Cornell University -- Assessing the Greenhouse Impact of Natural Gas

7/31/2019 Cornell University -- Assessing the Greenhouse Impact of Natural Gas

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Assessingthegreenhouseimpactof1naturalgas2L.M.Cathles,June6,20123

Abstract4Theglobalwarmingimpactofsubstitutingnaturalgasforcoalandoiliscurrentlyin5

debate.Weaddressthisquestionherebycomparingthereductionofgreenhousewarming6

thatwouldresultfromsubstitutinggasforcoalandsomeoiltothereductionwhichcould7

beachievedbyinsteadsubstitutingzerocarbonenergysources.Weshowthatsubstitution8

ofnaturalgasreducesglobalwarmingby40%ofthatwhichcouldbeattainedbythe9

substitutionofzerocarbonenergysources.Atmethaneleakageratesthatare~1%of10production,whichissimilartotodaysprobableleakagerateof~1.5%ofproduction,the11

40%benefitisrealizedasgassubstitutionoccurs.Forshorttransitionstheleakagerate12

mustbemorethan10to15%ofproductionforgassubstitutionnottoreducewarming,13

andforlongertransitionstheleakagemustbemuchgreater.Buteveniftheleakagewasso14

highthatthesubstitutionwasnotofimmediatebenefit,the40%ofzerocarbonbenefit15

wouldberealizedshortlyaftermethaneemissionsceasedbecausemethaneisremoved16

quicklyformtheatmospherewhereasCO2isnot.Thebenefitsofsubstitutionare17

unaffectedbyheatexchangetotheocean.CO2emissionsarethekeytoanthropogenic18

climatechange,andsubstitutinggasreducesthemby40%ofthatpossiblebyconversionto19

zerocarbonenergysources.Gassubstitutionalsoreducestherateatwhichzerocarbon20

energysourcesmustbeeventuallyintroduced.21

Introduction22Inarecentcontroversialpaper,Howarthetal.(2011)suggestedthat,becausemethaneisa23

farmorepotentgreenhousegasthancarbondioxide,theleakageofnaturalgasmakesits24

greenhouseforcingasbadandpossiblytwiceasbadascoal,andtheyconcludedthatthis25

underminesthepotentialbenefitofnaturalgasasatransitionfueltolowcarbonenergy26

sources.Others(Hayhoeetal.,2009;Wigley,2011)havepointedoutthatthewarming27causedbyreducedSO2emissionsascoalelectricalfacilitiesareretiredwillcompromise28

someofthebenefitsoftheCO2reduction.Wigley(2011)hassuggestedthatbecausethe29

impactofgassubstitutionforcoalonglobaltemperaturesissmallandtherewouldbe30


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somewarmingasSO2emissionsarereduced,thedecisionoffueluseshouldbebasedon31

resourceavailabilityandeconomics,notgreenhousegasconsiderations.32

Someofthesesuggestionshavebeenchallenged.ForexampleCathlesetal.(2012)have33

takenissuewithHowarthetal.forcomparinggasandcoalintermsoftheheatcontentof34

thefuelsratherthantheirelectricitygeneratingcapacity(coalisusedonlytogenerate35

electricity),forexaggeratingthemethaneleakagebyafactorof3.6,andforusingan36

inappropriatelyshort(20year)globalwarmingpotentialfactor(GWP).Neverthelessit37

remainsdifficulttoseeinthepublishedliteraturepreciselywhatbenefitmightberealized38

bysubstitutinggasforcoalandtheuseofmetricssuchasGWPfactorsseemstocomplicate39

ratherthansimplifytheanalysis.Thispaperseekstoremedythesedeficienciesby40

comparingthebenefitsofnaturalgassubstitutiontothoseofimmediatelysubstituting41lowcarbonenergysources.Thecomparativeanalysisgoesbacktothefundamental42

equationanddoesnotusesimplifiedGWPmetrics.Becauseitisanullanalysisitavoids43

thecomplicationsofSO2,carbonblack,andthecomplexitiesofCO2removalfromthe44

atmosphere.Itshowsthatthesubstitutionofnaturalgasforcoalandsomeoilwould45

realize~40%ofthegreenhousebenefitsthatcouldbehadbyreplacingfossilfuelswith46

lowcarbonenergysourcessuchaswind,solar,andnuclear.Inthelongtermthisgas47

substitutionbenefitdoesnotdependonthespeedofthetransitionorthemethaneleakage48

rate.Ifthetransitionisfaster,greenhousewarmingisless.Iftheleakageisless,the49

reductionofwarmingduringthesubstitutionperiodisgreater,butregardlessoftherateof50

leakageorthespeedofsubstitution,naturalgasachieves~40%ofthebenefitsoflow51

carbonenergysubstitutionafewdecadesaftermethaneemissionsassociatedwithgas52

productioncease.Thebenefitofnaturalgassubstitutionisadirectresultofthedecrease53

inCO2emissionsitcauses.54

ThecalculationmethodsusedherefollowWigley(2011),butarecomputedusing55programsofourowndesignfromtheequationsandparametersgivenbelow.Parameters56

aredefinedthatconvertscenariosfortheyearlyconsumptionofthefossilfuelstothe57

yearlyproductionofCO2andCH4.Thesegreenhousegasesarethenintroducedintothe58

atmosphereandremovedusingacceptedequations.Radiativeforcingsarecalculatedfor59

thevolumetricgasconcentrationsastheyincrease,theequilibriumglobaltemperature60


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changeiscomputedbymultiplyingthesumoftheseforcingsbytheequilibriumsensitivity61

factorcurrentlyfavoredbytheIPCC,andtheincrementsofequilibriumtemperature62

changeareconvertedtotransienttemperaturechangesusingatwolayeroceanthermal63

mixingmodel.64

EmissionScenarios65GreenhousewarmingisdrivenbytheincreaseintheatmosphericlevelsofCO2,CH4and66

othergreehousegasesthatresultfromtheburningoffossilfuels.Between1970and2002,67

worldenergyconsumptionfromallsources(coal,gas,oil,nuclear,hydroandrenewables)68

increasedattherateof2.1%peryear.Intheyear2005sixandahalfbillionpeople69

consumed~440EJ(EJ=exajoules=1018joules,1joule=1.055Btu;EIA,2011)ofenergy.70

Oilandgassupplied110EJeach,coal165EJ,andothersources(hydro,nuclear,and71

renewablessuchawindandsolar)55EJ(MiniCAMscenario,Clark,2007).In2100the72

worldpopulationisprojectedtoplateauat~10.5billion.Iftheperpersonconsumption73

thenisattodaysEuropeanaverageof~7kWp1,globalenergyconsumptionin210074

wouldbe2300EJperyear(74TW).Westartwiththefuelconsumptionpatternat200575

ADandgrowitexponentiallysothatitreaches2300EJperyearattheendofatransition76

period.Attheendofthetransitiontheenergyissuppliedalmostentirelybylowcarbon77

sourcesinallcases,butinthefirsthalfofthetransition,whichwecallthegrowthperiod,78

hydrocarbonconsumptioneitherincreasesonthecurrenttrajectory(thebusinessas79

usualscenario),increasesatthesameequivalentratewithgassubstitutedforcoalandoil80

(asubstitutegasscenario),ordeclinesimmediately(thelowcarbonfastscenario).Coal81

useisphasedoutatexactlythesamerateinthesubstitutegasandlowcarbonfast82

scenarios,sothatthereductionofSO2andcarbonblackemissionsisexactlythesamein83

thesetwoscenariosandthereforisnotafactorwhenwecomparethereductionin84

greenhousewarmingforthesubstitutegasandthelowcarbonfastscenarios.85

Figure1showsthethreefuelscenariosconsideredfora100yeartransition:86

Infirsthalf(growthperiod)ofthebusinessasusualscenario(AinFigure1),fossil87fuelconsumptionincreases2.9foldfrom440EJ/yrin2005to1265EJ/yroverthe88


