Methane: An Introduction to Emission Sources and Reduction Strategies
Released: 2016-11-18T00:00:00

Summary:
The Obama Administration's Strategy to Reduce Methane Emissions
On June 25, 2013, President Obama announced a national "Climate Action Plan" (CAP) to reduce emissions of carbon dioxide (CO2) and other greenhouse gases (GHGs), as well as to encourage adaptation to expected climate change. One of the initiatives within the CAP focused on the control of methane emissions, a potent short-lived climate pollutant. It called for the U.S. Environmental Protection Agency (EPA) and the Departments of Agriculture, Energy, the Interior, Labor, and Transportation to develop a comprehensive interagency "Strategy to Reduce Methane Emissions." The Strategy, released on March 28, 2014, committed to steps to cut methane emissions by an estimated 16% from 2012 levels by 2020 through both voluntary actions and agency rulemaking. It also outlined the Administration's efforts to improve the measurement and assessment of these emissions.
Perspectives on the Strategy
Some stakeholders, including many in the affected sectors (i.e., agriculture, fossil energy, and waste management), have raised concerns over federal proposals requiring more stringent controls. They argue that further regulation of methane emissions would not provide cost-effective health and environmental benefits. Some industries contend that they are already doing everything feasible to capture and reuse methane emissions (for requisite safety and economic reasons) and that state and local authorities—who share a closer understanding of the industries' specific circumstances—are best equipped to oversee and enforce emission reduction efforts within their jurisdictions.
Other stakeholders, including many health and environmental advocates, contend that the Strategy and its proposed rulemakings fall short. They argue that methane emissions can jeopardize worker safety, lead to ground-level ozone formation (commonly referred to as "smog"), and act as a potent GHG. Recent events in the United States (e.g., the rise in domestic oil and natural gas production, the encroachment of domestic oil and natural gas production on new or more populated areas, and the revitalization of the petrochemical manufacturing sector) have led these stakeholders to suggest the need for more enforceable standards. Likewise, they estimate that the Obama Administration's recent GHG reduction targets, offered under the U.S. commitments to the United Nations Framework Convention on Climate Change, would be unattainable without further controls.
The Role of Methane
Behind it all is methane—the world's simplest hydrocarbon and the primary component of natural gas. It is released into the atmosphere by both natural sources (such as wetlands and wildfires) and human activities (such as oil and natural gas systems, coal mines, landfills, and the raising of livestock). When captured, methane can be used as either a fuel or a chemical feedstock, with many advantages over other fossil fuels (e.g., it is more versatile and less polluting). Its dual nature as both a pollutant and a commodity makes efforts to control emissions potentially beneficial to both the environment and the economy.
For these reasons, as far back as the 1970s, the federal government has sought policies to help reduce, capture, and reuse methane emissions. Whether strategies to control emissions are effective and cost-efficient for a given industry may depend upon a number of factors, including the nature and extent of the emissions, the technology available for capture, and the market price for the recovered products. In this way, the cost-benefit considerations are similar to those of energy efficiency efforts, wherein high up-front investments and other market barriers, if confronted by producers, may be offset over time.
Recent federal policies have included a variety of funding programs for research and technology development as well as voluntary programs and tax incentives for industry. Historically, methane emissions were addressed directly by two federal rules: one on new municipal landfills and another on federal oil and gas leases. Since the Strategy's release, the Administration has proposed and finalized several additional rules—on oil and natural gas systems, coal mines, and municipal landfills. These rulemakings—as well as a variety of legislative efforts in Congress—attest to the continued interest in an appropriate policy response to the issue of methane emissions.

subsections: section_title: Introduction
paragraphs: Methane is the world's simplest hydrocarbon, with a chemical formula CH 4 (one atom of carbon and four atoms of hydrogen). It is gaseous under normal atmospheric conditions and is commonly produced through the decomposition of organic materials in the absence of oxygen. It is released into the atmosphere by natural sources such as wetlands, oceans, sediments, termites, volcanoes, and wildfires, as well as human activities such as oil and natural gas systems, coal mines, landfills, wastewater treatment facilities, and the raising of livestock. 
Methane, when captured, can be used as either a fuel or a chemical feedstock. When used as a fuel—for example, methane is the primary component of natural gas —it has many advantages over other hydrocarbons (e.g., coal and oil). Methane is more versatile: It can heat homes, fuel stoves, run vehicles, fire power plants, and, when liquefied, be exported to support the energy needs of U.S. allies and trading partners. Methane is cleaner-burning: It emits, on average, about half as much carbon dioxide (CO 2 ) as coal and one-quarter less than oil when consumed in a typical electric utility plant. Further, its combustion emits no mercury (a persistent, bioaccumulative neurotoxin), virtually no particulate matter or sulfur dioxide, and less nitrogen oxides, per unit of energy, than either coal or oil. Recent expansion in natural gas production, primarily as a result of improved technologies (e.g., hydraulic fracturing and directional drilling) used on unconventional resources (e.g., shale, tight sands, and coalbed methane), has made methane an increasingly significant component in the energy supply and security of the United States.
When used as a chemical feedstock, methane is a manufacturing component for a variety of household and industrial products including plastic, fertilizer, antifreeze, and fabrics. Abundant and economical supplies of methane may serve arguably to reinvigorate the U.S. petrochemical sector, bringing manufacturing industries back on shore and aiding in the creation of domestic jobs and economic development. 
For these reasons, many in both the public and private sectors have advocated for the increased production and use of methane (via natural gas extraction or other capture technologies) and have hailed it as a potential "cost-effective bridge" to a less polluting and lower greenhouse-gas-intensive economy. This position has been supported by many Members of Congress as well as the Obama Administration.
Methane, however, when released or allowed to escape into the atmosphere (commonly referred to as "vented" and "fugitive" emissions, respectively), has adverse impacts on human health, safety, and the environment. The U.S. Occupational Safety and Health Administration lists methane as both an asphyxiant and an explosive, as increased concentrations in local settings can jeopardize worker safety. Further, the U.S. Environmental Protection Agency (EPA) classifies methane as both a precursor to ground-level ozone formation (commonly referred to as "smog") and a potent greenhouse gas (GHG), albeit with a shorter atmospheric life than CO 2 . Methane's effect on climate change is up to 34 times greater than that of CO 2 when averaged over a 100-year time period and even greater when considered over the first 20 years after it is emitted. An increase in emissions may counteract some of the environmental benefits that the U.S. economy has to gain by switching from coal or oil to natural gas and other sources of methane. For these reasons, some stakeholders, including some Members of Congress, have called for increased controls on methane emissions in several sectors of the economy, including oil and natural gas production and transportation, coal mining, industrial processes, and agriculture.
