Assessing the greenhouse impact of natural gas - Energy in Depth â€”

Article

Volume 13, Number 6

19 June 2012

Q06013, doi:10.1029/2012GC004032

ISSN: 1525-2027

Assessing the greenhouse impact of natural gas

L. M. CathlesDepartment of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA([email protected])

[1] The global warming impact of substituting natural gas for coal and oil is currently in debate. We addressthis question here by comparing the reduction of greenhouse warming that would result from substituting gasfor coal and some oil to the reduction which could be achieved by instead substituting zero carbon energysources. We show that substitution of natural gas reduces global warming by 40% of that which could beattained by the substitution of zero carbon energy sources. At methane leakage rates that are�1% of produc-tion, which is similar to today’s probable leakage rate of�1.5% of production, the 40% benefit is realized asgas substitution occurs. For short transitions the leakage rate must be more than 10 to 15% of production forgas substitution not to reduce warming, and for longer transitions the leakage must be much greater. But evenif the leakage was so high that the substitution was not of immediate benefit, the 40%-of-zero-carbon benefitwould be realized shortly after methane emissions ceased because methane is removed quickly from theatmosphere whereas CO2 is not. The benefits of substitution are unaffected by heat exchange to the ocean.CO2 emissions are the key to anthropogenic climate change, and substituting gas reduces them by 40% ofthat possible by conversion to zero carbon energy sources. Gas substitution also reduces the rate at whichzero carbon energy sources must eventually be introduced.

Components: 11,000 words, 9 figures, 4 tables.

Keywords: global warming; greenhouse forcing; leakage; natural gas.

Index Terms: 3305 Atmospheric Processes: Climate change and variability (1616, 1635, 3309, 4215, 4513).

Received 10 January 2012; Revised 13 April 2012; Accepted 14 May 2012; Published 19 June 2012.

Cathles, L. M. (2012), Assessing the greenhouse impact of natural gas, Geochem. Geophys. Geosyst., 13, Q06013,doi:10.1029/2012GC004032.

1. Introduction

[2] In a recent controversial paper, Howarth et al.[2011] suggested that, because methane is a farmore potent greenhouse gas than carbon dioxide,the leakage of natural gas makes its greenhouseforcing as bad and possibly twice as bad as coal, andthey concluded that this undermines the potentialbenefit of natural gas as a transition fuel to lowcarbon energy sources. Others [Hayhoe et al., 2002;Wigley, 2011a] have pointed out that the warmingcaused by reduced SO2 emissions as coal electrical

facilities are retired will compromise some of thebenefits of the CO2 reduction. Wigley [2011a] hassuggested that because the impact of gas substitu-tion for coal on global temperatures is small andthere would be some warming as SO2 emissions arereduced, the decision of fuel use should be based onresource availability and economics, not greenhousegas considerations.

[3] Some of these suggestions have beenchallenged. For example, Cathles et al. [2012;see also Press Release: Response to Howarth

©2012. American Geophysical Union. All Rights Reserved. 1 of 18

et al.’s reply (February 29, 2012), http://www.geo.cornell.edu/eas/PeoplePlaces/Faculty/cathles/Natural%20Gas/Response%20to%20Howarth’s%20Reply%20Distributed%20Feb%2030,%202012.pdf, 2012] have taken issue with Howarth et al.[2011] for comparing gas and coal in terms of theheat content of the fuels rather than their electricitygenerating capacity (coal is used only to generateelectricity), for exaggerating the methane leakageby a factor of 3.6, and for using an inappropri-ately short (20 year) global warming potential factor(GWP). Nevertheless it remains difficult to see in thepublished literature precisely what benefit might berealized by substituting gas for coal and the use ofmetrics such as GWP factors seems to complicaterather than simplify the analysis. This paper seeks toremedy these deficiencies by comparing the benefitsof natural gas substitution to those of immediatelysubstituting low-carbon energy sources. The com-parative analysis goes back to the fundamentalequation and does not use simplified GWP metrics.Because it is a null analysis it avoids the complica-tions of SO2, carbon black, and the complexities ofCO2 removal from the atmosphere. It shows thatthe substitution of natural gas for coal and some oilwould realize�40% of the greenhouse benefits thatcould be had by replacing fossil fuels with lowcarbon energy sources such as wind, solar, andnuclear. In the long term this gas substitution benefitdoes not depend on the speed of the transition or themethane leakage rate. If the transition is faster,greenhouse warming is less, but regardless of therate of transition substituting natural gas achieves�40% of the benefits of low carbon energy sub-stitution a few decades after methane emissionsassociated with gas production cease. The benefitof natural gas substitution is a direct result of thedecrease in CO2 emissions it causes.

[4] The calculation methods used here followWigley [2011a], but are computed using programsof our own design from the equations and para-meters given below. Parameters are defined thatconvert scenarios for the yearly consumption ofthe fossil fuels to the yearly production of CO2 andCH4. These greenhouse gases are then introducedinto the atmosphere and removed using acceptedequations. Radiative forcings are calculated forthe volumetric gas concentrations as they increase,the equilibrium global temperature change is com-puted by multiplying the sum of these forcings bythe equilibrium sensitivity factor currently favoredby the IPCC, and the increments of equilibriumtemperature change are converted to transient

temperature changes using a two layer ocean ther-mal mixing model.

2. Emission Scenarios

[5] Greenhouse warming is driven by the increasein the atmospheric levels of CO2, CH4 and othergreenhouse gases that result from the burning offossil fuels. Between 1970 and 2002, world energyconsumption from all sources (coal, gas, oil, nuclear,hydro and renewables) increased at the rate of2.1% per year. In the year 2005 six and a half bil-lion people consumed �440 EJ (EJ = exajoules =1018 joules, 1 J = 1.055 Btu [U.S. EnergyInformation Administration, 2011]) of energy. Oiland gas supplied 110 EJ each, coal 165 EJ, andother sources (hydro, nuclear, and renewables sucha wind and solar) 55 EJ (MiniCAM scenario[Clarke et al., 2007]). In 2100 the world populationis projected to plateau at �10.5 billion. If the perperson consumption then is at today’s Europeanaverage of �7 kW p�1, global energy consumptionin 2100 would be 2300 EJ per year (74 TW). Westart with the fuel consumption pattern at 2005 ADand grow it exponentially so that it reaches 2300 EJper year at the end of a “transition” period. At theend of the transition the energy is supplied almostentirely by low carbon sources in all cases, but inthe first half of the transition, which we call thegrowth period, hydrocarbon consumption eitherincreases on the current trajectory (the “business-as-usual” scenario), increases at the same equiv-alent rate with gas substituted for coal and oil(a “substitute-gas” scenario), or declines immedi-ately (the “low-carbon-fast” scenario). Coal use isphased out at exactly the same rate in the substitute-gas and low-carbon-fast scenarios, so that thereduction of SO2 and carbon black emissions isexactly the same in these two scenarios and thereforis not a factor when we compare the reduction ingreenhouse warming for the substitute-gas and thelow-carbon-fast scenarios.

[6] Figure 1 shows the three fuel scenarios consid-ered for a 100 year transition:

[7] 1. In the first half (growth period) of the business-as-usual scenario (Figure 1a), fossil fuel consump-tion increases 2.9 fold from 440 EJ/yr in 2005 to 1265EJ/yr over the 50 year growth period, and thendeclines to 205.6 EJ/yr after the full transition. Themix of hydrocarbons consumed at the end of thetransition produces CO2 emissions at the same 4.13GtC/yr rate as at the end of the other scenarios. Thetotal energy consumption grows at 2.13% per year in

GeochemistryGeophysicsGeosystems G3G3 CATHLES: GREENHOUSE IMPACT OF NATURAL GAS 10.1029/2012GC004032

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the growth period, and at 1.2% over the declineperiod. The growth period is a shifted (to start in2005), slightly simplified, exponential version of theMiniCAM scenario in Clark et al. [2007]. Weincrease the hydrocarbon consumption by the samefactors as in the MiniCAM scenario, and determinethe renewable growth by subtracting the hydrocar-bon energy consumption from this total. Thegrowth-decline combination is similar to the basescenario used by Wigley [2011a].