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50yeargrowthperiod,andthendeclinesto205.6EJ/yrafterthefulltransition.The89

mixofhydrocarbonsconsumedattheendofthetransitionproducesCO2emissions90

atthesame4.13GtC/yrrateasattheendoftheotherscenarios.Thetotalenergy91

consumptiongrowsat2.13%peryearinthegrowthperiod,andat1.2%overthe92

declineperiod.Thegrowthperiodisashifted(tostartin2005),slightlysimplified,93

exponentialversionoftheMiniCAMscenarioinClark(2007).Weincreasethe94

hydrocarbonconsumptionbythesamefactorsasintheMiniCAMscenario,and95

determinetherenewablegrowthbysubtractingthehydrocarbonenergy96

consumptionfromthistotal.Thegrowthdeclinecombinationissimilartothebase97

scenariousedbyWigley(2011).98

Inthesubstitutegasscenario(BinFigure1),gasreplacescoalandnewoil99consumptionoverthegrowthperiod,andisreplacedbylowcarbonfuelsinthe100

declineperiod.Gasreplacescoalonanequalelectricitygenerationbasis101

(Hgas=HcoalRcoal/Rgas=234EJy1,seeTable1),andgasreplacesnewoil(165EJy1)102

onanequalheatcontentbasis.Gasuseattheendofthegrowthperiodisthus729103

EJy1,ratherthan330EJy1inthebusinessasusualscenario.Thegrowthof104

renewableenergyconsumptionisgreaterthanin(A).Overtheensuingdecline105

period,oilconsumptiondropsto75EJy1andgasto175EJy1.106

Inthelowcarbonfastscenario(CinFigure1),lowcarbonenergysourcesreplace107coal,newgas,andnewoiloverthegrowthperiod,andgasusegrowsandoiluse108

decreasessothattheconsumptionattheendisthesameasinthesubstitutegas109

scenario.110

Thesescenariosareintendedtoprovideasimplebasisforassessingthebenefitsof111

substitutinggasforcoal;theyareintendedtobeinstructiveandrealisticenoughtobe112

relevanttofuturesocietaldecisions.Thequestiontheyposeis:Howfarwillsubstituting113

gasforcoalandsomeoiltakeustowardthegreenhousebenefitsofanimmediateandrapid114

conversiontolowcarbonenergysources.115


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0

500

1000

1500

2000

2500

A. Business as Usual

0

500

1000

1500

2000

2500

EJpe

ryear

B. Subst iute Gas

no C

Coal

Gas

Oil

0

500

1000

1500

2000

2500

0 25 50 75

C. Low Carbon Fast

100

years af er 2005

55

110

165110

330

330

440

165

55.67575

2094.4

2050

2050

729

371

165175

75

110165

17575

Growth

Period

Decli

nePerio

d

990440 EJ

1265 EJ

2300 EJ

116

Figure1Threefuelconsumptionscenarioscomparedinthispaper:(A)Fossilfueluseinthebusinessasusual117

scenariocontinuesthepresentgrowthinfossilfuelconsumptionintheinitial50yeargrowthperiodbeforelow118

carbonenergysourcesreplacefossilfuelsinthedeclineperiod.(B)Inthesubstitutegasscenario,gasreplaces119

coalsuchthatthesameamountofelectricityisgenerated,andsubstitutesfornewoilonanequalheatenergy120

basis.(C)Inthelowcarbonfastscenario,lowcarbonenergysourcesimmediatelysubstituteforcoalandnewoil121

andgasinthegrowthperiod,andgasusedeclinesandsubstitutesforoilinthedeclineperiod.Numbersindicate122

theconsumptionofthefuelsinEJperyearatthestart,midpoint,andendofthetransitionperiod.Thetotal123

energyuseisthesameinallscenariosandisindicatedatthestart,midpoint,andendbytheboldblacknumbers124

in(C).125


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Table1.Parametersusedinthecalculations.Iistheenergycontentofthefuel,Rtheefficiencyofconversionto126

electricity,and and thecarbonandmethaneemissionsfactors.Seetextfordiscussion.127

I[EJGt1] R[EJeEJ1] [GtCEJ1] [GtCH4EJ1]

Gas 55 0.6 0.015 1.8x104foraleakageof1%ofproduction

Oil 43 0.020

Coal 29 0.32 0.027 1.2x104for5m3/t

128

ComputationMethodandParameters129Table1summarizestheparametersusedinthecalculations.I[EJGt1],givestheheat130

energyproducedwheneachfossilfuelisburnedinexajoules(1018

joules)pergigaton(109

131tons)ofthefuel.Thevaluesweusearefromhttp://www.natural132

gas.com.au/about/references.html.Theenergydensityofcoalvariesfrom2537GJ/t,133

dependingontherankofthecoal,but29GJ/tisconsideredagoodaveragevaluefor134

calculations.135

R[EJeEJ1]istheefficiencywithwhichgasandcoalcanbeconvertedtoelectricityin136

exajoulesofelectricalenergyperexajouleofheat.Gascangenerateelectricitywithmuch137

greaterefficiencythancoalbecauseitcandriveagasturbinewhoseeffluentheatcanthen138

beusedtodriveasteamgenerator.Lookingforward,olderlowefficiencycoalplantswill139

likelybereplacedbyhigherefficiencycombinedcyclegasplantsofthiskind.Theelectrical140

conversionefficienciesweadoptinTable1arethoseselectedbyHayhoeetal.(2002,their141

TableII).142

Thecarbonemissionfactorsingigatonsofcarbonreleasedtotheatmosphereperexajoule143

ofcombustionheat,[GtCEJ1],listedinthefourthcolumnofTable1arethefactors144

compiledbytheEPA(2005)andusedbyWigley(2011).145

Finally,themethaneemissionfactors,[GtCH4EJ1]inthelastcolumnofTable1are146

computedfromthefractionofmethanethatleaksduringtheproductionanddeliveryof147

naturalgasandthevolumeofmethanethatisreleasedtotheatmosphereduringmining148

andtransportofcoal:149


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]GtEJ[]GtGt[]EJGt[-1

burned-CH4

-1

burned-CH4vented-CH4

-1

CH4 ILgas (1a)150

]GtEJ[]m[]tm[]EJGt[-1

burned-coal

-3

CH44CH4

-1

mined-coal

3

CH4

-1

CH4 ItV CHcoal . (1b)151

Thedensityofmethanein(1b)CH4=0.71x103tonsperm3.Wetreatthemethanevented152

totheatmosphereduringtheproductionanddistributionofnaturalgas,L,parametrically153

inourcalculations.Thenaturalgasleakage,L,isdefinedasthemassfractionofnaturalgas154

thatisburned.155

Weassumeinourcalculationsthat5m3ofmethaneisreleasedpertonofcoalmined.The156

leakageofmethaneduringcoalmininghasbeenreviewedindetailbyHowarthetal.157

(2011)andWigley(2011).Combiningleakagesfromsurfaceanddeepmininginthe158

proportionsthatcoalisextractedinthesetwoprocesses,theyarriveat6.26m3/tand4.88159

m3/trespectively.Thevalueweuseliesbetweenthesetwoestimates,andappearstobea160

reasonableestimate(e.g.,seeSaghafietal.,1997),althoughsomehaveestimatedmuch161

highervalues(e.g,Hayhoeetal.,2002,suggest~23m3/t).162

TheyearlydischargeofCO2(measuredintonsofcarbon)andCH4totheatmosphere,163

QC[GtCy1]andQCH4[GtCH4y1],arerelatedtotheheatproducedinburningthefuels,H[EJy164

1

]inFigure1:165

]EJGt[]yEJ[]yGt[ -1C-1-1

C HQC (2a)166

]EJGt[]yEJ[]yGt[ -1CH4-1-1

C4 HQCH (2b)

167

ThevolumefractionsofCO2andCH4addedtotheatmosphereinyeartiby(1)are:168

tMV

V

W

W

W

WQ

yppmvtXatm

air

CO

CO

air

C

CO

C

iCO

2

2

2151-

C

12

10]yGt[

][ (3a)169

tM

V

V

W

WQ

yppbvtXatm

air

CH

CH

air

CH

iCH

4

4

181-

CH44

1

4

10]yGt[

][ . (3b)170


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HereMatm[t]=5.3x1015tonsisthemassoftheatmosphere,WCO2isthemolecularweightof171

