What are the facts?
A private developer sought a special-use permit from the U.S. Forest Service to construct a downhill ski resort in the Okanogan National Forest in Washington's Methow Valley. The Forest Service prepared a draft and final EIS evaluating environmental impacts, including wildlife, land use, traffic, socioeconomics, and especially winter air quality concerns tied to temperature inversions and increased vehicular emissions. The EIS identified potential adverse impacts and discussed various mitigation measures—such as phasing development, shuttle and traffic-management strategies, limits on parking, restrictions on wood-burning appliances, and coordination with state and local air quality permitting—but it did not contain a fully specified, enforceable mitigation plan or quantify the effectiveness of each measure. Local citizens and environmental groups challenged the sufficiency of the EIS under NEPA. The district court upheld the EIS, but the Ninth Circuit reversed, holding that NEPA required a complete mitigation plan and a "worst-case" analysis of uncertain air quality effects. The Supreme Court granted certiorari.
What is the legal issue?
Under NEPA, must an agency's EIS include (1) a detailed, fully developed and enforceable mitigation plan for identified harms and (2) a "worst-case" analysis when information is incomplete or uncertain; and, in light of these standards, was the Forest Service's EIS for the proposed ski resort adequate?
What rule applies?
NEPA imposes procedural—not substantive—obligations: an agency must take a "hard look" at environmental consequences and disclose them to the public before acting. An EIS must include a reasonably thorough discussion of significant environmental impacts and "possible mitigation measures," but NEPA does not require the adoption of any particular mitigation, nor a fully developed, enforceable mitigation plan at the EIS stage. CEQ's regulations interpreting NEPA are entitled to substantial deference. Under CEQ's 1986 revision of 40 C.F.R. § 1502.22, agencies need not prepare a "worst-case" analysis; instead, they must disclose when information is incomplete or unavailable, explain its relevance, summarize credible scientific evidence, and evaluate reasonably foreseeable significant adverse effects and their likelihood.
What did the court hold?
No. NEPA does not require a fully developed and enforceable mitigation plan in the EIS, and it does not require a "worst-case" analysis. The Forest Service's EIS, which discussed potential mitigation measures and disclosed uncertainties, satisfied NEPA's procedural requirements. The Ninth Circuit's decision was reversed.
What is the reasoning?
The Court emphasized that NEPA's core is procedural: it ensures that agencies make informed decisions by considering environmental consequences, not that they reach any particular substantive result or guarantee mitigation. Requiring a fully specified mitigation plan at the EIS stage would be incongruous with NEPA's timing and function, because many mitigation details are appropriately developed through later permitting and design processes in coordination with state and local authorities. What NEPA does require is a "reasonably complete" discussion of possible mitigation to inform the decisionmaker and the public. The EIS here identified concrete categories of mitigation—such as phasing, transportation demand management, and emission controls—and explained how further planning and permitting would refine those measures. That level of detail was sufficient to demonstrate the agency took a "hard look." On uncertainty, the Court deferred to CEQ's 1986 revision of § 1502.22, which replaced the earlier "worst-case" analysis requirement with an approach focused on disclosure of data gaps, scientific credibility, and evaluation of reasonably foreseeable adverse effects and their probabilities. The Court found CEQ's interpretation reasonable and entitled to substantial deference. Mandating a worst-case analysis, especially where it could distract from the best available evidence and overwhelm decisionmakers with speculation, was not compelled by NEPA's text or purposes. The Forest Service's EIS acknowledged uncertainties surrounding winter air quality inversions and described potential consequences and mitigation, thus satisfying the governing regulation and NEPA's "hard look" standard. Because NEPA does not require agencies to adopt mitigation, or to demonstrate mitigation's guaranteed efficacy before approval, the adequacy of the EIS turned on the sufficiency of discussion—not on enforceable commitments. The Court concluded the EIS's analysis and mitigation discussion were adequate and that the Ninth Circuit erred in imposing stricter, extra-statutory requirements.
Why is this case significant?
Robertson is a leading case on NEPA's scope. It cements that NEPA is a disclosure statute: agencies must analyze and disclose impacts and mitigation, but NEPA does not require them to select the most environmentally protective alternative or to finalize a mitigation plan in the EIS. The decision also establishes substantial judicial deference to CEQ's NEPA regulations and confirms that a "worst-case" analysis is not required. For students and practitioners, Robertson defines what makes a mitigation discussion adequate, highlights how agencies should handle incomplete information, and frames the "hard look" review that courts apply to EIS adequacy challenges.
Does NEPA require agencies to adopt or fund mitigation measures identified in an EIS?
No. NEPA is procedural. It requires agencies to disclose and consider environmental impacts and possible mitigation, but it does not impose a substantive duty to adopt mitigation or achieve any particular environmental result. Agencies must show they took a hard look; they need not commit to or guarantee mitigation's success in the EIS.
What level of detail about mitigation must an EIS include after Robertson?
An EIS must contain a reasonably complete discussion of possible mitigation—identifying types of measures, how they could work, and potential effectiveness to an extent that informs decisionmakers and the public. It need not provide a fully engineered, enforceable mitigation plan or quantify with precision the effectiveness of each measure at the EIS stage.
Is a "worst-case" analysis ever required under NEPA?
No. After CEQ's 1986 revision to 40 C.F.R. § 1502.22, which the Court upheld, agencies need not perform a worst-case analysis. Instead, when information is incomplete or unavailable, they must disclose the gap, explain its relevance, summarize credible scientific evidence, and evaluate reasonably foreseeable significant adverse effects and their likelihood.
How does Robertson affect judicial review of EIS adequacy?
It reinforces deferential, record-based review of whether the agency took a hard look and followed NEPA's procedures. Courts assess the reasonableness and completeness of the EIS discussion but do not second-guess policy choices or require specific outcomes. CEQ's interpretations receive substantial deference, shaping how courts evaluate uncertainty and mitigation analysis.
What practical steps should agencies take post-Robertson when information is incomplete?
Agencies should identify data gaps; explain why the information is important; describe efforts to obtain it and whether doing so is feasible; summarize existing scientific evidence; and analyze reasonably foreseeable significant adverse effects and their probabilities. They should also describe potential mitigation sufficiently to inform decisionmaking, even if details will be refined later.
Does Robertson limit challenges based on the effectiveness of proposed mitigation?
It narrows them. Plaintiffs can still argue that an EIS inadequately discussed mitigation's nature and potential effectiveness or ignored reasonable alternatives, but they cannot prevail merely because mitigation is uncertain or not guaranteed. The focus is on the sufficiency and candor of the analysis, not on binding commitments or results.