This report discusses the Environmental Protection Agency's (EPA's) authority to regulate greenhouse gas (GHG) emissions from mobile sources of pollution. Mobile sources include cars, trucks, aircraft, ships, locomotives, nonroad vehicles (e.g., tractors, bulldozers), portable equipment (i ncluding lawnmowers, chain saws, leaf blowers, and backup generators), and the fuels that provide power to each of these. Mobile sources account for close to 30% of the total U.S. emissions of GHGs. As shown in Figure 1 , more than three-quarters of the mobile source total comes from cars and trucks. Over the last eight years, the Obama Administration has worked with major stakeholders in the auto and truck industries and with states and other interested parties to develop and implement GHG standards. Because carbon dioxide (CO 2 ) from fuel combustion is the major GHG produced by cars and trucks, the White House directed EPA to work with the National Highway Traffic Safety Administration (NHTSA) to harmonize the GHG standards with fuel economy standards under development by NHTSA. In addition, under the Clean Air Act, the state of California enjoys unique status to issue motor vehicle emission standards provided that they are at least as stringent as federal standards and are necessary to meet "compelling and extraordinary conditions." California had already promulgated GHG emission standards prior to 2009, for which it had requested an EPA waiver. EPA granted California a waiver in June 2009, and President Obama directed EPA and NHTSA to harmonize the federal GHG and fuel efficiency standards with those developed by California. The mobile source GHG standards moved forward even as Congress and the Obama Administration reached an impasse over climate issues. EPA supported economy-wide GHG legislation that passed the House in 2009, but the legislation died in the Senate. Lacking what new authority might have been provided by congressional action, EPA moved ahead with sectoral GHG emission standards using the authority of the existing Clean Air Act (CAA). In Massachusetts v. EPA , the Supreme Court had held in 2007 that the existing CAA authorized the agency to address GHG emissions. The key to using this authority was for the EPA Administrator to find that GHG emissions are air pollutants that endanger public health or welfare, and that motor vehicles cause or contribute to that pollution. EPA Administrator Lisa Jackson issued these "endangerment and cause-or-contribute findings" in December 2009. Following the issuance of the findings, EPA promulgated four sets of standards for motor vehicles: 1. standards for Model Year (MY) 2012-2016 light-duty motor vehicles (cars, SUVs, crossovers, minivans, and most pickups), May 7, 2010; 2. standards for MY2017-MY2025 light-duty vehicles, October 15, 2012; 3. standards for MY 2014-2018 medium- and heavy-duty trucks and engines, September 15, 2011; and 4. standards for MY2021-MY2027 medium- and heavy-duty trucks and engines and MY2018-MY2027 trailers, October 25, 2016. Under the Trump Administration, it is unclear whether this authority will be put to further use, and whether some of the standards already promulgated but not yet implemented might be rolled back or modified. Much may depend on the views of the affected industries: auto, truck, and truck engine manufacturers; and their customers, especially those in the trucking industry. In general, the auto manufacturing and trucking industries have been supportive of EPA's GHG regulations, partly because of concerns that states would implement a patchwork of standards in the absence of federal action. As the standards have been implemented, however, industry concerns have arisen regarding a lack of harmonization between EPA, NHTSA, and related California GHG and fuel efficiency programs. The 2017-2025 light-duty vehicle regulations provided for a Midterm Evaluation of the 2022-2025 standards for these vehicles, which offered an opportunity to modify the standards. As discussed at greater length below, on January 12, 2017, EPA announced a final decision that the MY2022-MY2025 standards remained appropriate and that a rulemaking to change them was not warranted. Congress may take an interest in that final determination. Congress may also take an interest in the Phase 2 medium- and heavy-duty truck standards. These standards, which were promulgated in October 2016, would reduce oil consumption by up to 2 billion barrels over the lifetime of the vehicles sold under the program, saving vehicle owners fuel costs of about $170 billion according to EPA. The standards will increase the cost of a long haul tractor-trailer, but the buyer would recoup the investment in fuel-efficient technology in less than two years through fuel savings, according to the agency. In general, the truck standards have been well-received by the affected industries, but given the new Administration's general opposition to GHG regulations and the opposition to GHG rules among many in the leadership of the 115 th Congress, there could be efforts to review or modify the regulations. Another looming issue in the mobile source arena concerns GHG emission standards for aircraft. In October 2016, the International Civil Aviation Organization (ICAO) agreed on international CO 2 emission standards for aircraft, beginning in 2020. ICAO also agreed on a system for offsetting future carbon emissions from aviation. The emission standards and the offset system would be implemented in the United States by EPA regulations issued under the CAA. U.S. airlines and aircraft manufacturers participated in the ICAO negotiations and have been supportive of the resulting agreements; whether EPA actions to implement them would run contrary to the President's and the Administration's broader views on regulation and climate change is unclear. This report focuses on these three actions to limit GHG emissions from mobile sources. Following the discussion of these issues, the report provides background information on GHG emissions from other mobile sources, including ships, nonroad vehicles and engines, locomotives, and fuels. We begin with a brief discussion of the court action that led to EPA's regulatory decisions. Whether EPA could regulate GHGs through existing CAA authority was under consideration at EPA for more than a decade before the agency took action. In 1998, during the Clinton Administration, EPA General Counsel Jonathan Cannon concluded in a memorandum to the agency's Administrator that greenhouse gases were air pollutants within the CAA's definition of the term, and therefore could be regulated under the act. Relying on the Cannon memorandum as well as the statute itself, on October 20, 1999, a group of 19 organizations petitioned EPA to regulate greenhouse gas emissions from new motor vehicles under Section 202 of the act. Section 202 gives the EPA Administrator broad authority to set "standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles" if in the Administrator's judgment they cause or contribute to air