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Biogeosciences, 16, 3033–3046, 2019
https://doi.org/10.5194/bg-16-3033-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
Ideas and perspectives: is shale gas a major driver of recent increase
in global atmospheric methane?
Robert W. Howarth
Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, USA
Correspondence: Robert W. Howarth (howarth@cornell.edu)
Received: 10 April 2019 – Discussion started: 23 April 2019
Revised: 11 July 2019 – Accepted: 12 July 2019 – Published: 14 August 2019
Abstract. Methane has been rising rapidly in the atmosphere
over the past decade, contributing to global climate change.
Unlike the late 20th century when the rise in atmospheric
methane was accompanied by an enrichment in the heav-
ier carbon stable isotope (13C) of methane, methane in re-
cent years has become more depleted in 13C. This depletion
has been widely interpreted as indicating a primarily bio-
genic source for the increased methane. Here we show that
part of the change may instead be associated with emissions
from shale-gas and shale-oil development. Previous studies
have not explicitly considered shale gas, even though most
of the increase in natural gas production globally over the
past decade is from shale gas. The methane in shale gas is
somewhat depleted in 13C relative to conventional natural
gas. Correcting earlier analyses for this difference, we con-
clude that shale-gas production in North America over the
past decade may have contributed more than half of all of the
increased emissions from fossil fuels globally and approx-
imately one-third of the total increased emissions from all
sources globally over the past decade.
1 Introduction
Methane is the second most important greenhouse gas behind
carbon dioxide causing global climate change, contributing
approximately 1 W m−2to warming when indirect effects
are included compared to 1.66 W m−2for carbon dioxide
(IPCC, 2013). Unlike carbon dioxide, the climate system
responds quickly to changes in methane emissions, and re-
ducing methane emissions could provide an opportunity to
immediately slow the rate of global warming (Shindell et
al., 2012) and perhaps meet the United Nations Framework
Convention on Climate Change (UNFCCC) COP21 target of
keeping the planet well below 2 ◦C above the pre-industrial
baseline (IPCC, 2018). Methane also contributes to the for-
mation of ground-level ozone, with large adverse conse-
quences for human health and agriculture. Considering these
effects as well as climate change, Shindell (2015) estimated
that the social cost of methane is 40 to 100 times greater than
that for carbon dioxide: USD 2700 per ton for methane com-
pared to USD 27 per ton for carbon dioxide when calculated
with a 5 % discount rate and USD 6000 per ton for methane
compared to USD 150 per ton for carbon dioxide when cal-
culated with a 1.4 % discount rate.
Atmospheric methane levels rose steadily during the last
few decades of the 20th century before leveling off for
the first decade of the 21st century. Since 2008, how-
ever, methane concentrations have again been rising rapidly
(Fig. 1a). This increase, if it continues in coming decades,
will significantly increase global warming and undercut ef-
forts to reach the COP21 target (Nisbet et al., 2019). The
total atmospheric flux of methane for the period 2008–2014
was ∼24.7 Tg per year greater than for the 2000–2007 pe-
riod (Worden et al., 2017), an increase of 7 % in global
human-caused methane emissions. The change in the sta-
ble carbon δ13C ratio of methane in the atmosphere over
the past 35 years is striking and seems clearly related to the
change in the methane concentration (Fig. 1b). For the final
20 years of the 20th century, as atmospheric methane con-
centrations rose, the isotopic composition became more en-
riched in the heavier stable isotope of carbon, 13C, relative
to the lighter and more abundant isotope, 12C, resulting in a
less negative δ13C signal. The isotopic composition remained
constant from 1998 to 2008, when the atmospheric concen-
tration was constant. And the isotopic composition has be-
Published by Copernicus Publications on behalf of the European Geosciences Union.
3034 R. W. Howarth: Shale gas and global methane
Figure 1. (a) Global increase in atmospheric methane between
1980 and 2015. (b) Change in δ13C value of atmospheric methane
globally between 1980 and 2015. Both adapted from Schaefer et
al. (2016).
come lighter (depleted in 13C, more negative δ13C) since
2009, as atmospheric methane concentrations have been ris-
ing again (Schaefer et al., 2016; Nisbet et al., 2016). Since
biogenic sources of methane are lighter than the methane
released from fossil-fuel emissions, Schaefer et al. (2016)
concluded that the increase in atmospheric methane in the
late 20th century was due to increasing emissions from fos-
sil fuels but that the increase in methane since 2006 is due
to biogenic sources, most likely tropical wetlands, rice cul-
ture, or animal agriculture. Their model results indicated that
fossil-fuel sources have remained flat or decreased globally
since 2006, playing no major role in the recent atmospheric
rise of methane. Schaefer et al. (2016) noted that their con-
clusion contradicted many reports of increased emissions
from fossil-fuel sources over this time and stated that their
conclusion was “unexpected, given the recent boom in un-
conventional gas production and reported resurgence in coal
mining and the Asian economy”. Six months after the Schae-
fer et al. (2016) study was published in Science, Schwietzke
et al. (2016) presented a similar analysis in Nature that used
a larger and more comprehensive data set for the δ13C values
of methane emission sources. They too concluded that fossil-
fuel emissions have likely decreased during this century and
that biogenic emissions are the probable cause of any recent
increase in global methane emissions.