CO2(44g/mole),andVCO2isthemolarvolumeofCO2,etc.In(2a)thefirstmolecularweight172

ratioconvertstheyearlymassadditionofcarbontotheyearlymassadditionofCO2,and173

thesecondmassfractionratioconvertsthistothevolumefractionofCO2inthe174

atmosphere.WeassumethegasesareidealandthusVCO2=Vair.175

Eachyearlyinputofcarbondioxideandmethaneisassumedtodecaywithtimeasfollows:176

186.151.189.1722

222

186.0338.0259.0217.0ttt

CO

COiCOiCO

eeetf

tftXttX

(4a)177

124

444

tCH

CHiCHiCH

etf

tftXttX

, (4b)178

wheretistimeinyearsaftertheinputofayearlyincrementofgasatti.Thesedecayrates179

arethoseassumedbytheIPCC(2007,Table2.14).The12yeardecaytimeformethanein180

(4b)isaperturbationlifetimethattakesintoaccountchemicalreactionsthatincrease181

methaneslifetimeaccordingtotheIPCC(2007,2.10.3.1).ThedecayofCO2describedby182

(4a)doesnotaccountforchangeswithtimeinthecarbonatebicarbonateequilibrium183

(suchasdecreasingCO2solubilityasthetemperatureoftheoceansurfacewaters184

increases)whichbecomeimportantathigherconcentrationsofatmosphericCO2(seeNRC,185

2011;Ebyetal.,2009).Equation(4a)thusprobablyunderstatestheamountofCO2that186

willberetainedintheatmospherewhenwarminghasbecomesubstantial.187

Theconcentrationofcarbondioxideandmethaneintheatmosphereasafunctionoftimeis188

computedbysummingtheadditionseachyearandthedecayedcontributionsfromthe189

additionsinpreviousyears:190

1

1

4444

1

1

2222

i

j

jiCHjCHiCHiCH

i

j

jiCOjCOiCOiCO

ttftXtXtX

ttftXtXtX

, (5)191


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where iCO tX 2 and iCH tX 4 arevolumetricconcentrationofCO2andCH4inppmvandppbv192

respectively,irunsfrom1tottotwherettotisthedurationofthetransitioninyears,andthe193

sumtermsontherighthandsidesdoesnotcontributeunlessi2.194

Theradiativeforcingsforcarbondioxideandmethane,FCO2[Wm2]andFCO2[Wm2]are195

computedusingthefollowingformulaegivenintheIPCC(2001,6.3.5):196

52.11555

4444444

2

4

2

222

2

31.51001.21ln47.0,

,0,000036.0

0

0ln35.5

NMMMNMNNMf

NXfNXtXfXXtXmWF

tX

tXtXmWF

oCHoCHiCHCHCHiCHCHCH

CO

COiCOCO

(6)197

Westartourcalculationswiththeatmosphericconditionsin2005:XCO2[t=0]=379ppmv,198

XCH4[t=0]=1774ppbv,andtheN2Oconcentration,No=319ppbv.CH4isafactorthat199

magnifiesthedirectforcingofCH4totakeintoaccounttheindirectinteractionscausedby200

increasesinatmosphericmethane.TheIPCC(2007)suggeststheseindirectinteractions201

increasethedirectforcingfirstby15%andthenbyanadditional25%,withtheresultthat202

CH4=1.43.Shindelletal.(2009)havesuggestedadditionalindirectinteractionswhich203

increaseCH4to~1.94.ThereiscontinuingdiscussionofthevalidityofShindelletal.s204

suggestedadditionalincrease(seeHultmanetal.,2011).WegenerallyuseCH4=1.43in205

ourcalculations,butconsidertheimpactofCH4to~1.94whereitcouldbeimportant.206

Theradiativeforcingofthegreenhousegasadditionsin(6)drivesglobaltemperature207

change.Theultimatechangeinglobaltemperaturetheycauseis:208

421

42 CHCOSCHCO

equil FFTTT , (7)209

where 1S istheequilibriumclimatesensitivity.WeadopttheIPCC,2007value 8.01 S ,210

whichisequivalenttoassumingthatadoublingofatmosphericCO2[ppmv]causesa3C211

globaltemperatureincrease.212

Theheatcapacityoftheoceandelaysthesurfacetemperatureresponsetogreenhouse213

forcing.Assuming,followingSolomonetal(2011),atwolayeroceanwherethemixed214

layerisinthermalequilibriumwiththeatmosphere:215


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Itisunlikelythatthatheatwillbetransferredoutthebaseofthemixedlayermore234

efficientlythanitisintothetopofthemixedlayerbecausethetransferwillbemostly235

drivenbywindsandcoolingoftheoceansurface.Forthisreasontheheattransfer236

coefficientratio 1s isalmostcertainly1andthereductionoftemperatureisgreatestfor237

11 s .For 11 s ,theinitialtemperaturechangeinthemixedlayerwillbeabouthalf238

thechangethatwilloccurwhentheoceanlayersarefullywarmed,andtheresponsetime239

requiredtoreachthisequilibriumchange(thetimerequiredtoreach2/3rdsofthe240

equilibriumvalue)willbeaboutoftheresponsetimeofthemixedlayer(e.g.,2

11 mixe ).241

For 11 s ,theresponsetimeofthedeeplayeristwicetheheatstoragecapacityratio242

timestheresponsetimeofthemixedlayer: mixmixdeepCC 12 .243

Thetransienttemperaturechangecanbecomputedfromtheequilibriumtemperature244

changein(7)byconvolvinginafashionsimilartowhatwasdonein(5):245

1

111

exp1exp1i

j mixd

ji

mixm

ji

j

equil

ie

tta

e

ttatTtT

, (12)246

whereij.Wedonotusetheapproximationsofequation(11)whenwecarryoutthe247

convolutionin(12).Ratherwesolvefortheactualvaluesoftheeigenvaluesand248

parameterafromthematrixin(9)ateachyearlyincrementintemperaturechange.For249

mix=5years,Tmixwillreach0.483Tmixequilwithadecaytimeof2.5yearsandriseto250

Tmixequilwithadecaytimeof200years.251

Thecurrentconsensusseemstobethat 11 s andthetransientthermalresponseis252

abouthalfthefullequilibriumforcingvalue(NRC,2011,3.3).Theratiooftheheatstorage253

capacityofthedeeptomixedlayer,

1

mixdeepCC isprobablyatleast20,avalueadoptedby254

Solomonetal.(2011).Schwartz(2007)estimatedthethermalresponsetimeofthemixed255

layerat~5yearsfromthetemporalautocorrelationofseasurfacetemperatures.Thismay256

bethebestestimateofthisparameter,butSchwartznotesthatestimatesrangefrom2to257

30years.Fortunatelythemoderationoftemperaturechangebytheoceansdoesnot258


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impactthebenefitofsubstitutinggasforcoalandoilatall.Itisofinterestindefiningthe259

coolingthatsubstitutionwouldproduce,however.Wecalculatethetransienttemperature260

changesforthefullrangeofoceanmoderationparameters.261

Equations(1)to(10)plus(12),togetherwiththeparametersjustdiscusseddefine262

completelythemethodsweusetocalculatetheglobalwarmingcausedbythefueluse263

scenariosinFigure1.264

0 100 200

ConcentrationChange

[ppmvcarbondioxide]

0

500 Carbon Dioxide [ppmv]

434 ppmv

327

190

Business as usual

Substitute gas

Low carbon fast

Low carbon fast

Substitutegas

Businessasusual

404

470 ppbv

120

0 100 200

time [years] since 2005

ConcentrationChange

[ppbvmethane]

0

500 Methane [ppbv]

258

194

113

368

428 ppbv

115

397

359

108

154

115

66

50year

transition

100year

transition

200year

transitionA

B

265

Figure2Changesin(A)carbondioxideand(B)methaneconcentrationscomputedforthethreefuelscenarios266

showninFigure1andthreedifferenttransitionintervals(50100and200years).Inthisandsubsequentfigures267

thebluecurvesindicatethebusinessasusualfuelusescenario,thegreencurvesindicatethesubstitutegas268

scenario,andtheredcurvesthelowcarbonfastscenario.Thenumbersindicatethechangeinconcentrationsof269


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CO2andmethanefromthe379ppmvforCO2and1774ppbvforCH4levelspresentintheatmospherein2005.270

ThecalculationisbasedonL=1%ofgasconsumptionandV=5m3methanepertonofcoalburned.271

Results272Figure2showstheadditionsofCO2inppmvandmethaneinppbvthatoccurforthe273

differentfuelconsumptionscenariosshowinFigure1forthethreetransitionperiods(50,274

