Environ Resource Econ

DOI 10.1007/s10640-015-9937-6

Valuing the Ozone-Related Health Benefits of Methane

Emission Controls

Marcus C. Sarofim1 · Stephanie T. Waldhoff2 ·

Susan C. Anenberg3

Accepted: 11 June 2015

© The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract Methane is a greenhouse gas that oxidizes to form ground-level ozone, itself a

greenhouse gas and a health-harmful air pollutant. Reducing methane emissions will both

slow anthropogenic climate change and reduce ozone-related mortality. We estimate the

benefits of reducing methane emissions anywhere in the world for ozone-related premature

mortality globally and for eight geographic regions. Our methods are consistent with those

used by the US Government to estimate the social cost of carbon (SCC). We find that the

global short- and long-term premature mortality benefits due to reduced ozone production

from methane mitigation are (2011) $790 and $1775 per tonne methane, respectively. These

correspond to approximately 70 and 150% of the valuation of methane’s global climate

impacts using the SCC after extrapolating from carbon dioxide to methane using global

warming potential estimates. Results for monetized benefits are sensitive to a number of

factors, particularly the choice of elasticity to income growth used when calculating the value

of a statistical life. The benefits increase for emission years further in the future. Regionally,

most of the global mortality benefits accrue in Asia, but 10% accrue in the United States. This

methodology can be used to assess the benefits of methane emission reductions anywhere in

the world, including those achieved by national and multinational policies.

Marcus C. Sarofim, Stephanie T. Waldhoff and Susan C. Anenberg have contributed equally to this work.

B Marcus C. Sarofim

Sarofim.marcus@epa.gov

Stephanie T. Waldhoff

Stephanie.waldhoff@pnnl.gov

Susan C. Anenberg

susan.anenberg@airhealthanalytics.com

1 U.S. Environmental Protection Agency (USEPA, 6207A), 1200 Pennsylvania Ave NW,

Washington, DC 20460, USA

2 Joint Global Change Research Institute, Pacific Northwest National Laboratory,

College Park, MD, USA

3 Environmental Health Analytics, LLC, Washington, DC, USA

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Keywords Methane · Ozone · Mortality · Air pollution · SCC

1 Introduction

Methane (CH4) is a relatively short-lived and potent greenhouse gas. It also reacts in the

atmosphere to form ground-level ozone (O3) that is itself is a greenhouse gas as well as a

pollutant that is associated with adverse health effects such as impaired respiratory health and

premature mortality (US Environmental Protection Agency 2012) and damaged vegetation.

Methane is emitted by natural sources, such as wetlands, and anthropogenic sources, such as

agriculture, coal mines, municipal solid waste, oil and gas systems, and wastewater. Anthropogenic activities contribute approximately 50–65% of total global methane emissions (IPCC

2013).

While methane has long been considered to be an important greenhouse gas to target for

climate change mitigation and is included in the basket of climate pollutants under the United

Nations Framework Convention on Climate Change (UNFCCC), it has only recently been

appreciated as a potential method to control surface ozone concentrations (West et al. 2006).

Two voluntary international efforts, the Climate and Clean Air Coalition to Reduce ShortLived Climate Pollutants1 (CCAC) and the Global Methane Initiative,2 aim to reduce methane

emissions from anthropogenic sources. Many national efforts that achieve methane emission

reductions as a primary target or as a co-benefit are also underway, including regulations like

the Oil and Natural Gas Air Pollution Standards and voluntary programs like the Natural

Gas STAR Program, the AgSTAR Program, and the Landfill Methane Outreach Program in

the US Given the many methane mitigation activities ongoing at international and national

scales, valuing both the climate and health benefits of national and international actions to

control methane emissions3 can inform policy decision-making.

Although the 12-year methane lifetime is shorter than that of many greenhouse gases (e.g.,

N2O has a lifetime of 110 years, chloro- and perfluoro-carbons lifetimes of thousands of years,

and while CO2 doesn’t have a single lifetime, it is added to the carbon cycle for millennia), it

lives longer in the atmosphere than the other major ozone precursors (nitrogen oxides, nonmethane volatile organic compounds, and carbon monoxide), with atmospheric lifetimes on

the order of weeks or months depending on the gas (IPCC 2007). Ozone is formed as product

of the methane oxidation process, and as a result, ozone production via methane occurs on

the same decadal time scale as methane’s lifetime, long enough to be globally well-mixed in

the atmosphere [e.g., (West et al. 2006)]. Therefore, the ozone response to methane emission

reductions is mostly insensitive to the location in which the emissions were reduced (ibid). The

relative uniformity of the ozone response to methane emissions allows for the quantification

of ozone benefits for a particular location per tonne of methane emission reduced anywhere

in the world. Similarly, the climate benefits of marginal reductions in methane, known as

the Social Cost of Methane (SC–CH4) have been estimated in the literature [e.g. (Marten

et al. 2014; Marten and Newbold 2012; Waldhoff et al. 2014)]. The US Government has

also calculated benefits of methane reduction using the Social Cost of Carbon4 (SCC) and

1 www.unep.org/ccac.

2 https://www.globalmethane.org/.

3 Throughout this paper we refer to the benefits of methane mitigation, though our estimates are equally valid

to estimate the damages of increases in CH4 emissions. For simplicity, we restrict our language to focus on

the benefits of mitigation.