2 Sensitivity of emission models based on δ13C in
methane to biomass burning
Model analyses that use δ13C methane data to infer emis-
sion sources are highly sensitive to changes in the rate of
biomass burning: although biomass burning is a relatively
small contributor to global methane emissions, those emis-
sions are quite enriched in 13C relative to the atmospheric
methane signal (Rice et al., 2016; Sherwood et al., 2017).
Both Schaefer et al. (2016) and Schwietzke et al. (2016)
assumed that biomass burning had been constant in recent
years. However, Worden et al. (2017) estimated that biomass
burning globally went down for the period 2007–2014 com-
pared to 2001–2006, resulting in decreased methane emis-
sions of 3.7 Tg per year (±1.4 Tg per year) and contributing
to a lower δ13C for atmospheric methane. Using the data set
of Schwietzke et al. (2016) for δ13C values of methane emis-
sion sources, but including changes in biomass burning over
time, Worden et al. (2017) concluded that the recent increase
in methane emissions was likely driven more by fossil fu-
els than by biogenic sources, with an increase of 16.4 Tg per
year from fossil fuels (±3.6 Tg per year) compared to an in-
crease of 12 Tg per year from biogenic sources (±2.5 Tg per
year) when comparing 2007–2014 to 2001–2006.
Clearly global models for partitioning methane sources
based on the δ13C approach are sensitive to assumptions
about seemingly small terms such as decreases in biomass
burning. In this paper, we explore for the first time another as-
sumption: that the global increase in shale-gas development
may have caused some of the depletion of 13C in the global
average methane observed over the past decade. Shale-gas
emissions were not explicitly considered in the models pre-
sented by Schaefer et al. (2016) and Worden et al. (2017)
and were explicitly excluded in the analysis of Schwietzke et
al. (2016).
3 What is shale gas?
Shale gas is a form of unconventional natural gas (mostly
methane) held tightly in shale-rock formations. Conventional
natural gas, the dominant form of natural gas produced dur-
ing the 20th century, is composed largely of methane that
migrated upward from the underlying sources such as shale
rock over geological time, becoming trapped under a geo-
logical seal (Fig. 2a). Until this century, shale gas was not
commercially developable. The use of a new combination
of technologies in the 21st century – high-precision direc-
tional drilling, high-volume hydraulic fracturing, and clus-
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R. W. Howarth: Shale gas and global methane 3035
Figure 2. (a) Schematic comparing shale gas and conventional nat-
ural gas. For conventional natural gas, methane migrates from the
shale through semipermeable formations over geological time, be-
coming trapped under a geological seal. Shale gas is methane that
remained in the shale formation and is released through the com-
bined technologies of high-precision directional drilling and high-
volume hydraulic fracturing. (b) Global production of shale gas and
other forms of natural gas from 2000 to 2017, with projections into
the future from EIA (2016). Redrawn from EIA (2016) with data
from IEA (2017).
tered multi-well drilling pads – has changed this. In re-
cent years, global shale-gas production has exploded 14-fold,
from 31 billion cubic meters per year in 2005 to 435 billion
cubic meters per year in 2015 (Fig. 2b), with 89 % of this
production in the United States and 10 % in western Canada
(EIA, 2016). Shale gas accounted for 63 % of the total in-
crease in natural gas production globally over this time pe-
riod (EIA, 2016; IEA, 2017). The US Department of Energy
predicts rapid further growth in shale-gas production glob-
ally, reaching 1500 billion cubic meters per year by 2040
(EIA, 2016; Fig. 2b).
Several studies have suggested that the δ13C signal of
methane from shale gas can often be lighter (more depleted
in 13C) than that from conventional natural gas (Golding et
al., 2013; Hao and Zou, 2013; Turner et al., 2017; Botner
et al., 2018). This should not be surprising. In the case of
conventional gas, the methane has migrated over geological
time frames from the shale and other source rocks through
permeable strata until trapped below a seal (Fig. 2a). Dur-
ing this migration, some of the methane can be oxidized both
by bacteria, perhaps using iron (III) or sulfate as the source
of the oxidizing power, and by thermochemical sulfate re-
duction (Whelan et al., 1986; Burruss and Laughrey, 2010;
Rooze et al., 2016). This partial oxidation fractionates the
methane by preferentially consuming the lighter 12C isotope
and gradually enriching the remaining methane in 13C (Hao
and Zou, 2013; Baldassare et al., 2014), resulting in a δ13C
signal that is less negative. The methane in shales, on the
other hand, is tightly held in the highly reducing rock for-
mation and therefore very unlikely to have been subject to
oxidation and the resulting fractionation. The expectation,
therefore, is that methane in conventional natural gas should
be heavier and less depleted in 13C than the methane in shale
gas.
