2
Assessing the greenhouse impact of natural gas 3
L.M.Cathles,June6,2012
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Abstract 1
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Theglobalwarmingimpactofsubstitutingnaturalgasforcoalandoiliscurrentlyin
debate.Weaddressthisquestionherebycomparingthereductionofgreenhousewarming
thatwouldresultfromsubstitutinggasforcoalandsomeoiltothereductionwhichcould
beachievedbyinsteadsubstitutingzerocarbonenergysources.Weshowthatsubstitution
ofnaturalgasreducesglobalwarmingby40%ofthatwhichcouldbeattainedbythe
substitutionofzerocarbonenergysources.Atmethaneleakageratesthatare~1%of
production,whichissimilartotoday’sprobableleakagerateof~1.5%ofproduction,the
40%benefitisrealizedasgassubstitutionoccurs.Forshorttransitionstheleakagerate
mustbemorethan10to15%ofproductionforgassubstitutionnottoreducewarming,
andforlongertransitionstheleakagemustbemuchgreater.Buteveniftheleakagewasso
highthatthesubstitutionwasnotofimmediatebenefit,the40%‐of‐zero‐carbonbenefit
wouldberealizedshortlyaftermethaneemissionsceasedbecausemethaneisremoved
quicklyformtheatmospherewhereasCO2isnot.Thebenefitsofsubstitutionare
unaffectedbyheatexchangetotheocean.CO2emissionsarethekeytoanthropogenic
climatechange,andsubstitutinggasreducesthemby40%ofthatpossiblebyconversionto
zerocarbonenergysources.Gassubstitutionalsoreducestherateatwhichzerocarbon
energysourcesmustbeeventuallyintroduced.
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Introduction 23
Inarecentcontroversialpaper,Howarthetal.(2011)suggestedthat,becausemethaneisa
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farmorepotentgreenhousegasthancarbondioxide,theleakageofnaturalgasmakesits
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greenhouseforcingasbadandpossiblytwiceasbadascoal,andtheyconcludedthatthis
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underminesthepotentialbenefitofnaturalgasasatransitionfueltolowcarbonenergy
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sources.Others(Hayhoeetal.,2009;Wigley,2011)havepointedoutthatthewarming
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causedbyreducedSO2emissionsascoalelectricalfacilitiesareretiredwillcompromise
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someofthebenefitsoftheCO2reduction.Wigley(2011)hassuggestedthatbecausethe
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impactofgassubstitutionforcoalonglobaltemperaturesissmallandtherewouldbe
1
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somewarmingasSO2emissionsarereduced,thedecisionoffueluseshouldbebasedon
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resourceavailabilityandeconomics,notgreenhousegasconsiderations.
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Someofthesesuggestionshavebeenchallenged.ForexampleCathlesetal.(2012)have
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takenissuewithHowarthetal.forcomparinggasandcoalintermsoftheheatcontentof
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thefuelsratherthantheirelectricitygeneratingcapacity(coalisusedonlytogenerate
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electricity),forexaggeratingthemethaneleakagebyafactorof3.6,andforusingan
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inappropriatelyshort(20year)globalwarmingpotentialfactor(GWP).Neverthelessit
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remainsdifficulttoseeinthepublishedliteraturepreciselywhatbenefitmightberealized
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bysubstitutinggasforcoalandtheuseofmetricssuchasGWPfactorsseemstocomplicate
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ratherthansimplifytheanalysis.Thispaperseekstoremedythesedeficienciesby
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comparingthebenefitsofnaturalgassubstitutiontothoseofimmediatelysubstituting
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low‐carbonenergysources.Thecomparativeanalysisgoesbacktothefundamental
43
equationanddoesnotusesimplifiedGWPmetrics.Becauseitisanullanalysisitavoids
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thecomplicationsofSO2,carbonblack,andthecomplexitiesofCO2removalfromthe
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atmosphere.Itshowsthatthesubstitutionofnaturalgasforcoalandsomeoilwould
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realize~40%ofthegreenhousebenefitsthatcouldbehadbyreplacingfossilfuelswith
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lowcarbonenergysourcessuchaswind,solar,andnuclear.Inthelongtermthisgas
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substitutionbenefitdoesnotdependonthespeedofthetransitionorthemethaneleakage
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rate.Ifthetransitionisfaster,greenhousewarmingisless.Iftheleakageisless,the
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reductionofwarmingduringthesubstitutionperiodisgreater,butregardlessoftherateof
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leakageorthespeedofsubstitution,naturalgasachieves~40%ofthebenefitsoflow
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carbonenergysubstitutionafewdecadesaftermethaneemissionsassociatedwithgas
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productioncease.Thebenefitofnaturalgassubstitutionisadirectresultofthedecrease
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inCO2emissionsitcauses.
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ThecalculationmethodsusedherefollowWigley(2011),butarecomputedusing
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programsofourowndesignfromtheequationsandparametersgivenbelow.Parameters
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aredefinedthatconvertscenariosfortheyearlyconsumptionofthefossilfuelstothe
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yearlyproductionofCO2andCH4.Thesegreenhousegasesarethenintroducedintothe
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atmosphereandremovedusingacceptedequations.Radiativeforcingsarecalculatedfor
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thevolumetricgasconcentrationsastheyincrease,theequilibriumglobaltemperature
2
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changeiscomputedbymultiplyingthesumoftheseforcingsbytheequilibriumsensitivity
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factorcurrentlyfavoredbytheIPCC,andtheincrementsofequilibriumtemperature
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changeareconvertedtotransienttemperaturechangesusingatwolayeroceanthermal
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mixingmodel.
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Emission Scenarios 66
GreenhousewarmingisdrivenbytheincreaseintheatmosphericlevelsofCO2,CH4and
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othergreehousegasesthatresultfromtheburningoffossilfuels.Between1970and2002,
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worldenergyconsumptionfromallsources(coal,gas,oil,nuclear,hydroandrenewables)
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increasedattherateof2.1%peryear.Intheyear2005sixandahalfbillionpeople
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consumed~440EJ(EJ=exajoules=1018joules,1joule=1.055Btu;EIA,2011)ofenergy.
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Oilandgassupplied110EJeach,coal165EJ,andothersources(hydro,nuclear,and
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renewablessuchawindandsolar)55EJ(MiniCAMscenario,Clark,2007).In2100the
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worldpopulationisprojectedtoplateauat~10.5billion.Iftheperpersonconsumption
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thenisattoday’sEuropeanaverageof~7kWp‐1,globalenergyconsumptionin2100
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wouldbe2300EJperyear(74TW).Westartwiththefuelconsumptionpatternat2005
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ADandgrowitexponentiallysothatitreaches2300EJperyearattheendofa“transition”
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period.Attheendofthetransitiontheenergyissuppliedalmostentirelybylowcarbon
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sourcesinallcases,butinthefirsthalfofthetransition,whichwecallthegrowthperiod,
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hydrocarbonconsumptioneitherincreasesonthecurrenttrajectory(the“business‐as‐
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usual”scenario),increasesatthesameequivalentratewithgassubstitutedforcoalandoil
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(a“substitute‐gasscenario),ordeclinesimmediately(thelow‐carbon‐fastscenario).Coal
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useisphasedoutatexactlythesamerateinthesubstitute‐gasandlow‐carbon‐fast
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scenarios,sothatthereductionofSO2andcarbonblackemissionsisexactlythesamein
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thesetwoscenariosandthereforisnotafactorwhenwecomparethereductionin
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greenhousewarmingforthesubstitute‐gasandthelow‐carbon‐fastscenarios.
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Figure1showsthethreefuelscenariosconsideredfora100yeartransition:

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Infirsthalf(growthperiod)ofthebusiness‐as‐usualscenario(AinFigure1),fossil
fuelconsumptionincreases2.9foldfrom440EJ/yrin2005to1265EJ/yroverthe
3
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50yeargrowthperiod,andthendeclinesto205.6EJ/yrafterthefulltransition.The
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mixofhydrocarbonsconsumedattheendofthetransitionproducesCO2emissions
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atthesame4.13GtC/yrrateasattheendoftheotherscenarios.Thetotalenergy
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consumptiongrowsat2.13%peryearinthegrowthperiod,andat1.2%overthe
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declineperiod.Thegrowthperiodisashifted(tostartin2005),slightlysimplified,
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exponentialversionoftheMiniCAMscenarioinClark(2007).Weincreasethe
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hydrocarbonconsumptionbythesamefactorsasintheMiniCAMscenario,and
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determinetherenewablegrowthbysubtractingthehydrocarbonenergy
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consumptionfromthistotal.Thegrowth‐declinecombinationissimilartothebase
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scenariousedbyWigley(2011).

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Inthesubstitute‐gasscenario(BinFigure1),gasreplacescoalandnewoil
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consumptionoverthegrowthperiod,andisreplacedbylowcarbonfuelsinthe
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declineperiod.Gasreplacescoalonanequalelectricity‐generationbasis
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(∆Hgas=∆HcoalRcoal/Rgas=234EJy‐1,seeTable1),andgasreplacesnewoil(165EJy‐1)
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onanequalheatcontentbasis.Gasuseattheendofthegrowthperiodisthus729
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EJy‐1,ratherthan330EJy‐1inthebusiness‐as‐usualscenario.Thegrowthof
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renewableenergyconsumptionisgreaterthanin(A).Overtheensuingdecline
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period,oilconsumptiondropsto75EJy‐1andgasto175EJy‐1.