[8] 2. In the substitute-gas scenario (Figure 1b), gasreplaces coal and new oil consumption over thegrowth period, and is replaced by low carbon fuelsin the decline period. Gas replaces coal on an equalelectricity-generation basis (DHgas = DHcoal Rcoal/Rgas = 234 EJ y�1, see Table 1), and gas replacesnew oil (165 EJ y�1) on an equal heat content basis.Gas use at the end of the growth period is thus729 EJ y�1, rather than 330 EJ y-1 in the business-as-usual scenario. The growth of renewable energyconsumption is greater than in Figure 1a. Over theensuing decline period, oil consumption drops to75 EJ y�1 and gas to 175 EJ y�1.

[9] 3. In the low-carbon-fast scenario (Figure 1c),low carbon energy sources replace coal, new gas,and new oil over the growth period, and gas usegrows and oil use decreases so that the consump-tion at the end is the same as in the substitute-gasscenario.

[10] These scenarios are intended to provide asimple basis for assessing the benefits of sub-stituting gas for coal; they are intended to beinstructive and realistic enough to be relevant tofuture societal decisions. The question they pose is:How far will substituting gas for coal and some oiltake us toward the greenhouse benefits of animmediate and rapid conversion to low carbonenergy sources.

3. Computation Method and Parameters

[11] Table 1 summarizes the parameters used in thecalculations. I[EJ Gt�1], gives the heat energy pro-duced when each fossil fuel is burned in exajoules

Figure 1. Three fuel consumption scenarios comparedin this paper: (a) Fossil fuel use in the business-as-usualscenario continues the present growth in fossil fuel con-sumption in the initial 50 year growth period before lowcarbon energy sources replace fossil fuels in the declineperiod. (b) In the substitute-gas scenario, gas replacescoal such that the same amount of electricity is generated,and substitutes for new oil on an equal heat energy basis.(c) In the low-carbon-fast scenario, low carbon energysources immediately substitute for coal and new oil andgas in the growth period, and gas use declines and sub-stitutes for oil in the decline period. Numbers indicatethe consumption of the fuels in EJ per year at the start,midpoint, and end of the transition period. The totalenergy use is the same in all scenarios and is indicatedat the start, midpoint, and end by the bold black numbersin Figure 1c.

Table 1. Parameters Used in the Calculationsa

I[EJ Gt�1] R[EJe EJ�1] x[GtC EJ�1] z[GtCH4 EJ

�1]

Gas 55 0.6 0.015 1.8 � 10�4 for a leakage of 1% of productionOil 43 0.020Coal 29 0.32 0.027 1.2 � 10�4 for 5 m3/t

aI is the energy content of the fuel, R the efficiency of conversion to electricity, and x and z the carbon and methane emissions factors. See text fordiscussion.


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(1018 joules) per gigaton (109 tons) of the fuel. Thevalues we use are from http://www.natural-gas.com.au/about/references.html. The energy density ofcoal varies from 25 to 37 GJ/t, depending on therank of the coal, but 29 GJ/t is considered a goodaverage value for calculations.

[12] R[EJe EJ�1] is the efficiency with which gasand coal can be converted to electricity in exajoulesof electrical energy per exajoule of heat. Gas cangenerate electricity with much greater efficiencythan coal because it can drive a gas turbine whoseeffluent heat can then be used to drive a steamgenerator. Looking forward, older low efficiencycoal plants will likely be replaced by higher effi-ciency combined cycle gas plants of this kind. Theelectrical conversion efficiencies we adopt inTable 1 are those selected by Hayhoe et al. [2002,Table 2].

[13] The carbon emission factors in gigatons ofcarbon released to the atmosphere per exajoule ofcombustion heat, x [GtC EJ�1], listed in the fourthcolumn of Table 1 are the factors compiled by theU.S. Environmental Protection Agency (EPA) [2005]and used by Wigley [2011a].

[14] Finally, the methane emission factors, z[GtCH4EJ�1] in the last column of Table 1 are computedfrom the fraction of methane that leaks during theproduction and delivery of natural gas and thevolume of methane that is released to the atmo-sphere during mining and transport of coal:

xgas½GtCH4 EJ�1 � ¼ L½GtCH4-vented Gt

�1

CH4-burned�=I ½EJ Gt�1

CH4-burned�ð1aÞ

xcoal ½GtCH4 EJ�1�¼ V ½m3

CH4 t�1

coal-mined�rCH4½tCH4 m�3CH4�=I ½EJ Gt

�1

coal-burned�:ð1bÞ

The density of methane in (1b) rCH4 = 0.71 � 10�3

tons per m3. We treat the methane vented to theatmosphere during the production and distributionof natural gas, L, parametrically in our calculations.The natural gas leakage, L, is defined as the massfraction of natural gas that is burned.

[15] We assume in our calculations that 5 m3 ofmethane is released per ton of coal mined. Theleakage of methane during coal mining has beenreviewed in detail by Howarth et al. [2011] andWigley [2011a]. Combining leakages from surfaceand deep mining in the proportions that coal isextracted in these two processes, they arrive at6.26 m3/t and 4.88 m3/t respectively. The value we

use lies between these two estimates, and appears tobe a reasonable estimate [e.g., see Saghafi et al.,1997], although some have estimated much highervalues (e.g., Hayhoe et al. [2002] suggest�23 m3/t).

[16] The yearly discharge of CO2 (measured in tonsof carbon) and CH4 to the atmosphere, QC[GtC y�1]and QCH4[GtCH4 y�1], are related to the heat pro-duced in burning the fuels, H[EJ y�1] in Figure 1:

QC ½GtCy�1� ¼ H ½EJ y�1 �x ½GtC EJ�1� ð2aÞ

QCH4½GtCy�1� ¼ H ½EJ y�1�z ½GtCH4 EJ�1�: ð2bÞ

The volume fractions of CO2 and CH4 added tothe atmosphere in year ti by (1a) and (1b) are asfollows:

DXCO2 tið Þ½ppmv y�1� ¼QC ½GtCy�1�1015 WCO2

WC

Wair

WCO2

VCO2

Vair

Matm t½ �ð3aÞ

DXCH4 tið Þ½ppbv y�1� ¼QCH4½GtCH4y�1�1018 Wair

WCH4

VCH4

Vair

Matm t½ � :

ð3bÞ

Here Matm[t] = 5.3 � 1015 tons is the mass of theatmosphere, WCO2 is the molecular weight of CO2

(44 g/mole), and VCO2 is the molar volume ofCO2, etc. In (2a) the first molecular weight ratioconverts the yearly mass addition of carbon to theyearly mass addition of CO2, and the secondmass fraction ratio converts this to the volumefraction of CO2 in the atmosphere. We assumethe gases are ideal and thus VCH4 = VCO2 = Vair.