4 Calculating the effect of 13C signal of shale gas on
emission sources: conceptual framework
To explore the contribution of methane emissions from shale
gas, we build on the analysis of Worden et al. (2017). Fig-
ure 3a shows the δ13C values used by them as well as their
mean estimates for changes in emissions since 2008 (as they
estimated using the δ13C data of Schwietzke et al., 2016).
Figure 3a represents a weighting for the change in emis-
sions (yaxis) and the δ13C values of those emissions (x
axis) by individual sources. Our addition is to separately con-
sider shale-gas emissions, recognizing that methane emis-
sions from shale gas are more depleted in 13C than for con-
ventional natural gas or other fossil fuels as considered by
Worden et al. (2017). For this analysis, we accept that net to-
tal emissions have increased by 24.7 Tg per year (±14.0 Tg
per year) since 2007, driven by an increase of ∼28.4 Tg per
year for the sum of biogenic emissions and emissions from
fossil fuels and a decrease of ∼3.7 Tg per year for emissions
from biomass burning (Worden et al., 2017).
We start with Eq. (1), which expresses the findings of Wor-
den et al. (2017):
BW+FFW=28.4 Tg yr−1,(1)
where BWand FFWare the estimates from Worden et
al. (2017) for the increase respectively in biogenic emissions
and fossil-fuel emissions of methane globally since 2007.
Equation (2) explicitly considers methane emissions from
shale gas:
BN+FFN+SG =28.4 Tg yr−1,(2)
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3036 R. W. Howarth: Shale gas and global methane
Figure 3. (a) On the xaxis, δ13C values for methane from biogenic sources, fossil fuel, and biomass burning as presented in Worden et
al. (2017) for values from Schwietzke et al. (2016); width of horizontal bars represents the 95 % confidence limits for these values. Triangle
indicates the flux-weighted mean input of methane to the atmosphere. The yaxis shows mean estimates from Worden et al. (2017) for
the increase and decrease in methane emissions from particular sources since 2007 as calculated using the δ13C values of Schwietzke et
al. (2016). (b) On the xaxis, δ13C values as in Fig. 3a, except the value for fossil fuels does not include shale gas and a separate estimate for
shale-gas value is included (see text). The yaxis indicates estimates developed in this paper for the increase or decrease in methane emissions
since 2008.
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R. W. Howarth: Shale gas and global methane 3037
where BNis our new estimate for the increase in the biogenic
fluxes since 2007, FFNis our new estimate for the increase
in fossil-fuel emissions other than shale gas since 2007, and
SG is our estimate for emissions from shale gas since 2007.
Subtracting Eq. (2) from Eq. (1),
(BW−BN)+(FFW−FFN)−SG =0.(3)
Equation (4) builds on Eq. (3) and reweights the informa-
tion in Fig. 3a for the difference between most fossil fuels
and shale gas, multiplying global mass fluxes for each source
by the difference between the δ13C ratio of each source and
the flux-weighted mean for all sources:
(BW−BN)·DB−A+(FFW−FFN)·DFF−A
−(SG ·DSG−A)=0,(4)
whereDB−A,DFF−A, and DSG−Aare the differences in the
δ13C ratio of biogenic emissions, fossil fuels, and shale
gas compared to the flux-weighted mean δ13C ratio for all
sources (A). The xaxis of Fig. 3b shows the δ13C for each
source; note that the yaxis is the estimate of the change in
emissions for each of the sources that we derive below. Next,
we multiply both sides of Eq. (3) byDB−A,
(BW−BN)·(DB−A)+(FFW−FFN)·DB−A
−SG ·(DB−A)=0.(5)
Subtracting Eq. (5) from Eq. (4),
(FFW−FFN)·(DFF−A−DB−A)
−SG ·(DSG−A−DB−A)=0.(6)
Rearranging Eq. (6) to solve for SG,
SG =(FFW−FFN)·(DFF−A−DB−A)/
(DSG−A−DB−A).(7)
Note that from Worden et al. (2017), FFWis 16.4 Tg per
year.
Although our expectation is that the methane in shale gas
is depleted in 13C relative to conventional natural gas, the
δ13C ratios for the methane in both conventional gas reser-
voirs and in shale gas vary substantially, changing with the
maturity of the gas and several other factors (Golding et al.,
2013; Hao and Zou, 2013; Tilley and Muehlenbachs, 2013).
