I didn't see this posted anywhere, but I think it's fairly significant.
In particular, I appreciate the Court's comparison of "uncertainty" about the harm caused by greenhouse gases with the harm caused by leaded gasoline:
Industry Petitioners do not find fault with much of the substantial record EPA amassed in support of the Endangerment Finding. Rather, they contend that the record evidences too much uncertainty to support that judgment. But the existence of some uncertainty does not, without more, warrant invalidation of an endangerment finding. If a statute is “precautionary in nature” and “designed to protect the public health,” and the relevant evidence is “difficult to come by, uncertain, or conflicting because it is on the frontiers of scientific knowledge,” EPA need not provide “rigorous step-by-step proof of cause and effect” to support an endangerment finding. Ethyl Corp. v. EPA, 541 F.2d 1, 28 (D.C. Cir. 1976). As we have stated before, “Awaiting certainty will often allow for only reactive, not preventive, regulation.” Id. at 25.
Congress did not restrict EPA to remedial regulation when it enacted CAA § 202(a). That section mandates that EPA promulgate new emission standards if it determines that the air pollution at issue “may reasonably be anticipated to endanger public health or welfare.” 42 U.S.C. § 7521(a)(1). This language requires a precautionary, forward-looking scientific judgment about the risks of a particular air pollutant, consistent with the CAA’s “precautionary and preventive orientation.” Lead Indus. Ass’n, Inc. v. EPA, 647 F.2d 1130, 1155 (D.C. Cir. 1980). Requiring that EPA find “certain” endangerment of public health or welfare before regulating greenhouse gases would effectively prevent EPA from doing the job Congress gave it in § 202(a)—utilizing emission standards to prevent reasonably anticipated endangerment from maturing into concrete harm.