I n 2013, President Obama released a Climate Action Plan (CAP) to reduce greenhouse gases (GHG) emissions and to encourage adaptation to expected climate change. Subsequently, the Obama Administration implemented the CAP pursuant to presidential powers and existing statutory authority. The President, the U.S. Environmental Protection Agency (EPA), and other agencies issued various executive orders, guidance, and regulations to reduce GHG emissions from motor vehicles, new and existing power plants, facilities in the oil-and-gas sector, and other GHG-emitting sources and federal facilities. On March 28, 2017, President Trump signed Executive Order 13783, which rescinds the CAP and requires agencies to review their existing regulations and "appropriately suspend, revise, or rescind those that unduly burden" domestic energy production and use. The executive order, entitled Promoting Energy Independence and Economic Growth , rescinds or withdraws various Obama Administration orders and directions that were intended to address climate change and regulate GHG emissions, including the following: Presidential Orders/Reports Executive Order 13653, Preparing the United States for the Impacts of Climate Change (Nov. 1, 2013) Power Sector Carbon Pollution Standards, Pres. Memo. (June 25, 2013) Mitigating Impacts on Natural Resources from Development and Encouraging Related Private Investment, Pres. Memo. (Nov. 3, 2015) Climate Change and National Security, Pres. Memo. (Sept. 21, 2016) The President's Climate Action Plan (June, 2013) Climate Action Plan Strategy to Reduce Methane Emissions (March 2014) Agency Guidance Final Guidance for Federal Departments and Agencies on Consideration of Greenhouse Gas Emissions and the Effects of Climate Change in National Environmental Policy Act Reviews, 81 Fed. Reg. 51,866 (Aug. 5, 2016) All technical documents related to the Social Cost of Carbon Agency Orders All moratoria on federal land coal leasing activities related to Order 3338, Discretionary Programmatic Environmental Impact Statement (PEIS) to Modernize the Federal Coal Program (Jan. 15, 2016) The executive order also directs EPA to review GHG regulations for power plants and methane regulations for existing oil and gas sources, and "as soon as practicable, suspend, revise, or rescind the guidance, or publish for notice and comment proposed rules suspending, revising, or rescinding those rules." Because the evolution and scope of U.S. climate change regulation and litigation are extensive, this report will focus on existing federal laws and regulations and will not address international agreements or legislation related to climate change. This report will provide a brief history of U.S. climate change regulation; review the different types of regulation and legal actions that have been pursued in the national debate over GHGs; examine selected legal issues and related litigation; and address what these legal and regulatory developments mean for Congress. Since the late 1970s, Congress has provided authority and funds for federal agencies to research and better understand climate science. In 1978, Congress passed the National Climate Program Act (NCPA) that created the National Climate Program in the Department of Commerce. The NCPA required the program it established to assess climate change effects, improve understanding of climate change science, and collect global and domestic climate data. Congress subsequently passed additional legislation authorizing federal climate change research, interagency collaboration, and collection of GHG emissions data. These statutes authorized federal agencies to investigate climate change and its potential impacts, but did not direct agencies to reduce GHG emissions. Various executive orders and environmental, energy, and natural resources statutes address climate change in different ways. For example, under the Clean Air Act (CAA), EPA has sought to reduce GHG emissions from stationary and mobile sources. Similarly, the Department of Energy has established efficiency standards for certain consumer products to reduce energy consumption, while the Council on Environmental Quality has issued guidance on how agencies should consider potential GHG impacts resulting from proposed federal actions under the National Environmental Policy Act. Federal authority to regulate GHGs under existing law and policies has been challenged in court. Many of these challenges have focused on EPA's authority to regulate GHGs under the CAA. In 1998, EPA General Counsel Jonathan Cannon concluded in a memorandum that GHGs were air pollutants within the CAA definition of the term, and therefore could be regulated under the CAA. On October 20, 1999, relying on the Cannon memorandum as well as on the CAA, a group of 19 organizations petitioned EPA to regulate GHG emissions from new motor vehicles under CAA Section 202, which directs EPA to develop emission standards for "any air pollutant" from new motor vehicles "which, in his judgment cause[s], or contribute[s] to air pollution which may reasonably be anticipated to endanger public health or welfare." On August 28, 2003, EPA denied the petition