This article will endeavor to explain the regulation of greenhouse gases under the Clean Air Act, focusing on and explaining the provisions of the Clean Air Act relied on by the Environmental Protection Agency to justify or support that regulation. The article will examine the CAA’s applicable definitions and provisions, the Supreme Court’s interpretation of those provisions in the seminal case of Massachusetts v. EPA, the various findings and rules published in the wake of Massachusetts v. EPA, the recent case law interpreting the validity of these findings and rules, the major rules regulating greenhouse gases which have been issued or proposed by the Environmental Protection Agency, and the likely future of greenhouse gases regulation.
The Clean Air Act (CAA) provides broad authority for the Environmental Protection Agency (EPA) to impose regulations designed to reduce emissions of air pollutants which endanger human health and the environment. For decades, EPA focused on and addressed a known set of air pollutants, called criteria pollutants and hazardous air pollutants, emitted from an established set of sources.
The gradual realization that greenhouse gases, such as carbon dioxide, methane, nitrous oxide, and certain fluorinated gases (sometimes GHG), play an important role in climate change, and thus our health and welfare, caused many to turn to the CAA seeking to invoke its provisions in the effort to reduce emissions of these gases and thus limit the potential adverse effects of climate change. The CAA, though, does not readily lend itself to the direct regulation of greenhouse gases. Instead, a series of findings must be made and subsequent steps taken in order to encompass greenhouse gases under the regulatory umbrella afforded by the CAA.
Since 2007, EPA has utilized a variety of provisions in the CAA to incorporate greenhouse gases within its ambit. After the Supreme Court found that the broad definition of ‘air pollutant’ clearly included greenhouse gases, EPA was required to decide, or make a ‘judgment,’ whether greenhouse gases cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare. EPA’s judgment that it did so contribute mandated, under the explicit provisions of the CAA, the issuance of standards for motor vehicles under Title II of the CAA. EPA’s long-standing interpretation of the CAA that regulation of an air pollutant under Title II also mandated regulation of that air pollutant under other titles of the CAA, in turn, led to the inclusion of greenhouse gases within the traditional permitting programs previously used to reduce and regulate emissions of criteria pollutants and hazardous air pollutants from stationary sources. Other CAA provisions, such as those mandating standards of performance and reporting, have been relied on by EPA to regulate greenhouse gases.
EPA’s actions have not been met with universal acceptance. The ‘judgment’ made by EPA and the inclusion of greenhouse gases under other titles spawned litigation that resulted in a second sweeping decision by the Supreme Court on the subject. Judicial review of proposed rules regulating greenhouse gases from power plants will likely be sought.
A Brief Overview of The Clean Air Act
The modern Clean Air Act (CAA) has its origins in the enactment of the Clean Air Amendments of 1970.  Major amendments in 1977 and 1990 substantially rewrote the CAA.  Currently codified as 42 USC §§7401 – 7671q, the CAA is a comprehensive statute the purpose of which is to “protect and enhance the quality of the Nation’s air resources so as to promote the public health and welfare and the productive capacity of its population.” 
In very general terms, the CAA is centered on the regulation and minimization of ‘air pollutants’ from mobile and stationary sources. The term ‘air pollutant’ is found throughout the CAA and in its most important provisions. It is very broadly defined: “any air pollution agent or combination of such agents, including any physical, chemical, biological, radioactive (including source material, special nuclear material, and byproduct material) substance or matter which is emitted into or otherwise enters the ambient air.”  This broad definition applies to the entirety of the CAA. 
To accomplish its purpose of protecting public health and welfare, the CAA is divided into several titles. For the purposes of this article, the relevant titles are: Title I, Programs and Activities, contains provisions regarding National Ambient Air Quality Standards, state implementation plans, hazardous air pollutants, Prevention of Significant Deterioration (PSD), and nonattainment areas. Title II, Emission Standards for Moving Sources, contains provision s regarding new motor vehicle emission and fuel standards. Title V, Permits, contains provisions regarding operating permits. 
A. Title I – Programs and Activities
The CAA requires that EPA establish primary and secondary National Ambient Air Quality Standards (NAAQS) for “each air pollutant for which air quality criteria have been issued.”  To date, NAAQS have been established for six air pollutants: ozone (regulated through control of volatile organic compounds and oxides of nitrogen), particulate matter, carbon monoxide, nitrogen dioxide, sulfur dioxide, and lead.  These six air pollutants have become known as the ‘criteria pollutants.’