[17] Each yearly input of carbon dioxide andmethane is assumed to decay with time as follows:

DXCO2 ti þ tð Þ ¼ DXCO2 tið Þ fCO2 tð ÞfCO2 tð Þ ¼ 0:217þ 0:259e�t=172:9 þ 0:338e�t=18:51 þ 0:186e�t=1:186

ð4aÞ

DXCH4 ti þ tð Þ ¼ DXCH4 tið Þ fCH4 tð ÞfCH4 tð Þ ¼ e�t=12 ; ð4bÞ

where t is time in years after the input of a yearlyincrement of gas at ti. These decay rates are thoseassumed by the Intergovernmental Panel on ClimateChange (IPCC) [2007, Table 2.14]. The 12 yeardecay time for methane in (4b) is a perturbationlifetime that takes into account chemical reactionsthat increase methane’s lifetime according to the


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IPCC [2007, §2.10.3.1]. The decay of CO2 describedby (4a) does not account for changes with timein the carbonate-bicarbonate equilibrium (such asdecreasing CO2 solubility as the temperature ofthe ocean surface waters increases) which becomeimportant at higher concentrations of atmosphericCO2 [see National Research Council (NRC), 2011;Eby et al., 2009]. Equation (4a) thus probablyunderstates the amount of CO2 that will be retainedin the atmosphere when warming has becomesubstantial.

[18] The concentration of carbon dioxide andmethane in the atmosphere as a function of time iscomputed by summing the additions each year andthe decayed contributions from the additions inprevious years:

XCO2 tið Þ ¼ DXCO2 tið Þ þXi�1

j¼1

DXCO2 tj� �

fCO2 ti � tj� �

XCH4 tið Þ ¼ DXCH4 tið Þ þXi�1

j¼1

DXCH4 tj� �

fCH4 ti � tj� �

;

ð5Þ

where XCO2(ti) and XCH4(ti)are volumetric concen-tration of CO2 and CH4 in ppmv and ppbv respec-tively, i runs from 1 to ttot where ttot is the durationof the transition in years, and the sum terms on theright hand sides do not contribute unless i ≥ 2.

[19] The radiative forcings for carbon dioxide andmethane, DFCO2[W m�2] and DFCO2[W m�2] arecomputed using the following formulae given inthe IPCC [2001, §6.3.5]:

DFCO2 W m�2� � ¼ 5:35 ln

XCO2 tið Þ þ XCO2 t ¼ 0ð ÞXCO2 t ¼ 0ð Þ

DFCH4 W m�2� � ¼ 0:036YCH4

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXCH4 tið Þ þ XCH4 0ð Þ

p��

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXCH4 0ð Þ

p �� f XCH4 tið Þ þ XCH4 0ð Þð Þ;Noð Þð

� f XCH4 0ð Þð Þ;NoÞ�

f M ;Nð Þ ¼ 0:47 ln 1þ 2:01� 10�5 MNð Þ�5�

þ 5:31 MN�15� �þM NMð Þ1:52

�: ð6Þ

We start our calculations with the atmosphericconditions in 2005: XCO2[t = 0] = 379 ppmv,XCH4[t = 0] = 1774 ppbv, and the N2O concen-tration, No = 319 ppbv. =CH4 is a factor thatmagnifies the direct forcing of CH4 to take intoaccount the indirect interactions caused by increasesin atmospheric methane. IPCC [2007] suggeststhese indirect interactions increase the direct forcingfirst by 15% and then by an additional 25%, with theresult that =CH4 = 1.43. Shindell et al. [2009] havesuggested additional indirect interactions which

increase =CH4 to �1.94. There is continuing dis-cussion of the validity of Shindell et al.’s suggestedadditional increase [see Hultman et al., 2011]. Wegenerally use =CH4 = 1.43 in our calculations, butconsider the impact of =CH4 to �1.94 where itcould be important.

[20] The radiative forcing of the greenhouse gasadditions in (6) drives global temperature change.The ultimate change in global temperature theycause is as follows:

DT equil ¼ DTCO2 þDTCH4 ¼ l�1S DFCO2 þDFCH4ð Þ; ð7Þ

where lS�1 is the equilibrium climate sensitivity.

We adopt the IPCC [2007] value lS�1 = 0.8,

which is equivalent to assuming that a doublingof atmospheric CO2[ppmv] causes a 3�C globaltemperature increase.

[21] The heat capacity of the ocean delays the sur-face temperature response to greenhouse forcing.Assuming, following Solomon et al. [2011], a twolayer ocean where the mixed layer is in thermalequilibrium with the atmosphere:

Cmix∂DTmix

∂t¼ ls DT equil

mix �DTmix� �

� g DTmix �DTdeep� �

Cdeep∂DTmix

∂t¼ g DTmix �DTdeep

� �: ð8Þ

Here g is the heat transfer coefficient for the flowof heat from the mixed layer into the deep layer inWK�1 m�2, and ls is the heat transfer coefficient intothe mixed layer from the atmosphere (and theinverse of the equilibrium climate sensitivity). Cmix

and Cdeep are the heat storage capacities per unitsurface area of the mixed and deep layers in J K�1

m�2. Defining DT ′mix = DT mixequil � DTmix, DT ′deep =

DT mixequil � DTdeep, �t ¼ t=tmix , and tmix = Cmixls

�1,we can write the following:

∂∂�t

DT ′mix

DT ′deep

!

¼ � gl�1s þ 1

� �gl�1

s

gl�1s CmixC�1

deep �gl�1s CmixC�1

deep

!DT ′

mix

DT ′deep

!: ð9Þ

For the imposition of a sudden increase in green-house forcing that will ultimately produce an equi-librium temperature change of DT mix

equil as describedby (7), the solution to (8) is as follows:

DTmix ¼ DT equilmix

n1�

�a exp �t

.e�1m �mix

� �þ 1� að Þ exp �t

.e�1d �mix

� ��o: ð10Þ


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Here em and ed are the magnitudes of the eigenva-lues of the matrix in (9), and the coefficient, a, isdetermined by the initial condition that the layersare not thermally perturbed before the increment ofgreenhouse forcing is imposed.

[22] Insight is provided by noting that the eigenva-lues and parameter a in (10) are functions of theratios of heat transfer and heat storage parametersgls

�1 and CdeepCmix�1 only, and can be approximated

to within �10%:

a ¼ 0:483þ 0:344 1� gl�1s

� �;0:2 < gl�1

s ≤ 1

e�1m ¼ 1þ gl�1

s

� ��1

e�1d ¼ 2CdeepC

�1mix

.ð��1

s Þ0:7 : ð11Þ

It is unlikely that that heat will be transferred outthe base of the mixed layer more efficiently than itis into the top of the mixed layer because thetransfer will be mostly driven by winds and coolingof the ocean surface. For this reason the heattransfer coefficient ratio gls

�1is almost certainly ≤1and the reduction of temperature is greatest forgls

�1 = 1. For gls�1 = 1, the initial temperature

change in the mixed layer will be about half thechange that will occur when the ocean layers arefully warmed, and the response time required toreach this equilibrium change (the time required toreach 2/3rds of the equilibrium value) will be about1/2 of the response time of the mixed layer (e.g.,e�1mix ¼ 1

2= ). For gls�1 = 1, the response time of the

deep layer is twice the heat storage capacity ratiotimes the response time of the mixed layer:2CdeepCmix

�1tmix. For tmix = 5 yrs DTmix will reach0.483DTmixequil with a response time of 2.5 years andrise to DTmix

equil with a response time of 200 years.

[23] The transient temperature change can be com-puted from the equilibrium temperature change in(7) by convolving in a fashion similar to what wasdone in (5):

T tið Þ ¼Xi�1

j¼1

DT equil tj� �

1� a exp� ti � tj� �e�1m tmix

�

þ 1� að Þ exp � ti � tj� �e�1d tmix

�; ð12Þ

where i ≥ j. We do not use the approximations ofequation (11) when we carry out the convolutionin (12). Rather we solve for the actual values ofthe eigenvalues and parameter a from the matrix in(9) at each yearly increment in temperature change.