The large data set of Sherwood et al. (2017) suggests no sys-
tematic difference between the average ratio for shale gas and
the average for conventional gas. However, some of the data
listed as shale gas in that data set are actually for methane
that has migrated from shale to reservoirs (Tilley et al., 2011)
and therefore may have been partially oxidized and fraction-
ated (Hao and Zou, 2013). In other cases, the data appear
to come both from conventional vertical wells and shale-
gas horizontal wells in the same region, making interpreta-
tion ambiguous (Rodriguez and Philp, 2010; Zumberge et
al., 2012). Note that in the Barnett shale region, Texas, the
δ13C ratio for methane emitted to the atmosphere (−46.5 ‰;
Townsend-Small et al., 2015) is more depleted than the av-
erage for wells reported in the Sherwood et al. (2017) data
set: −44.8 ‰ for “group 2A and 2B” wells and −38.5 ‰ for
“group 1” wells (Rodriguez and Philp, 2010) and a −41.1 ‰
average value (Zumberge et al., 2012). For our analysis, we
use the mean of the δ13C ratio (−46.9 ‰) from three stud-
ies where the methane clearly came from horizontal, high-
volume fractured shale wells: −47.0 ‰ for Bakken shale,
North Dakota (Schoell et al., 2011), −46.5 ‰ for Barnett
shale, Texas (Townsend-Small et al., 2015), and −47.3 ‰
for Utica shale, Ohio (Botner et al., 2018). Note that sev-
eral studies have reported mean δ13C ratios for methane
from organic-rich shales that are more depleted in 13C (more
negative) than this: −50.7 (Martini et al., 1998) for Antrim
shale, Michigan, −53.3 (McIntosh et al., 2002) and −51.1
(Schlegel et al., 2011) for New Albany shale, Illinois, and
−49.3 (Osborn and McIntosh, 2010) for a Devonian shale
in Ohio. However, these shales are not typical of the major
shale plays supporting the huge increase in gas production
over the past decade.
The average δ13C ratio for methane in the atmosphere (A)
in 2005 was −47.2 ‰ (Schaefer et al., 2016), which reflects
a flux-weighted mean input of methane with a δ13C ratio of
−53.5 ‰. This flux-weighted mean value is approximately
6.3 ‰ more depleted in 13C because of fractionation during
the oxidation of methane in the atmosphere (Schwietzke et
al., 2016; Sherwood et al., 2017). In our analysis, we use this
flux-weighted mean value of −53.5 ‰. Therefore, the mean
value for DFF−Ais −9.5 ‰, the value for DB−Ais 9.0 ‰, and
the value for DSG−Ais −6.6 ‰ (Fig. 3b). Substituting these
values into Eq. (7), we see that
SG = −1.19 ·FFN+19.4.(8)
5 Estimating increased methane fluxes for coal, oil, and
natural gas
Next, we estimate the likely contributions from coal and oil
to the increased methane emissions over the past decade. We
estimate the increase in methane emissions from coal be-
tween 2006 and 2016 to be 1.3 Tg per year, based on the rise
in global coal production of 27 %, with almost all of this due
to surface-mined coal in China (IEA, 2008, 2017) and us-
ing a well-accepted emission factor of 870 g methane per ton
of surface-mined coal (Howarth et al., 2011). Methane emis-
sions from surface-mined coal tend to be low, as much of the
methane that was once associated with the coal has degassed
over geological time. This estimate is very close to the 1.1 Tg
per year increase from coal emissions in China between 2009
and 2015 as estimated based on satellite observations (Miller
et al., 2019). For oil, global production increased by 9.6 %
(IEA, 2008, 2017), thereby increasing methane emissions by
approximately 1.6 Tg per year (using emission factors from
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3038 R. W. Howarth: Shale gas and global methane
NETL 2008; as detailed in Howarth et al., 2011). Therefore,
of the increase in 28.4 Tg per year from fossil fuels plus bio-
genic sources since 2005 (see discussion above), we estimate
2.9 Tg per year to be from increased emissions from coal and
oil, leaving an increase of approximately 25.5 Tg per year
from natural gas (including shale gas) plus biogenic sources.
As noted above, shale gas accounted for 63 % of the global
increase in all natural gas production between 2005 and 2015
(EIA, 2016; IEA, 2017). If we make the simplifying assump-
tion that for both shale gas and conventional natural gas,
emissions are equal as a percentage of the gas produced, then
SG =0.63 ·TG,(9)
and
CG =0.37 ·TG,(10)
where TG is total increase in emissions from all natural gas.
Note that we test this assumption later in our sensitivity anal-
yses, since some research indicates that emissions from shale
gas are higher than for conventional gas as a percentage of
gas production. Rearranging Eq. (9) for TG and substituting
into Eq. (10),
CG =0.37 ·(SG/0.63), or CG =0.59 ·SG.(11)
FFNis the sum of CG (0.59 ·SG) plus the emissions from oil
and coal (2.9 Tg per year), or
FFN=(0.59 ·SG)+2.9.(12)
Substituting Eq. (12) into Eq. (8) and solving for SG, we
estimate that the increase in shale-gas emissions between
2005 and 2015 was 9.4 Tg per year (Table 1; Fig. 3b). From
Eq. (11), increased emissions from conventional natural gas
are then estimated to be 5.5 Tg per year, emissions from
all natural gas (shale plus conventional) are estimated to be
14.9 Tg per year, and emissions from all fossil fuels (includ-
ing coal and oil) are estimated to be 17.8 Tg per year. From
Eq. (3), increased emissions from biogenic sources are es-
timated to be 10.6 Tg per year. While the biogenic sources
are important, the increase in fossil-fuel emissions has been
greater, and shale gas makes up more than half of these in-
creased fossil-fuel emissions.