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Inthelow‐carbon‐fastscenario(CinFigure1),lowcarbonenergysourcesreplace
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coal,newgas,andnewoiloverthegrowthperiod,andgasusegrowsandoiluse
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decreasessothattheconsumptionattheendisthesameasinthesubstitute‐gas
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scenario.
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Thesescenariosareintendedtoprovideasimplebasisforassessingthebenefitsof
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substitutinggasforcoal;theyareintendedtobeinstructiveandrealisticenoughtobe
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relevanttofuturesocietaldecisions.Thequestiontheyposeis:Howfarwillsubstituting
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gasforcoalandsomeoiltakeustowardthegreenhousebenefitsofanimmediateandrapid
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conversiontolowcarbonenergysources.
4
2500
A. Business as Usual
2000
1500
165
1000
55
500
330
110
110
55.6
75
75
330
0 165
2500
EJ per year
2094.4
440
d
erio
P
e
clin
e
D
B. Subst i ute Gas
2000
1500
1000
w
Gro
500
riod 371
e
P
th
2050
729
175
165
0
2500
1500
75
2300 EJ
C. Low Carbon Fast
2000
no C
Coal
Gas
Oil
1265 EJ
2050
1000
990
440 EJ
500
0
0
175
75
75
100
years af er 2005
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117
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120
121
122
123
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125
25
110
165
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Figure1Threefuelconsumptionscenarioscomparedinthispaper:(A)Fossilfueluseinthebusiness‐as‐usual
scenariocontinuesthepresentgrowthinfossilfuelconsumptionintheinitial50yeargrowthperiodbeforelow
carbonenergysourcesreplacefossilfuelsinthedeclineperiod.(B)Inthesubstitute‐gasscenario,gasreplaces
coalsuchthatthesameamountofelectricityisgenerated,andsubstitutesfornewoilonanequalheatenergy
basis.(C)Inthelow‐carbon‐fastscenario,lowcarbonenergysourcesimmediatelysubstituteforcoalandnewoil
andgasinthegrowthperiod,andgasusedeclinesandsubstitutesforoilinthedeclineperiod.Numbersindicate
theconsumptionofthefuelsinEJperyearatthestart,midpoint,andendofthetransitionperiod.Thetotal
energyuseisthesameinallscenariosandisindicatedatthestart,midpoint,andendbytheboldblacknumbers
in(C).
5
126
127
Table1.Parametersusedinthecalculations.Iistheenergycontentofthefuel,Rtheefficiencyofconversionto
electricity,andandthecarbonandmethaneemissionsfactors.Seetextfordiscussion.
I[EJGt‐1]
R[EJeEJ‐1]
ξ[GtCEJ‐1] ζ[GtCH4EJ‐1]
Gas
55
0.6
0.015
1.8x10‐4foraleakageof1%ofproduction
Oil
43
0.020
0.32
0.027
1.2x10‐4for5m3/t
Coal 29
128
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Computation Method and Parameters 130
Table1summarizestheparametersusedinthecalculations.I[EJGt‐1],givestheheat
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energyproducedwheneachfossilfuelisburnedinexajoules(1018joules)pergigaton(109
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tons)ofthefuel.Thevaluesweusearefromhttp://www.natural‐
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gas.com.au/about/references.html.Theenergydensityofcoalvariesfrom25‐37GJ/t,
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dependingontherankofthecoal,but29GJ/tisconsideredagoodaveragevaluefor
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calculations.
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R[EJeEJ‐1]istheefficiencywithwhichgasandcoalcanbeconvertedtoelectricityin
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exajoulesofelectricalenergyperexajouleofheat.Gascangenerateelectricitywithmuch
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greaterefficiencythancoalbecauseitcandriveagasturbinewhoseeffluentheatcanthen
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beusedtodriveasteamgenerator.Lookingforward,olderlowefficiencycoalplantswill
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likelybereplacedbyhigherefficiencycombinedcyclegasplantsofthiskind.Theelectrical
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conversionefficienciesweadoptinTable1arethoseselectedbyHayhoeetal.(2002,their
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TableII).
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Thecarbonemissionfactorsingigatonsofcarbonreleasedtotheatmosphereperexajoule
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ofcombustionheat,ξ[GtCEJ‐1],listedinthefourthcolumnofTable1arethefactors
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compiledbytheEPA(2005)andusedbyWigley(2011).
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Finally,themethaneemissionfactors,ζ[GtCH4EJ‐1]inthelastcolumnofTable1are
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computedfromthefractionofmethanethatleaksduringtheproductionanddeliveryof
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naturalgasandthevolumeofmethanethatisreleasedtotheatmosphereduringmining
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andtransportofcoal:
6
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 gas [Gt CH4 EJ -1 ]  L[Gt CH4-vented Gt -1CH4-burned ] I [EJ Gt -1CH4-burned ] 151
-1
-3
-1
 coal [Gt CH4 EJ -1 ]  V [m 3CH4 t coal
- mined ] CH4 [t CH 4 m CH4 ] I [ EJ Gt coal-burned ] 152
Thedensityofmethanein(1b)CH4=0.71x10‐3tonsperm3.Wetreatthemethanevented
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totheatmosphereduringtheproductionanddistributionofnaturalgas,L,parametrically
154
inourcalculations.Thenaturalgasleakage,L,isdefinedasthemassfractionofnaturalgas
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thatisburned.
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Weassumeinourcalculationsthat5m3ofmethaneisreleasedpertonofcoalmined.The
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leakageofmethaneduringcoalmininghasbeenreviewedindetailbyHowarthetal.
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(2011)andWigley(2011).Combiningleakagesfromsurfaceanddeepmininginthe
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proportionsthatcoalisextractedinthesetwoprocesses,theyarriveat6.26m3/tand4.88
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m3/trespectively.Thevalueweuseliesbetweenthesetwoestimates,andappearstobea
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reasonableestimate(e.g.,seeSaghafietal.,1997),althoughsomehaveestimatedmuch
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highervalues(e.g,Hayhoeetal.,2002,suggest~23m3/t).
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TheyearlydischargeofCO2(measuredintonsofcarbon)andCH4totheatmosphere,
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QC[GtCy‐1]andQCH4[GtCH4y‐1],arerelatedtotheheatproducedinburningthefuels,H[EJy‐
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1]inFigure1:
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QC [Gt C y -1 ]  H [EJ y -1 ] [Gt C EJ -1 ] (2a)
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QCH 4 [Gt C y -1 ]  H [ EJ y -1 ]  [Gt CH4 EJ -1 ] (2b)
(1a)
.
(1b)
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169
170
ThevolumefractionsofCO2andCH4addedtotheatmosphereinyeartiby(1)are:
X CO 2 t i [ ppmv y 1 ] 
X CH 4 t i [ ppbv y 1 ] 
WCO 2 Wair VCO 2
WC WCO 2 Vair
M atm t 
QC [Gt C y -1 ]1015
QCH 4 [Gt CH4 y -1 ]1018
M atm t 
Wair VCH 4
WCH 4 Vair
7
.
(3a)
(3b)
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HereMatm[t]=5.3x1015tonsisthemassoftheatmosphere,WCO2isthemolecularweightof
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CO2(44g/mole),andVCO2isthemolarvolumeofCO2,etc.In(2a)thefirstmolecularweight
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ratioconvertstheyearlymassadditionofcarbontotheyearlymassadditionofCO2,and
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thesecondmassfractionratioconvertsthistothevolumefractionofCO2inthe
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atmosphere.WeassumethegasesareidealandthusVCO2=Vair.
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Eachyearlyinputofcarbondioxideandmethaneisassumedtodecaywithtimeasfollows:
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X CO 2 t i  t   X CO 2 t i  f CO 2 t 
f CO 2 t   0.217  0.259 e
t
172.9
 0.338 e
X CH 4 t i  t   X CH 4 t i  f CH 4 t 
f CH 4 t   e
t
, t
18.51
 0.186 e
t
(4a)
1.186
(4b)
12
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wheretistimeinyearsaftertheinputofayearlyincrementofgasatti.Thesedecayrates
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arethoseassumedbytheIPCC(2007,Table2.14).The12yeardecaytimeformethanein
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(4b)isaperturbationlifetimethattakesintoaccountchemicalreactionsthatincrease
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methane’slifetimeaccordingtotheIPCC(2007,§2.10.3.1).ThedecayofCO2describedby
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(4a)doesnotaccountforchangeswithtimeinthecarbonate‐bicarbonateequilibrium
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(suchasdecreasingCO2solubilityasthetemperatureoftheoceansurfacewaters
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increases)whichbecomeimportantathigherconcentrationsofatmosphericCO2(seeNRC,
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2011;Ebyetal.,2009).Equation(4a)thusprobablyunderstatestheamountofCO2that
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willberetainedintheatmospherewhenwarminghasbecomesubstantial.
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Theconcentrationofcarbondioxideandmethaneintheatmosphereasafunctionoftimeis
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computedbysummingtheadditionseachyearandthedecayedcontributionsfromthe
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additionsinpreviousyears:
i 1
X CO 2 t i   X CO 2 t i    X CO 2 t j  f CO 2 t i  t j 
j 1
191
i 1
X CH 4 t i   X CH 4 t i    X CH 4 t j  f CH 4 t i  t j 
, j 1
8
(5)
192
where X CO2 ti  and X CH 4 t i  arevolumetricconcentrationofCO2andCH4inppmvandppbv
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respectively,irunsfrom1tottotwherettotisthedurationofthetransitioninyears,andthe
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sumtermsontherighthandsidesdoesnotcontributeunlessi≥2.
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Theradiativeforcingsforcarbondioxideandmethane,∆FCO2[Wm‐2]and∆FCO2[Wm‐2]are
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computedusingthefollowingformulaegivenintheIPCC(2001,§6.3.5):




