Climate Benefits of U.S. EPA Programs and Policies That Reduced

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Policy Analysis

Climate benefits of U.S. EPA programs and policies that reduced methane emissions 1993-2013 April M Melvin, Marcus Sarofim, and Allison R. Crimmins Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00367 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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Environmental Science & Technology

Climate benefits of U.S. EPA programs and policies that reduced methane emissions 1993-2013

April M. Melvin, [email protected], [email protected] AAAS S&T Policy Fellow hosted by the Climate Change Division, U.S. Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC 20460, USA

Marcus C. Sarofim*, [email protected] Climate Change Division, U.S. Environmental Protection Agency U.S. Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC 20460, USA *corresponding author

Allison R. Crimmins, [email protected] Climate Change Division, U.S. Environmental Protection Agency U.S. Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC 20460, USA

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Table of Contents and Abstract Graphic Methane Global concentration

1993

Atm. conc.

Reductions

1993

2013

Temperature rise 1993

2013

∆T

Domestic reductions

2013

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Abstract

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The United States (U.S.) Environmental Protection Agency (EPA) has established voluntary

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programs to reduce methane (CH4) emissions, and regulations that either directly reduce CH4 or

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provide co-benefits of reducing CH4 emissions while controlling for other air pollutants. These

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programs and regulations address four sectors that are among the largest domestic CH4 emissions

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sources: municipal solid waste landfills, oil and natural gas, coal mining, and agricultural manure

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management. Over the 1993-2013 time period, 127.9 Tg of CH4 emissions reductions were

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attributed to these programs, equal to about 18% of the counterfactual (or potential) emissions

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over that time, with almost 70% of the abatement due to landfill sector regulations. Reductions

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attributed to the voluntary programs increased nearly continuously during the study period. We

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quantified how these reductions influenced atmospheric CH4 concentration and global

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temperature, finding a decrease in concentration of 28 ppb and an avoided temperature rise of

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0.006 °C by 2013. Further, we monetized the climate and ozone-health impacts of the CH4

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reductions, yielding an estimated benefit of $255 billion. These results indicate that EPA

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programs and policies have made a strong contribution to CH4 abatement, with climate and air

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quality benefits.

17 18

Introduction

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Atmospheric methane (CH4) concentrations have increased by about 150% since the industrial

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revolution,1 due largely to human activities2 associated with fossil fuel extraction and

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distribution, agricultural practices, waste management, and biomass burning.3 Methane is a

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greenhouse gas (GHG) that contributes directly to climate warming due to its radiative

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properties, as well as indirectly due to the photochemical production of other GHGs, including

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stratospheric water vapor and tropospheric ozone (O3).4 Tropospheric O3 in turn, is a pollutant

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linked to reduced air quality, human mortality and hospitalizations,5-9 and damage to agricultural

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crops and other terrestrial plants.10-12 After carbon dioxide (CO2), CH4 is the largest driver of the

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anthropogenic increase in radiative forcing (RF) and therefore contributes substantially to

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climate warming.4

29 30

Methane has an atmospheric perturbation lifetime of about 12 years.4 Therefore, decreasing CH4

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emissions could reduce the atmospheric CH4 concentration over relatively short time scales and

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provide near-term climate benefits,3, 13, 14 while also lessening impacts of tropospheric O3

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pollution on human health and terrestrial ecosystems.11 The United States (U.S.) Environmental

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Protection Agency (EPA) has developed numerous programs and policies to reduce CH4

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emissions. These include the establishment of voluntary emissions reductions programs,

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regulations that directly limit vehicle CH4 emissions, and regulations that require control of other

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pollutants, which provide the co-benefit of also capturing CH4. Collectively, these regulatory and

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voluntary activities have focused on many of the largest CH4 emissions sources in the U.S.

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including oil and natural gas systems, municipal solid waste landfills, coal mining, and animal

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manure management, and have increased the fraction of CH4 emissions being controlled through

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capture and/or combustion, instead of being emitted directly to the atmosphere. The EPA has

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recently proposed additional regulations that would directly limit CH4 emissions in landfill and

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oil and gas sectors. The analysis presented here is retrospective and does not quantitatively

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address proposed rules or any future reductions from existing programs.

