On January 5, 2001, the Clinton administration finalized a new policy called the Roadless Rule, which put 58 million acres of national forest lands off limits to mining, logging, drilling and road-building. Industry and many states balked at the restrictions, environmentalists cheered, and everyone wondered: Would the protections survive? George W. Bush, a former oilman whose campaign promises included opening Arctic lands to drilling, would take office just 15 days later.

Donald Trump’s dark-horse finish in the presidential race last month is raising similar concerns: Is Barack Obama’s environmental legacy doomed? Trump hasn’t articulated a detailed environmental agenda, but what he has said has the fossil fuel industry celebrating and environmentalists girding for battle. And it’s given the Roadless Rule saga new resonance. When Bush took office, he delayed implementation of the rule and lengthy court battles ensued. But after more than a decade of litigation, the rule largely held up. It’s not always easy for a new administration to undo the work of the last.

Nevertheless, Trump has said he will approve the Keystone XL pipeline, rescind the Clean Power Plan, scrap a stream and wetland protection rule, and end a temporary moratorium on leasing of federal coal reserves. He’s also made broad promises to “lift restrictions” on energy development on public lands. Overall though, his ambitions are murky at best. “The current administration is more unpredictable than any administration that has ever come into office, at least within my lifetime,” says Erik Schlenker-Goodrich, executive director of the Western Environmental Law Center. “We are in uncharted waters.”

Whether Trump can deliver on his pledges depends in part on how the Obama administration advanced its own agenda. The coal-leasing moratorium, for instance, can be reversed immediately, says Matt Lee-Ashley, a public lands expert with the Center for American Progress. Where coal seams underlie federal land, agencies within the Department of the Interior offer leases to mining companies, which then pay royalties on the coal they dig. Last January, current Interior Secretary Sally Jewell instituted a moratorium on new leases via a “secretarial order”—the cabinet members’ version of the president’s executive order. It was part of a broader review of the federal coal leasing program, which has been criticized by environmentalists, the Government Accountability Office and others for failing to deliver fair returns to taxpayers and being inconsistent with Obama’s goal to combat climate change, since burning coal emits carbon. Trump’s Interior Secretary could perform the same move in reverse, with the stroke of a pen. Candidates for the post reportedly include former vice presidential candidate and “drill baby drill” sloganeer Sarah Palin; Washington Rep. Cathy McMorris Rodgers, and Oklahoma Gov. Mary Fallin.

Likewise, Obama’s State Department had authority to approve the Keystone XL pipeline, which would deliver crude oil from the Canadian tar sands to refineries and ports on the U.S. Gulf Coast. His administration rejected TransCanada’s application for a permit to allow construction across an international border. If the company resubmits its application, Trump could green-light it. Still, actually getting the pipeline built might not be simple. Jason Hutt, an attorney with the firm Bracewell, that represents oil and gas clients, points out that Trump has said he wants the U.S. to earn a share of the pipeline’s profits, and what exactly he’ll demand as president is unknown. And the pipeline has been the centerpiece of climate activists’ campaign against fossil fuels. They’ll continue to fight it.

The new Republican Congress can also ask Trump to overturn regulations finalized late in Obama’s term. The Congressional Review Act gives the House and Senate 60 days in session to scrutinize rules developed and finalized by executive branch agencies like the Environmental Protection Agency (EPA). In that time, Congress can pass a “joint resolution of disapproval” for a new rule. If the President signs the resolution, the rule is vacated. Because Congress takes so many breaks, rules finalized after May 30 of this year will likely be subject to congressional review, according to the Congressional Research Service.

Among them are the Bureau of Land Management’s (BLM) new rules for how oil and gas operations on federal land handle methane, a potent greenhouse gas and an energy source. (Methane is the primary component of natural gas.) The rules require companies to minimize methane leakage from pipelines and storage equipment, and to limit the amount of methane that’s vented or burned as waste at oil wells. Oil is the more valuable commodity, and flaring and venting methane that comes up with it is common practice. But leaks, vents and flares release greenhouse gases, and the public doesn’t earn royalties on gas that isn't captured and sold. Industry opposes the rules as unnecessary, arguing that it’s already working to reduce methane waste. When the rules were finalized in mid-November, Republican Sen. Jim Inhofe responded by saying he would work with the new administration to rescind them.