4 The SCC, as used by the US Government, is valued in units of $/t–CO2.

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the Global Warming Potential (GWP) of methane. Using a GWP of 25 (IPCC 2007) yields

climate change benefits of (2011)$1,150/t–CH4 in 2020. The global ozone mortality benefits

of methane mitigation estimated in this paper are of similar magnitude to the monetized

climate benefits of methane mitigation. As we refer to it in this paper, the “social cost” of

methane refers only to the climate impacts, though the term could be used to refer to all the

impacts of methane including the health and agricultural impacts resulting from methane

oxidation. A cost-benefit analysis can then be done by comparing these marginal benefits—

SC–CH4 and ozone-related mortality—to the marginal cost of an emission control strategy.

Previously, studies have used global chemical transport modeling to estimate the premature

mortality impact of ozone reductions resulting from large reductions in methane emissions

or concentration. These studies estimated that reductions of 20% in anthropogenic methane

emissions (West et al. 2006) or in methane concentrations (Anenberg et al. 2009) would result

in about 17,000 global premature ozone-related cardiopulmonary deaths avoided (and 1.7

times that number of total avoided non-accidental deaths), or about 150–260 cardiopulmonary

deaths avoided per Mt CH4 reduced. Using similar methods but focusing on long-term ozone

concentration-mortality relationships, further implementing seven specific methane emission

control measures around the world was also estimated to avoid 47,000 premature deaths

annually in 2030, or about 340 deaths avoided valued at approximately $1,000/tonne of

methane emission reduced (Anenberg et al. 2012; Shindell et al. 2012).

The approach taken here has been designed to produce monetized benefits of reductions

that are consistent with the methodology used for development of the US Government calculations of the SCC. In addition, this approach is novel in comparison to previous analyses that

quantify the ozone-related health impacts of CH4 reductions in that it estimates avoided respiratory mortality associated with mitigating a marginal tonne of CH4 emissions in a given year.

The paper proceeds as follows. Section 2 presents the methodology and sources used

to estimate the indirect health benefits of CH4 mitigation. Section 3 presents the results

and discussion of multiple sensitivity tests on the short-term mortality estimates, as well as

estimated benefits of CH4 mitigation on long-term O3-related mortality. Section 4 concludes.

2 Methods

The approach taken here begins by calculating avoided respiratory mortality over several

decades associated with mitigating a marginal tonne of CH4 emissions in a given year. The

mortality reduction is then valued and discounted back to the year of emissions. The result is

a value that is well suited to regulatory analysis, enabling the estimation of the ozone-related

mortality benefits of policies that reduce CH4 emissions.

Our method makes use of the existing literature on the relationships between CH4 emissions and O3 concentrations, changes in O3 concentrations and short-term mortality, future

scenarios of population and income, and the value of lives lost. The roles of each of these on

the estimates are described in turn below.

2.1 Methane Emissions and Ozone Concentrations

The analysis begins with an instantaneous pulse of 1 tonne of methane. This methane is

assumed to have a lifetime of 12 years, consistent with the perturbation lifetime from the

IPCC AR4 (IPCC 2007). We consider the effect of 1 tonne of methane on short- and longterm ozone exposure metrics (annual average ozone for short-term and the 6month average

of the 1 h daily maximum ozone for long-term, consistent with the epidemiology studies

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from which ozone-mortality relationships were derived, as described below). We derive the

24-h average surface ozone response to an elevated atmospheric loading of methane from

(Fiore et al. 2008), who estimated a response of 11–15 ppt O3 as a steady state response to

1 Tg reduction in annual methane emissions. Using a mean response of 13 ppt O3 and taking

into account the fact that at steady-state 1 Tg year−1 of methane is equivalent to 12 Tg of

atmospheric methane loading, we calculate the ozone response in year t to a 1 Tg pulse of

methane at time zero to be

O3(t) = 13/12∗exp(−(t)/12). (1)

The April–September average of the 1-h daily maximum ozone concentration response to

an elevated atmospheric loading of methane is derived from a 3.0 to 3.6 ppb change as a

steady-state response to a 139 Tg reduction in annual methane emissions (Anenberg et al.

2012). We use a 70 year timeframe, capturing more than 99% of the total integrated methane

loading over all time.

2.2 Ozone-Mortality Concentration-Response Relationships

We use epidemiologically-derived health impact functions to estimate changes in both premature short-term respiratory mortality related to transient changes in ozone concentration

and longer-term mortality from chronic exposure to peak ozone concentrations in response

to a pulse of methane emissions.

To estimate the mortality impacts we assume log-linear relationships between ozone concentration and relative risk, following (Anenberg et al. 2010). The fraction of baseline deaths

attributable to a given change in concentration (attributable fraction, AF) is:

AF = (RR − 1)/RR = 1 − exp(− β O3) (2)

where β is the slope of the log-linear relation between ozone concentration and mortality

(concentration-response factor) and O3 is the change in ozone in response to a pulse of

methane emissions. AF is multiplied by the baseline cause-specific mortality rate (y0) and

population size (pop) to estimate the change in mortality (Mort) associated with a given

change in concentration:

Mort = y0 x pop x (1 − exp(− β O3) (3)

Equation 3 is applied in each world region using the corresponding population size, baseline

mortality rate, and change in ozone concentration.

