Changing much of the Obama-era environmental agenda is at the top of President-elect Donald Trump’s list of priorities.

President Obama called the Clean Power Plan, which pushes states to move away from coal-fired power plants, the most significant step the U.S. has taken to fight global warming. Trump could direct the Environmental Protection Agency (EPA) to chuck the regulations, but that would require a years-long rulemaking process and scientific evidence to back up a new policy. And even that effort would face litigation that at the very least would slow things down. Trump could ask Congress to draft legislation gutting the regulation more quickly, but such efforts–like a GOP attempt to weaken the Clean Air Act in the 1990s–have often backfired.

Every environmental regulation has its quirks, but Trump will face a similar set of challenges with each rule he tries to undo–and there are many. But Trump appears ready to fight, choosing Scott Pruitt, a key foe of EPA regulations, to lead the agency. Pruitt has sued the agency on several occasions as Oklahoma attorney general and will bring that zeal to his new job.

"He's very proud of his leadership challenging the EPA," says Brendan Collins, an environmental lawyer at the firm Ballard Spahr. "He's got some experience and knows how to get things done." That's a CV that should have greens worried.