100and200years).Themethaneleakageisassumedtobe1%ofconsumption.Fivecubic275

metersofmethaneareassumedtoleaktotheatmosphereforeachtonofcoalmined.The276

atmosphericmethaneconcentrationstrackthepatternofmethanereleasequiteclosely277

becausemethaneisremovedquicklyfromtheatmospherewithanexponentiallydecay278

constantof12years(equation4b).Ontheotherhand,becauseonlyaportionoftheCO2279

introducedintotheatmospherebyfuelcombustionisremovedquickly(seeequation4a),280

CO2accumulatesacrossthetransitionperiodsand,aswewillshowbelow,persistsfora281

longtimethereafter.282

0 100 200

time [years]

RadiativeForcing[W

m-2]

0

5

Methane

Carbon Dioxidebusiness as usual

substitute gas

low carbon fast

40%

41%

42%

283

Figure3Radiative

forcings

calculated

for

the

carbon

dioxide

and

methane

additions

shown

inFigure

2using

284equation(6)andassumingCH4=1.43.Thebluecurvesindicatethebusinessasusualscenarioforthe50,100285

and200yeartransitionperiods,thegreenthesubstitutegasscenario,andtheredthelowcarbonfastscenario.286

ThenumbersindicatethereductioninCO2forcingachievedbysubstituting gas,expressedasapercentageofthe287

reductionachievedbythelowcarbonfastscenario.288


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Figure3showstheradiativeforcingscorrespondingtotheatmosphericgasconcentrations289

showninFigure2usingequation(6).ThemethaneforcingisafewpercentoftheCO2290

forcing,andthusisunimportantindrivinggreenhousewarmingforagasleakagerateof291

1%.292

Figure4showstheglobalwarmingpredictedfromtheradiativeforcingsinFigure3for293

variousdegreesofheatlosstotheocean.Wetaketheequilibriumclimatesensitivity 1s =294

0.8(e.g.,adoublingofCO2causesa3Cofglobalwarming).Thefastertransitionsproduce295

lessglobalwarmingbecausetheyputlessCO2intotheatmosphere.Thethermal296

modulationoftheoceanscanreducethewarmingbyuptoafactoroftwo.Forexample,297

Figure4Ashowstheglobalwarmingthatwouldresultfromthebusinessasusualscenario298

iftherewerenoheatlossestotheoceanrangesfrom1.5Cforthe50yeartransitionto299

3.3Cforthe200yeartransition.Figure4Cindicatesthatheatexchangetotheoceans300

couldreducethiswarmingbyafactoroftwoforthelongtransitionsandthreeforthe50301

yeartransition.Awarmingreductionthislargeisunlikelybecauseitassumesextreme302

parametervalues:adeepoceanlayerwithaheatstorage50timestheshallowmixedlayer,303

andalongmixingtimefortheshallowlayer(mix=50years).Figure4Bindicatesthemore304

likelyoceantemperaturechangemoderationbasedonmidrangedeeplayerstorage305

(1

mixdeepCC =20)andmixedlayerresponsetime(mix=5years)parametervalues.306

Theimportantmessageofthisfigureforthepurposesofthispaper,however,isnotthe307

amountofwarmingthatmightbeproducedbythevariousfuelscenariosofFigure1,but308

theindicationthatthereductioningreenhousewarmingfromsubstitutinggasforcoaland309

oilisnotsignificantlyaffectedbyheatexchangewiththeoceanorbythedurationofthe310

transitionperiod.Thesamepercentreductioninglobalwarmingfromsubstitutinggasfor311

coalandoilisrealizedregardlessofthedurationofthetransitionperiodorthedegreeof312

thermalmoderationbytheocean.Thebenefitofsubstitutinggasisapercentorsolessfor313

theshorttransitions,andtheoceanmoderationreducesthebenefitbyapercentorso,but314

thebenefitinallcircumstancesremains~38%.Heatlossintotheoceansmayreducethe315

warmingbyafactoroftwo,butthebenefitofsubstitutinggasisnotsignificantlyaffected.316


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15

317

0

4business as usual

substitute gas

low carbon fast

1% gas leakage

no ocean mixing

1.5 C

2.3 C

3.3 C

1.8 C

2.7 C

TemperatureChange[C]

32.4 C

2.0 C

1.3 C1.4 C

0.8 C

Cd/Cm=20, mix=5 yrs

0

0 50 100 200

time [years]

3

0150

1.7 C

1.4 C

0.9 C

1.1 C

0.5 C

Cd/Cm=50, mix=50 yrs

39%

38%

38%

39%

38%

37%

38%

37%

36%1

2

1

2

1

2

3

A.

B.

C.

318

Figure4.GlobalwarmingproducedbytheforcingsinFigure3computedusingequations(7,10,and12).The319

bluecurvesindicatetemperaturechangesunderthebusinessasusualscenariofor50,100and200year320

transitiondurations,andthegreenandredcurvesindicatethetemperaturechangesforthesubstitutegasand321

low

carbon

fast

scenarios.

The

colored

numbers

indicate

the

temperature

changes,

and

the

black

numbers

the

322reductionintemperatureachievedbythesubstitutegasscenarioexpressedasafractionofthetemperature323

reductionachievedbythelowcarbonfastscenario.(A)Thewarmingwhenthereisnothermalinteractionwith324

theocean(ortheoceanlayersthermallyequilibrateveryquickly).(B)Warmingunderalikelyoceaninteraction.325

(C)Warmingwithaveryhighoceanthermalinteraction.Theoceanmixingparametersareindicatedin(B)and326

(C).Allcalculations assumegasleakageis1%ofconsumptionandtheIPCCmethaneclimatesensitivity.327


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16

Figure5comparesthemethaneforcingofthesubstitutegasscenariototheCO2forcingof328

thebusinessasusualscenarioforthe50and100yeartransitiondurations.Theforcing329

forthe1%methanecurvesarethesameasinFigure3,butiscontinuedoutto200years330

assumingthefueluseremainsthesameasattheendoftheofthetransitionperiod.331

SimilarlythebusinessasusualcurveisthesameasinFigure3continuedoutto200years.332

Thefigureshowsthatthemethaneforcingincreasesasthepercentmethaneleakage333

increases,andbecomesequaltotheCO2forcinginthebusinessasusualscenariowhenthe334

leakageis~15%ofconsumptionforthe50yeartransitionand30%ofconsumptionforthe335

100yeartransition.Attheendofthetransitionthemethaneradiativeforcingsfalltothe336

levelthatcanbesteadilymaintainedbytheconstantmethaneleakageassociatedwiththe337

smallcontinuednaturalgasconsumption.TheCO2forcingunderthebusinessasusual338

scenariofallabitandthenriseataslowsteadyrate,reflectingtheproscriptionthat26%of339

theCO2releasedtotheatmosphereisonlyveryslowlyremovedand22%isnotremovedat340

all(equation3a).ThisslowriseemphasizesthatevenverylowreleasesofCO2canbeof341

concern.Themethaneintheatmospherewouldrapidlydisappearinafewdecadesifthe342

methaneventingwerestopped,whereastheCO2curveswouldflattenbutnotdrop343

significantly.Finally,Figure5Ashowsthatthegreatermethaneclimatesensitivity344

proposedbyShindelletal(2009)(CH4=1.94)wouldmakea10%methaneventing345

equivalenttoa15%ventingwithCH4=1.43(theIPCCmethaneclimatesensitivity).346


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17

2.5

0

1%

5%

10%

15%

20% Busines

s as Usual

Substitute Gas

Leakage rate

[% of consumption]

100 200

time [years]

RadiativeForcing[W

m-2]

2.5

0

0

0.5

1.0

1.5

2.0

3.0

0.5

1.0

1.5

2.0

50 150

Business as Usual

Substitute Gas

1%

5%

10%

15%

20%

25%

30%

50 year transition

100 year transition

A.

B.