6 Comparison with prior estimates
Our best estimate for the increase in methane emissions from
all fossil fuels since 2008 (shale gas, conventional natural
gas, coal, and oil) of 17.8 Tg per year is 9 % larger than the
mean estimate of Worden et al. (2017) of 16.4 Tg per year
(Table 1). Our estimate for the increased emissions from bio-
genic sources, 10.6 Tg per year, is 12 % lower than the Wor-
den et al. (2017) estimate of 12 Tg per year (Table 1). Thus,
our estimates are not greatly different from those of Worden
et al. (2017), although our estimate for fossil fuels is larger
and our estimate for biogenic fluxes lower than their esti-
mates. On the other hand, comparing emissions for the 2003–
2013 period with those from the late 20th century, Schwi-
etzke et al. (2016) concluded that biogenic emissions had
risen by ∼27 Tg per year, while fossil-fuel emissions had
decreased by ∼18 Tg per year. And Schaefer et al. (2016)
concluded that increased methane emissions since 2006 have
been “predominantly biogenic” and that fossil-fuel emissions
likely have fallen.
We estimate that shale gas has contributed 33 % of the
global increase in all methane emissions in recent years
(Table 1). Since virtually all shale-gas development glob-
ally through 2015 occurred in North America (mostly in the
United States but also western Canada), we conclude that
at least 33 % of the increase in methane fluxes came from
North America. This is consistent with the work of Turner
et al. (2016), who used satellite data to conclude that 30 %
to 60 % of the global increase in methane emissions between
2002 and 2014 came from the United States. On the other
hand, Nisbet et al. (2016, 2019) used monitoring data to in-
fer spatial changes in methane emissions over time and em-
phasized that much of the increase in recent years originated
in the tropics and Southern Hemisphere, although they noted
that the northern temperate latitude played a major role in
the large increase in emissions in 2014 (Nisbet et al., 2019),
a time of major increase in shale-gas development (EIA,
2016). While our estimate for increased emissions from fos-
sil fuels is only marginally greater than that of the Worden
et al. (2017) paper upon which we build our analysis, we
demonstrate the importance of shale gas as a major part of
these increased fossil-fuel emissions and thereby explicitly
link the increased emissions to North America.
Our estimate of increased emissions of 9.4 Tg per year
from shale-gas development is quite reasonable in light of the
growing body of evidence from measurements made at local
to regional scales. Between 2005 and 2015, global shale-gas
production rose by 404 billion cubic meters per year (Fig. 2b;
EIA, 2016). Assuming that 93 % of natural gas is composed
of methane (Schneising et al., 2014), our estimate of the in-
crease in methane emissions from shale gas represents 3.5 %
of the shale-gas production (270 Tg per year of methane pro-
duced from shale-gas operations on average in 2015). This
estimate of 3.5 % (based on global change in the 13C con-
tent of methane) represents full life-cycle emissions, includ-
ing those from the gas well site, transportation, processing,
storage systems, and final distribution to customers. Our es-
timate is well within the range reported in several recent stud-
ies for shale gas and in fact is at the low end for many (but
not all) of these studies (Howarth et al., 2011; Pétron et al.,
2014; Karion et al., 2013; Caulton et al., 2014; Schneising et
al., 2014; Howarth, 2014). Alvarez et al. (2018) recently pre-
sented a summary estimate for natural gas emissions in the
United States (both conventional and shale gas) of 2.3% us-
ing bottom-up, facility-based data. However, they noted that
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R. W. Howarth: Shale gas and global methane 3039
Table 1. Estimates for sources of increased or decreased methane emissions to the atmosphere in recent years (teragrams per year). All values
are positive, except as specified.
This studyaWorden et al.bSchwietzke et al.c
(2017) (2016)
All fossil fuels 17.8 16.4 Negative ∼18
(±3.6)
– Shale gas 9.4
– Conventional gas 5.5
– Oil 1.6
– Coal 1.3
Biogenic sources 10.6 12.0 ∼27
(±2.5)
aTime period is 2008–2014 compared to 2000–2007. bTime period is 2008–2014 compared to
2000–2007, using the Schwietzke et al. (2016) data set with values from their Fig. 4 and assuming a
decrease in biomass burning of 3.7 Tg per year. Uncertainty is as shown in original publication.
cTime period is for 2003–2013 compared to 1985–2002, with values from their Fig. 2b.
Uncertainties are large, and only mean differences shown here.
Table 2. Exploration of sensitivity to assumptions for estimates of increase in global methane emissions in recent years (teragrams per year).