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Domestic efforts to reduce emissions from dominant sources date back to the early 1990s, when

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researchers at the EPA noted that capturing and utilizing CH4 could provide a cost-effective

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strategy to reduce CH4 emissions while also providing a new fuel source, since CH4 is the

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primary component of natural gas.13, 14 In 1993, the Clinton Administration put forth a Climate

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Change Action Plan15 that directed the EPA to expand a newly formed Natural Gas STAR

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partnership program (www.epa.gov/gasstar) that fosters public/private partnerships and promotes

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the use of cost-effective technologies and practices to capture and use CH4 within the natural gas

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sector.15 The Plan further called for the development of CH4 outreach and assistance programs to

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facilitate the capture and use of CH4 produced at landfills and coal mines, and from animal

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manure, leading to the creation of three additional voluntary CH4 emissions reductions programs

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in 1994. The Landfill Methane Outreach Program (LMOP, www.epa.gov/lmop) and Coalbed

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Methane Outreach Program (CMOP, www.epa.gov/cmop) were developed to address CH4

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emissions from small municipal solid waste landfills and coal mining operations, respectively.

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The AgSTAR program (www.epa.gov/agstar) was developed in collaboration with the U.S.

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Department of Agriculture and was tasked with reducing CH4 emissions produced during liquid

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manure management by encouraging biogas recovery and use. In each of these programs, strong

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emphasis was placed on the development of voluntary partnerships between the federal, state,

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and local governments, as well as with businesses and industry. These voluntary programs

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subsequently provided the foundation for the creation of the Global Methane Initiative

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(www.globalmethane.org), an international voluntary CH4 emissions abatement program

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established in 2004. Programs also maintain a close relationship with the Climate and Clean Air

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Coalition (www.ccacoalition.org), launched in 2012.

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Federal regulations have provided both direct and indirect approaches to addressing CH4

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emissions. Limits on CH4 emissions from light-duty16, 17 and medium- and heavy-duty vehicles18

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were established to prevent increases in CH4 emissions from these sources. These are the only

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existing U.S. regulations that directly regulate CH4 for climate purposes. Indirect CH4 emissions

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reductions have resulted from the required control of other pollutants that pose a risk to human

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health. Methane is co-emitted with volatile organic compounds and hazardous air pollutants from

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some sources, including landfills and oil and natural gas production. Because the methane is co-

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emitted, the capture and combustion targeting these other pollutants has the co-benefit of also

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capturing CH4. This gas can then be either flared or used as an energy source. Regulations

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providing this co-benefit include the 1996 “Landfill Rule”19 (and associated amendments20-22),

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requiring the reduction of non-methane organic compounds from gas produced in large

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municipal solid waste landfills, and the National Emissions Standards for Hazardous Air

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Pollutants (NESHAP) Rule established in 1999 for oil and natural gas activities23 and expanded

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in 200724 and 2012.25 In 2012, New Source Performance Standards (NSPS) were also finalized

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for crude oil and natural gas production.25 In the coal mining and manure management sectors,

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no federal regulations are currently in place (or proposed) that limit CH4 emissions.

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The EPA continues to build on existing voluntary programs and regulations to further reduce

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domestic CH4 emissions and address climate change. In March 2014, The White House released

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the “Strategy to Reduce Methane Emissions”26 as part of President Obama’s Climate Action

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Plan. This policy statement called for continued support and expansion of all domestic voluntary

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CH4 programs and development of new regulatory actions. In July 2015, the EPA announced the

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Natural Gas STAR Methane Challenge, a new voluntary initiative designed to increase voluntary

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CH4 abatement from oil and gas. New regulations have also been proposed that would increase

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restrictions on landfill gas emissions at existing landfills,27 limit emissions from new and

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modified landfills,28 and would require control of CH4 emissions from certain new and modified

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sources in the oil and gas sector.29

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As the U.S. continues to take steps to reduce domestic CH4 emissions to mitigate climate

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warming, it is important to evaluate the effectiveness of EPA’s existing voluntary programs and

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regulations that have influenced U.S. CH4 emissions. The EPA quantifies and reports CH4

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emissions reductions from each of the voluntary programs on an annual basis. In many cases,

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these data, along with CH4 emissions reductions values resulting from regulations and other

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incentives or programs, are used in the development of the Inventory of U.S. Greenhouse Gas

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Emissions and Sinks (hereafter GHG Inventory), released annually30 and submitted to the United

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Nations Framework Convention on Climate Change (UNFCCC). These CH4 emissions

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reductions data are also submitted to the UNFCCC as part of the U.S. Climate Action Report as

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documentation of national actions and progress made in combating climate change. While the

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emissions reductions values documented in these reports provide valuable information about

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emissions trends, the climate benefits (i.e. the reduced warming resulting from fewer CH4

