GHG Mitigation under the Clean Air Act

GHG Emission Standards for Mobile Sources

Greenhouse Gas Emission Standards by Model Year

EPA is required by section 202(a) of the Clean Air Act to set technology-based standards for classes of vehicles that cause or contribute to air pollution that endangers the public health or welfare. Motor vehicles emissions are regulated by the EPA in coordination with the National Highway Traffic Safety Administration (NHTSA), an agency within the Department of Transportation, which regulates fuel economy standards.

Model Year 2027–2032 (Heavy-Duty)

On January 22, 2025, H.J. Res. 26, a joint resolution of disapproval under the Congressional Review Act, was introduced in the House. If passed, it would void EPA's final rule on greenhouse gas emissions standards for heavy-duty vehicles under the Clean Air Act. 89 Fed. Reg. 29,440 (Apr. 22, 2024).

Second Trump Administration (2025-2028)

Model Year 2027–2032 (Heavy-Duty)

On January 22, 2025, H.J. Res. 26, a joint resolution of disapproval under the Congressional Review Act, was introduced in the House. If passed, it would void EPA's final rule on greenhouse gas emissions standards for heavy-duty vehicles under the Clean Air Act. 89 Fed. Reg. 29,440 (Apr. 22, 2024).

Biden Administration (2021-2024)

Model Year 2027–2032 (Heavy-Duty)

On April 22, 2024, EPA published a final rule establishing  phase 3 GHG emissions standards for model year 2027 to 2032 heavy-duty vehicles. 89 Fed. Reg. 29,440 (Apr. 22, 2024). The rule sets more stringent carbon dioxide emissions standards for heavy-duty vehicles—such as school buses, delivery trucks, tractor-trailers, and more—than the previous phase 2 regulation. (See Nebraska v. EPA in “Litigation” below.)

Model Year 2027–2032 (Light- and Medium-Duty)

On April 18, 2024, EPA published a final rule establishing emissions standards for model year 2027-2032 light- and medium-duty vehicles. 89 Fed. Reg. 27,842 (Apr. 18, 2024). The standards cover multiple pollutants, including GHGs. The standards take a fleetwide-averaging approach to set more stringent greenhouse gas and criteria pollutant standards than previous regulations. (See Kentucky v. EPA in “Litigation” below.)

Model Year 2027 and later (Heavy-Duty)

On January 24, 2023, EPA published a final rule with emissions standards for model year 2027 and later  highway heavy-duty vehicles and engines. 88 Fed. Reg. 4,296 (Jan. 24, 2023). The standards cover GHGs and other pollutants, including ozone and particulate matter. The rule alters many aspects of the agency’s heavy-duty emission control program, including standards, test procedures, useful life, warranty, and other requirements, and update the agency’s Phase 2 Heavy-Duty Vehicle Greenhouse Gas Emissions program by proposing additional greenhouse gas reductions in the model year 2027 timeframe.

Model Year 2023–2026 (Light-Duty)

On December 30, 2021, EPA published a final rule with new GHG emission standards for model year 2023 and later light-duty vehicles. The rule revises current GHG standards beginning in model year 2023 and increases in stringency between 5 and 10 percent each year from 2023 through 2026. The standards are expected to result in average fuel economy label values of 40 mpg, while the previous standards would achieve only 32 mpg in MY 2026. Further, NHTSA promulgated related CAFE standards. (See Texas v. EPA in “Litigation” below.)

Model Year 2021 and Later (Heavy-Duty)

On June 29, 2021, EPA published a final rule revising the test procedures for heavy-duty engines and vehicles and the certification procedures for exhaust emissions standards applicable to light-duty vehicles, heavy-duty vehicles, highway motorcycles, locomotives, marine engines, other nonroad engines and vehicles, and stationary engines. 86 Fed. Reg. 34,308 (June 29, 2021).

Model Year 2018-2027 (Medium- and Heavy-Duty)

On July 28, 2022, EPA published corrections, clarifications, flexibilities, and adjustments to a 2016 final rule establishing the Phase 2 Heavy-Duty Vehicle Greenhouse Gas Emissions program.

