Then there are the longer-term rollbacks. It could take two years or more for the administration to restore Obama-era climate change regulations, rolled back by the Trump administration, that were designed to cut emissions of planet-warming pollution across major sectors of the economy. It could also take that long to restore rules on industrial emissions of toxic pollutants such as mercury, as well as a major rule, Waters of the United States, designed to protect wetlands and waterways.

“When it comes to restoring the vast majority of these Obama environmental policies, the rollbacks were actually done by putting in place new regulations,” Jeffrey Holmstead, a lawyer representing fossil fuel companies who served in the E.P.A. in both Bush administrations. “In some cases that took two to three years. They will have to be replaced with new regulations. There is a legal process that has to be followed.”

The work of reinstating the federal government’s more comprehensive regulations on air, water and climate pollution will take even longer. A key reason, explained legal experts: when the Trump administration rolled back those rules, they almost never eliminated them entirely. Rather, they replaced strict federal pollution regulations with new, weaker pollution regulations.

The Biden administration, in turn, will seek to legally undo those weak regulations and replace them with tougher ones. And that legal process typically takes 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.

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