REVIEW SUMMARY

ENERGY

Net-zero emissions energy systemsSteven J Davis Nathan S Lewis Matthew Shaner Sonia Aggarwal Doug ArentInecircs L Azevedo Sally M Benson Thomas Bradley Jack Brouwer Yet-Ming ChiangChristopher T M Clack Armond Cohen Stephen Doig Jae Edmonds Paul FennellChristopher B Field Bryan Hannegan Bri-Mathias Hodge Martin I HoffertEric Ingersoll Paulina Jaramillo Klaus S Lackner Katharine J MachMichael Mastrandrea Joan Ogden Per F Peterson Daniel L SanchezDaniel Sperling Joseph Stagner Jessika E Trancik Chi-Jen Yang Ken Caldeira

BACKGROUND Net emissions of CO2 byhuman activitiesmdashincluding not only en-ergy services and industrial production butalso land use and agriculturemdashmust ap-proach zero in order to stabilize globalmean temperature Energy services suchas light-duty transportation heating coolingand lighting may be relatively straight-forward to decarbonize by elec-trifying and generating electricityfrom variable renewable energysources (such as wind and solar)and dispatchable (ldquoon-demandrdquo)nonrenewable sources (includingnuclear energy and fossil fuels withcarbon capture and storage) How-ever other energy services essentialto modern civilization entail emis-sions that are likely to be moredifficult to fully eliminate Thesedifficult-to-decarbonize energy ser-vices include aviation long-distancetransport and shipping productionof carbon-intensive structural mate-rials such as steel and cement andprovision of a reliable electricitysupply that meets varying demandMoreover demand for such ser-vices and products is projectedto increase substantially over thiscentury The long-lived infrastruc-ture built today for better or worsewill shape the futureHere we review the special chal-

lenges associated with an energysystem that does not add any CO2

to the atmosphere (a net-zeroemissions energy system) Wediscuss prominent technolog-ical opportunities and barriersfor eliminating andor managingemissions related to the difficult-to-decarbonize services pitfallsin which near-term actions maymake it more difficult or costly toachieve the net-zero emissionsgoal and critical areas for re-

search development demonstration and de-ployment It may take decades to researchdevelop and deploy these new technologies

ADVANCES A successful transition to afuture net-zero emissions energy systemis likely to depend on vast amounts of in-expensive emissions-free electricity mecha-

nisms to quickly and cheaply balance largeand uncertain time-varying differences be-tween demand and electricity generationelectrified substitutes for most fuel-usingdevices alternative materials and manu-facturing processes for structural materialsand carbon-neutral fuels for the parts of theeconomy that are not easily electrified Re-cycling and removal ofcarbon from the atmo-sphere (carbon manage-ment) is also likely to bean important activity ofany net-zero emissionsenergy system The spe-cific technologies that will be favored infuture marketplaces are largely uncertainbut only a finite number of technology choicesexist today for each functional role To takeappropriate actions in the near term it isimperative to clearly identify desired endpoints To achieve a robust reliable and af-fordable net-zero emissions energy systemlater this century efforts to research developdemonstrate and deploy those candidatetechnologies must start now

OUTLOOK Combinations of known tech-nologies could eliminate emissions relatedto all essential energy services and pro-cesses but substantial increases in costsare an immediate barrier to avoiding emis-sions in each category In some cases in-novation and deployment can be expectedto reduce costs and create new options Morerapid changes may depend on coordinat-ing operations across energy and industrysectors which could help boost utilizationrates of capital-intensive assets but thiswill require overcoming institutional andorganizational challenges in order to createnew markets and ensure cooperation amongregulators and disparate risk-averse busi-nesses Two parallel and broad streams ofresearch and development could prove use-ful research in technologies and approachesthat can decarbonize provision of the mostdifficult-to-decarbonize energy services andresearch in systems integration that wouldallow reliable and cost-effective provision ofthese services

RESEARCH

Davis et al Science 360 1419 (2018) 29 June 2018 1 of 1

The list of author affiliations is available in the full article onlineCorresponding author Email sjdavisuciedu (SJD)nslewiscaltechedu (NSL) kcaldeiracarnegiescienceedu(KC)Cite this article as S J Davis et al Science 360 eaas9793(2018) DOI 101126scienceaas9793

A shower of moltenmetal in a steel foundry Industrialprocesses such as steelmaking will be particularlychallenging to decarbonize Meeting future demand forsuch difficult-to-decarbonize energy services and industrialproducts without adding CO2 to the atmosphere may dependon technological cost reductions via research and innovationas well as coordinated deployment and integration ofoperations across currently discrete energy industries

ON OUR WEBSITE

Read the full articleat httpdxdoiorg101126scienceaas9793

TOMORROWrsquoS EARTHRead more articles onlineat scimagTomorrowsEarth

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REVIEW

ENERGY

Net-zero emissions energy systemsSteven J Davis12 Nathan S Lewis3 Matthew Shaner4 Sonia Aggarwal5Doug Arent67 Inecircs L Azevedo8 Sally M Benson91011 Thomas Bradley12Jack Brouwer1314 Yet-Ming Chiang15 Christopher T M Clack16 Armond Cohen17Stephen Doig18 Jae Edmonds19 Paul Fennell2021 Christopher B Field22Bryan Hannegan23 Bri-Mathias Hodge62425 Martin I Hoffert26 Eric Ingersoll27Paulina Jaramillo8 Klaus S Lackner28 Katharine J Mach29 Michael Mastrandrea4Joan Ogden30 Per F Peterson31 Daniel L Sanchez32 Daniel Sperling33Joseph Stagner34 Jessika E Trancik3536 Chi-Jen Yang37 Ken Caldeira32

Some energy services and industrial processesmdashsuch as long-distance freight transportair travel highly reliable electricity and steel and cement manufacturingmdashare particularlydifficult to provide without adding carbon dioxide (CO2) to the atmosphere Rapidlygrowing demand for these services combined with long lead times for technologydevelopment and long lifetimes of energy infrastructure make decarbonization of theseservices both essential and urgentWe examine barriers and opportunities associated withthese difficult-to-decarbonize services and processes including possible technologicalsolutions and research and development priorities A range of existing technologies couldmeet future demands for these services and processes without net addition of CO2 tothe atmosphere but their use may depend on a combination of cost reductions viaresearch and innovation as well as coordinated deployment and integration of operationsacross currently discrete energy industries