First Trump Administration (2017-2020)

Model Year 2017-2025 (Light-Duty)

In 2020, EPA finalized a series of technical corrections to the final rule promulgated by the Obama EPA in October 2012 establishing GHG emission standards from model year 2017 and later light-duty vehicles. A first set of technical corrections were published on April 23, 2020 and a second set on August 31, 2020. 85 Fed. Reg. 22,609 (Apr. 23, 2020); 85 Fed. Reg. 53,676 (Aug. 31, 2020). Also on August 31, 2020, EPA published a proposed rule with additional technical corrections to the test procedures used in the Corporate Average Fuel Economy program. 85 Fed. Reg. 53,722 (Aug. 31, 2020).

Model Year 2021 and Later (Heavy-Duty)

On May 12, 2020, EPA published a proposed rule to make “changes to test procedures for heavy-duty engines and vehicles to improve accuracy and reduce testing burden.” 85 Fed. Reg. 28,140 (May 12, 2020). The new testing procedures would apply to criteria pollutants and GHG emissions.

Model Year 2021–2026 (Light-Duty)

On March 31, 2020, EPA and NHTSA finalized the SAFE Rule. 85 Fed. Reg. 24,174 (Mar. 31, 2020). The rule was proposed to weaken the light-duty emission and fuel economy standards for model year 2021-2026 light duty vehicles and revoke the waiver allowing California to establish more stringent standards which other states could then adopt. Under the final rule, GHG emissions and fuel economy standards will become 1.5% more stringent every year from 2020-2026. (See Competitive Enterprise Institute v. National Highway Traffic Safety Administration in “Litigation” below.)

On September 19, 2019, EPA and NHTSA finalized the first portion of the SAFE Rule revoking the California waiver. 84 Fed. Reg. 51,310 (Sept. 27, 2019).

Model Year 2022–2025 (Light-Duty)

On March 22, 2017, EPA and NHTSA announced their intention to reconsider the GHG emissions standards for model year 2022-2025 light duty vehicles. 82 Fed. Reg. 14,671 (Mar. 22, 2017). On August 21, 2017, NHTSA and EPA published a request for public comment on the reconsideration of GHG emissions standards for model year 2022-25 light-duty vehicles. 82 Fed. Reg. 39,551 (Aug. 21, 2017). On April 13, 2018, EPA announced that it had completed its mid-term evaluation of the GHG emissions standards for light-duty vehicles, and determined the standards were too stringent, and that it would initiate a new rulemaking proceeding to revise the standards. 83 Fed. Reg. 16,077 (Apr. 13, 2018). In the notice, EPA also withdrew its previous final determination issued on January 12, 2017.

Obama Administration (2009-2016)

Model Year 2018-2027 (Medium- and Heavy-Duty)

On October 25, 2016, EPA and NHTSA extended GHG and fuel economy standards for medium- and heavy-duty vehicles through model years 2018-2027 for certain trailers, and model years 2021-2027 for semi-trucks, large pickup trucks, vans, buses, and work trucks. 81 Fed. Reg. 73,478 (Oct. 25, 2016). (See Truck Trailer Manufacturers Association v. EPA in “Litigation” below.)

Model Year 2017-2025 (Light-Duty)

On October 15, 2012, EPA and NHTSA extended GHG emission and fuel economy standards for light-duty vehicles to model years 2017-2025. 77 Fed. Reg. 62,624 (Oct. 15, 2012). The rule called for a mid-term evaluation to determine whether the standards should be revised for light duty vehicles MY 2022-2025.

Model Year 2014-2018 (Medium- and Heavy-Duty)

On September 15, 2011, EPA and NHTSA finalized GHG emission standards and fuel economy standards for model year 2014 - 2018 heavy-duty highway vehicles. 76 Fed. Reg. 57,106 (Sept. 15, 2011).

Fuel Economy Labeling of Motor Vehicles

On July 6, 2011, EPA and NHTSA finalized a rule to redesign and add information to the fuel economy label that is posted on the window sticker of all new cars and light-duty trucks sold in the U.S. 76 Fed. Reg. 39,477 (Jul. 6, 2011). The redesigned label provides new information to American consumers about fuel economy and consumption, fuel costs, and environmental impacts associated with purchasing new vehicles beginning with model year 2013 cars and trucks. The rule also develops new labels for certain advanced technology vehicles poised to enter the U.S. market, including plug-in hybrid electric vehicles and electric vehicles.

Model Year 2012-2016 (Light- and Medium-Duty)

On May 7, 2010, EPA and NHTSA promulgated GHG emissions and fuel economy standards applicable to model year 2021-2016 passenger cars, light-duty trucks, and medium-duty passenger vehicles. 75 Fed. Reg. 25,324 (May 7, 2010).

