TOWARD A HYDROGEN ECONOMY
REVIEW
Stabilization Wedges: Solving the Climate Problem
for the Next 50 Years with Current Technologies
S. Pacala1* and R. Socolow2*
Humanity already possesses the fundamental scientific, technical, and industrial andclimateproblemoverthenexthalf-century”
know-how to solve the carbon and climate problem for the next half-century. A meanstodeploythetechnologiesand/orlifestyle
portfoliooftechnologiesnowexiststomeettheworld’senergyneedsoverthenext changesnecessarytofillallsevenwedgesofthe
50 years and limit atmospheric CO to a trajectory that avoids a doubling of the stabilizationtriangle.
2
preindustrial concentration. Every element in this portfolio has passed beyond the Stabilizationatanylevelrequiresthatnet
laboratorybenchanddemonstrationproject;manyarealreadyimplementedsome- emissionsdonotsimplyremainconstant,but
whereatfullindustrialscale.Althoughnoelementisacrediblecandidatefordoing eventuallydroptozero.Forexample,inone
theentirejob(orevenhalfthejob)byitself,theportfolioasawholeislargeenough simple model (9) that begins with the stabi-
thatnoteveryelementhastobeused. lizationtrianglebutlooksbeyond2054,500-
ppm stabilization is achieved by 50 years of
Thedebateinthecurrentliteratureaboutstabi- (BAU)trajectory],thequantitativedetailsofthe flatemissions,followedbyalineardeclineof
lizingatmosphericCO atlessthanadoubling stabilization target, and the future behavior of about two-thirds in the following 50 years,
2
of the preindustrial concentration has led to natural sinks for atmospheric CO (i.e., the andaveryslowdeclinethereafterthatmatch-
2
needless confusion about current options for oceansandterrestrialbiosphere).Wefocusex- es the declining ocean sink. To develop the
mitigation.Ononeside,theIntergovernmental clusively on CO , because it is the dominant revolutionary technologies required for such
2
PanelonClimateChange(IPCC)hasclaimed anthropogenicgreenhousegas;industrial-scale largeemissionsreductionsinthesecondhalf
that“technologiesthatexistinoperationorpilot mitigation options also exist for subordinate ofthecentury,enhancedresearchanddevel-
stagetoday”aresufficienttofollowaless-than- gases,suchasmethaneandN O. opmentwouldhavetobeginimmediately.
2
doubling trajectory “over the next hundred Very roughly, stabilization at 500 ppm Policies designed to stabilize at 500 ppm
yearsormore”[(1),p.8].Ontheotherside,a requires that emissions be held near the wouldinevitablyberenegotiatedperiodically
recentreviewinScienceassertsthattheIPCC present level of 7 billion tons of carbon per to take into account the results of research
claimdemonstrates“misperceptionsoftechno- year (GtC/year) for the next 50 years, even and development, experience with specific
logical readiness” and calls for “revolutionary thoughtheyarecurrentlyoncoursetomore wedges, and revised estimates of the size of
changes”inmitigationtechnology,suchasfu- thandouble(Fig.1A).Thenext50yearsis the stabilization triangle. But not filling the
sion,space-basedsolarelectricity,andartificial a sensible horizon from several perspec- stabilizationtrianglewillput500-ppmstabi-
photosynthesis(2).Weagreethatfundamental tives. It is the length of a career, the life- lization out of reach. In that same simple
research is vital to develop the revolutionary time of a power plant, and an interval for model (9), 50 years of BAU emissions fol-
mitigationstrategiesneededinthesecondhalf which the technology is close enough to lowed by 50 years of a flat trajectory at 14
ofthiscenturyandbeyond.Butitisimportant envision. The calculations behind Fig. 1A GtC/yearleadstomorethanatriplingofthe
not to become beguiled by the possibility of areexplainedinSection1ofthesupporting preindustrialconcentration.