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emissions) and additional qualitative impacts of the voluntary reductions programs have never

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been evaluated. Further, no studies have quantified the contribution of U.S. CH4 emissions

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reductions resulting from these programs and policies to the observed decrease in the rate of CH4

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accumulation in the atmosphere in the 1990s, or the near constant atmospheric CH4

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concentration observed from 1999 to 2006.3, 31

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Using annual U.S. CH4 emissions reductions data reported by the EPA, we quantified how

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reductions attributed to existing federal programs and policies influenced CH4 emissions trends

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in the municipal solid waste landfill, oil and natural gas, coal mining, and manure management

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sectors from 1993 through 2013. We compared these reductions to EPA estimates of national

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CH4 emissions within the study sectors and discuss the relative impact of the programs and

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policies. Further, we assessed how these U.S. CH4 emissions reductions influenced the global

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atmospheric CH4 concentration and consequent changes in temperature over this time period.

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Finally, we summarized the qualitative aspects of the voluntary programs within their respective

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sectors, as well as the monetary benefits of reductions from all sources, estimated using the

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Social Cost of Methane and O3-health damage estimates.

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Methods

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Methane emissions reductions data sources

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Annual U.S. CH4 emissions reductions values used in this analysis have been previously

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published in EPA reports. For the voluntary programs, only emissions reductions associated with

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voluntary activities may be attributed to programs and therefore, the voluntary program data does

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not include reductions resulting from regulations. It is possible that the CH4 emissions reductions

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credited to these voluntary programs may have occurred without the programs, especially when

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it is cost-effective to capture and use the gas. The programs therefore evaluate their successes

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using many metrics that determine emissions reductions and qualitative benefits of the programs.

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Each program has developed sector-specific methodologies to estimate these reductions which

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incorporate reported and modeled CH4 emissions reductions, as well as information on project

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assistance, level of partner involvement, and for some sectors, Intergovernmental Panel on

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Climate Change methodologies.

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All CH4 emissions reductions data used in this analysis for the domestic voluntary programs

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(AgSTAR, CMOP, LMOP, and Natural Gas STAR) were values previously published in EPA

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reports. Emissions reductions for 2000-2013 were contained in the Climate Protection

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Partnerships 2013 Annual Report32 and the 1993-1999 data were included in the 2010 Climate

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Protection Partnerships Report.33 In some cases, direct CH4 emissions reductions values were not

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presented independently for each program and these program-specific values were obtained

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directly from EPA staff. Data from the 2010 Report were reported as a single annual value that

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combined reductions from all programs voluntary programs. In the 2013 Annual Report,

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emissions from AgSTAR and LMOP, were presented as the sum of direct CH4 emissions

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reductions and avoided CO2 emissions, and in this quantitative analysis only the direct reductions

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are included, because avoided CO2 emissions represent an indirect benefit of the programs.

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Throughout this analysis, results for the AgSTAR program include only years 2000-2013

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because data were unavailable prior to this time period. Qualitative voluntary program

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information was acquired from EPA’s annual Climate Protection Partnerships Reports, program

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websites, and communication with EPA staff. The CH4 emissions reductions calculated by the

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individual voluntary programs used in this analysis are not explicitly reported in the GHG

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Inventory for some sectors. In some cases, the GHG Inventory reports values for CH4 capture

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and use without attributing the capture and use to specific programs. Methodological details for

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GHG Inventory estimates are provided in the report.

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Estimates of domestic CH4 emissions reductions resulting primarily from federal regulations

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were obtained from the GHG Inventory.30 The GHG Inventory is the U.S. official annual GHG

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inventory report to the UNFCCC. This report provides the most comprehensive estimates

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available across domestic emission source sectors, provides a long-term record, and is

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continuously updated to reflect the best information and methods available. We included CH4

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emissions reductions resulting from regulations for municipal solid waste landfills and oil and

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natural gas systems only, since these are the only large source sectors with existing regulations

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that impact CH4 emissions. For landfills, the GHG Inventory reported gas-to-energy and flared

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“Recovered” CH4 emissions that included reductions credited to both voluntary programs and

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regulatory reductions stemming from the Landfill Rule. To avoid double counting emissions

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credited to the voluntary LMOP, we subtracted the LMOP values obtained from the Climate

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Protection Partnerships reports from the GHG Inventory values and used this difference for the

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total emissions reductions attributed to the Landfill Rule. For oil and gas, the GHG Inventory