FCO 2 W m  2  5.35 ln
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X CO 2 ti   X CO 2 t  0 
X CO 2 t  0 
FCH 4 W m  2  0.036 CH 4




X CH 4 ti   X CH 4 0   X CH 4 0    f  X CH 4 ti   X CH 4 0 , N o   f  X CH 4 0 , N o 


f M , N   0.47 ln 1  2.01  10  5 MN   5.31 MN 15  M  NM 
5
1.52

(6)
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Westartourcalculationswiththeatmosphericconditionsin2005:XCO2[t=0]=379ppmv,
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XCH4[t=0]=1774ppbv,andtheN2Oconcentration,No=319ppbv.ψCH4isafactorthat
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magnifiesthedirectforcingofCH4totakeintoaccounttheindirectinteractionscausedby
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increasesinatmosphericmethane.TheIPCC(2007)suggeststheseindirectinteractions
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increasethedirectforcingfirstby15%andthenbyanadditional25%,withtheresultthat
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ψCH4=1.43.Shindelletal.(2009)havesuggestedadditionalindirectinteractionswhich
204
increaseψCH4to~1.94.ThereiscontinuingdiscussionofthevalidityofShindelletal.’s
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suggestedadditionalincrease(seeHultmanetal.,2011).WegenerallyuseψCH4=1.43in
206
ourcalculations,butconsidertheimpactofψCH4to~1.94whereitcouldbeimportant.
207
Theradiativeforcingofthegreenhousegasadditionsin(6)drivesglobaltemperature
208
change.Theultimatechangeinglobaltemperaturetheycauseis:
209
T equil  TCO 2  TCH 4  S1 FCO 2  FCH 4  ,
210
where S1 istheequilibriumclimatesensitivity.WeadopttheIPCC,2007value  S1  0.8 ,
211
whichisequivalenttoassumingthatadoublingofatmosphericCO2[ppmv]causesa3°C
212
globaltemperatureincrease.
213
Theheatcapacityoftheoceandelaysthesurfacetemperatureresponsetogreenhouse
214
forcing.Assuming,followingSolomonetal(2011),atwolayeroceanwherethemixed
215
layerisinthermalequilibriumwiththeatmosphere:
9
(7)
216
Tmix
equil
  s Tmix
 Tmix   Tmix  Tdeep 
t
.
Tmix
C deep
  Tmix  Tdeep 
t

C mix

(8)
217
Hereistheheattransfercoefficientfortheflowofheatfromthemixedlayerintothedeep
218
layerinWK‐1m‐2,andsistheheattransfercoefficientintothemixedlayerfromthe
219
atmosphere(andtheinverseoftheequilibriumclimatesensitivity).CmixandCdeeparethe
220
heatstoragecapacitiesperunitsurfaceareaofthemixedanddeeplayersinJK‐1m‐2.
221
equil
equil
  Tmix
  Tmix
 Tdeep , t  t  mix ,and  mix  C mix s 1 ,wecan
Defining Tmix
 Tmix , Tdeep
222
write:
223

t
224
Fortheimpositionofasuddenincreaseingreenhouseforcingthatwillultimatelyproduce
225
equil
asdescribedby(7),thesolutionto(8)is:
anequilibriumtemperaturechangeof Tmix
226


 


equil 
Tmix  Tmix
1   a exp  t e 1   1  a  exp  t e 1   . m mix 
d mix   


 
227
Hereemandedarethemagnitudesoftheeigenvaluesofthematrixin(9),andthe
228
coefficient,a,isdeterminedbytheinitialconditionthatthelayersarenotthermally
229
perturbedbeforetheincrementofgreenhouseforcingisimposed.
230
Insightisprovidedbynotingthattheeigenvaluesandparameterain(10)arefunctionsof
231
1
theratiosofheattransferandheatstorageparameters s 1 and C deep C mix
only,andcanbe
232
approximatedtowithin±10%:


    s 1  1
 Tmix

 
 T     1 C C 1
deep

  s mix deep

 
 Tmix


 .
1 


T

 s 1 C mix C deep
deep


s 1
(9)
(10)

a  0.483  0.344 1  s 1 , 0.2  s 1  1
233

em1  1  
ed1 

1 1
s
1
2C deep C mix
.
 
1 0.7
s
10
(11)
234
Itisunlikelythatthatheatwillbetransferredoutthebaseofthemixedlayermore
235
efficientlythanitisintothetopofthemixedlayerbecausethetransferwillbemostly
236
drivenbywindsandcoolingoftheoceansurface.Forthisreasontheheattransfer
237
coefficientratio s 1 isalmostcertainly≤1andthereductionoftemperatureisgreatestfor
238
s 1  1 .For s 1  1 ,theinitialtemperaturechangeinthemixedlayerwillbeabouthalf
239
thechangethatwilloccurwhentheoceanlayersarefullywarmed,andtheresponsetime
240
requiredtoreachthisequilibriumchange(thetimerequiredtoreach2/3rdsofthe
241
1
equilibriumvalue)willbeabout½oftheresponsetimeofthemixedlayer(e.g., emix
 1 ).
2
242
For s 1  1 ,theresponsetimeofthedeeplayeristwicetheheatstoragecapacityratio
243
1
 mix .
timestheresponsetimeofthemixedlayer: 2C deep C mix
244
Thetransienttemperaturechangecanbecomputedfromtheequilibriumtemperature
245
changein(7)byconvolvinginafashionsimilartowhatwasdonein(5):
246
i 1
 
