Mr. Trump’s biggest target may be emission rules that would force the closing of hundreds of coal-fired power plants meant to be replaced by wind and solar power. But they are caught up in court battles that could run for months or years.

By contrast, he could more quickly soften Mr. Obama’s rules requiring tougher vehicle emission standards. Mr. Trump met on Tuesday with executives of major American automakers, who complained that before leaving office, Mr. Obama finalized an ambitious E.P.A. rule requiring that vehicles average 54.5 miles per gallon by 2026. Mr. Trump said he would help with burdensome regulations, but offered no specifics.

Mr. Trump could lift a moratorium instituted last year by Mr. Obama on new coal mining leases on public lands. As soon as next month, the Republican-led Congress may pass legislation undoing Mr. Obama’s regulations on the practice of mountaintop-removal coal mining and on leaks of planet-warming methane emissions from oil and gas drilling rigs.

In the meantime, the Keystone and Dakota pipelines provided Mr. Trump with visible ways to demonstrate action. As proposed by TransCanada, an Alberta firm, Keystone would carry 800,000 barrels a day from the Canadian oil sands to the Gulf Coast. Republicans and some Democrats said that it would create jobs and expand energy resources, while environmentalists said it would encourage a form of oil extraction that produces more gases that warm the planet than normal petroleum.

Studies showed that the pipeline would not have a momentous effect on jobs or the environment, but both sides made it into a symbolic test case. The State Department estimated that Keystone would support 42,000 temporary jobs for two years — about 3,900 of them in construction and the rest through indirect support, like food service — but only 35 permanent jobs. Similarly, the government concluded that Keystone’s carbon emissions would equal less than 1 percent of the total greenhouse gas emissions in the United States.

“Keystone has never been a significant issue from an environmental point of view in substance, only in symbol,” said David L. Goldwyn, an energy market analyst and a former head of the State Department’s energy bureau in the Obama administration.

But it was a symbol Mr. Trump found important enough to seize on early in his presidency. He signed an executive memorandum inviting TransCanada “to promptly resubmit its application to the Department of State for a presidential permit” for the pipeline, although the document did not guarantee approval.