revolutionarytechnology.Humanitycansolve online material (SOM) text. The BAU and It is important to understand that each of
thecarbonandclimateprobleminthefirsthalf stabilization emissions in Fig. 1A are near thesevenwedgesrepresentsaneffortbeyond
of this century simply by scaling up what we the center of the cloud of variation in the what would occur under BAU. Our BAU
alreadyknowhowtodo. large published literature (8). simply continues the 1.5% annual carbon
emissions growth of the past 30 years. This
What Do We Mean by “Solving the The Stabilization Triangle historic trend in emissions has been accom-
Carbon and Climate Problem for the Weidealizethe50-yearemissionsreductions paniedby2%growthinprimaryenergycon-
Next Half-Century”? asaperfecttriangleinFig.1B.Stabilization sumption and 3% growth in gross world
Proposals to limit atmospheric CO to a con- is represented by a “flat” trajectory of fossil product (GWP) (Section 1 of SOM text). If
2
centration that would prevent most damaging fuel emissions at 7 GtC/year, and BAU is carbonemissionsweretogrow2%peryear,
climate change have focused on a goal of represented by a straight-line “ramp” trajec- then(cid:2)10wedgeswouldbeneededinsteadof
500(cid:1)50partspermillion(ppm),orlessthan toryrisingto14GtC/yearin2054.The“sta- 7, and if carbon emissions were to grow at
double the preindustrial concentration of 280 bilization triangle,” located between the flat 3% per year, then (cid:2)18 wedges would be
ppm(3–7).Thecurrentconcentrationis(cid:2)375 trajectory and BAU, removes exactly one- required (Section 1 of SOM text). Thus, a
ppm.TheCO emissionsreductionsnecessary thirdofBAUemissions. continuation of the historical rate of decar-
2
toachieveanysuchtargetdependontheemis- Tokeepthefocusontechnologiesthathave bonization of the fuel mix prevents the need
sionsjudgedlikelytooccurintheabsenceofa thepotentialtoproduceamaterialdifferenceby forthreeadditionalwedges,andongoingim-
focus on carbon [called a business-as-usual 2054, we divide the stabilization triangle into provements in energy efficiency prevent the
seven equal “wedges.” A wedge represents an needforeightadditionalwedges.Mostread-
activitythatreducesemissionstotheatmosphere erswillrejectatleastoneofthewedgeslisted
1Department of Ecology and Evolutionary Biology, that starts at zero today and increases linearly here, believing that the corresponding de-
2DepartmentofMechanicalandAerospaceEngineer-
untilitaccountsfor1GtC/yearofreducedcar- ployment is certain to occur in BAU, but
ing,PrincetonUniversity,Princeton,NJ08544,USA.
bon emissions in 50 years. It thus represents a readerswilldisagreeaboutwhichtorejecton
*To whom correspondence should be addressed. E-
cumulativetotalof25GtCofreducedemissions such grounds. On the other hand, our list of
mail:[email protected](S.P.);socolow@princeton.
edu(R.S.) over50years.Inthispaper,to“solvethecarbon mitigationoptionsisnotexhaustive.
968 13AUGUST2004 VOL305 SCIENCE www.sciencemag.org
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TOWARD A HYDROGEN ECONOMY
What Current Options Could Be pergallon(mpg)onconventionalfuel,carsin bonatarateof1GtC/year,awedgewouldbe
Scaled Up to Produce at Least One
2054 averaged 60 mpg, with fuel type and achievedbydisplacing1400GWofbaseloadcoal
Wedge?
distancetraveledunchanged. withbaseloadgasby2054.Thepowershiftedto
Wedges can be achieved from energy effi-
Option 2: Reduced reliance on cars. A gasforthiswedgeisfourtimesaslargeasthetotal
ciency, from the decarbonization of the sup-
wedgewouldalsobeachievediftheaverage currentgas-basedpower.
plyofelectricityandfuels(bymeansoffuel
fueleconomyofthe2billion2054carswere Option 6: Storage of carbon captured in
shifting, carbon capture and storage, nuclear
30mpg,buttheannualdistancetraveledwere power plants. Carbon capture and storage
energy,andrenewableenergy),andfrombi-
5000milesinsteadof10,000miles. (CCS)technologypreventsabout90%ofthe
ological storage in forests and agricultural
Option3:Moreefficientbuildings.According fossil carbon from reaching the atmosphere,
soils. Below, we discuss 15 different exam-
to a 1996 study by the IPCC, a wedge is the soawedgewouldbeprovidedbytheinstal-
plesofoptionsthatarealreadydeployedatan
difference between pursuing and not pursuing lation of CCS at 800 GW of baseload coal
industrial scale and that could be scaled up
“known and established approaches” to energy- plants by 2054 or 1600 GW of baseload
further to produce at least one wedge (sum-
efficientspaceheatingandcooling,waterheating, natural gas plants. The most likely approach
marized in Table 1). Although several op-
lighting,andrefriger-
tions could be scaled up to two or more
ation in residential
wedges, we doubt that any could fill the
and commercial
stabilizationtriangle,orevenhalfofit,alone.
