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Aircraft Greenhouse Gas Emissions Regulation Gets Off the Ground—Are Other Nonroad Mobile Sources Next?

September 2014

On September 3, 2014, the U.S. Environmental Protection Agency (EPA) announced a timeframe to begin the rulemaking process to regulate greenhouse gas (GHG) emissions from aircraft.1  In a paper submitted to the United Nations International Civil Aviation Organization Committee on Aviation and Environmental Protection (ICAO/CAEP), EPA announced that it is working with the U.S. Federal Aviation Administration (FAA) within the ICAO/CAEP standard setting process to “establish international emission standards2 and related requirements” for aircraft engines and subsequently to initiate rulemakings to establish equivalent domestic standards.

The announcement is significant because it is one of the first concrete steps EPA has taken to regulate GHG emissions from any nonroad mobile source, which include, in addition to aircraft, marine vessels, off-road construction equipment, and recreational vehicles, among many others.  The announcement is also significant, however, because it is limited to aircraft and does not mention any other nonroad mobile sources of GHG emissions that EPA may have an interest in regulating.  There is a good explanation for this.  As discussed in more detail below, differences of wording in different sections of the Clean Air Act (Act), and one court's interpretation of the differences, have created the conditions under which EPA has apparently decided to tackle domestic aircraft GHG emissions first and further delay consideration of other nonroad mobile sources for another day.  When that day will be, however, is anyone's guess.

In its U.N. Paper, EPA indicated that it would propose domestic endangerment and cause/contribute findings pursuant to Section 231 of the Act by April 2015.3  Under this plan, EPA would release an Advanced Notice of Proposed Rulemaking (ANPRM) at the same time “to provide an overview of ICAO/CAEP efforts to reduce greenhouse gas emissions.”4  An endangerment finding is a determination whether a particular air pollutant “causes, or contributes to, air pollution which may reasonably be anticipated to endanger public health or welfare.”5  EPA estimated that it would take about another year after the ANPRM to finalize its findings.6  Only then would EPA be able to establish GHG emissions standards for aircraft, as it has already done for cars and trucks.

EPA noted that “there are domestic pressures from stakeholders” to initiate the rulemaking.7  It also acknowledged that it was responding to a federal district court case, Center for Biological Diversity v. EPA, which ruled that under Section 231 of the Act the EPA is required to make an endangerment finding with respect to emissions of GHGs from aircraft.8  The plaintiffs in that case recently submitted a notice of intent to sue EPA pursuant to the citizen suit provisions of the Act because of the Agency's delay in issuing the endangerment finding.9

Domestic pressure certainly remains on EPA to regulate other nonroad sources of GHG emissions, including marine vessels and tractors.  But, as explained further below, apparently because the district court in Center for Biological Diversity v. EPA interpreted EPA's endangerment finding obligations under the Act differently for those other sources, EPA is delaying additional GHG emissions rulemaking for those sources until a later date.  Some background on the case and EPA's response is below.

In Center for Biological Diversity v. EPA, plaintiffs sued EPA for failing to respond to three separate petitions submitted in 2007 for GHG emissions rulemaking to cover marine vessels, nonroad vehicles, and aircraft engines.10  They also argued that EPA was required to issue endangerment findings for all of these sources pursuant to Section 213(a)(4) (for marine vessels and nonroad vehicles) and Section 231(a)(2)(A) (for aircraft).11  In a partial motion to dismiss, EPA argued that its delay in responding to the petitions was not unreasonable and that its obligation to make endangerment finding was discretionary under both sections.12

The court ruled that EPA did indeed have discretion under Section 213 as to whether to issue endangerment findings for GHG emissions from marine vessels and nonroad vehicles.13  Section 213(a)(4) states that “[i]f the Administrator determines that any emissions . . . from new nonroad engines or vehicles significantly contribute to air pollution which may reasonably be anticipated to endanger public health or welfare, the Administrator may promulgate . . . such regulations as the Administrator deems appropriate.”14  The court stated that “[u]nlike its counterparts, [Section 213(a)](4) is simply silent as to when—or whether—EPA must make endangerment findings; it merely says what EPA ‘may' do ‘if' an affirmative finding is made.”15  The court ruled that EPA did not have discretion under Section 231 as to whether to issue an endangerment finding for GHG emissions from aircraft.  Section 231(a)(2) states that EPA “shall, from time to time, issue proposed emission standards applicable to the emission of any air pollutant . . . which in his judgment causes, or contributes to, air pollution which may reasonably be anticipated to endanger public health or welfare.”16  The court found that this directive established not only a post-endangerment duty to regulate GHG emissions, but also a non-discretionary duty to perform the endangerment finding in the first place.17