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provided reductions attributed specifically to certain regulations (i.e. 1999 NESHAP) and

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therefore we used those values. The 2012 amendments to NESHAP and the 2012 NSPS are

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expected to have influenced 2013 CH4 emissions reductions values, however these reductions are

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not explicitly quantified in the GHG Inventory (some reductions, e.g. for hydraulically fractured

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gas well completions and workovers) are implicitly included in technology-specific calculations

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and therefore the CH4 emissions reductions attributed to them could not be isolated for this

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analysis. In this way, our analysis does not fully account for all reductions attributable to EPA

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programs. We also obtained annual domestic CH4 emissions data by sector from the 2015 GHG

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Inventory,30 to determine the impact of the programs and policies on sectoral emissions

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throughout the study period.

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In recent years, substantial amounts of new information on CH4 emissions have become

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available through a number of channels, including EPA’s Greenhouse Gas Reporting Program

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(GHGRP) and studies conducted by various organizations, government and academic

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researchers, and industry.34-36 The GHGRP and some of the new studies provide information that

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can be used to update or assess emission factors or activity data in the GHG Inventory. Other

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recent studies evaluated source-based, bottom-up approaches to quantifying CH4 emissions (as is

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used in EPA’s GHG Inventory), as well as atmospheric, top-down methods, and found that the

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former approach tends to yield lower emissions estimates.37, 38 Others have also noted

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discrepancies between the GHG Inventory and emissions estimates at local or regional scales

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stemming from various factors including differences in equipment counts,39 whether emissions

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from “super-emitters” are characterized,34 and the emissions factors used in calculating total

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emissions from some sources.37 EPA reviewed new data and has made several updates to its CH4

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estimates in the GHG Inventory. See for example, the public review draft of the Inventory of

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U.S. Greenhouse Gas Emissions and Sinks: 1990-2014.40 As data and methods are updated to

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reflect improvements, historical values are revised as needed to ensure consistent methods are

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applied to all inventory years. Therefore, the most recent GHG Inventory report emissions

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estimates may differ from those presented here.

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The emissions reductions presented in this analysis would be less sensitive to differences in

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emissions values stemming from the abovementioned factors or from updates to the GHG

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Inventory than the total sectoral emissions estimates. Therefore, the relative impact of CH4

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emissions reductions on the sectoral and national CH4 emissions discussed in this analysis would

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likely be smaller when using the emissions estimates developed by some of these other sources,

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or when using the values in the public review draft of the GHG Inventory. The implications of

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any future updates to the GHG inventory for this analysis would depend on the precise nature of

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the updates, but would be expected to be more relevant for estimating baseline emissions than

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reductions in CH4 emissions, which are the focus of this paper.

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Atmospheric CH4 concentration, radiative forcing, and temperature

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To estimate the impact of the CH4 emissions reductions on climate, we compared observed

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atmospheric CH4 concentrations to a counterfactual world with no U.S. reductions (voluntary or

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regulatory) in CH4 emissions from 1993 to 2013. Observed global CH4 concentration data used

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here were obtained from the Advanced Global Atmospheric Gases Experiment.41-43 Similar

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global CH4 values were available from the National Oceanic and Atmospheric Administration’s

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Marine Boundary Layer44 and Annual Greenhouse Gas Index45 datasets. Mean monthly CH4

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concentrations were averaged to estimate annual global mean concentration. Counterfactual

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concentrations for each year were calculated by adding total CH4 emissions reductions values to

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the observed CH4 concentrations. The cumulative change in atmospheric CH4 concentration, or

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atmospheric burden (in Tg), attributed to domestic CH4 reductions was calculated by adding each

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year’s emissions reduction value to the change in atmospheric burden from the previous year

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using a decay rate of 12.4 years.4

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Observed and counterfactual RF were estimated through the application of a simplified

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expression provided in Myhre et al.4 (Supporting Information (SI), equation 1) and an adjustment

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factor of 1.65 was applied to account for tropospheric O3 and stratospheric water vapor produced

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through the degradation of CH4 in the atmosphere. Change in global mean air temperature

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attributed to reduced CH4 emissions was then derived from the calculated RF based on a

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simplified equation presented in Shine et al.46 (SI, equation 2) using a best estimate of climate

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sensitivity of 347 and bounds of 1.5 and 4.5.48

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Results and Discussion

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Methane emissions reductions and sectoral impacts

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Annual CH4 abatement increased throughout most of the study period for both regulations and

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voluntary programs, with some slowing in recent years (Figure 1). Cumulatively over this period,

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70% of annual emissions reductions were the result of regulations and 30% were credited to

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voluntary programs, with small inter-annual variation. Regulations were responsible for a total

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reduction of 89.3 Tg CH4, while voluntary programs contributed an additional 38.6 Tg, for an

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18% reduction in total domestic CH4 emissions between 1993 and 2013. Together, this

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cumulative CH4 reduction is equivalent to approximately 5 times the CH4 emissions reported

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from all U.S. sources in 2013.