In many cases, efforts to control air pollution can compete against the economic considerations of the affected industries. However, in methane's case, its dual nature as both a commodity and a pollutant provides a unique set of incentives. Under certain conditions, the value of fugitive methane and other byproducts that can be recovered and sold at market may be able to offset the cost of their capture. Further, the value of these recovered products during oil and gas extraction could contribute to increased royalty payments to state and federal governments.
The difficulty, however, is that methane emissions are not always easy to capture. Methane, unlike some other pollutants (e.g., sulfur dioxide or CO 2 ), is not commonly emitted in a concentrated stream from industrial processes. Rather, it is released into the atmosphere through dispersion, leaks, vents, accidents, and ruptures. In this way, methane emissions are most similar to those of other volatile organic compounds (VOCs), both in manner and control. Efforts to capture or abate these emissions are generally more difficult and costly than for other pollutants. Whether recovery of methane is profitable for producers may depend upon a number of factors, including the nature and extent of the release, the technology available for capture, and the market price for the recovered products. In this way, the cost-benefit consideration of methane capture becomes very similar to that of energy efficiency efforts, wherein high up-front investments and other market barriers, if confronted by producers, may have the potential to be offset over time.
This report examines the many facets of methane: from commodity to coproduct to byproduct to waste. It begins with a survey of past and present attempts by Congress and the executive branch to address methane emissions for the purposes of energy policy and pollution control. It then provides a general overview of methane before focusing on specific sectors of the economy in order to (1) characterize different sources of methane and the data available on their emissions; (2) discuss current practices, opportunities, and challenges for emission controls; and (3) outline recent initiatives proposed by Congress and the Administration.
section_title: Issues for Congress
paragraphs: Through the years, the federal government has sought policies to control methane emissions for a variety of economic, environmental, and public health and safety reasons. Some justifications for federal involvement have included the following:
1. Promoting domestic energy production and energy security, 2. Protecting the property rights of mineral owners (including federal resources and associated royalties to the American taxpayer), 3. Assuring the operational safety of employees who work with or near significant emission sources, and 4. Safeguarding the general population from air pollution that may reasonably be anticipated to endanger public health or welfare. 
Initially, policies to capture methane emissions were motivated in part by the Organization of Arab Petroleum Exporting Countries oil embargo of 1973 and the subsequent calls for U.S. energy independence. During this time, the United States saw natural gas and other sources of methane as a potential alternative to imported crude oil. Efforts to incentivize the capture of methane and use it as an alternative fuel were proposed by both Congress and the Administration across the full range of commercial sectors. They included a variety of funding programs for research and technology development, voluntary guidelines and tax incentives for industry, and/or rules for mineral rights lessees on federal lands.
As an understanding of methane's role in ozone formation and climate change grew during the 1990s, some state and federal authorities turned their attention to reducing methane emissions as a form of pollution control. Once again, the key policy tools used for pollution abatement took the form of voluntary guidelines and tax incentives. However, in a few instances, where reductions in other pollutants could serve the co-benefit of aiding in the reduction of methane, regulatory emission standards were proposed and/or promulgated. In the 2000s, as Congress considered comprehensive market-based strategies to reduce GHG emissions across the entire U.S. economy, more innovative proposals for methane reduction became prevalent. Methane capture was commonly suggested as an "offset" credit for higher GHG-emitting industries, as the net costs of reducing methane emissions, in some instances, could be more favorable than directly controlling for CO 2 emissions.
Recent events in the United States (e.g., the rise in domestic oil and natural gas production, its encroachment on new or more populated areas, and the revitalization of the petrochemical manufacturing sector) have led some stakeholders to suggest the need for more enforceable standards. At the state level, Colorado, Wyoming, Ohio, California, and Pennsylvania have recently promulgated or proposed rules to control for methane emissions from their oil and gas sectors. Further, many states have standards to control for methane emissions from coal mining and landfill operations. Some have regulations that focus on methane's potential as an energy source, and a few classify methane as a renewable energy source.
Under the Clean Air Act (CAA), EPA has the authority to regulate methane emissions as both an ozone precursor and a GHG. Currently, EPA has no standards in place to regulate methane as an ozone precursor, and it has shown a disinclination for doing so in the past. Conversely, EPA has regulated methane as a GHG since 1996 under its standards for municipal landfills. The agency's authority to regulate methane as a GHG was upheld by the Supreme Court's 2007 decision in Massachusetts v. EPA , which determined that GHGs fall under the definition of "air pollutant" as used in the CAA. Following this decision, EPA determined that six GHGs, including methane, endangered public health and welfare and issued several rules focused primarily on CO 2 . 
Recently, under the directive of the Obama Administration's 2014 "Strategy to Reduce Methane Emissions" (discussed in further detail in the remainder of this report), EPA has proposed and promulgated a suite of voluntary and regulatory programs to address methane emissions across a range of industrial sectors. Additionally, the Departments of Agriculture, Energy, the Interior, Labor, and Transportation have some authorities to monitor, give guidance for, and make rules to control for methane emissions (e.g., see the Bureau of Land Management's [BLM's] 1980 notice on venting and flaring for oil and gas leases on federal lands and its 2016 revision). As with EPA, some of these agencies have also proposed initiatives under the directive of the Administration's Strategy.
Some stakeholders, including many in the affected sectors (i.e., agriculture, fossil energy, and waste management), have raised concerns over federal proposals requiring more stringent controls. They argue that further regulation of methane emissions would not provide cost-effective health and environmental benefits. Some industries contend that they are already doing everything feasible to capture and reuse methane emissions (for requisite safety and economic reasons) and that state and local authorities—who share a closer understanding of the industries' specific circumstances—are best equipped to oversee and enforce emission reduction efforts within their jurisdictions.
Efforts by the federal government to incentivize the reduction, capture, and reuse of methane—including the Obama Administration's Strategy—are summarized in the following two sections: " Legislative Initiatives " and " Administrative Initiatives ." Further, Table A-1 of Appendix A provides a detailed list of recent congressional proposals both in support of and in opposition to increased methane emission controls. Finally, Appendix B provides a selected chronology of recent executive branch initiatives.
subsections: section_title: Legislative Initiatives
paragraphs: Congress has pursued policies in support of methane emissions reduction since the 1970s. Legislation aimed at capturing methane emissions from agricultural activities and promoting the use of the recovered gas dates back, at least, to the 94 th Congress. Similar bills targeting emissions from coal mines and municipal landfills were introduced in the 96 th and 97 th Congresses, respectively. These efforts often promoted methane as an alternative fuel source, specifically as a replacement for imported crude oil. Legislation addressing methane's role as an air pollutant (e.g., as a GHG) reaches back to the 101 st Congress, wherein several bills were introduced with specific methane control provisions. These included one in 1989 by then-Senator Al Gore to analyze "the contribution of methane to global climate change, the sources and sinks of methane, and the methods of controlling emissions of methane. " A similar set of studies was codified by the Clean Air Act Amendments of 1990, which required EPA to report on the "activities, substances, processes, or combinations thereof that could reduce methane emissions and that are economically and technologically justified." Methane reduction was also included as a qualifying activity in market-based GHG control proposals as far back as the 101 st Congress.
