline X

550 ppm X

450 ppm

450 ppm sensitivities 5%-75%

5%-75%

35%-75%

5%-75%

5%-75%

15%-75% 75%

5%-75% X

Color Legend:X Scenario not attempted Infeasible

Scenario not part of study protocol Feasible with greater transport electrification Feasible Infeasible with lower transport electrification

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indicated by the coloring. The last row of the table refers to a sensitivity analysis on transportsector electrification that was conducted specifically for this paper, in order to furtherexplore the feasibility of the 450 ppm climate stabilization target. The percentages describethe degree of electrification in transport in these additional scenarios, as discussed in depth inSection 3. In this paper electrification refers specifically to vehicles powered by battery-electric drivetrains, not from fuel cells.

2.1 Feasibility of reaching stringent climate targets

One of the main insights from the MESSAGE interpretation of the EMF27 scenarios is thatstringent climate stabilization can in principle be achieved with energy technologies that arecurrently available or have been demonstrated to work beyond the laboratory scale, as longas a number of key conditions hold. An important assumption of the analysis presented hereis that concerted global action to limit climate change starts before 2020. Given the years-long deadlock of climate negotiations, this assumption may appear rather heroic; neverthe-less, it is still a useful starting point for isolating the role of technology in the required energysystem transformation. When we refer to the feasibility of such transitions in the remainderof this article, it is strictly the technological dimension that we refer to. Despite theirimportance (Anderson and Bows 2011), social, institutional, and political barriers to thistransition are outside the scope of the analysis, although many of these factors serve as amotivation to explore the reduced technology portfolio scenarios in the study (e.g., publicopposition to nuclear energy in the NucOff cases).

For the less ambitious 550 ppm target, essentially all variations of technology availability,cost and performance do not endanger the feasibility of the energy system transformation.The only exception is the LimTech scenario in which all pessimistic technology assumptionsare combined and thereby do not allow reaching the stabilization target. This is notsurprising given that almost all supply-side low-carbon optionsCCS, nuclear, wind, solar,and bioenergyare excluded or at least severely constrained in the LimTech scenario whichessentially leaves demand reduction (from a relatively high baseline demand) as the onlyviable option to reduce GHG emission from the energy system.

More interesting are the technology portfolio variations under the more ambitious450 ppm CO2-eq target which illustrate the importance of having a wide portfolio ofsupply-side technologies available to meet these low targets as well as successfully reducingfinal energy demand levelsthrough a combination of energy efficiency improvements andbehavioral adjustmentswhich then allows for greater flexibility of excluding options onthe supply side (cf. Riahi et al. (2012) and McCollum et al. (2012)). Based on theMESSAGE scenarios, the single most important technological option for reaching the lowstabilization target is carbon capture and storage (CCS), a result consistent with findingsfrom previous exercises that have explored the role of technology for stringent GHGmitigation (Edenhofer et al. 2010; Krey and Riahi 2009; Luderer et al. 2012; Riahi et al.2012). Several reasons bear mentioning in this context: (i) the versatile use of CCS indifferent processes, from electricity generation to liquid fuel production and cement pro-duction, (ii) the possible combination of CCS with electricity, liquid fuel, or gas productionfrom biomass, which can lead to negative emissions in the long term (Azar et al. 2006;Clarke et al. 2009; Tavoni and Tol 2010; van Vuuren et al. 2007), and (iii) the compatibilityof CCS with existing supply chains, infrastructure and business models, which could lead toa faster upscaling of the technology. However, the importance of CCS is to a good degreetied to the structure of fuel demand in the end-use sectors, notably in transport. The degree towhich electrification is possible in the transportation sector has profound implications on the

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feasibility and mitigation costs of the 450 ppm target under less optimistic supply-sidetechnology assumptions (see Section 3.4).