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Figure 1. Annual CH4 emissions reductions for the study period 1993-2013. Voluntary

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reductions include reductions credited to EPA’s voluntary programs in the municipal solid waste

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landfills, oil and natural gas, coal mining, and manure management sectors. Regulatory

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reductions indicate CH4 reductions attributed primarily to federal regulation in the landfill sector.

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Municipal solid waste landfills

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The 1996 Landfill Rule addressing emissions in the municipal solid waste sector is the single

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largest source of CH4 abatement considered in this analysis and accounts for 98% of annual

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regulatory reductions during the period 1999-2013 (when the NESHAP rule was also in effect)

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and for 69% of CH4 emissions reductions from all voluntary programs and regulations during the

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1993-2013 time period. A large increase in estimated CH4 abatement in 2010 can be seen in

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Figure 2A. This increase is driven at least in part by use of a new municipal solid waste landfill

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database created following EPA issuance of the 2009 Greenhouse Gas Reporting Rule.49 This

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database is considered to contain the best verified data among available sources,30 indicating that

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regulatory CH4 reductions values published in the GHG Inventory prior to the incorporation of

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this database in 2010 may have been an underestimate of reductions from this sector. There has

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also been a trend toward consolidation of municipal solid waste landfill services, resulting in

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fewer, larger landfill sites. As landfills become larger, they are more likely to reach the threshold

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for regulatory compliance. This could result in greater collection and control of emissions and

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influence the observed increase in CH4 abatement in recent years.

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Figure 2. Total annual domestic CH4 emissions (obtained from the U.S. EPA Greenhouse Gas

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Inventory) and CH4 emissions reductions within the municipal solid waste landfill (A), oil and

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natural gas (B), coal mining (C), and manure management (D) sectors. For landfills and oil and

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natural gas, data includes reductions resulting from existing regulations and voluntary programs.

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For coal mines and manure management, only voluntary programs currently result in CH4

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emissions abatement. Note the difference in scale between A/B, C and D.

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Voluntary activities attributed to LMOP contributed an additional emissions reduction of 8.5 Tg

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CH4 during the study period, with increasing annual reductions over time (Figure 3). Since

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LMOP began in 1994, the program has assisted more than 600 domestic landfill gas energy

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projects. In 2013, the program reported combined participation from 1,070 partners and

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endorsers (nonprofit organizations that encourage members to capture and use landfill gas). This

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includes a broad range of partnerships with communities, landfill owners, utility companies,

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power marketers, state governments, project developers, tribes, and nonprofit organizations.

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Figure 3. Annual reductions in CH4 emissions attributed to EPA’s voluntary CH4 programs in oil

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and natural gas (Natural Gas STAR), municipal solid waste landfills (LMOP), coal mining

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(CMOP), and manure management (AgSTAR) from 1993 to 2013.

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When considering both regulations and voluntary programs together, landfills were responsible

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for about 75% of the total estimated national CH4 emissions abatement that occurred from 1993

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to 2013. Data from the GHG Inventory indicates that national CH4 emissions from the landfill

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sector have decreased by 40% over this time. Under a counterfactual scenario with no regulatory

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or voluntary CH4 reductions, emissions from this sector would have increased instead of

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decreased (Figure 2A). The largest difference between observed and counterfactual occurred in

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2013, when programs and regulations resulted in a 66% (9.0 Tg CH4) reduction from what CH4

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emissions would have been otherwise that year. We made the assumption for this analysis that all

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CH4 emissions reductions provided in the GHG Inventory were the result of either federal

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regulation or LMOP. Reductions categorized as regulatory prior to the implementation of the

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Landfill Rule in 1996 shown in Figure 2A are likely the result of landfill gas energy projects that

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began prior to the establishment of LMOP and the Landfill Rule, as a result of federal tax credits

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and demonstrated efficiency, dependability, and cost savings resulting from adoption of CH4

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capture and use technologies.50

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Oil and natural gas

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The oil and natural gas sector has seen reductions from both regulatory and voluntary actions.