We quantify ozone-mortality impacts using two different ozone exposure metrics found

by the epidemiological literature to be associated with premature mortality: daily exposure

(“short-term”) and repeated exposure to hourly peak concentrations over the warm season

(“long-term”). For short-term exposure, we calculate β from the (Bell et al. 2004) study of

95 US urban communities, who found that a 10 ppb increase in 24-h average ozone was

associated with a 0.64% (95% posterior interval, 0.31–0.98%) increase in cardiovascular

and respiratory mortality. For long-term mortality, we calculate β from the (Jerrett et al. 2009)

study of the American Cancer Society study cohort of adults aged 30 and older.We use the two

pollutant model that controlled for fine particulate matter (PM2.5), which found that a 10 ppb

increase in the April-September average of the 1-h daily maximum ozone concentration was

associated with a 4% (95% confidence interval, 1.0–6.7%) increase in respiratory mortality.

Following (Anenberg et al. 2010, 2012), we apply these concentration-response relationships

found in the US globally, despite differences in exposure, susceptibility, and vulnerability

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around the world. We focus on cause-specific mortality since it is more comparable among

populations in different countries than is all-cause mortality.

2.3 Future Population Projections

To estimate the changes in mortality over time due to a reduction in CH4, it is necessary to

project population (Eq. 3, pop) and baseline mortality (Eq. 3, y0) over the coming century.

While a number of long-term projections for global population exist, e.g. (Nakicenovic

2000; United Nations 2004), in order to promote consistency and comparability with the

SCC approach, the same scenarios used by the US Government in developing and updating

the SCC (US Department of Energy 2010; US Government 2013) are used here.5 These

scenarios are Business as Usual (BAU) simulations from four integrated assessment models—

MiniCAM, MESSAGE, MERGE, and IMAGE—that participated in the Energy Modeling

Forum 22 (EMF 22) exercise (Clarke et al. 2009). The population for each of the four models’

scenarios is aggregated to eight global regions as described in (US Department of Energy

2010). Our base estimate uses the MiniCAM scenario and the other three scenarios are used

to test the sensitivity of the results to changes in population and GDP.

We estimate the effects of long-term exposure to peak ozone concentrations only for the

population ages 30 and above, consistent with the American Cancer Society cohort used

by Jerrett et al. (2009). In order to generate this value for each of the four socioeconomic

scenarios, we first estimated the fraction of the population in this demographic using UN

country-level population projections (United Nations 2004) by age category and then aggregated to our eight regions. We then applied this ratio to the total population for each of our

four socio-economic scenarios.6

2.4 Future Baseline Mortality Rates

Our methodology to estimate mortality due to ozone concentrations changes also requires

baseline rates of death due to cardiovascular and respiratory disease for the total population

and respiratory disease for the population age 30 and above.

These rates vary greatly by region and would be expected to change over time as

economies develop and mortality rates shift from infectious disease to chronic disease. This

type of information is not included in the EMF 22 scenarios used for the SCC estimation. Country-level projections of total cardiovascular and pulmonary deaths through 21007

[International Futures Project (IFs) base projections (International Futures 2013)] are aggregated to the eight regions corresponding to the population and GDP scenarios, weighted

by 2004 country population and cardiovascular and pulmonary death rates (Mathers et al.

2008). We apply these baseline death rates to all socio-economic scenarios. We test the

5 The SCC estimates were developed using the four models’ BAU scenarios, plus a fifth set of parameters

that are the averages of these four models’ population, GDP, and non-CO2 emissions for a GHG mitigation

policy case. Because the estimation of damages from CH4 emissions is linearly related to population and GDP,

and does not depend on background concentrations (and therefore emissions) of GHGs, the results from this

fifth set of parameters would be a simple average of the four scenarios being used here and is therefore not

included.

6 Although scenarios with different total populations in 2100 are likely to have differing fractions of the

population in the age 30 and above category, detailed demographic projections are not available for the

individual scenarios. Thus these values should be interpreted as a sensitivity test of the total number of

individuals in the demographic category, rather than a specific scenario projection.

7 The timeframe for analysis is 70 years, so emissions in the years 2040 and 2050 require baseline death rates

through 2110 and 2120, respectively. We hold death rates constant at 2100 levels through 2120.

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sensitivity of the results to baseline mortality rates using constant 2010 baseline mortality

rates.

Estimating long-term ozone mortality impacts requires baseline respiratory mortality

rates for the subset of the population ages 30 and above. We first use the Global Burden of Disease (Lim et al. 2012)

8 country-level, ages 30+, respiratory mortality rates for

2010, weighted and aggregated to the eight regions, then approximate annual rates through

2100 by scaling 2010 respiratory mortality with the cardiovascular and pulmonary mortality rates from the IFs. We also test the sensitivity of results to constant 2010 mortality

rates.

2.5 Valuation of Changes in Mortality

In order to monetize the reduction in mortality estimated above, we apply a Value of Statistical

Life (VSL) to the change in mortality in each future year. These values are then discounted

back to the year of emission and summed, producing a Net Present Value (NPV) of the

indirect health benefits of CH4 mitigation, measured in (2011)$/t–CH4.

Since present and future estimates of VSL for every country or region are not available,

we test the sensitivity of results to three methods of extrapolating the VSL. Each method

is anchored to the value used by the US Environmental Protection Agency for the US VSL

(US Environmental Protection Agency 2010) and then extrapolated across region and time

by assuming an elasticity (ε) of the VSL to income:

VSLx,t = VSLref,0

∗(Incomex,t/Incomeref,0)

ε (4)

where VSLx,t is the VSL in region x in time period t. VSLx=ref,t=0 is the VSL in a reference

region and year. In this case we reference to the EPA VSL in the US in 2010, adjusted to 2011$,

of $8.84 million per premature death. Income is defined in 2011$ per capita,9 and estimated

using GDP and population data from the EMF 22 scenarios (income = GDP/population).