  t i  t j  
  t  t j   
  1  a  exp 1i
  , T t i    T equil t j 1   a exp 1

 
j 1
 e m  mix 
 ed  mix  
(12)
247
wherei≥j.Wedonotusetheapproximationsofequation(11)whenwecarryoutthe
248
convolutionin(12).Ratherwesolvefortheactualvaluesoftheeigenvaluesand
249
parameterafromthematrixin(9)ateachyearlyincrementintemperaturechange.For
250
mix=5years,Tmixwillreach0.483Tmixequilwithadecaytimeof2.5yearsandriseto
251
Tmixequilwithadecaytimeof200years.
252
Thecurrentconsensusseemstobethat s 1  1 andthetransientthermalresponseis
253
abouthalfthefullequilibriumforcingvalue(NRC,2011,§3.3).Theratiooftheheatstorage
254
1
capacityofthedeeptomixedlayer, C deep C mix
isprobablyatleast20,avalueadoptedby
255
Solomonetal.(2011).Schwartz(2007)estimatedthethermalresponsetimeofthemixed
256
layerat~5yearsfromthetemporalautocorrelationofseasurfacetemperatures.Thismay
257
bethebestestimateofthisparameter,butSchwartznotesthatestimatesrangefrom2to
258
30years.Fortunatelythemoderationoftemperaturechangebytheoceansdoesnot
11
259
impactthebenefitofsubstitutinggasforcoalandoilatall.Itisofinterestindefiningthe
260
coolingthatsubstitutionwouldproduce,however.Wecalculatethetransienttemperature
261
changesforthefullrangeofoceanmoderationparameters.
262
Equations(1)to(10)plus(12),togetherwiththeparametersjustdiscusseddefine
263
completelythemethodsweusetocalculatetheglobalwarmingcausedbythefueluse
264
scenariosinFigure1.
200 year
transition
Carbon Dioxide [ppmv]
500
A
Concentration Change
[ppmv carbon dioxide]
Business as usual 434 ppmv
100 year
transition
50 year
transition
Substitute gas
327
st
Low carbon fa
190
258
194
154
115
66
113
0
0
100
Methane [ppbv]
Concentration Change
[ppbv methane]
500
428 ppbv
359
470 ppbv
404
368
397
108
200
115
Su
bs
tit
Bu ute
sin ga
s
es
sa
su
su
al
st
Low carbon fa
B
120
0
0
200
time [years] since 2005
265
266
267
268
269
100
Figure2Changesin(A)carbondioxideand(B)methaneconcentrationscomputedforthethreefuelscenarios
showninFigure1andthreedifferenttransitionintervals(50100and200years).Inthisandsubsequentfigures
thebluecurvesindicatethebusiness‐as‐usualfuelusescenario,thegreencurvesindicatethesubstitute‐gas
scenario,andtheredcurvesthelow‐carbon‐fastscenario.Thenumbersindicatethechangeinconcentrationsof
12
270
271
CO2andmethanefromthe379ppmvforCO2and1774ppbvforCH4levelspresentintheatmospherein2005.
272
Results 273
Figure2showstheadditionsofCO2inppmvandmethaneinppbvthatoccurforthe
274
differentfuelconsumptionscenariosshowinFigure1forthethreetransitionperiods(50,
275
100and200years).Themethaneleakageisassumedtobe1%ofconsumption.Fivecubic
276
metersofmethaneareassumedtoleaktotheatmosphereforeachtonofcoalmined.The
277
atmosphericmethaneconcentrationstrackthepatternofmethanereleasequiteclosely
278
becausemethaneisremovedquicklyfromtheatmospherewithanexponentiallydecay
279
constantof12years(equation4b).Ontheotherhand,becauseonlyaportionoftheCO2
280
introducedintotheatmospherebyfuelcombustionisremovedquickly(seeequation4a),
281
CO2accumulatesacrossthetransitionperiodsand,aswewillshowbelow,persistsfora
282
longtimethereafter.
Radiative Forcing [W m-2]
ThecalculationisbasedonL=1%ofgasconsumptionandV=5m3methanepertonofcoalburned.
5
business as usual
Carbon Dioxide
substitute gas
41%
n
low carbo
40%
fast
42%
Methane
0
0
200
time [years]
283
284
285
286
287
288
100
Figure3RadiativeforcingscalculatedforthecarbondioxideandmethaneadditionsshowninFigure2using
equation(6)andassumingΨCH4=1.43.Thebluecurvesindicatethebusinessasusualscenarioforthe50,100
and200yeartransitionperiods,thegreenthesubstitute‐gasscenario,andtheredthelow‐carbon‐fastscenario.
ThenumbersindicatethereductioninCO2forcingachievedbysubstitutinggas,expressedasapercentageofthe
reductionachievedbythelow‐carbon‐fastscenario.
13
289
Figure3showstheradiativeforcingscorrespondingtotheatmosphericgasconcentrations
290
showninFigure2usingequation(6).ThemethaneforcingisafewpercentoftheCO2
291
forcing,andthusisunimportantindrivinggreenhousewarmingforagasleakagerateof
292
1%.
293
Figure4showstheglobalwarmingpredictedfromtheradiativeforcingsinFigure3for
294
variousdegreesofheatlosstotheocean.Wetaketheequilibriumclimatesensitivity  s 1 =
295
0.8(e.g.,adoublingofCO2causesa3°Cofglobalwarming).Thefastertransitionsproduce
296
lessglobalwarmingbecausetheyputlessCO2intotheatmosphere.Thethermal
297
modulationoftheoceanscanreducethewarmingbyuptoafactoroftwo.Forexample,
298
Figure4Ashowstheglobalwarmingthatwouldresultfromthebusiness‐as‐usualscenario
299
iftherewerenoheatlossestotheoceanrangesfrom1.5°Cforthe50yeartransitionto
300
3.3°Cforthe200yeartransition.Figure4Cindicatesthatheatexchangetotheoceans
301
couldreducethiswarmingbyafactoroftwoforthelongtransitionsandthreeforthe50
302
yeartransition.Awarmingreductionthislargeisunlikelybecauseitassumesextreme
303
parametervalues:adeepoceanlayerwithaheatstorage50timestheshallowmixedlayer,
304
andalongmixingtimefortheshallowlayer(mix=50years).Figure4Bindicatesthemore
305
likelyoceantemperaturechangemoderationbasedonmid‐rangedeeplayerstorage
306
1
( C deep C mix
=20)andmixedlayerresponsetime(mix=5years)parametervalues.
307
Theimportantmessageofthisfigureforthepurposesofthispaper,however,isnotthe
308
amountofwarmingthatmightbeproducedbythevariousfuelscenariosofFigure1,but
309
theindicationthatthereductioningreenhousewarmingfromsubstitutinggasforcoaland
310
oilisnotsignificantlyaffectedbyheatexchangewiththeoceanorbythedurationofthe
311
transitionperiod.Thesamepercentreductioninglobalwarmingfromsubstitutinggasfor
312
coalandoilisrealizedregardlessofthedurationofthetransitionperiodorthedegreeof
313
thermalmoderationbytheocean.Thebenefitofsubstitutinggasisapercentorsolessfor
314
theshorttransitions,andtheoceanmoderationreducesthebenefitbyapercentorso,but
315
thebenefitinallcircumstancesremains~38%.Heatlossintotheoceansmayreducethe
316
warmingbyafactoroftwo,butthebenefitofsubstitutinggasisnotsignificantlyaffected.
14
317
4
1% gas leakage
  
no ocean mixing
3
business as usual
3.3 C
A.
substitute gas
2.7 C
39%
2.3 C
2
n fast 1.8 C
low carbo
38%
1
Temperature Change [C]
39%
1.5 C
0
3
 Cd/Cm=20, mix=5 yrs
B.
2.4 C
2.0 C 38%
2
1.4 C
1
37%
0
3
1.3 C
38%
0.8 C
 Cd/Cm=50, mix=50 yrs
C.
2
1.7 C
1.4 C 38%
1.1 C
1
0.5 C
0.9 C
37%
36%
0
0
100
150
time [years]
318
319
320
321
322
323
324
325
326
327
50
200
Figure4.GlobalwarmingproducedbytheforcingsinFigure3computedusingequations(7,10,and12).The
bluecurvesindicatetemperaturechangesunderthebusiness‐as‐usualscenariofor50,100and200year
transitiondurations,andthegreenandredcurvesindicatethetemperaturechangesforthesubstitute‐gasand
low‐carbon‐fastscenarios.Thecolorednumbersindicatethetemperaturechanges,andtheblacknumbersthe
reductionintemperatureachievedbythesubstitute‐gasscenarioexpressedasafractionofthetemperature
reductionachievedbythelow‐carbon‐fastscenario.(A)Thewarmingwhenthereisnothermalinteractionwith
theocean(ortheoceanlayersthermallyequilibrateveryquickly).(B)Warmingunderalikelyoceaninteraction.
(C)Warmingwithaveryhighoceanthermalinteraction.Theoceanmixingparametersareindicatedin(B)and
(C).Allcalculationsassumegasleakageis1%ofconsumptionandtheIPCCmethaneclimatesensitivity.
15
328
Figure5comparesthemethaneforcingofthesubstitute‐gasscenariototheCO2forcingof
329
thebusiness‐as‐usualscenarioforthe50and100yeartransitiondurations.Theforcing
330
forthe1%methanecurvesarethesameasinFigure3,butiscontinuedoutto200years
331
assumingthefueluseremainsthesameasattheendoftheofthetransitionperiod.
332
Similarlythebusiness‐as‐usualcurveisthesameasinFigure3continuedoutto200years.
333
Thefigureshowsthatthemethaneforcingincreasesasthepercentmethaneleakage
334
increases,andbecomesequaltotheCO2forcinginthebusiness‐asusualscenariowhenthe
335
leakageis~15%ofconsumptionforthe50yeartransitionand30%ofconsumptionforthe
336
100yeartransition.Attheendofthetransitionthemethaneradiativeforcingsfalltothe
337
levelthatcanbesteadilymaintainedbytheconstantmethaneleakageassociatedwiththe
338
smallcontinuednaturalgasconsumption.TheCO2forcingunderthebusiness‐as‐usual
339
scenariofallabitandthenriseataslowsteadyrate,reflectingtheproscriptionthat26%of
340
theCO2releasedtotheatmosphereisonlyveryslowlyremovedand22%isnotremovedat
341
all(equation3a).ThisslowriseemphasizesthatevenverylowreleasesofCO2canbeof
342
concern.Themethaneintheatmospherewouldrapidlydisappearinafewdecadesifthe
343
methaneventingwerestopped,whereastheCO2curveswouldflattenbutnotdrop
344
significantly.Finally,Figure5Ashowsthatthegreatermethaneclimatesensitivity
345
proposedbyShindelletal(2009)(CH4=1.94)wouldmakea10%methaneventing
346
equivalenttoa15%ventingwithCH4=1.43(theIPCCmethaneclimatesensitivity).
16
A.
50 year transition
2.5
20%
2.0
Leakage rate
[% of consumption]
ss a
Busine
15%
1.5
  

10%
Substitute Gas
  
1.0
Radiative Forcing [W m-2]
l
s Usua
5%
0.5
1%
0
100 year transition
30%
Business as Usual
25%
3.0
B.
20%
2.5
15%
2.0
10%
Substitute Gas
1.5
5%
1.0
0.5
1%
0
0
50
time [years]
347
348
349
350
351
352
353
354
355
356
150
100
200
Figure5.RadiativeforcingsofCO2forthebusinessasusualscenario(bluecurves)andforCH4forvariousgas
leakageratesinthesubstitute‐gasscenario(greencurves).The1%methanecurvesandthebusinessasusual
curvesarethesameasinFigure3excepttheverticalscaleisexpandedandthecurvesareextendedfromtheend
ofthetransitionto200yearsassumingthegasemissionsarethesameasattheendofthetransitionpast100
years.Themethaneforcingsplateauatthelevelscorrespondingtotheatmosphericconcentrationsupportedby
thesteadyCH4emissions.TheCO2forcingincreasesbecauseanappreciablefractionoftheCO2emissionsare
removedslowlyornotatallfromtheatmosphere.ThemethaneforcingsallassumetheIPCCmethaneclimate
sensitivity(CH4=1.43)exceptthesingleredcurve,whichassumesthemethaneclimatesensitivitysuggestedby
Shindelletal.(2009)(CH4=1.94).
17
  