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The NESHAP Rule in the oil and natural gas sector, which requires controls on hazardous air

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pollutants, accounted for 2% of the regulatory CH4 reductions occurring since rule finalization in

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1999, and 1% of reductions from all CH4 abatement activities included in this analysis for the

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entire study period. Although the NESHAP rule made a relatively small contribution to

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emissions reductions, a nearly continuous increase in annual CH4 emissions reductions resulting

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from this regulation was reported. Conversely, the voluntary Natural Gas STAR program had a

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relatively large impact on U.S. CH4 abatement, accounting for 18% of total CH4 emissions

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reductions between 1993 and 2013. Natural Gas STAR was also the primary source of reductions

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among the voluntary programs (Figure 3), totaling 23.3 Tg CH4 for the study period. The Natural

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Gas STAR program has grown to address CH4 emissions from all sectors of the natural gas

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supply chain, and from oil production. The success of the domestic Natural Gas STAR program

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led to a 2006 expansion to include international partners (though this analysis includes only

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domestic reductions). In 2013, the program reported more than 130 partnerships in the U.S. and

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abroad and the cumulative implementation of over 150 cost-effective technologies and best

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management practices that reduce CH4 emissions as a result Natural Gas STAR program

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activities.

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According to the EPA GHG Inventory, national CH4 emissions from oil and natural gas showed

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a 14% decline between 1993 and 2013, with some small variability among years (Figure 2B).

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This overall decline has been attributed to the voluntary programs, regulations, and replacement

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and installation of new equipment.30 The counterfactual indicates that without existing programs

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and policies, U.S. CH4 emissions from this sector would have increased slightly over the study

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period. In recent years, the Natural Gas STAR program has shown a decrease in reported

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voluntary CH4 abatement (Figure 3). This is likely driven by new federal and state-level

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regulations in some oil and gas producing states that have resulted in fewer operations being able

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to report reductions as voluntary. This decrease does not necessarily reflect a decline in CH4

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abatement in the oil and gas sector, but a shift in abatement for some categories from voluntary

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to regulatory activities. The regulation-driven CH4 emissions reductions quantified in this

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analysis reflect only reductions attributed to the NESHAP regulations established in 1999, as

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these regulations are explicitly quantified for the GHG Inventory. Reductions driven by state-

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level policies and other regulatory activities, including the 2012 NSPS, have not been quantified

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explicitly. In this way, this analysis may underestimate the reductions resulting from EPA

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programs in this sector. For example, reductions of CH4 from meeting 2012 NSPS emissions

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reduction requirements for hydraulically fractured gas well completions are taken into account in

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the technology-specific calculations for this emission source in the GHG Inventory, and no

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specific reduction values were available for use in this analysis.

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Coal mining

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Voluntary activities in the coal mining sector were responsible for 5% (6.6 Tg) of the total CH4

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abatement between 1993 and 2013. Reductions tended to increase across the study period, with

345

some variation among years (Figure 3). Initially, CMOP focused on CH4 reduction from

346

degasification systems in active underground mines, where only about 25% of systems captured

347

and used CH4 in the mid-1990s. By 2010, over 80% of the CH4 captured in desgasification

348

systems was utilized as a fuel source.33 The success of capture in degasification systems led to

349

expansion into ventilation systems and projects in abandoned underground mines, as well as

350

some surface mines. In 2002, CMOP began implementation of the first domestic commercial-

351

scale demonstration of ventilation air oxidation technology in collaboration with the U.S.

352

Department of Energy and a publicly owned energy company. This technology has now been

353

incorporated into some new CMOP-related projects. In 2013, CH4 capture and use systems were

354

used in 17 active coal mines and 18 abandoned mines.32

355 356

National CH4 emissions from the coal mining sector reported in the GHG Inventory showed a

357

slight decrease during most study years, with the exception of increased emissions occurring in

358

2008-2010 (Figure 2C), the cause of which remains unclear. In a counterfactual situation without

359

CMOP, annual CH4 emissions would have been 6-14% higher between 1993 and 2013, with

360

greater reductions observed in more recent years.

361 362

Manure management

363

The AgSTAR program was responsible for 0.19 Tg of CH4 emissions reduction between 1993

364

and 2013 and accounted for 0.1% of reductions credited to voluntary programs. This estimate is

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likely conservative because it does not include reductions during the 1994-1999 time period.