For each such pollutant, EPA established a primary standard “the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect” the public health.”  Additionally, EPA established secondary standards “the attainment and maintenance of which in the judgment of the Administrator, based on such criteria, is requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of such air pollutant in the ambient air.” 
EPA is required to create a “list of categories of stationary sources” which “cause, or contribute significantly to, air pollution which may reasonably be anticipated to endanger public health or welfare.”  A ‘stationary source’ is a facility “which emits or may emit any air pollutant.”  For each category of stationary sources, EPA must establish “standards of performance for new sources within such category.”  Standards of performance may be prescribed for existing sources as well. The long list of these Standards of Performance for New Stationary Sources may be found in 40 CFR Part 60. After a standard of performance is promulgated, it is unlawful for any source to operate in violation of that standard. 
The CAA also requires regulation of ‘hazardous air pollutants,’ which are a specific list of air pollutants, such as benzene, which Congress delineated, by name, as hazardous.  Similar to the ‘standards of performance,’ EPA is required to publish regulations establishing emission standards for each category of major sources and area sources (which are non-major sources). 
Each state is required to prepare a “plan which provides for implementation, maintenance, and enforcement of” the primary and secondary NAAQS within each state. These State Implementation Plans, or SIPs as they are called, must include a procedure for “implementing and enforcing standards of performance” published by the EPA.  Additionally, they must include, among many other things, enforceable emission limitations, necessary control measures, and monitoring of ambient air quality. 
Based on the level of the air pollutants measured in the ambient air, an area may be designated as either ‘attainment’ or ‘non-attainment.’ If the amount of the measured pollutant is below the NAAQS for that pollutant, the area is in attainment. If the amount of the measured pollutant is above the NAAQS for that pollutant, the area is in non-attainment. This designation is important as it determines the regulatory program applicable in each area.
In attainment areas, the Prevention of Significant Deterioration (PSD) Program applies. Enacted in 1977, it is designed to protect air quality in an attainment area so that the air quality remains below the NAAQS. Three key concepts relevant here assist in achieving this goal.
The PSD Program applies to major emitting facilities, as that term is defined. A major emitting facility is: 1) a source included in a list of 28 sources that emits or has the potential to emit 100 Tons Per Year (TPY) or more of any air pollutant; or 2) any other source with the potential to emit 250 TPY or more any air pollutant. These emission limits are referred to as the 100/250 TPY Threshold. The PSD Program also applies to major modifications at major sources. 
The PSD Program is a pre-construction program.  This means that the appropriate permit must be in place prior to the beginning of construction for a new major facility or a modification of a major facility.
Finally, the major emitting facility may not be constructed unless “the proposed facility is subject to the best available control technology for each pollutant subject to regulation under this chapter emitted from, or which results from, such facility.”  BACT, as it is known, “means an emission limitation based on the maximum degree of reduction of each pollutant subject to regulation under this chapter emitted from or which results from any major emitting facility, which the permitting authority, on a case-by-case basis, taking into account energy, environmental, and economic impacts and other costs, determines is achievable for such facility.” 
B. Title V – Permits
Title V is an operating permit program enacted in 1990 whose key goal was to “gather into one permitting mechanism the CAA requirements applicable to a source and impose conditions necessary to assure compliance with such requirements.”  It applies to major sources, which for these purposes includes sources which emit or has the potential to emit 100 TPY or more of any air pollutant.  Each major source is required to have a comprehensive ‘Title V’ permit.
C. Title II – Emission Standards for Moving Sources
Title II applies to mobile sources. Pursuant to Title II, EPA is generally authorized to regulate emissions from vehicles and the content of fuels. As it relates to greenhouse gases, however, it requires that the EPA “shall by regulation prescribe (and from time to time revise) in accordance with the provisions of this section, standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines, which in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” 
Massachusetts v. EPA 
Thus, under the CAA, EPA has traditionally regulated the emissions of criteria pollutants and hazardous air pollutants through the NAAQS, the standards of performance for criteria pollutants, emission standards for hazardous air pollutants, and the implementation of these requirements by the states as set forth in their SIPs. Against this regulatory backdrop spanning decades, the importance of, and threat from, greenhouse gases arose.