347

Figure5.RadiativeforcingsofCO2forthebusinessasusualscenario(bluecurves)andforCH4forvariousgas348

leakageratesinthesubstitutegasscenario(greencurves).The1%methanecurvesandthebusinessasusual349

curvesarethesameasinFigure3excepttheverticalscaleisexpandedandthecurvesareextendedfromtheend350

of

the

transition

to

200

years

assuming

the

gas

emissions

are

the

same

as

at

the

end

of

the

transition

past

100

351years.Themethaneforcingsplateauatthelevelscorresponding totheatmosphericconcentrationsupportedby352

thesteadyCH4emissions.TheCO2forcingincreasesbecauseanappreciablefractionoftheCO2emissionsare353

removedslowlyornotatallfromtheatmosphere.ThemethaneforcingsallassumetheIPCCmethaneclimate354

sensitivity( CH4=1.43)exceptthesingleredcurve,whichassumesthemethaneclimatesensitivitysuggestedby355

Shindelletal.(2009)( CH4=1.94).356


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18

0 50

time [years]

1.5

0

Cd/Cm=20, mix=5 yrs

1%

0.96 C

5%

1.07 C

10%

1.19 C

15%

1.30 C

0 100

1.5

0

1%

1.60 C

10%

1.84 C

20%

2.05 C

0 200

1.5

0

2.69 C

1%

35%

3.48 C

A. 50yr Transition

B. 100yr Transition

C. 200 yr Transition

40%

1%

40%

21%

5%

-6%

17%

39%

-1%

Tem

peratureChange[C]

Busin

ess

asU

sual

Su

bstitute

Gas

Low

Carrb

onFast

357

Figure6.Impactofmethaneleakageonglobalwarmingfortransitionperiodsof(A)50,(B)100,and(C)200358

years.Astheleakagerate(greenpercentagenumbers)increase,thewarmingofthesubstitutegasscenario359

(greencurves)increases,thebluebusinessasusualandgreensubstitutegascurvesapproachoneanotherand360

thencross,andthepercentageofthewarmingreductionattainedbythefastsubstitutionoflowcarbonenergy361

sourcesdecreaseandthenbecomenegative.Thewarmingsassumethesameexchangewiththeoceanasin362

Figure4B.363

Figures6illustrateshowthebenefitsofsubstitutinggasforcoalandoildisappearasthe364

methaneleakageincreasesabove1%oftotalmethaneconsumption.Thefigureshowsthe365

globalwarmingcalculatedfortheoceanheatexchangeshowinFigure4B.Asthemethane366

leakageincreases,thegreensubstitutegasscenariocurvesrisetowardandthenexceedthe367

bluebusinessasusualcurves,andthebenefitofsubstitutinggasdisappears.Thegas368


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19

leakageatwhichsubstitutinggasforoilandcoalwarmstheearthmorethanthebusiness369

asusualscenarioissmallest(L~10%)forthe50yeartransitionperiodandlargest370

(L~35%)forthe200yeartransitionperiod.371

Figure7summarizeshowthebenefitofgassubstitutiondependsonthegasleakagerate.372

FortheIPCCmethaneclimatesensitivity(CH4=1.43),thebenefitofsubstitutinggasgoesto373

zerowhenthegasleakageis44%ofconsumption(30%ofproduction)forthe200year374

transition,24%ofconsumption(19%ofproduction)forthe100yeartransition,and13%375

ofconsumption(12%orproduction)forthe50yeartransition.FortheShindelletal.376

climatesensitivitycorrespondingtoCH4=1.94,thecrossoverforthe50yeartransition377

occursatagasleakageof~9%ofconsumption,andreasonableoceanthermalmixing378

reducesthisslightlyto~8%ofconsumption(7.4%ofproduction).Thislastis379approximatelythecrossoverdiscussedbyHowarthetal.(2011and2012).Intheirpapers380

theysuggestamethaneleakagerateashighas8%ofproductionispossible,andtherefor381

thatnaturalgascouldbeasbad(ifcomparedonthebasisofelectricitygeneration)ortwice382

asbad(ifcomparedonaheatcontentbasis)ascoaloverashorttransitionperiod.As383

discussedinthenextsection,aleakagerateashighas8%isdifficulttojustify.Figure7384

thusshowsthesignificanceofShindellshighermethaneclimatesensitivitytoHowarths385

proposition.Withoutit,anevenlessplausiblemethaneleakagerateof13%wouldbe386

requiredtomakegasasbadortwiceasbadascoalintheshortterm.Overthelongerterm,387

substitutionofgasisbeneficialevenathighleakageratesapointcompletelymissedby388

Howarthetal.389


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20

0

10

20

30

40

50

0 5 10 15

%Percentlow

C

fast

Leakage[%ofconsumption]

BenefitofGasSubstitution1to2%leakage

390

Figure7.Thereductionofgreenhousewarmingattainedbysubstitutingnaturalgasforcoalandoil(substitute391

gasscenario),expressedasapercentageofthereductionattainedbyimmediatelysubstitutinglowcarbonfuels392

(lowCfastscenario),plottedasafunctionofthegasleakagerate.Atleakagerateslessthan~1%,thebenefitof393

substituting naturalgasis>40%thatofimmediatelysubstitutinglowcarbonenergysources.Thebenefit394

declinesmorerapidlywithleakageforshorttransitions.ThetopthreecurvesassumeanIPCCmethaneclimate395

sensitivity( CH4=1.43).Thebottomtwoshowtheimpactofthegreatermethaneclimatesensitivitysuggestedby396

Shindelletal(2009)( CH4=1.94).Theoceanmixingcurveaddsthesmalladditionalimpactofthermalexchange397

withtheoceansattherateshowninFigure4Btothe CH4=1.94curveimmediatelyaboveit.398

Whatisthegasleakagerate399Themostextensivesynthesesofdataonfugitivegasesassociatedwithunconventionalgas400

recoveryisanindustryreporttotheEPAcommissionedbyTheDevonEnergyCorporation401

(Harrison,2012).Itdocumentsgasleakageduringthecompletionof1578unconventional402

(shalegasortightsand)gaswellsby8differentcompanieswithareasonable403

representationacrossthemajorunconventionalgasdevelopmentregionsoftheU.S.Three404

percentofthewellsinthestudyventedmethanetotheatmosphere.Ofthe1578405unconventional(shalegasortightsand)gaswellsintheDevonstudy,1475(93.5%)were406

greencompletedthatistheywereconnectedtoapipelineinthepreinitialproduction407

stagesotherewasnoneedforthemtobeeitherventedorflared.Ofthe6.5%ofallwells408

thatwerenotgreencompleted,54%wereflared.Thus3%ofthe1578wellsstudied409

ventedmethaneintotheatmosphere.410


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21

Thewellsthatventedmethanetotheatmospheredidsoattherateof765411

Mcsf/completion.Themaximumgasthatcouldbeventedfromthenongreencompleted412

wellswasestimatedbycalculatingthesonicventingratefromthechoke(orifice)sizeand413

sourcegastemperatureofthewell,usingaformularecommendedbytheEPA.Sincemany414

wellsmightventatsubsonicrates,whichwouldbeless,thisisanupperboundonthe415

ventingrate.Thetotalventedvolumewasobtainedbymultiplyingthisventingratebythe416

knowndurationofventingduringwellcompletion.Theseventedvolumesrangedfrom417

340to1160Mscf,withanaverageof765Mscf.Theventingfromanaverage418

unconventionalshalegaswellindicatedbytheDevonstudyisthus~23Mscf(=0.03x765419

Mscf),whichissimilartothe18.33McfEPA(2010)estimatesisventedduringwell420

completionofaconventionalgaswell(halfventedandhalfflared).Sinceventingduring421

wellcompletionandworkoverconventionalgaswellsisestimatedat0.01%ofproduction422

(e.g.,Howarthetal.,2011),thiskindofventingisinsignificantforbothunconventionaland423

conventionalwells.424

TheunconventionalgasleakagerateindicatedbytheDevondataisverydifferentfromthe425

4587MscftheEPA(2010)inferredwasventedduringwellcompletionandworkoverfor426

unconventionalgaswellsfromtheamountofgascapturedinaverylimitednumberof427

greencompletionsreportedtothembyindustrythroughtheirGasSTARprogram.In428

their2010backgroundtechnicalsupportdocumenttheEPAassumedthatthiskindof429

greencapturewasveryrare,andthatthegaswasusuallyeitherventedorflared.430

Assumingfurtherthatthegaswasvented50%ofthetime,theEPAconcludedthat4587431

Mscfwasventedtotheatmosphereandthatunconventionalwellsvent250times432

(=4587/18.3)moremethaneduringwellcompletionandworkoverthanconventionalgas433

wells.TheEPA(2010)studyisaBackgroundTechnicalSupportDocumentandnotan434

officialreport.Itwasprobablyneverintendedtobemorethananoutlineofanapproach435

andaninitialestimate,andtheEPAhassincecautionedthattheyhavenotreviewedtheir436

analysisindetailandcontinuetobelievethatnaturalgasisbetterfortheenvironmentthan437