buildings. These ap-
BecausethesameBAUcarbonemissions
proachesreducemid-
cannot be displaced twice, achieving one
century emissions
wedgeofteninteractswithachievinganother.
from buildings by
Themoretheelectricitysystembecomesdecar-
about one-fourth.
bonized,forexample,thelesstheavailablesav-
About half of poten-
ingsfromgreaterefficiencyofelectricityuse,and
tialsavingsareinthe
vice versa. Interactions among wedges are dis-
buildingsindevelop-
cussedintheSOMtext.Also,ourfocusisnoton
ingcountries(1).
costs.Ingeneral,theachievementofawedgewill
Option 4: Im-
requiresomepricetrajectoryforcarbon,thede-
proved power plant
tailsofwhichdependonmanyassumptions,in-
efficiency. In 2000,
cludingfuturefuelsprices,publicacceptance,and
coal power plants,
costreductionsbymeansoflearning.Instead,our
operatingonaverage
analysisisintendedtocomplementthecompre-
at 32% efficiency,
hensivebutcomplex“integratedassessments”(1)
produced about one-
ofcarbonmitigationbylettingthefull-scaleex-
fourth of all carbon
amplesthatarealreadyinthemarketplacemakea
emissions: 1.7 GtC/
simplecasefortechnologicalreadiness.
year out of 6.2 GtC/
CategoryI:EfficiencyandConservation
year.Awedgewould
Improvementsinefficiencyandconservation
becreatediftwiceto-
probably offer the greatest potential to pro-
day’s quantity of
videwedges.Forexample,in2002,theUnit-
coal-based electricity
edStatesannouncedthegoalofdecreasingits
in 2054 were pro-
carbon intensity (carbon emissions per unit
ducedat60%instead
GDP) by 18% over the next decade, a de-
of40%efficiency.
crease of 1.96% per year. An entire wedge
Category II: Decar-
would be creatediftheUnitedStateswereto
bonization of Elec-
resetitscarbonintensitygoaltoadecreaseof
tricityandFuels
2.11%peryearandextenditto50years,andif
(Seereferencesand
everycountryweretofollowsuitbyaddingthe
details in Section 3
same 0.15% per year increment to its own
oftheSOMtext.)
carbonintensitygoal.However,efficiencyand
Option 5: Substi-
conservation options are less tangible than Fig.1.(A)ThetopcurveisarepresentativeBAUemissionspathforglobal
tutingnaturalgasfor
thosefromtheothercategories.Improvements carbonemissionsasCO fromfossilfuelcombustionandcementmanufac-
coal. Carbon emis- 2
in energy efficiency will come from literally ture:1.5%peryeargrowthstartingfrom7.0GtC/yearin2004.Thebottom
sionsperunitofelec- curveisaCO emissionspathconsistentwithatmosphericCO stabilization
hundredsofinnovationsthatrangefromnew 2 2
tricity are about half at500ppmby2125akintotheWigley,Richels,andEdmonds(WRE)family
catalysts and chemical processes, to more
aslargefromnatural ofstabilizationcurvesdescribedin(11),modifiedasdescribedinSection1of
efficientlightingandinsulationforbuildings, gas power plants as theSOMtext.Thebottomcurveassumesanoceanuptakecalculatedwiththe
to the growth of the service economy and High-LatitudeExchangeInteriorDiffusionAdvection(HILDA)oceanmodel
fromcoalplants.As-
telecommuting. Here, we provide four of (12)andaconstantnetlanduptakeof0.5GtC/year(Section1oftheSOM
sumethatthecapaci-
many possible comparisons of greater and text). The area between the two curves represents the avoided carbon
tyfactoroftheaver- emissionsrequiredforstabilization.(B)Idealizationof(A):Astabilization
less efficiency in 2054. (See references and
age baseload coal triangle of avoided emissions (green) and allowed emissions (blue). The
detailsinSection2oftheSOMtext.)