pollution which "may reasonably be anticipated to endanger public health or welfare." Under the Bush Administration, EPA denied the petition August 28, 2003, on the basis of a new General Counsel memorandum the same day. The new memorandum 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. Denial of the petition was challenged by Massachusetts, 11 other states, and various other petitioners in a case that ultimately reached the Supreme Court. In an April 2, 2007, decision ( Massachusetts v. EPA ), the Court found by 5-4 that EPA does have authority to regulate greenhouse gas emissions, since the emissions are clearly air pollutants under the CAA's definition of that term. The Court's majority concluded that EPA must, therefore, decide whether emissions of these pollutants from new motor vehicles contribute to air pollution that may reasonably be anticipated to endanger public health or welfare, or provide a reasonable explanation why it cannot or will not make that decision, such as that there is insufficient information to make the decision. If it makes a finding of endangerment, the act requires the agency to establish standards for emissions of the pollutants. In nearly two years following the Court's decision, the Bush Administration's EPA did not respond to the original petition or make a finding regarding endangerment. Its only formal action following the Court decision was to issue a detailed information request, called an Advance Notice of Proposed Rulemaking (ANPR), on July 30, 2008. The Obama Administration's EPA, however, made review of the endangerment issue a high priority. On December 15, 2009, it promulgated a finding that GHGs do endanger both public health and welfare and that GHGs from new motor vehicles contribute to that endangerment. Four greenhouse gases are emitted by motor vehicles (CO 2 , methane, nitrous oxide, and hydrofluorocarbons). According to EPA, emissions of the four gases from motor vehicles (including trucks) accounted for 23.6% of the total inventory of U.S. GHG emissions in 2006 (latest data available when the endangerment finding was being considered). Most of the emissions were (and are) in the form of CO 2 (see Figure 2 ), which is the product of combusting any fuel containing carbon. Hydrofluorocarbons (HFCs), the chemicals used as coolants in vehicle air conditioning systems, are the second-most important motor vehicle GHG; but, as the figure shows, they are a distant second. Even before it finalized the endangerment finding, the Obama Administration reached agreement with nine auto manufacturers and California (which had developed its own GHG emission standards for motor vehicles), as well as other interested parties regarding the major outlines of a joint greenhouse gas/fuel economy rulemaking. As announced by President Obama, May 19, 2009, EPA and the National Highway Traffic Safety Administration (which administers fuel economy standards for cars and trucks under authorities that began with the Energy Policy and Conservation Act of 1975) would integrate corporate average fuel economy (CAFE) standards for new cars and light trucks (collectively known as "light-duty motor vehicles") with national greenhouse gas emission standards to be issued by EPA. The objective of the joint standards was to achieve "One National Program," with GHG reduction levels similar to those adopted by California, which harmonized its own standards with EPA's as part of the agreement. As has happened on many previous occasions when EPA set emission standards for conventional pollutants from motor vehicles, the GHG standards have forced the development or implementation of new technology. In adopting technology-forcing regulations, EPA has generally followed the lead of California. Because of its more severe air pollution and its pioneering role in establishing motor vehicle emission control requirements in the 1960s, California is allowed to adopt standards more stringent than federal requirements. The state must apply for a waiver of federal preemption under CAA Section 209(b) in order to enforce its more stringent standards, which EPA is to grant if the state meets certain criteria, primarily a showing that the standards are needed to meet "compelling and extraordinary conditions." If California is granted a waiver, other states with air quality problems may adopt identical requirements, thus reinforcing the potential impact of California's standards. Besides adopting national standards for GHG emissions, EPA has granted California waivers beginning in 2009 to enforce its own GHG standards. The EPA/NHTSA joint regulations for light-duty motor vehicles were finalized April 1, 2010, and published in the Federal Register the following month. They required the vehicles (cars, SUVs, crossovers, minivans, and other light trucks) to meet combined emissions levels that EPA estimated would average 250 grams/mile of CO 2 in model year 2016. The result would be a 21% reduction in fleet-wide emissions by 2030 compared to the level that would occur in the absence of the regulations, according to EPA. NHTSA set corresponding fuel economy standards, achieving a combined estimated fuel economy of 34.1 miles per gallon for cars and light trucks in 2016. In both cases, the standards were gradually phased in; the first reduction targets affected model year 2012. In setting the GHG standards, EPA used the concept of a vehicle's "footprint" to set differing standards for different size vehicles. As explained by EPA, These standards are based on CO 2 emissions-footprint curves, where each vehicle has a different CO 2 emissions compliance target depending on its footprint value (related to the size of the vehicle). Generally, the larger the vehicle footprint, the higher the corresponding vehicle CO 2 emissions target. As a result, the burden of compliance is distributed across all vehicles and all manufacturers. Manufacturers are not compelled to build light vehicles of any particular size or type, and each manufacturer will have its own standard which reflects the vehicles it chooses it [sic] produce. (For a further discussion of vehicle footprints and fuel economy standards, see CRS Report R42721, Automobile and Truck Fuel Economy (CAFE) and Greenhouse Gas Standards .) In general, manufacturers have reduced CO 2 emissions by improving the vehicles' fuel economy, but they can also take advantage of options to generate CO 2 -equivalent credits by reducing emissions of hydrofluorocarbons (HFCs) and CO 2 through improvements in their air conditioner systems or by the use of idle reduction technologies, among other strategies. Manufacturers are also allowed to average, bank, and trade emission credits. On October 15, 2012, EPA promulgated a second phase of GHG emission standards, for MY2017-MY2025 light duty vehicles. Like the earlier standards, these were preceded by a multi-party agreement, brokered by the White House; the agreement included 13 auto manufacturers, the United Auto Workers, the state of California, and other interested parties. The