Since 2007, the Supreme Court has ruled on two separate occasions that the Clean Air Act authorizes the Environmental Protection Agency (EPA) to set standards for emissions of greenhouse gases (GHGs). In the first case, Massachusetts v. EPA , the Court held that GHGs are air pollutants within the Clean Air Act's definition of that term and that EPA must regulate their emissions from motor vehicles if the agency found that such emissions cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare. In the second case, American Electric Power, Inc. v. Connecticut , the Court held that corporations cannot be sued for GHG emissions under federal common law, because the Clean Air Act delegates the management of carbon dioxide and other GHG emissions to EPA: "... Congress delegated to EPA the decision whether and how to regulate carbon-dioxide emissions from power plants; the delegation is what displaces federal common law." GHGs are a disparate group of pollutants —the most significant of them being carbon dioxide (CO 2 ). According to a widely held scientific consensus, these gases trap the sun's energy in the Earth's atmosphere and contribute to climate change. In 2009, the House passed comprehensive legislation that would have limited numerous elements of EPA's existing authority over GHGs, substituting economy-wide cap-and-trade systems to reduce GHG emissions. Companion legislation emerged from committee in the Senate, but failed to reach the floor. Since then, Congress has made no serious effort to revive the legislation. Instead, in the 112 th -114 th Congresses, attention to the issue has focused on various bills to repeal or limit EPA's authority over GHG emissions without providing a substitute. Meanwhile, EPA has taken action, using its existing Clean Air Act (CAA) authority: In April 2010, then-EPA-Administrator Lisa Jackson signed final regulations that required auto manufacturers to limit emissions of GHGs from new Model Year (MY) 2012-2016 cars and light trucks. Effective in January 2011, EPA began requiring new and modified major stationary sources to undergo pre-construction review under the Prevention of Significant Deterioration/New Source Review (PSD/NSR) program with respect to their GHG emissions in addition to any other pollutants subject to CAA regulation that they emit. This review requires affected new and modified sources to obtain permits and install Best Available Control Technology (BACT) to address their GHG emissions. At the same time, EPA began requiring large existing stationary sources of GHG emissions (in addition to new sources) to obtain operating permits under Title V of the Clean Air Act (or have existing permits modified to include their GHG emissions). In September 2011, EPA promulgated GHG emission standards for MY2014-2018 medium- and heavy-duty trucks. In October 2012, EPA promulgated a second round of GHG emission standards for cars and light trucks, applicable to MY2017-2025 vehicles. In July 2015, EPA proposed a second round of GHG emission standards for medium- and heavy-duty trucks, applicable to MY2021-2027 trucks and engines, and MY2018 and later trailers. In August 2015, EPA promulgated emission standards for new and existing fossil-fueled electric generating units (EGUs) under Section 111 of the Clean Air Act. EPA's potential regulation of GHG emissions (particularly from stationary sources) has led many in Congress to suggest that the agency delay taking action or be stopped from proceeding. In each Congress since the 111 th , bills have been introduced to rescind or limit EPA's greenhouse gas authority. Early on, EPA attempted to respond to congressional and stakeholder concerns by clarifying the direction and schedule of its actions. The agency provided three clear responses to implementation issues as it was taking the first regulatory actions described above: The first came on March 29, 2010, when the Administrator reinterpreted a 2008 memorandum concerning the effective date of the stationary source permit requirements. Facing a possibility of having to begin the permitting process on April 1, 2010 (the date the first GHG standard for automobiles was finalized), the March 29 decision delayed for nine months (to January 2, 2011) the date on which EPA would consider stationary source GHGs to be subject to regulation, and thus subject to the permitting requirements of PSD/NSR and Title V. On May 13, 2010, the Administrator signed the GHG "Tailoring" Rule, which provided for a phasing in of Title V and PSD/NSR permitting requirements, so that only a limited number of very large sources would initially have to meet requirements. On November 10, 2010, EPA released a package of guidance and technical information to assist local and state permitting authorities in implementing PSD and Title V permitting for greenhouse gas emissions. The EPA Administrator and the President also repeatedly expressed their preference for Congress to take the lead in designing a GHG regulatory system. However, EPA simultaneously stated that, in the absence of congressional action, it must proceed to regulate GHG emissions: the April 2007 Supreme Court decision in Massachusetts v. EPA compelled EPA to address whether