 Cd/Cm=20, mix=5 yrs
C. 200 yr Transition
3.48 C
B. 100yr Transition
2.05 C
35%
Substit
ute Ga
s
Busine
Ca
ss as U
rrb
sual
on
Fa
st
A. 50yr Transition
1%
20%
1.30 C
-6%
15%
Lo
w
1.84 C
1.19 C
17%
5%
-1%
2.69 C
10%
10%
39%
1.07 C
Temperature Change [C]
21%
5%
40%
40%
0
0
0
0
50
0
100
time [years]
357
1.5
1%
0.96 C
1%
1%
1.60 C
1.5
1.5
0
200
358
359
360
361
362
363
Figure6.Impactofmethaneleakageonglobalwarmingfortransitionperiodsof(A)50,(B)100,and(C)200
364
Figures6illustrateshowthebenefitsofsubstitutinggasforcoalandoildisappearasthe
365
methaneleakageincreasesabove1%oftotalmethaneconsumption.Thefigureshowsthe
366
globalwarmingcalculatedfortheoceanheatexchangeshowinFigure4B.Asthemethane
367
leakageincreases,thegreensubstitute‐gasscenariocurvesrisetowardandthenexceedthe
368
bluebusiness‐as‐usualcurves,andthebenefitofsubstitutinggasdisappears.Thegas
years.Astheleakagerate(greenpercentagenumbers)increase,thewarmingofthesubstitute‐gasscenario
(greencurves)increases,thebluebusiness‐as‐usualandgreensubstitute‐gascurvesapproachoneanotherand
thencross,andthepercentageofthewarmingreductionattainedbythefastsubstitutionoflowcarbonenergy
sourcesdecreaseandthenbecomenegative.Thewarmingsassumethesameexchangewiththeoceanasin
Figure4B.
18
369
leakageatwhichsubstitutinggasforoilandcoalwarmstheearthmorethanthebusiness‐
370
as‐usualscenarioissmallest(L~10%)forthe50yeartransitionperiodandlargest
371
(L~35%)forthe200yeartransitionperiod.
372
Figure7summarizeshowthebenefitofgassubstitutiondependsonthegasleakagerate.
373
FortheIPCCmethaneclimatesensitivity(CH4=1.43),thebenefitofsubstitutinggasgoesto
374
zerowhenthegasleakageis44%ofconsumption(30%ofproduction)forthe200year
375
transition,24%ofconsumption(19%ofproduction)forthe100yeartransition,and13%
376
ofconsumption(12%orproduction)forthe50yeartransition.FortheShindelletal.
377
climatesensitivitycorrespondingtoCH4=1.94,thecrossoverforthe50yeartransition
378
occursatagasleakageof~9%ofconsumption,andreasonableoceanthermalmixing
379
reducesthisslightlyto~8%ofconsumption(7.4%ofproduction).Thislastis
380
approximatelythecross‐overdiscussedbyHowarthetal.(2011and2012).Intheirpapers
381
theysuggestamethaneleakagerateashighas8%ofproductionispossible,andtherefor
382
thatnaturalgascouldbeasbad(ifcomparedonthebasisofelectricitygeneration)ortwice
383
asbad(ifcomparedonaheatcontentbasis)ascoaloverashorttransitionperiod.As
384
discussedinthenextsection,aleakagerateashighas8%isdifficulttojustify.Figure7
385
thusshowsthesignificanceofShindell’shighermethaneclimatesensitivitytoHowarth’s
386
proposition.Withoutit,anevenlessplausiblemethaneleakagerateof13%wouldbe
387
requiredtomakegasasbadortwiceasbadascoalintheshortterm.Overthelongerterm,
388
substitutionofgasisbeneficialevenathighleakagerates‐apointcompletelymissedby
389
Howarthetal.
19
Benefit of Gas Substitution
%Percent low‐C‐fast
50
1 to 2% leakage
40
30
20
10
0
0
5
10
15
Leakage [% of consumption]
390
391
392
393
394
395
396
397
398
Figure7.Thereductionofgreenhousewarmingattainedbysubstitutingnaturalgasforcoalandoil(substitute‐
399
What is the gas leakage rate 400
Themostextensivesynthesesofdataonfugitivegasesassociatedwithunconventionalgas
401
recoveryisanindustryreporttotheEPAcommissionedbyTheDevonEnergyCorporation
402
(Harrison,2012).Itdocumentsgasleakageduringthecompletionof1578unconventional
403
(shalegasortightsand)gaswellsby8differentcompanieswithareasonable
404
representationacrossthemajorunconventionalgasdevelopmentregionsoftheU.S.Three
405
percentofthewellsinthestudyventedmethanetotheatmosphere.Ofthe1578
406
unconventional(shalegasortightsand)gaswellsintheDevonstudy,1475(93.5%)were
407
greencompleted‐thatistheywereconnectedtoapipelineinthepre‐initialproduction
408
stagesotherewasnoneedforthemtobeeitherventedorflared.Ofthe6.5%ofallwells
409
thatwerenotgreencompleted,54%wereflared.Thus3%ofthe1578wellsstudied
410
ventedmethaneintotheatmosphere.
gasscenario),expressedasapercentageofthereductionattainedbyimmediatelysubstitutinglowcarbonfuels
(low‐C‐fastscenario),plottedasafunctionofthegasleakagerate.Atleakagerateslessthan~1%,thebenefitof
substitutingnaturalgasis>40%thatofimmediatelysubstitutinglowcarbonenergysources.Thebenefit
declinesmorerapidlywithleakageforshorttransitions.ThetopthreecurvesassumeanIPCCmethaneclimate
sensitivity(CH4=1.43).Thebottomtwoshowtheimpactofthegreatermethaneclimatesensitivitysuggestedby
Shindelletal(2009)(CH4=1.94).Theoceanmixingcurveaddsthesmalladditionalimpactofthermalexchange
withtheoceansattherateshowninFigure4BtotheCH4=1.94curveimmediatelyaboveit.
20
411
Thewellsthatventedmethanetotheatmospheredidsoattherateof765
412
Mcsf/completion.Themaximumgasthatcouldbeventedfromthenon‐greencompleted
413
wellswasestimatedbycalculatingthesonicventingratefromthechoke(orifice)sizeand
414
sourcegastemperatureofthewell,usingaformularecommendedbytheEPA.Sincemany
415
wellsmightventatsub‐sonicrates,whichwouldbeless,thisisanupperboundonthe
416
ventingrate.Thetotalventedvolumewasobtainedbymultiplyingthisventingratebythe
417
knowndurationofventingduringwellcompletion.Theseventedvolumesrangedfrom
418
340to1160Mscf,withanaverageof765Mscf.Theventingfromanaverage
419
unconventionalshalegaswellindicatedbytheDevonstudyisthus~23Mscf(=0.03x765
420
Mscf),whichissimilartothe18.33McfEPA(2010)estimatesisventedduringwell
421
completionofaconventionalgaswell(halfventedandhalfflared).Sinceventingduring
422
wellcompletionandworkoverconventionalgaswellsisestimatedat0.01%ofproduction
423
(e.g.,Howarthetal.,2011),thiskindofventingisinsignificantforbothunconventionaland
424
conventionalwells.
425
TheunconventionalgasleakagerateindicatedbytheDevondataisverydifferentfromthe
426
4587MscftheEPA(2010)inferredwasventedduringwellcompletionandworkoverfor
427
unconventionalgaswellsfromtheamountofgascapturedinaverylimitednumberof
428
“greencompletions”reportedtothembyindustrythroughtheirGasSTARprogram.In
429
their2010backgroundtechnicalsupportdocumenttheEPAassumedthatthiskindof
430
“green”capturewasveryrare,andthatthegaswasusuallyeitherventedorflared.
431
Assumingfurtherthatthegaswasvented50%ofthetime,theEPAconcludedthat4587
432
Mscfwasventedtotheatmosphereandthatunconventionalwellsvent250times
433
(=4587/18.3)moremethaneduringwellcompletionandworkoverthanconventionalgas
434
wells.TheEPA(2010)studyisa“BackgroundTechnicalSupportDocument”andnotan
435
officialreport.Itwasprobablyneverintendedtobemorethananoutlineofanapproach
436
andaninitialestimate,andtheEPAhassincecautionedthattheyhavenotreviewedtheir
437
analysisindetailandcontinuetobelievethatnaturalgasisbetterfortheenvironmentthan
438
coal(Fulton,2011).NeverthelesstheEPA(2010)reportsuggestedtomanythatthe
439
leakageduringwellcompletionandworkoverforunconventionalgaswellscouldbea
440
substantialpercentage(~2.5%)ofproduction,andmanyacceptedthissuggestionwithout
21
441
furthercriticalexaminationdespitethefactthatthesafetyimplicationsofthemassive
442
ventingimpliedbytheEPAnumbersshouldhaveraisedquestions(e.g.,Cathlesetal.,
443
2012a,b).
444
Onceawellisinplace,theleakageinvolvedinroutineoperationofthewellsiteandin
445
transportingthegasfromthewelltothecustomeristhesameforanunconventionalwell
446
asitisfromaconventionalwell.WhatweknowaboutthisleakageissummarizedinTable
447
2.Routinesiteleaksoccurwhenvalvesareopenedandclosed,andleakageoccurswhen
448
thegasisprocessedtoremovingwaterandinertcomponents,duringtransportationand
449
storage,andintheprocessofdistributiontocustomers.Thefirstmajorassessmentof
450
theseleakswascarriedoutbytheGasResearchInstitute(GRI)andtheEPAin1997and
451
theresultsareshowninthesecondcolumnofTable2.AppendixAofEPA(2010)givesa
452