Trump’s options for erasing Obama’s earlier environmental achievements are more complicated. Take the stream and wetland protections known as the Clean Water Rule, issued by the EPA and the Army Corp of Engineers. The 2015 rule clarified the federal government’s jurisdiction to protect small and intermittent streams and wetlands from pollution. Developers, business and agriculture groups oppose the rule, saying it infringes on private property rights and creates undue regulatory burdens. Implementation is on hold while the courts consider the rule, and Trump could let the legal system decide the issue. Or, he could ask the court to send the rule back to the EPA for revision. In that scenario, Trump’s EPA would presumably weaken it. (In a 2015 Fox News interview, Trump said of the agency, “what they do is a disgrace.”) But any administrative effort to rewrite the rule would be subject to public comment and litigation, and would require the EPA to justify its actions. Alternatively, Congress could try to pass legislation to rescind the rule, which Trump would surely sign.

The Trump administration’s options for dismantling the Clean Power Plan, Obama’s signature effort to cut carbon emissions, are similar. “It will not go gently," says David Doniger, director of the Natural Resources Defense Council’s clean air and climate program. Senate Democrats can filibuster legislative attempts to gut the regulations, and environmentalists will litigate administrative attempts to kill it. “They have to undo it the same way it was built, with rulemaking and opportunity for public comment,” Doniger says. “It will take a long time, and every step in that process we will fight.”

And what of Trump’s more nebulous promise to “lift restrictions” on energy development on public lands? Kathleen Sgamma, vice president of government and public affairs for the Western Energy Alliance, an oil and gas industry group focused on public lands issues, says it’s difficult to discern specific priorities at this stage. She’d welcome any changes that sped up leasing and permitting, but exactly how a new administration might do that is unclear. For now, Trump’s tone is enough to make her optimistic. “What we’re simply looking for is an administration that does not actively try to drive us off public lands,” Sgamma says.

Environmentalists are skeptical that Trump can spark an energy boom, though, disputing the idea that regulation significantly inhibits energy development. An analysis done by the Wilderness Society this year found that 90 percent of BLM lands and minerals in the Western U.S. are open to oil and gas leasing—or about 192 million acres. Nada Culver, director of the society’s BLM Action Center, says the market is what really dictates drilling activity at any given time. According to the group, between 2009 and 2013, producers bid on only 24 percent of acreage the BLM offered at auction.

Yet it’s also true that environmentalists have succeeded under Obama in limiting development in certain landscapes. In November, with the goal of protecting wildlife habitat, the BLM adopted a management plan for Colorado's Roan Plateau that bars oil and gas drilling across some 35,000 acres. The deal resolved years of litigation and was agreed to by conservation groups and a company that bought drilling leases on the plateau during the Bush administration. “It’s a consensus plan," says Mike Freeman, a lawyer with Earthjustice who doubts the plan will face trouble in the new administration. “It just wouldn’t make sense to reopen this.”

But it might make sense elsewhere. Interior Secretary Jewell also announced in November the controversial cancellation of natural gas leases in the Thompson Divide, an area near Colorado resort towns like Aspen that is prized by recreationists, who oppose development. In a statement following the decision, Colorado Oil and Gas Association president Dan Haley expressed hope “that the new administration can work to right this wrong.” To reopen the area to development, says Freeman, the U.S. Forest Service would have to go through a public process to amend its management plan. That would take a while and face stiff public opposition, but it could be done.

And there was still more November news: The Obama administration announced its five-year plan for offering offshore drilling leases, which stipulated that no leases will be offered through 2022 on the Atlantic and Pacific coasts, or in the Arctic Ocean. Alaska Sen. Lisa Murkowski was “infuriated” at the decision on Arctic leases, and vowed to fight it. The Trump administration could draw up a new offshore plan, but it would likely take time, and it might not have much practical effect on near-term drilling activity. Shell already bought leases in the Chukchi Sea, but abandoned exploration there last year.

All in all, there are still more questions than answers on how Trump will actually approach environmental and public lands policy in the early days of his administration. Candidate Trump promised to withdraw the U.S. from the Paris climate accord, but during an interview last week with the New York Times said, “I have an open mind to it.” Though the Republican Party platform included support for legislation to transfer or sell federal lands to states, Trump rejected the idea on the campaign trail. Then again, the Western environmental magazine High Country News reported in October that in private conversations with proponents of land transfers, Trump expressed openness to the idea. Trump’s interest in improving infrastructure—including drinking water systems, the failures of which the crisis in Flint, Michigan laid bare—appears to be one of the scant areas where the environmentally-minded might find common ground with the new administration.

“The question in front of us now is whether President Trump intends to govern as divisively as he campaigned, or to moderate some,” says Doniger, of the Natural Resources Defense Council. But, he adds, “I don’t have rational expectations about how pragmatic” the future president might turn out to be.