2.2 Timing of CO2 mitigation

In addition to costs and feasibility, the variation of technology assumptions also hassignificant implications for the timing of mitigation and thus on the pace at which actionneeds to be taken in the near term (Rogelj et al. 2013). As shown in Fig. 1, particularly largedeviations among the alternative CO2 emission pathways exist over the coming two decades.For example, there is a total fossil fuel and industrial emissions gap of more than 5 GtCO2between the LimBio and the FullTech case in 2020 which further opens up to about 9 GtCO2by 2030. This near-term deviation is largely a function of long-term emission levels, whichare considerably higher in the LimBio variant compared to the FullTech scenario due to the

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reduced potential to generate negative CO2 emissions in the former. Delay of mitigationactions in the short-term therefore implicitly means putting a bet on the availability ofoptions such as bioenergy with CCS that actively remove carbon from the atmosphere. Itshould, however, be noted that some of the other technology variations also lead to changesin the emission profiles, although not as pronounced as in the case of limited CCS orbioenergy. This is true for the low energy intensity (450 LowEI) case in particular where thereduced demand in the long-term decreases emissions because of the relatively moresignificant contribution from bioenergy and other RE which in turn allows for moreheadroom over the coming decades.

3 Implications of electrifying transport

As previously mentioned, the potential for electrifying end-use technologies in transport(primarily road vehicles and trains) has profound implications on the feasibility and mitiga-tion costs of reaching the deep emission reduction targets described in Section 2, especiallyin the less optimistic supply-side technology cases. Transport electrification also has far-reaching impacts throughout the rest of the energy system by freeing up valuable resources(physical and financial) that can then be used elsewhere and by redefining the marginal valueof particular mitigation options in certain sectors.

3.1 Transport sector electrification sensitivity analysis

In order to systematically explore the impact of transport electrification on the energysystem, we carried out a sensitivity analysis that expands upon the standard set of EMF27scenarios. For each of the 450 ppm caseswith their varying assumptions for supply-sidetechnologieswe sampled a portion of the scenario space in the transport dimension,specifically with respect to how that sector could potentially be transformed by electric-drive technologies over the twenty-first century. Several recent, detailed transport studies(considering all transport modes) conducted at varying levels of aggregation (from global toregional/national to sub-national) have indicated a large uncertainty range for transportelectrification by mid-century. Interestingly, even though each of these studies frames itsresults within the context of transformational changeassuming, by extension, a reasonablyhigh level of optimism for future technological development and consumer behavior theconclusions of the studies nevertheless vary quite widely. For instance, whereas theInternational Energy Agency (2012) [Global], Skinner et al. (2010) [European Union], andYang et al. (2011) [California] each find that roughly 15 % of final energy in transport couldcome from electricity by 2050, Teske et al. (2012) [Global] and Williams et al. (2012)[California] show that upwards of 4555 % of the sector could be electrified. On the moreoptimistic end, this could require partial electrification of heavy-duty trucking (potentiallyvia overhead wires) and certain port-based aviation and marine activities. The generalexpectation across all the studies is that from 2050 onward the electricity shares would risequickly, as explicitly calculated and shown in IEA (2012). Moreover, in some of theseanalyses, the contribution from electric-drive technologies in 2050 grows even higher whenconsidering the use of hydrogen for those transport applications (both road and non-road)where electricity is not well suited (e.g., aviation). Despite the vast potential of hydrogen totransform the global transport sector (as discussed in Turton (2006), Grahn et al. (2007), andelsewhere), we restrict the scope of our analysis to electricity as an energy carrier (eitherfrom the grid or distributed generation). This choice was made for two reasons. First, while

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hydrogen has been proposed for a range of transport applications (including for aviation andlong-distance trucking), a number of unique vehicle- and infrastructure-level challenges stillremain (IEA 2012; Ogden and Anderson 2011), and these challenges could ultimatelyconstrain the scale and scope of hydrogens application to essentially the same transportmodes as for electricity. Given this somewhat similar potential (from a whole transportperspective) and because a number of studies in the literature find that electrification couldplay a major role in climate mitigation (Edmonds et al. 2006; Sugiyama 2012), we decidedto focus this paper on the electricity option. From the outset, our ambition was not to explorethe myriad possible futures that could unfold in the transport sector.