People do not want energy itself but ratherthe services that energy provides and theproducts that rely on these services Evenwith substantial improvements in efficiencyglobal demand for energy is projected to

increase markedly over this century (1) Mean-while net emissions of carbondioxide (CO2) fromhuman activitiesmdashincluding not only energyand industrial production but also land use andagriculturemdashmust approach zero to stabilize glo-bal mean temperature (2 3) Indeed interna-tional climate targets such as avoiding morethan 2degC of mean warming are likely to requirean energy systemwith net-zero (or net-negative)emissions later this century (Fig 1) (3)Energy services such as light-duty transpor-

tation heating cooling and lighting may berelatively straightforward to decarbonize byelectrifying and generating electricity from var-iable renewable energy sources (such as windand solar) and dispatchable (ldquoon-demandrdquo) non-

renewable sources (including nuclear energyand fossil fuels with carbon capture and storage)However other energy services essential to mo-dern civilization entail emissions that are likelyto be more difficult to fully eliminate Thesedifficult-to-decarbonize energy services includeaviation long-distance transport and shippingproduction of carbon-intensive structural materi-als such as steel and cement and provision ofa reliable electricity supply that meets varyingdemand To the extent that carbon remains in-volved in these services in the future net-zeroemissions will also entail active managementof carbonIn 2014 difficult-to-eliminate emissions related

to aviation long-distance transportation andshipping structuralmaterials andhighly reliableelectricity totaled ~92 Gt CO2 or 27 of globalCO2 emissions from all fossil fuel and industrialsources (Fig 2) Yet despite their importancedetailed representation of these services in in-

tegrated assessment models remains challeng-ing (4ndash6)Here we review the special challenges asso-

ciated with an energy system that does not addany CO2 to the atmosphere (a net-zero emissionsenergy system) We discuss prominent techno-logical opportunities and barriers for eliminat-ing andor managing emissions related to thedifficult-to-decarbonize services pitfalls in whichnear-term actions may make it more difficult orcostly to achieve the net-zero emissions goaland critical areas for research developmentdemonstration and deployment Our scope isnot comprehensive we focus on what now seemthe most promising technologies and pathwaysOur assertions regarding feasibility throughoutare not the result of formal quantitative econo-mic modeling rather they are based on compar-ison of current and projected costs with statedassumptions about progress and policyA major conclusion is that it is vital to integrate

currently discrete energy sectors and industrialprocesses This integration may entail infrastruc-tural and institutional transformations as well asactive management of carbon in the energy system

Aviation long-distance transportand shipping

In 2014 medium- and heavy-duty trucks withmean trip distances of gt160 km (gt100 miles)accounted for ~270 Mt CO2 emissions or 08of global CO2 emissions from fossil fuel com-bustion and industry sources [estimated byusing (7ndash9)] Similarly long trips in light-dutyvehicles accounted for an additional 40 Mt CO2and aviation and other shipping modes (suchas trains and ships) emitted 830 and 1060 MtCO2 respectively Altogether these sources wereresponsible for ~6 of global CO2 emissions(Fig 2) Meanwhile both global energy demandfor transportation and the ratio of heavy- tolight-duty vehicles is expected to increase (9)Light-duty vehicles can be electrified or run

on hydrogen without drastic changes in perfor-mance except for range andor refueling timeBy contrast general-use air transportation andlong-distance transportation especially by trucksor ships have additional constraints of revenuecargo space and payload capacity that mandateenergy sources with high volumetric and grav-imetric density (10) Closed-cycle electrochemicalbatteries must contain all of their reactants andproducts Hence fuels that are oxidized with

RESEARCH

Davis et al Science 360 eaas9793 (2018) 29 June 2018 1 of 9

1Department of Earth System Science University of California Irvine Irvine CA USA 2Department of Civil and Environmental Engineering University of California Irvine Irvine CA USA3Division of Chemistry and Chemical Engineering California Institute of Technology Pasadena CA USA 4Near Zero Carnegie Institution for Science Stanford CA USA 5Energy Innovation SanFrancisco CA USA 6National Renewable Energy Laboratory Golden CO USA 7Joint Institute for Strategic Energy Analysis Golden CO USA 8Engineering and Public Policy Carnegie MellonUniversity Pittsburgh PA USA 9Global Climate and Energy Project Stanford University Stanford CA USA 10Precourt Institute for Energy Stanford University Stanford CA USA 11Departmentof Energy Resource Engineering Stanford University Stanford CA USA 12Department of Mechanical Engineering Colorado State University Fort Collins CO USA 13Department of Mechanicaland Aerospace Engineering University of California Irvine Irvine CA USA 14Advanced Power and Energy Program University of California Irvine CA USA 15Department of Material Science andEngineering Massachusetts Institute of Technology Cambridge MA USA 16Vibrant Clean Energy Boulder CO USA 17Clean Air Task Force Boston MA USA 18Rocky Mountain InstituteBoulder CO USA 19Pacific National Northwestern Laboratory College Park MD USA 20Department of Chemical Engineering South Kensington Campus Imperial College London London UK21Joint Bioenergy Institute 5885 Hollis Street Emeryville CA USA 22Woods Institute for the Environment Stanford University Stanford CA USA 23Holy Cross Energy Glenwood Springs COUSA 24Department of Electrical Computer and Energy Engineering University of Colorado Boulder Boulder CO USA 25Department of Chemical and Biological Engineering Colorado School ofMines Golden CO USA 26Department of Physics New York University New York NY USA 27Lucid Strategy Cambridge MA USA 28The Center for Negative Carbon Emissions Arizona StateUniversity Tempe AZ USA 29Department of Earth System Science Stanford University Stanford CA USA 30Environmental Science and Policy University of California Davis Davis CA USA31Department of Nuclear Engineering University of California Berkeley Berkeley CA USA 32Department of Global Ecology Carnegie Institution for Science Stanford CA USA 33Institute ofTransportation Studies University of California Davis Davis CA USA 34Department of Sustainability and Energy Management Stanford University Stanford CA USA 35Institute for DataSystems and Society Massachusetts Institute of Technology Cambridge MA USA 36Santa Fe Institute Santa Fe NM USA 37Independent researcherCorresponding authors Email sjdavisuciedu (SJD) nslewiscaltechedu (NSL) kcaldeiracarnegiescienceedu (KC)

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nloaded from

ambient air and then vent their exhaust to theatmosphere have a substantial chemical advan-tage in gravimetric energy densityBattery- and hydrogen-powered trucks are now

used in short-distance trucking (11) but at equal

range heavy-duty trucks powered by currentlithium-ion batteries and electric motors can car-ry ~40 less goods than can trucks poweredby diesel-fueled internal combustion enginesThe same physical constraints of gravimetric

and volumetric energy density likely precludebattery- or hydrogen-powered aircraft for long-distance cargo or passenger service (12) Auto-nomous trucks and distributed manufacturingmay fundamentally alter the energy demands of

Davis et al Science 360 eaas9793 (2018) 29 June 2018 2 of 9

Fig 1 Schematic of an integrated system that can provideessential energy services without adding any CO2 to the atmo-sphere (A to S) Colors indicate the dominant role of specifictechnologies and processes Green electricity generation and trans-

mission blue hydrogen production and transport purplehydrocarbon production and transport orange ammonia productionand transport red carbon management and black end uses ofenergy and materials

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the freight industry but if available energy-denseliquid fuels are likely to remain the preferredenergy source for long-distance transportationservices (13)Options for such energy-dense liquid fuels in-

clude the hydrocarbons we now use as well ashydrogen ammonia and alcohols and ethersIn each case there are options for producingcarbon-neutral or low-carbon fuels that couldbe integrated to a net-zero emissions energysystem (Fig 1) and each can also be intercon-verted through existing thermochemical processes(Table 1)