plantin2054hasin- allowedemissionsarefixedat7GtC/yearbeginningin2004.Thestabili-
Option 1: Improved fuel economy. Sup- creased to 90% and zation triangle is divided into seven wedges, each of which reaches 1
posethatin2054,2billioncars(roughlyfour GtC/year in 2054. With linear growth, the total avoided emissions per
thatitsefficiencyhas
timesasmanyastoday)average10,000miles wedgeis25GtC,andthetotalareaofthestabilizationtriangleis175GtC.
improved to 50%.
peryear(astheydotoday).Onewedgewould The arrow at the bottom right of the stabilization triangle points down-
Because 700 GW of ward to emphasize that fossil fuel emissions must decline substantially
beachievedif,insteadofaveraging30miles
suchplantsemitcar- below7GtC/yearafter2054toachievestabilizationat500ppm.
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has two steps: (i) precombustion capture of currentenhancedoilrecovery,orcurrentseason- Sleipner project in the North Sea strips CO
2
CO , in which hydrogen and CO are pro- alstorageofnaturalgas,orthefirstgeological from natural gas offshore and reinjects 0.3
2 2
duced and the hydrogen is then burned to storagedemonstrationproject.Today,about0.01 milliontonsofcarbonayear(MtC/year)into
produceelectricity,followedby(ii)geologic GtC/yearofcarbonasCO isinjectedintogeo- a non–fossil-fuel–bearing formation, so a wedge
2
storage, in which the waste CO is injected logicreservoirstospurenhancedoilrecovery,so wouldbe3500Sleipner-sizedprojects(orfew-
2
into subsurface geologic reservoirs. Hydro- a wedge of geologic storage requires that CO er,largerprojects)overthenext50years.
2
gen production from fossil fuels is already a injectionbescaledupbyafactorof100overthe Aworldwideeffortisunderwaytoassess
very large business. Globally, hydrogen next50years.Tosmoothoutseasonaldemand the capacity available for multicentury stor-
plants consume about 2% of primary energy in the United States, the natural gas industry ageandtoassessrisksofleakslargeenough
and emit 0.1 GtC/year of CO . The capture annually draws roughly 4000 billion standard toendangerhumanorenvironmentalhealth.
2
partofawedgeofCCSelectricitywouldthus cubic feet (Bscf) into and out of geologic Option 7: Storage of carbon captured in
require only a tenfold expansion of plants storage, and a carbon flow of 1 GtC/year hydrogenplants.Thehydrogenresultingfrom
resembling today’s large hydrogen plants (whether as methane or CO ) is a flow of precombustioncaptureofCO canbesentoff-
2 2
overthenext50years. 69,000 Bscf/year (190 Bscf per day), so a sitetodisplacetheconsumptionofconvention-
The scale of the storage part of this wedge wedge would be a flow to storage 15 and 20 al fuels rather than being consumed onsite to
can be expressed as a multiple of the scale of times as large as the current flow. Norway’s produceelectricity.Thecapturepartofawedge
Table1.Potentialwedges:Strategiesavailabletoreducethecarbonemissionratein2054by1GtC/yearortoreducecarbonemissionsfrom
2004to2054by25GtC.
