President-elect Donald Trump has vowed to drastically alter the U.S.’s direction on climate and energy. His promises include actions like “canceling” the Paris agreement and dismantling the U.S. Environmental Protection Agency (EPA) as well as repealing restrictions on domestic energy development. Trump infamously tweeted that global warming is a “hoax,” and has selected Myron Ebell of the conservative Competitive Enterprise Institute’s Center for Energy and Environment, an outspoken climate skeptic, to lead his EPA transition team. Trump also made specific (and ambiguous) promises about energy and climate on his Web site and on the campaign trail, such as canceling funding for the United Nations Green Climate Fund and lifting restrictions on fossil fuel development. He has sworn to make some of these changes early on, even within his first 100 days in office. But what can he actually accomplish from the get-go?

The president-elect will not have as much power as his declarations suggest. Many of his plans will take a huge amount of time and energy—and sometimes, the cooperation of Congress. He is also likely to encounter countless lawsuits and bureaucratic roadblocks that will frustrate his efforts. “I don't want to dismiss the impact of Trump's victory, it will slow down action [on] climate change,” explains Jason Bordoff, professor of professional practice in international and public affairs at Columbia University, “But I would say that for many of the things he’s promised, we will only see an impact on the margin—it'll pale in comparison to Trump’s rhetoric.”

Other experts agree. “He may be announcing a bunch of things on his first day, it’s totally possible for him to sign a bunch of executive orders that say he’s going to do things,” says Jody Freeman, a professor of law at Harvard University and director of its Environmental Law and Policy Program, “But it’s very difficult to effectuate everything without going through the process.” That said, the U.S.’s course on energy and the environment will inevitably shift under the Trump administration.

Canceling the Paris Agreement and U.N. Climate Change Fund

During a campaign speech in North Dakota in May, Trump told a cheering crowd, “We’re going to cancel the Paris climate agreement and stop all payments of U.S. tax dollars to U.N. global warming programs.” The agreement, however, was negotiated and ratified by the global community, so Trump cannot simply call it off. (So far, 193 countries have signed the accord, and 113 have ratified it). But Trump still has perhaps the most power here to follow through on his promise, compared with his other pledges.

Pres. Barack Obama unilaterally ratified the Paris accord without the Senate, and Trump could withdraw the U.S. from the agreement nearly as easily by signing an executive order, even on his first day in office. Experts note that it will take the U.S. four years to fully back out of the agreement, due to the way it is structured: Under the Paris accord, any country that wants to leave must wait three years after the agreement takes force (which happened on Nov. 4) —and then there is an additional yearlong notice period. Yet “as a practical matter, the announcement and paperwork would have the effect of removing the U.S., setting it on a certain course for withdrawal,” says Scott Fulton, president of the Environmental Law Institute.

Trump will have another option if he doesn’t want to wait four years: He could pull out of the U.N. Framework Convention on Climate Change (UNFCCC) with an executive order, which would simultaneously withdraw the U.S. from the Paris agreement. It would take only one year to complete, although it is viewed as a more drastic move. “It would be a very dramatic thing to withdraw from the UNFCCC,” Freeman says, “He could do it, but it would annoy an awful lot of our allies.” Trump, though, has recently appeared to soften his stance on the Paris accord. In a recent interview with the New York Times, he said he had “an open mind” about the agreement.

The Trump administration will also have the choice to simply ignore U.S. emission targets under the Paris agreement—the accord does not include any formal punishment for countries that do not meet their goals. Pulling out of one or any international treaties or ignoring their mandates would likely damage the U.S.’s relationship with the international community. Experts say that if the U.S. withdraws from the Paris accord, there could be diplomatic and economic repercussions—some countries, such as Mexico and Canada, are reportedly considering imposing a carbon tariff on U.S. products.

Trump has also declared that he wants to “cancel billions in payments to U.N. climate change programs and use the money to fix America’s water and environmental infrastructure.” This funding is meant to provide support to developing countries so they can take action to reduce carbon emissions and adapt to climate change, something they probably could not afford otherwise. Under the Obama administration the U.S. has committed to giving $3 billion to the U.N. Green Climate Fund, and as of November 4, it had already handed over $500 million. Whereas Trump will not be able to get that money back, he and Congress could withhold the remainder of those funds from the U.N. “The prospect of that money has dimmed, because neither Trump nor Congress is particularly interested in providing it,” Freeman says. Although Trump has promised to cancel climate fund payments to the U.N. on his first day, it is more a matter of doing nothing over time—he and Congress simply would not designate any more money for the Green Climate Fund. As for reallocating the U.N. payments to fix America’s water and environmental infrastructure, Trump would need Congressional buy in, as Congress determines how federal money is spent.

Lifting Restrictions on Energy Production

Chopping many of Obama’s key climate change efforts is part of Trump's plan to encourage fossil fuel production. “I will lift the restrictions on the production of 50 trillion dollars' worth of job-producing American energy reserves, including shale, oil, natural gas and clean coal,”he pledged in his Contract with the American Voter.

To achieve this, the president-elect plans to revoke the Clean Power Plan, open onshore and offshore leasing on federal lands for energy development, end the ban on new federal coal leases and lift various other regulations. He will likely attempt to repeal several EPA or Department of the Interior rules, according to experts—such as the Obama administration’s regulations for offshore drilling, its rules for limiting methane emissions from the oil and gas industry’s operations, and others.