Hydrogen and ammonia fuels

The low volumetric energy density of hydrogenfavors transport and storage at low temperatures(ndash253degC for liquid hydrogen at atmospheric pres-sure) andor high pressures (350 to 700 bar)thus requiring heavy and bulky storage contain-ers (14) To contain the same total energy as adiesel fuel storage system a liquid hydrogenstorage system would weigh roughly six timesmore and be about eight times larger (Fig 3A)However hydrogen fuel cell or hybrid hydrogen-battery trucks can be more energy efficient thanthose with internal combustion diesel engines(15) requiring less onboard energy storage toachieve the same traveling range Toyota hasrecently introduced a heavy-duty (36000 kg)500-kW fuel cellbattery hybrid truck designedto travel 200 miles on liquid hydrogen and storedelectricity and Nikola has announced a similarbatteryfuel cell heavy-duty truck with a claimedrange of 1300 to 1900 km which is comparablewith todayrsquos long-haul diesel trucks (16) If hy-drogen can be produced affordably without CO2

emissions its use in the transport sector couldultimately be bolstered by the fuelrsquos importancein providing other energy servicesAmmonia is another technologically viable

alternative fuel that contains no carbon and

may be directly used in an engine or may becracked to produce hydrogen Its thermolysismust be carefully controlled so as to minimizeproduction of highly oxidized products such asNOx (17) Furthermore like hydrogen ammo-niarsquos gravimetric energy density is considerablylower than that of hydrocarbons such as diesel(Fig 3A)

Biofuels

Conversion of biomass currently provides themost cost-effective pathway to nonfossil carbon-containing liquid fuels Liquid biofuels at presentrepresent ~42 EJ of the roughly 100 EJ of energyconsumed by the transport sector worldwideCurrently the main liquid biofuels are ethanolfrom grain and sugar cane and biodiesel and re-newable diesel from oil seeds and waste oilsThey are associated with substantial challengesrelated to their life-cycle carbon emissions costand scalability (18)Photosynthesis converts lt5 of incident ra-

diation to chemical energy and only a fractionof that chemical energy remains in biomass (19)Conversion of biomass to fuel also requires en-ergy for processing and transportation Landused to produce biofuels must have water nu-trient soil and climate characteristics suitablefor agriculture thus putting biofuels in competi-tion with other land uses This has implicationsfor food security sustainable rural economies andthe protection of nature and ecosystem services(20) Potential land-use competition is heightenedby increasing interest in bioenergy with carboncapture and storage (BECCS) as a source of nega-tive emissions (that is carbon dioxide removal)which biofuels can provide (21)Advanced biofuel efforts include processes that

seek to overcome the recalcitrance of cellulose toallow use of different feedstocks (such as woodycrops agricultural residues and wastes) in orderto achieve large-scale production of liquid trans-

portation fuels at costs roughly competitive withgasoline (for example US $19GJ or US $151gallon of ethanol) (22) As technology maturesand overall decarbonization efforts of the energysystem proceed biofuels may be able to largelyavoid fossil fuel inputs such as those related toon-farm processes and transport as well as emis-sions associated with induced land-use change(23 24) The extent to which biomass will supplyliquid fuels in a future net-zero emissions energysystem thus depends on advances in conversiontechnology competing demands for bioenergyand land the feasibility of other sources of carbon-neutral fuels and integration of biomass produc-tion with other objectives (25)

Synthetic hydrocarbons

Liquid hydrocarbons can also be synthesizedthrough industrial hydrogenation of feedstockcarbon such as the reaction of carbon monoxideand hydrogen by the Fischer-Tropsch process(26) If the carbon contained in the feedstockis taken from the atmosphere and no fossil en-ergy is used for the production processing andtransport of feedstocks and synthesized fuelsthe resulting hydrocarbons would be carbon-neutral (Fig 1) For example emissions-free elec-tricity could be used to produce dihydrogen (H2)by means of electrolysis of water which wouldbe reacted with CO2 removed from the atmo-sphere either through direct air capture or photo-synthesis (which in the latter case could includeCO2 captured from the exhaust of biomass orbiogas combustion) (27 28)At present the cost of electrolysis is a major

barrier This cost includes both the capital costsof electrolyzers and the cost of emissions-freeelectricity 60 to 70 of current electrolytic hy-drogen cost is electricity (Fig 3C) (28 29) Thecheapest and most mature electrolysis technologyavailable today uses alkaline electrolytes [such aspotassium hydroxide (KOH) or sodium hydroxide

Davis et al Science 360 eaas9793 (2018) 29 June 2018 3 of 9

Table 1 Key energy carriers and the processes for interconversion Processes listed in each cell convert the row energy carrier to the column energy

carrier Further details about costs and efficiencies of these interconversions are available in the supplementary materials

To

From endash H2 CxOyHz NH3

endash Electrolysis ($5 to 6kg H2) Electrolysis + methanation Electrolysis + Haber-Bosch

Electrolysis + Fischer-Tropsch

H2 Combustion Methanation

($007 to 057m3 CH4)

Haber-Bosch ($050 to

060kg NH3)

Oxidation via fuel cell Fischer-Tropsch ($440

to $1500gallon of

gasoline-equivalent)

CxOyHz Combustion Steam reforming

($129 to 150kg H2)

Steam reforming +

Haber-Bosch

Biomass gasification

($480 to 540kg H2)

NH3 Combustion Metal catalysts

(~$3kg H2)

Metal catalysts + methanation

Fischer-Tropsch

Sodium amide

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(NaOH)] together with metal catalysts to pro-duce hydrogen at an efficiency of 50 to 60 anda cost of ~US $550kg H2 (assuming industrialelectricity costs of US $007kWh and 75 uti-lization rates) (29 30) At this cost of hydrogenthe minimum price of synthesized hydrocarbonswould be $150 to $170liter of diesel equivalent[or $550 to $650gallon and $42 to $50 per GJassuming carbon feedstock costs of $0 to 100 perton of CO2 and very low process costs of $005liter or $150 per GJ (28)] For comparison H2

from steam reforming of fossil CH4 into CO2 andH2 currently costs $130 to 150 per kg (Fig 3Dred line) (29 31) Thus the feasibility of syn-thesizing hydrocarbons from electrolytic H2 maydepend on demonstrating valuable cross-sectorbenefits such as balancing variability of renew-able electricity generation or else a policy-imposedprice of ~$400 per ton of CO2 emitted (whichwould also raise fossil diesel prices by ~$100literor ~$400gallon)In the absence of policies or cross-sector coor-

dination hydrogen costs of $200kg (approachingthe cost of fossil-derived hydrogen and synthe-sized diesel of ~$079liter or $300gallon) couldbe achieved for example if electricity costs were$003kWh and current electrolyzer costs werereduced by 60 to 80 (Fig 3B) (29) Such reduc-tions may be possible (32) but may require central-ized electrolysis (33) and using less mature butpromising technologies such as high-temperaturesolid oxide or molten carbonate fuel cells orthermochemical water splitting (30 34) Fuelmarkets are vastly more flexible than instan-taneously balanced electricity markets because

of the relative simplicity of large long-termstorage of chemical fuels Hence using emissions-free electricity to make fuels represents a criticalopportunity for integrating electricity and trans-portation systems in order to supply a persistentdemand for carbon-neutral fuels while boostingutilization rates of system assets

