May 07, 2020

Scientific uncertainty and the Council on Environmental Quality’s proposed changes to its National Environmental Policy Act regulations

Steph Tai


On January 10, 2020, the Council on Environmental Quality (CEQ) proposed major changes to its approach toward the National Environmental Policy Act (NEPA), in its Update to the Regulations Implementing the Procedural Provisions of NEPA (hereinafter Update). These proposed changes include changes to key aspects of how federal agencies are to comply with NEPA, including the threshold applicability analysis, categorical exclusions, environmental assessments (EAs), scoping, and environmental impact statements (EISs). To date, no one has addressed what these proposed changes mean for assessing and disclosing scientific uncertainties in the NEPA process.

The Update seeks to ensure that environmental documents prepared under NEPA “serve their purpose of informing decision makers regarding the significant potential environmental effects of proposed major Federal actions and the public of environmental issues in the pending decision-making process.” The evaluation of scientific uncertainties is a necessary part of this process; as the Institute of Medicine Committee report, Environmental Decisions in the Face of Uncertainty (hereinafter IOM Uncertainty Report), observed: “the informed identification and use of the uncertainties inherent in the process is an essential feature of environmental decision making.” This is because “[s]ystematically considering uncertainties and their potential to affect a decision from the onset of the decision-making process will improve the decision, focus uncertainty analyses on the decision at hand, facilitate the identification of uncertainties in factors in addition to health risk estimates, improve the planning of uncertainty analyses, and set the stage for the consideration of uncertainties in decisions.” Three aspects of the Update warrant careful consideration: (1) proposed changes that could affect an agency’s ability to evaluate fully scientific uncertainty, (2) proposed changes that could require additional assessments of scientific uncertainty, and (3) proposed changes that could alter the presentation or evaluation of scientific uncertainty.

Proposed changes that could affect an action agency’s procedural ability to evaluate scientific uncertainty

The first change that could affect an agency’s procedural ability to evaluate scientific uncertainty is the creation of procedural time limits for the NEPA evaluation process. In particular, the Update proposes “a presumptive time limit for EAs of 1 year and a presumptive time limit for EISs of 2 years.” While these limits can be waived by senior officials, the use of presumptive time limits could limit an agency’s ability to fully evaluate the scientific uncertainties involved with a given proposed action. That said, the IOM Uncertainty Report recognizes that “[in an] ideal world time and resources would not be limiting, and the requisite information and analyses would be available at the time needed, and in the quality and quantity needed, so that decision makers would be able to make decisions consistently using relevant data. In reality, however, decision makers do not have perfect information upon which to base decisions or to predict the impact and consequences of such decisions.”

More significantly, the Update proposes that EISs be required to evaluate the environmental impacts of the “proposed action and reasonable alternatives” rather than “the proposed action and the reasonable alternatives,” reasoning that, “an EIS need not include every available alternative where the consideration of a spectrum of alternatives allows for the selection of any alternative within that spectrum.” The removal of the word “the,” therefore, is intended to allow agencies to consider a more limited range of alternatives than it might have otherwise. To the extent this change might reduce the number of alternatives an agency must analyze, it may impact how much an agency is able to evaluate “deep uncertainty,” defined as the “uncertainty about the fundamental processes or assumptions underlying a risk assessment” involved with a given action. As the IOM Uncertainty Report acknowledged, evaluating or even describing deep uncertainties can be especially challenging when there is “uncertainty or disagreement about what regulatory alternatives are available.” What the report recommends instead is a robust scenario analysis, assessing outcomes using a broad range of possible alternatives. This may be hindered when the range of alternatives to be presented in an EIS is limited.

Proposed changes that could require additional assessments of scientific uncertainty

The Update proposes changes to 40 C.F.R. § 1501.2(b)(2) and § 1502.15 to clarify that agencies should consider economic and technical analyses with environmental effects. Such a focus could require additional uncertainty analyses. The IOM Uncertainty Report provides a strong recommendation to evaluate economic and technical uncertainties, recognizing that “[a]lthough different estimates of technology availability are sometimes used, the uncertainties that stem from those estimates are not often carried through to the final outputs in a regulatory impact assessment.” In addition, “[u]ncertainties in economic analyses are sometimes conducted, but they are not necessarily presented.” Sufficient follow-through with this Update change, therefore, should implicate Recommendations 2 and 3 of the IOM Uncertainty Report, which state:

[An agency] should develop methods to systematically describe and account for uncertainties in decision relevant factors in addition to estimates of health risks—including technological and economic factors—in its decision-making process. When influential in a decision, those new methods should be subject to peer review.
Analysts and decision makers should describe in decision documents and other public communications uncertainties in cost–benefit analyses that are conducted, even if not required by statute for decision making, and the analyses should be described at levels that are appropriate for technical experts and non-experts.

Proposed changes that could alter the presentation of scientific uncertainty

The Update also proposes several changes to how agencies must produce EAs and EISs that may affect agencies’ ability to fully describe the scientific uncertainties involved in their environmental evaluations. First are the presumptive space limitations for EAs and EISs. The Update proposes a presumptive 75-page limit for EAs (a “new”ish requirement for EAs, although the earlier CEQ Forty Most Asked Questions document states that EAs are generally around 10 to 15 pages), 150 pages for regular EISs, and 300 pages for EISs of “unusual scope or complexity” (the same presumptive limits as the earlier page limits, but with an additional requirement of approval in writing for extending beyond those presumptive limits). While both presumptive limits are available for modification, the limits may impair an agency’s ability to fully describe the scientific uncertainties in its evaluations. As the IOM Uncertainty Report recommended, at least for the Environmental Protection Agency (EPA), “[t]o better inform the public and decision makers, . . . communications to the public should systematically include information on what uncertainties in the health risk assessment are present and which need to be addressed, discuss how the uncertainties affect the decision at hand, and include an explicit statement that uncertainty is inherent in science, including the science that informs EPA decisions.” Such statements can require space in documents.

Finally, the Update proposes the use of other evaluative documents as the functional equivalent of an EIS, allowing such use when “(1) There are substantive and procedural standards that ensure full and adequate consideration of environmental issues; (2) There is public participation before a final alternative is selected; and (3) A purpose of the analysis that the agency is conducting is to examine environmental issues.” To the extent that such functional equivalents can provide replacements for EISs, different norms in addressing scientific uncertainties in the production of other evaluative documents may affect how scientific uncertainties are presented in EISs.


The Update is a major one for federal agency compliance with NEPA. While other effects are also important, evaluation of scientific uncertainties is one area in which the Update will have particularly significant ramifications.

Steph Tai


Steph Tai is a law professor at the University of Wisconsin Law School, whose work focuses on the intersection of risk, regulation, and science. She coauthored the Institute of Medicine Committee report, “Environmental Decisions in the Face of Uncertainty,” cited in this article.