Three elasticity sensitivities are used in this study: 0.0, 0.4, and 1.0. The elasticity of 0.4,

which is used in our base health benefit estimate, is used by (Shindell et al. 2012) . An

elasticity of 0 generates a VSL that is constant across region and time and equal to the

US 2010 VSL of (2011)$8.84 million, while an elasticity of 1.010 produces a VSL that is

proportional to income. While an elasticity of 1.0 is the largest elasticity considered in this

study, there are some studies that indicate that the income elasticity may be greater than 1 in

some cases (Hammitt and Robinson 2011).

The total annual value of reduced mortality must be discounted back to the year of emissions in order to calculate the NPV. Estimates of benefits that accrue over long time horizons

(such as the SCC) are highly sensitive to the discount rate. We therefore test seven discount

rate estimates. The first four are constant discount rates of 2.5, 3.0, 5.0, and 7.0%. The first

three correspond to the discount rates used to develop the US SCC estimates and 3 and 7%

correspond to the discount rates of consumption and capital as used in US regulatory analyses

(US Office of Management and Budget 2003).

Three additional sensitivities, using a Ramsey discounting framework in which the growth

rate of income in a given region and period influence the discount rate, are also tested. The

8 Data are available at http://www.healthmetricsandevaluation.org/.

9 All values are in 2011$ unless otherwise noted. GDP data for the EMF 22 scenarios is in 2005$ and is

adjusted to 2011$ using a multiplier of 1.0608.

10 This is the value utilized by the FUND model (Anthoff and Tol 2013), which is one of the three models

used to develop the US Government’s SCC estimate.

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Ramsey discounting framework (Eq. 5) decomposes the discount rate into two parts:

rx,t = ρ + η ∗gx,t (5)

where rx,t is the discount rate for region x in time period t, η is the marginal utility of

consumption, and gx,t is the income growth rate in that region and period. The “pure rate of

time preference,” ρ, represents a strict preference for a dollar today over a dollar tomorrow

and the elasticity of the marginal utility of consumption times the GDP growth rate, η∗ gx,t,

has the effect of increasing the discount rate for rapidly growing economies. When the

rate of growth is constant across all regions and periods, the Ramsey formula will result

in a constant value. In shorter-term, regional analyses, a constant discount rate may be a

reasonable approximation. The Ramsey formula is more appropriate when growth rates vary

greatly over time and across regions. We test the sensitivity of our results to the Ramsey

discount method with three additional discount rates. There is no clearly agreed upon value

for ρ, and very low values, like 0.1% have been used in valuing long-term climate damages,

though 1% is a more common value and estimates up to 3% have also been used. These three

estimates of ρ and η = 1 are used to estimate the three Ramsey discounting sensitivity tests.

The base estimates for both short- and long-term ozone-related mortality due to a 1 tonne

change in methane are calculated using an emission year of 2020, the MiniCAM scenario

inputs for population and GDP, average O3, central estimates of β, the IFs baseline for

y0, and a constant discount rate of 3%. We test the sensitivity of the ozone-related mortality benefits of a reduction of a marginal tonne of methane over all input parameters. These

include four additional estimates of the attributable fraction of mortality from methane emissions, including both a high and low estimate of O3 and the bounds of the 95 percentile

range estimate of β, an alternate specification of y0, GDP and population from three additional socioeconomic scenarios, six additional discount rates, two additional methods of VSL

estimation, and four other years of emission.

3 Results and Discussion

Our base estimate11 of the global avoided premature cardiovascular and pulmonary deaths due

to short-term peak ozone exposure, over a 70 year period, due to a one million tonne decrease

in methane emissions in 2020, is 239 deaths, with a monetized value of (2011)$790/t–CH4.

We estimate that a one million tonne decrease in methane emissions in 2020 will avoid 591

premature global respiratory deaths among the population aged 30+ due to long-term peak

ozone exposure, with a value of (2011)$1,775/t–CH4.

12

To place these estimates in context, they can be compared to the social cost of methane.

Since 2008, US Government Regulatory Impact Analyses have used the Social Cost of

Carbon Dioxide (SCC) to estimate the climate change benefits due to a marginal (1 tonne)

reduction in carbon dioxide. Standardized values were developed in 2010 (US Department

of Energy 2010) and revised in 2013 (US Government 2013). Although the US Government

11 As described in Sect. 2, our base estimates for the value of avoided short-term mortality are discounted at a

constant 3% rate, using the MiniCAM GDP and population scenario, average O3 of 13 ppt annual average,

and β of 0.64% per 10 ppb ozone, the IFs baseline mortality projections, and a 0.4 income elasticity for VSL.

12 The base estimate of long-term mortality uses identical parameters to the short-term exposure estimate,

with the exception of O3 and β, where instead the base estimate relies on the maximum six-month average

of the 1-h daily maximum response of 24 ppt per Tg methane and the concentration-response function is 4%

per 10 ppb ozone, respectively.

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does not have a standardized SC–CH4 estimate, recent US EPA Regulations13 have included

monetized climate benefits from methane emissions reductions.14 These estimates of the SC–

CH4 were based on the current SCC estimate, transformed using the GWP for methane. Using

the current 2020 SCC central15 value of (2011)$46 and GWP of 25 yields (2011)$1,150/t–

CH4. The SC–CH4 can also be estimated directly by the models used to estimate the SCC.