[24] The current consensus seems to be thatgls

�1 = 1 and the transient thermal response is

about half the full equilibrium forcing value [NRC,2011, §3.3]. The ratio of the heat storage capacityof the deep to mixed layer, CdeepCmix

�1 is probably atleast 20, a value adopted by Solomon et al. [2011].Schwartz [2007] estimated the thermal responsetime of the mixed layer at �5 years from the tem-poral autocorrelation of sea surface temperatures.This may be the best estimate of this parameter,but Schwartz notes that estimates range from 2 to30 years. Fortunately the moderation of tempera-ture change by the oceans does not impact thebenefit of substituting gas for coal and oil at all.It is of interest in defining the cooling that sub-stitution would produce, however. We calculate thetransient temperature changes for the full range ofocean moderation parameters.

[25] Equations (1) to (10) plus (12), together withthe parameters just discussed define completely themethods we use to calculate the global warmingcaused by the fuel use scenarios in Figure 1.

4. Results

[26] Figure 2 shows the additions of CO2 in ppmvand methane in ppbv that occur for the differentfuel consumption scenarios show in Figure 1 for thethree transition periods (50, 100 and 200 years).The methane leakage is assumed to be 1% of con-sumption. Five cubic meters of methane are assumedto leak to the atmosphere for each ton of coal burned.The atmospheric methane concentrations track thepattern of methane release quite closely becausemethane is removed quickly from the atmospherewith an exponentially decay constant of 12 years(equation (4b)). On the other hand, because only aportion of the CO2 introduced into the atmosphere byfuel combustion is removed quickly (see equation(4a)), CO2 accumulates across the transition periodsand, as we will show below, persists for a long timethereafter.

[27] Figure 3 shows the radiative forcings corre-sponding to the atmospheric gas concentrationsshown in Figure 2 using equation (6). The methaneforcing is a few percent of the CO2 forcing, andthus is unimportant in driving greenhouse warmingfor a gas leakage rate of 1%.

[28] Figure 4 shows the global warming predictedfrom the radiative forcings in Figure 3 for variousdegrees of heat loss to the ocean. We take theequilibrium climate sensitivity ls

�1 = 0.8 (e.g., adoubling of CO2 causes a 3�C of global warming).The faster transitions produce less global warmingbecause they put less CO2 into the atmosphere. The


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thermal modulation of the oceans can reduce thewarming by up to a factor of two. For example,Figure 4a shows the global warming that wouldresult from the business-as-usual scenario if therewere no heat losses to the ocean ranges from 1.5�Cfor the 50 year transition to 3.3�C for the 200 yeartransition. Figure 4c indicates that heat exchangeto the oceans could reduce this warming by afactor of two for the long transitions and three forthe 50 year transition. A warming reduction thislarge is unlikely because it assumes extremeparameter values: a deep ocean layer with a heatstorage 50 times the shallow mixed layer, and a longmixing time for the shallow layer (tmix = 50 years).Figure 4b indicates the more likely ocean tempera-ture change moderation based on mid-range deeplayer storage (CdeepCmix

�1 = 20) and mixed layerresponse time (tmix = 5 years) parameter values.

[29] The important message of this figure for thepurposes of this paper, however, is not the amountof warming that might be produced by the variousfuel scenarios of Figure 1, but the indication that thereduction in greenhouse warming from substitutinggas for coal and oil is not significantly affected byheat exchange with the ocean or by the duration ofthe transition period. The same percent reduction inglobal warming from substituting gas for coal andoil is realized regardless of the duration of thetransition period or the degree of thermal modera-tion by the ocean. The benefit of substituting gas is apercent or so less for the short transitions, and theocean moderation reduces the benefit by a percentor so, but the benefit in all circumstances remains�38%. Heat loss into the oceans may reduce thewarming by a factor of two, but the benefit of sub-stituting gas is not significantly affected.

[30] Figure 5 compares the methane forcing of thesubstitute-gas scenario to the CO2 forcing of thebusiness-as-usual scenario for the 50 and 100 yeartransition durations. The forcing for the 1%methanecurves are the same as in Figure 3, but is continuedout to 200 years assuming the fuel use remains thesame as at the end of the of the transition period.Similarly the business-as-usual curve is the same asin Figure 3 continued out to 200 years. The figureshows that the methane forcing increases as the per-cent methane leakage increases, and becomes equalto the CO2 forcing in the business-as usual scenariowhen the leakage is �10% of consumption forthe 50 year transition and 30% of consumption

Figure 2. Changes in (a) carbon dioxide and (b) methaneconcentrations computed for the three fuel scenarios shownin Figure 1 and three different transition intervals (50 100and 200 years). In this and subsequent figures the bluecurves indicate the business-as-usual fuel use scenario,the green curves indicate the substitute-gas scenario, andthe red curves the low-carbon-fast scenario. The numbersindicate the change in concentrations of CO2 and methanefrom the 379 ppmv for CO2 and 1774 ppbv for CH4 levelspresent in the atmosphere in 2005. The calculation is basedon L = 1% of gas consumption and V = 5 m3 methane perton of coal burned.

Figure 3. Radiative forcings calculated for the carbondioxide and methane additions shown in Figure 2 usingequation (6) and assuming YCH4 = 1.43. The blue curvesindicate the business as usual scenario for the 50, 100and 200 year transition periods, the green the substitute-gas scenario, and the red the low-carbon-fast scenario.The numbers indicate the reduction in CO2 forcingachieved by substituting gas, expressed as a percentageof the reduction achieved by the low-carbon-fast scenario.


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for the 100 year transition. At the end of the transi-tion the methane radiative forcings fall to the levelthat can be steadily maintained by the constantmethane leakage associated with the small continuednatural gas consumption. The CO2 forcing under thebusiness-as-usual scenario fall a bit and then rise ata slow steady rate, reflecting the proscription that26% of the CO2 released to the atmosphere is onlyvery slowly removed and 22% is not removed atall (equation (3a)). This slow rise emphasizes thateven very low releases of CO2 can be of concern.The methane in the atmosphere would rapidly dis-appear in a few decades if the methane venting

were stopped, whereas the CO2 curves would flattenbut not drop significantly. Finally, Figure 5a showsthat the greater methane climate sensitivity pro-posed by Shindell et al. [2009] (=CH4 = 1.94) wouldmake a 10% methane venting equivalent to a 15%venting with =CH4 = 1.43 (the IPCC methane cli-mate sensitivity).

[31] Figures 6 illustrates how the benefits of sub-stituting gas for coal and oil disappear as the methaneleakage increases above 1% of total methane con-sumption. The figure shows the global warmingcalculated for the ocean heat exchange show in

Figure 4. Global warming produced by the forcings in Figure 3 computed using equations (7), (10), and (12). Theblue curves indicate temperature changes under the business-as-usual scenario for 50, 100 and 200 year transitiondurations, and the green and red curves indicate the temperature changes for the substitute-gas and low-carbon-fastscenarios. The colored numbers indicate the temperature changes, and the black numbers the reduction in temperatureachieved by the substitute-gas scenario expressed as a fraction of the temperature reduction achieved by the low-carbon-fast scenario. (a) The warming when there is no thermal interaction with the ocean (or the ocean layers thermallyequilibrate very quickly). (b) Warming under a likely ocean interaction. (c) Warming with a very high ocean thermalinteraction. The ocean mixing parameters are indicated in Figures 4b and 4c. All calculations assume gas leakage is1% of consumption and the IPCC methane climate sensitivity.