The detailed transport scenario studies highlighted above were used to benchmark ourelectric transport (ET) sensitivity cases in MESSAGE. The solution framework ofMESSAGE is such that it attempts to meet demand for transportation activity with theleast-cost technological options (from a systems and full supply chain perspective), based onthe myriad assumptions embedded in the model and subject to a variety of constraints. Oneset of these constraints puts limits on the maximum amount of electricity that may beconsumed in a given regions transport sector in a given year. These limits take the formof share constraints: i.e., electricity as a share of total final energy demand. In our analysiswe systematically varied these upper limits over a wide range of valuesto be clear, thesewere the only assumptions that were modified in the ET sensitivity cases described herein.(As later results indicate, in climate mitigation scenarios with sufficiently high carbon prices,the model prefers to use as much electricity as is permitted by the constraints; in baselinescenarios the constraints are non-binding.) Across our ET scenarios, the maximum globalshares range from 5 % to 75 % (at roughly equally-spaced intervals) over the course of thecentury. (By 2050, electrification is typically still on the rise (see Section 3.2); shares in thisyear span from

infrastructure largely exists alreadywhereas the former would take decades to mature. Twoof our sensitivity cases illustrate these diverging futures quite clearly. By explicitly keepingtransport electrification to a low level, the 450 FullTech ET5% scenario represents thecontinued dominance of the conventional system; the 450 FullTech ET75%, on the otherhand, is far more optimistic regarding the penetration of electric vehicles and their requisiterecharging infrastructure. Figure 2 shows the evolution of the transport fuel mix in these twoscenarios over the next several decades. In the 450 FullTech ET5% (upper panels),decarbonization of the transport sector is achieved mostly through low-carbon liquid fuels(biofuels and natural gas-based liquids produced with CCS), with a fair contribution fromgaseous fuels (natural gas2 and bio-gas with some hydrogen blending) in the second half ofthe century. Electricity never exceeds 5 % (by design) of final energy demand at any point.Meanwhile, in the 450 FullTech ET75% scenario (lower panels) electricity is allowed tocontribute to the decarbonization effort to a far greater degree. The dominance of liquid fuelsyields to the new electric paradigm: liquid fuels are consumed in lower quantities (andessentially only in the applications where electrification is exceedingly challenging, e.g.,aviation, long-distance trucking, and shipping), while the share of final energy from elec-tricity grows to almost 30 % by mid-century and to roughly 75 % by 2070. Though high in2050, this electricity share is well within the upper limits estimated by the more detailedtransport studies in the literature, as discussed above.

These trends have obvious implications on the diversity of the transport fuel mix (seelines in fuel mix charts), which becomes important when discussing energy security in aregional context, as diversity affects the ability of the transport system to respond tounforeseen shocks (Cherp et al. 2012; Riahi et al. 2012). Thanks to rapid electrification,the 450 FullTech ET75% sees its fuel diversity grow quickly (globally-aggregated; measuredby the Shannon-Wiener diversity index (SWDI); see SM); in fact, the level of diversity in2050 is roughly twice that of the low electrification 450 FullTech ET5% scenario. Onceelectricity becomes the dominant energy carrier, however, the diversity of the transport fuelmix begins to decline and eventually saturate. Whether this backward trend would eventu-ally arouse energy security concerns is not yet clear. It largely depends on where the differentenergy carriers are sourced and from which specific primary resources. For a region with adecarbonized electricity system comprised largely of renewables (a likely eventuality in a450 ppm future), most of its electricity could actually be considered domestic and, thus,relatively secure, at least from an energy dependence point of view. Indeed, this is what wefind in the 450 FullTech ET75% scenario: more electrification of transport means morerenewables being consumed at the primary energy level to meet the final energy demands inthe sector (right panels of Fig. 2). This, in turn, contributes to far higher primary resourcediversity of the transport sector (see SWDI lines in right-hand charts) than in the 450FullTech ET5%, which becomes increasingly reliant on biomass (for biofuels) and naturalgas (for gas-powered transport applications and for conversion to synthetic liquids). A keyconclusion here is that transport electrification opens up a door through which nuclearenergy and non-biomass renewables (solar, wind, hydro, geothermal) can flow. (Andalthough not the focus of this analysis, a similar finding holds for hydrogen.) In a conven-tional liquid fuels system, the potential contribution of these low-carbon energy providers ismarginalized because their primary output (electricity) cannot be easily converted to liquidform.