detailedandveryspecificaccountingofleaksofmanydifferentkinds.Thesenumbersare
453
summedintothesamecategoriesanddiaplayedincolumn3ofTable2.EPA(2011)found
454
similarleakagerates(column4).Skone(2011)assessedleakagefrom6classesofgas
455
wells.WeshowhisresultsforunconventionalgaswellsintheBarnettShaleincolumn5of
456
Table2.Hisotherwellclassesaresimilar.Venkatishetal(2011)carriedoutan
457
independentassessmentthatisgivenincolumn6.Therearevariationsinthese
458
assessments,butoverallaleakageof~1.5%ofproductionissuggested.Additional
459
discussionofthisdataanditscompilationcanbefoundinCathlesetal.(2012)andCathles
460
(2012).
461
462
Table2.Leakageofnaturalgasthatiscommontobothconventionalandunconventionalgaswellsinperentof
gasproduction.
GRI‐EPA
EPA
EPA
Skone
Venkatish
(1997)
(2010)
(2011)
(2011)
eta.(2011)
Routinesiteleaks
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%
22
463
464
Basedontheabovereviewthenaturalgasleakagerateappearstobenodifferentduring
465
thedrillingandwellpreparationofunconventional(tightshalesdrilledhorizontallyand
466
hydrofractured)gaswellsthanforconventionalgaswells,andtheoverallleakagefromgas
467
wellsisprobably<1.5%ofgasproduction.Intheircontroversialpapersuggestingthatgas
468
couldbetwiceasbadacoalfromagreenhousewarmingperspective,Howarthetal(2011,
469
2012)suggestedroutinesiteleakscouldbeupto1.9%ofproduction,leakageduring
470
transportation,storage,anddistributioncouldbeupto3.6%orproduction,andgas
471
leakagefromunconventionalgaswellsduringwellcompletionandworkovercouldbe
472
1.9%ofproduction.Adding0.45%leakageforliquidunloadingandgasprocessing,the
473
suggestedgasleakagecouldbe7.9%ofproduction,enoughto“undercutthelogicofitsuse
474
asabridgingfuelinthecomingdecades,ifthegoalistoreduceglobalwarming.”
475
ThebasisgivenbyHowarthetal.(2011)fortheirmorethan5foldincreaseinleakage
476
duringtransportation,storage,anddistributionis:(a)aleakageinRussianpipelinesthat
477
occurredduringthebreakupoftheSovietUnionwhichisirrelevanttogaspipelinesinthe
478
U.S.,and(b)adebateontheaccountingofgasinTexaspipelinesthatconcernsroyalties
479
andtaxreturns(Percival,2010).Howarthetal.suggestinthisTexascasethattheindustry
480
isseekingtohidemethanelossesofmorethan5%ofthegastransmitted,butthe
481
proponentsinthearticlestate“Wedon’tthinkthey’rereallylosingthegas,wejustthink
482
they’renotpayingforit”.Intheir5foldincreaseinroutinegasleaks(fromtheaverage
483
levelinTable2of0.38%to1.9%),Howarth’setal.(2011)citeaGAOstudyofventingfrom
484
wellsinonshoreandoffshoregovernmentleasesthatdoesnotdistinguishventingfrom
485
flaring.Lackingthisdistinction,itisnotsurprisingthatitconflictsdramaticallywiththe
486
summariesinTable2.Wehavealreadydiscussedleakageduringwellcompletionand
487
workoverandnotedthattheDevondataindicateHowarthetal.’s1.9%leakageatthis
488
stageishugelyexaggerated(theDevondataindicatestheleakageis~0.01%andsimilarto
489
thatfromconventionalgaswellcompletionsandworkovers).
490
TherehavebeenanumberofpaperspublishedrecentlythatoffersupportforHowarth’s
491
highleakageestimates.Hughes(2011)re‐interpreteddatapresentedinawidely
23
492
distributedNETLpowerpointanalysisbySkone(2011).ByloweringSkone’sEstimated
493
UltimateRecoveries(EUR)fortheBarnellShalefrom3Bcfto0.84Bcfwhilekeepingthe
494
sameestimateofleakageduringwellcompletionandgasdelivery,Hughesincreased
495
Skone’sleakageestimatesfrom2to6%ofproduction‐alevelwhichfallsmidwaybetween
496
Howarth’slowandhighgasleakageestimates.Howeverleakageisafractionofwell
497
production(awellthatdoesnotproducecannotemit),andthusisitbogustoreducethe
498
EUR(thedenominator)withoutalsoreducingthenumerator(theabsoluteleakageofthe
499
well).Skone’sdatamustbeevaluatedonitsownterms,notsimplyadjustedtofitsomeone
500
else’sconclusions.
501
Petronetal.(2012)analyzedairsamplesatthe300mhighBolderAtmospheric
502
Observatory(BAO)towerwhenthewindwastowarditfromacrosstheDenver‐Julesburg
503
Basin(DJB).Gasesventingfromcondensate(condensedgasfromoilandwetgaswells)
504
stocktanksintheDJBarerichinpropanerelativetomethane,whereastherawnaturalgas
505
ventingfromgaswellsintheDJBcontainverylittlepropane.Fromtheintermediateratio
506
ofpropanetomethaneobservedattheBAOtowerandestimatesofleakagefromthestock
507
tanks,Petroneetal.calculatethattodilutethepropaneleakingfromthestocktankstothe
508
propane/methaneratioobservedatthetower,~4%ofmethaneproducedbygaswellsin
509
theDJBmustventintotheatmosphere.TheairsampledattheBAOtoweriscertainlynot
510
simplyamixofrawnaturalgasandstocktankemissionsfromtheDJBasPetronetal.
511
assume,however.IfthiswerethecasetherewouldbenooxygenintheairattheBAO
512
towerlocation.Thebackgroundatmospheremustcertainlymixinwiththesetwo(and
513
perhapsother)gassources.BackgroundairintheDenverareacontains~1800ppb
514
methaneandverylittlepropane.Mixingwiththebackgroundatmospherecoulddilutethe
515
stocktankemissionstothepropane/methaneratioobservedattheBAOtowerwithno
516
leakagefromgaswellsintheDJBrequiredatall.Contrarytotheirsuggestion,theBAO
517
towerdatareportedbyPetroneetal.placenoconstraintsatallonthegasleakageratesin
518
theDJBwhatsoever.MoredetailsareinCathles(2012).
519
Certainlythereismorewecouldlearnaboutnaturalgasleakagerates.Theissueis
520
complicatedbecausegasisusedinthetransmissionprocesssoshrinkageofproductdoes
521
notequatetoventing.Inadditionthereareconventionsandpracticesthatmakescientific
24
522
assessmentdifficult.Despitethedifficulties,however,itappearsthattheleakagerateis
523
lessthan2%ofproduction.
524
Discussion 525
WehaveverifiedourcomputationsbycomparingthemtopredictionsbyWigley’s(2011)
526
publicallyavailableandwidelyusedMAGICCprogram.Althoughtherearesomeinternal
527
differences,Table3showsthatthe~40%reductioningreenhousewarmingwepredictis
528
alsopredictedbyMAGICCwhenscenariossimilartotheoneweconsiderhereareinputto
529
bothMAGICCandourprograms.TheMAGICCcalculationsstartat1990ADsoweconsider
530
thetemperatureincreasesfrom2000totheendoftheperiod.Fueluseisincreasedand
531
reducedlinearlyratherthanexponentially,andthefueluseatthestart,midpoint,andend
532
ofthetransitionsimulationsareslightlydifferentthaninFigure1.Thetemperature
533
changesforthe200yearcycleagreeverywell.Wigley’sMAGICCtemperaturechange
534
predictionsbecomeprogressivelylowerthanoursasthetransitionintervalisshortened.
535
ThismaybebecauseMAGICCincludesasmalloceanthermalinteraction,whereasthe
536
calculationswereportinTable3donot.
537
538
539
540
541
542
Table3.TemperaturechangespredictedbyWiglely’s(2011)MAGICCprogramforlinearchangesinfueluse
similartothescenariosinFigure1comparedtoequilibrium(nooceanthermalinteraction)globalwarming
predictionsbytheprogramdescribedandusedinthispaper.Thefirstthreerowscomparethetemperature
changesofthetwoprograms.Thelastrowshowsthereductioningreenhousewarmingachievableby
substitutingnaturalgasforcoalandoilasapercentageofthereductionthatwouldbeachievedbytherapid
substitutionofallfossilfuelswithlowcarbonenergysources.
543
200yearcycle
100yearcycle
40yearcycle
Program
MAGICC
Thispaper
MAGICC
Thispaper
MAGICC
Thispaper
B‐as‐usual
3.85
3.68
2.3
2.56
1.05
1.5
Swapgas
2.85
2.85
1.65
1.94
0.80
1.12
LowCfast
1.7
1.70
0.85
1.09
0.38
0.58
%reduction
42%
42%
45%
42%
37%
41%
25
544
Incorporationoftheindirectcontributionstomethane’sradiativeforcingthroughψCH4in
545
equation(6)wasvalidatedbycomparingvaluesofGWPcomputedby(13)topublished
546
valuessummarizedinTable4.
CH 4
GWP 
547
t
FCH 4
MWCO 2  f CH 4 dt
C CH 4 [ ppbv]
t 0
t
FCO 2
MWCH 4  f CO 2 dt
C CO 2 [ ppbv]
t 0
(13)
548
GWPistherelativeglobalwarmingimpactofakgofCH4comparedtoakgofCO2addedto
549
theatmosphere,whenconsideredoveraperiodoftimet.Theradiativeforcings(∆F)are
550