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Figure 4b. As the methane leakage increases, thegreen substitute-gas scenario curves rise toward andthen exceed the blue business-as-usual curves, andthe benefit of substituting gas disappears. The gasleakage at which substituting gas for oil and coalwarms the earth more than the business-as-usualscenario is smallest (L � 10%) for the 50 yeartransition period and largest (L � 35%) for the200 year transition period.

[32] Figure 7 summarizes how the benefit of gassubstitution depends on the gas leakage rate. Forthe IPCC methane climate sensitivity (=CH4 =1.43), the benefit of substituting gas goes to zerowhen the gas leakage is 44% of consumption(30% of production) for the 200 year transition,24% of consumption (19% of production) for the100 year transition, and 13% of consumption(12% or production) for the 50 year transition. Forthe Shindell et al. [2009] climate sensitivitycorresponding to =CH4 = 1.94, the crossover forthe 50 year transition occurs at a gas leakage of�9% of consumption, and reasonable ocean thermalmixing reduces this slightly to�8% of consumption(7.4% of production). This last is approximately thecrossover discussed byHowarth et al. [2011, 2012].In their papers they suggest a methane leakage rateas high as 8% of production is possible, and there-fore that natural gas could be as bad (if compared onthe basis of electricity generation) or twice as bad (ifcompared on a heat content basis) as coal over ashort transition period. As discussed in the nextsection, a leakage rate as high as 8% is difficult tojustify. Figure 7 thus shows the significance ofShindell’s higher methane climate sensitivity toHowarth’s proposition. Without it, an even lessplausible methane leakage rate of 12% would berequired to make gas as bad or twice as bad as coalin the short term. Over the longer term, substitutionof gas is beneficial even at high leakage rates- apoint completely missed by Howarth et al.

5. What is the Gas Leakage Rate

[33] The most extensive syntheses of data on fugitivegases associated with unconventional gas recoveryis an industry report to the EPA commissioned byThe Devon Energy Corporation [Harrison, 2012].It documents gas leakage during the completionof 1578 unconventional (shale gas or tight sand)gas wells by 8 different companies with a reasonablerepresentation across the major unconventional gasdevelopment regions of the U.S. Three percent ofthe wells in the study vented methane to the atmo-sphere. Of the 1578 unconventional (shale gas ortight sand) gas wells in the Devon study, 1475(93.5%) were green completed - that is they wereconnected to a pipeline in the pre-initial produc-tion stage so there was no need for them to beeither vented or flared. Of the 6.5% of all wellsthat were not green completed, 54% were flared.Thus 3% of the 1578 wells studied vented methaneinto the atmosphere.

Figure 5. Radiative forcings of CO2 for the businessas usual scenario (blue curves) and for CH4 for variousgas leakage rates in the substitute-gas scenario (greencurves). The 1% methane curves and the business asusual curves are the same as in Figure 3 except the verticalscale is expanded and the curves are extended from theend of the transition to 200 years assuming the gas emis-sions are the same as at the end of the transition past100 years. The methane forcings plateau at the levelscorresponding to the atmospheric concentration supportedby the steady CH4 emissions. The CO2 forcing increasesbecause an appreciable fraction of the CO2 emissionsare removed slowly or not at all from the atmosphere.The methane forcings all assume the IPCC methane cli-mate sensitivity (=CH4 = 1.43) except the single red curve,which assumes the methane climate sensitivity suggestedby Shindell et al. [2009] (=CH4 = 1.94).


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[34] The wells that vented methane to the atmo-sphere did so at the rate of 765 Mcsf/completion.The maximum gas that could be vented from thenon-green completed wells was estimated by cal-culating the sonic venting rate from the choke(orifice) size and source gas temperature of thewell, using a formula recommended by the EPA.Since many wells might vent at sub-sonic rates,which would be less, this is an upper bound on theventing rate. The total vented volume was obtainedby multiplying this venting rate by the knownduration of venting during well completion. Thesevented volumes ranged from 340 to 1160 Mscf,with an average of 765 Mscf. The venting from anaverage unconventional shale gas well indicated

by the Devon study is thus �23 Mscf ( = 0.03 �765 Mscf), which is similar to the 18.33 Mscf EPA[2010] estimates is vented during well completionof a conventional gas well (half vented and halfflared). Since venting during well completion andworkover conventional gas wells is estimated at0.01% of production [e.g., Howarth et al., 2011],this kind of venting is insignificant for bothunconventional and conventional wells.

[35] The unconventional gas leakage rate indi-cated by the Devon data is very different fromthe 4587 Mscf that EPA [2010] inferred wasvented during well completion and workover forunconventional gas wells from the amount of gascaptured in a very limited number of “green

Figure 6. Impact of methane leakage on global warming for transition periods of (a) 50, (b) 100, and (c) 200 years.As the leakage rate (green percentage numbers) increase, the warming of the substitute-gas scenario (green curves)increases, the blue business-as-usual and green substitute-gas curves approach one another and then cross, and thepercentage of the warming reduction attained by the fast substitution of low carbon energy sources (black number)decrease and then become negative. The warmings assume the same exchange with the ocean as in Figure 4b.


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completions” reported to them by industry throughtheir GasSTAR program. In their 2010 backgroundtechnical support document the EPA assumed thatthis kind of “green” capture was very rare, and thatthe gas was usually either vented or flared. Assum-ing further that the gas was vented 50% of the time,the EPA concluded that 4587 Mscf was vented tothe atmosphere and that unconventional wells vent250 times (= 4587/18.3) more methane during wellcompletion and workover than conventional gaswells. The EPA [2010] study is a “Background

Technical Support Document” and not an officialreport. It was probably never intended to be morethan an outline of an approach and an initial esti-mate, and the EPA has since cautioned that theyhave not reviewed their analysis in detail and con-tinue to believe that natural gas is better for theenvironment than coal (M. Fulton et al., Comparinggreenhouse gas emissions from natural gas andcoal, http://lockthegate.org.au/documents/doc-305-comparing-life-cycle-greenhouse-gas-db.pdf, World-watch Institute/Deutsche Bank, 25 August 2011).Nevertheless the EPA [2010] report suggested tomany that the leakage during well completion andworkover for unconventional gas wells could be asubstantial percentage (�2.5%) of production, andmany accepted this suggestion without further crit-ical examination despite the fact that the safetyimplications of the massive venting implied by theEPA numbers should have raised questions [e.g.,Cathles et al., 2012; see also online press release,2012].

[36] Once a well is in place, the leakage involved inroutine operation of the well site and in transportingthe gas from the well to the customer is the same foran unconventional well as it is from a conventionalwell. What we know about this leakage is summa-rized in Table 2. Routine site leaks occur whenvalves are opened and closed, and leakage occurswhen the gas is processed to removing water andinert components, during transportation and storage,and in the process of distribution to customers. Thefirst major assessment of these leaks was carried outby the Gas Research Institute (GRI) and the EPA in1997 and the results are shown in the second columnof Table 2. Appendix A of EPA [2010] gives adetailed and very specific accounting of leaks ofmany different kinds. These numbers are summedinto the same categories and displayed in column 3of Table 2. EPA [2011] found similar leakagerates (column 4). Skone [2011] assessed leakagefrom 6 classes of gas wells. We show his resultsfor unconventional gas wells in the Barnett Shale

Figure 7. The reduction of greenhouse warmingattained by substituting natural gas for coal and oil(substitute-gas scenario), expressed as a percentage ofthe reduction attained by immediately substituting lowcarbon fuels (low-C-fast scenario), plotted as a functionof the gas leakage rate. At leakage rates less than �1%,the benefit of substituting natural gas is >40% that ofimmediately substituting low carbon energy sources. Thebenefit declines more rapidly with leakage for short transi-tions. The top three curves assume an IPCC methaneclimate sensitivity (=CH4 = 1.43). The bottom two showthe impact of the greater methane climate sensitivity sug-gested by Shindell et al. [2009] (=CH4 = 1.94). Theocean mixing curve adds the small additional impactof thermal exchange with the oceans at the rate shownin Figure 4B to the =CH4 = 1.94 curve immediatelyabove it.