Base analysisaIncreased emission factor Explicit consideration
for shale gas of shale oil
(sensitivity test no. 1)b(sensitivity test no. 2)c
All fossil fuels 17.8 18.0 18.2
– All natural gas 14.9 15.1 12.0
– Shale gas 9.4 10.8 7.8
– Conventional gas 5.5 4.3 4.2
– All oil 1.6 1.6 4.9
– Shale oil 4.2
– Conventional oil 0.7
– Coal 1.3 1.3 1.3
Biogenic sources 10.6 10.4 10.2
aBase analysis is from equations Eq. (1) to Eq. (12) and is also presented in Table 1. Assumptions include equivalent percentage
emissions as a function of production for shale gas and conventional natural gas and no contribution of 13C-depleted methane from
tight shale-oil production. bSame assumptions as for the base analysis, except shale-gas emissions are assumed to be 50 % greater
than those from conventional natural gas, expressed as a percentage of production. cSame assumptions as for the base analysis,
except emission of 13C-depleted methane from shale oil is explicitly considered.
top-down estimates from approaches such as airplane fly-
overs give higher values than the bottom-up estimates they
emphasized. In fact, a careful comparison of bottom-up and
top-down approaches for one shale-gas field showed 45%
higher emissions from the top-down approach due to under-
sampling of some emission events by the bottom-up, facility-
based approach (Vaughn et al., 2018). Further, Alvarez et
al. (2018) used a very low value for the methane emissions
from local distribution pipelines, only 0.08 % (see discussion
in Howarth et al., 2011). Many studies suggest that distri-
bution emissions in Boston, Los Angeles, Indianapolis, and
Texas cities may be as high as 2.5 % or more, not 0.08 %
(Howarth et al., 2011; McKain et al., 2015; Lamb et al., 2016;
Wunch et al., 2016), so a full life cycle of 3.5% emissions
from shale gas over the past decade is quite plausible and
perhaps even low.
7 Sensitivity analyses
Our analysis contains two major assumptions: (1) that
methane emissions as a percentage of gas produced are the
same for shale gas and conventional natural gas (Eqs. 9 and
10); and (2) that emissions from oil have remained propor-
tional to the global rate of oil production. Here we explore
the sensitivity of our analysis to these assumptions. With
regard to the first assumption, some evidence suggests that
percent emissions may be higher from shale gas than from
conventional natural gas, perhaps due to venting at the time
of flowback following high-volume hydraulic fracturing of
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3040 R. W. Howarth: Shale gas and global methane
Figure 4. (a) Gas blowdown for maintenance on a pipeline in Yates County, New York. While methane is invisible, the cooling caused by
the blowdown condenses water vapor, leading to the obvious cloud. Photo courtesy of Jack Ossont. (b, c) Gas storage tanks receiving natural
gas from feeder pipelines before compression for transport in high-pressure pipelines at the Haynseville shale formation, Texas. Photo on left
was taken with a normal camera. Photo on the right was taken with a forward-looking infrared (FLIR) camera tuned to the infrared spectrum
of methane, allowing visualization of methane, which is invisible in the normal camera view and to the naked eye. Photo courtesy of Sharon
Wilson.
shale-gas wells (Howarth et al., 2011) and also due to release
of methane from trapped pockets when drilling down through
a very long legacy (often a century or more) of prior fossil-
fuel operations (coal, oil, and gas) to reach the deeper shale
formations (Caulton et al., 2014; Howarth, 2014). For this
first sensitivity analysis, we modify equations Eq. (9) through
Eq. (12) with new equations Eq. (A1) through Eq. (A4) to
reflect a 50 % higher emission factor for shale gas than for
conventional gas, as proposed in Howarth et al. (2011; see
Appendix A). With this change in assumptions, estimated
shale-gas emissions increase by 12 % (10.8 instead of 9.4 Tg
per year), which corresponds to a life-cycle emission factor
of 4.0 % rather than 3.5 %. Biogenic emissions remain vir-
tually unchanged (10.4 instead of 10.6 Tg per year), as do
total fossil-fuel emissions (18 instead of 17.8 Tg per year;
Table 2).
Our second major assumption in the base analysis is that
methane emission factors for oil production have remained
constant over time as a function of production. This may
not be true, since 60 % of the increase in global oil produc-
tion between 2005 and 2015 was due to tight oil produc-
tion from shales using the same technologies that allowed
shale-gas development, high-precision directional drilling
and high-volume hydraulic fracturing (calculated from data
in EIA, 2015, 2018). Large quantities of methane are often
co-produced with this tight shale oil, and because oil is a
much more valuable product than natural gas, for shale-oil
fields removed from easy access to natural gas markets, much
Biogeosciences, 16, 3033–3046, 2019 www.biogeosciences.net/16/3033/2019/
R. W. Howarth: Shale gas and global methane 3041
of the methane may be vented or flared rather than delivered
to the market. This may be part of the reason for the large
increase in methane emissions between 2008 and 2011 in
the Bakken shale fields of North Dakota (Schneising et al.,
2014).