2 Leakage from natural gas extraction, transport/distribution, storage, and use is accounted for in MESSAGE.Assumed leakage rates add up to

Not shown in Fig. 2 are the absolute levels of transport electricity, final energy, andenergy service demands in the two diverging transport futures discussed here.3 Consumptionof electricity across all transport modes grows to be quite high in the 450 FullTech ET75%scenario, for instance: from approximately 0.8 EJ/yr today to 1.5 EJ/yr in 2020 and 33.1EJ/yr in 2050 (20 % of total electricity consumption for all uses in 2050). To put thesenumbers in perspective, this would roughly translate to between 35 and 100 million light-duty battery-electric vehicles (BEV) on the worlds roads by 2020, growing to 17005000million by mid-century (see SM for assumptions)a truly ambitious task given the pres-ently low starting point. (Note that in 2010 the global passenger light-duty vehicle stocktotaled 841 million (OECD/ITF 2012), of which BEVs numbered only in the thousands.) Instark contrast, the pace of transport electrification would be far less rapid in the 450 FullTech

3 For reference, total final energy consumption in the transport sector is ~87 EJ/yr today. In the 450 FullTechET75% scenario, this grows to 109 EJ/yr in 2020 and 123 EJ/yr in 2050; in 450 FullTech ET5%, thesenumbers are 107 EJ/yr and 140 EJ/yr, respectively.

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Fig. 2 Evolution of transportation fuel mix over time, in terms of both final energy carriers (left panels, a & c)and transport sector final energy shares allocated to the primary energy sources from which they originate(right panels, b & d). Lines on charts capture the diversity of the fuel/resource mix in each case, as measuredby the Shannon-Wiener diversity index and calculated based on the respective shares. Upper panels showresults for the 450 FullTech ET5% scenario; lower panels for the 450 FullTech ET75%

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ET5%: essentially no BEVs in 2020 and 290860 million by 2050 (transport electricitydemand of 0.8 EJ/yr and 6.3 EJ/yr, respectively). The transport sector would obviously lookquite different in these two contrasting scenarios, even though each still allows for thefeasible attainment of the 450 ppm stabilization target (see Table 1). Moreover, through themuch higher end-use efficiencies that electric-drive technologies make possible, post-2050final energy demands in transport are significantly reduced in the 450 FullTech ET75%compared to the 450 FullTech ET5% (see footnote). Service demands (e.g., vehicle-km) arehigher in the former case, however, thanks to the reduced costs of vehicle operation. In otherwords, there is a rebound effect: because mitigation costs are lower when there is a greaterpotential for transport electrification (see Section 3.4), there is less of an incentive forconsumers to reduce their demand for mobility.4 By our calculations, the lower price signalin the high electrification scenario would spur much less demand reduction (9 % in 2050relative to the no-climate policy baseline) than in the low electrification scenario (16 %).Given that other considerations besides climate change mitigation will be on the minds ofpolicy makers and transportation planners in the years to come (e.g., concerns over noise, airpollution, network capacity, and equity), these rebound effects should not be ignored.

3.3 Re-allocation of biomass and biofuels

Biomass is a unique energy resource, the only one in fact that can potentially be used as a netcarbon sink. Hence, deciding how best to distribute constrained biomass supplies is impor-tant. (See Grahn et al. (2007), Rose et al. (2013), and Calvin et al. (2013) for cross-modelcomparisons on this topic.) In the MESSAGE framework, the freeing-up of valuablebiomass resources to be used in other sectors is found to be one of the key system-wideconsequences of electrifying transport. This is illustrated quite clearly in Fig. 3, which tracksthe flows of biomass and biomass-derived energy carriers all the way from the primary tosecondary to final energy sectors for two relatively similar 450 ppm scenarios with differentlevels of transport sector electrification.