manufacturers agreed to reduce GHG emissions from new cars and light trucks by about 50% by 2025, compared to 2010, with fuel economy standards rising to nearly 50 miles per gallon. CO 2 emissions will be reduced to about 160 grams/mile by 2025, under the agreement. Because of the long lead-time for the later years of these standards, as part of the MY2017-MY2025 rulemaking, EPA made a commitment to conduct a Mid-Term Evaluation (MTE) for the MY2022-MY2025 standards. Through the MTE, EPA was to determine whether the standards for MY2022-MY2025 were still appropriate given the latest available data and information, with the option of strengthening, weakening, or retaining the standards as promulgated. On November 30, 2016, EPA released a proposed determination under the MTE stating that the MY2022-MY2025 standards remained appropriate and that a rulemaking to change them was not warranted. EPA based its findings on a Technical Support Document, a previously released Draft Technical Assessment Report (which was issued jointly by EPA, NHTSA, and the California Air Resources Board (CARB)), and input from the auto industry and other stakeholders. The proposed determination opened a public comment period that ran through December 30, 2016. On January 12, 2017, the EPA Administrator made a final determination to retain the MY2022-MY2025 standards as originally promulgated. This action has significantly accelerated the original timeline for the MTE (which called for a final determination by April 2018), and EPA announced it separately from any NHTSA (fuel economy) or California (GHG standard) process. Critics reacted to the accelerated timetable swiftly, vowing to work with the new Administration to revisit EPA's determination—citing a "rush to judgment" that they argued contradicted the objectives of the One National Program. Among the potential revisions suggested by critics have been better harmonization of the existing EPA/NHTSA/CARB standards, easing the MY2022-MY2025 standards, or eliminating them entirely. It is unclear whether the Trump Administration will reconsider EPA's Mid-Term Evaluation, and if so, how quickly. The standards were promulgated in 2012 and were not modified by the MTE, so the deadlines for judicial review and for the fast-track review authority in the Congressional Review Act would appear to have lapsed. The promulgated standards could be changed through a new rulemaking, a process that normally would take a year or more and would itself be subject to judicial and congressional review upon completion. A rulemaking would probably need to be justified by new or additional data or a reasonable justification to avoid being considered arbitrary and capricious if challenged in court. With options for continued review uncertain, the role of Congress may be one of oversight, at least for now. (For additional information, see CRS Insight IN10619, EPA's Mid-Term Evaluation of Vehicle Greenhouse Gas Emissions Standards .) On the other hand, some changes that have been suggested by industry stakeholders—such as better harmonization of NHTSA's standards to align with those of EPA and California—may be accomplished without major changes to the promulgated EPA standards. Several inconsistencies to the One National Program have been noted in stakeholder petitions to the rulemaking, and include the accounting for off-cycle technologies, air conditioning efficiencies, Lifetime Vehicle Miles Traveled (VMT) values, and credit transfer and adjustment factors. For many of the suggested changes, Congress would likely need to revisit NHTSA's statutory authorities. (For additional information, see CRS Insight IN10550, Automakers Seek to Align Fuel Economy and Greenhouse Gas Regulations , by [author name scrubbed].) The light-duty vehicle rules affect a large group of emission sources that accounts for a significant percentage of total U.S. GHG emissions, but the effectiveness of the standards in reducing total GHG emissions is limited in that they apply only to new motor vehicles. The car and light truck fleets turn over slowly: in 2015, the average age of U.S. cars and light trucks was 11.5 years. Given this durability, the impact of GHG standards on the total emissions of the motor vehicle fleet will take a long time to be felt. Furthermore, if historic experience is any guide, much of the potential reduction in GHG emissions per new vehicle may be offset by growth in vehicle miles traveled. Section 202(a) of the CAA, the section that provided authority for the light-duty vehicle GHG standards, requires the Administrator to set "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" (emphasis added). This authority covers medium- and heavy-duty trucks: in fact, the December 15, 2009, endangerment and cause-or-contribute findings specifically identified the medium- and heavy-duty truck categories as among those that contributed to the GHG emissions for which it found endangerment. As a result, EPA promulgated standards for these vehicles on September 15, 2011. These standards cover MY2014-MY2018 trucks. A second round of standards, covering trucks from MY2021-MY2027 and trailers from MY2018-MY2027, was promulgated in October 2016. Separate from EPA's authority to set GHG standards, the Energy Independence and Security Act of 2007 (EISA, P.L. 110-140 ), directed NHTSA to study the potential for fuel efficiency standards for medium- and heavy-duty trucks, and, if feasible, set efficiency standards reflecting the "maximum feasible improvement." Thus, as with light duty vehicles, in order to harmonize their GHG and fuel efficiency requirements, EPA and NHTSA have cooperated on the setting of standards. Medium- and heavy-duty trucks are trucks with a gross vehicle weight of 8,500 pounds or more. The largest emitters, tractor-trailers (Class 8b trucks), account for roughly 30% of the total number of medium- and heavy-duty trucks, but, because they are heavier and are driven longer distances, they consume 67% of all fuel used by these vehicles and presumably emit about the same percentage of medium- and heavy-duty trucks' GHGs. As shown in Table 1 , medium- and heavy-duty trucks emitted more than 400 million metric tons of GHGs in 2014, nearly 27% of GHG emissions from motor vehicles. Between 1990 and 2014, emissions from these trucks grew 76%, the fastest growth for any major category of GHG sources. (See Table 1 .) The EPA Administrator is given substantial leeway in the design and implementation of motor vehicle regulations. The act states that the Administrator may establish categories for purposes of regulation based on "gross vehicle weight, horsepower, type of fuel used or other appropriate factors." In addition, he may delay the effective date of regulations as long as he finds necessary "to permit the development and application of the requisite technology, giving appropriate consideration to the cost of compliance within such period." Using this authority in regulating conventional pollutants, EPA has used weight or power classifications to set