366

Although this was a small reduction compared to those reported for other programs, a relatively

367

large increase in CH4 abatement was observed within the program, increasing from 0.001 Tg

368

CH4 in 2000 to 0.03 Tg in 2013. Numerous activities contributed to these reductions, including

369

the development of demonstration farms to showcase biogas recovery systems, creation of

370

national standards for biogas technologies, and outreach with renewable energy industry leaders,

371

state and local governments, universities, and non-governmental organizations. AgSTAR has

372

also assisted states in the development of programs and policies that support greater adoption of

373

cost-effective biogas technologies. In addition to avoiding CH4 emissions and providing a

374

cleaner fuel source, AgSTAR notes the benefits of digester systems for reducing local air and

375

water pollution, generating byproducts such as manure fibers that can act as an additional

376

revenue source, and providing opportunities for rural economic development. Between 1994 and

377

2013, the AgSTAR program reported adoption of biogas recovery systems at 239 livestock farms

378

in the U.S.32

379 380

National CH4 emissions from the animal manure management sector reported in the GHG

381

Inventory increased by 56% between 1993 and 2013 (Figure 2D). This pattern was attributed to

382

an increase in the use of liquid manure management, especially on swine and dairy cow farms.30

383

The AgSTAR program addresses livestock manure handled as liquid and slurry, which makes up

384

the largest fraction of CH4 emissions from the manure management sector. In a counterfactual

385

situation without AgSTAR, annual emissions from this sector would have been 1.3% higher in

386

2013.

387

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Impacts of U.S. programs and policies on global CH4 concentrations and temperature

389

In a counterfactual world without existing U.S. federal regulations and voluntary programs to

390

reduce CH4 emissions, global atmospheric CH4 concentrations would have been approximately

391

28 ppb higher than observed in 2013 (Figure 4). Regulations were responsible for approximately

392

20 ppb and the voluntary programs for 8 ppb. The difference between observed and

393

counterfactual CH4 concentrations grew throughout the study period due to the annual increase

394

in CH4 emissions reductions and the associated effect on atmospheric burden. The change in

395

global atmospheric CH4 concentration credited to U.S. actions resulted in a decrease in RF and

396

reduction in global mean temperature increase throughout the study period. In 2013, the

397

decreased RF reached 0.01 W m-2 and translated to a reduction in mean temperature of

398

approximately 0.006oC, assuming a climate sensitivity of 3 (Figure 5). The bounds for sensitivity

399

(1.5 and 4.5) indicated a global temperature reduction of 0.004oC and 0.007oC, respectively in

400

2013. Regulations were responsible for approximately 0.004oC of this change, and voluntary

401

programs for 0.002oC. For context, the global surface temperature increase over the 1993-2013

402

period, based on global land and ocean anomalies, was about 0.3 oC.51 This suggests that the U.S.

403

CH4 programs addressed in this paper have reduced global warming by about 2% over the past

404

two decades, which is a substantial contribution for mitigation activities addressing a subset of

405

emissions from a single nation. With further domestic or international action, a larger near-term

406

climate benefit could be achieved. These programs may also have indirect benefits in the form of

407

avoided CO2 emissions if CH4 burned for electricity generation displaces the use of other fossil

408

fuels, but this effect was not quantified in this analysis. The results presented here do not include

409

the effect on CO2 concentrations resulting from immediate production of CO2 due to combustion

410

of CH4, rather than delayed production resulting from oxidation of CH4 in the atmosphere. While

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the appropriate methodology for valuing a delay in release of a gas is an ongoing area of

412

research, a sensitivity analysis using the assumption that all CH4 reductions accounted for in this

413

analysis are immediately converted to atmospheric CO2 found a temperature impact in 2013 of

414

about 4% of the CH4 reductions (SI).

415

416 417

Figure 4. Observed global mean CH4 concentration and estimated counterfactual CH4

418

concentration in the absence of regulations and voluntary CH4 emissions reductions programs

419

included in this study.

420

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421 422

Figure 5. Estimated reduction in the global temperature increase attributed to U.S. CH4 emissions

423

reductions resulting from regulations and voluntary programs for the study period 1993-2013.

424

Values were calculated using a best estimate climate sensitivity of 3 (solid line), and bounds of

425

1.5 and 4.5 (dashed lines).