GHGs were air pollutants that endanger public health and welfare, and if it found they were, to embark on a regulatory course prescribed by statute. Having made an affirmative decision in response to the endangerment question, EPA proceeded with regulations. Thus, EPA and many Members of Congress have been on a collision course. EPA is proceeding to regulate emissions of GHGs under the Clean Air Act, as it maintains it must. Opponents of this effort in Congress continue to explore approaches to alter the agency's course. The President has made clear that he intends to take action to control GHG emissions. In his second inaugural address, he promised to "respond to the threat of climate change." He has reiterated his determination to address the issue in multiple State of the Union addresses. In June 2013, he directed EPA to propose New Source Performance Standards for greenhouse gas emissions from new fossil-fueled power plants by September 20, 2013, and to propose guidelines for existing power plants by June 1, 2014. EPA met both of these deadlines and finalized the power plant rules in August 2015—leaving Congress to consider whether and how best to respond. This report discusses elements of this controversy, providing background on stationary sources of greenhouse gas pollution and identifying options Congress has if it chooses to address the issue. The report discusses four sets of options: (1) resolutions of disapproval under the Congressional Review Act; (2) freestanding legislation directing, delaying, or prohibiting EPA action; (3) the use of appropriations bills as a vehicle to influence EPA activity; and (4) amendments to the Clean Air Act, including legislation to establish a new GHG control regime. The report considers each of these in turn, but first provides additional detail regarding the sources of GHG emissions, the requirements of the Clean Air Act, and a brief description of the two rules that would limit power plant GHG emissions. Although this report focuses on Congress and the Executive Branch, there is, of course, a third branch of government, the judiciary, which continues to be involved in the issues discussed here. The courts have upheld EPA's regulations related to GHG emissions from motor vehicles. The Supreme Court has issued a stay, however, of the Clean Power Plan, EPA's August 2015 regulations governing GHG emissions from existing fossil-fueled power plants. Challenges to both EPA power plant rules are proceeding in the U.S. Court of Appeals for the D.C. Circuit, with the potential for appeals thereafter to the Supreme Court. Stationary sources are the major sources of the country's GHG emissions. As shown in Figure 1 , 64% of U.S. emissions of greenhouse gases comes from stationary sources (the remainder comes from mobile sources, primarily cars and trucks, and from agriculture, which has elements in both the stationary and mobile source groups). If EPA (or Congress) is to embark on a serious effort to reduce greenhouse gas emissions, stationary sources, and in particular large stationary sources such as power plants, will have to be included. The substantial amount of greenhouse gas emissions emanating from stationary source categories is even more important from a policy standpoint: reducing greenhouse gas emissions from these sources is likely to be more timely and cost-effective than attempts to reduce emissions from the transport sector. A relatively small number of stationary sources (a few thousand power plants, for example) account for a large percentage of total emissions, making regulation easier to administer and enforce. By contrast, there are more than 200 million sources in the fleet of cars and trucks. EPA has no legal authority to impose more stringent standards on these existing vehicles. Even for new vehicles, standards have to be phased in: the manufacturers can't be expected to develop new vehicle designs and engines for every model simultaneously. The fleet of vehicles turns over slowly. Thus, although new vehicles will be required to reduce GHG emissions by about 50% in Model Year 2025 and later vehicles, it will be 2040 or later before the full effect of that requirement is felt. Existing power plants and other stationary sources—although not easily modified—are somewhat more amenable to changes than mobile sources: fuel sources can be switched; processes can be made more efficient, reducing fuel consumption; and demand for power can be modified through a variety of measures. Section 111 of the Clean Air Act provides authority for EPA to impose performance standards on stationary sources of air pollution directly in the case of new (or modified) stationary sources (Section 111(b)), and through the states in the case of existing sources (Section 111(d)). Because power plants and other stationary sources are the largest sources of GHG emissions, EPA has begun the process of regulating their emissions through the two authorities. New Source Performance Standards (NSPS) are emission limitations imposed on designated categories of new (or substantially modified) stationary sources of air pollution. A new source is subject to NSPS regardless of its location or ambient air conditions.