In October, 1999, a group of private organizations petitioned EPA requesting that EPA regulate “greenhouse gases from new motor vehicles under §202 of the Clean Air Act.”  As support for the petition, the organizations noted that 1998 was the warmest year on record, that the United Nations’ Intergovernmental Panel on Climate Change (IPCC) had warned that carbon dioxide was the most important contributor to climate change, and that EPA had already concluded that it had the statutory power under the CAA to regulate carbon dioxide.  Two of EPA’s General Counsel had previously opined, in an internal legal memorandum and in testimony before Congress, that the CAA provided EPA the authority to regulate carbon dioxide but that EPA had, to date, declined to do so. 
Despite its own counsels’ opinions, EPA decided, in September, 2003, to deny the petition, concluding that “it cannot and should not regulate GHG emissions from U.S. motor vehicles under the CAA.”  EPA concluded that it did not have the statutory authority to regulate GHG and that, even if it did, regulation was not appropriate.
EPA stated that it “believes that the CAA does not authorize regulation to address global climate change” based on “a thorough review of the CAA, its legislative history, other congressional action and Supreme Court precedent.”  However, even if it could regulate GHG, “Congress has not authorized the Agency to regulate CO2 emissions from motor vehicles to the extent such standards would effectively regulate car and light truck fuel economy, which is governed by a comprehensive statute administered by DOT.”  Further, EPA also concluded that “setting GHG emission standards for motor vehicles is not appropriate at this time” based on “EPA’s lack of CAA regulatory authority to address global climate change, DOT’s authority to regulate fuel economy, the President’s policy, and the potential foreign policy implications.”  Therefore, EPA declined the petitioners’ request to regulate GHG emissions from motor vehicles. 
After the Court of Appeals upheld the denial, the petitioners asked the Supreme Court to “answer two questions concerning the meaning of §202(a)(1) of the Act: whether EPA has the statutory authority to regulate greenhouse gas emissions from new motor vehicles; and if so, whether its stated reasons for refusing to do so are consistent with the statute.”  The Supreme Court agreed to do so, persuaded by the “unusual importance of the underlying issue.” 
As to the first question, the Supreme Court “had little trouble concluding” that CAA §202(a)(1) authorized EPA to regulate GHG from new motor vehicles “in the event it forms a ‘judgment’ that such emission contribute to climate change.”  Although the EPA concluded that carbon dioxide is not an air pollutant, the Court found that the text of the CAA “forecloses EPA’s reading” as the CAA provides a “sweeping definition” of air pollutants. The Court emphasized that the definition applies to “any air pollution agent” which specifically included “any physical [or] chemical … substance which is emitted.”  The definition, “on its face” “embraces all airborne compounds of any stripe, and underscores that intent through the repeated use of the word ‘any.'” The statutory definition “is unambiguous.” 
Having easily concluded that carbon dioxide was an air pollutant within the meaning of the CAA, the Court soundly rejected EPA’s supporting arguments. EPA argued that its denial of the petition was supported by congressional actions after the enactment of the CAA and Supreme Court precedent and the potential infringement of regulatory efforts by the Department of Transportation (DOT).
EPA argued that certain congressional actions occurring after the enactment of the CAA, such as congressional rejection of binding emission limitations on GHG and promotion of interagency collaboration and research to better understand climate change, commanded EPA to refrain from regulating GHG. The Court noted that, even if post-enactment actions could shed light on a previously enacted statute, any such collaboration or research served to complement regulatory efforts to address climate change. 
EPA also relied on Supreme Court precedent relating to tobacco to support its argument. In Brown v. Williamson Tobacco Corp., the Supreme Court found that tobacco products are not drugs or devices subject to Food and Drug Administration regulation under the Food, Drug and Cosmetic Act.  Likewise, EPA argued, GHG are not air pollutants subject to regulation under the CAA. The Court, though, noted distinct differences between the two situations. A ban of tobacco would have been required had tobacco been deemed a drug or device; however, regulation under the CAA requires no such ban but rather the regulation or curtailing of emissions. Further, there was an unbroken series of congressional enactments strongly suggesting that no ban of tobacco was ever intended; however, no such enactments existed to suggest that Congress thought there should be no regulation of GHG. 
Finally, EPA argued that the regulation of GHG would require it to tighten mileage standards for vehicles, a job Congress had assigned to the DOT. The Court, however, noted that whatever DOT’s role, EPA could not “shirk [the] environmental responsibilities” assigned to it regardless of the duties otherwise assigned to the DOT. 