differing levels of emission standards, particularly for trucks; it has given manufacturers as much as four years lead time to develop emission controls; and it has set different standards based on the type of fuel an engine uses. Except for specific conventional pollutants mentioned in Section 202, the act does not specify a level of stringency (e.g., best available control technology) for prospective regulations. EPA and NHTSA jointly promulgated GHG and fuel economy standards on September 15, 2011. The standards divided trucks into three main categories: (1) heavy-duty pickup trucks and vans; (2) combination tractors (the power unit of a tractor-trailer combined vehicle); and (3) vocational vehicles. The standards for heavy-duty pickups and vans use an approach similar to that for light duty vehicles, in which each manufacturer would be required to meet an average standard that would vary depending on its sales mix, with higher capacity vehicles (based on payload, towing capacity, and 4-wheel drive) having less stringent targets. The standards, which are being phased in from 2014 to 2018, are estimated by EPA to cut GHG emissions an average of 17% in diesel vehicles when fully implemented in 2018, and 12% in comparable gasoline-powered vehicles, compared to a model year 2010 baseline. For the other categories of trucks, referred to as vocational vehicles and combination tractors, the standards vary significantly depending on the size of the truck. These standards are expected to reduce GHG emissions up to 23% for combination tractors and 6% to 9% for vocational vehicles by model year 2017, according to the final rule. In addition to engine emission standards, the phase 1 truck rule set a standard for refrigerant leaks, in order to address emissions of HFC greenhouse gases. But trailer design, a major source of efficiency losses (and, thus, higher GHG emissions), was not addressed in the MY2014-MY2018 standards. According to EPA, Trailers are not covered under these rules, due to the first-ever nature of this program and the agencies' limited experience working in a compliance context with the trailer manufacturing industry. However, because trailers do impact the fuel consumption and CO2 emissions from combination tractors, and because of the opportunities for reductions, we intend to include them in a future rulemaking. On February 18, 2014, President Obama directed the two agencies to develop a second round of GHG and fuel efficiency standards for medium- and heavy-duty trucks. The standards were promulgated on October 26, 2016. The new standards cover MY2018-2027 for certain trailers and MY2021-2027 for semi-trucks, large pickup trucks, vans, and all types and sizes of buses and work trucks. According to EPA, The Phase 2 standards are expected to lower CO 2 emissions by approximately 1.1 billion metric tons, save vehicle owners fuel costs of about $170 billion, and reduce oil consumption by up to 2 billion barrels over the lifetime of the vehicles sold under the program. EPA projects the total cost of the rule at $29-$31 billion over the lifetime of MY2018-MY2029 trucks. The standards will increase the cost of a long haul tractor-trailer by as much as $13,500 in MY2027, according to the agency, but the buyer would recoup the investment in fuel-efficient technology in less than two years through fuel savings. In EPA's analysis, fuel consumption of 2027 model tractor-trailers will decline by 34% as a result of the rule. Given the new Administration's general opposition to GHG regulations and the opposition to GHG rules among many in the leadership of the 115 th Congress, there could be efforts to review or modify the truck regulations. If revocation is the goal, one route would be to disapprove the standards under the expedited procedures of the Congressional Review Act (CRA). The CRA provides that, if Congress passes a joint resolution disapproving a rule and the resolution becomes law, the rule cannot take effect or continue in effect. Also, the agency may not reissue that rule or any substantially similar one, except under authority of a subsequently enacted law. As explained in more detail in other CRS analyses, under the CRA, an expedited procedure for a resolution disapproving a rule may be used in the Senate at any time within 60 days of Senate session after the rule in question has been published in the Federal Register and received by both houses of Congress. During this period of time, a CRA resolution may not be amended or filibustered. CRS has concluded elsewhere that regulations promulgated and received in Congress on or after June 13, 2016, are eligible to use these fast-track Senate procedures during the first 60 legislative days of the 115 th Congress. After that, the truck and trailer rule could still be modified through ordinary legislation or through a rider on an appropriations bill, but doing so might require 60 votes for Senate consideration. In general, however, the truck standards—with the exception of the portion dealing with trailers—have been well-received, leaving in question whether general opposition to GHG rules will shape Congress's reaction more than the views of the affected industries. The American Trucking Associations, for example, described themselves as "cautiously optimistic" that the rule would achieve its targets: "We are pleased that our concerns such as adequate lead-time for technology development, national harmonization of standards, and flexibility for manufacturers have been heard and included in the final rule." The Truck and Engine Manufacturers Association, while describing itself as "in the process of reviewing" the final rule, highlighted its work providing input to assure that EPA and DOT established a single national program, and concluded: "A vitally important outcome is that EPA and DOT have collaborated to issue a single final rule that includes a harmonized approach to greenhouse gas reductions and fuel efficiency improvements." As of the filing deadline (December 27, 2016—60 days after the rule's promulgation in the Federal Register ), only two organizations had filed petitions for judicial review: the Truck Trailer Manufacturers Association and the Racing Enthusiasts and Suppliers Coalition. On August 15, 2016, EPA issued a finding that greenhouse gas emissions (including CO 2 emissions) from commercial aircraft contribute to the pollution that causes climate change and endangers the health and welfare of Americans. The finding, under Section 231 of the Clean Air Act, is the precondition for GHG emission standards for commercial aircraft. U.S. aircraft emit roughly 11% of GHG emissions from the U.S. transportation sector and 29% of GHG emissions from all aircraft globally. In October 2016, the International Civil Aviation Organization (ICAO) agreed on international CO 2 standards for aircraft, beginning in 2020. ICAO is a specialized body of the United Nations with 191 member states. For the past five years, ICAO had been working with the aviation industry and other stakeholders to develop