EPA subsequently agreed to respond to the petitions, and in doing so it detailed its reasons for further delay in regulating GHG emissions from any of the nonroad mobile sources.18  With respect to marine vessels and nonroad vehicles, for example, EPA stated that investing EPA resources in an endangerment finding “would detract from more pressing environmental issues in the mobile source area, including greenhouse emissions from sources that represent a greater portion of the greenhouse gas inventory.”19  In this regard, EPA was very likely referring to emissions from cars and trucks, which contribute more GHG emissions than all other mobile sources combined.20  EPA also stated that the nonroad sector encompassed a wide range of mobile sources and that rulemaking would be a very complex undertaking.21  With respect to marine vessels, EPA stated because of the “international nature of marine shipping in the United States,” EPA is pursuing a “coordinated approach” with the International Maritime Organization rather than attempt to regulate marine GHG emissions domestically through Section 213.22

With respect to aircraft, EPA stated that it intended to initiate an endangerment notice and comment proceeding after litigation and regulatory activities related to other mobile sources—principally cars and trucks—was resolved.  In particular, EPA stated that it wanted to wait for the outcome of litigation in Coalition for Responsible Regulation v. EPA.23  That D.C. Circuit case involved a challenge to EPA's endangerment finding with respect to GHG emissions from cars and trucks, which are regulated under Section 202 of the Act.  EPA stated that in any case it had no intention at the time to “commit to rulemaking with regard to controlling greenhouse gas emissions from aircraft engines.”24

The D.C. Circuit upheld EPA's endangerment finding for cars and trucks in 2012,25 and the Supreme Court denied a petition for certiorari on the case at the beginning of last year's term.26  Given that the broader scientific bases for an endangerment finding regarding GHG emissions from aircraft may be similar to those for cars and trucks, EPA may have decided that it was finally on firmer legal footing to move forward with regulation of aircraft GHG emissions now, and to set a course for finalizing the regulations over the next few years.  Absent is a clear statutory obligation to make endangerment findings with respect to other nonroad mobile sources, however, it remains to be seen when EPA will begin to consider regulating GHG emissions from those sources and what additional domestic pressure may be put on them to do so.  That, as they say, is still up in the air.


1 EPA, Greenhouse Gas Emissions from Aircraft, available at

2 EPA, U.S. Aircraft Greenhouse Gas Rulemaking Process (“U.N. Paper”), available at

3 Id.

4 Id.

5 42 U.S.C. § 7547(a)(4).  The U.S. Supreme Court held in 2007 that carbon dioxide, a major GHG, was an “air pollutant” under the Clean Air Act.  See Massachusetts v. EPA, 549 U.S. 497 (2007). 

6 U.N. Paper.

7 Id

8 794 F. Supp. 2d 151 (D.D.C. 2011) (“CBD”)

9 See (citing Section 304(a)(3) of the Act, 42 U.S.C. § 7604(a)(3)). 

10 See Complaint, Center for Biological Diversity et al. v. EPA, No. 10-00985 (D.D.C. Jun. 6, 2010).

11 See id.

12 Memorandum in Support of Partial Motion to Dismiss, Center for Biological Diversity v. EPA, No. 10-00985 (D.D.C. Aug. 2, 2010). 

13 CBD, 794 F. Supp. 2d at 157-158.

14 42 U.S.C. § 7547(a)(4). 

15 CBD, 794 F. Supp. 2d at 157-158.

16 42 U.S.C. § 7571(a)(2) (emphasis added).

17 CBD, 794 F. Supp. 2d at 159-162. 

18 See Order, Center for Biological Diversity et al. v. EPA, No. 10-00985 (D.D.C. Mar. 21, 2012).

19 Memorandum in Response to Petitions Regarding Greenhouse Gas and other Emissions from Marine Vessels and Nonroad Engines and Vehicles (June 18, 2012) (Nonroad Petition Memo), at 4, available at

20 See EPA, U.S. Transportation Sector Greenhouse Gas Emissions 1990-2011, available at

21 Nonroad Petition Memo at 7-8. 

22 Id. at 10.

23 Memorandum in Response to Petition Regarding Greenhouse Gas Emissions from Aircraft (June 18, 2012), at 2, available at

24 Id. at 3.

25 Coalition for Responsible Regulation, Inc. v. EPA, 684 F.3d 102 (D.C. Cir. 2012). 

26 Coalition for Responsible Regulation, Inc. v. EPA, 134 S.Ct. 468 (2013).