President-elect Donald Trump has been crystal clear: His second term will be an assault on climate policy. On the campaign trail, he vowed to “drill, baby, drill” for more oil and gas and inveighed against offshore wind farms. He attacked President Joe Biden’s Inflation Reduction Act, the largest climate investment in history, as the “green new scam.” And he pledged to withdraw the US once again from the Paris climate agreement.

Abroad, it’s feared that the incoming administration will “take a wrecking ball to climate diplomacy,” says Rachel Cleetus, climate and energy policy director at the Union of Concerned Scientists. Even before Trump takes office, his victory has cast a shadow over the United Nations climate conference COP29. Delegates there are grappling with how to navigate global climate action around the US in coming years.

There will be “a void for America’s geopolitical competitors to fill,” says John Morton, a former adviser to Treasury Secretary Janet Yellen and a managing director at climate investment and advisory firm Pollination. The climate leadership role relinquished by the US could be taken up by China, the world’s biggest emitter—the US is No. 2—and a renewable energy powerhouse.

At home, thanks to a Republican trifecta in Washington, Trump will be able to enact more sweeping changes than if Democrats controlled one or both chambers of Congress. Even so, it’s doubtful he can scrap the IRA altogether. The drafters of the law “did everything they possibly could” to make it resilient to attacks from the right, says Bob Keefe, executive director at clean energy advocacy group E2.

Most of the hundreds of billions of dollars unlocked by IRA tax credits have flowed into red congressional districts, bringing investment and jobs that some Republican lawmakers are now loath to forgo. In fact, before the election, a group of them wrote to House Speaker Mike Johnson to argue against revoking the subsidies. Darren Woods, the chief executive officer of Exxon Mobil Corp., said at COP29, “We think the IRA makes sense.”

But if Congress doesn’t nix them, Trump’s Treasury Department could still rewrite some rules to make a hydrogen production tax credit easier to obtain, even when natural gas is used to create the clean-burning fuel. Similar changes also could shrink the number of electric vehicles that qualify for a $7,500 credit.

The IRA also encourages investment in nuclear energy. That sector has bipartisan support and is unlikely to be targeted, says Costa Samaras, director of the Scott Institute for Energy Innovation at Carnegie Mellon University. Biden “has made the biggest investment in nuclear power in a generation,” says Samaras, who was an official in the White House Office of Science and Technology Policy until earlier this year. “The next administration needs to continue those investments.”

The US has been the world’s top oil producer for six years straight, stretching from Trump 1.0 through Biden’s tenure. Trump will almost certainly lower barriers to drilling on public lands and waters and make more of those places available for leasing. He has vowed to end Biden’s pause on new licenses to export liquefied natural gas, perhaps immediately. Biden signed a slew of executive orders early in his term to move the whole federal government to prioritize climate change; those are all vulnerable from Day 1.

Trump’s pick to run the Environmental Protection Agency, former New York Representative Lee Zeldin, is expected to follow through on promises to undo or loosen many environmental regulations, repeating the broad rollback of Trump’s first term. In a social media post, Zeldin said he would work to “restore US energy dominance, revitalize our auto industry to bring back American jobs, and make the US the global leader of AI.”

That’s likely another way of saying his targets are Biden-era rules that created stringent limits on vehicle emissions and on climate pollution from coal and new gas power plants. The EPA is currently developing emissions rules for existing gas-fired power plants; under the next administration, the effort may be abandoned.

Trump’s campaign platform promised to “shatter” the bureaucratic state, and an exodus of career staff from the EPA and other agencies could be in the cards. Many hundreds of scientists left the EPA during the first Trump administration. Longtime climate skeptics are pushing the transition team to challenge the science that underpins EPA decision-making.

Trump might also opt to kill or redefine the “green bank” inside the Department of Energy—the Loan Programs Office—which he also targeted in his first term. The office was infused with hundreds of billions of IRA dollars for clean technology loans. Not all that money has gone out the door, and Trump could reroute it to fossil-fuel projects.

Whatever his moves on the environment, there will be legal challenges. Trump reshaped the judiciary in his first term, appointing three conservative justices to the US Supreme Court as well as dozens of lower-court judges, many of them cultivated by the right-wing Federalist Society. These appointees could be sympathetic to groups that challenge existing environmental regulations. But the courts may stand in Trump’s way at least some of the time. In an ironic twist, the new administration could now be constrained by the Supreme Court’s blockbuster decision in June curbing the latitude of agencies to interpret unclear statutes.

If there is any good news for the climate, it’s that the clean energy economy has become too strong to stop. Solar is now the cheapest way to generate power, says Joseph Shapiro, a professor of trade and environment at the University of California at Berkeley. “Big picture,” he says, “the US energy transition may not be full steam ahead. But momentum will continue.” —With Jennifer A. Dlouhy, Ari Natter and Greg Stohr