Recent congressional interest continues to focus on methane's role as a GHG, with legislative efforts aimed at both supporting EPA's authority to regulate methane emissions and revoking it. Recent bills and amendments in the 114 th Congress have proposed several different policy tools as strategies for reduction. They include (1) providing economic incentives (e.g., tax benefits) for activities that capture and use fugitive gas (e.g., H.R. 2142 , the Capitalizing on American Methane Act of 2015), (2) authorizing the Administration or a specific agency to investigate or directly regulate methane emissions (e.g., H.R. 508 , the SUPER Act of 2015), and (3) providing a market-based mechanism (e.g., fee) to incentivize methane reduction (e.g., S. 1548 , the American Opportunity Carbon Fee Act of 2015).
Conversely, many bills in recent Congresses have aimed to remove the executive branch's authority to regulate methane emissions based predominantly on arguments for economic growth and employment. Some examples of these efforts include (1) amending the CAA to remove "methane" and other GHGs from the definition of "air pollutant" (e.g., H.R. 1806 , America COMPETES Reauthorization Act of 2015) and (2) prohibiting appropriated funds from being used by agencies to regulate methane (e.g., H.R. 2822 , Department of the Interior, Environment, and Related Agencies Appropriations Act, 2016). 
For a selected list of recent bills and amendments that address methane, see Table A-1 of Appendix A .
section_title: Administrative Initiatives
paragraphs: Historically, many of the methane control initiatives administered by the federal government have taken the form of either research funding or voluntary public-private partnerships with industry. Federal funding has been provided for the research and development of new technologies aimed at enabling more cost-effective emission reductions across various sectors of the economy. Offices that have provided financial and technical assistance in the past include the Department of Agriculture (USDA) Conservation Innovation Grants, Environmental Quality Incentive Program, Rural Energy for America Program, Bioenergy Program for Advanced Biofuels, and Biorefinery Assistance Program; the Department of Energy (DOE) Office of Fossil Fuels, Office of Energy Policy and Systems Analysis, and Section 1703 Loan Guarantee Program; the Department of Labor (DOL) Mine Safety and Health Administration; the Department of Transportation (DOT) Pipeline and Hazardous Materials Safety Administration; the EPA Office of Air and Radiation; and the Department of the Interior (DOI) BLM.
Similarly, voluntary partnerships managed by federal agencies have aimed to leverage the resources of the federal government to assist the private sector in overcoming the economic barriers to methane capture. They include the EPA's Natural Gas STAR Program and the Coalbed Methane Outreach Program for the energy sector, EPA/USDA's AgSTAR Program for the agricultural sector, EPA's Landfill Methane Outreach Program for the waste sector, and EPA's Global Methane Initiative for international activities. The goals of these programs are to (1) raise awareness of emission levels and the value of lost fuel, (2) provide information and training on new technologies and practices, and (3) discuss the barriers embedded in traditional operations, limited infrastructure, and uncertain investment climates. As with many voluntary initiatives, these programs have returned mixed results.
subsections: section_title: The Obama Administration's Strategy to Reduce Methane Emissions
paragraphs: On June 25, 2013, President Obama refocused his Administration's efforts to address GHG emissions with the release of the "Climate Action Plan" (CAP). Federal activities in support of methane emission reductions became one of the cornerstones of the CAP. During its presentation, the President stated that "curbing emissions of methane is critical to our overall effort to address global climate change." Many stakeholders have suggested that the Administration's recent GHG reduction targets, offered under the U.S. commitments to the United Nations Framework Convention on Climate Change, would be unattainable without significant methane controls. The CAP set guidelines for EPA and the Departments of Agriculture, Energy, the Interior, Labor, and Transportation to develop a comprehensive interagency methane strategy, which was released on March 28, 2014, under the title "Strategy to Reduce Methane Emissions." 
Key initiatives of the Strategy include the following:
1. Agriculture. A joint USDA, EPA, and DOE "Biogas Roadmap" outlining voluntary strategies to accelerate adoption of methane digesters and other cost-effective technologies to reduce U.S. dairy sector GHG emissions by 25% by 2020 (released on August 1, 2014).
2. Petroleum and Natural Gas.
EPA standards and guidelines to (1) build on the 2012 New Source Performance Standards (NSPS) for VOC emissions to address methane emissions from new and modified activities and equipment in the sector uncovered by the previous rule (published on June 3, 2016); (2) provide VOC reduction guidelines to state, local, and tribal air agencies for existing oil and gas sources in ozone nonattainment areas and states in the Ozone Transport Region (issued on October 20, 2016); and (3) expand voluntary efforts under the Natural Gas STAR program. BLM updated standards to reduce venting and flaring from oil and gas production on public lands (finalized on November 16, 2016). Several Department of Transportation Pipeline and Hazardous Materials Safety Administration (PHMSA) proposals for natural gas pipeline safety standards. DOE-convened roundtables, as part of the Quadrennial Energy Review, to identify "downstream" methane reduction opportunities (the summary of which was released on July 29, 2014).
3. Coal Mines. A BLM Advance Notice of Proposed Rulemaking (ANPRM) to gather public input on the development of a program for the capture and sale or disposal of waste mine methane on lands leased by the federal government (released on April 28, 2014).
4. Landfills. EPA standards to reduce methane from new and existing municipal solid waste landfills (published on August 29, 2016).
5. Improving Methane Measurement . Data quality improvement, including developing new measurement technologies, addressing areas of higher uncertainty in bottom-up inventories, and enhancing top-down modeling and monitoring based on direct measurement of atmospheric concentrations.
These initiatives are summarized in greater detail, by sector, in the remainder of this report. For a selected chronology of executive branch initiatives related to the White House's Strategy, see Appendix B .
section_title: Methane: A Primer
paragraphs: Methane is both a precursor to ground-level ozone formation and a potent GHG. As a precursor to ground-level ozone formation, methane reacts with nitrogen oxides in the presence of sunlight to form what is commonly referred to as smog. Methane, however, is generally less reactive than other hydrocarbons. For this reason—and at this time—EPA has excluded it from the definition of regulated hydrocarbons called volatile organic compounds (VOCs). 