coal(Fulton,2011).NeverthelesstheEPA(2010)reportsuggestedtomanythatthe438

leakageduringwellcompletionandworkoverforunconventionalgaswellscouldbea439

substantialpercentage(~2.5%)ofproduction,andmanyacceptedthissuggestionwithout440


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22

furthercriticalexaminationdespitethefactthatthesafetyimplicationsofthemassive441

ventingimpliedbytheEPAnumbersshouldhaveraisedquestions(e.g.,Cathlesetal.,442

2012a,b).443

Onceawellisinplace,theleakageinvolvedinroutineoperationofthewellsiteandin444

transportingthegasfromthewelltothecustomeristhesameforanunconventionalwell445

asitisfromaconventionalwell.WhatweknowaboutthisleakageissummarizedinTable446

2.Routinesiteleaksoccurwhenvalvesareopenedandclosed,andleakageoccurswhen447

thegasisprocessedtoremovingwaterandinertcomponents,duringtransportationand448

storage,andintheprocessofdistributiontocustomers.Thefirstmajorassessmentof449

theseleakswascarriedoutbytheGasResearchInstitute(GRI)andtheEPAin1997and450

theresultsareshowninthesecondcolumnofTable2.AppendixAofEPA(2010)givesa451detailedandveryspecificaccountingofleaksofmanydifferentkinds.Thesenumbersare452

summedintothesamecategoriesanddiaplayedincolumn3ofTable2.EPA(2011)found453

similarleakagerates(column4).Skone(2011)assessedleakagefrom6classesofgas454

wells.WeshowhisresultsforunconventionalgaswellsintheBarnettShaleincolumn5of455

Table2.Hisotherwellclassesaresimilar.Venkatishetal(2011)carriedoutan456

independentassessmentthatisgivenincolumn6.Therearevariationsinthese457

assessments,butoverallaleakageof~1.5%ofproductionissuggested.Additional458

discussionofthisdataanditscompilationcanbefoundinCathlesetal.(2012)andCathles459

(2012).460

Table2.Leakageofnaturalgasthatiscommontobothconventionalandunconventionalgaswellsinperentof461

gasproduction.462

GRIEPA

(1997)

EPA

(2010)

EPA

(2011)

Skone

(2011)

Venkatish

eta.(2011)

Routinesite

leaks

0.37% 0.40% 0.39%

Processing 0.15% 0.12% 0.16% 0.21% 0.42%

Transportation&storage 0.48% 0.37% 0.40% 0.40% 0.26%

Distribution 0.32% 0.22% 0.26% 0.22%

Totals 1.32% 1.11% 1.21%


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23

463

Basedontheabovereviewthenaturalgasleakagerateappearstobenodifferentduring464

thedrillingandwellpreparationofunconventional(tightshalesdrilledhorizontallyand465

hydrofractured)gaswellsthanforconventionalgaswells,andtheoverallleakagefromgas466

wellsisprobably


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24

distributedNETLpowerpointanalysisbySkone(2011).ByloweringSkonesEstimated492

UltimateRecoveries(EUR)fortheBarnellShalefrom3Bcfto0.84Bcfwhilekeepingthe493

sameestimateofleakageduringwellcompletionandgasdelivery,Hughesincreased494

Skonesleakageestimatesfrom2to6%ofproductionalevelwhichfallsmidwaybetween495

Howarthslowandhighgasleakageestimates.Howeverleakageisafractionofwell496

production(awellthatdoesnotproducecannotemit),andthusisitbogustoreducethe497

EUR(thedenominator)withoutalsoreducingthenumerator(theabsoluteleakageofthe498

well).Skonesdatamustbeevaluatedonitsownterms,notsimplyadjustedtofitsomeone499

elsesconclusions.500

Petronetal.(2012)analyzedairsamplesatthe300mhighBolderAtmospheric501

Observatory(BAO)towerwhenthewindwastowarditfromacrosstheDenverJulesburg502Basin(DJB).Gasesventingfromcondensate(condensedgasfromoilandwetgaswells)503

stocktanksintheDJBarerichinpropanerelativetomethane,whereastherawnaturalgas504

ventingfromgaswellsintheDJBcontainverylittlepropane.Fromtheintermediateratio505

ofpropanetomethaneobservedattheBAOtowerandestimatesofleakagefromthestock506

tanks,Petroneetal.calculatethattodilutethepropaneleakingfromthestocktankstothe507

propane/methaneratioobservedatthetower,~4%ofmethaneproducedbygaswellsin508

theDJBmustventintotheatmosphere.TheairsampledattheBAOtoweriscertainlynot509

simplyamixofrawnaturalgasandstocktankemissionsfromtheDJBasPetronetal.510

assume,however.IfthiswerethecasetherewouldbenooxygenintheairattheBAO511

towerlocation.Thebackgroundatmospheremustcertainlymixinwiththesetwo(and512

perhapsother)gassources.BackgroundairintheDenverareacontains~1800ppb513

methaneandverylittlepropane.Mixingwiththebackgroundatmospherecoulddilutethe514

stocktankemissionstothepropane/methaneratioobservedattheBAOtowerwithno515

leakagefromgaswellsintheDJBrequiredatall.Contrarytotheirsuggestion,theBAO516

towerdatareportedbyPetroneetal.placenoconstraintsatallonthegasleakageratesin517

theDJBwhatsoever.MoredetailsareinCathles(2012).518

Certainlythereismorewecouldlearnaboutnaturalgasleakagerates.Theissueis519

complicatedbecausegasisusedinthetransmissionprocesssoshrinkageofproductdoes520

notequatetoventing.Inadditionthereareconventionsandpracticesthatmakescientific521


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25

assessmentdifficult.Despitethedifficulties,however,itappearsthattheleakagerateis522

lessthan2%ofproduction.523

Discussion

524

WehaveverifiedourcomputationsbycomparingthemtopredictionsbyWigleys(2011)525

publicallyavailableandwidelyusedMAGICCprogram.Althoughtherearesomeinternal526

differences,Table3showsthatthe~40%reductioningreenhousewarmingwepredictis527

alsopredictedbyMAGICCwhenscenariossimilartotheoneweconsiderhereareinputto528

bothMAGICCandourprograms.TheMAGICCcalculationsstartat1990ADsoweconsider529

thetemperatureincreasesfrom2000totheendoftheperiod.Fueluseisincreasedand530

reducedlinearlyratherthanexponentially,andthefueluseatthestart,midpoint,andend531

ofthetransitionsimulationsareslightlydifferentthaninFigure1.Thetemperature532

changesforthe200yearcycleagreeverywell.WigleysMAGICCtemperaturechange533

predictionsbecomeprogressivelylowerthanoursasthetransitionintervalisshortened.534

