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III

A

Flexibility is a hallmark of equity jurisdiction. “The essence of equity jurisdiction has been the power of the Chancellor to do equity and to mould each decree to the necessities of the particular case. Flexibility rather than rigidity has distinguished it.” Weinberger v. Romero-Barcelo, 456 U. S. 305, 312 (1982) (quoting Hecht Co. v. Bowles, 321 U. S. 321, 329 (1944)). Consistent with equity’s character, courts do not insist that litigants uniformly show a particular, predetermined quantum of probable success or injury before awarding equitable relief. Instead, courts have evaluated claims for equitable relief on a “sliding scale,” sometimes awarding relief based on a lower likelihood of harm when the likelihood of success is very high. 11A C. Wright, A. Miller, & M. Kane, Federal Practice and Procedure §2948.3, p. 195 (2d ed.1995). This Court has never rejected that formulation, and I do not believe it does so today.
Equity’s flexibility is important in the NEPA context. Because an EIS is the tool for uncovering environmental harm, environmental plaintiffs may often rely more heavily on their probability of success than the likelihood of harm. The Court is correct that relief is not warranted “simply to prevent the possibility of some remote future injury.” Ante, at 12 (quoting Wright & Miller, supra, §2948.1, at 155). “However, the injury need not have been inflicted when application is made or be certain to occur; a strong threat of irreparable injury before trial is an adequate basis.” Wright & Miller, supra, §2948.1, at 155–156(footnote omitted). I agree with the District Court that NRDC made the required showing here.

B

The Navy’s own EA predicted substantial and irreparable harm to marine mammals. Sonar is linked to mass stranding of marine mammals, hemorrhaging around the brain and ears, acute spongiotic changes in the central nervous system, and lesions in vital organs. E.g., App. 600–602; 360–362; 478–479. As the Ninth Circuit noted, the EA predicts that the Navy’s “use of MFA sonar in the SOCAL exercises will result in 564 instances of physical injury including permanent hearing loss (Level A harassment) and nearly 170,000 behavioral disturbances (Level B harassment), more than 8,000 of which would also involve temporary hearing loss.” 518 F. 3d, at 696; see App. 223–224. Within those totals, “the EA predicts 436 Level A harassments of Cuvier’s beaked whales, of which, according to NOAA, as few as 1,121 may exist in California, Oregon and Washington combined. Likewise, the EA predicts 1,092Level B harassments of bottlenose dolphins, of which only 5,271 may exist in the California Coastal and Offshore stocks.” 518 F. 3d, at 691–692.
The majority acknowledges the lower courts’ findings, ante, at 9, but also states that the EA predicted “only eight Level A harassments of common dolphins each year” and “274 Level B harassments of beaked whales per year, none of which would result in permanent injury,” ante, at 6. Those numbers do not fully capture the EA’s predictions.
The EA classified the harassments of beaked whales as Level A, not Level B. The EA does indeed state that “modeling predicts non-injurious Level B exposures.” App.
185. But, as the majority correctly notes, ante, at 6, the EA also states that “all beaked whale exposures are counted as Level A,” App. 185. The EA counted the predicted exposures as Level A “[b]y Navy policy developed in conjunction with NMFS.” Id., at 200. The record reflects “the known sensitivity of these species to tactical sonar,” id., at 365 (National Oceanic and Atmospheric Administration letter), and as the majority acknowledges, beaked whales are difficult to study, ante, at 6. Further, as the Ninth Circuit noted, “the EA . . . maintained that the methodology used was based on the ‘best available science.’” 518 F. 3d, at 669.4
In my view, this likely harm—170,000 behavioral disturbances, including 8,000 instances of temporary hearing loss; and 564 Level A harms, including 436 injuries to a beaked whale population numbering only 1,121—cannot be lightly dismissed, even in the face of an alleged risk to the effectiveness of the Navy’s 14 training exercises. There is no doubt that the training exercises serve critical interests. But those interests do not authorize the Navy to violate a statutory command, especially when recourse to the Legislature remains open. “Of course, military interests do not always trump other considerations, and we have not held that they do.” Ante, at 16.
In light of the likely, substantial harm to the environment, NRDC’s almost inevitable success on the merits of its claim that NEPA required the Navy to prepare an EIS, the history of this litigation, and the public interest, I cannot agree that the mitigation measures the District Court imposed signal an abuse of discretion. Cf. Amoco Production Co. v. Gambell, 480 U. S. 531, 545 (1987) (“Environmental injury, by its nature, can seldom be adequately remedied by money damages and is often permanent or at least of long duration, i.e., irreparable. If such injury is sufficiently likely, therefore, the balance of harms will usually favor the issuance of an injunction to protect the environment.”). For the reasons stated, I would affirm the judgment of the Ninth Circuit.

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