Litigation

Challenge to 2024 Heavy-Duty Rule - Nebraska v. EPA

On May 13, 2024, a coalition of states filed a petition for review in the Court of Appeals for the D.C. Circuit, challenging EPA’s heavy-duty emissions standards. Briefing in the case, Nebraska v. EPA (No. 24-1129), will conclude in March 2024 and oral argument has not been scheduled.

Challenge to 2024 Light- and Medium-Duty Rule - Kentucky v. EPA

On April 18, 2024, Kentucky and other states filed a petition for review in the Court of Appeals for the D.C. Circuit, challenging EPA’s light- and medium-duty emissions standards. The case has been briefed but oral argument has not yet been scheduled. 

Challenge to 2021 Light-Duty Rule - Texas v. EPA

On February 28, 2022, Texas and other states and private parties filed a petition for review of EPA’s Revised 2023 and Later Model Year Light-Duty Vehicle Greenhouse Gas Emissions Standards in the Court of Appeals for the D.C. Circuit. The consolidated case was briefed and oral argument was held in September 2023. Most recently, the parties completed supplemental briefing in August 2024.

Challenge to 2020 SAFE Rule - Competitive Enterprise Institute v. National Highway Traffic Safety Administration

On May 27, 2020, a coalition of states and a coalition of environmental groups challenged the SAFE Rule in the D.C. Circuit.

Challenge to 2016 Medium- and Heavy-Duty Rule Rule - Truck Trailer Manufacturers Association v. EPA

The Truck Trailer Manufacturers Association challenged the 2016 extended GHG and CAFE standards applying to trailers. The Truck Trailer Manufacturers Association has argued that EPA and NHTSA lack authority to regulate GHG emissions from trailers under the Clean Air Act and the Energy Independence and Security Act. On September 29, 2020, the D.C. Circuit Court of Appeals stayed the compliance dates for fuel economy regulations to the extent the regulations apply to truck trailers. On November 12, 2021, the D.C. Circuit Court of Appeals vacated all portions of the rule that apply to trailers.

California Greenhouse Gas Regulation Waivers

The Clean Air Act allows California to enact air pollution standards for motor vehicles that are more stringent than those established by EPA through the section 209 waiver program. EPA must approve a waiver before California’s rules can go into effect.

On February 14, 2025 the EPA announced that it plans to send California's waivers to Congress. The announcement indicated that "the EPA will be transmitting to Congress the Biden Administration’s rules granting waivers that allowed California to preempt federal car and truck standards promulgated by EPA and the U.S. Department of Transportation’s National Highway Traffic Safety Administration." The announcement impliedly disputes a 2023 Government Accountability Office opinion concluding that an EPA notice accepting a California waiver request is not a rule subject to review under the Congressional Review Act. 

Second Trump Administration (2025-2028)

On February 14, 2025 the EPA announced that it plans to send California's waivers to Congress. The announcement indicated that "the EPA will be transmitting to Congress the Biden Administration’s rules granting waivers that allowed California to preempt federal car and truck standards promulgated by EPA and the U.S. Department of Transportation’s National Highway Traffic Safety Administration." The announcement impliedly disputes a 2023 Government Accountability Office opinion concluding that an EPA notice accepting a California waiver request is not a rule subject to review under the Congressional Review Act. 

Biden Administration (2021-2024)

On October 20, 2023, EPA issued a notice of decision granting an authorization request by California Air Resources Board (CARB). CARB had requested authorization from EPA to implement amendments to CARB’s Ocean-Going Vessels At-Berth regulation and Commercial Harbor Craft regulation. EPA published the notice of the authorization request on March 17, 2023. 

On April 6, 2023, EPA granted three requests for waivers from California related to motor vehicle pollution control standards, including some for zero-emission vehicles. 88 Fed. Reg. 20,688 (Apr. 6, 2023). The three requests were published in the Federal Register on June 13, 2022. (See Western States Trucking Association v. EPA in “Litigation” below.)