international CO 2 emission standards for aircraft engines. EPA and the Federal Aviation Administration, representing the United States, participated in ICAO's process. In October 2016, ICAO's governing council also agreed on a system for offsetting future carbon emissions from aviation—dubbed the Market-Based Mechanism, or MBM. EPA's endangerment finding lays the necessary foundation for the adoption and implementation of domestic aircraft CO 2 standards, in accordance with the Clean Air Act and the ICAO agreement. The MBM agreed to in the ICAO process is voluntary for the next decade and has been agreed to by the U.S. industry. In the United States, aircraft of all kinds were estimated to emit between 2.2% and 3.2% of the nation's total greenhouse gas emissions in 2014. When other factors are considered, the impact of U.S. aviation on climate change is perhaps twice that size. These factors include the contribution of aircraft emissions to ozone formation, the water vapor and soot that aircraft emit, and the high altitude location of the bulk of aircraft emissions. (For additional information on aircraft GHG emissions, see CRS Report R40090, Aviation and Climate Change .) The EPA and ICAO actions come years after five states and seven other parties petitioned EPA to address aircraft GHG emissions (see Table 2 ). Specifically, the petitions asked that EPA make a finding that aircraft GHG emissions endanger public health or welfare, and that the agency adopt regulations that allow a range of compliance approaches: these might include emission limits, operational practices, fees, a cap-and-trade system, minimizing engine idling time, employing single engine taxiing, or use of ground-side electricity measures to replace the use of fuel-burning auxiliary power units at airport gates. EPA has authority to regulate emissions from aircraft under Section 231 of the CAA. The language is similar to that for other mobile sources. It requires the Administrator to issue standards for the emission of any air pollutant from any class or classes of aircraft engines which, in his judgment, causes or contributes to air pollution which may reasonably be anticipated to endanger public health or welfare. The regulations are to take effect "after such period as the Administrator finds necessary ... to permit the development and application of the requisite technology, giving appropriate consideration to the cost of compliance." Compared to other mobile sources, EPA's CAA authority vis-à-vis aircraft and aircraft engines contains an important difference: the Administrator must consult with the Administrator of the Federal Aviation Administration and the Secretary of Transportation in developing emission standards, and is not allowed to impose new standards if doing so would significantly increase noise and adversely affect safety. The President may also disapprove any such standards if the Secretary of Transportation finds that they would create a hazard to aircraft safety. EPA has rarely regulated emissions from aircraft without first negotiating international agreements through ICAO. ICAO's agreements on standards for conventional pollutants from aircraft, unlike EPA's regulation of the same pollutants from motor vehicles, has consistently avoided forcing technology. The most recent standards for nitrogen oxides, for example, essentially ratified what the principal aircraft manufacturers had already achieved. Besides petitioning EPA for action on aviation emissions, environmental groups brought suit in the District Court for the District of Columbia seeking to force EPA to respond to their petitions on aircraft (as well as their petitions on marine vessels, and nonroad engines and vehicles). On July 5, 2011, the court found that EPA has a mandatory duty to determine whether aircraft emissions endanger public health or welfare, and may be sued for unreasonable delay in doing so. In March 2012, however, the court ruled that plaintiffs had not shown that EPA had unreasonably delayed such a decision. By issuing an endangerment finding for GHG emissions from commercial aircraft, EPA has now taken the first step toward setting aircraft GHG emission standards. The endangerment finding itself, although it does not impose standards, is a final rule. As explained earlier, final rules promulgated on or after June 13, 2016, are eligible for fast-track procedures in the Senate under the Congressional Review Act; a CRA resolution of disapproval overturning the rule could be approved by a simple majority, rather than the 60 votes generally needed to invoke cloture. Even more than the truck standards discussed earlier, however, the endangerment finding was welcomed by the affected industries, who participated in the ICAO process that has led to EPA's action. In response to EPA's issuance of the finding, an Airlines for America representative was reported as saying: As aviation is a global industry, with airlines operating internationally and manufacturers selling their aircraft in international markets, it is critical that aircraft emissions standards be set at the international level and not imposed unilaterally by one country or set of countries. ... Thus, we commend EPA's action, which will enable the ICAO [carbon dioxide] certification standard for future aircraft to be adopted into U.S. law consistent with the Clean Air Act, U.S. treaty obligations and in harmony with the international community." Following the Massachusetts v. EPA decision, the agency received at least 12 petitions asking it to regulate GHGs from other categories—all but two of the 12 focused on mobile sources and their fuels (see Table 2 ). These petitions covered aircraft, ocean-going ships and their fuels, motor fuels in general, locomotives, and nonroad vehicles and engines—a category that includes construction equipment, farm equipment, logging equipment, outdoor power equipment, forklifts, marine vessels, recreational vehicles, and lawn and garden equipment. The specifics of the CAA sections that give EPA authority to regulate pollution from these sources vary somewhat, but it has been generally believed that the endangerment finding and decision to regulate GHGs in response to the motor vehicle petition could provide precedents for GHG emission standards from other categories of sources. With that in mind, we look at other mobile source categories, the authorities provided under Title II for each, and what EPA's use of these authorities for conventional pollutants emitted by these sources indicates with regard to its ability to regulate greenhouse gases. Three of the petitions to EPA asking the agency to control greenhouse gas emissions concern ocean-going ships (also referred to as marine engines and vessels) and (in two of the petitions) their fuel. Although there is a wide range of estimates, the International Maritime Organization's consensus is that international shipping emitted 796 million metric tons of carbon dioxide, 2.2% of global CO 2 emissions, in 2012. Including domestic shipping and