Direct solar fuels

Photoelectrochemical cells or particulatemolecularphotocatalysts directly split water by using sunlightto produce fuel through artificial photosynthesiswithout the land-use constraints associated withbiomass (35) Hydrogen production efficienciescan be high but costs capacity factors and life-times need to be improved in order to obtain anintegrated cost-advantaged approach to carbon-neutral fuel production (36) Short-lived labora-tory demonstrations have also produced liquidcarbon-containing fuels by using concentratedCO2 streams (Fig 1H) (37) in some cases byusing bacteria as catalysts

Outlook

Large-scale production of carbon-neutral andenergy-dense liquid fuels may be critical to achiev-ing a net-zero emissions energy system Such fuelscould provide a highly advantageous bridge be-tween the stationary and transportation energy pro-duction sectors and may therefore deserve specialpriority in energy research and development efforts

Structural materials

Economic development and industrializationare historically linked to the construction of in-

frastructure Between 2000 and 2015 cement andsteel use persistently averaged 50 and 21 tons permillion dollars of global GDP respectively (~1 kgper person per day in developed countries) (4)Globally ~1320 and 1740 Mt CO2 emissions em-anated from chemical reactions involved with themanufacture of cement and steel respectively(Fig 2) (8 38 39) altogether this equates to~9 of global CO2 emissions in 2014 (Fig 1purple and blue) Although materials intensityof construction could be substantially reduced(40 41) steel demand is projected to grow by 33per year to 24 billion tons in 2025 (42) and ce-ment production is projected to grow by 08 to12 per year to 37 billion to 44 billion tons in2050 (43 44) continuing historical patterns ofinfrastructure accumulation andmaterials use seenin regions such as China India and Africa (4)Decarbonizing the provision of cement and

steel will require major changes in manufac-turing processes use of alternative materialsthat do not emit CO2 during manufacture orcarbon capture and storage (CCS) technologiesto minimize the release of process-related CO2

to the atmosphere (Fig 1B) (45)

Steel

During steel making carbon (coke from cokingcoal) is used to reduce iron oxide ore in blastfurnaces producing 16 to 31 tons of processCO2 per ton of crude steel produced (39) Thisis in addition to CO2 emissions from fossil fuelsburned to generate the necessary high temper-atures (1100 to 1500degC) Reductions in CO2 emis-sions per ton of crude steel are possible through

Davis et al Science 360 eaas9793 (2018) 29 June 2018 4 of 9

Fig 2 Difficult-to-eliminateemissions in current context(A and B) Estimates of CO2

emissions related to differentenergy services highlighting[for example by longer piepieces in (A)] those servicesthat will be the most difficultto decarbonize and themagnitude of 2014 emissionsfrom those difficult-to-eliminate emissionsTheshares and emissions shownhere reflect a global energysystem that still reliesprimarily on fossil fuels andthat serves many developingregions Both (A) the sharesand (B) the level of emissionsrelated to these difficult-to-decarbonize services arelikely to increase in the futureTotals and sectoral break-downs shown are basedprimarily on data from theInternational Energy Agencyand EDGAR 43 databases(838)The highlighted iron and steel and cement emissions are those relatedto the dominant industrial processes only fossil-energy inputs to thosesectors that are more easily decarbonized are included with direct emissionsfrom other industries in the ldquoOther industryrdquo category Residential and

commercial emissions are those produced directly by businesses andhouseholds and ldquoElectricityrdquo ldquoCombined heat amp electricityrdquo and ldquoHeatrdquorepresent emissions from the energy sector Further details are provided inthe supplementary materials

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the use of electric arc furnace (EAF) ldquominimillsrdquothat operate by using emissions-free electricityefficiency improvements (such as top gas recovery)new process methods (such as ldquoultra-low CO2

direct reductionrdquo ULCORED) process heat fuel-switching and decreased demand via betterengineering For example a global switch toultrahigh-strength steel for vehicles would avoid~160 Mt CO2 annually The availability of scrapsteel feedstocks currently constrains EAF pro-duction to ~30 of global demand (46 47) andthe other improvements reducemdashbut do noteliminatemdashemissionsProminent alternative reductants include char-

coal (biomass-derived carbon) and hydrogenCharcoal was used until the 18th century and theBrazilian steel sector has increasingly substitutedcharcoal for coal in order to reduce fossil CO2

emissions (48) However the ~06 tons of char-coal needed per ton of steel produced require01 to 03 ha of Brazilian eucalyptus plantation(48 49) Hundreds of millions of hectares ofhighly productive land would thus be necessaryto meet expected charcoal demands of the steelindustry and associated land use change emis-sions could outweigh avoided fossil fuel emissionsas has happened in Brazil (48) Hydrogen mightalso be used as a reductant but quality could becompromised because carbon imparts strengthand other desirable properties to steel (50)Cost notwithstanding capture and storage of

process CO2 emissions has been demonstratedand may be feasible particularly in designs suchas top gas recycling blast furnaces where con-centrations and partial pressures of CO and CO2

are high (40 to 50 and 35 by volume re-spectively) (Fig 1 G and E) (51 52)

Cement

About 40 of the CO2 emissions during cementproduction are from fossil energy inputs with theremaining CO2 emissions arising from the calcina-tion of calcium carbonate (CaCO3) (typically lime-stone) (53) Eliminating the process emissionsrequires fundamental changes to the cement-making process and cement materials andorinstallation of carbon-capture technology (Fig 1G)(54) CO2 concentrations are typically ~30 byvolume in cement plant flue gas [compared with~10 to 15 in power plant flue gas (54)] improv-ing the viability of post-combustion carbon cap-ture Firing the kiln with oxygen and recycled CO2

is another option (55) but it may be challengingto manage the composition of gases in existingcement kilns that are not gas-tight operate atvery high temperatures (~1500degC) and rotate (56)A substantial fraction of process CO2 emis-

sions from cement production is reabsorbed ona time scale of 50 years through natural car-bonation of cement materials (57) Hence captureof emissions associated with cement manufacturemight result in overall net-negative emissionsas a result of the carbonation of produced cementIf complete carbonation is ensured captured pro-cess emissions could provide an alternative feed-stock for carbon-neutral synthetic liquid fuels

Outlook

A future net-zero emissions energy systemmustprovide a way to supply structural materials such

as steel and cement or close substitutes withoutadding CO2 to the atmosphere Although alter-native processes might avoid liberation and useof carbon the cement and steel industries areespecially averse to the risk of compromising themechanical properties of produced materialsDemonstration and testing of such alternativesat scale is therefore potentially valuable Unlessand until such alternatives are proven eliminatingemissions related to steel and cement will de-pend on CCS