As a GHG, methane emitted into the atmosphere absorbs terrestrial infrared radiation, which contributes to increased global warming and continuing climate change. According to the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report 2013 (AR5), in 2011, methane concentrations in the atmosphere exceeded preindustrial levels by 150%. Further, they contributed about 16% to global warming due to anthropogenic GHG sources, making methane the second-leading climate forcer after CO 2 globally. While the perturbation lifetime for methane is only 12 years (compared to CO 2 's, which is considerably longer and does not undergo a simple decline over a single predictable timescale), its immediate impacts are significantly greater (see Text Box ). For this reason, methane is commonly characterized as a "short-lived climate forcer," along with black carbon and various hydrofluorocarbons (HFCs).
subsections: section_title: Emissions
paragraphs: According to EPA, methane is the second-most prevalent GHG emitted by the United States (behind CO 2 ), and in 2014 it accounted for 730.8 million metric tons of CO 2 equivalent, or about 10.6% of all domestically produced emissions from human activities (see Figure 1 ). Some academic studies have put these emissions higher. Of the total, over 44% was emitted from sources in the energy production sector, over 30% from sources in the agricultural sector, and over 20% from sources in the waste management sector (see Figure 2 ).
section_title: Historical Trends
paragraphs: Between 1990 and 2014, methane emissions in the United States decreased by a little over 5%. However, trends have fluctuated over the past decade, with slight increases reported over the last few years. Since 1990, emissions from sources associated with agricultural activities have increased, while emissions from sources associated with waste management and energy and industrial processes have decreased (see Figure 2 ). Comparatively, the source categories for landfills and coal mining have seen the most notable reductions over the past 25 years (-18% and -30%, respectively) and manure management and petroleum systems the most notable increases (+65% and +76%, respectively), but many other subcategories have seen little or no change (see Figure 3 ).
section_title: Source Sectors and Mitigation Activities
paragraphs: Emissions of methane can be categorized into three broad source categories: agriculture, energy and industrial processes, and waste management. The following section reviews each of these categories, and
characterizes the major sources in the sector and the respective data on emissions, discusses current practices, opportunities, and challenges for emission control, summarizes current and applicable government programs and program performance data, and outlines the proposed initiatives in the White House's recent Strategy.
subsections: section_title: Agriculture Sector51
paragraphs: Agricultural sources of methane emissions include the following:
Enteric f ermentation. Methane is produced as part of normal digestive processes in animals, which is moreso an issue with ruminant livestock (e.g., cattle). Microbes that reside in the animal's digestive system ferment food consumed by the animal and produce methane as a byproduct, which can be eructated (i.e., belching or flatulence) by the animal. Manure m anagement. Methane is produced from manure management systems, primarily liquid and slurry systems. The treatment and storage of livestock manure can produce methane through its anaerobic decomposition. Rice c ultivation. Methane is produced from the anaerobic environment resulting from flooded fields used for rice cultivation. Decomposition of organic material gradually depletes most of the oxygen present in the soil, causing anaerobic soil conditions. Field b urning of a gricultural r esidues. Methane is emitted from the field burning of agricultural residues, which is done usually for disposal purposes. Field burning of agricultural residues occurs more frequently in some parts of the United States and is regulated or monitored depending on state and local law. Internationally, slash-and-burn agriculture is a common form of field burning in tropical and forested areas.
The agriculture sector constituted approximately 33% of U.S. anthropogenic methane emissions in 2014. From 1990 to 2014, methane emissions from agricultural sources increased by nearly 11% (see Figure 2 ). Enteric fermentation is the leading source of agricultural methane emissions and the second-leading source of methane emissions from all industry sectors. Livestock manure management is the second-leading agricultural source (see Figure 3 ).
While best practices exist to reduce methane emitted from enteric fermentation (e.g., diet modification), it has been economically and technically challenging to systematically capture a significant portion of the methane emitted at this stage. There are, however, opportunities to reduce methane emissions from other agricultural sources, and efforts have focused on the second-largest agricultural source, manure management.
Anaerobic digestion (AD) systems employed on stockpiles of manure at animal feeding operations may offer the most practical and economic method of capture. Operators have experience with AD systems partly because, for at least the last 20 years, USDA, DOE, and EPA have supported their use with financial and technical assistance (e.g., EPA/USDA's AgSTAR Program, established in 1994). There are, however, some economic, operational, and safety concerns associated with the use of AD systems.
The Obama Administration's Strategy takes a two-pronged approach to the reduction of agricultural methane from manure management. First, the Strategy supports a Biogas Roadmap—issued by USDA, EPA, and DOE on August 1, 2014—that outlines voluntary strategies to accelerate the adoption of AD systems and other technologies. The Biogas Roadmap is a deliverable of an April 2013 Memorandum of Understanding between USDA and the Innovation Center for U.S. Dairy. Second, the Strategy supports the continued use of previously established voluntary efforts (e.g., AD system deployment through assistance from numerous USDA programs).
If the primary goal of the Administration's Strategy is methane emission reduction, then the omission of the sector's major source of emissions—enteric fermentation—is noteworthy. With this omission, some may wonder how much impact methane reduction from the agricultural sector can have. However, if the primary goal is cost-effective methane emission reduction, addressing manure management may be the most viable option for the agriculture sector at the moment.
Although federal support for AD systems using voluntary measures is not new, it is difficult to calculate the full impact of past and continued federal support. It is not clear that an adequate emissions baseline has been established among the appropriate federal entities for AD systems that receive federal support. An emissions baseline could allow for long-term analysis, which is necessary to gauge future impacts (e.g., number of AD systems, number of AD systems that are fully operational, amount of financial assistance provided, amount of methane captured, amount of methane flared, amount of methane used to generate electricity). Federal program data about AD systems tend to be disparate. The Strategy may give the federal government an opportunity to improve methods to document the impact of AD systems. 
Beyond the availability and impact of adequate mitigation technologies for the agricultural sector, economic factors may also dampen the adoption of best practices. For these reasons, it could be argued that the establishment of a carbon market, the use of direct government payment programs for mitigating technologies such as anaerobic digestion systems, and the development of voluntary mitigation-related contracts could help alleviate costs and incentivize innovation. On the other hand, expansion of mitigation technologies such as anaerobic digestion systems may face challenges larger than economics, such as national infrastructure and cooperation with utilities or other industries that can use—but do not necessarily agree that they need—the product being sold.
section_title: Fossil Energy Sector59
paragraphs: Fossil energy sources of methane emissions include the following:
Petroleum s ystems . Methane emissions from petroleum systems are primarily associated with crude oil production, transportation, and refining operations. During each of these activities, methane is released to the atmosphere as fugitive emissions, vented emissions, emissions from operational accidents, and emissions from incomplete fuel combustion. Natural g as s ystems . The U.S. natural gas system encompasses hundreds of thousands of wells, hundreds of processing and liquefaction facilities, and over 1 million miles of transmission and distribution pipelines. Emissions of methane (i.e., the principal component of natural gas) arise from vented and fugitive emissions from system components, natural gas engine and turbine uncombusted exhaust, bleed and discharge emissions from pneumatic devices, and emissions from operational accidents. Coal m ining . Three types of coal-mining-related activities release methane to the atmosphere: underground mining, surface mining, and post-mining (i.e., coal-handling) activities. While surface mines account for the majority of U.S. coal production, underground coal mines contribute the largest share of methane emissions due to the higher methane concentrations in deeper coal seams.