  • Notice of Decision, 88 Fed. Reg. 20,688 (Apr. 6, 2023)
  • California State Motor Vehicle Pollution Control Standards; Heavy-Duty Vehicle and Engine Emission Warranty and Maintenance Provisions; Request for Waiver of Preemption; Opportunity for Public Hearing and Public Comment, 87 Fed. Reg. 35,760 (June 13, 2022)
  • California State Motor Vehicle Pollution Control Standards and Nonroad Engine Pollution Control Standards; The “Omnibus” Low NOX Regulation; Request for Waivers of Preemption; Opportunity for Public Hearing and Public Comment, 87 Fed. Reg. 35,765 (June 13, 2022)
  • California State Motor Vehicle Pollution Control Standards; Advanced Clean Trucks; Zero Emission Airport Shuttle; Zero-Emission Power Train Certification; Request for Waiver of Preemption; Opportunity for Public Hearing and Public Comment, 87 Fed. Reg. 35,768 (June 13, 2022)

On March 14, 2022, EPA took final action to restore California’s authority to set its own vehicle emissions standards that are more stringent than federal requirements. 87 Fed. Reg. 14,332 (Mar. 14, 2022). The March 14 Notice of Decision rescinds the 2019 waiver withdrawal, bringing the 2013 Advanced Clean Car program waiver back into force. It also rescinds the 2019 interpretation that prevented other states from adopting California’s greenhouse gas standards. (See Ohio v. EPA in “Litigation” below.)

First Trump Administration (2017-2020)

On September 19, 2019, EPA and NHTSA finalized the first part of the Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule. 84 Fed. Reg. 51,310 (Sept. 27, 2019). The rule revoked California's preemption waiver as applied to the GHGemissions standards and zero-emission vehicle mandate. One basis for EPA's action was NHTSA's new interpretation of its authority to set fuel efficiency standards as preempting states' authority to regulate GHG emissions from cars. (See Union of Concerned Scientists v. National Highway Traffic Safety Administration in “Litigation” below.)

Obama Administration (2009-2016)

On December 29, 2016, EPA granted CARB’s request to regulate GHG emissions for model year 2014 later on-road medium- and heavy-duty engines and vehicles. 81 Fed. Reg. 95,982 (Dec. 29, 2016).

On August 7, 2014, EPA granted CARB’s request to regulate GHG emissions for certain 2011-2013 model year tractors, and dry- and refrigerated-van trailers pulled by such tractors. 79 Fed. Reg. 46,256 (Aug. 7, 2014).

On January 9, 2013, EPA granted California a waiver to implement the state’s Advanced Clean Car Program. 78 Fed. Reg. 2,112 (Jan. 9, 2013). The program establishes increasingly stringent GHG emissions standards for  model year 2015-2025 model year cars and light-duty trucks, and mandates that a certain percentage of new vehicles sold produce no carbon dioxide tailpipe emissions. 

On July 8, 2009, EPA granted CARB’s waiver request to regulate GHG emissions from new motor vehicles. 74 Fed. Reg. 32,744 (July 8, 2009). CARB requested the waiver in December 2005, which EPA initially denied in March 2008. In January 2009, CARB requested that EPA reconsider its waiver denial.

Litigation

Challenge to 2023 Waiver - Western States Trucking Association v. EPA

On June 6, 2023, several states and private parties filed a petition for review in the Court of Appeals for the D.C. Circuit challenging EPA’s combined approval of California’s three waiver requests announced on June 13, 2022. In the consolidated case, the Court of Appeals issued an order on December 21, 2023 holding the case in abeyance pending resolution of Ohio v. EPA (No. 22-1081) and Texas v. EPA (No. 22-1031).

Challenge to 2022 Waiver - Ohio v. EPA

On May 13, 2022, seventeen Republican states petitioned the Court of Appeals for the D.C. Circuit for review of EPA's California waiver, alleging that the waiver violates the constitutional doctrine of equal sovereignty. On April 9, 2024, the Court of Appeals issued an opinion upholding EPA’s reinstatement of the federal preemption waiver. On July 5, 2024, Ohio and other states opposing the rule filed a petition for writ of certiorari in the Supreme Court.

Challenge to 2019 Waiver Withdrawal - Union of Concerned Scientists v. National Highway Traffic Safety Administration

On October 28, 2019, a coalition of environmental groups challenged the waiver revocation in the D.C. Circuit. California and twenty-two other states—plus Washington, D.C., New York City and Los Angeles—followed suit on November 15, 2019. On February 8, 2021, the court granted the government's motion to hold the case in abeyance while the agencies reconsider the SAFE Rule pursuant to President Biden's executive order on “Protecting Public Health and the Environment and Restoring Science to Tackle the Climate Crisis.”