For sensitivity scenario no. 2, we modify Eq. (9) through
Eq. (12) with new Eq. (B1) through Eq. (B4) to allow for
higher emissions associated with shale oil than from conven-
tional oil production (see Appendix B). For this, we follow
the approach of Schneising et al. (2014) in combining shale
gas and shale oil, scaling the increase in production since
2005 by the energy value of the two products. As in our base-
line analysis developed in equations Eq. (1) through Eq. (12),
we assume that conventional natural gas and shale gas have
the same percentage methane emission per unit of produced
gas. Here we further assume that shale oil has the same emis-
sion rate as well, scaled to the energy content of oil compared
to natural gas. This sensitivity analysis again has very little
influence on either total emissions from fossil fuels (18.2 in-
stead of 17.8 Tg per year) or biogenic emissions (10.2 instead
of 10.6 Tg per year; Table 2). The contribution from shale
gas falls somewhat (from 9.4 to 7.8 Tg per year), as does
that from conventional natural gas (from 5.5 to 4.2Tg per
year), while shale oil becomes an important emission source
(4.2 Tg per year). Overall in this scenario, increased emis-
sions from fossil fuels extracted from shales (gas plus oil)
are 12 Tg per year, two-thirds of the total increase due to fos-
sil fuels.
8 Conclusions
We conclude that increased methane emissions from fossil
fuels likely exceed those from biogenic sources over the past
decade (since 2007). The increase in emissions from shale
gas (perhaps in combination with those from shale oil) makes
up more than half of the total increased fossil-fuel emis-
sions. That is, the commercialization of shale gas and oil in
the 21st century has dramatically increased global methane
emissions.
Note that while methane emissions are often referred to as
“leaks”, some of the emissions include purposeful venting,
including the release of gas during the flowback period im-
mediately following hydraulic fracturing, the rapid release of
gas from blowdowns during emergencies but also for routine
maintenance on pipelines and compressor stations (Fig. 4a),
and the steadier but more subtle release of gas from storage
tanks (Fig. 4b) and compressor stations to safely maintain
pressures (Howarth et al., 2011). This suggests large oppor-
tunities for reducing emissions, but at what cost? Do large
capital investments for rebuilding natural gas infrastructure
make economic sense, or would it be better to move towards
phasing natural gas out as an energy source and instead invest
in a 21st-century energy infrastructure that embraces renew-
able energy and much more efficient heat and transportation
through electrification (Jacobson et al., 2013)?
In October 2018, the Intergovernmental Panel on Climate
Change issued a special report, responding to the call of the
United Nations COP21 negotiations to keep the planet well
below 2 ◦C of the pre-industrial baseline (IPCC, 2018). They
noted the need to reduce both carbon dioxide and methane
emissions, and they recognized that the climate system re-
sponds more quickly to methane: reducing methane emis-
sions offers one of the best routes for immediately slowing
the rate of global warming (Shindell et al., 2012). Given our
finding that natural gas (both shale gas and conventional gas)
is responsible for much of the recent increases in methane
emissions, we suggest that the best strategy is to move as
quickly as possible away from natural gas, reducing both
carbon dioxide and methane emissions. Natural gas is not a
bridge fuel (Howarth, 2014).
Finally, in addition to contributing to climate change,
methane emissions lead to increased ground-level ozone lev-
els, with significant damage to public health and agriculture.
Based on the social cost of methane emissions of USD 2700
to USD 6000 per ton (Shindell, 2015), our baseline estimate
for increased emissions from shale gas of 9.4 Tg per year cor-
responds to damage to public health, agriculture, and the cli-
mate of USD 25 billion to USD 55 billion per year for each
of the past several years. This is comparable to the wholesale
value for this shale gas over these years.
Data availability. No data sets were used in this article.
www.biogeosciences.net/16/3033/2019/ Biogeosciences, 16, 3033–3046, 2019
3042 R. W. Howarth: Shale gas and global methane
Appendix A: Sensitivity case no. 1: emissions per unit of
gas produced assumed to be 50 % greater for shale gas
than for conventional gas
First we modify Eqs. (9) and (10) as follows to reflect that
methane emissions per unit of gas produced are 50 % greater
for shale gas than for conventional natural gas:
SG =1.2·(0.63 ·TG), or SG =0.76 ·TG,(A1)
and
CG =0.8·(0.37 ·TG), or CG =0.30 ·TG.(A2)
Rearranging Eq. (A1) for TG and substituting into
Eq. (A2),
CG =0.30 ·(SG/0.76), or CG =0.40 ·SG.(A3)
Since FFNis the sum of CG and the 2.9 Tg per year emis-
sions for oil and coal,
FFN=0.40 ·SG +2.9.(A4)
Substituting Eq. (A4) into Eq. (8) and solving for SG,
we estimate that the increase in shale-gas emissions be-
tween 2005 and 2015 was 10.8 Tg per year (Table 2). From
Eq. (A3), for conventional natural gas, CG=4.3 Tg per year.
The increase in total fossil-fuel emissions are estimated to
be the contributions from coal (1.3 Tg per year) and oil
(1.6 Tg per year) plus SG and CG, or 18 Tg per year. From
Eq. (3), biogenic emissions are estimated to have increased
by 10.4 Tg per year. These values are reported in Table 2.