In short, the “broad language of §202(a)(1) reflects an intentional effort to confer the flexibility necessary to forestall … obsolescence” of the CAA.  While Congress may not have understood that fossil fuels may lead to global warming, they “did understand that without regulatory flexibility, changing circumstances and scientific development would soon render the Clean Air Act obsolete.”  GHG “fit well within the [Act’s] capacious definition of ‘air pollutant'” and as a result, EPA had “statutory authority to regulate the emission of such gases from new motor vehicles.” 
The Court then turned to the EPA’s decision that it was not appropriate or was unwise to regulate GHG, even if it had such authority. The Court stated that EPA’s decision in this regard “rests on reasoning divorced from the statutory text.” 
The CAA clearly provides some discretion to EPA in that it requires EPA to make a ‘judgment.’
However, that judgment must be made within the bounds set forth in the CAA, namely a judgment that the air pollutant in question causes or contributes to air pollution that may endanger public health or welfare. EPA “refused to comply with this clear statutory command. Instead, it has offered a laundry list of reasons not to regulate” none of which were related to the statutory text.  In responding to a petition for rulemaking, EPA’s “reason for action or inaction must conform to the authorizing statute.” 
Having been asked to make a ‘judgment,’ the Court made it clear that EPA’s rationale for not regulating GHG simply did not ‘conform to the authorizing statute.’ Indeed, the Court stated that “EPA can avoid taking further action only if it determines that greenhouse gases do not contribute to climate change or if it provides some reasonable explanation as to why it cannot or will not exercise its discretion to determine whether they do.”
The Court remanded the matter. However, it was careful to state that no specific outcome was required. Instead, EPA must “ground its reasons for action or inaction in the statute.” 
 P.L. 91-604, 84 Stat. 1676.
 Clean Air Amendments of 1977, P.L. 95-95, 91 Stat. 685; Clean Air Amendments of 1990, P.L. 101-549, 104 Stat. 2399.
 42 USC §7401(b)(1).
 42 USC §7602(g).
 42 USC §7602: “When used in this chapter….” The chapter referenced is Chapter 85 of Title 42 of the United States Code, which is the CAA.
 Other titles include: Title III, General Provisions; Title IV, Noise Pollution; Title IV-A, Acid Deposition Control; and Title VI, Stratospheric Ozone Protection.
 42 USC §7409(a)(1).
 See 40 CFR Part 50.
 42 USC §7409(b)(1).
 42 USC §7409(b)(2).
 42 USC §7411(b)(A).
 42 USC §7411(a)(3).
 42 USC §7411(b)(B).
 §7411(d). 42 USC
 42 USC §7411(e).
 42 USC §7412(b).
 42 USC §7412(d).
 42 USC §7410(a)(1).
 42 USC §7411(c).
 42 USC §7410(a)(2).
 The statutory and regulatory requirement for non-attainment areas are not directly relevant to this issue and will not be detailed herein.
 42 USC §§7470 – 7479.
 42 USC §7475(a).
 42 USC §7479(1).
 See, e.g., 40 CFR 52.21(b)(2), (b)(3), and (b)(40).
See 42 USC §7475(a): “No major emitting facility … may be constructed in any area to which this part applies.”
 42 USC §7475(a)(4).
 42 USC §7479(3).
 75 Fed. Reg. 31551 (June 3, 2010).
 42 USC §§7661(2) and 7602(j).
 42 USC §7521(a)(1).
 Massachusetts v. EPA, 549 US 497, 127 S.Ct 1438 (2007)
 Massachusetts, 127 S.Ct @ p. 1449.
 See 68 Fed. Reg. 52925 (September 8, 2003).
 Massachusetts, 127 S.Ct @ p. 1446.
 Massachusetts, 127 S.Ct @ p. 1447.
 Massachusetts, 127 S.Ct @ p. 1459.
 Massachusetts, 127 S.Ct @ p. 1460.
 42 USC 7602(g); Massachusetts, 127 S.Ct @ p. 1460 (emphasis in original).
 Massachusetts, 127 S.Ct @ p. 1460.
 Massachusetts, 127 S.Ct @ pp. 1460-1.
 Brown v. Williamson Tobacco Corp., 529 U.S. 120, 120 S.Ct. (2000); Massachusetts, 127 S.Ct @ p. 1461.
 Massachusetts, 127 S.Ct @ p. 1462.
 Massachusetts, 127 S.Ct @ p. 1462.
 Massachusetts, 127 S.Ct @ p. 1463.