Highly reliable electricity

Modern economies demand highly reliable elec-tricity for example demand must be met gt999of the time (Fig 1A) This requires investment inenergy generation or storage assets that will beused a small percentage of the time when demandis high relative to variable or baseload generationAs the share of renewable electricity has grown

in the United States natural gas-fired generatorshave increasingly been used to provide generat-ing flexibility because of their relatively low fixedcosts (Fig 3B) their ability to ramp up and downquickly (58) and the affordability of natural gas(59) In other countries other fossil-fuel sourcesor hydroelectricity are used to provide flexibilityWe estimate that CO2 emissions from such ldquoload-followingrdquo electricity were ~4000 Mt CO2 in 2014(~12 of global fossil-fuel and industry emis-sions) based loosely on the proportion of elec-tricity demand in excess of minimum demand(Fig 2) (60)The central challenge of a highly reliable net-

zero emissions electricity system is thus to achieve

Davis et al Science 360 eaas9793 (2018) 29 June 2018 5 of 9

Fig 3 Comparisons of energy sources andtechnologies A) The energy density of energysources for transportation including hydrocar-bons (purple) ammonia (orange) hydrogen(blue) and current lithium ion batteries (green)(B) Relationships between fixed capital versusvariable operating costs of new generationresources in the United States with shadedranges of regional and tax credit variation andcontours of total levelized cost of electricityassuming average capacity factors and equip-ment lifetimes NG cc natural gas combinedcycle (113) (C) The relationship of capital cost(electrolyzer cost) and electricity price on thecost of produced hydrogen (the simplest possi-ble electricity-to-fuel conversion) assuming a25-year lifetime 80 capacity factor 65operating efficiency 2-year construction timeand straight-line depreciation over 10 years with$0 salvage value (29) For comparison hydrogenis currently produced by steam methane refor-mation at costs of ~$150kg H2 (~$10GJ redline) (D) Comparison of the levelized costs ofdischarged electricity as a function of cyclesper year assuming constant power capacity20-year service life and full discharge over8 hours for daily cycling or 121 days for yearlycycling Dashed lines for hydrogen and lithium-ion reflect aspirational targets Further detailsare provided in the supplementary materials

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the flexibility scalability and low capital costsof electricity that can currently be provided bynatural gasndashfired generatorsmdashbut without emit-ting fossil CO2 This might be accomplished by amix of flexible generation energy storage anddemand management

Flexible generation

Even when spanning large geographical areasa system in which variable energy from windand solar are major sources of electricity willhave occasional but substantial and long-termmismatches between supply and demand Forexample such gaps in the United States arecommonly tens of petajoules (40 PJ = 108 TWh =24 hours of mean US electricity demand in 2015)and span multiple days or even weeks (61) Thuseven with continental-scale or global electricityinterconnections (61ndash63) highly reliable electricityin such a system will require either very sub-stantial amounts of dispatchable electricity sources(either generators or stored energy) that operateless than 20 of the time or correspondingamounts of demand management Similar chal-lenges apply if most electricity were producedby nuclear generators or coal-fired power plantsequipped with carbon capture and storage sug-gesting an important role for generators withhigher variable cost such as gas turbines thatuse synthetic hydrocarbons or hydrogen as fuel(Fig 1P) (64)Equipping dispatchable natural gas biomass

or syngas generators with CCS could allow con-tinued system reliability with drastically reducedCO2 emissions When fueled by syngas or bio-mass containing carbon captured from the at-mosphere such CCS offers an opportunity fornegative emissions However the capital costsof CCS-equipped generators are currently consi-derably higher than for generators without CCS(Fig 3B) Moreover CCS technologies designedfor generators that operate a large fraction ofthe time (with high ldquocapacity factorsrdquo) such ascoal-burning plants may be less efficient andeffective when generators operate at lower capa-city factors (65) Use of CCS-equipped gener-ators to flexibly produce back-up electricity andhydrogen for fuel synthesis could help alleviatetemporal mismatches between electricity gener-ation and demandNuclear fission plants can operate flexibly to

follow loads if adjustments are made to coolantflow rate and circulation control and fuel rodpositions andor dumping steam (66ndash68) In theUnited States the design and high capital costsof nuclear plants have historically obligated theirnear-continuous ldquobaseloadrdquo operation often atcapacity factors gt90 If capital costs could bereduced sufficiently nuclear power might alsobecome a cost-competitive source of load-followingpower but costs may have increased over time insome places (69ndash71) Similar to CCS-equippedgas generators the economic feasibility of next-generation advanced nuclear plants may dependon flexibly producing multiple energy productssuch as electricity high-temperature heat andorhydrogen

Energy storage

Reliable electricity could also be achieved throughenergy storage technologies The value of todayrsquosenergy storage is currently greatest when frequentcycling is required such as for minute-to-minutefrequency regulation or price arbitrage (72) Cost-effectively storing and discharging much largerquantities of energy over consecutive days and lessfrequent cycling may favor a different set ofinnovative technologies policies and valuation(72 73)

Chemical bonds

Chemical storage of energy in gas or liquid fuelsis a key option for achieving an integrated net-zero emissions energy system (Table 1) Storedelectrolytic hydrogen can be converted back toelectricity either in fuel cells or through com-bustion in gas turbines [power-to-gas-to-power(P2G2P)] (Figs 1 F and P and 3D red curve)commercial-scale P2G2P systems currently exhibita round-trip efficiency (energy out divided byenergy in) of gt30 (74) Regenerative fuel cellsin which the same assets are used to interconvertelectricity and hydrogen could boost capacityfactors but would benefit from improvementsin round-trip efficiency (now 40 to 50 in proton-exchange membrane designs) and chemical sub-stitutes for expensive precious metal catalysts(75 76)Hydrogen can also either be combined with

nonfossil CO2 via methanation to create renew-able methane or can be mixed in low concen-trations (lt10) with natural gas or biogas forcombustion in existing power plants Existingnatural gas pipelines turbines and end-use equip-ment could be retrofitted over time for use withpure hydrogen or richer hydrogen blends (77 78)although there may be difficult trade-offs of costand safety during such a transitionCurrent mass-market rechargeable batteries

serve high-value consumer markets that prizeround-trip efficiency energy density and highchargedischarge rates Although these batteriescan provide valuable short-duration ancillaryservices (such as frequency regulation and back-up power) their capital cost per energy capacityand power capacity makes them expensive forgrid-scale applications that store large quantitiesof energy and cycle infrequently For an examplegrid-scale use case with an electricity cost of$0035kWh (Fig 3D) the estimated cost ofdischarged electricity by using current lithium-ion batteries is roughly $014kWh ($39GJ) ifcycled daily but rises to $050kWh ($139GJ)for weekly cycling Assuming that targets forhalving the energy capacity costs of lithium-ionbatteries are reached (for example ~$130kWhof capacity) (73 79 80) the levelized cost of dis-charged electricity would fall to ~$029kWh($81GJ) for weekly cycling Cost estimates forcurrent vanadium redox flow batteries are evenhigher than for current lithium-ion batteries butlower cost flow chemistries are in development(81) Efficiency physical size chargedischargerates and operating costs could in principle besacrificed to reduce the energy capacity costs of

stationary batteries Not shown in Fig 3D less-efficient (for example 70 round-trip) batteriesbased on abundant materials such as sulfur mightreduce capital cost per unit energy capacity to$8kWh (with a power capacity cost of $150kW)leading to a levelized cost of discharged electri-city for the grid-scale use case in the range of$006 to 009kWh ($17 to 25 per GJ) assuming20 to 100 cycles per year over 20 years (81)Utilization rates might be increased if elec-

tric vehicle batteries were used to support theelectrical grid [vehicle-to-grid (V2G)] presumingthat the disruption to vehicle owners from dim-inished battery charge would be less costly thanan outage would be to electricity consumers (82)For example if all of the ~150 million light-dutyvehicles in the United States were electrified10 of each batteryrsquos 100 kWh charge wouldprovide 15 TWh which is commensurate with~3 hours of the countryrsquos average ~05 TW powerdemand It is also not yet clear how ownerswould be compensated for the long-term impactson their vehiclesrsquo battery cycle life whether pe-riods of high electricity demand would be co-incident with periods of high transportationdemand whether the ubiquitous charging infras-tructure entailed would be cost-effective whetherthe scale and timing of the consent control andpayment transactions would be manageable atgrid-relevant scales (~30 million transactionsper 15 min period) or how emerging techno-logies and social norms (such as shared auton-omous vehicles) might affect V2G feasibility