definedby(6),theremovalofthegasesfromtheatmosphere(f)by(4aandb),andMWCO2
551
isthemolecularweightofCO2.TheψCH4factorof1.43inthesecondcolumnofTable4
552
combinestheindirectforcingcausedbyCH4‐inducedproductionofozone(25%according
553
toIPCC,2007)andwatervaporinthestratosphere(additional15%accordingtotheIPCC,
554
2007).WiththisfactortheGWPlistedinTable2.14oftheIPCC(2007)arereplicatedas
555
showninthesecondrowofTable4.TheψCH4factorof1.94inthesecondcolumnwas
556
determinedbyussuchthatitapproximatelypredictstheincreasedforcingssuggestedby
557
Shindelletal.(2009)asshowninthebottomrowofTable4.WedonotuseGWPsinour
558
analysisandusethemhereonlytojustifythevaluesofψCH4usedinourcalculations.
559
Table4TheGWPcalculatedfrom(6and13)forthevalueofψCH4incolumn2arecomparedtoGWP(in
560
parentheses)givenbytheIPCC(2007)andShindelletal.(2009).
ψCH4
t=20years
t=100years
t=500years
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
Directmethaneforcingfrom(6)
561
562
Themostimportantmessageofthecalculationsreportedhereisthatsubstitutingnatural
563
gasforcoalandoilisasignificantwaytoreducegreenhouseforcingregardlessofhowlong
564
(withinafeasiblerange)thesubstitutiontakes(Figure4).Formethaneleakagesof~1%of
26
565
totalconsumption,replacingcoalusedinelectricitygenerationand50%oftheoilusedin
566
transportationwithnaturalgas(veryfeasiblestepsthatcouldbedrivenbythelowcostof
567
methanealonewithnogovernmentencouragement)wouldachieve~40%ofthe
568
greenhousewarmingreductionthatcouldbeachievedbytransitioningimmediatelytolow
569
carbonenergysourcessuchaswind,nuclear,orsolar.Afastertransitiontolow‐carbon
570
energysourceswoulddecreasegreenhousewarmingfurther,butthesubstitutionof
571
naturalgasfortheotherfossilfuelsisequallybeneficialinpercentagetermsnomatterhow
572
fastthetransition.
573
Thebasisforthe~40%reductioningreenhouseforcingissimplythereductionoftheCO2
574
putintotheatmosphere.Whengasleakageislow,thecontributionofmethaneto
575
greenhousewarmingisnegligible(Figure3),andonlytheCO2inputcounts.Thereduction
576
inCO2ventedbetweenthebusiness‐as‐usualandthesubstitute‐gasscenariosis44.1%of
577
thereductionbetweenthebusiness‐as‐usualtothelow‐carbon‐fastscenarios.This
578
fractionisindependentofthetransitionperiod;itisthesamewhetherthetransition
579
occursover50yearsor200years.BecausethelossesofCO2fromtheatmosphere
580
(equation4a)areproportionaltotheamountofCO2intheatmosphere,therelative
581
amountsofCO2attheendofthetransitionaresimilartotheproportionsadded.Forthe
582
sametransitionintervalalmostthesameproportionalamountsofCO2areremovedforall
583
scenarios.Thusthefractionalsubstitute‐gasreductioninCO2intheatmosphereatthe
584
endofallthetransitionintervalsremains44.1%althoughtherearesomevariationsinthe
585
seconddecimalplace.ThecurvesshowninFigure7intersectthey‐axis(0%gasleakage)
586
atfractionsslightlydifferentfrom44.1%becausetheradiativeforcingisnon‐linearwith
587
respecttoCO2concentration(equation5a).Thelongertransitionperiodsshowlargernon‐
588
lineareffectsbecausetheyputmoreCO2intotheatmosphere.Thenearlydirect
589
relationshipbetweenreductionsinthemassofCO2ventedandthedecreaseinglobal
590
warmingisapowerfulconceptualsimplificationthatisparticularlyusefulbecauseitisso
591
easytocalculate,apointmadebyAllen(2009).
592
Theglobalwarmingreductionfromswappinggasfortheotherfossilfuelsofcourse
593
decreasesasmethaneleakageincreases.Butatlowleakagerates,thebenefitof
594
substitutingnaturalgasremainscloseto40%.Inthecontextofswappinggasforcoal,the
27
595
extramethaneemittedbylowlevelsofleakagehassuchatrivialclimateeffectthatitneed
596
notbeconsideredatall.
597
Sulfurdioxideadditionsarenotafactorinouranalysisbecausethesubstitute‐gasandlow‐
598
carbon‐fastscenariosreducetheburningofcoaloverthegrowthperiodinanidentical
599
fashion.ThusbothintroduceSO2identically,andthesmallwarmingeffectsoftheSO2,
600
whichwilloccurnomatterhowcoalisretired,cancelinthecomparison.Intherealworld
601
the“aerosolbenefit”ofcoalmustberemovedeventually(unlesswearetoburncoal
602
forever),andthesooneritisremovedthebetterbothbecausethesmallwarmingits
603
removalwillcausewillhavelessimpactwhentemperaturesarecooler,and,muchmore
604
importantly,becausereplacingcoalsoonwillreduceCO2emissionsandleadtomuchless
605
globalwarminginthelongerterm.
606
Wigley’s(2011)decreaseingreenhousewarmingforthenaturalgassubstitutionhe
607
definesissimilartothatwecomputehere.At0%leakage,Wigley(2011,hisFigure3)
608
calculatesa0.35°Ccoolingwhichwouldbea0.45°CcoolingabsentthereducedSO2
609
emissionsheconsiders.Wecalculateacoolingof~0.62°Cfor0%leakage.Ourcoolingis
610
greaterthanhisatleastinpartbecauseourgassubstitutionscenarioreducestheCO2
611
emissionsmorethanhis.Fromnearlythesamestart,ourgassubstitutionreducesCO2
612
emissionsfromthebusiness‐as‐usual200yeartransitioncycleby743GtCwhereasWigley
613
reducesCO2by425GtC.
614
Thereareofcourseuncertaintiesinthekindofcalculationscarriedouthere,butthese
615
uncertaintiesareunlikelytochangetheconclusionsreached.Carbondioxideisalmost
616
certainlynotremovedfromtheatmosphereexactlyasdescribedbyequation(3).The
617
uptakeofCO2maywellslowastheclimatewarms.Carbondioxideislesssolubleinwarm
618
waterandthehalinecirculationmayslowastheseasurfacetemperatureincreases.The
619
increaseinterrestrialCO2uptakefromCO2fertilizationmaybereducedbynitrogen
620
limitations.AgooddiscussionoftheseissuesisprovidedinNRC(2011).Ebyetal.(2009)
621
havesuggestedbasedonsophisticatedcoupledglobalmodelsthat~50%oftheintroduced
622
CO2mayberemovedwithatimeconstantof130yearsand50%withanexponentialtime
623
constantof2900years.Modificationsofequation(3)thatreduceCO2uptakeastheclimate
28
624
warmswillmakethebenefitsofnotputtingCO2intotheatmosphere,forexampleby
625
substitutinggasforcoal,evengreater,andtheargumentspresentedherestronger.
626
Thetransmissionofheatfromthemixedtothedeeplayeroftheoceansisanunknown
627
whichhasastrongimpactontransientglobalwarming.Forexample,ifheatenteredthe
628
deeplayerwith10%oftheeasewithwhichitentersitfromtheatmospheresothat
629
s 1 ~0.1,thedeeplayerwouldlargelylooseitscoolingeffectiveness(e.g.,ainequation11
630
wouldhaveavalueof0.91).ThetransientresponsetoCO2forcingwouldberapid(occur
631
at0.91mix),andtheoceanwouldreducetheequilibriumglobaltemperaturechangeby
632
only9%.Therelativeratesatwhichheatistransferredintothemixedlayerandoutofit
633
intothedeeplayerwouldappeartobeanimportantareaforfurtherinvestigation,
634
especiallybecauseitimpactsourabilitytoinferpropervaluesintheequilibriumclimate
635
forcing(seediscussioninNRC,2011).Oceanheatexchangedoesnotaffectthe
636
comparativebenefitofsubstitutinggas,souncertaintiesintheoceanheatexchangeaernot
637
ofconcerntotheconclusionswereachhere.
638
ThecalculationsmadehereavoidtheuseofGWPfactors.ThedeficienciesintheGWP
639
approacharediscussedwellbySolomonetal.(2011).Asisapparentfrom(13),theGWP
640
metricrequiresthatthetimeperiodofcomparisonbespecified.Forashorttimeperiod,a
641
shortlivedgaslikemethanehasahighGWP(e.g.,itis72timesmorepotentintermsof
642
globalwarmingthanCO2whencomparedovera20year).Thenotionthatmethane
643
emissionshave72timestheglobalwarmingimpactofCO2wouldtempteliminating
644
methaneemissionsimmediately,andworryingaboutreducingCO2emissionslater.Onthe
645
otherhandfora500yearperiod,theglobalwarmingimpactofakilogramofvented
646
methaneisonly7.6thatofakilogramofCO2(GWPCH4=7.6,seeTable4),andthislow
647
impactwouldsuggestdealingwithCO2emissionsfirstandthemethaneemissionslater,
648
perhapsevensubstitutinggasforcoalandoil.AsSolomonetal.pointouttheGWPmetric
649
speaksonlytothetimeperiodforwhichitiscalculatedandshedsnolightonthewhether
650
CO2orCH4shouldbereducedfirst.
29
2
no fossil fuels
Temperaure Change [C]