Table 2. Leakage of Natural Gas That is Common to Both Conventional and Unconventional Gas Wells in Percentof Gas Production

Gas ResearchInstitute–U.S. EnvironmentalProtection Agency [1997] EPA [2010] EPA [2011] Skone [2011]

Venkatesh et al.[2011]

Routine site 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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in column 5 of Table 2. His other well classes aresimilar. Venkatesh et al. [2011] carried out anindependent assessment that is given in column 6.There are variations in these assessments, but overalla leakage of �1.5% of production is suggested.Additional discussion of this data and its compi-lation can be found in Cathles et al. [2012; seealso online press release, 2012] and L. M. Cathles(Perspectives on the Marcellus gas resource: Whatbenefits and risks are associated with Marcellusgas development?, online blog, http://blogs.cornell.edu/naturalgaswarming/, 2012).

[37] Based on the above review the natural gasleakage rate appears to be no different during thedrilling and well preparation of unconventional(tight shales drilled horizontally and hydrofractured)gas wells than for conventional gas wells, and theoverall leakage from gas wells is probably <1.5% ofgas production. In their controversial paper sug-gesting that gas could be twice as bad a coal from agreenhouse warming perspective, Howarth et al.[2011, 2012] suggested routine site leaks could beup to 1.9% of production, leakage during transpor-tation, storage, and distribution could be up to 3.6%or production, and gas leakage from unconventionalgas wells during well completion and workovercould be 1.9% of production. Adding 0.45% leak-age for liquid unloading and gas processing, thesuggested gas leakage could be 7.9% of production,enough to “undercut the logic of its use as a bridgingfuel in the coming decades, if the goal is to reduceglobal warming.”

[38] The basis given by Howarth et al. [2011] fortheir more than fivefold increase in leakage duringtransportation, storage, and distribution is: (1) aleakage in Russian pipelines that occurred duringthe breakup of the Soviet Union which is irrelevantto gas pipelines in the U.S., and (2) a debate on theaccounting of gas in Texas pipelines that concernsroyalties and tax returns [Percival, 2010]. Howarthet al. suggest in this Texas case that the industry isseeking to hide methane losses of more than 5% ofthe gas transmitted, but the proponents in the articlestate “We don’t think they’re really losing the gas,we just think they’re not paying for it.” In theirfivefold increase in routine gas leaks (from theaverage level in Table 2 of 0.38% to 1.9%),Howarth et al.’s [2011] cite a GAO study of ventingfrom wells in onshore and offshore governmentleases that does not distinguish venting from flaring.Lacking this distinction, it is not surprising thatit conflicts dramatically with the summaries in

Table 2. We have already discussed leakage duringwell completion and workover and noted that theDevon data indicate Howarth et al.’s 1.9% leakageat this stage is hugely exaggerated (the Devon dataindicates the leakage is �0.01% and similar tothat from conventional gas well completions andworkovers).

[39] There have been a number of papers publishedrecently that offer support for Howarth’s high leak-age estimates. Hughes [2011] re-interpreted datapresented in a widely distributed NETL powerpointanalysis by Skone [2011]. By lowering Skone’sEstimated Ultimate Recoveries (EUR) for theBarnell Shale from 3 Bcf to 0.84 Bcf while keepingthe same estimate of leakage during well completionand gas delivery, Hughes increased Skone’s leakageestimates from 2 to 6% of production- a level whichfalls midway between Howarth’s low and high gasleakage estimates. However, leakage is a fraction ofwell production (a well that does not produce cannotemit), and thus is it bogus to reduce the EUR (thedenominator) without also reducing the numerator(the absolute leakage of the well). Skone’s data mustbe evaluated on its own terms, not simply adjustedto fit someone else’s conclusions.

[40] Pétron et al. [2012] analyzed air samples at the300 m high Bolder Atmospheric Observatory (BAO)tower when the wind was toward it from across theDenver-Julesburg Basin (DJB). Gases venting fromcondensate (condensed gas from oil and wet gaswells) stock tanks in the DJB are rich in propanerelative to methane, whereas the raw natural gasventing from gas wells in the DJB contain very littlepropane. From the intermediate ratio of propane tomethane observed at the BAO tower and estimatesof leakage from the stock tanks, Pétron et al. calcu-late that to dilute the propane leaking from the stocktanks to the propane/methane ratio observed at thetower, �4% of methane produced by gas wells inthe DJB must vent into the atmosphere. The airsampled at the BAO tower is certainly not simply amix of raw natural gas and stock tank emissionsfrom the DJB as Pétron et al. assume, however. Ifthis were the case there would be no oxygen in theair at the BAO tower location. The backgroundatmosphere must certainly mix in with these two(and perhaps other) gas sources. Background air inthe Denver area contains �1800 ppb methane andvery little propane. Mixing with the backgroundatmosphere could dilute the stock tank emissions tothe propane/methane ratio observed at the BAOtower with no leakage from gas wells in the DJB


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required at all. Contrary to their suggestion, theBAO tower data reported by Pétron et al. place noconstraints at all on the gas leakage rates in the DJB.More details are in Cathles (online blog, 2012).

[41] Certainly there is more we could learn aboutnatural gas leakage rates. The issue is complicatedbecause gas is used in the transmission process soshrinkage of product does not equate to venting. Inaddition there are conventions and practices thatmake scientific assessment difficult. Despite thedifficulties, however, it appears that the leakagerate is less than 2% of production.

6. Discussion

[42] We have verified our computations by com-paring them to predictions by Wigley’s [2011b]publically available and widely used MAGICCprogram. Although there are some internal differ-ences, Table 3 shows that the �40% reduction ingreenhouse warming we predict is also predictedby MAGICC when scenarios similar to the onewe consider here are input to both MAGICC andour programs. The MAGICC calculations start at1990 AD so we consider the temperature increasesfrom 2000 to the end of the period. Fuel use isincreased and reduced linearly rather than expo-nentially, and the fuel use at the start, midpoint,and end of the transition simulations are slightlydifferent than in Figure 1. The temperature changesfor the 200 year cycle agree very well. Wigley’s

MAGICC temperature change predictions becomeprogressively lower than ours as the transition inter-val is shortened. This may be because MAGICCincludes a small ocean thermal interaction, whereasthe calculations we report in Table 3 do not.

[43] Incorporation of the indirect contributionsto methane’s radiative forcing through =CH4 inequation (6) was validated by comparing valuesof GWP computed by (13) to published valuessummarized in Table 4.