ThismaybebecauseMAGICCincludesasmalloceanthermalinteraction,whereasthe535

calculationswereportinTable3donot.536

Table3.TemperaturechangespredictedbyWiglelys(2011) MAGICCprogramforlinearchangesinfueluse537

similartothescenariosinFigure1comparedtoequilibrium(nooceanthermalinteraction)globalwarming538

predictionsbytheprogramdescribedandusedinthispaper.Thefirstthreerowscomparethetemperature539

changesofthetwoprograms.Thelastrowshowsthereductioningreenhousewarmingachievableby540

substituting naturalgasforcoalandoilasapercentageofthereductionthatwouldbeachievedbytherapid541

substitution ofallfossilfuelswithlowcarbonenergysources.542

200yearcycle 100yearcycle 40yearcycle

Program MAGICC Thispaper MAGICC Thispaper MAGICC Thispaper

Basusual 3.85 3.68 2.3 2.56 1.05 1.5

Swapgas

2.85 2.85 1.65 1.94 0.80 1.12LowCfast 1.7 1.70 0.85 1.09 0.38 0.58

%reduction 42% 42% 45% 42% 37% 41%

543


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26

IncorporationoftheindirectcontributionstomethanesradiativeforcingthroughCH4in544

equation(6)wasvalidatedbycomparingvaluesofGWPcomputedby(13)topublished545

valuessummarizedinTable4.546

t

t

COCH

CO

CO

t

t

CHCO

CH

CH

CH

dtfMWppbvC

F

dtfMWppbvC

F

GWP

0

24

2

2

0

42

4

4

4

][

][

(13)547

GWPistherelativeglobalwarmingimpactofakgofCH4comparedtoakgofCO2addedto548

theatmosphere,whenconsideredoveraperiodoftimet.Theradiativeforcings(F)are549

definedby(6),theremovalofthegasesfromtheatmosphere(f)by(4aandb),andMWCO2550

isthemolecularweightofCO2.TheCH4factorof1.43inthesecondcolumnofTable4551

combinestheindirectforcingcausedbyCH4inducedproductionofozone(25%according552

toIPCC,2007)andwatervaporinthestratosphere(additional15%accordingtotheIPCC,553

2007).WiththisfactortheGWPlistedinTable2.14oftheIPCC(2007)arereplicatedas554

showninthesecondrowofTable4.TheCH4factorof1.94inthesecondcolumnwas555

determinedbyussuchthatitapproximatelypredictstheincreasedforcingssuggestedby556

Shindelletal.(2009)asshowninthebottomrowofTable4.WedonotuseGWPsinour557

analysisandusethemhereonlytojustifythevaluesofCH4usedinourcalculations.558

Table4TheGWPcalculatedfrom(6and13)forthevalueofCH4incolumn2arecomparedtoGWP(in559

parentheses)givenbytheIPCC(2007)andShindelletal.(2009).560

CH4 t=20years t=100years t=500years

Directmethaneforcingfrom(6) 1 51.5 17.9 5.45

IPCC(2007,2.10.3.1,Table2.14) 1.43 73.5(72) 25.8(25) 7.8(7.6)

Shindelletal.(2009) 1.94 99(105) 35(33) 10.5

561

Themostimportantmessageofthecalculationsreportedhereisthatsubstitutingnatural562

gasforcoalandoilisasignificantwaytoreducegreenhouseforcingregardlessofhowlong563

(withinafeasiblerange)thesubstitutiontakes(Figure4).Formethaneleakagesof~1%of564


27/38


28/38

28

extramethaneemittedbylowlevelsofleakagehassuchatrivialclimateeffectthatitneed595

notbeconsideredatall.596

Sulfurdioxideadditionsarenotafactorinouranalysisbecausethesubstitutegasandlow597

carbonfastscenariosreducetheburningofcoaloverthegrowthperiodinanidentical598

fashion.ThusbothintroduceSO2identically,andthesmallwarmingeffectsoftheSO2,599

whichwilloccurnomatterhowcoalisretired,cancelinthecomparison.Intherealworld600

theaerosolbenefitofcoalmustberemovedeventually(unlesswearetoburncoal601

forever),andthesooneritisremovedthebetterbothbecausethesmallwarmingits602

removalwillcausewillhavelessimpactwhentemperaturesarecooler,and,muchmore603

importantly,becausereplacingcoalsoonwillreduceCO2emissionsandleadtomuchless604

globalwarminginthelongerterm.605

Wigleys(2011)decreaseingreenhousewarmingforthenaturalgassubstitutionhe606

definesissimilartothatwecomputehere.At0%leakage,Wigley(2011,hisFigure3)607

calculatesa0.35Ccoolingwhichwouldbea0.45CcoolingabsentthereducedSO2608

emissionsheconsiders.Wecalculateacoolingof~0.62Cfor0%leakage.Ourcoolingis609

greaterthanhisatleastinpartbecauseourgassubstitutionscenarioreducestheCO2610

emissionsmorethanhis.Fromnearlythesamestart,ourgassubstitutionreducesCO2611

emissionsfromthebusinessasusual200yeartransitioncycleby743GtCwhereasWigley612

reducesCO2by425GtC.613

Thereareofcourseuncertaintiesinthekindofcalculationscarriedouthere,butthese614

uncertaintiesareunlikelytochangetheconclusionsreached.Carbondioxideisalmost615

certainlynotremovedfromtheatmosphereexactlyasdescribedbyequation(3).The616

uptakeofCO2maywellslowastheclimatewarms.Carbondioxideislesssolubleinwarm617

waterandthehalinecirculationmayslowastheseasurfacetemperatureincreases.The618

increaseinterrestrialCO2uptakefromCO2fertilizationmaybereducedbynitrogen619

limitations.AgooddiscussionoftheseissuesisprovidedinNRC(2011).Ebyetal.(2009)620

havesuggestedbasedonsophisticatedcoupledglobalmodelsthat~50%oftheintroduced621

CO2mayberemovedwithatimeconstantof130yearsand50%withanexponentialtime622

constantof2900years.Modificationsofequation(3)thatreduceCO2uptakeastheclimate623


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29

warmswillmakethebenefitsofnotputtingCO2intotheatmosphere,forexampleby624

substitutinggasforcoal,evengreater,andtheargumentspresentedherestronger.625

Thetransmissionofheatfromthemixedtothedeeplayeroftheoceansisanunknown626

whichhasastrongimpactontransientglobalwarming.Forexample,ifheatenteredthe627

deeplayerwith10%oftheeasewithwhichitentersitfromtheatmospheresothat628

1s ~0.1,thedeeplayerwouldlargelylooseitscoolingeffectiveness(e.g.,ainequation11629

wouldhaveavalueof0.91).ThetransientresponsetoCO2forcingwouldberapid(occur630

at0.91mix),andtheoceanwouldreducetheequilibriumglobaltemperaturechangeby631

only9%.Therelativeratesatwhichheatistransferredintothemixedlayerandoutofit632

intothedeeplayerwouldappeartobeanimportantareaforfurtherinvestigation,633

especiallybecauseitimpactsourabilitytoinferpropervaluesintheequilibriumclimate634

forcing(seediscussioninNRC,2011).Oceanheatexchangedoesnotaffectthe635

comparativebenefitofsubstitutinggas,souncertaintiesintheoceanheatexchangeaernot636

ofconcerntotheconclusionswereachhere.637

ThecalculationsmadehereavoidtheuseofGWPfactors.ThedeficienciesintheGWP638

approacharediscussedwellbySolomonetal.(2011).Asisapparentfrom(13),theGWP639

metricrequiresthatthetimeperiodofcomparisonbespecified.Forashorttimeperiod,a640

shortlivedgaslikemethanehasahighGWP(e.g.,itis72timesmorepotentintermsof641

globalwarmingthanCO2whencomparedovera20year).Thenotionthatmethane642

emissionshave72timestheglobalwarmingimpactofCO2wouldtempteliminating643

methaneemissionsimmediately,andworryingaboutreducingCO2emissionslater.Onthe644

otherhandfora500yearperiod,theglobalwarmingimpactofakilogramofvented645

methaneisonly7.6thatofakilogramofCO2(GWPCH4=7.6,seeTable4),andthislow646

impactwouldsuggestdealingwithCO2emissionsfirstandthemethaneemissionslater,647

perhapsevensubstitutinggasforcoalandoil.AsSolomonetal.pointouttheGWPmetric648

speaksonlytothetimeperiodforwhichitiscalculatedandshedsnolightonthewhether649

CO2orCH4shouldbereducedfirst.650


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100 500

time [years]

200 300 400

0.5

1

1.5

2

Tem

peraureChange[C]

00

39.8%

10%

leakage

n

ofossilfuels

Business as Usual

Substitute Gas

Low Carbon Fast

Cd/Cm=20, mix=5 yrs

CH4

651

Figure8.TemperaturechangeforscenariosinFigure1whenatransitionperiodis100yearsisfollowedbya400652

yearperiodwithnoburningoffossilfuels.Methaneleakageinthetransitionis10%ofgasconsumptionand653

Shindellsgreatermethaneforcingandheatexchangewiththeoceanareincluded.Extramethaneventinginthe654

substitutegasscenarioproduceswarminggreaterthanthebusinessasusualscenariouptoalmosttheendof655

thetransition,butthebenefitsofreducingcarbonemissionsbysubstitutinggasemergeveryquicklythereafter.656