Although the energy industry must follow EPA or Interior rules, the president has sway over how strictly they are enforced—which means Trump could decide to weakly implement some of the Obama regulations. Or if Trump decides to change or revoke the current rules, he would not need Congress’s approval to do so.

Finalized regulations have already gone through the agency rulemaking process, including a notice and comment period—which takes at least a year, if not several. In order to change or rescind a final rule, Trump’s EPA or Interior Department would have to go through the same process as well. “This is not an overnight task, this is not something that the president can do with a stroke of the pen,” Freeman says, “You can't sign an executive order that undoes an agency rule.” Trump, however, could easily kill any rules that are not yet finalized.

Although this means Trump still has the power to revoke regulations for the energy industry, it is also likely that environmental groups and others will sue the administration for changing the rules, experts say. Lawsuits mean time, energy and rationale on the part of the Trump administration. “Legal action by environmental groups will be back with a vengeance if Trump carries through with his promises,” says Ann Carlson, a professor of environmental law at the University of California, Los Angeles.

One of the biggest targets in Trump's sight is the Clean Power Plan, which requires power plants across the country to cut their carbon emissions. It is an EPA rule, finalized August 2015. As Scientific American reported, the rule would be difficult to undo, although it still has an uncertain future; it is currently being challenged in the U.S. Court of Appeals for the District of Columbia Circuit. If the D.C. court has not made its decision by the time Trump takes office, his administration could ask the court to send the case back to the EPA so it can rework the rule. In that case, Trump’s EPA would still need to go through the rulemaking process, and would likely run into lawsuits, according to Fulton. If the court makes its decision before Trump’s inauguration or the new administration does not ask for the case back, then the Clean Power Plan may head to the U.S. Supreme Court if either side appeals. By then, Trump may have appointed a replacement for the late conservative Justice Antonin Scalia.

If the highest court upholds the Clean Power Plan, Trump could still replace it with a new rule—but again, would have to go through the lengthy rulemaking process. Another possibility is that even if the Clean Power Plan stays, Trump’s EPA could put little energy into enforcing it.

Meanwhile, Trump has also said he would end the moratorium on new federal coal leases, revive the Keystone XL pipeline, and take other actions to promote energy development—all actions that would impact public lands.

Dismantling the EPA

One major feature of Trump's plan is to strip down the EPA’s regulatory power. “Department of Environmental Protection [sic]—we’re going to get rid of it in almost every form,” Trump said during a Fox News debate. Besides appointing Ebell, who has called Obama’s Clean Power Plan “illegal,” Trump at times has called for abolishing the agency altogether. But he cannot eliminate the EPA on his own—he needs Congress both to introduce and pass legislation, according to Freeman. And even with Republicans in control of the House and Senate, he likely would not have enough votes. “Eliminating EPA altogether looks to me to be profoundly difficult to do,” Fulton says. Other experts agree that it is highly unlikely that Trump could kill the EPA. “Environmental statutes like the Clean Air Act and the Clean Water Act are really popular, and I don't think most members in Congress are interested in fully repealing them,” Carlson explains. “And if you don't repeal them, then you need an agency to implement them.”

Trump does have other ways to curb the EPA’s power. He could slash the agency’s budget by asking for significantly less money from Congress, which would make it harder for the EPA to enforce its rules—although he cannot end its funding altogether. Congress will ultimately decide how much money an agency gets through the standard budgeting process. “Every president has different priorities. Some ask for more for EPA, some ask for less,” Freeman explains, “But Congress does what it wants—the president’s budget is a request.” Trump could also direct the EPA not to issue any new regulations, with the exception of statutes that legally require them (such as the Clean Air Act). He could also ask Congress to curb the EPA’s authority, rather than eliminating the agency altogether. “We could see some pretty draconian efforts to reduce the EPA by limiting its budget and power,” Carlson says. “It depends on where Congress wants to place its energy, but it wouldn't be shocking to see that kind of move.”

If the newly elected president and Congress make diminishing the EPA’s power their priority, then Carlson says we would likely see environmental regulation shift largely to state governments. “This is going to make the states super-important again,” she explains, “When the [George W.] Bush administration stepped out of the business of regulating greenhouse gas emissions, you saw a lot of states take action.”

The American Energy and Infrastructure Act

One of the more cryptic of Trump's claims involves passing a new piece of legislation called the American Energy and Infrastructure Act within his first 100 days in office. This would, “Leverage public-private partnerships, and private investments through tax incentives, to spur $1 trillion in infrastructure investment over 10 years. It is revenue neutral.” It is hard to say exactly what the details of his plan are and how it would work. “It's very mysterious at this point in time, we don't really know what's contemplated there,” Fulton says, “If you piece together the different things that have been said by the president-elect about fossil fuels, and encouraging fossil fuel development, you'd expect this would have something to do with that.”

Trump has stated some of his goals for energy and infrastructure on his Web site, which include encouraging fossil fuel development and updating aging water systems. Trump will, of course, have to work with Congress in order to pass this legislation.

Ultimately, Trump can alter environmental and energy policy—but he cannot fulfill most of his promises as quickly as he has claimed. And in addition to the political, bureaucratic and legal obstacles the incoming administration will need to overcome, Trump is also up against strong market forces that are fueling the growth of natural gas and renewable energy, and devastating coal—which will likely make some of his campaign promises hard to accomplish.