Under the new rules, “foreseeable future” extends only as far as officials can “reasonably determine” that the future threats are “likely.” This leaves substantial room to dismiss the far-off threats of climate change as excessively speculative. It might have made it difficult, for instance, back in 20 08, to designate the polar bear as a threatened species and, two years later, set aside more than 187,000 square miles of sea ice and barrier islands in Alaska as critical habit and thus off limits to development. At the time, officials relied on climate models that extended many decades into the future, which under the new rules might not have been acceptable.

The ability of species to adapt to global warming could be further undermined by new language addressing critical habitat, the land that can be kept off limits to development to help species recover. Current rules define critical habitat as not only the land where the threatened or endangered species live but also the land they once occupied and land they might need in the future. As the world warms, species will move, their ranges will shift and that unoccupied land could become important to their survival. But that land gets short shrift in the new rules, and it could now be opened for mining, timbering, oil drilling or other forms of exploitation.

For all of Mr. Bernhardt’s talk of transparency and efficiency, some of the new rules seemed deliberately designed to throw sand in the machinery (which may, of course, be the whole point). Under present practice, known as the “blanket section 4(d) rule,” the United States Fish and Wildlife Service, which administers the law, automatically extends many of the same protections to threatened species as it does to endangered ones. The point of the act is to keep species from becoming endangered or, even worse, blinking out.

This rule is now history, replaced by a system in which the agency will, case by case, develop species-specific protection plans. This could be a cumbersome and time-consuming task, of little benefit to the species awaiting help. Mr. Bernhardt’s team has already slowed the listing process. The Obama administration added an average of 49 species a year to the threatened and endangered lists. The Trump administration has added no more than 11 in one year. In 2019, it has added only the trispot darter, a type of fish from the Southeast listed as threatened in late January.

Like other Trump environmental rule changes and rollbacks, this one will undoubtedly face challenges. Environmentally inclined senators have been talking about invoking congressional oversight procedures to overturn the rules, which is possible if they move quickly enough, but unlikely. The best hope lies with the courts, which in several cases — notably Mr. Trump’s efforts to roll back Obama-era protections against drilling in Arctic waters — have not been at all reluctant to slam on the brakes, and in general have been a bulwark against Trumpian overreach. The California attorney general, Xavier Becerra, and his Massachusetts counterpart, Maura Healey, citing findings from the alarming United Nations extinctions study in May, have said they will take the administration to court for what they believe are multiple violations of the law’s basic purpose.

We hope they, and the environmental groups that join them, will persist. Ms. Healey likens the new rules to “a plan from a cartoon villain,” ignoring everything we know about biodiversity, its uses and its alarming decline in pursuit of Mr. Trump’s reckless energy agenda. At issue is the full expression of one of the country’s noblest and most ambitious environmental laws.