Waldhoff et al. (2014); (Marten et al. 2014) and Marten and Newbold (2012) both found

that the directly modeled relative damage of CH4 to CO2 was greater than the GWP-based

damages.16 Marten et al. (2014) use a methodology consistent with the US Government

SCC and the sensitivities tested in this paper and found that for CH4 emissions in 2020, at a

discount rate of 3%, the Social Cost of Methane was (2007)$1200 (or $1302 in 2011$). The

global ozone mortality benefits estimated in this paper are 0.7 or 1.5 times the estimate of

climate benefits using the updated SCC and GWP, and 0.6 or 1.4 times the estimate of climate

benefits from Marten et al. (2014), for the short and long-term ozone mortality estimates,

respectively.

3.1 Sensitivity Analyses

We tested the sensitivity of the results to a number of parameter assumptions and explored

the relative importance of different factors in determining the mortality benefit, as described

above. These sensitivities were all calculated relative to the base case. Sensitivity ranges were

chosen based on ranges for parameters provided within underlying studies (O3, β, Ramsey

discount rates), sensitivities used within the US Government SCC approach (discount rates

2.5, 3, and 5%, different scenarios, emission years), OMB Circular A-4 (discount rate of

7), and reasonable bounding parameters chosen by the authors (VSL elasticities and CVP

mortality rate). Therefore, while these calculations are informative, this sensitivity analysis

is not comprehensive nor are the ranges as comparable as they would be if they were all

equivalent standard deviation ranges.

Figure 1 shows the difference between the base estimate, (2011)$790/t–CH4, and each

sensitivity and Table 1 reports the estimates for each of the alternate measures for the specified

parameters. The estimates are very sensitive to the elasticity parameter used to extrapolate

the VSL over region and time. The use of a constant VSL (ε = 0) nearly doubles the base

estimate in 2020, while the VSL estimated as a strict ratio of per capita income (ε = 1)

produces an estimate only half of the base value.

Figure 2 shows the effect of the different VSL metrics on the CH4 mortality benefit over

time. The estimate using ε = 1 is one-half the base estimate in 2020 and increases to twothirds the base estimate in 2050. Similarly, an elasticity of 0 produces a mortality benefit

roughly twice as large as the base estimate in 2020, but only 1.5 times larger in 2050. That

13 Federal Register Vol 77. No. 159, 49536, 2012 and Federal Register Vol 77, No. 199, 62930, 2012.

14 Recent rules have relied on the SCC estimates developed in 2010 (central estimate of (2011)$28/t–CO2)

and two different GWP values: the Second Assessment Report GWP of 21 [for consistency with UNFCCC

guidelines (Oil and Gas)] and a GWP of 25 [(for consistency with the most recent IPCC report at the time

(Light-Duty Vehicle)]. The use of the former results in a SC–CH4 estimate of (2011)$585 and the latter in a

value of (2011)$697.

15 Due to many uncertainties in the calculation of the SCC, the US Government provides and

uses a range of SCC values. For CO2 emissions in 2020, these values range from (2011)$13

to $137. See http://www.whitehouse.gov/sites/default/files/omb/assets/inforeg/technical-update-social-costof-carbon-for-regulator-impact-analysis.pdf for additional detail.

16 Differences between the GWP and explicit approaches stem mainly from the different temporal valuation

methods (combined with the fact that methane has a shorter lifetime than CO2), and the inclusion of positive

yield effects on agriculture from CO2 fertilization in some of the underlying integrated assessment models.

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Fig. 1 Differences between the short-term mortality base estimate (2011)$790/t–CH4 and multiple parameter

sensitivities. See Table 1 for a listing of sensitivity runs for each parameter

the estimates using a constant VSL (ε = 0) are much larger than with the base method is not

particularly surprising, as the US 2010 per capita income is substantially greater than most

regions in the world at present and greater than what some regions reach even by the end of

the century in these scenarios.

The year of emission also has a large effect on the estimate, with benefits per tonne more

than doubling for methane emitted between 2010 and 2050 (Fig. 2). Within the base set of

parameters, this change is due primarily to increases in population and VSL changes due to

income growth over the time period.

Reasonable estimates of future CVP mortality rates have a comparably smaller effect on

the base case estimate in 2020: under an assumption of constant CVP mortality rates from

2010 to 2120, the benefit per tonne estimate in 2010 is only 6% lower. By 2050, however,

this assumption decreases the estimate by 30%. As shown in Table 1, the 2020 estimate is

roughly 12% lower using a constant mortality rate than under the base assumptions. Given

the extreme nature of an assumption of constant CVP mortality rates through 2120, it is likely

that other, more plausible estimations of mortality rates would have a relatively small effect

on the methane indirect health benefits.