Figure8illustratesthefundamentaldilemma.Itshowsthatevenwhenmethaneleakageis657

solarge(L=10%ofconsumption)thatsubstitutinggasforcoalandoilincreasesglobal658

warmingintheshortterm,thebenefitofgassubstitutionreturnsinthelongterm.The659

shorttermheatingcausedbymethaneleakagerapidlydissipatesafteremissionsofCO2660

andCH4ceaseat100years.CH4israpidlyremovedfromtheatmosphere,butCO2isnot.661

Theresultisthat50yearsorsoaftertheterminationofventing(beyond150yearsin662

Figure8),thebenefitofgasemergesunscathed.Ata10%leakagerateanda100year663

transitionperiod,thesubstitutegasscenarioproducesasmallamountmorewarmingthan664

thebusinessasusualscenarioat70years,butafter150yearsthegassubstitutionreduces665

globalwarmingmuchmorebecauseithasreducedtheamountofCO2ventedtothe666

atmosphere.Figure8showshowdangerousametricsuchasGWPcanbe.Evenfor667


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methaneemissionsof9%ofproductionandShindellsforcings,substitutinggasforcoalis668

worthwhileinthelongterm.AnalysesthatrelyonlyonGWPfactors,suchasthatof669

Howarthetal.(2011),missthismixofimpactscompletely,andseeonlythedamageof670

extramethaneemissionsintheshorttermorthebenefitsofgassubstitutioninthelong671

term,dependingontheGWPintervalselected.Fortunatelyitisveryeasytocarryoutthe672

necessaryconvolutionintegrals(equations5and11)asdonehereandavoidGWPmetrics673

altogether.AsstatedbySolomonetal.(2011)andotherswhotheycite,GWPfactors674

shouldsimplynotbeusedtoevaluatefuelconsumptionscenarios.675

Finally,framingthefuelusescenariosintermsofexponentialgrowthanddeclineaswe676

havedonehereallowsthefeasibilityofimplementingthevariousscenariostobeexamined677

inapreliminaryfashion.Figure9showstherateofgrowthoflowcarbonenergyresources678thatisrequiredbythefuelhistoriesinFigure1fora100yeartransition.Growthatmore679

than5%peryearwouldbechallenging.Figure9showsthatthelowcarbonfastscenario680

inFigure1requiresanimmediate~16%peryear(butrapidlydeclining)growthinlow681

carbonenergysources.Thegrowthrateoflowcarbonenergysourcesattheendofthe682

growthperiodofthebusinessasusualscenarioisanevengreater24%peryear.Because683

thereistimetoplan,thiscouldbereducedbyphasinginlowcarbonenergysourcestoward684

theendofthefossilfuelgrowthperiod.Thesubstitutegasscenariohasamuchlower685

growthrequirementatthisstage,whichwouldmakethisscenariosubstantiallyeasierto686

accommodate.687

Anydecisiontosubstitutegasforcoalandoilofcourseinvolveseconomicandsocial688

consideration,aswellasclimateanalysis.Naturalgascanenablethetransitiontowindor689

solarenergybyprovidingthesurgecapacitywhenthesesourcesfluctuateandbackup690

whenthesesourceswane.Becauseofitswideavailabilityandlowcost,economicfactors691

willencouragegasreplacingcoalinelectricitygenerationandoilinsegmentsof692transportation.ItisafueltheU.S.andmanyothercountriesneednotimport,soits693

developmentcouldincreaseemployment,nationalsecurity,andamorepositivebalanceof694

payments.Ontheotherhand,cheapandavailablegasmightunderminetheeconomic695

viabilityoflowcarbonenergysourcesanddelayatransitiontolowcarbonsources.Froma696

greenhousepointofviewitwouldbebettertoreplacecoalelectricalfacilitieswithnuclear697


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32

plants,windfarms,orsolarpanels,butreplacingthemwithnaturalgasstationswillbe698

faster,cheaperandachieve40%ofthelowcarbonfastbenefitiftheleakageislow.How699

thisbalanceisstruckisamatterofpoliticsandoutsidethescopeofthispaper.Whatcan700

besaidhereisthatgasisanaturaltransitionfuelthatcouldrepresentsthebiggest701

availablestabilizationwedgeavailabletous.702

50 100

time [years]

Growthrate[%/yr]

0

0

20

24

4

8

12

16

BusinessasUsual

LowCarb

onFast SubstituteGas

Growth Rate Low Carbon Energy Sources

703

Figure9.ThegrowthrateoflowcarbonenergysourcesdeducedfromFigure1plottedasafunctionoftimefora704

100yeartransition.Growthratesmorethan5%peryearsuchwillbechallengingtoachieveonaglobalbasis.705

Conclusions706Thecomparativeapproachtakeninthispapershowsthatthebenefitofsubstituting707

naturalgasdependsonlyonitsleakagerate.708

1.Forleakagerates~1%orless,thesubstitutionofnaturalgasforthecoalusedin709

electricitygenerationandfor55%oftheoilusedintransportationandheatingachieves710


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33

40%ofthereductionthatcouldbeattainedbyanimmediatetransitiontolowcarbon711

energysources.712

2.This40%reductiondoesnotdependonthedurationofthetransition.A40%reduction713

isattainedwhetherthetransitionisover50yearsor200years.714

3.Forleakagerates~1%orless,thereductionofgreenhousewarmingatalltimesis715

relateddirectlytothemassofCO2putintotheatmosphere,andthereforetoreduce716

greenhouseforcingwemustreducethisCO2input.ComplexitiesofhowCO2isremoved717

andreductionsinSO2emissionsandincreasesincarbonblackandthelikedonotchange718

thissimpleimperativeandshouldnotbeallowedtoconfusethesituation.719

4.Atlowmethaneleakagerates,substitutingnaturalgasisalwaysbeneficialfroma720greenhousewarmingperspective,evenforforcingsashighashavebeensuggestedby721

Shindelletal.(2009)andusedbyHowarthetal.(2011).Underthefastesttransitionthatis722

probablyfeasible(our50yeartransitionscenario),substitutionofnaturalgaswillbe723

beneficialiftheleakagerateislessthanabout7%ofproduction.Foramorereasonable724

transitionof100years,substitutinggaswillbebeneficialiftheleakagerateislessthan725

~19%ofproduction(Figure7).Thenaturalgasleakagerateappearstobepresentlyless726

than2%ofproductionandprobably~1.5%ofproduction.727

5.Evenifthenaturalgasleakageratewerehighenoughtoincreasegreenhousewarming728

(e.g.,theleakagewas10%ofmethaneconsumptionor9%ofmethaneproduction),729

substitutinggaswouldstillhavebenefitsbecausethereductionofCO2emissionswould730

leadtoagreaterreductioningreenhousewarminglater(Figure8).731

6.Gasisanaturaltransitionfuelbecauseitssubstitutionreducestherateatwhichlow732

carbonenergysourcesmustbelaterintroduced(Figure9)andbecauseitcanfacilitatethe733

introductionoflowcarbonenergysources.734

Thepolicyimplicationsofthisanalysisare:(1)reducetheleakageofnaturalgasfrom735

productiontoconsumptionsothatitis~1%ofproduction,(2)encouragetherapid736

substitutionofnaturalgasforcoalandoil,and(3)encourageasrapidaconversiontolow737

carbonsourcesofenergyaspossible.738


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34

Acknowledgements739Thispaperwasgreatlyimprovedbythreeexcellentreviews,twoanonymousandoneby740

RayPierrehumbert.Raypointedouttheimportanceofoceanmixing,suggestedcasting741

fueluseintermsofexponentialgrowthanddecline,anddrewmyattentiontoimportant742references(asdidtheotherreviewers).Iamindebtedtomypriorcoauthorsinthis743

subject(MiltonTaam,LarryBrown,andAndrewHunter)forcontinuingveryhelpful744

discussions,andtomembersofthegasindustrywhopointedoutdataandhelpedme745

understandthecomplexitiesofgasproduction.MiltonTaamdrewmyattentiontothe746

MAGICCprogramandshowedmehoweasyitwastouse,andalsopushedpersistentlyfor747

thebroaderviewofmethanesubstitutionshowninFigure8.Thepaperwouldnotbewhat748

itiswithoutthecontributionoftheseindividualsandIthankthemfortheirinput.749

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