GWP ¼Y

CH4

∂DFCH4

∂CCH4½ppbv�MWCO2

Z t

t¼0

fCH4 dt

∂DFCO2

∂CCO2½ppbv�MWCH4

Z t

t¼0

fCO2 dt

: ð13Þ

GWP is the relative global warming impact of a kgof CH4 compared to a kg of CO2 added to theatmosphere, when considered over a period oftime t. The radiative forcings (DF) are defined by(6), the removal of the gases from the atmosphere( f ) by (4a and 4b), and MWCO2 is the molecularweight of CO2. The =CH4 factor of 1.43 in thesecond column of Table 4 combines the indirectforcing caused by CH4-induced production of ozone(25% according to IPCC [2007]) and water vapor inthe stratosphere (additional 15% according to IPCC[2007]). With this factor the GWP listed inTable 2.14 of IPCC [2007] are replicated as shownin the second row of Table 4. The =CH4 factor of

Table 3. Temperature Changes Predicted by Wiglely’s [2011a, 2011b] MAGICC Program for Linear Changes inFuel Use Similar to the Scenarios in Figure 1 Compared to Equilibrium (No Ocean Thermal Interaction) GlobalWarming Predictions by the Program Described and Used in This Papera

200 Year Cycle 100 Year Cycle 40 Year Cycle

Program MAGICC This paper MAGICC This paper MAGICC This paperB-as-usual 3.85 3.68 2.3 2.56 1.05 1.5Swap gas 2.85 2.85 1.65 1.94 0.80 1.12Low C fast 1.7 1.70 0.85 1.09 0.38 0.58% reduction 42% 42% 45% 42% 37% 41%

aThe first three rows compare the temperature changes of the two programs. The last row shows the reduction in greenhouse warming achievableby substituting natural gas for coal and oil as a percentage of the reduction that would be achieved by the rapid substitution of all fossil fuels withlow carbon energy sources.

Table 4. The GWP Calculated From (6) and (13) for the Value of =CH4 in Column 2 are Compared to GWP(in Parentheses) Given by the IPCC [2007] and Shindell et al. [2009]

=CH4 t = 20 Years t = 100 Years t = 500 Years

Direct methane forcing from (6) 1 51.5 17.9 5.45IPCC [2007, §2.10.3.1, Table 2.14] 1.43 73.5 (72) 25.8(25) 7.8 (7.6)Shindell et al. [2009] 1.94 99(105) 35(33) 10.5


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1.94 in the second column was determined by ussuch that it approximately predicts the increasedforcings suggested by Shindell et al. [2009] asshown in the bottom row of Table 4. We do notuse GWPs in our analysis and use them here onlyto justify the values of =CH4 used in ourcalculations.

[44] The most important message of the calcula-tions reported here is that substituting natural gasfor coal and oil is a significant way to reducegreenhouse forcing regardless of how long (withina feasible range) the substitution takes (Figure 4).For methane leakages of �1% of total consump-tion, replacing coal used in electricity generationand 50% of the oil used in transportation withnatural gas (very feasible steps that could be drivenby the low cost of methane alone with no govern-ment encouragement) would achieve �40% ofthe greenhouse warming reduction that could beachieved by transitioning immediately to low car-bon energy sources such as wind, nuclear, or solar.A faster transition to low-carbon energy sourceswould decrease greenhouse warming further, butthe substitution of natural gas for the other fossilfuels is equally beneficial in percentage terms nomatter how fast the transition.

[45] The basis for the �40% reduction in green-house forcing is simply the reduction of the CO2

put into the atmosphere. When gas leakage is low,the contribution of methane to greenhouse warmingis negligible (Figure 3), and only the CO2 inputcounts. The reduction in CO2 vented between thebusiness-as-usual and the substitute-gas scenariosis 44.1% of the reduction between the business-as-usual to the low-carbon-fast scenarios. Thisfraction is independent of the transition period; itis the same whether the transition occurs over50 years or 200 years. Because the losses of CO2

from the atmosphere (equation (4a)) are propor-tional to the amount of CO2 in the atmosphere, therelative amounts of CO2 at the end of the transitionare similar to the proportions added. For the sametransition interval almost the same proportionalamounts of CO2 are removed for all scenarios.Thus the fractional substitute-gas reduction in CO2

in the atmosphere at the end of all the transitionintervals remains 44.1% although there are somevariations in the second decimal place. The curvesshown in Figure 7 intersect the y axis (0% gasleakage) at fractions slightly different from 44.1%because the radiative forcing is nonlinear withrespect to CO2 concentration (equation (5a)). The

longer transition periods show larger nonlineareffects because they put more CO2 into the atmo-sphere. The nearly direct relationship betweenreductions in the mass of CO2 vented and thedecrease in global warming is a powerful concep-tual simplification that is particularly useful becauseit is so easy to calculate, a point made by Allen et al.[2009].

[46] The global warming reduction from swappinggas for the other fossil fuels of course decreases asmethane leakage increases. But at low leakage rates,the benefit of substituting natural gas remains closeto 40%. In the context of swapping gas for coal, theextra methane emitted by low levels of leakage hassuch a trivial climate effect that it need not beconsidered at all.

[47] Sulfur dioxide additions are not a factor in ouranalysis because the substitute-gas and low-carbon-fast scenarios reduce the burning of coal over thegrowth period in an identical fashion. Thus bothreduce SO2 identically, and the small warmingeffects of reducing SO2 emissions, which will occurno matter how coal is retired, cancel in the com-parison. In the real world the “aerosol benefit” ofcoal must be removed eventually (unless we are toburn coal forever), and the sooner it is removed thebetter both because the small warming its removalwill cause will have less impact when temperaturesare cooler, and, much more importantly, becausereplacing coal soon will reduce CO2 emissions andlead to much less global warming in the longer term.

[48] Wigley’s [2011a] decrease in greenhousewarming for the natural gas substitution he definesis similar to that we compute here. At 0% leakage,Wigley [2011a, Figure 3] calculates a 0.35�C cool-ing which would be a 0.45�C cooling absent thereduced SO2 emissions he considers. We calculatea cooling of �0.62�C for 0% leakage. Our coolingis greater than his at least in part because our gassubstitution scenario reduces the CO2 emissionsmore than his. From nearly the same start, ourgas substitution reduces CO2 emissions from thebusiness-as-usual 200 year transition cycle by743 GtC whereas Wigley reduces CO2 by 425 GtC.

[49] There are of course uncertainties in the kind ofcalculations carried out here, but these uncertaintiesare unlikely to change the conclusions reached.Carbon dioxide is almost certainly not removedfrom the atmosphere exactly as described byequations (3a) and (3b). The uptake of CO2 maywell slow as the climate warms. Carbon dioxide is


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less soluble in warm water and the haline circulationmay slow as the sea surface temperature increases.The increase in terrestrial CO2 uptake from CO2

fertilization may be reduced by nitrogen limitations.A good discussion of these issues is provided inNRC [2011]. Eby et al. [2009] have suggestedbased on sophisticated coupled global models that�50% of the introduced CO2 may be removedwith a time constant of 130 years and 50% with anexponential time constant of 2900 years. Modifica-tions of equation (3a) and (3b) that reduce CO2

uptake as the climate warms will make the benefitsof not putting CO2 into the atmosphere, for exampleby substituting gas for coal, even greater, and thearguments presented here stronger.

[50] The transmission of heat from the mixed tothe deep layer of the oceans is an unknown whichhas a strong impact on transient global warming.For example, if heat entered the deep layer with10% of the ease with which it enters it from theatmosphere so that gls

�1 �0.1, the deep layerwould largely loose its cooling effectiveness (e.g.,a in equation (11) would have a value of 0.91).The transient response to CO2 forcing would berapid (occur at 0.91 tmix), and the ocean wouldreduce the equilibrium global temperature changeby only 9%. The relative rates at which heat istransferred into the mixed layer and out of it intothe deep layer would appear to be an important

area for further investigation, especially because itimpacts our ability to infer proper values in theequilibrium climate forcing (see discussion inNRC [2011]). Ocean heat exchange does notaffect the comparative benefit of substituting gas,so uncertainties in the ocean heat exchange arenot of concern to the conclusions we reach here.