We test a wide range of discount rates and methods and find that the estimate of the indirect

short-term mortality impacts from a tonne of methane emitted in 2020 using a constant 2.5%

discount rate is about 1.3 times the estimate with a constant 5% discount rate. In contrast, the

SCC is very sensitive to the choice of discount rate and methodology; the SCC is nearly 5.5

times greater when discounted at 2.5 versus 5%. However, Waldhoff et al. (2014) show that

direct estimates of the SC–CH4 are also much less sensitive to discounting than the SCC,

due to methane’s comparatively short atmospheric lifetime and the resulting nearer-term

nature of the damages. The ozone-mediated mortality benefits from methane emissions are

even less sensitive to discounting. This is likely due to two reasons. First, the inertia of the

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Table 1 Value of short-term ozone-related mortality benefits (2011$/t − CH4), sensitivity resultsParameter Base Test 1 Test 2 Test 3 Test 4 Test 5 Test 6Discount rate 3.00% $790 2.50% $831 5.00% $660 7.00% $569 prtp 0.1% $858 prtp 1.0% $782 prtp 3.0% $655Emission year 2020 $790 2010 $634 2030 $980 2040 $1,198 2050 $1,429VSL income elasticity ε = 0.4 $790 ε = 0 $1,523 ε = 1 $393

O3/t-CH4 13 $790 11 $668 15 $911

β(%CVPM/10 ppb − 03) 0.64 $790 0.31 $382 0.98 $1,209

y0 (CVP mortality rate) IFS Base $790 Constant $691

Scenario MiniCAM $790 MESSAGE $866 MERGE $798 IMAGE $843

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Fig. 2 Sensitivity of short-term ozone-related mortality benefits to VSL and year of emission

climate system will cause temperature changes that last longer than concentration changes

and therefore the climate damages, but not the ozone-mediated mortality damages, will

continue to increase with future temperature changes. Second, the social cost of methane is

a function of the reference temperature as well as population and GDP. Because temperature

is increasing in the reference scenarios (even without the addition of a marginal tonne of

CH4), the climate damages from future climate changes are larger than they would be under

present temperature conditions so the climate damages decay slightly more slowly than the

mortality damages that are not dependent on temperature.

We examine four different future socio-economic scenarios, namely reference scenarios

from the EMF 22 exercise from the IMAGE, MERGE, MESSAGE, and MiniCAM models.

The use of different socioeconomic scenarios can increase the base estimate from up to 6%

in 2010 to up to 12% for emissions in 2050. The increasing sensitivity across scenarios

over time is a result of the growing differences across the models’ projections of GDP and

population as the scenarios extend into the latter part of this century.

We also examine the sensitivity of incidence results to year of emission, CVP mortality

response, O3 concentration-response factor, CVP mortality rates, and socioeconomic scenario

(Figure 3).

The concentration-response factor (β) and the year of emission reduction have the largest

effect on avoided premature mortality. The 95th percentile ranges of β bound the avoided

premature mortality between 116 and 366. Over time, a reduction of 1 MMT–CH4 in a single

year increases the total avoided deaths, from 210 in 2010 to 329 in 2050.

To put these values in perspective, the recent EPA Oil and Natural Gas Sector: New

Source Performance Standards and National Emission Standards for Hazardous Air Pollutants Reviews rule17 estimated that actions to reduce VOC emissions from oil and gas

production would also reduce methane emissions by 1.0 million tonnes annually, with an

17 40 CFR Parts 60 and 63.

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Fig. 3 Avoided short-term ozone-related mortality per MMT–CH4 reduction, sensitivity results

additional 0.7 million tonnes attributed to voluntary reductions. The net effect of these

reduced CH4 emissions in the year 2020 is to avoid 406 premature deaths globally from

the resulting reduction in ozone concentrations over the following 70 years. The net present

value of the avoided deaths due to reductions in 2020 CH4 emissions is $1.3 billion using

our base estimate.

3.2 Regional Estimates of Cardiovascular and Pulmonary Mortality Damages

In order to examine the distributional impacts of methane on ozone health, we present estimates of avoided mortality and benefits by region. As seen in Fig. 4, the estimates vary greatly

across regions, ranging from a low of $44/t–CH4 in Africa and the Middle East to a high of

$181/t–CH4 in East Asia under base assumptions. That the US falls somewhat in the middle

of this range may seem somewhat surprising. However, the estimates are dependent not only

on the VSL within a region, but also on the total population and the baseline cardiovascular

and pulmonary death rates. Large populations and high cardiovascular and pulmonary mortality may cause the total benefits to exceed those in regions with much higher per capita

incomes. This is demonstrated in Table 2: a one million tonne decrease in emissions in 2020

avoids 12 deaths in the United States, compared to 68 avoided deaths in East Asia. The effects

of per capita income (the basis for the VSL estimates) are worth noting, however. While only

4.5% of the world’s population lives in the United States, 10.6% of the benefits are accrued

there, primarily due to differences in the estimated VSL.

As with the global values, the benefits accrued by individual regions is very sensitive to

the VSL methodology, though the magnitude, and even sign, varies across regions. When

VSL is a constant value (ε = 0), the US value is roughly 5% of the global estimate. In sharp

contrast, a VSL estimate that varies linearly with per capita income (ε = 1) increases the

fraction of benefits that accrue in the US to 25%. The effect is opposite to the effects of VSL

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Fig. 4 Percent difference in short-term ozone-related mortality results using a VSL elasticity of η = 0.4, by

region (base estimates in parentheses)

Table 2 Total avoided mortality

per 1 MMT decrease in CH4 in a

given year (base case)

Due to rounding, global totals

may not equal the sums of

regional values

Year 2010 2020 2030 2040 2050

European Union 26 29 30 32 31

United States 10 12 14 16 17

Other OECD 8 10 12 13 14

Eastern Europe 21 23 25 27 29

East Asia 56 68 81 93 102

Southeast Asia 57 64 73 83 93

Africa-Middle East 19 18 18 18 19

Latin America 13 16 18 21 24

Global 210 239 271 302 329

methodology on all other regions, where the use of a constant (ε = 0) VSL increases benefits

and the linear method (ε = 1) decreases benefits compared to the base assumption ofε = 0.4.