Appendix B: Sensitivity case no. 2: explicit
consideration of shale oil (tight oil)
For the base analysis presented in the main text using equa-
tions Eq. (1) through Eq. (12), we assumed that increased
emissions from the additional oil development over the past
decade were proportional to the increase in that rate of de-
velopment. That is, the oil produced in recent years had the
same emission factor as that for oil produced a decade or
more ago. However, 60 % of the increase in oil production
globally between 2005 and 2015 was for tight oil from shale
formations (calculated from data in EIA, 2015, 2018), and
methane emissions from this shale oil may be greater than
for conventional oil. In sensitivity case no. 2, we consider in-
creased emissions from conventional oil and from tight shale
oil separately. For conventional oil, the increase in emissions
is 40 % of the total oil emissions from the base analysis (40 %
of 1.6 Tg per year, or 0.65 Tg per year, rounded to 0.7 in Ta-
ble 2), reflecting that conventional oil contributed 40 % to the
growth in oil production between 2005 and 2015.
For the tight shale oil, we follow the approach used by
Schneising et al. (2014): the increases in methane emissions
from shale gas and shale oil are considered together, normal-
ized to the energy content of the two fuels. Shale-gas produc-
tion increased by 405 billion cubic meters per year between
2005 and 2015 (EIA, 2016). With an energy content of 37MJ
per cubic meter, this reflects an increase in 15.9 trillion MJ
per year. For shale oil, production increased by 230 liters per
year between 2005 and 2015 (EIA, 2015, 2018). With an en-
ergy content of 38 MJ per liter, this reflects an increase in
8.9 trillion MJ per year. Conventional natural gas production
increased by 238 billion cubic meters per year between 2005
and 2015 (EIA, 2016). With an energy content of 37MJ m−3,
this reflects an increase in 8.8 trillion MJ per year. Therefore,
the sum of the increase in production for shale gas, shale oil,
and conventional natural gas is 33.6 trillion megajoules per
year. Shale gas represents 48% of this, shale oil represents
26 %, and conventional natural gas represents 26 %. The sum
of shale gas and shale oil represents 74 % of the total.
For this sensitivity analysis, we further assume that shale
gas and conventional natural gas have the same percentage
emissions, as in our base case analysis in the main text, and
that the 13C content of methane from shale oil is the same as
for shale gas. Using these assumptions, we modify Eqs. (9)
and (10) as follows:
SG&O =0.74 ·TG&SO,(B1)
and
CG =0.26 ·TG&SO,(B2)
where SG&O is shale gas plus shale oil and TG&SO is total
natural gas plus shale oil. Rearranging Eq. (B1) for TG&SO
and substituting into Eq. (B2),
CG =0.26·(SG&O/0.74), or CG =0.35·SG&O.(B3)
Since FFNis the sum of CG and the 2.0 Tg per year emis-
sions for coal and conventional oil,
FFN=0.35 ·SG +2.0.(B4)
Substituting Eq. (B4) into Eq. (8) and solving for SG, we
estimate that the increase in methane emissions from shale
oil plus shale gas between 2005 and 2015 was 12 Tg per year.
From Eq. (B3), increased emissions from conventional natu-
ral gas are 4.2 Tg per year (Table 2).
The total increase in fossil-fuel emissions is estimated to
be the contributions from coal (1.3 Tg per year), conventional
oil (0.7 Tg per year), and conventional natural gas (4.2 Tg
per year) plus the sum for shale gas plus shale oil (12 Tg per
year), or 18.2 Tg per year. We can separately estimate shale
gas and shale oil, estimating the proportion of the sum of the
two made up by shale gas as follows:
15.9 trillion MJ yr−1/
(15.9 trillion MJ yr−1
+8.9 MJ yr−1)=0.64 (B5)
Biogeosciences, 16, 3033–3046, 2019 www.biogeosciences.net/16/3033/2019/
R. W. Howarth: Shale gas and global methane 3043
Therefore, of the increased emissions of 12 Tg per year
for SG&O, the increase for shale-gas emissions is 7.8 Tg per
year and that for shale-oil emissions is 4.2 Tg per year. These
values are reported in Table 2.
www.biogeosciences.net/16/3033/2019/ Biogeosciences, 16, 3033–3046, 2019
3044 R. W. Howarth: Shale gas and global methane
Competing interests. The authors declare that they have no conflict
of interest.
Acknowledgements. We thank Tony Ingraffea, Amy Townsend-
Small, Alexander Turner, Euan Nisbet, Martin Manning, Den-
nis Swaney, Roxanne Marino, three anonymous reviewers, and as-
sociate editor Jack Middelburg for comments on earlier versions of
this paper. We particularly thank Dennis Swaney for help with the
analyses we report. We thank Gretchen Halpert for the artwork in
Figs. 1 and 2, Sharon Wilson for the photographs in Fig. 4b, c, and
Jack Ossont for the photograph in Fig. 4a. Tony Ingraffea helped
interpret these photographs.
Financial support. This research has been supported by the Park
Foundation (grant no. 16-612) and an endowment given by
David R. Atkinons to support the professorship at Cornell Univer-
sity held by Robert W. Howarth.
Review statement. This paper was edited by Jack Middelburg and
reviewed by three anonymous referees.
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