The fossil energy sector constituted nearly 45% of U.S. anthropogenic methane emissions in 2014. From 1990 through 2014, methane emissions from fossil energy sources have decreased by approximately 10% (see Figure 2 ). Natural gas systems are the leading source of emissions from the sector, and they have historically vied with enteric fermentation as the leading man-made source of methane emissions in the United States (see Figure 3 ). Methane emissions from coal mining have fluctuated over the past two decades—but for the most part declined—and currently account for approximately 10% of made-made emissions in the United States.
Taken together, the petroleum and natural gas industry (as shown in Figure 4 ) is one of the largest sources of methane emissions in the country, contributing in excess of 33% of U.S. anthropogenic methane emissions in 2014. In the 2016 Inventory, EPA revisited its calculations for the oil and gas industry and reported that methane emitted by the sector had increased by 9% since 2010 and had remained virtually unchanged since 1990. (Conversely, the 2015 Inventory reported that emissions from the sector generally declined by 12% since 1990.)
Sources of emissions in the oil and gas sector include the following:
Upstream p roduction . Methane may be emitted while drilling through gas-bearing geologic formations, during drilling mud circulation, during well development (following well stimulation by hydraulic fracturing) when formation fluids and fracture fluids flow back to the surface, and from field treatment equipment that separates oil, gas, and water. Midstream p rocessing and t ransmission . Gathering lines connecting the wellhead to field treatment equipment that separates gas, oil, and water into product streams represent another source for fugitive methane and gas condensate emissions. Leaking valves, transmission lines, and pump stations add to this sector's emissions. Downstream d istribution . Emissions from leaking distribution pipelines are most likely to occur from older pipelines. In 2014, there were more than 1.2 million miles of distribution mains in the United States. Of these, more than 32,000 miles of mains were older cast iron or wrought iron, and more than 61,000 miles were unprotected steel.
Some companies in the oil and gas industries have made significant voluntary reductions in methane emissions over the past decade. By volume, some of the largest reductions have come from using reduced emissions completions (or "green completions") during hydraulic fracturing activities, leak detection and repair technologies at processing facilities and compressor stations, reduced venting of associated gas at oil wells, and the replacement of high‐emitting pneumatic devices. A number of these technologies and practices have been promoted and supported by EPA's public-private partnerships with industry, including the Natural Gas STAR Program and the Coalbed Methane Outreach Program. However, voluntary adoption of control techniques has been uneven across companies and regions. Consequently, in 2012, EPA promulgated emission standards for conventional pollutants (e.g., volatile organic compounds) for the oil and gas sector through a series of New Source Performance Standards (NSPS) and National Emissions Standards for Hazardous Air Pollutants. These standards have the co-benefit of reducing methane emissions from certain new sources in some segments of the gas industry. Further, some states have established or proposed regulations that specifically address methane emissions from the oil and gas industry (e.g., Colorado, California, Ohio, Wyoming, and Pennsylvania, as well as a Western Governors' Association policy resolution). Notwithstanding, many sources have remained uncontrolled by state or federal standards.
Additional to EPA's standards, BLM has issued rulemakings that indirectly address methane emissions on federal lands under the Mineral Leasing Act (MLA). The MLA authorizes the Secretary of the Interior to lease onshore lands owned by the United States that contain fossil fuel deposits, with the federal government retaining title to the lands. The framework of the MLA provides BLM and the federal government with flexibility to use federal lands to help satisfy the nation's energy needs while generating revenue for the federal government and protecting environmentally sensitive areas. Existing BLM rulemakings affecting methane emissions include BLM's 1980 "Notice to Lessees and Operators of Onshore Federal and Indian Oil and Gas Leases (NTL-4A): Royalty or Compensation for Oil and Gas Loss," which outlines appropriate payment terms for losses of natural resources under the authority of the MLA. The notice lists circumstances wherein operators are authorized to vent or flare methane without incurring royalty obligations.
The Obama Administration's Strategy targets methane control in the fossil energy sector through a number of agencies. Since its release, the Administration has proposed and promulgated a number of rulemakings, including
An EPA rule to build on the 2012 NSPS "to set standards for methane and VOC emissions from new and modified oil and gas production sources, and natural gas processing and transmission sources" (published on June 3, 2016). The new standards set first-ever controls for methane emissions and extend controls for VOC emissions beyond the existing requirements to include new or modified hydraulically fractured oil wells, pneumatic pumps, compressor stations, and leak detection and repair at well sites, gathering and boosting stations, and processing plants. EPA estimates that the standards for new and modified sources are expected to reduce 510,000 short tons of methane in 2025—the equivalent of reducing 11 million metric tons of carbon dioxide—and yield net climate benefits of $170 million in 2025. The final rule also includes the issuance for public comment of an Information Collection Request (ICR) that would require companies to provide extensive information that would be instrumental for developing comprehensive regulations to reduce methane emissions from existing oil and gas sources. The final version of the ICR was released on November 10, 2016. EPA guidelines for VOC reduction requirements on existing oil and gas sources in ozone nonattainment areas and states in the Ozone Transport Region (published on October 20, 2016). These requirements would be in the form of Control Techniques Guidelines and would be similar to the proposed NSPS. However, Control Techniques Guidelines do not apply any requirements directly to facilities; rather, they provide recommendations for state and local air agencies to consider in determining reasonably available control technology for reducing emissions from covered processes and equipment. States may use different technology and approaches, subject to EPA approval, provided they achieve the same level of emissions reductions as would be achieved under the guidelines. An EPA rule to clarify the definitions for "major source" categories in the oil and natural gas sector for the purpose of permitting (published on June 3, 2016). An EPA initiative to expand voluntary efforts under the Natural Gas STAR program and provide several new mechanisms through which oil and gas companies could make and track commitments to reduce methane emissions (launched on March 30, 2016, with 41 founding partners). An EPA rule to strengthen its Greenhouse Gas Reporting Program to require reporting in all segments of the industry (published on October 22, 2015). A BLM rule to update standards to reduce venting and flaring from oil and gas production on federal lands (finalized on November 16, 2016) and an ANPRM to develop a program for the capture and sale or disposal of waste mine methane on lands leased by the federal government (released on April 28, 2014). Several Department of Transportation PHMSA proposals for natural gas pipeline safety standards. The President's FY2016 budget request for $15 million in funding for DOE to develop and demonstrate more cost-effective technologies to detect and reduce losses from natural gas transmission and distribution systems and $10 million in funding to launch a program to enhance the quantification of emissions from natural gas infrastructure. DOE proposals to issue energy efficiency standards for natural gas and air compressors, advance research and development to bring down the cost of detecting leaks, work with Federal Energy Regulatory Commission to modernize natural gas infrastructure, and partner with local distribution companies to accelerate pipeline repair and replacement at the local level. DOE's Quadrennial Energy Review, which includes "additional policy recommendations and analysis on the environmental, safety, and economic benefits of investments that reduce natural gas system leakage."