This is because the US has a relatively high VSL compared with other regions, particularly in

the near-term. A constant 2010 US VSL raises other regions’ VSLs by more than it decreases

the US value over time. When ε = 1, however, the US VSL increases linearly, rather than

exponentially at 0.4. Since other regions have lower incomes, the exponent(ε = 0.4)increases

VSL relatively more, and future increases in income in the US do not change the VSL as

much as with a ε = 1 method.

Further differences can be seen across the magnitude of response to VSL methodology

by region (Fig. 4). The use of a constant VSL across region and time has the largest effect

on the Africa-Middle East and Southeast Asia regions, increasing the benefits in each region

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M. C. Sarofim et al.

nearly 175% compared to the base estimate. Generally, regions with relatively low per capita

incomes have much larger increases in benefits when ε = 0 than corresponding decrease

when ε = 1. Higher income regions (e.g. European Union and Other OECD) have comparable

changes between the two methods.

For further comparison, Table 2 shows the total avoided deaths for a one million tonne

decrease in methane emissions in each year. While economic growth, across regions and

over time, has a large effect on the monetized benefits, the drivers of avoided mortality are

population and baseline mortality rates, both of which vary by region and over time.

Total avoided deaths for 1 MMT–CH4 emissions reduction increases over time most

rapidly for three regions: Other OECD, East Asia, and Latin America, all seeing avoided

mortalities increase by more than 80% between 2010 and 2050. These increases are driven

primarily by rapidly increasing cardiovascular and pulmonary death rates. Similarly, the

Africa and Middle East region shows practically no change in total avoided mortality over

time. This is because the increase in population in this region is almost perfectly offset by

the decrease in cardiovascular and pulmonary death rates through mid-century.18 This region

begins the century with the lowest cardiovascular and pulmonary death rates, and though

these rates are projected to begin increasing again around 2050, by century’s end the rate is

still lower than in 2010.

3.3 Long-Term Ozone Mortality and Damages

In addition to cardiovascular and pulmonary mortality from short-term exposure to peak

ozone concentrations, changes in methane emissions can also impact respiratory mortality

from long-term exposure to average ozone concentrations. We examine the mortality effects

and damages of chronic ozone exposure due to changes in methane emissions and test the

sensitivity of the results to several parameters. While the long-term mortality results respond

similarly to the short-term estimates for most of the sensitivity tests, there are two differences

worth noting. The long-term concentration-response factor has a relatively large 95% confidence interval. Because of this, the long-term respiratory mortality results are very sensitive

to this parameter, with a range of (2011)$444–$2973.

Additionally, as seen in Fig. 5, the benefits of avoided respiratory morality due to long-term

ozone exposure increase over time at a faster rate than for the avoided cardiovascular and pulmonary mortality due to short-term ozone exposure. The long-term benefits per tonne of CH4

mitigated increase from $1301 in 2010 to $3846 in 2050, while the short-term benefits increase

from $634 to $1,428 in the same period. The primary reason for the faster increase in benefits

is that the long-term mortality impacts are based only on the sub-set of the population ages 30

and above. Globally, the affected demographic (population 30+) for the long-term impacts

is growing faster than the total population, upon which the short-term estimates are based.

The long-term mortality relationships include both acute and chronic effects of ozone

exposure, so the significantly larger long-term mortality estimates suggest that considering

only short-term mortality may exclude a substantial portion of ozone-related risk. However,

because the short-term mortality relationships include a larger population (all ages versus

adults ages 30 and older only) and mortality due to any cardiovascular and pulmonary disease

18 Although well beyond the scope of this paper to examine in detail, if other causes of mortality, such

as communicable diseases and conflict-related deaths, were to decrease significantly through the century,

the region would see a commensurate increase in cardiovascular-pulmonary mortality and total population,

thus increasing total benefits and avoided premature deaths. So although sensitivity with respect to baseline

mortality rates is tested, using constant rates over time, changes in other underlying trends in mortality causes

and rates may impact these results.

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Fig. 5 Global short-term and long-term health benefits per tonne-CH4, 2010–2050

rather than just respiratory disease, the short-term mortality estimates may include some

ozone mortality effects that are not captured in the long-term results. As the extent of the

overlap between these estimates is unknown, the short-term and long-term mortality results

should be viewed as complementary estimates, rather than additive or one as a subset of the

other.

These results can be compared to work by (Shindell et al. 2012) that shows long-term

global methane health benefit estimates in the range of $1080 ± $721/t–CH4 (in 2006$).19

There are several of differences between the Shindell et al. approach and the methods used

in this paper. The key difference is that the estimates presented in this paper follow the decay

of a pulse of methane over a period of time and reflect the value of the change in mortality

due to the resulting ozone concentration to the population in the same time period. For a

pulse of methane emitted in 2020, the ozone increment in 2020 is applied to the population

in 2020, the ozone increment in 2021 is applied to the population in 2021, and so forth, and

the changes in mortality in each region and year are valued with a region and time consistent

with estimated VSL. In contrast, Shindell et al. used a more sophisticated chemistry-climate

model to calculate the ozone changes driven by emission changes in 2030 and averaging the

ozone response in years 30–50 of the simulation. This ozone increment is then applied to the

population of 2030 in order to estimate changes in mortality. The Shindell et al. approach

would be expected to yield a result more similar to the integrated valuation of a single

year’s emissions (as in this paper) with no future changes in population, GDP, or mortality

response, and if future benefits were not discounted. In addition, Shindell et al. include PM2.5

concentration changes and health impacts resulting from CH4 emission reductions, whereas

we focus here on ozone only.