[51] The calculations made here avoid the useof GWP factors. The deficiencies in the GWPapproach are discussed well by Solomon et al.[2011]. As is apparent from (13), the GWP metricrequires that the time period of comparison bespecified. For a short time period, a short-lived gaslike methane has a high GWP (e.g., it is 72 timesmore potent in terms of global warming than CO2

when compared over a 20 year). The notion thatmethane emissions have 72 times the globalwarming impact of CO2 would tempt eliminatingmethane emissions immediately, and worrying aboutreducing CO2 emissions later. On the other hand fora 500 year period, the global warming impact of akilogram of vented methane is only 7.6 that of akilogram of CO2 (GWPCH4 = 7.6, see Table 4),and this low impact would suggest dealing with CO2

emissions first and the methane emissions later,perhaps even substituting gas for coal and oil. AsSolomon et al. point out the GWP metric speaksonly to the time period for which it is calculatedand sheds no light on the whether CO2 or CH4

should be reduced first.

[52] Figure 8 illustrates the fundamental dilemma.It shows that even when methane leakage is solarge (L = 10% of consumption) that substitutinggas for coal and oil increases global warming in theshort term, the benefit of gas substitution returns inthe long-term. The short-term heating caused bymethane leakage rapidly dissipates after emissionsof CO2 and CH4 cease at 100 years. CH4 is rapidlyremoved from the atmosphere, but CO2 is not.The result is that 50 years or so after the terminationof venting (beyond 150 years in Figure 8), thebenefit of gas emerges unscathed. At a 10% leakagerate and a 100 year transition period, the substitute-gas scenario produces a small amount more warm-ing than the business-as-usual scenario at 70 years,but after 150 years the gas substitution reducesglobal warming much more because it has reducedthe amount of CO2 vented to the atmosphere.Figure 8 shows how dangerous a metric such asGWP can be. Even for methane emissions of 9% ofproduction and Shindell’s forcings, substituting gasfor coal is worthwhile in the long-term. Analysesthat rely only on GWP factors, such as that ofHowarth et al. [2011], miss this mix of impacts

Figure 8. Temperature change for scenarios in Figure 1when a transition period is 100 years is followed by a400 year period with no burning of fossil fuels. Methaneleakage in the transition is 10% of gas consumption,Shindell’s greater methane forcing (yCH4 = 1.94), andheat exchange with the ocean is included. Extra methaneventing in the substitute-gas scenario produces warminggreater than the business-as-usual scenario up to almostthe end of the transition, but the benefits of reducingcarbon emissions by substituting gas emerge very quicklythereafter.


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completely, and see only the damage of extramethane emissions in the short term or the benefitsof gas substitution in the long-term, depending onthe GWP interval selected. Fortunately it is veryeasy to carry out the necessary convolution integrals(equations (5) and (11)) as done here and avoidGWPmetrics altogether. As stated by Solomon et al.[2011] and others who they cite, GWP factorsshould simply not be used to evaluate fuel con-sumption scenarios.

[53] Finally, framing the fuel use scenarios in termsof exponential growth and decline as we have donehere allows the feasibility of implementing thevarious scenarios to be examined in a preliminaryfashion. Figure 9 shows the rate of growth of lowcarbon energy resources that is required by the fuelhistories in Figure 1 for a 100 year transition.Growth at more than 5% per year would be chal-lenging. Figure 9 shows that the low-carbon-fastscenario in Figure 1 requires an immediate �16%per year (but rapidly declining) growth in lowcarbon energy sources. The growth rate of lowcarbon energy sources at the end of the growthperiod of the business-as-usual scenario is aneven greater 24% per year. Because there is timeto plan, this could be reduced by phasing in lowcarbon energy sources toward the end of thefossil fuel growth period. The substitute-gas sce-nario has a much lower growth requirement atthis stage, which would make this scenario sub-stantially easier to accommodate.

[54] Any decision to substitute gas for coal and oil ofcourse involves economic and social consideration,

as well as climate analysis. Natural gas can enablethe transition to wind or solar energy by providingthe surge capacity when these sources fluctuate andbackup when these sources wane. Because of itswide availability and low cost, economic factors willencourage gas replacing coal in electricity genera-tion and oil in segments of transportation. It is a fuelthe U.S. and many other countries need not import,so its development could increase employment,national security, and a more positive balance ofpayments. On the other hand, cheap and availablegas might undermine the economic viability of lowcarbon energy sources and delay a transition to lowcarbon sources. From a greenhouse point of view itwould be better to replace coal electrical facilitieswith nuclear plants, wind farms, or solar panels, butreplacing them with natural gas stations will befaster, cheaper and achieve 40% of the low-carbon-fast benefit if the leakage is low. How this balance isstruck is a matter of politics and outside the scopeof this paper. What can be said here is that gas isa natural transition fuel that could represents thebiggest available stabilization wedge available to us.

7. Conclusions

[55] The comparative approach taken in this papershows that the benefit of substituting natural gasdepends only on its leakage rate.

[56] 1. For leakage rates �1% or less, the substitu-tion of natural gas for the coal used in electricitygeneration and for 55% of the oil used in trans-portation and heating achieves 40% of the reduc-tion that could be attained by an immediatetransition to low-carbon energy sources.

[57] 2. This 40% reduction does not depend on theduration of the transition. A 40% reduction isattained whether the transition is over 50 years or200 years.

[58] 3. For leakage rates �1% or less, the reductionof greenhouse warming at all times is relateddirectly to the mass of CO2 put into the atmosphere,and therefore to reduce greenhouse forcing we mustreduce this CO2 input. Complexities of how CO2 isremoved and reductions in SO2 emissions andincreases in carbon black and the like do not changethis simple imperative and should not be allowed toconfuse the situation.

[59] 4. At low methane leakage rates, substitutingnatural gas is always beneficial from a greenhousewarming perspective, even for forcings as high ashave been suggested by Shindell et al. [2009] and

Figure 9. The growth rate of low carbon energysources deduced from Figure 1 plotted as a functionof time for a 100 year transition. Growth rates morethan 5% per year will be challenging to achieve on aglobal basis.


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used by Howarth et al. [2011]. Under the fastesttransition that is probably feasible (our 50 yeartransition scenario), substitution of natural gas willbe beneficial if the leakage rate is less than about7% of production. For a more reasonable transitionof 100 years, substituting gas will be beneficial ifthe leakage rate is less than �19% of production(Figure 7). The natural gas leakage rate appears tobe presently less than 2% of production and prob-ably �1.5% of production.

[60] 5. Even if the natural gas leakage rate werehigh enough to increase greenhouse warming (e.g.,the leakage was 10% of methane consumption or9% of methane production), substituting gas wouldstill have benefits because the reduction of CO2

emissions would lead to a greater reduction ingreenhouse warming later (Figure 8).

[61] 6. Gas is a natural transition fuel because itssubstitution reduces the rate at which low carbonenergy sources must be later introduced (Figure 9)and because it can facilitate the introduction of lowcarbon energy sources.

[62] The policy implications of this analysis are:(1) reduce the leakage of natural gas from pro-duction to consumption so that it is �1% ofproduction, (2) encourage the rapid substitution ofnatural gas for coal and oil, and (3) encourage asrapid a conversion to low carbon sources of energyas possible.

Acknowledgments

[63] This paper was greatly improved by three excellentreviews, two anonymous and one by Ray Pierrehumbert. Raypointed out the importance of ocean mixing, suggested castingfuel use in terms of exponential growth and decline, and drewmy attention to important references (as did the other reviewers).I am indebted to my prior co-authors in this subject (MiltonTaam, Larry Brown, and Andrew Hunter) for continuing veryhelpful discussions, and to members of the gas industry whopointed out data and helped me understand the complexities ofgas production. Milton Taam drew my attention to the MAGICCprogram and showed me how easy it was to use, and also pushedpersistently for the broader view of methane substitution shownin Figure 8. The paper would not be what it is without the con-tribution of these individuals and I thank them for their input.

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