The Administration states that these proposals are key components under the CAP to put the United States on track to reduce methane emissions from the oil and gas sector by 40%-45% from 2012 levels by 2025. The oil and natural gas industry has argued against the need for additional standards, contending that they are unnecessary (due to the historical decline in the sector's emissions), duplicative (of many state requirements), and a burden (as many domestic producers are already doing everything feasible to capture and reuse methane emissions for requisite safety and economic reasons). Health and environmental advocates welcomed the proposed and promulgated rules but suggested that the NSPS does not go far enough in addressing existing sources of emissions. 
With the issuance of the ICR, the possibility remains open for EPA to propose performance standards on methane emissions for existing sources in the future. That is, for certain pollutants, promulgation of NSPS under Section 111(b) triggers a mandatory EPA duty under CAA Section 111(d) to address existing sources in the same source category. At present, however, there is a looming legal question as to precisely what those "certain pollutants" are. This question of which pollutants trigger the Section 111(d) program for existing sources is likely to be front and center in the litigation over EPA's Clean Power Plan. The answer provided by the courts could affect EPA's ability to move forward with regulation of existing sources of methane from the oil and natural gas sector.
section_title: Waste Management Sector84
paragraphs: Waste management sources of methane emissions include the following:
Landfills. Landfill gas—a mixture of roughly 50% methane, 50% CO 2 , and small amounts of other gases—is released into the atmosphere if not captured. The amount of gas produced at any given landfill depends on the amount of organic material in the waste, the landfill's design, the climate at the site of the landfill, and the operating practices used by the site's operator. In general, large amounts of organic waste and high levels of moisture in a landfill lead to greater gas production. Wastewater t reatment. Wastewater from domestic and industrial sources is commonly treated to remove soluble organic matter and other contaminants. Soluble organic matter may be removed using biological processes in which microorganisms consume the organic matter for maintenance and growth. On occasion, these processes may be accidentally or deliberately managed under anaerobic conditions, producing methane. Composting. Composting of organic waste—such as food waste, garden (yard) and park waste, and sludge—is a common practice in the United States. Methane is formed in anaerobic sections of the compost, but its impacts are generally mitigated due to oxygenation in the aerobic sections of the compost.
Waste management and treatment activities constituted approximately 23% of U.S. anthropogenic methane emissions in 2014. Landfills accounted for just over 20% of this total, the third-largest contribution of any methane source in the United States. Landfills emitted 148.0 million metric tons of CO 2 equivalents, or 2.2% of total U.S. GHG emissions (see Figure 1 ). Although substantial, estimated methane emissions from landfills have declined 18.5% in recent years from a high of 181.7 million metric tons (MMT) in 1991 (see Figure 3 ). Additionally, wastewater treatment and composting of organic waste accounted for 2.0% and less than 0.3% of U.S. methane emissions, respectively. 
Currently, landfill gas is captured at the nation's largest landfills. A common landfill gas capture system consists of an arrangement of vertical wells and horizontal collectors usually installed after a landfill cell has been capped. A 1996 CAA regulation known as the "Landfill Gas Rule" established NSPS and guidelines that require landfills with a 2.5 MMT design capacity that accepted waste after November 8, 1987, to capture and burn the gas. The gas can be either flared or used for energy production—it is often used as fuel for electricity generation. In promulgating the 1996 rule, EPA said that the 2.5 MMT minimum "corresponds to cities greater than 100,000 people." The agency also stated that the regulations "will only affect less than 5 percent of all landfills" but would reduce emissions of methane by 37% at new landfills and by 39% at existing facilities. Partly as a result of the 1996 regulation, and partly due to tax incentives and voluntary programs, there were 648 operational methane capture projects at landfills as of March 2016. This represents roughly one-third of the 1,800 to 1,900 municipal solid waste landfills reported in operation by EPA.
Even under ideal conditions, the capture of landfill gas is a technical challenge in an operationally dynamic environment. Whatever success existing regulations, tax incentives, and voluntary programs may be having, a significant amount of methane continues to be emitted even at landfills subject to the Landfill Gas Rule. In addition, there are few methane capture projects at smaller landfills and at landfills that ceased operation before November 1987 (those not covered under the CAA). The latter group, numbering in the tens of thousands of sites, poses a particular challenge. But EPA notes that production of landfill gas diminishes over time, especially after a landfill is closed, and there is often no responsible party who might implement a methane collection system if the site's original owner is no longer in business.
In response to the Administration's "Strategy to Reduce Methane Emissions," EPA has reviewed the 1996 Landfill Gas Rule and Guideline. On August 29, 2016, the agency published revisions to the NSPS for new and modified landfills and to the Emission Guidelines for existing landfills. The revised NSPS makes no change in the universe of new or modified landfills subject to its requirements: The threshold remains at 2.5 MMT of design capacity (or 2.5 million cubic meters of waste). But it does change the emission threshold at which landfills will be required to begin capturing landfill gases. Under the rule, EPA will require that a gas collection control system be installed and operational within 30 months after landfill gas emissions reach 34 metric tons of nonmethane organic compounds (NMOC) or more per year. Under the 1996 NSPS, this threshold was 50 metric tons per year.
EPA expects relatively few landfills to be affected by the revised NSPS, because few new and modified landfills will be constructed. According to an agency fact sheet, 115 new, modified, or reconstructed landfills will be subject to the emission control requirements of the revised standards by 2025. Compared to the requirements of the 1996 standards, the revised standards would reduce annual methane emissions by an additional 44,000 metric tons beginning in 2025. This would be the equivalent of reducing carbon dioxide emissions by 1.1 MMT per year, less than 1% of current estimated landfill emissions. 