the CO 2 -equivalent of other GHGs emitted by ships, the amount would increase to 961 million metric tons, 2.5% of global emissions. At these levels, only five countries (the United States, China, Russia, India, and Japan) individually account for a higher percentage of the world total of CO 2 emissions. In addition to the CO 2 emissions, the low-quality bunker fuel that ships use and the general absence of pollution controls result in significant emissions of black carbon, which also contributes to climate change. Thus, the total impact of ships on climate may be greater than the 2.5% estimate above. The authority to control pollution from ships is found in Section 213(a)(4) of the CAA, which provides general authority to the Administrator to promulgate standards for emissions other than carbon monoxide, oxides of nitrogen, and volatile organic compounds from "nonroad engines and vehicles." Fuels are regulated separately under Section 211 of the act. The language of Section 213 is similar to that for new motor vehicles in Section 202, except that in place of the words "cause, or contribute," Section 213 uses the phrase "significantly contribute": if the Administrator determines that emissions of GHGs from ships significantly contribute to air pollution which may reasonably be anticipated to endanger public health or welfare, he may promulgate such regulations as he deems "appropriate." Except for the specific conventional pollutants mentioned in Section 213(a)(2), there is no level of stringency (such as best available control technology) specified for prospective regulations. The Administrator may establish classes or categories of ships for the purposes of regulation. There is no deadline for the promulgation of standards, and in setting them, the Administrator may take into account costs, noise, safety, and energy factors associated with the application of technology. If the Administrator were to conclude that GHG emissions from ships significantly contribute to air pollution reasonably anticipated to endanger public health or welfare, a wide variety of measures might be undertaken to reduce emissions from shipping, ranging from simple operational measures, such as reducing speed or using cleaner fuels, to various hull and propeller design features that would increase fuel economy. Reducing speed can save substantial amounts of fuel. A.P. Moller-Maersk, which operates the world's largest fleet of containerships, has reported that "reducing speed 5-10% does increase the number of days at sea, but reduces fuel consumption and CO 2 emissions by more than 15%." Over the period 2007-2015, Maersk reduced its CO 2 emissions per container shipped by 42%. The company has set a target of 60% reduction per container as compared to 2007 emissions by 2020. In addition to reducing speed, the company has reduced emissions through "operational optimization," newer (more efficient) vessels, and reductions of energy use in ports. The petitions to EPA mentioned improved fleet deployment planning, use of shore-side power while in port, heat recovery systems, the use of sails as supplemental propulsion sources, and NOx controls, such as selective catalytic reduction (SCR) or exhaust gas recirculation, as potential emission control measures. A complicating factor in the regulation of emissions from ocean-going vessels would be that, for the most part, their GHG emissions occur in international waters, and the sources (the ships) are not registered in the United States: according to California's petition, 95% of the fleet calling on U.S. ports is foreign-flagged. The petitioners asserted that these factors are not a bar to EPA regulation, however, citing as precedent a Supreme Court case that held that the Americans with Disabilities Act (ADA) could be applied to foreign-flagged cruise ships so long as the ADA-required accommodations did not interfere with the ships' internal affairs or require major, permanent modifications to the ships. The shipping industry has generally opposed international regulation of ships' GHG emissions, but following the United Nations' climate agreement reached in Paris in December 2015, the International Chamber of Shipping urged the International Maritime Organization (IMO) to adopt a sector-wide pledge to reduce international shipping emissions. The IMO's Environment Protection Committee met in April and October 2016, but took no action on GHGs other than to require large ships (ships of greater than 5,000 gross tons) to report their annual CO 2 emissions and fuel consumption. In addition to petitioning for regulation of emissions from ships, the petitions submitted by California and Oceana et al., stated that EPA should regulate the composition of marine shipping vessel fuel to control global-climate-change-related emissions, or should require use of marine diesel fuel oil instead of bunker fuel. The purpose would be to limit the sulfur content of marine fuels and reduce NOx emissions. We discuss EPA's authority to regulate fuels in a separate section below, but note here that EPA, the state of California, and the International Maritime Organization have all moved forward with regulations to limit the sulfur content of bunker fuel for the purpose of reducing conventional pollutants. California's low sulfur fuel requirements went into effect July 1, 2009. In addition, on March 26, 2010, the International Maritime Organization (IMO) approved an EPA proposal that the entire U.S. coastline except portions of Alaska be designated as an Emission Control Area, subject to lower sulfur limits in bunker fuel. On July 15, 2011, the IMO officially added the waters around Puerto Rico and the U.S. Virgin Islands as Emission Control Areas. Sulfur emissions form fine particles of sulfate in the atmosphere, with significant impacts on public health and welfare. (For a further discussion of these impacts, see CRS Report RL34548, Air Pollution and Greenhouse Gas Emissions from Ships .) Although harmful as a conventional pollutant, the impact of sulfur emissions on climate is more complicated. Sulfate particles are thought by most experts to have a cooling effect on the atmosphere, since they tend to reflect solar radiation back into space rather than absorbing it. On the other hand, removing sulfur might be necessary to prevent the fouling of pollution control equipment that reduces other pollutants that do lead to warming. Section 213 of the Clean Air Act can also be used to regulate other nonroad vehicles and engines. An endangerment finding similar to that promulgated for motor vehicles would first be required, following which the Administrator could promulgate such regulations as he deems appropriate to control emissions from the classes or categories of nonroad engines that he determines "significantly contribute" to the air pollution that endangers public health or welfare. The Administrator is to take into account costs, noise, safety, and energy factors in setting standards. There is no deadline for setting standards. The nonroad sector is a