All of that would require aggressive domestic climate policies, but just undoing the Trump policies would typically take two years or more. For example, the Trump E.P.A. undid the Obama administration’s single largest policy aimed at curbing climate change, a rule that forced automakers to rapidly increase the fuel economy of passenger vehicles, and, in so doing, drastically lowering their pollution of heat-trapping carbon dioxide pollution. To do that, the E.P.A. had to follow a long legal path, formally publishing a proposal to change the rule, opening it to public comments, drafting legal, economic and scientific justifications for the rule, and performing complex technical analyses of the impacts of the new rule on highway safety, air quality and consumer behavior.
Although the Trump administration began its rollback of the Obama auto pollution rule within Mr. Trump’s first days of taking office, it was not completed until last spring.
The same timetable could await Mr. Biden as he seeks to reinstate the rule.
“It’s a laborious, time-consuming process,” said Richard Revesz, a professor of environmental law at New York University, who was on Mr. Biden’s short list to run the E.P.A.
“No one doubts the E.P.A.’s authority to put these regulations on auto pollution back in place,” Mr. Revesz said. “But they can’t just make the Trump rules go away by executive order. They have to follow the same process — preparing all the scientific and economic analyses, and you have to get all that right.”
Mr. Revesz noted that while the Obama administration had performed such analyses a decade ago when it first put the auto pollution rule in place, much has changed since then, including the economy, the auto industry and the amount of planet-warming carbon dioxide already in the atmosphere. So Mr. Biden’s E.P.A. staff will need to conduct all that work again, with present-day data.
And he noted that after four years in which the staff and budget of the E.P.A. has shrunk, the agency is now being asked to do far more with far fewer resources.
“They want to do a lot, but they can’t do it all at the same time,” Mr. Revesz said. “E.P.A. has suffered the loss of a lot of senior staff. They’ll need to prioritize. So they may not get to some of these until the end of a first term.”