The revised guidelines for existing landfills will require the installation of landfill gas collection and control systems at active landfills that emit more than 34 metric tons of NMOC annually, as well. The annual threshold has been 50 metric tons under the 1996 guidelines. Closed landfills will remain subject to the 1996 threshold of 50 metric tons per year, however. 
As a result of the lower threshold for active landfills, EPA estimates that the emission control requirements will apply at 731 existing open and closed landfills, as compared to 638 facilities currently subject to emission control requirements. Methane emissions are expected to be reduced by 290,000 metric tons annually beginning in 2025 at these existing facilities, compared to the 1996 guideline requirements (the equivalent of reducing 7.1 MMT of CO 2 ). This would be a 5% reduction from the current level of landfill methane emissions.
section_title: Issues in Measurement93
paragraphs: Unlike CO 2 , whose emissions are reported using well-tracked energy statistics, methane is emitted to the atmosphere primarily through fugitive releases of the gas (e.g., leaks in infrastructure, vapors from landfills, eructation [i.e., belching or flatulence] from livestock). By definition, fugitive emissions are diffuse, transitory, and elusive. Thus, one of the greater difficulties in understanding the impacts of methane emissions is acquiring comprehensive and consistent observational data. Broadly, there are two approaches to measuring fugitive emissions of methane: "bottom-up" and "top-down." Each approach has its respective strengths, weaknesses, and uncertainties. At present, the difference in data acquisition and analysis between these two approaches has returned competing—and occasionally conflicting—emission estimates.
Bottom- u p a pproaches . Bottom-up methodologies begin by directly measuring the emissions from a number of randomly selected pieces of equipment or activities to determine an average "emission factor" for each type. Emissions for the entire industry are then estimated by multiplying these emission factors by the activity levels for each component (e.g., the total population of livestock and its diet, the number of oil and gas wellheads and other components, or the volume of landfill material). Thus, while the inventory is supported by initial direct measurements, the final results are statistical averages derived through computation and may not reflect actual emissions in the field. Because the quality of methane data for some sources can be either absent or highly variable, bottom-up emission estimates entail considerable uncertainty. Top- d own a pproaches . Other studies use "top-down" methodologies for the calculation of leakage (e.g., satellite observations, ambient atmospheric measurements, and geostatistical inverse modeling). Atmospheric studies use data sets of ambient concentrations of methane and related hydrocarbons in the vicinity of the targeted industry, along with the known emission profiles for these gases from industry operations, to infer the emissions from the sectors. (That is, these methodologies capture methane emissions from all natural, agricultural, and industrial activities. Researchers must then parse data estimates for attribution to their appropriate sources using such analyses as isotopic ratios or prevalence signatures from accompanying nonmethane hydrocarbons.) Due to the technology requirements, these studies are rarer than bottom-up approaches. As with the bottom-up approaches, different top-down studies have returned different emission estimates. Further, reported emission rates have varied considerably across different regions, making source attribution highly uncertain at the national level.
In general, top-down methodologies have returned higher emission estimates than bottom-up approaches. Reasons for this discrepancy include (1) researchers may be attributing naturally occurring methane emissions to man-made sources; (2) researchers may be attributing emissions inaccurately from one man-made sector to another; (3) atmospheric measurements may capture emissions that are not accounted for in EPA's Inventory (e.g., leakage from abandoned gas wells); (4) atmospheric measurements capture all the gross emitters, accidents, spills, and human errors, whereas component measurements use emission factors averaged over instances of "normal operation"; and (5) atmospheric studies may be biased to regions where there is known leakage.
Currently, the primary source of information on methane emissions in the United States is EPA's annually published Inventory of U.S. Greenhouse Gas Emissions and Sinks . EPA's Inventory is a "bottom-up" approach, employing commonly accepted emission factors and activity levels to calculate aggregate estimates for all source categories. Methodologies for the Inventory are based primarily on 2006 guidelines released by the IPCC and supplemented with additional domestic information where available. Bottom-up methodologies are used also for EPA's Greenhouse Gas Reporting Program, as well as the Energy Information Administration's Natural Gas Annual . Further, there are many examples of state, local, and nongovernmental inventories commissioned by a range of stakeholders—from regional and municipal agencies to community groups and academic institutions.
Due to the differences in emission factors, industry reporting, and levels of uncertainty, current inventories have returned a variety of emission estimates. These differences have also contributed to periodic revisions to EPA's Inventory, and these revisions have returned occasionally significant fluctuations in reporting. (For example, emission estimates in the Inventory for natural gas systems have fluctuated between 96.4 MMTCO 2 e and 221.2 MMTCO 2 e over the past several years due primarily to changes in reporting methodology.) These periodic revisions have made it challenging to analyze trends in emissions. Furthermore, EPA's Inventory has been challenged by a number of academic studies as both over- and under-reporting methane releases from man-made sources. (As examples, a 2015 study by Yale researchers concludes that methane emissions from U.S. landfills may be double EPA's estimates; a 2014 study by federal and academic researchers suggests that methane emissions from gas-producing areas in Colorado are as much as three times higher than EPA inventories; a 2013 paper published by Harvard University researchers and federal scientists reports that EPA's oil and gas figures may be underestimated in some cases by as much as 50%; a February 2014 study by Stanford University researchers estimates that methane leakage from natural gas lines and other sources could be 50% higher than current EPA estimates; and, conversely, an April 2016 paper by New Zealand researchers reports that increases in global methane emissions since 2006 are predominantly biogenic, as fossil fuel emissions were seen to have stagnated or diminished.)
The White House Strategy proposes actions to enhance U.S. methane measurement in support of two broad goals: (1) improving the bottom-up emission data relevant for mitigation, and (2) advancing the science and technology for monitoring and validating atmospheric concentrations. Actions in the Strategy include efforts to (1) enhance EPA's Inventory through new scientific evidence and data sources, (2) encourage the development of cost-effective measurement technologies through funding at DOE's Advanced Research Projects Agency—Energy, (3) maintain and further develop a nationwide methane monitoring network through funding at the National Oceanic and Atmospheric Administration, and (4) improve local, regional, and global emission modeling at EPA and DOE. EPA is already in the process of outlining a comprehensive strategy for significantly improving its methodology for estimating emissions from the oil and natural gas sector. This effort is in response to recommendations made by an EPA inspector general report. Moving forward, the Strategy will need to find a way to harmonize the differences in reporting between the bottom-up and top-down studies, dampen the artificial annual fluctuations in reported estimates, and provide more transparent and unbiased source data in order to guarantee credibility in EPA's Inventory for all stakeholders and fairness in any subsequent rulemaking.
section_title: Cost-Benefit Analysis
paragraphs: Efforts to protect the environment and conserve natural resources are generally assessed by how much they benefit human health and welfare. Often, these benefits are weighed against the economic costs incurred by the affected industries in an effort to determine the "net costs" or "net benefits" of a given action. Such an accounting is commonly referred to as cost-benefit analysis (CBA). One challenge in conducting CBA is that costs and benefits may not be quantified and translated comprehensively into monetary terms. Further, even if the costs and benefits can be accurately estimated, criticisms arise from the fact that benefits may accrue to individuals or groups other than those paying the up-front costs. 
