WASHINGTON — President Trump sharply changed the federal government’s approach to the environment on Tuesday as he cleared the way for two major oil pipelines that had been blocked, and set in motion a plan to curb regulations that slow other building projects.
In his latest moves to dismantle the legacy of his predecessor, Mr. Trump resurrected the Keystone XL pipeline that had stirred years of debate, and expedited another pipeline in the Dakotas that had become a major flash point for Native Americans. He also signed a directive ordering an end to protracted environmental reviews.
“I am, to a large extent, an environmentalist, I believe in it,” Mr. Trump said during a meeting with auto industry executives. “But it’s out of control, and we’re going to make it a very short process. And we’re going to either give you your permits, or we’re not going to give you your permits. But you’re going to know very quickly. And generally speaking, we’re going to be giving you your permits.”
The decisions expanded an effort to unravel much of the policy structure left by former President Barack Obama, who made fighting climate change a central priority. Just a day earlier, Mr. Trump formally abandoned the Trans-Pacific Partnership, an ambitious 12-nation trade pact negotiated by Mr. Obama.
In his opening days in office, Mr. Trump has also modified or reversed Mr. Obama’s policies on health care, abortion and housing while ordering a freeze of any pending regulations left behind by the former administration.
The pipelines were more about symbol than substance but generated enormous passion on both sides of the debate. Mr. Obama rejected the proposed Keystone pipeline in 2015, arguing that it would undercut American leadership in curbing the reliance on carbon energy. The Army sidetracked the Dakota Access pipeline in North Dakota last month in the waning days of the Obama administration.
Environmental activists quickly denounced Mr. Trump’s decisions. “Donald Trump has been in office for four days, and he’s already proving to be the dangerous threat to our climate we feared he would be,” said Michael Brune, the executive director of the Sierra Club.
Mr. Trump made clear on the campaign trail that he saw Mr. Obama’s environmental policies as a threat to the economy and dismissed climate change as a hoax perpetrated by China. Myron Ebell, a climate change denier who headed Mr. Trump’s Environmental Protection Agency transition team, has drafted a 50-page blueprint for how he could eliminate Mr. Obama’s climate change policies. “It is designed to implement all of the president’s campaign trail promises — every single one,” Mr. Ebell said this week in an interview.
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.
The president told reporters he would “renegotiate some of the terms” — including possibly an insistence that the pipeline be built with American steel — but left little doubt that he wanted it approved. “We’ll see if we can get that pipeline built,” he said. “A lot of jobs.”
In a statement, TransCanada accepted his invitation to seek permission again. “We are currently preparing the application and intend to do so,” the company said, vowing that it would create jobs and still protect waterways and other sensitive resources.
The Dakota Access pipeline in North Dakota became the focus of protests when the Standing Rock Sioux Tribe objected to its construction less than a mile from its reservation. The tribe and its allies won victory last month when the Army Corps of Engineers announced that it would look for alternative routes for the $3.7 billion pipeline instead of allowing it to be drilled under a dammed section of the Missouri River.
Mr. Trump signed an executive memorandum directing the Army “to review and approve in an expedited manner” the pipeline, “to the extent permitted by law and as warranted.” In his session with reporters, he added, “Again, subject to terms and conditions to be negotiated by us.”
Mr. Trump owned stock in Energy Transfer Partners, the company that is building the Dakota Access pipeline, according to his most recent filing with the Federal Election Commission. Last month, a spokesman for Mr. Trump said he sold all of his stock in June, but there is no way of verifying that sale, and Mr. Trump has not provided documentation of it.
Critics vowed to keep resisting the projects. Jan Hasselman, a lawyer for Earthjustice, an environmental law group representing the tribe, said Mr. Trump was discarding the findings of a review. “They’re just ignoring the problems that the government has already found,” he said, “and that is the kind of thing that courts need to review very closely.”
In Canada, the government of Prime Minister Justin Trudeau welcomed Mr. Trump’s decision. “We have been supportive of this since the day we were sworn into government,” Jim Carr, the natural resources minister, told reporters. Mr. Carr said the American reversal will lead “to a deepening of the relationship across the border.”
In addition to the Keystone and Dakota directives, Mr. Trump signed three others intended to ease the way for businesses and to promote American manufacturing. One instructed the Commerce Department to develop a plan to ensure that future pipelines built in the United States be constructed of American-made materials.
Another was aimed at streamlining what he called “the incredibly cumbersome, long, horrible permitting process and reducing regulatory burdens for domestic manufacturing.” The last directive was intended to expedite environmental reviews for “high-priority infrastructure projects” like highways and bridges.
Some news reports on Tuesday said that the E.P.A. and other departments had issued orders forbidding employees from issuing news releases or posting on social media. But longtime officials in multiple agencies said the guidance was similar to that of when Mr. Obama took office eight years ago.