Effortby2054foronewedge,relativeto14
Option Comments,issues
GtC/yearBAU
Energyefficiencyandconservation
Economy-widecarbon-intensity Increasereductionbyadditional0.15%peryear Canbetunedbycarbonpolicy
reduction(emissions/$GDP) (e.g.,increaseU.S.goalof1.96%reductionper
yearto2.11%peryear)
1. Efficientvehicles Increasefueleconomyfor2billioncarsfrom30to Carsize,power
60mpg
2. Reduceduseofvehicles Decreasecartravelfor2billion30-mpgcarsfrom Urbandesign,masstransit,telecommuting
10,000to5000milesperyear
3. Efficientbuildings Cutcarbonemissionsbyone-fourthinbuildings Weakincentives
andappliancesprojectedfor2054
4. Efficientbaseloadcoalplants Producetwicetoday’scoalpoweroutputat60% Advancedhigh-temperaturematerials
insteadof40%efficiency(comparedwith32%
today)
Fuelshift
5. Gasbaseloadpowerforcoal Replace1400GW50%-efficientcoalplantswith Competingdemandsfornaturalgas
baseloadpower gasplants(fourtimesthecurrentproductionof
gas-basedpower)
CO CaptureandStorage(CCS) 2
6. CaptureCO atbaseloadpower IntroduceCCSat800GWcoalor1600GWnatural TechnologyalreadyinuseforH production
2 2
plant gas(comparedwith1060GWcoalin1999)
7. CaptureCO atH plant IntroduceCCSatplantsproducing250MtH /year H safety,infrastructure 2 2 2 2
fromcoalor500MtH /yearfromnaturalgas 2
(comparedwith40MtH /yeartodayfromall
2
sources)
8. CaptureCO atcoal-to-synfuels IntroduceCCSatsynfuelsplantsproducing30 IncreasedCO emissions,ifsynfuelsare
2 2 plant millionbarrelsadayfromcoal(200timesSasol), producedwithoutCCS
ifhalfoffeedstockcarbonisavailablefor
capture
Geologicalstorage Create3500Sleipners Durablestorage,successfulpermitting
Nuclearfission
9. Nuclearpowerforcoalpower Add700GW(twicethecurrentcapacity) Nuclearproliferation,terrorism,waste
Renewableelectricityandfuels
10. Windpowerforcoalpower Add2million1-MW-peakwindmills(50timesthe Multipleusesoflandbecausewindmillsare
currentcapacity)“occupying”30(cid:3)106ha,on widelyspaced
landoroffshore
11. PVpowerforcoalpower Add2000GW-peakPV(700timesthecurrent PVproductioncost
capacity)on2(cid:3)106ha
12. WindH infuel-cellcarfor Add4million1-MW-peakwindmills(100timesthe H safety,infrastructure
2 2
gasolineinhybridcar currentcapacity)
13.Biomassfuelforfossilfuel Add100timesthecurrentBrazilorU.S.ethanol Biodiversity,competinglanduse
production,withtheuseof250(cid:3)106ha
(one-sixthofworldcropland)
Forestsandagriculturalsoils
14. Reduceddeforestation,plus Decreasetropicaldeforestationtozeroinsteadof Landdemandsofagriculture,benefitsto
reforestation,afforestation,and 0.5GtC/year,andestablish300Mhaofnewtree biodiversityfromreduceddeforestation
newplantations. plantations(twicethecurrentrate)
15. Conservationtillage Applytoallcropland(10timesthecurrentusage) Reversibility,verification
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TOWARD A HYDROGEN ECONOMY
wouldrequiretheinstallationofCCS,by2054, from photovoltaic (PV) electricity would re- be created by reforesting or afforesting ap-
at coal plants producing 250 MtH /year, or at quire 2000 GW of installed capacity that proximately 250 million hectares in the
2 p
natural gas plants producing 500 MtH /year. displaces coal electricity in 2054. Although tropics or 400 million hectares in the tem-
2
The former is six times the current rate of only3GW ofPVarecurrentlyinstalled,PV perate zone (current areas of tropical and
p
hydrogen production. The storage part of this electricityhasbeengrowingatarateof30% temperateforestsare1500and700million
optionisthesameasinOption6. per year. A wedge of PV electricity would hectares, respectively). A third half-wedge
Option 8: Storage of carbon captured in require 700 times today’s deployment, and would be created by establishing approxi-
synfuelsplants.Loomingovercarbonmanage- about2millionhectaresoflandin2054,or2 mately 300 million hectares of plantations
ment in 2054 is the possibility of large-scale to3m2perperson. on nonforested land.