Potential and kinetic energy

Water pumped into superposed reservoirs forlater release through hydroelectric generatorsis a cost-effective and technologically matureoption for storing large quantities of energy withhigh round-trip efficiency (gt80) Although cap-ital costs of such pumped storage are substantialwhen cycled at least weekly levelized costs ofdischarged electricity are competitive (Fig 3D)Major barriers are the availability of water andsuitable reservoirs social and environmental op-position and constraints on the timing of waterreleases by nonenergy considerations such asflood protection recreation and the storage anddelivery of water for agriculture (83) Under-ground and undersea designs as well as weight-based systems that do not use water might expandthe number of possible sites avoid nonenergyconflicts and allay some social and environmentalconcerns (84ndash86)Electricity may also be stored by compressing

air in underground geologic formations under-water containers or above-ground pressure ves-sels Electricity is then recovered with turbineswhen air is subsequently released to the atmo-sphere Diabatic designs vent heat generatedduring compression and thus require an external(emissions-free) source of heat when the air isreleased reducing round-trip efficiency to lt50Adiabatic and isothermal designs achieve higherefficiencies (gt75) by storing both compressedair and heat and similarly efficient underwatersystems have been proposed (84)

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Thermal energy

Thermal storage systems are based on sensibleheat (such as in water tanks building envelopesmolten salt or solid materials such as bricks andgravel) latent heat (such as solid-solid or solid-liquid transformations of phase-change materials)or thermochemical reactions Sensible heat storagesystems are characterized by low energy densities[36 to 180 kJkg or 10 to 50 watt-hour thermal(Whth)kg] and high costs (84 87 88) Futurecost targets are lt$15kWhth (89) Thermal stor-age is well suited to within-day shifting of heat-ing and cooling loads whereas low efficiencyheat losses and physical size are key barriers tofilling week-long large-scale (for example 30 ofdaily demand) shortfalls in electricity generation

Demand management

Technologies that allow electricity demand to beshifted in time (load-shifting or load-shaping) orcurtailed to better correlate with supply wouldimprove overall system reliability while reducingthe need for underused flexible back-up generators(90 91) Smart charging of electric vehicles shiftedheating and cooling cycles and scheduling ofappliances could cost-effectively reduce peakloads in the United States by ~6 and thusavoid 77 GW of otherwise needed generatingcapacity (~7 of US generating capacity in2017) (92) Managing larger quantities of energydemand for longer times (for example tens ofpetajoules over weeks) would involve idling largeindustrial uses of electricitymdashthus underutilizingother valuable capitalmdashor effectively curtailingservice Exploring and developing new technol-ogies that can manage weekly or seasonal gapsin electricity supply is an important area forfurther research (93)

Outlook

Nonemitting electricity sources energy-storagetechnologies and demand management optionsthat are now available and capable of accom-modating large multiday mismatches in elec-tricity supply and demand are characterized byhigh capital costs compared with the currentcosts of some variable electricity sources or na-tural gasndashfired generators Achieving affordablereliable and net-zero emissions electricity sys-tems may thus depend on substantially reducingsuch capital costs via continued innovation anddeployment emphasizing systems that can beoperated to provide multiple energy services

Carbon management

Recycling and removal of carbon from the atmo-sphere (carbon management) is likely to be an im-portant activity of any net-zero emissions energysystem For example synthesized hydrocarbonsthat contain carbon captured from the atmospherewill not increase atmospheric CO2 when oxidizedIntegrated assessment models also increasinglyrequire negative emissions to limit the increasein global mean temperatures to 2degC (94ndash97)mdashfor example via afforestationreforestation en-hanced mineral weathering bioenergy with CCSor direct capture of CO2 from the air (20)

Capture and storage will be distinct carbonmanagement services in a net-zero emissionsenergy system (for example Fig 1 E and J)Carbon captured from the ambient air could beused to synthesize carbon-neutral hydrocarbonfuels or sequestered to produce negative emis-sions Carbon captured from combustion of bio-mass or synthesized hydrocarbons could berecycled to produce more fuels (98) Storage ofcaptured CO2 (for example underground) willbe required to the extent that uses of fossil car-bon persist andor that negative emissions areneeded (20)For industrial CO2 capture research and de-

velopment are needed to reduce the capital costsand costs related to energy for gas separationand compression (99) Future constraints onland water and food resources may limit bio-logically mediated capture (20) The main chal-lenges to direct air capture include costs tomanufacture sorbents and structures energizethe process and handle and transport the cap-tured CO2 (100 101) Despite multiple demon-strations at scale [~15 Mt CO2year are nowbeing injected underground (99)] financingcarbon storage projects with high perceivedrisks and long-term liability for discharge re-mains a major challenge (102)

Discussion

We have estimated that difficult-to-eliminateemissions related to aviation long-distance trans-portation and shipping structural materials andhighly reliable electricity represented more thana quarter of global fossil fuel and industry CO2

emissions in 2014 (Fig 2) But economic andhuman development goals trends in interna-tional trade and travel the rapidly growing shareof variable energy sources (103) and the large-scale electrification of other sectors all suggestthat demand for the energy services and pro-cesses associated with difficult-to-eliminate emis-sions will increase substantially in the future Forexample in some of the Shared SocioeconomicPathways that were recently developed by theclimate change research community in order toframe analysis of future climate impacts globalfinal energy demand more than doubles by 2100(104) hence the magnitude of these difficult-to-eliminate emissions could in the future be com-parable with the level of total current emissionsCombinations of known technologies could

eliminate emissions related to all essential en-ergy services and processes (Fig 1) but sub-stantial increases in costs are an immediatebarrier to avoiding emissions in each categoryIn some cases innovation and deployment canbe expected to reduce costs and create new op-tions (32 73 105 106) More rapid changes maydepend on coordinating operations across energyand industry sectors which could help boostutilization rates of capital-intensive assets Inpractice this would entail systematizing andexplicitly valuing many of the interconnectionsdepicted in Fig 1 which would also mean over-coming institutional and organizational chal-lenges in order to create newmarkets and ensure

cooperation among regulators and disparate risk-averse businesses We thus suggest two parallelbroad streams of RampD effort (i) research intechnologies and processes that can provide thesedifficult-to-decarbonize energy services and (ii)research in systems integration that would allowfor the provision of these services and productsin a reliable and cost-effective wayWe have focused on provision of energy ser-

vices without adding CO2 to the atmosphereHowever many of the challenges discussed herecould be reduced by moderating demand suchas through substantial improvements in energyand materials efficiency Particularly crucial arethe rate and intensity of economic growth indeveloping countries and the degree to whichsuch growth can avoid fossil-fuel energy whileprioritizing human development environmentalprotection sustainability and social equity(4 107 108) Furthermore many energy servicesrely on long-lived infrastructure and systems sothat current investment decisions may lock inpatterns of energy supply and demand (andthereby the cost of emissions reductions) forhalf a century to come (109) The collective andreinforcing inertia of existing technologies pol-icies institutions and behavioral norms mayactively inhibit innovation of emissions-free tech-nologies (110) Emissions of CO2 and other ra-diatively active gases and aerosols (111) from landuse and land-use change could also cause sub-stantial warming (112)

