Despite the vulnerability of many of his own properties to sea level rise, on the campaign trail President-elect Donald Trump stuck with Republican orthodoxy in questioning human-driven climate change, and criticizing the steps the Obama administration has taken to combat it.

Trump has promised to "cancel" the Paris agreement, the recently adopted global deal to curb global warming, and to curb climate regulations from the Environmental Protection Agency (EPA), including the Clean Power Plan to cut emissions from coal-fired power plants, during his first 100 days in office.

Leaving the U.N. Framework Convention on Climate Change 'would be the most symbolic antiestablishment move.' David Victor, University of California, San Diego

How much of this can he really do after he assumes the presidency on 20 January, along with a Republican majority in the House of Representatives and the Senate?

Here’s some analysis:

The Paris agreement

There are multiple paths Trump can take to undermining the U.S. ratification of the deal, which saw the country pledge to cut greenhouse gas emissions by up to 28% below 2005 levels by 2025. Under its requirements—already agreed to by the United States—Trump could not immediately withdraw from the deal, but he could do so by 2020.

There's a more drastic option as well, says David Victor, an expert on international climate policy at the University of California, San Diego. Trump could have the United States immediately submit a written notice to the Secretary General of the United Nations that it is leaving the U.N. Framework Convention on Climate Change, which includes nearly all the world's nations among its membership and coordinates international deals on climate. After a year, the departure would take effect. Victor suspects this as the most likely step. "It would be the most symbolic antiestablishment move," he says.

More simply, "They could just fail to deliver on U.S. commitment and reverse strong leadership at the international level that the Obama administration provided," says Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University.

None of these moves would kill the Paris agreement, but they would severely undermine it. At its foundation, the deal relied on an accord between the world's two largest emitters, China and the United States, to cut emissions, and in the run up to the election, China has been vocal about the need for the United States to live up to its international obligations. Indeed, a U.S. departure could see China "become one of the guiding forces for the Paris process," Victor says. And though the U.S.-China deal could go into a "deep freeze," it could still be revived under a new administration in 2020 or later.

What could put Paris into a deep freeze is money. The United States had promised $800 million a year to help finance climate adaptation for the least developed nations, and that money is unlikely to come through in a Trump presidency—although Congress has the final say on how money is spent. Without that incentive, nations could see little reason to meet their pledges. Action could instead shift to more bilateral deals between China, India, and the Europeans, or through the Arctic Council of nations with interests in that polar region. It would not surprise Victor if senior U.S. political and business leaders make a point of traveling the world to let them know that the country has a plan for rejoining global climate efforts for when Trump leaves.

If the United States does abandon the Paris accord, it will certainly mark a moment for nations that have not traditionally led international climate efforts to step up, as the United States and Europe deal with difficult political situations at home. "All the traditional leaders in global politics are distracted or sidelined," Victor says.

The Clean Power Plan

Domestically, Trump has appointed a prominent climate skeptic, Myron Ebell, to lead his transition team at EPA. Ebell, who leads the Center for Energy and Environment at the right-leaning Competitive Enterprise Institute, has warned against climate "alarmism" and called the agency's Clean Power Plan, issued in August 2015, "illegal."

As Trump's team takes over, however, they may find it more difficult than it might seem to change the power plan and other climate regulations that have already gone through lengthy review and release processes. There's much more to it than simply issuing an executive order, says Jody Freeman, director of the Environmental Law Program at Harvard Law School.

"You can't rescind a rule with the stroke of a pen," she says. "They'd have to engage a focused effort to undo regulations and replace them with something else."

Any changes to regulations that have existed for some time and moved into implementation, like fuel-efficiency standards for cars and trucks or rules controlling the carbon dioxide emissions from new power plants, would certainly face legal challenge from environmental groups, some state governments, and even industry groups seeking certainty or to protect investments. And courts aren't likely to look favorably on efforts to change regulations without sound scientific or technical reasoning, Freeman says.

The Clean Power Plan is currently being challenged in the U.S. Court of Appeals for the D.C. Circuit. The court could rule before the end of Obama's term, and then the Supreme Court, which already issued a stay on enacting the regulation, would have to consider whether to take up the case. Efforts by the Trump administration to change EPA's position would likely face more litigation, stretching out for many years. And even if the administration did find a way to stop the plan, the collapse of the coal market would likely continue, driven by the low price of natural gas, a competing fuel.

More drastic changes at EPA would require collaboration with House and Senate Republicans, as much of the agency's budget and regulations stem from legal requirements. It's possible that Congress could slash these laws and the agency's budget, as Trump and some lawmakers have promised. But it's far from certain that Republicans would be united enough to go through with such cuts, especially given the potential of a Democratic filibuster in the Senate. (It requires just 41 votes to prevent the body from voting on legislation or appointments.) "I don't know if there's a consensus [in Congress] to do that," Freeman says. "Certainly I don't think the election was about these specific issues."

A Trump administration could more easily block, or water down, EPA regulations that are not yet finalized, including limits on greenhouse gas emissions at existing oil and gas sites, or clean water regulations on hydraulic fracturing. Trump appointees could also target regulations and policies that have a good deal of agency discretion baked into them, as courts often defer to agency expertise, Freeman says. "There's a precedent that agencies can change their minds, as long as it’s rational and reasonable."

Reagan Revolution, redux?

With oilmen like Harold Hamm, CEO of Continental Resources, and Forrest Lucas, the founder of Lucas Oil, named as potential candidates to lead the departments of energy and the interior, respectively, in a Trump administration, the mostly likely historical analog for the next few years could be the start of Ronald Reagan’s presidency, when he appointed senior officials who were often hostile to the policies of their own agencies. For example, Reagan’s secretary of the interior, James Watt, wanted to sell off public lands and reduce forest protections, and his EPA head, Anne Gorsuch, moved to soften clear air and water rules. Some agency staff fought back, and there were frequent leaks, resignations, and lawsuits. Both Watt and Gorsuch ultimately resigned amidst political chaos, and were replaced by less polarizing appointments. If Trump follows a similar path, “there could be a whole lot of churn," Victor predicts.

Indeed, Trump may quickly learn the limits of the presidency, Victor adds. "The Oval Office will be a lonely place," he says, if the White House attempts to make radical changes that agency professional staff fiercely opposes.

States may step up

If Trump does follow through with his stated climate skepticism, expect the states to continue their activism through limited partnerships like California's greenhouse gas laws, or the Northeast's Regional Greenhouse Gas Initiative, adds Vicki Arroyo, executive director of the Georgetown University Climate Center in Washington, D.C.

"Based on early conversations with senior state officials [Wednesday] from some red- and blue-led states," she says, "I expect we will see a number of states and cities continuing to lead in promoting clean energy and increasing resilience to climate impacts."

The most important thing scientists can do, however, is find a way to help Trump understand their pressing concerns of climate change, Oppenheimer says. "The community has a job to do here. It can't go off and sulk about the outcome of the election."