As discussed previously, the 450 FullTech ET5% attempts to represent the continueddominance of conventional liquid-based fuel delivery infrastructures for vehicles; thus,biomass becomes highly valued as a source for low, or even negative-, carbon biofuels.These biofuels are directed almost entirely to the transport sector for final use. What biomassremains is consumed directly in the industrial sector, particularly as a feedstock for gener-ating high-temperature heat and in applications where carbonaceous fuels are difficult tosubstitute (e.g., primary steel production), thereby replacing coal and natural gas.

The 450 FullTech ET75% is far more optimistic regarding the penetration of electricvehicles and their requisite recharging infrastructure; hence, the situation with biomass looksquite different. Thanks to the lower demand for biofuels in the transport sector (though stillnecessary for certain applications: aviation, marine, heavy-duty trucking), the industrialsector becomes the main recipient of biomass-derived energy carriers. Specifically, directuse of biomass in industry scales up, and the sector starts to make greater use of biofuels, atrend that accelerates quickly after 2050 (not shown in Fig. 3; see further discussion below).In contrast, neither this scenario nor the low transport electrification variant (450 FullTechET5%) sees a measurable amount of biomass-derived energy carriers directed toward theresidential and commercial sector. Electricity and heat become the preferred carriers in thissector in a carbon-constrained world, though producing them does not appear to be the most

4 MESSAGE accounts for these price-induced demand feedbacks through its linkage to an aggregated macro-economic model.

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optimal use of biomass when considering the competing demands for this valuable resourcein other parts of the system. Moreover, it should also be noted that in the high transportelectrification scenario (450 FullTech ET75%), a greater percentage of the energy containedin the raw (primary) biomass feedstock is available for use at the final energy level. Thisprovides an indication of the higher biomass supply-chain efficienciesacross sectors andfrom the primary to secondary to final energy levelthat electrification of transportationend-use demand technologies makes possible. (Note that a full analysis of system-wideefficiencies would also account for bio-products conversion from final to useful energy ineach of the different end-use sectors.)

One of the principal reasons biofuels become so attractive as an industrialinputonce biofuel demands in transport are relaxedhas much to do with howMESSAGE accounts for emissions from fossil-based feedstocks (coal, oil, natural gas,synthetic liquid fuels) used to produce industrial goods, such as poly-ethylene andother petro-chemicals. For this reason, we conducted a separate sensitivity analysiswith MESSAGE, varying the assumed share of feedstock carbon that ultimately isreleased to the atmosphere as CO2. A robust finding of this analysis is that theseemission-share assumptions have a considerable impact on the costs of mitigation and,ultimately, on the feasibility of ambitious climate targets; thus, the topic deservesadditional attention in future research. (See SM for further details.)

aLow transport electrification

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Fig. 3 Flows of biomass and biomass-derived energy carriers in 2050. Relative thickness of lines indicatesquantities. Panel (a) shows results for the 450 FullTech ET5% scenario; Panel (b) for the 450 FullTech ET75%

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3.4 Mitigation costs and scenario feasibility

Similar to the availability of supply-side technologies and stringency of energy efficiencyand conservation efforts, transport sector electrification has important implications for thefeasibility of the ambitious 450 ppm CO2-eq stabilization target. As shown in Table 1, someof the 450 ppm MESSAGE scenarios from the standard EMF27 set turned out to be quiteclose to the feasibility threshold of the model. By varying the assumptions on transportationsector electrification, two of these scenarios became feasible (in the case of higher electri-fication), and one became infeasible (lower electrification). An expanded discussion can befound in the SM.

The scale of electrification of the transport sector also has a major impact on the totalcosts of climate mitigation. Supplementary Figure 3 shows for the 450 FullTech family ofscenarios two different mitigation cost measures and how these measures vary depending onthe scale of electrification. In both cases, mitigation costs dramatically decrease at higherelectrification levels. These dynamics are due to the ripple effects that transport electrifica-tion has on fuel consumption in the industry and buildings sectors. More specifically, byobviating the need for biofuels in transport, electrification frees up valuable biomassresources that can then be used in other parts of the energy system (see Section 3.3).Further details are in the SM.