Conclusion

We have enumerated here energy services thatmust be served by any future net-zero emissionsenergy system and have explored the technolo-gical and economic constraints of each A success-ful transition to a future net-zero emissions energysystem is likely to depend on the availability ofvast amounts of inexpensive emissions-free elec-tricity mechanisms to quickly and cheaply bal-ance large and uncertain time-varying differencesbetween demand and electricity generation elec-trified substitutes for most fuel-using devicesalternative materials and manufacturing pro-cesses including CCS for structural materials andcarbon-neutral fuels for the parts of the economythat are not easily electrified The specific tech-nologies that will be favored in future market-places are largely uncertain but only a finitenumber of technology choices exist today foreach functional role To take appropriate actionsin the near-term it is imperative to clearly iden-tify desired endpoints If we want to achieve arobust reliable affordable net-zero emissionsenergy system later this century we must beresearching developing demonstrating and de-ploying those candidate technologies now

REFERENCES AND NOTES

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2 H D Matthews K Caldeira Stabilizing climate requiresnear-zero emissions Geophys Res Lett 35 L04705 (2008)doi 1010292007GL032388

Davis et al Science 360 eaas9793 (2018) 29 June 2018 7 of 9

RESEARCH | REVIEW

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nloaded from

3 J Rogelj et al Zero emission targets as long-term globalgoals for climate protection Environ Res Lett 10 105007(2015) doi 1010881748-93261010105007

4 J C Steckel R J Brecha M Jakob J StreflerG Luderer Development without energy Assessing futurescenarios of energy consumption in developing countriesEcol Econ 90 53ndash67 (2013) doi 101016jecolecon201302006

5 S Collins et al Integrating short term variations of the powersystem into integrated energy system models Amethodological review Renew Sustain Energy Rev 76839ndash856 (2017) doi 101016jrser201703090

6 S Yeh et al Detailed assessment of global transport-energymodelsrsquo structures and projections Transp Res Part DTransp Environ 55 294ndash309 (2017) doi 101016jtrd201611001

7 S C Davis S W Diegel R G Boundy Transportation EnergyData Book (Center for Transportation Analysis ed 34 2015)

8 International Energy Agency (IEA) ldquoCO2 emissions from fuelcombustionrdquo (IEA 2016)

9 IEA Energy Technology Perspectives 2017 (IEA 2017)10 L M Fulton L R Lynd A Koumlrner N Greene L R Tonachel

The need for biofuels as part of a low carbon energy futureBiofuels Bioprod Biorefin 9 476ndash483 (2015) doi 101002bbb1559

11 J Impullitti ldquoZero emission cargo transport II San Pedro Bayports hybrid amp fuel cell electric vehicle projectrdquo wwwenergygovsitesprodfiles201606f33vs158_impullitti_2016_o_webpdf

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22 L R Lynd et al Cellulosic ethanol Status and innovationCurr Opin Biotechnol 45 202ndash211 (2017) doi 101016jcopbio201703008 pmid 28528086

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33 DOE ldquoTechnical targets for hydrogen production fromelectrolysisrdquo (2018) wwwenergygoveerefuelcellsdoe-technical-targets-hydrogen-production-electrolysis

34 S M Saba M Muller M Robinius D Stolten The investmentcosts of electrolysismdashA comparison of cost studies from thepast 30 years Int J Hydrogen Energy 43 1209ndash1223 (2018)doi 101016jijhydene201711115

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41 J Tollefson The wooden skyscrapers that could help to coolthe planet Nature 545 280ndash282 (2017) doi 101038545280a pmid 28516941

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45 NETL ldquoCost of capturing CO2 from Industrial Sourcesrdquo(NETL 2014)

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49 M-G Piketty M Wichert A Fallot L Aimola Assessing landavailability to produce biomass for energy The case ofBrazilian charcoal for steel making Biomass Bioenergy 33180ndash190 (2009) doi 101016jbiombioe200806002

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51 T Kuramochi A Ramiacuterez W Turkenburg A FaaijComparative assessment of CO2 capture technologies forcarbon-intensive industrial processes Pror Energy CombustSci 38 87ndash112 (2012) doi 101016jpecs201105001

52 M C Romano et al Application of advanced technologies forCO2 capture from industrial sources Energy Procedia 377176ndash7185 (2013) doi 101016jegypro201306655

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68 J D Jenkins et al The benefits of nuclear flexibility in powersystem operations with renewable energy Appl Energy 222872ndash884 (2018) doi 101016japenergy201803002

69 J R Lovering A Yip T Nordhaus Historical constructioncosts of global nuclear power reactors Energy Policy 91371ndash382 (2016) doi 101016jenpol201601011

70 A Grubler The costs of the French nuclear scale-up A caseof negative learning by doing Energy Policy 38 5174ndash5188(2010) doi 101016jenpol201005003

71 J Koomey N E Hultman A reactor-level analysis of busbarcosts for US nuclear plants 1970ndash2005 Energy Policy 355630ndash5642 (2007) doi 101016jenpol200706005

72 W A Braff J M Mueller J E Trancik Value of storagetechnologies for wind and solar energy Nat Clim Chang 6964ndash969 (2016) doi 101038nclimate3045

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77 M Melaina O Antonia M Penev ldquoBlending hydrogen into naturalgas pipeline networks A review of key issuesrdquo (NREL 2013)

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83 J I Peacuterez-Diacuteaz M Chazarra J Garciacutea-Gonzaacutelez G CavazziniA Stoppato Trends and challenges in the operation ofpumped-storage hydropower plants Renew Sustain EnergyRev 44 767ndash784 (2015) doi 101016jrser201501029

84 A B Gallo J R Simotildees-Moreira H K M CostaM M Santos E Moutinho dos Santos Energy storage in theenergy transition context A technology review RenewSustain Energy Rev 65 800ndash822 (2016) doi 101016jrser201607028

85 T Letcher Storing Energy with Special Reference toRenewable Energy Sources (Elsevier 2016)

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energy storage systems Open Renew Ener J 4 42ndash46(2010) doi 1021741876387101004010042

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90 E Hale et al ldquoDemand response resource quantification withdetailed building energy modelsrdquo (NREL 2016)

91 P Alstone et al ldquoCalifornia demand response potentialstudyrdquo (CPUCLBNL 2016)

92 P Bronski et al ldquoThe economics of demand flexibility Howldquoflexiwattsrdquo create quantifiable value for customers and thegridrdquo (Rocky Mountain Institute 2015)

