On July 5, 2011, the United States District Court for the District of Columbia issued an opinion suggesting that U.S. EPA has an obligation to make an "endangerment finding" with respect to greenhouse gas emissions from aircraft. In Center for Biological Diversity v. United States Environmental Protection Agency, No. 10-00985 (HHK) (D.D.C.), plaintiffs have raised claims that EPA unreasonably delayed in responding to plaintiffs' rulemaking petitions, filed in 2007 and 2008, and in failing to determine whether greenhouse gas emissions from marine vessels, non-road vehicles and engines, and aircraft engines "cause or contribute" to dangerous air pollution. EPA moved to dismiss the claims regarding failure to make endangerment findings.

On July 5, 2011, the district court granted in part and denied in part EPA's motion to dismiss. The court found that the permissive language addressing emissions from non-road vehicles and engines in Section 213 of the Clean Air Act, 42 U.S.C. § 7547, did not require EPA to conduct endangerment findings. The court therefore dismissed the claims related to marine vessels and non-road vehicles and engines.

Emissions from aircraft engines, however, are addressed in Section 231 of the Clean Air Act, 42 U.S.C. § 7571, which provides that EPA "shall, from time to time, issue proposed emission standards applicable to the emission of any air pollutant from any class or classes of aircraft engines which in [its] judgment causes, or contributes to, air pollution which may reasonably be anticipated to endanger public health or welfare." In denying EPA's motion to dismiss the claim related to aircraft engines, the district court stated that this mandatory language "strongly suggest[s] that Congress intended the predicate endangerment finding to be a compulsory step."

The statutory language cited by the district court is almost identical to language in Section 202 of the Clean Air Act, dealing with emission standards from on-road motor vehicles. In April 2010, EPA relied upon that language to establish greenhouse gas standards for emissions from passenger vehicles and light-duty trucks. According to the U.S. Department of Transportation, domestic aircraft account for nine percent of all transportation-related greenhouse gas emissions in the United States.

The court did not, however, take the step at this stage of the litigation of ordering EPA to make such an endangerment determination for aircraft emissions. A motion for summary judgment on plaintiffs' remaining claims is pending, with EPA's response due by July 27, 2011.