Since 2009, the Environmental Protection Agency (EPA) has begun to address emissions of greenhouse gases (GHGs) from both mobile and stationary sources, using broad regulatory authority provided by Congress decades ago in the Clean Air Act. Although Congress has never specifically directed EPA to regulate emissions of GHGs, the Clean Air Act as enacted in 1970 and as amended in 1977 and 1990 gave the agency authority to identify air pollutants and promulgate regulations to limit their emission. From the late 1990s until 2007, EPA and various interested parties debated whether that authority covered greenhouse gases. This debate was settled by the Supreme Court in April 2007, in Massachusetts v. EPA . In a 5-4 decision, the Court found that greenhouse gases are unambiguously air pollutants: The Clean Air Act's sweeping definition of 'air pollutant' includes ' any air pollution agent or combination of such agents, including any physical, chemical ... substance or matter which is emitted into or otherwise enters the ambient air... (emphasis added). ... Carbon dioxide, methane, nitrous oxide, and hydrofluorocarbons are without a doubt 'physical [and] chemical ... substances[s] which [are] emitted into ... the ambient air.' The statute is unambiguous. Since the Court's Massachusetts decision, EPA has addressed GHG emissions in a number of steps, among them: In December 2009, the agency laid the groundwork for regulations by finding that emissions of greenhouse gases may reasonably be anticipated to endanger public health and welfare, and that GHGs from new motor vehicles cause or contribute to that endangerment. In May 2010, the agency promulgated GHG emission standards for model year 2012-2016 cars and light trucks. In January 2011, the agency began requiring permits and the imposition of Best Available Control Technology on new stationary sources (and major modifications of existing sources) that emit more than a threshold amount of GHGs. In September 2011, EPA promulgated GHG emission standards for model year 2014-2018 medium- and heavy-duty trucks. In October 2012, the agency promulgated a second phase of GHG emission standards for cars and light trucks, covering model years 2017-2025. As extensive as these actions may seem, they have had relatively minor impacts on GHG emissions to date. The rules are prospective, and in most cases have not yet taken effect. The auto and truck manufacturing industries have been the major focus of the GHG regulations; in both cases, they are eager to improve fuel economy (coincidentally reducing GHG emissions), because the high cost of fuel has affected consumer purchasing decisions over the last five years. The stationary source permitting requirement has yet to affect most sources. As of early September 2013, EPA and the states have issued only 110 GHG permits to stationary sources since the requirement was implemented in January 2011. (For comparison, EPA estimates that there are more than 6 million stationary sources.) EPA set the emission threshold for requiring permits at a high level, exempting most new sources of GHGs; and few new facilities have been constructed in the recession's aftermath. Ultimately, if EPA is to reduce the nation's GHG emissions, as the President has committed to do, it will have to issue emission standards for broad categories of existing stationary sources. EPA took the first step toward setting such standards on April 13, 2012, with the proposal of standards for new electric generating units (EGUs). In a June 25, 2013, memorandum to the EPA Administrator, the President directed the agency to re-propose those standards by September 20, 2013, finalize them "in a timely fashion after considering all public comments," and propose guidelines for existing EGUs by June 1, 2014. The re-proposed standards for new sources were announced on September 20, 2013. As shown in Table 1 , EGUs—principally, coal-fired power plants—are the most significant U.S. source of greenhouse gases, accounting for about one-third of the nation's total emissions. With the principal mobile source categories already subject to GHG regulations, EPA will have addressed the sources of more than half of all U.S. emissions once it promulgates regulations for existing EGUs. To control GHG emissions from stationary sources, EPA intends to use Section 111 of the Clean Air Act, which requires the agency to set New Source Performance Standards (NSPS) when, in the Administrator's judgment, a category of sources causes, or contributes significantly to, air pollution which may reasonably be anticipated to endanger public health or welfare. As noted, EPA proposed the first such NSPS/GHG standard, for electric generating units, on April 13, 2012, and following extensive public comment, at the President's direction, re-proposed the standard on September 20, 2013. The agency has also committed to the promulgation of NSPS for petroleum refineries, although it is unclear when those standards will be proposed. Under Section 111, the EPA Administrator is required to set standards for categories of new (or substantially modified) major