4 Conclusions

As part of the EMF27 model inter-comparison exercise, we have analyzed the role oftechnology in meeting climate stabilization targets of differing stringency (550 and450 ppm CO2-eq) using the MESSAGE integrated assessment model. A systematic variationof availability, cost, and performance of particular energy supply technologies has beenundertaken to derive implications for feasibility of climate stabilization goals and theassociated costs of mitigation. Moreover, in order to complement the supply-side focus ofthe overall study design with a demand-side perspective, we have explored a wide range ofassumptions regarding the future configuration of the transportation sector. Specifically,while a number of detailed transport scenario studies have looked into the role of electrifi-cation in this sector, there has been comparatively little consideration of the implications ofsuch transformational change for the rest of the energy system. In this paper, we aim to fillthis gap.

Our analysis of the EMF27 scenarios shows that the 550 ppm climate target can still bemet even if there are restrictions to the utilization of nuclear power, CCS, intermittentrenewables, or bioenergy supply. Only when all of these technologies are restricted simul-taneously does the 550 target become infeasible. The 450 ppm target can also be achieved inmost cases, except when CCS is completely unavailable or when both renewables andbioenergy are simultaneously restricted. The former case highlights the importance ofCCS as a carbon mitigation option; however, this importance is closely tied to the structureof fuel demand in the end-use sectors, most notably transport. Our standard set of EMF27scenarios assumes a moderately high limit to future transport electrification (a maximum25 % share of final energy over the century), consistent with the middle of the range oftransport studies in the literature. Interestingly, if greater electrification is allowed, then boththe 450 ppm scenario without CCS and with limited intermittent renewables and bioenerybecome feasibleat 35 % and 75 % maximum electrification, respectively. In contrast, the450 scenario with limited bioenergy turns infeasible at our lowest maximum electrification

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rate of 5 %. The implications of electrification for scenario feasibility also play out in termsof climate mitigation costs. For instance, by our estimation the difference in costs between a450 ppm scenario with low transport electrification compared to one with high electrificationis roughly comparable to the imposition of technological restrictions on the supply side, suchas on wind and solar electricity generation.

Electrifying transport leads to ripple effects up the transportation fuel supply chain andthroughout the rest of the energy system. Valuable resources (both physical and financial) arefreed up that can then be used elsewhere: this resulting from an implicit redefinition of themarginal value of certain sectoral mitigation options. In the MESSAGE framework, for example,we find that the freeing-up of biomass resources to be used in other sectors is one of the keysystem-wide consequences of electrifying transport. The industrial sector subsequently becomesthe main recipient of biomass-derived energy carriers, for both energy and non-energy uses (e.g.,as a feedstock for petrochemicals). Rapid and pervasive electrification also leads to increaseddiversity of the transport fuel mix, an important indicator for energy security in a regional context.By extension, electrification contributes to far higher primary resource diversity for the transportsector. Whereas a low-carbon transport system built upon conventional liquid-based fuel deliveryinfrastructures is destined to become increasingly reliant on biomass (for biofuels) and natural gas(for gas-powered applications and for conversion to synthetic liquids), electrification opens up adoor through which nuclear energy and non-biomass renewables (solar, wind, hydro, geothermal)can flow. A similar finding also holds for hydrogen, although not the focus of this paper.

Acknowledgments We recognize the technical contributions of Patrick Sullivan to this analysis. TheSankey-type flow diagrams were developed using the Fineo software made available by the DensityDesignResearch Lab of the Politecnico di Milano. The comments of the editor and anonymous reviewers helped tosubstantially improve this paper.

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Transport electrification: A key element for energy system transformation and climate stabilizationAbstractIntroduction and motivationThe MESSAGE interpretation of the EMF27 scenariosFeasibility of reaching stringent climate targetsTiming of CO2 mitigation

Implications of electrifying transportTransport sector electrification sensitivity analysisDiversity of the transport fuel mixRe-allocation of biomass and biofuelsMitigation costs and scenario feasibility

ConclusionsReferences