broad category that includes construction equipment, farm equipment, forklifts, outdoor power equipment, lawn and garden equipment, and recreational vehicles. This group accounted for 199.7 million metric tons of CO 2 emissions in 2007, according to the two petitions requesting regulation (see Table 3 ), 3.3% of total U.S. emissions of CO 2 in that year. According to the ICTA petition, GHG emissions from the nonroad sector increased 49% between 1990 and 2005, a higher rate of emissions increase over the same period than for on-road vehicles (32%), aircraft (3%), boats and ships (36%), and rail (32%). Given their smaller impact on overall emission levels, EPA has been slower to regulate conventional (criteria) pollutants from nonroad engines than from motor vehicles. Many of these engines had few emission control requirements for as many as 25 years after the regulation of automobiles. In the last decade, however, often following the lead of California, EPA has promulgated standards for many nonroad categories. Some of these standards, particularly for diesel-powered equipment and for lawn and garden equipment, have been technology-forcing. Others, such as for snowmobiles, have been less so. In general, given the wide variety of engine types and sizes and the configurations of the equipment itself, the agency has based its standards on a review of individual subcategories and the technologies available to reduce emissions from specific types of machinery or equipment, rather than applying one across the board standard. Presumably, any GHG standards for this sector would take the same approach. On September 21, 2010, EPA received a petition from three environmental organizations to regulate GHG emissions and black carbon from locomotives. In 2014, locomotives emitted 47.6 million metric tons of greenhouse gases. Although this is less than 1% of total U.S. GHG emissions, GHG emissions from railroads increased by 22% between 1990 and 2014, three times the rate of increase for total U.S. emissions. In addition, locomotives emit substantial amounts of black carbon, which is thought to have significant global warming potential through its ability to absorb solar radiation and to reduce the reflectivity of snow and ice. According to a report from NASA's Goddard Institute for Space Studies cited in the locomotive petition, "… black soot may be responsible for 25 percent of observed global warming over the past century." As a result, in addition to requesting that EPA set GHG emission standards for locomotives, the petition asked EPA to set standards for locomotives' black carbon emissions. The CAA requires EPA to set emission standards for new locomotives (and new engines used in locomotives) in Section 213(a)(5). Unlike almost every other CAA section dealing with mobile sources, the locomotive subsection does not require an endangerment finding for the Administrator to act. Instead, it requires the Administrator to set standards that achieve the greatest degree of emission reduction achievable through the application of technology which he determines will be available, giving appropriate consideration to cost, noise, energy, and safety factors. As it did with the medium- and heavy-duty truck category, EPA discussed several potential strategies for reducing GHG emissions from locomotives in its July 2008 Advance Notice of Proposed Rulemaking (ANPR). The ANPR identified more than 20 strategies for reducing emissions from rail transport, including idle reduction equipment, auxiliary power units, hybrid engines, regenerative braking, and reduction of refrigerant leaks from railcars. EPA has not taken action to implement these GHG reduction strategies. Fuel regulation, whether of bunker fuel, gasoline, or any other type of fuel used in motor vehicles, their engines, or non-road vehicles and engines, is authorized under Section 211 of the CAA. Section 211 gives the Administrator authority to control or prohibit the manufacture and sale of any fuel or fuel additive if the Administrator concludes that its emission products may endanger public health or welfare. As with the regulation of engines and vehicles themselves, the Administrator is given substantial leeway in the design and implementation of fuel regulations, and there is no deadline for their promulgation even after an endangerment finding is made. GHG emissions from fuels have already been targeted for regulation by the state of California. On April 15, 2010, California's Office of Administrative Law approved regulations to implement the California Low Carbon Fuel Standard (LCFS), which had been under development since 2007. The standard's goal is to reduce GHG emissions from transportation fuels per unit of energy 10% by 2020. The regulations address emissions from the production, transportation, and consumption of gasoline, diesel fuel, and their alternatives, including biofuels. They envision compliance both through the use of lower carbon fuels and through the development of more efficient, advanced-technology vehicles, such as plug-in hybrids, electric vehicles, and hydrogen fuel cells. As has been the case with motor vehicles, California has often led the way in the development of cleaner fuels through technology-forcing regulation, with U.S. EPA later adopting similar standards. Thus, many have viewed the Low Carbon Fuel Standard as the prototype of another possible use of existing CAA authority to regulate greenhouse gas emissions nationally. On July 29, 2009, the Institute for Policy Integrity at NYU Law School petitioned EPA to establish a cap-and-trade system to limit greenhouse gas emissions from fuels used in motor vehicles, nonroad vehicles, and aircraft. Regulation of fuels would be a way for California or U.S. EPA to obtain reductions from existing vehicles and engines. As noted earlier, the slow turnover of the vehicle fleet means that emission reductions from new vehicles will only gradually affect emission levels from the fleet as a whole. By requiring low carbon fuels, California and EPA could obtain GHG reductions from the entire fleet more quickly. On the other hand, measuring the carbon content of fuels is more complicated than it may seem, particularly if one considers the life-cycle emissions, including indirect impacts of production. EPA has been embroiled in a controversy over this issue already, as it attempted to develop a methodology for measuring greenhouse gas emissions from biofuels, as required by the Energy Independence and Security Act of 2007 ( P.L. 110-140 ). For regulations implementing that provision, EPA developed and later modified a methodology to measure the GHG effects of indirect land-use changes, such as the switching of land from forest to cropland. EPA did not pursue an LCFS under the Obama Administration and, given President Trump 's general position on GHG regulation, would appear unlikely to do so in a Trump Administration. Table 4 summarizes EPA's existing authorities over mobile source GHG emissions and the emissions of the sources discussed in this