10%
leakage
 Cd/Cm=20, mix=5 yrs
CH4  
Business as Usual
Substitute Gas
1.5
1
39.8%
Low Carbon Fast
0.5
0
0
100
200
300
400
500
time [years]
651
652
653
654
655
656
Figure8.TemperaturechangeforscenariosinFigure1whenatransitionperiodis100yearsisfollowedbya400
657
Figure8illustratesthefundamentaldilemma.Itshowsthatevenwhenmethaneleakageis
658
solarge(L=10%ofconsumption)thatsubstitutinggasforcoalandoilincreasesglobal
659
warmingintheshortterm,thebenefitofgassubstitutionreturnsinthelongterm.The
660
shorttermheatingcausedbymethaneleakagerapidlydissipatesafteremissionsofCO2
661
andCH4ceaseat100years.CH4israpidlyremovedfromtheatmosphere,butCO2isnot.
662
Theresultisthat50yearsorsoaftertheterminationofventing(beyond150yearsin
663
Figure8),thebenefitofgasemergesunscathed.Ata10%leakagerateanda100year
664
transitionperiod,thesubstitute‐gasscenarioproducesasmallamountmorewarmingthan
665
thebusiness‐as‐usualscenarioat70years,butafter150yearsthegassubstitutionreduces
666
globalwarmingmuchmorebecauseithasreducedtheamountofCO2ventedtothe
667
atmosphere.Figure8showshowdangerousametricsuchasGWPcanbe.Evenfor
yearperiodwithnoburningoffossilfuels.Methaneleakageinthetransitionis10%ofgasconsumptionand
Shindell’sgreatermethaneforcingandheatexchangewiththeoceanareincluded.Extramethaneventinginthe
substitute‐gasscenarioproduceswarminggreaterthanthebusiness‐as‐usualscenariouptoalmosttheendof
thetransition,butthebenefitsofreducingcarbonemissionsbysubstitutinggasemergeveryquicklythereafter.
30
668
methaneemissionsof9%ofproductionandShindell’sforcings,substitutinggasforcoalis
669
worthwhileinthelongterm.AnalysesthatrelyonlyonGWPfactors,suchasthatof
670
Howarthetal.(2011),missthismixofimpactscompletely,andseeonlythedamageof
671
extramethaneemissionsintheshorttermorthebenefitsofgassubstitutioninthelong
672
term,dependingontheGWPintervalselected.Fortunatelyitisveryeasytocarryoutthe
673
necessaryconvolutionintegrals(equations5and11)asdonehereandavoidGWPmetrics
674
altogether.AsstatedbySolomonetal.(2011)andotherswhotheycite,GWPfactors
675
shouldsimplynotbeusedtoevaluatefuelconsumptionscenarios.
676
Finally,framingthefuelusescenariosintermsofexponentialgrowthanddeclineaswe
677
havedonehereallowsthefeasibilityofimplementingthevariousscenariostobeexamined
678
inapreliminaryfashion.Figure9showstherateofgrowthoflowcarbonenergyresources
679
thatisrequiredbythefuelhistoriesinFigure1fora100yeartransition.Growthatmore
680
than5%peryearwouldbechallenging.Figure9showsthatthelow‐carbon‐fastscenario
681
inFigure1requiresanimmediate~16%peryear(butrapidlydeclining)growthinlow
682
carbonenergysources.Thegrowthrateoflowcarbonenergysourcesattheendofthe
683
growthperiodofthebusiness‐as‐usualscenarioisanevengreater24%peryear.Because
684
thereistimetoplan,thiscouldbereducedbyphasinginlowcarbonenergysourcestoward
685
theendofthefossilfuelgrowthperiod.Thesubstitute‐gasscenariohasamuchlower
686
growthrequirementatthisstage,whichwouldmakethisscenariosubstantiallyeasierto
687
accommodate.
688
Anydecisiontosubstitutegasforcoalandoilofcourseinvolveseconomicandsocial
689
consideration,aswellasclimateanalysis.Naturalgascanenablethetransitiontowindor
690
solarenergybyprovidingthesurgecapacitywhenthesesourcesfluctuateandbackup
691
whenthesesourceswane.Becauseofitswideavailabilityandlowcost,economicfactors
692
willencouragegasreplacingcoalinelectricitygenerationandoilinsegmentsof
693
transportation.ItisafueltheU.S.andmanyothercountriesneednotimport,soits
694
developmentcouldincreaseemployment,nationalsecurity,andamorepositivebalanceof
695
payments.Ontheotherhand,cheapandavailablegasmightunderminetheeconomic
696
viabilityoflowcarbonenergysourcesanddelayatransitiontolowcarbonsources.Froma
697
greenhousepointofviewitwouldbebettertoreplacecoalelectricalfacilitieswithnuclear
31
698
plants,windfarms,orsolarpanels,butreplacingthemwithnaturalgasstationswillbe
699
faster,cheaperandachieve40%ofthelow‐carbon‐fastbenefitiftheleakageislow.How
700
thisbalanceisstruckisamatterofpoliticsandoutsidethescopeofthispaper.Whatcan
701
besaidhereisthatgasisanaturaltransitionfuelthatcouldrepresentsthebiggest
702
availablestabilizationwedgeavailabletous.
Growth Rate Low Carbon Energy Sources
e
Busin
20
s u al
s s as U
16
w
Lo
12
r
Ca
t
as
nF
bo
Growth rate [%/yr]
24
8
Substitu
te Gas
4
0
0
50
100
time [years]
703
704
705
Figure9.ThegrowthrateoflowcarbonenergysourcesdeducedfromFigure1plottedasafunctionoftimefora
706
Conclusions 707
Thecomparativeapproachtakeninthispapershowsthatthebenefitofsubstituting
708
naturalgasdependsonlyonitsleakagerate.
709
1.Forleakagerates~1%orless,thesubstitutionofnaturalgasforthecoalusedin
710
electricitygenerationandfor55%oftheoilusedintransportationandheatingachieves
100yeartransition.Growthratesmorethan5%peryearsuchwillbechallengingtoachieveonaglobalbasis.
32
711
40%ofthereductionthatcouldbeattainedbyanimmediatetransitiontolow‐carbon
712
energysources.
713
2.This40%reductiondoesnotdependonthedurationofthetransition.A40%reduction
714
isattainedwhetherthetransitionisover50yearsor200years.
715
3.Forleakagerates~1%orless,thereductionofgreenhousewarmingatalltimesis
716
relateddirectlytothemassofCO2putintotheatmosphere,andthereforetoreduce
717
greenhouseforcingwemustreducethisCO2input.ComplexitiesofhowCO2isremoved
718
andreductionsinSO2emissionsandincreasesincarbonblackandthelikedonotchange
719
thissimpleimperativeandshouldnotbeallowedtoconfusethesituation.
720
4.Atlowmethaneleakagerates,substitutingnaturalgasisalwaysbeneficialfroma
721
greenhousewarmingperspective,evenforforcingsashighashavebeensuggestedby
722
Shindelletal.(2009)andusedbyHowarthetal.(2011).Underthefastesttransitionthatis
723
probablyfeasible(our50yeartransitionscenario),substitutionofnaturalgaswillbe
724
beneficialiftheleakagerateislessthanabout7%ofproduction.Foramorereasonable
725
transitionof100years,substitutinggaswillbebeneficialiftheleakagerateislessthan
726
~19%ofproduction(Figure7).Thenaturalgasleakagerateappearstobepresentlyless
727
than2%ofproductionandprobably~1.5%ofproduction.
728
5.Evenifthenaturalgasleakageratewerehighenoughtoincreasegreenhousewarming
729
(e.g.,theleakagewas10%ofmethaneconsumptionor9%ofmethaneproduction),
730
substitutinggaswouldstillhavebenefitsbecausethereductionofCO2emissionswould
731
leadtoagreaterreductioningreenhousewarminglater(Figure8).
732
6.Gasisanaturaltransitionfuelbecauseitssubstitutionreducestherateatwhichlow
733
carbonenergysourcesmustbelaterintroduced(Figure9)andbecauseitcanfacilitatethe
734
introductionoflowcarbonenergysources.
735
Thepolicyimplicationsofthisanalysisare:(1)reducetheleakageofnaturalgasfrom
736
productiontoconsumptionsothatitis~1%ofproduction,(2)encouragetherapid
737
substitutionofnaturalgasforcoalandoil,and(3)encourageasrapidaconversiontolow
738
carbonsourcesofenergyaspossible.
33
739
Acknowledgements 740
Thispaperwasgreatlyimprovedbythreeexcellentreviews,twoanonymousandoneby
741
RayPierrehumbert.Raypointedouttheimportanceofoceanmixing,suggestedcasting
742
fueluseintermsofexponentialgrowthanddecline,anddrewmyattentiontoimportant
743
references(asdidtheotherreviewers).Iamindebtedtomypriorco‐authorsinthis
744
subject(MiltonTaam,LarryBrown,andAndrewHunter)forcontinuingveryhelpful
745
discussions,andtomembersofthegasindustrywhopointedoutdataandhelpedme
746
understandthecomplexitiesofgasproduction.MiltonTaamdrewmyattentiontothe
747
MAGICCprogramandshowedmehoweasyitwastouse,andalsopushedpersistentlyfor
748
thebroaderviewofmethanesubstitutionshowninFigure8.Thepaperwouldnotbewhat
749
itiswithoutthecontributionoftheseindividualsandIthankthemfortheirinput.
750
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