	Introduction

This report discusses the Environmental Protection Agency's (EPA's) authority to regulate greenhouse gas (GHG) emissions from mobile sources of pollution. 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. 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. 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. 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. 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. 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 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. 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. It is unclear whether that determination, which left the already promulgated GHG standards in place, could be subject to the Congressional Review Act.
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.