productionofsyntheticfuel(synfuel)fromcoal. Option 12: Renewable hydrogen. Re- Option 15: Agricultural soils manage-
Carbon emissions, however, need not exceed newable electricity can produce carbon- ment.Whenforestornaturalgrasslandiscon-
those associated with fuel refined from crude free hydrogen for vehicle fuel by the elec- verted to cropland, up to one-half of the soil
oil if synfuels production is accompanied by trolysis of water. The hydrogen produced carbonislost,primarilybecauseannualtilling
CCS.Assumingthathalfofthecarbonentering by 4 million 1-MW windmills in 2054, if increasestherateofdecompositionbyaerating
p
a 2054 synfuels plant leaves as fuel but the used in high-efficiency fuel-cell cars, undecomposedorganicmatter.About55GtC,
otherhalfcanbecapturedasCO ,thecapture would achieve a wedge of displaced gaso- ortwowedges’worth,hasbeenlosthistorically
2
partofawedgein2054wouldbethedifference line or diesel fuel. Compared with Option inthisway.Practicessuchasconservationtill-
between capturing and venting the CO from 10,thisistwiceasmany1-MW windmills age(e.g.,seedsaredrilledintothesoilwithout
2 p
coalsynfuelsplantsproducing30millionbar- as would be required to produce the elec- plowing), the use of cover crops, and erosion
relsofsynfuelsperday.(Theflowofcarbonin tricity that achieves a wedge by displacing controlcanreversethelosses.By1995,conser-
24 million barrels per day of crude oil is 1 high-efficiency baseload coal. This inter- vationtillagepracticeshadbeenadoptedon110
GtC/year;weassumethesamevalueforthe esting factor-of-two carbon-saving advan- millionhectaresoftheworld’s1600million
flow in synfuels and allow for imperfect tage of wind-electricity over wind-hydro- hectaresofcropland.Ifconservationtillage
capture.) Currently, the Sasol plants in gen is still larger if the coal plant is less could be extended to all cropland, accom-
South Africa, the world’s largest synfuels efficient or the fuel-cell vehicle is less panied by a verification program that en-
facility, produce 165,000 barrels per day spectacular. forces the adoption of soil conservation
from coal. Thus, a wedge requires 200 Option13:Biofuels.Fossil-carbonfuelscan practicesthatactuallyworkasadvertised,a
Sasol-scale coal-to-synfuels facilities with alsobereplacedbybiofuelssuchasethanol.A good case could be made for the IPCC’s
CCS in 2054. The storage part of this op- wedge of biofuel would be achieved by the estimate that an additional half to one
tion is again the same as in Option 6. productionofabout34millionbarrelsperday wedge could be stored in this way.
Option9:Nuclearfission.Onthebasisof ofethanolin2054thatcoulddisplacegasoline,
the Option 5 estimates, a wedge of nuclear provided the ethanol itself were fossil-carbon Conclusions
electricity would displace 700 GW of effi- free. This ethanol production rate would be In confronting the problem of greenhouse
cient baseload coal capacity in 2054. This about50timeslargerthantoday’sglobalpro- warming,thechoicetodayisbetweenaction
wouldrequire700GWofnuclearpowerwith ductionrate,almostallofwhichcanbeattrib- and delay. Here, we presented a part of the
thesame90%capacityfactorassumedforthe uted to Brazilian sugarcane and United States caseforactionbyidentifyingasetofoptions
coalplants,orabouttwicethenuclearcapac- corn. An ethanol wedge would require 250 that have the capacity to provide the seven
ity currently deployed. The global pace of million hectares committed to high-yield (15 stabilization wedges and solve the climate
nuclearpowerplantconstructionfrom1975 dry tons/hectare) plantations by 2054, an area problem for the next half-century. None of
to 1990 would yield a wedge, if it contin- equal to about one-sixth of the world’s crop- the options is a pipe dream or an unproven
ued for 50 years (10). Substantial expan- land.Anevenlargerareawouldberequiredto idea. Today, one can buy electricity from a
sion in nuclear power requires restoration theextentthatthebiofuelsrequirefossil-carbon windturbine,PVarray,gasturbine,ornucle-
of public confidence in safety and waste inputs.Becauselandsuitableforannuallyhar- ar power plant. One can buy hydrogen pro-
disposal, and international security agree- vestedbiofuelscropsisalsooftensuitablefor duced with the chemistry of carbon capture,
ments governing uranium enrichment and conventional agriculture, biofuels production biofuel to power one’s car, and hundreds of
plutonium recycling. couldcompromiseagriculturalproductivity. devices that improve energy efficiency. One
Option 10: Wind electricity. We account CategoryIII:NaturalSinks can visit tropical forests where clear-cutting
for the intermittent output of windmills by Althoughtheliteratureonbiologicalseques- hasceased,farmspracticingconservationtill-
equating 3 GW of nominal peak capacity (3 trationincludesadiversearrayofoptionsand age,andfacilitiesthatinjectcarbonintogeo-
GW )with1GWofbaseloadcapacity.Thus, some very large estimates of the global po- logicreservoirs.Everyoneoftheseoptionsis
p
awedgeofwindelectricitywouldrequirethe tential, here we restrict our attention to the already implemented at an industrial scale
deploymentof2000GW thatdisplacescoal pairofoptionsthatarealreadyimplemented andcouldbescaledupfurtherover50years
p
electricityin2054(or2million1-MW wind atlargescaleandthatcouldbescaledupto toprovideatleastonewedge.