19 Or roughly (2011)$1186 ± $792, when converted using a GDP deflator of 1.098.

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M. C. Sarofim et al.

3.4 Caveats and Limitations

A few of the assumptions made to develop these benefits estimates have the potential to

slightly increase the range of possible outcomes. For simplicity these estimates assume that

the ozone concentration response to methane is homogenous. However, (Anenberg et al. 2012)

suggest accounting for the heterogeneity in ozone response to CH4 may lead to differences

in mortality estimation on the order of 15% globally, with uncertain sign. The decay rate of

methane is also not a constant, with some papers suggesting that it might change by up to

10% depending on emissions of VOCs, NOX, and methane itself (Sarofim 2012). A change in

the overall decay rate would presumably also have an effect on the rate of ozone production.

However, these effects are relatively small compared to the uncertainties involved in the

choices of VSL and discount rates considered above.

Due to lack of regional studies, we also apply concentration-response functions found in

the US globally, though differences in exposure and population susceptibility characteristics

(e.g. time spent outdoors, home ventilation rates, medical care, etc.) could cause differences in

this factor across regions. As discussed in Anenberg et al.(2010), short-term O3 epidemiology

studies in developing nations produce fairly similar results to short-term studies in North

America and Europe (Health Effects Institute International Scientific Oversight Committee

2004). Furthermore, concentration–mortality relationships do not vary significantly by sex,

age, and race (Jerrett et al. 2009; Krewski et al. 2009; Zanobetti et al. 2000), although some

sensitive populations may be at a higher risk. We base our estimates on cardiovascular and

respiratory mortality rates rather than all-cause mortality rates because this will reduce errors

due to differences in causes of deaths across regions.

These estimates also do not include the benefits of avoided morbidity. The additional

value of reduced morbidity from reductions in ozone concentrations due to lower methane

emissions would likely increase the total health benefit valuation, though precisely how much

is left to future work. Additional effects that would serve as a complement to both the climate

effects from the social cost of methane and the health effects from this analysis include the

effect of ozone concentrations on agricultural and forestry yields (Avnery et al. 2013) and on

the carbon uptake of natural systems (Feltzer et al. 2005).

One effect that could lead to our results being overestimates is our exclusion of particulate matter concentrations and health impacts. Reduced methane emissions may affect

atmospheric oxidant concentrations, leading to increased production of sulfate, a component

of particulate matter pollution (Anenberg et al. 2012). Increased particulate matter-related

health impacts resulting from reduced methane emissions may counteract some of the ozonerelated health benefits.

4 Conclusions and Future Directions

This paper estimates the premature mortality benefits of reducing ozone concentrations

through reductions in methane emissions. This estimate is the first one that is consistent

methodologically with the approach used in estimating the social cost of carbon values used

by the US Government for regulatory analyses. This paper also examines the sensitivity of

the estimates to several key parameters and methodologies. We estimate that a one million

tonne decrease in methane emissions in 2020 will avoid 239 premature cardiovascular and

pulmonary deaths globally due to short-term peak ozone exposure and 591 premature respiratory deaths globally due to long-term peak ozone exposure. The values of these mortality

benefits are (2011)$790/t–CH4and $1,775/t–CH4, respectively. Ozone mortality benefits are

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Valuing the Ozone-Related Health Benefits of Methane Emission...

0.7–1.5 times the estimate of climate benefits based on the US Government’s central SCC

estimate and GWP, or 0.6–0.4 times the estimate of climate benefits based on Marten et al.

(2014), with the range depending on whether the short or long-term ozone mortality estimate is used. Both measures are uncertain. With the sensitivities examined here, the highest

estimate of the short-term health benefits is four times greater than the lowest. Including the

benefits estimates of long-term ozone exposure-related mortality increases the upper bound

of the range even further. The climate benefits of CH4 emissions reductions, as measured

by the SC–CH4 (calculated using the US Government’s SCC estimates and the CH4 GWP),

span a range of 10 times the lowest to the highest estimate.20

Significant differences in avoided mortality and monetized benefits exist across regions.

Differences in the number of avoided premature deaths are due to regional populations,

growth rates, and baseline cardiovascular and pulmonary death rates. While the monetized

damages are partially dependent upon the number of avoided premature deaths in a region,

the method used for the extrapolation of the VSL across regions and time has a very large

impact on the estimated monetized benefits; in some regions it has the potential to increase

monetized benefits by more than 10 times.

We find that the benefits of avoided cardiovascular and pulmonary mortality due to reductions in methane emissions are substantial and are an important benefit to include when

assessing the benefits of methane mitigation policies.

Acknowledgments The authors would like to thank Neal Fann, Amy Lamson, Bryan Hubbell, Jason West,

and Steve Smith for reviews and comments on drafts of this paper. Stephanie Waldhoff gratefully acknowledges

the U.S. Environmental Protection Agency for support of this work. Susan Anenberg performed the work on

this paper while employed by the U.S. Environmental Protection Agency. The views expressed in this paper are

those of the authors and do not reflect those of the U.S. Environmental Protection Agency, Pacific Northwest

National Laboratory, nor the U.S. Department of Energy.

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution,

and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source,

provide a link to the Creative Commons license, and indicate if changes were made.

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