sources if, in her judgment, they cause or contribute significantly to air pollution which may reasonably be anticipated to endanger public health or welfare. Over the past four decades, EPA Administrators have used this authority to set emission standards for numerous sources of conventional pollutants, such as sulfur dioxide or nitrogen oxides. The standards are to reflect the degree of emission limitation achievable through application of the best "adequately demonstrated" system of emission reduction. The Administrator can take costs, health impacts, environmental impacts, and energy requirements into account in setting the standards; she can distinguish among classes, types, and sizes of sources; and she must review the standards at least every eight years. In addition to standards for new sources, Section 111 requires that EPA develop guidelines applicable to GHG emissions from existing units whenever it promulgates GHG standards for new sources in a category (Section 111(d)). Using the guidelines, states would be required to develop performance standards for existing sources. These standards could be less stringent than the NSPS, taking into account, among other factors, the remaining useful life of the existing source to which the standard applies. Nevertheless, these standards might have far greater impact than the NSPS, given that existing power plants are the largest U.S. source of GHG emissions. The authority to control existing sources is particularly important in sectors like the electric utility industry, where old units can continue operating for decades. The average coal-fired power plant in the United States is more than 40 years old. Without the authority to control emissions from such existing facilities, it could be decades before emissions from most power plants would be controlled. How quickly Section 111(d) standards will be applied to existing sources has been an open question, however. EPA must first propose and promulgate guidelines, following which the states will be given time to develop implementation plans. In the President's June 25 memorandum, he requested that EPA: (i) issue proposed guidelines for modified, reconstructed, and existing power plants by no later than June 1, 2014; (ii) issue final guidelines by no later than June 1, 2015; and (iii) include in the guidelines addressing existing power plants a requirement that states submit to EPA the implementation plans required under section 111(d) of the Clean Air Act by no later than June 30, 2016. Following approval of the plans, the act envisions case-by-case determinations of emission limits. Thus, it is likely to be several years before existing power plants or other stationary sources are subject to emission limits for GHGs. As noted, the President directed EPA in his June 25 memorandum to re-propose the NSPS standards for power plant carbon pollution. To understand what issues the agency has addressed in the re-proposed standards, this report discusses both the 2012 proposal and the September 2013 re-proposal. The re-proposed standard would set a limit of 1,100 pounds of carbon dioxide (CO 2 ) per megawatt-hour (MWh) of electricity generated for coal-fired EGUs, and a standard of 1,000 or 1,100 lbs/MWh (depending on the size of the unit) for new natural gas-fired plants. The standard is to be measured on a gross output basis; it requires reduction of emissions whether the electricity is used on site (e.g., for the operation of pollution control equipment) or is sold to customers. The standards can be met by new natural gas combined cycle plants without add-on emission controls. Coal-fired plants, however, would find it impossible to meet the 1,100 lb. standard without controls to capture and store some of the CO 2 they produce. EPA estimates that a supercritical pulverized coal-fired power plant without such controls produces roughly 1,800 lbs. CO 2 /MWh of electricity, so a plant subject to the standard would need to reduce emissions by about 40%. Carbon capture and storage (CCS) technology that might be used to reduce CO 2 emissions has been the subject of much recent research and demonstration. It poses a number of challenges, not the least of which is the additional energy it consumes. The energy required to run equipment that can remove CO 2 from an emission stream (referred to as the "parasitic load") is currently in the range of 30% on most demonstration projects. In addition, a CCS-equipped unit might incur costs for underground storage of the captured CO 2 (unless it is used for enhanced oil recovery) and possibly significant costs for building and operating a pipeline to transport the CO 2 to the storage location. To address the concerns of those who maintain that CCS technology is not yet available, or who expect the technology to improve (bringing down costs) as research and demonstration continue, the agency's 2012 proposal included an alternative under which coal-fired facilities would have been allowed to average their emissions over a 30-year period: during the first 10 years, such facilities could have emitted up to 1,800 lbs.