93 B Pierpont D Nelson A Goggins D Posner ldquoFlexibility The path tolow-carbon low-cost electricity gridsrdquo (Climate Policy Initiative2017)

94 L Clarke et al in Mitigation of Climate Change Contributionof Working Group III to the IPCC 5th Fifth Assessment Reportof the Intergovernmental Panel on Climate Change(Cambridge Univ Press 2014)

95 D P van Vuuren et al The role of negative CO2 emissionsfor reaching 2degCmdashInsights from integrated assessmentmodelling Clim Change 118 15ndash27 (2013) doi 101007s10584-012-0680-5

96 E Kriegler et al The role of technology for achieving climatepolicy objectives Overview of the EMF 27 study on globaltechnology and climate policy strategies Clim Change 123353ndash367 (2014) doi 101007s10584-013-0953-7

97 C Azar et al The feasibility of low CO2 concentration targetsand the role of bio-energy with carbon capture and storage(BECCS) Clim Change 100 195ndash202 (2010) doi 101007s10584-010-9832-7

98 J M D MacElroy Closing the carbon cycle through rationaluse of carbon-based fuels Ambio 45 (Suppl 1) S5ndashS14(2016) doi 101007s13280-015-0728-7 pmid 26667055

99 H de Coninck S M Benson Carbon dioxide capture andstorage Issues and prospects Annu Rev Environ Resour 39243ndash270 (2014) doi 101146annurev-environ-032112-095222

100 R Socolow et al ldquoDirect air capture of CO2 with chemicals Atechnology assessment for the APS Panel on Public Affairsrdquo(American Physical Society 2011)

101 K S Lackner et al The urgency of the development of CO2

capture from ambient air Proc Natl Acad Sci USA 10913156ndash13162 (2012) doi 101073pnas1108765109pmid 22843674

102 Z Kapetaki J Scowcroft Overview of carbon capture andstorage (CCS) demonstration project business models Risksand enablers on the two sides of the Atlantic EnergyProcedia 114 6623ndash6630 (2017) doi 101016jegypro2017031816

103 IEA Renewables 2017 Analysis and Forecasts to 2022 (IEA2017)

104 N Bauer et al Shared socio-economic pathways of theenergy sector-quantifying the narratives Glob EnvironChange 42 316ndash330 (2017) doi 101016jgloenvcha201607006

105 J D Farmer F Lafond How predictable is technologicalprogress Res Policy 45 647ndash665 (2016) doi 101016jrespol201511001

106 L M A Bettencourt J E Trancik J Kaur Determinants ofthe pace of global innovation in energy technologiesPLOS ONE 8 e67864 (2013) doi 101371journalpone0067864 pmid 24155867

107 K Riahi et al The Shared Socioeconomic Pathways and theirenergy land use and greenhouse gas emissions implicationsAn overview Glob Environ Change 42 153ndash168 (2017)doi 101016jgloenvcha201605009

108 E Holden K Linnerud D Banister The imperatives ofsustainable development Sustain Dev 101002sd1647(2016)

109 S J Davis K Caldeira H D Matthews Future CO2 emissionsand climate change from existing energy infrastructureScience 329 1330ndash1333 (2010) doi 101126science1188566 pmid 20829483

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111 D E H J Gernaat et al Understanding the contribution ofnon-carbon dioxide gases in deep mitigation scenariosGlob Environ Change 33 142ndash153 (2015) doi 101016jgloenvcha201504010

112 D P van Vuuren et al Energy land-use and greenhouse gasemissions trajectories under a green growth paradigmGlob Environ Change 42 237ndash250 (2017) doi 101016jgloenvcha201605008

113 EIA ldquoLevelized Cost and Levelized Avoided Cost of NewGeneration Resources in the Annual Energy Outlook 2018rdquo(2018) wwweiagovoutlooksaeopdfelectricity_generationpdf

ACKNOWLEDGMENTS

The authors extend a special acknowledgment to MIH forinspiration on the 20th anniversary of publication of (1) Theauthors also thank M Dyson L Fulton L Lynd G Janssens-MaenhoutM McKinnon J Mueller G Pereira M Ziegler andM Wang for helpful input This Review stems from anAspen Global Change Institute meeting in July 2016 convenedwith support from NASA the Heising-Simons Foundationand the Fund for Innovative Climate and Energy Research SJDand JB also acknowledge support of the US National ScienceFoundation (INFEWS grant EAR 1639318) DA BH and B-MHacknowledge Alliance for Sustainable Energy the manager andoperator of the National Renewable Energy Laboratory for the USDepartment of Energy (DOE) under contract DE-AC36-08GO28308Funding was in part provided by the DOE Office of Energy Efficiencyand Renewable Energy The views expressed in the article do notnecessarily represent the views of the DOE or the US governmentThe US government retains and the publisher by accepting thearticle for publication acknowledges that the US government retainsa nonexclusive paid-up irrevocable worldwide license to publish orreproduce the published form of this work or allow others to do sofor US government purposes

SUPPLEMENTARY MATERIALS

wwwsciencemagorgcontent3606396eaas9793supplDC1Materials and MethodsReferences (114ndash161)

11 January 2018 accepted 25 May 2018101126scienceaas9793

Davis et al Science 360 eaas9793 (2018) 29 June 2018 9 of 9

RESEARCH | REVIEW

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Net-zero emissions energy systems

Jessika E Trancik Chi-Jen Yang and Ken CaldeiraKatharine J Mach Michael Mastrandrea Joan Ogden Per F Peterson Daniel L Sanchez Daniel Sperling Joseph Stagner

LacknerChristopher B Field Bryan Hannegan Bri-Mathias Hodge Martin I Hoffert Eric Ingersoll Paulina Jaramillo Klaus S FennellBradley Jack Brouwer Yet-Ming Chiang Christopher T M Clack Armond Cohen Stephen Doig Jae Edmonds Paul

Steven J Davis Nathan S Lewis Matthew Shaner Sonia Aggarwal Doug Arent Inecircs L Azevedo Sally M Benson Thomas

DOI 101126scienceaas9793 (6396) eaas9793360Science

this issue p eaas9793Scienceto achieve minimal emissionstechnologies and pathways show promise but integration of now-discrete energy sectors and industrial processes is vitalaviation long-distance transport steel and cement production and provision of a reliable electricity supply Current

includingdecarbonization of the energy system Some parts of the energy system are particularly difficult to decarbonize review what it would take to achieveet allater this century Most of these emissions arise from energy use Davis

Models show that to avert dangerous levels of climate change global carbon dioxide emissions must fall to zeroPath to zero carbon emissions

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MATERIALSSUPPLEMENTARY httpsciencesciencemagorgcontentsuppl201806273606396eaas9793DC1

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REFERENCES

httpsciencesciencemagorgcontent3606396eaas9793BIBLThis article cites 101 articles 4 of which you can access for free

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is a registered trademark of AAASSciencelicensee American Association for the Advancement of Science No claim to original US Government Works The title Science 1200 New York Avenue NW Washington DC 20005 2017 copy The Authors some rights reserved exclusive

(print ISSN 0036-8075 online ISSN 1095-9203) is published by the American Association for the Advancement ofScience

on June 10 2019

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