report. Given the Supreme Court's remand in Massachusetts v. EPA , the agency initially focused its efforts on motor vehicles, which account for three-quarters of all U.S. mobile source GHG emissions. By issuing endangerment findings similar to the one it issued for motor vehicles, EPA could move forward to control GHG emissions from other categories of mobile sources and/or their fuels. As discussed, the agency took a step along this path by issuing an endangerment finding for aircraft on August 15, 2016. This action was not particularly controversial, and was not challenged by aircraft manufacturers or airlines. The aircraft manufacturing and aviation industries have participated in international negotiations that have produced agreement on emission standards for new aircraft and a voluntary system to offset what might otherwise be a growth in emissions from air travel. As shown in Table 4 , aircraft are the largest source of mobile source GHG emissions after cars and trucks. Although a framework for regulating GHG emissions from the largest categories of mobile sources is now in place, Congress and the Trump Administration face a number of questions regarding GHG emissions from mobile sources in the first months of 2017. In campaigning for election, President Trump promised to overturn EPA regulations, including those addressing GHGs. Congressional Republicans have also been critical of Obama Administration actions addressing climate change. The early months of the new Congress present the opportunity under the Congressional Review Act to rescind EPA regulations promulgated since June 2016, which could include the Phase 2 emission standards for medium- and heavy-duty trucks and the endangerment finding for aircraft. These rules are not particularly controversial with the regulated industries. Somewhat more controversial is the Mid-Term Evaluation of the light-duty vehicle GHG emissions standards for MY2022-MY2025. It is unclear whether that determination, which left the already promulgated GHG standards in place, could be subject to the Congressional Review Act. As shown in Table 4 , after motor vehicles and aircraft, other mobile source categories are less significant: most of them accounted for less than 1% of total U.S. GHG emissions in 2014. Thus, rather than develop standards for additional mobile sources, EPA has expanded its focus to stationary sources. Stationary sources account for about 70% of the nation's GHG emissions; within that group electric power plants account for about 30% of all U.S. GHG emissions, a higher percentage of the nation's total than all mobile sources combined. To address power plant emissions, EPA promulgated the Clean Power Plan (CPP) on October 23, 2015. Although the CPP is currently stayed by the Supreme Court pending the outcome of judicial proceedings, resolving its future arguably poses a higher priority for the Trump Administration and many in Congress than the pending decisions regarding mobile sources. (For additional discussion of the Clean Power Plan, see CRS Report R44341, EPA's Clean Power Plan for Existing Power Plants: Frequently Asked Questions . ) 

Summary: This report discusses EPA's authority to regulate greenhouse gas (GHG) emissions as it pertains to mobile sources, including cars, trucks, aircraft, ships, locomotives, nonroad vehicles and engines, and their fuels. The Supreme Court held in 2007 that the Clean Air Act (CAA) authorizes the agency to address GHG emissions. The key to using this CAA authority was for the EPA Administrator to find that GHG emissions endanger public health or welfare, a step taken in December 2009. Under the Trump Administration, it is unclear whether this authority will be put to further use. Other questions concern what steps EPA and Congress may take with regard to already promulgated—but not yet implemented—standards for GHG emissions from cars and trucks. In principle, the auto manufacturing and trucking industries have been supportive of EPA's GHG regulations, in part because of concerns that states would implement a patchwork of standards in the absence of federal action. As the standards have been implemented, however, industry concerns have arisen regarding a lack of harmonization between EPA's GHG standards, fuel efficiency (CAFE) standards administered by the National Highway Traffic Safety Administration (NHTSA), and related California GHG and fuel efficiency programs. One issue concerns the Model Year (MY) 2017-2025 light-duty vehicle regulations. Under these standards, GHG emissions from new light-duty vehicles (i.e., cars, SUVs, crossovers, minivans, and most pickup trucks) will be reduced about 50% compared to 2010 levels, and average fuel economy will rise to nearly 50 miles per gallon by 2025. When EPA and NHTSA promulgated the standards in 2012, EPA committed to a Midterm Evaluation (MTE) of the 2022-2025 portion of the GHG standards. This evaluation was completed on January 12, 2017, with EPA deciding to maintain the standards as promulgated. Given industry concerns about the standards, there is speculation as to whether the Administration will reconsider the MTE decision. A second issue concerns GHG emission and fuel economy standards for medium- and heavy-duty trucks. EPA and NHTSA promulgated a second phase of these standards on October 25, 2016, covering trucks and engines beginning with the 2021 model year and truck trailers beginning in 2018. These standards could be reconsidered by the two agencies, or Congress could review them under the Congressional Review Act. GHG emissions are directly related to fuel combustion. In order to reduce GHG emissions, EPA expects the standards to increase fuel efficiency, lowering oil consumption by up to 2 billion barrels over the lifetime of 2018-2029 trucks and saving vehicle owners about $170 billion in fuel costs as a result. In general, the truck standards – with the exception of the portion dealing with trailers—have been well-received, leaving in question whether general opposition to GHG rules will shape Congress's and the new Administration's reaction to the rules more than the views of the affected industries. A third potential issue concerns GHG emission standards for aircraft. In October 2016, the International Civil Aviation Organization (ICAO) agreed on international carbon dioxide (CO2) emission standards for aircraft, beginning in 2020, and on a system for offsetting future CO2 emissions from aviation. The emission standards would be implemented in the United States by EPA regulations issued under the CAA. U.S. airlines and aircraft manufacturers participated in the ICAO negotiations and have been supportive of the resulting agreements; whether EPA actions to implement them would run contrary to the President's and the Administration's broader views on regulation and climate change is unclear. In addition to a discussion of these three issues, this report provides background on GHG emissions from other mobile sources, including ships, nonroad vehicles and engines, locomotives, and fuels.