p
turbines). Installed wind capacity has been a wedge or more without a lot of new
growingatabout30%peryearformorethan research. (See Section 4 of the SOM text
10 years and is currently about 40 GW . A for references and details.) ReferencesandNotes
p 1. IPCC,ClimateChange2001:Mitigation,B.Metzetal.,
wedgeofwindelectricitywouldthusrequire Option14:Forestmanagement.Conserva-
Eds. (IPCC Secretariat, Geneva, Switzerland, 2001);
50 times today’s deployment. The wind tur- tiveassumptionsleadtotheconclusionthatat available at www.grida.no/climate/ipcc_tar/wg3/
bineswould“occupy”about30millionhect- least one wedge would be available from re- index.htm.
ares (about 3% of the area of the United duced tropical deforestation and the manage- 2. M.I.Hoffertetal.,Science298,981(2002).
3. R.T.Watsonetal.,ClimateChange2001:Synthesis
States), some on land and some offshore. mentoftemperateandtropicalforests.Atleast
Report.ContributiontotheThirdAssessmentReport
Because windmills are widely spaced, land onehalf-wedgewouldbecreatedifthecurrent of the Intergovernmental Panel on Climate Change
withwindmillscanhavemultipleuses. rate of clear-cutting of primary tropical forest (CambridgeUniv.Press,Cambridge,UK,2001).
4. B. C. O’Neill, M. Oppenheimer, Science 296, 1971
Option 11: Photovoltaic electricity. Sim- werereducedtozeroover50yearsinsteadof
(2002).
ilar to a wedge of wind electricity, a wedge being halved. A second half-wedge would 5. RoyalCommissiononEnvironmentalPollution,En-
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8. IPCC,SpecialReportonEmissionsScenarios(2001); 12. G.Shaffer,J.L.Sarmiento,J.Geophys.Res.100,2659 Figs.S1andS2
available at www.grida.no/climate/ipcc/emission/ (1995). TablesS1toS5
index.htm. 13. TheauthorsthankJ.Greenblatt,R.Hotinski,andR. References
VIEWPOINT
Sustainable Hydrogen Production
John A. Turner
Identifying and building a sustainable energy system are perhaps two of the most 250-yearcoalreservesdropto75yearsorso(6),
criticalissuesthattoday’ssocietymustaddress.Replacingourcurrentenergycarrier whichisnotatallsustainable.Thatleavessolar-
mixwithasustainablefuelisoneofthekeypiecesinthatsystem.Hydrogenasan derived,wind,nuclear,andgeothermalenergyas
energy carrier, primarily derived from water, can address issues of sustainability, majorresourcesforsustainablehydrogenproduc-
environmentalemissions,andenergysecurity.Issuesrelatingtohydrogenproduction tion. The hydrogen production pathways from
pathways are addressed here. Future energy systems require money and energy to theseresourcesincludeelectrolysisofwater,ther-
build.GiventhattheUnitedStateshasafinitesupplyofboth,harddecisionsmust malchemicalcyclesusingheat,andbiomasspro-
be made about the path forward, and this path must be followed with a sustained cessing (using a variety of technologies ranging
andfocusedeffort. fromreformingtofermentation).