Both EPA and BLM considered the costs and the benefits of their respective rulemakings stemming from the Administration's Methane Strategy. These considerations were conducted as required by statute (e.g., CAA, MLA ) and by orders and guidance from the Administration (e.g., Executive Order 12866: Regulatory Planning and Review, Executive Order 13563: Improving Regulation and Regulatory Review, and Circular A-4 from the Office of Management and Budget). Neither the CAA nor the MLA requires that the regulatory agency set the level of control based on CBA. However, developing and considering CBA, as well as producing a Regulatory Impact Analysis and other administrative addenda, is required by executive orders. Specifically, the executive orders encourage agencies to "propose or adopt a regulation only upon a reasoned determination that the benefits of the intended regulation justify its costs." As such, to the extent allowed by law, the agencies have generally sought options that yield only net benefits (i.e., that are worth more to society than they cost).
In their respective analyses, the agencies calculated regulatory compliance costs for the affected industry to include initial capital costs and annualized engineering costs. These calculations incorporated estimates for new technology investment, increased monitoring and reporting requirements, and the adoption of additional management or workplace practices. Costs were then adjusted for the estimated revenues generated from the recovered natural gas and other products that would otherwise have been vented or flared. 
The agencies calculated regulatory benefits in both monetized and non-monetized terms. Monetized benefits included those from reductions in methane emissions, which were valued using the social cost of methane (SC-CH 4 ). Non-monetized benefits included estimates for improvements in ambient air quality and reductions in negative health effects associated with exposure to hazardous air pollutants, ozone, and particulate matter, which the agencies determined could not be adequately monetized with the data currently available. In addition to these health improvements, non-monetized benefits included improvements in visibility, ecosystem effects, and additional natural gas recovery. 
EPA reported the net monetized benefits of the final oil and gas NSPS to be $180 million in 2025 and the net monetized benefits of the final landfill NSPS and emissions guidelines to be $452 million in 2025. BLM reported the net monetized benefits of the proposed venting and flaring rule to be $43 million to $48 million in 2025. The agencies stated that these estimates were conservative (i.e., that assumptions and errors were more likely to minimize benefits than maximize them). However, some stakeholders have called into question several of the agencies' assumptions, including the choice of discount rate, the projected volume of and prices for recovered products, the value and validity of the SC-CH 4 metric, and the failure to monetize reductions in other pollutants. 
section_title: Conclusion
paragraphs: For a variety of economic, environmental, and public health and safety reasons, various stakeholders have sought policies to reduce, capture, and reuse methane emissions for the past several decades. But emissions of methane have proven difficult to measure and hard to control. Their naturally occurring presence in the environment, their wide and varied sources of emissions, and the fugitive nature of their release have contributed to these difficulties. Nevertheless, methane is a valuable resource. Its dual nature as both pollutant and commodity has offered a unique opportunity for control, and many strategies have attempted to capitalize on the economics of recovery. Whether a given control strategy is effective and cost-efficient for a given industry depends upon a number of factors, including (1) the nature and extent of the emissions, (2) the technology available for capture, and (3) the market price for the recovered products. (For example, with declining natural gas prices, the economics of capture technology are less favorable.) Some significant efforts have been made by industry and some state regulators to address methane emissions in their particular localities. For its part, the federal government has contributed funding for research and technology development, voluntary guidelines and tax incentives for industry, rules for mineral rights lessees on federal lands, and, on occasion, air pollution standards.
While the most current data on domestic methane emissions show a 5% decrease over the past two decades, the source categories that have contributed to these reductions are few (i.e., landfills, coal mines, and natural gas systems). While these industries have made noteworthy strides in emission reductions through a combination of best management practices and the co-benefits provided by other air pollution standards, they may represent only the "low-hanging fruit." Other sources of methane emissions have confronted greater challenges. They may lack adequately demonstrated control technologies or cost-effective opportunities. They may not co-emit methane with other air pollutants and thus may lack the "co-benefits" accrued through other air quality standards. Some of these sources have seen recent or sustained increases in emissions (e.g., petroleum systems and manure management, respectively). Other sources (e.g., enteric fermentation and wastewater treatment) have gone unaddressed for decades, as no economically viable technology solution has been offered.
The Obama Administration's recent Strategy—as well as a variety of recent proposals in Congress—attests to the continued interest in better emission assessments and appropriate policy responses. In considering strategies moving forward, it may be useful to ask the following questions:
1. Is the current set of methodologies used for measurement adequate to rationalize and/or prioritize the appropriate controls? 2. Is the projected rise in domestic fossil fuel production and petrochemical manufacturing significant enough to rationalize and/or prioritize additional controls? 3. Is the recent rate of increase in observed emissions expected to continue; or, if not, are the long-run decreases sufficient enough to discharge the public health, safety, and environmental concerns? 4. To what extent might fluctuations in the market price of crude oil and natural gas affect industry and regulatory efforts to mitigate and/or recover methane emissions? Will changes in the natural gas industry result in fewer small companies and a greater market share for large companies, which may already have control practices in place? 5. To what extent may recently promulgated and proposed rulemaking for air pollutants commonly co-emitted with methane also serve the co-benefit of reducing methane emissions (e.g., the NSPS for VOCs on the oil and gas production sector and the petroleum refinery sector, and the revised National Ambient Air Quality Standard for ozone)? 6. To what extent may the recently promulgated and proposed rulemakings for methane emissions in the oil and natural gas sector and the landfill sector serve to (1) curb emissions from both new and existing sources in each sector and (2) curb the aggregate of all domestic emissions? 7. If further reductions are under consideration for a given source category, should the response come from the federal government, state governments, the industries, or the market? 8. If further reductions are under consideration for a given source category, which policy tool(s) would be most appropriate: (1) increased funding for technology research, (2) expanded public-private demonstration projects with industry, (3) regionally targeted or state-sponsored guidance or rulemaking, (4) methane-specific state or federal command-and-control air pollution standards, or (5) economy-wide market-based mechanisms for either ozone or GHG controls? 9. How should the burden of GHG reductions be distributed among the various GHG emissions sources? 10. How should methane's other environmental benefits (in comparison to oil and coal combustion) be weighed in the context of its GHG control?
Appendix A. Recent Legislative Proposals
Appendix B. Recent Executive Branch Initiatives