426 427

Figure 4 suggests a fairly constant atmospheric CH4 concentration between 1999 and 2006,

428

where the lines are relatively flat. This observation has been documented in other studies and

429

linked to both natural and anthropogenic causes, including changes in wetland emissions and

430

agricultural practices, the collapse of the Russian economy post 1992, and reductions in the

431

growth of CH4 emissions, including from the U.S. and Europe.31, 52, 53 Methane emissions

432

abatement in the U.S. increased throughout this period (Figure 1), suggesting that the programs

433

and policies included in our analysis, particularly landfill regulation, may have had a previously

434

underappreciated contribution to stabilizing the global CH4 concentration during this time .

435

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Estimated Monetized Benefits of Methane Reductions

437

In addition to physical metrics that demonstrate the impact of historical CH4 reductions on

438

climate, there have been recent advances in monetizing the benefits of CH4 abatement. Marten et

439

al.54, 55 calculated a Social Cost of Methane, which estimated that the net present value of CH4

440

emissions reductions in 2020 resulting from mitigation of climate damages is $1200/Mg (2011$)

441

at a discount rate of 3%. In addition to the climate damages of CH4, a number of studies have

442

analyzed impacts on health5, 56 and agriculture11 due to the creation of O3 by CH4 oxidation in the

443

atmosphere. Sarofim et al.9 found the net present value of methane reductions in 2020 resulting

444

from mitigation of O3-health damages to be $790/Mg (2011$) at a discount rate of 3%, using

445

assumptions that were designed to complement the Social Cost of Methane approach.54

446

Combining the two methods, the value of one Mg of CH4 reduced in 2020 would be $1990 at a

447

discount rate of 3%, with a range of $1200 to $2431 for discount rates of 5 and 2.5%,

448

respectively. Assuming a constant value of CH4 reductions, and not accounting for uncertainties

449

beyond the discount rate, the monetized health and climate benefits of 128 Tg of CH4 reductions

450

credited to U.S. voluntary programs and policies over the period 1993-2013 would be $255

451

billion at a discount rate of 3%, with a range of $154 to $311 billion depending on discount rate.

452

For comparison, 128 Tg of CH4 would be worth approximately $15 billion if it were captured

453

and sold (based on the mean price of natural gas in 2015 of $2.63 per million Btu,57 and

454

assuming natural gas is 100% CH4). More comprehensive cost-benefit analyses that incorporate

455

costs incurred to install and operate CH4 capture and use technologies would further refine

456

estimates of the monetary benefits of reducing CH4 emissions.

457 458

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Conclusions

460

This quantification of the benefits of U.S. EPA regulations and voluntary CH4 programs

461

demonstrates substantial avoided increases in global CH4 concentration, RF, and temperature. In

462

sum, these programs accounted for a reduction of 127.9 Tg of CH4 over a 20-year period, leading

463

to a reduction of 28 ppb atmospheric CH4 concentration and an avoided 0.006 °C temperature

464

rise in 2013. The monetized climate and health benefits of CH4 and associated O3 reductions are

465

estimated to be $255 billion. Historically, the 1996 Landfill Rule accounted for 70% of the total

466

reductions, with the voluntary programs accounting for about 30%. Recently finalized rules,

467

including the 2012 NSPS for oil and gas, have also influenced emissions which are not

468

quantified here. Accounting for non-federal programs, and for international reductions resulting

469

from U.S. actions, would also increase estimated CH4 emissions reductions. There will also be

470

future benefits resulting from historical reductions and the continuation of existing programs.

471

Recently proposed rules within the landfill and oil and gas sectors would also have CH4

472

reduction benefits. In addition to these quantifiable CH4 benefits, the voluntary programs have

473

further facilitated the adoption of more cost-effective technologies and practices. Collectively,

474

these findings illustrate the benefits of a wide range of approaches taken historically to address

475

the risks of climate change and provide important insights moving forward, as programs and

476

policies to address U.S. CH4 emissions remain a priority for the Federal Government.

477 478

Supporting Information (SI)

479

Supporting information available: the Supporting Information describes the equations and

480

procedures used for calculating concentration, radiative forcing, and temperature changes for a

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pulse of methane emissions, or for the acceleration of CO2 production resulting from flaring

482

methane. This material is available free of charge via the Internet at http://pubs.acs.org.

483 484

Acknowledgements

485

The authors would like to thank Melissa Weitz and additional EPA staff who provided

486

information used in this analysis and reviewed drafts of this manuscript. Thanks also to three

487

anonymous reviewers for providing constructive feedback to improve this manuscript. The

488

Landfill Methane Outreach Program generously provided the photographs use in the TOC

489

graphic. The views expressed in this paper are those of the authors and do not reflect those of the

490

U.S. Environmental Protection Agency.

491 492

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