Biomassprocessingtechniquescanbene-
Inhis2003StateoftheUnionAddress,U.S. States chooses a hydrogen-based future it fit greatly from the wealth of research that
President Bush proposed “$1.2 billion in re- needs to think carefully about how much hasbeencarriedoutovertheyearsonrefin-
search funding so that America can lead the energy we need and where it is going to ing and converting liquid and gaseous fossil
world in developing clean, hydrogen- comefrom.Inaddition,sustainabilitymust fuels. Some of these processes require con-
powered automobiles.” Since that time, arti- beahallmarkofanyproposedfutureinfra- siderableamountsofhydrogen,andmanyof
clesbothproandconhavebuffetedthewhole structure. What energy-producing technol- thesefossil-derivedprocessescanbeadapted
concept.Thehydrogeneconomy(1)isnota ogies can be envisioned that will last for for use with a large variety of biomass-
new idea. In 1874, Jules Verne, recognizing millennia, and just how many people can derived feedstocks. Biomass can easily be
thefinitesupplyofcoalandthepossibilities they support (6–8)? converted into a number of liquid fuels, in-
ofhydrogenderivedfromwaterelectrolysis, cludingmethanol,ethanol,biodiesel,andpy-
made the comment that “water will be the Technologies for Hydrogen Production rolysis oil, which could be transported and
coal of the future” (2). Rudolf Erren in the Hydrogencanbegeneratedfromwater,bio- used to generate hydrogen on site. For the
1930s suggested using hydrogen produced mass,naturalgas,or(aftergasification)coal. high-biomass-yieldprocesses,suchascornto
from water electrolysis as a transportation Today, hydrogen is mainly produced from ethanol, hydrogen is required in the form of
fuel (3). His goal was to reduce automotive naturalgasviasteammethanereforming,and ammonia for fertilizer. Although biomass is
emissionsandoilimportsintoEngland.Sim- although this process can sustain an initial clearly (and necessarily) sustainable, it can-
ilarly,FrancisBaconsuggestedusinghydro- foray into the hydrogen economy, it repre- notsupplyhydrogenintheamountsrequired.
gen as an energy storage system (4). The sents only a modest reduction in vehicle Itremainstobeseen,inaworldthatisboth
vision of using energy from electricity and emissions as compared to emissions from food-limited and carbon-constrained, wheth-
electrolysistogeneratehydrogenfromwater current hybrid vehicles, and ultimately only er the best use of biomass is for food, as a
for transportation and energy storage to re- exchangesoilimportsfornaturalgasimports. chemicalfeedstock,orasanenergysource.
duce environmental emissions and provide Itisclearlynotsustainable. Because the direct thermal splitting of
energy security is compelling, but as yet re- Coalgasificationcouldproduceconsider- waterrequirestemperaturesof(cid:5)2000°Cand
mainsunrealized. able amounts of hydrogen and electricity produces a rapidly recombining mixture of
If one assumes a full build-out of a hy- merelybecauseofthelargesizeofavailable hydrogenandoxygen(10),anumberofther-
drogen economy, the amount of hydrogen coaldeposits(9).Additionally,becauseofitsrel- malchemicalcycleshavebeenidentifiedthat
needed just for U.S. transportation needs atively low cost, it is often cited as the best re- can use lower temperatures and produce hy-
wouldbeabout150milliontonsperyear(5). sourceforeconomicallyproducinglargequanti- drogenandoxygeninseparatesteps.Theone
Onemustquestiontheefficacyofproducing, ties of hydrogen. However, the energy required that has received the greatest attention in-
storing,anddistributingthatmuchhydrogen. for the necessary sequestration of CO would volves sulfuric acid (H SO ) at 850°C and
2 2 4
Because energy is required to extract hydro- increasetherateatwhichcoalreservesaredeplet- hydrogeniodide(HI)at450°C(11).Thenext
gen from either water or biomass so that it ed;convertingthevehiclefleettoelectricvehicles generation of fission reactors includes de-
canbeusedasanenergycarrier,iftheUnited andgeneratingthatelectricityfrom“cleancoal”or signs that can provide the necessary heat;
making hydrogen as a possible energy carrier however,anumberofcriticalmaterialprop-
wouldacceleratethatdepletion.Couplethattoa erties must be satisfied to meet the required
National Renewable Energy Laboratory, Golden, CO
80401–3393,USA.E-mail:[email protected] modesteconomicgrowthrateof(cid:2)1%,andU.S. stabilityundertheoperatingconditionsofHI
972 13AUGUST2004 VOL305 SCIENCE www.sciencemag.org
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