based on a new General Counsel memorandum that concluded that the CAA does not grant EPA authority to regulate carbon dioxide (CO 2 ) and other GHG emissions based on their climate change impacts. Massachusetts, 11 other states, and various other petitioners challenged EPA's denial of the petition in a case that ultimately reached the Supreme Court. In 2007, the Supreme Court defined the contours of EPA's authority to regulate GHGs under the Clean Air Act in Massachusetts v. EPA . In its 5-4 decision, the Supreme Court held that EPA has authority to regulate GHGs as "air pollutants" under the CAA. The Court determined that the CAA authorized EPA to regulate GHG emissions from new motor vehicles because CO 2 and other GHGs "fit well within the CAA's capacious [and unambiguous] definition of 'air pollutant.'" Rejecting EPA's argument that the agency could not regulate vehicle GHG emissions because to do so would conflict with the Department of Transportation's (DOT's) fuel economy standards, the Court explained that EPA's statutory obligation to protect public health and welfare under the CAA is "wholly independent of DOT's mandate to promote energy efficiency." In addition, the Court held that CAA Section 202 does not allow EPA to refuse to regulate GHG emissions from new motor vehicles based on policy considerations, but that EPA must regulate GHG emissions if EPA finds that GHGs "cause or contribute to air pollution that may reasonably be anticipated to endanger public health or welfare." Based on this reasoning, the Court identified three ways EPA could respond to the rulemaking petition: (1) make a finding that motor vehicle GHG emissions may "endanger public health or welfare" and issue emissions standards; (2) make a finding that there is no endangerment from these emissions; (3) or provide a "reasonable explanation" as to why the Agency cannot or will not make an endangerment finding. Because EPA "offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change," the Court held that EPA's denial of the petition for rulemaking was arbitrary and capricious, reversed the D.C. Circuit opinion, and remanded the case to that court for further proceedings. Following the Supreme Court's decision in Massachusetts v. EPA , EPA found that GHG emissions from motor vehicles endanger public health and welfare and subsequently issued a series of regulations under the CAA to reduce GHG emissions from both mobile and stationary sources. EPA has consequently regulated GHG emissions from new motor vehicles under CAA Section 202. EPA has also regulated GHG emissions from various stationary sources, including new and existing fossil fuel-fired power plants and GHG-emitting sources from municipal landfills, and the oil and gas sector under other CAA sections. Most of EPA's GHG regulatory actions have been challenged in court. These challenges generally concern the scope of EPA's authority and its methods for regulating GHG emissions—not whether EPA has authority to regulate GHG emissions under the CAA, which the Supreme Court held in Massachusetts v. EPA . Although the Supreme Court has clarified EPA's authority to regulate GHGs under specific CAA programs in subsequent cases, Massachusetts v. EPA remains the governing law. This report will focus on two types of GHG regulations: (1) laws that seek to reduce GHG emissions from stationary and mobile sources, and (2) laws that require that climate change effects be considered when the federal government takes certain actions. This section will review examples of these types of regulations and related legal challenges. The majority of regulations addressing GHG emissions from stationary and mobile sources have been promulgated under the CAA. Following the Massachusetts decision, EPA began to explore "whether and how greenhouse gases could be effectively controlled under the Clean Air Act" in an advanced notice of proposed rulemaking (ANPR). In 2009, EPA made two findings under CAA Section 202: (1) that GHGs currently in the atmosphere potentially endanger public health and welfare, and (2) that new motor vehicle emissions cause or contribute to that pollution (collectively these findings are known as the "endangerment finding"). The endangerment finding triggered EPA's duty under CAA Section 202(a) to promulgate emission standards for new motor vehicles. In general, the CAA divides mobile sources into three broad categories: on-road vehicles (cars, trucks, and buses); aircraft; and nonroad vehicles and engines. Following the Massachusetts v. EPA decision, EPA has exercised its authority under Title II of the CAA to address GHG emissions from some mobile sources. This section will focus on EPA's regulation of motor vehicles, and briefly discuss EPA's ability to regulate GHG emissions from aircrafts and other nonroad mobile sources and engines. While EPA's endangerment finding was pending, President Obama announced that EPA would develop its GHG emission standards in coordination with the National Highway Traffic Safety Administration's (NHTSA) development of corporate average fuel economy (CAFE) standards, which NHTSA administers for new cars and light trucks.