Held: [...]
3. Because greenhouse gases fit well within the Act’s capacious definition of “air pollutant,” EPA has statutory authority to regulate emission of such gases from new motor vehicles. That definition—which includes “any air pollution agent … , including any physical, chemical, … substance … emitted into … the ambient air … ,” §7602(g) (emphasis added)—embraces all airborne compounds of whatever stripe. Moreover, carbon dioxide and other greenhouse gases are undoubtedly “physical [and] chemical … substance.” Ibid. EPA’s reliance on postenactment congressional actions and deliberations it views as tantamount to a command to refrain from regulating greenhouse gas emissions is unavailing. Even if postenactment legislative history could shed light on the meaning of an otherwise-unambiguous statute, EPA identifies nothing suggesting that Congress meant to curtail EPA’s power to treat greenhouse gases as air pollutants. The Court has no difficulty reconciling Congress’ various efforts to promote interagency collaboration and research to better understand climate change with the agency’s pre-existing mandate to regulate “any air pollutant” that may endanger the public welfare. FDA v. Brown & Williamson Tobacco Corp., 529 U. S. 120 , distinguished. Also unpersuasive is EPA’s argument that its regulation of motor-vehicle carbon dioxide emissions would require it to tighten mileage standards, a job (according to EPA) that Congress has assigned to the Department of Transportation. The fact that DOT’s mandate to promote energy efficiency by setting mileage standards may overlap with EPA’s environmental responsibilities in no way licenses EPA to shirk its duty to protect the public “health” and “welfare,” §7521(a)(1). Pp. 25–30.
4. EPA’s alternative basis for its decision—that even if it has statutory authority to regulate greenhouse gases, it would be unwise to do so at this time—rests on reasoning divorced from the statutory text. While the statute conditions EPA action on its formation of a “judgment,” that judgment must relate to whether an air pollutant “cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” §7601(a)(1). Under the Act’s clear terms, EPA can avoid promulgating regulations only if it determines that greenhouse gases do not contribute to climate change or if it provides some reasonable explanation as to why it cannot or will not exercise its discretion to determine whether they do. It has refused to do so, offering instead a laundry list of reasons not to regulate, including the existence of voluntary Executive Branch programs providing a response to global warming and impairment of the President’s ability to negotiate with developing nations to reduce emissions. These policy judgments have nothing to do with whether greenhouse gas emissions contribute to climate change and do not amount to a reasoned justification for declining to form a scientific judgment. Nor can EPA avoid its statutory obligation by noting the uncertainty surrounding various features of climate change and concluding that it would therefore be better not to regulate at this time. If the scientific uncertainty is so profound that it precludes EPA from making a reasoned judgment, it must say so. The statutory question is whether sufficient information exists for it to make an endangerment finding. Instead, EPA rejected the rulemaking petition based on impermissible considerations. Its action was therefore “arbitrary, capricious, or otherwise not in accordance with law,” §7607(d)(9). On remand, EPA must ground its reasons for action or inaction in the statute. Pp. 30–32.