“Significantly” is defined based on context – no uniform definition.

Charles R. Breyer, “Sierra Club v. Bosworth,” 2005 U.S. Dist. LEXIS 27573, 14 November 2005, pp. 27-28, Lexis.

The central concern of the Court's analysis is not whether the new information supports the Forest Service's original conclusions, or whether the effect of the project on the fisher is more likely to lead toward listing, but whether there are "significant new circumstances or information relevant to environmental concerns and bearing on the proposed action or its impacts." Friends of the Clearwater, 222 F.3d at 557 (emphasis added). If so, NEPA requires the agency to conduct a supplemental environmental analysis. Id.; see also Marsh, 490 U.S. at 374. Regulations promulgated pursuant to NEPA by the Council on Environmental Quality ("CEQ") define significantly' as "requir[ing] considerations of both context and intensity." Friends of the Clearwater, 222 F.3d at 557 n.4 (quoting 40 C.F.R. § 1508.27). Context means that "the significance of an action must be analyzed in several contexts," including both site-specific, regional and national. Id. Intensity "refers to the severity of the impact," and, in relevant part, the following factors should be considered:

(4) The degree to which the effects on the quality of the human environment are likely to be highly controversial; (5) The degree to which the possible effects on the human environment are highly uncertain or involve unique or unknown risks; and (7) whether the action is related to other actions with individually insignificant but cumulatively significant impacts.