During his first moments in the Oval Office on Wednesday, President Biden returned the United States to the Paris climate accord and directed federal agencies to begin unraveling Donald Trump’s environmental policies — the first step in what Biden has vowed will be a sustained effort to safeguard the nation’s air and water, protect endangered species and combat climate change at home and abroad.

Biden’s executive order recommitting the United States to the international struggle to slow global warming fulfilled a campaign promise and represented a stark repudiation of the “America First” approach of Trump, who officially withdrew the nation from the Paris agreement Nov. 4 after years of disparaging it.

Biden also ordered federal agencies to review scores of climate and environmental policies enacted during the Trump years and, if possible, to quickly reverse them. Nearly half of the regulations the new administration is targeting come from the Environmental Protection Agency, on issues as varied as drinking water, dangerous chemicals and gas-mileage standards.

“A cry for survival comes from the planet itself. A cry that can’t be any more desperate or any more clear,” Biden, who has listed tackling climate change as one of his core priorities, said in his inaugural address Wednesday. Listing the challenges the nation faces, he pointed to “the battle to save our planet by getting the climate under control.”

Biden’s new national climate adviser, Gina McCarthy, told reporters this week that the moves will “begin undoing some of the harmful actions that happened in the previous administration’s watch, so that we can move forward in combating the climate crisis.”

McCarthy said Biden would sign “a broad executive order that takes steps that are imperative to address our climate crisis, and will also create good union jobs and advance environmental justice while reversing more than 100 of the previous administration’s harmful policies.”

Biden is expected to take even more sweeping action next Wednesday, according to a document obtained by The Washington Post. He plans to sign an executive order elevating climate in domestic and national security policy; direct “science and evidence based decision-making” in federal agencies; reestablish the Presidential Council of Advisers on Science and Technology and announce that U.S. data that will help underpin the Climate Leadership Summit that Biden will host in Washington in late April.

Though incoming presidents often sign executive orders on their first day, the chasm between Biden’s agenda and Trump’s legacy is one of the widest in recent decades. Nowhere is that contrast more pronounced than on climate change — which Trump largely dismissed altogether — and the environment, where Trump and his deputies scaled back a range of protections to benefit the fossil fuel industry.

Biden comes to office with a sense of urgency about climate change that is unmatched by any previous occupant of the White House, and he is installing throughout the government people who share his views. The regulations he is instructing agencies to review include a recent Labor Department rule preventing environmentally sustainable mutual funds from being default retirement investments and a Transportation Department regulation making it easier to transport liquefied natural gas by rail.

“At this moment of profound crisis, we have the opportunity to build a more resilient, sustainable economy, one that will put the United States on an irreversible path to achieve net-zero emissions economywide no later than 2050,” McCarthy said.

That pledge came as welcome news to many in the international community, which has forged ahead with efforts to combat climate change in recent years without cooperation from the world’s second-largest emitter of greenhouse gases.

“The United States is such an important actor internationally. So this day today has been creating a lot of expectation everywhere,” Patricia Espinosa, executive secretary of the U.N. Framework Convention on Climate Change, said in an interview. “Very few generations have the opportunity to really influence a historical change in the way that humanity evolves. This is one of those moments. So it means a lot to have the U.S. back.”

Though many of Biden’s actions Wednesday will take effect over time — the country will again formally become a party to the Paris agreement 30 days from now — his most immediate action will be to rescind the presidential permit Trump granted the Keystone XL pipeline to transport crude oil from Canada across the border into the United States. The project became a flash point for climate activists during the Obama administration, and Biden pledged during the campaign to block it.

Industry executives made it clear Wednesday that they were prepared to work with Biden but warned him against pushing to abolish fossil fuels.

The American Petroleum Institute’s chief executive, Mike Sommers, said in a statement that though his members “support the ambitions of the Paris Agreement,” the new administration should keep in mind “models show that this agreement between nations cannot be achieved without access to natural gas.”

And he took issue with Biden’s decision on the Keystone pipeline permit, saying, “Revoking the Keystone XL pipeline is a significant step backwards both for environmental progress and our economic recovery.”

Sen. Shelley Moore Capito (R-W.Va.), a senior member of the Senate Environment and Public Works Committee, foreshadowed the opposition Biden’s agenda could face in Congress. “In these next four years, it is imperative that Congress aggressively exercises oversight and pushes back on the worst impulses of Washington bureaucrats,” she said.

Conservationists, by contrast, lauded the quick push to roll back so many of Trump’s policies.

“Today we are feeling the first rays of hope after four dark years where racial violence and injustice, destruction of our environment and disdain for climate science became standard operating procedure for a government that was supposed to represent us all,” said Jamie Williams, president of the Wilderness Society.

Dozens of steps that Biden began during his first hours in office will take months, if not longer, to complete.

He is instructing the EPA and the Transportation Department to strengthen fuel-efficiency standards for cars and light trucks, which Trump weakened. He asked two other departments, Interior and Commerce, to review the boundaries and protections Trump had scaled back for national monuments in Utah and off the coast of New England.

Biden can change the boundaries of a national monument with the stroke of a pen, and though U.S. automakers probably are willing to strike a deal with the new administration on more-ambitious gas-mileage standards for the nation’s cars and pickup trucks, reversing other Trump policies will be more challenging.

Biden plans to impose a temporary moratorium on all oil and natural gas leasing activities in the Arctic National Wildlife Refuge, which is home to caribou, polar bears and Indigenous people. But on Tuesday, Interior’s Bureau of Land Management signed and issued nine leases it auctioned off earlier this month, spanning 437,804 acres on the refuge’s coastal plain.

All but two of the new leases were won by the Alaska Industrial Development and Export Authority, an arm of the Alaska state government. The sale raised a total of roughly $14 million, less than 2 percent of what congressional Republicans projected when they opened the near-pristine reserve to drilling in 2017.

BLM Alaska State Director Chad Padgett said Tuesday that the leases “reflect a solid commitment by both the state and industry to pursue responsible oil and gas development on the Alaska’s North Slope.”

In some instances, Biden is already setting the stage for changes that will reverberate throughout the federal government. For instance, he will revive an interagency working group Trump disbanded in 2017 that sets the “social cost of carbon,” an estimation of the economic damage caused by the release of a ton of carbon dioxide into the atmosphere.

Policymakers factor this figure into the cost-and-benefit calculations they make on a host of issues, including permitting new oil wells and imposing stricter pollution controls on coal- and gas-fired power plants. Under the Obama administration’s formula, the price per ton would now stand at $52, but Trump officials reduced it to between $1 and $7 per ton.

Many economists suggest that the Biden administration start at $125 per ton to better reflect new research on the economic damage of climate change and market realities.

“It is the one way you can uniformly incorporate the cost of climate change into decisions across all of government,” Kevin Rennert, who directs the Social Cost of Carbon Initiative at the environmental think tank Resources for the Future, said in an interview.

Biden’s environmental push on Day 1 far surpassed that of any other president, but only time will show how much of his agenda he can actually finish — and how successfully he can rebuild the nation’s image around the world, particularly when it comes to leading on climate action.

“Rejoining the Paris agreement is only the beginning, and the incoming administration appears to know this. In order to make truly meaningful progress towards global cooperation on climate, the Biden administration must follow up with concrete actions,” Brandon Wu, director of policy and campaigns at ActionAid USA, said in an email. “There’s simply no reason for other countries to trust our word if we fail, again, to act on our own emissions and use the significant resources at our disposal to support the world’s most vulnerable countries.”