The stunning results of the 2016 election have prompted headlines suggesting that Trump will, with the help of the Republican Congress, dramatically reverse the Obama legacy on climate, energy and the environment. But how realistic is this threat? The short answer is: the picture is significantly more complicated, and markedly less bleak, than the headlines suggest.

While Trump may try to rescind a number of regulations, the process would be long, arduous and only partially successful. Moreover, any broad legislative attack on environmental statutes is unlikely to succeed without a filibuster-proof majority in the Senate, which the Republicans do not have.

The analysis below is candid about the bad news (some of which is bad indeed) but also includes the better news about the substantial legal, political, and practical obstacles a Trump-led regulatory rollback would face.

The Bad News

It is possible for Trump unilaterally to withdraw from the Paris Agreement on Climate Change. (It is not a treaty; it required only the exercise of Executive power). Even without formal withdrawal, he could effectively abandon the U.S.’s commitment, and its leadership role, by making little effort to deliver on the U.S. pledge (to cut greenhouse gas emissions up to 28% below 2005 levels by 2025).

It is possible for Trump’s EPA to try to rescind and replace certain climate, environmental or energy-related regulations. (Congress is not necessary to this process.) It is standard practice for an incoming administration to immediately halt work on so-called “midnight” regulations—rules that may have been adopted in the last months of an outgoing administration—in order to review and potentially rescind or amend them. The Trump White House would almost certainly do this, and all it requires is a stroke of the president’s pen.

In addition, a Trump White House could also initiate a more sweeping review of agency rules, even those that were not adopted in the waning days of the Obama administration, to consider which to rescind and revise. One obvious target would be the Clean Power Plan (see more on that below.) But other environmental rules are also vulnerable. The highly controversial Army Corps of Engineers’ regulation, which defines the government’s jurisdiction to regulate wetlands (known as the “Waters of the United States” rule), now being challenged in the courts, could be rescinded and revised as well, perhaps dramatically cutting back on its reach. During the campaign, Trump singled out the WOTUS rule, pledging to reverse it. And that step could moot any ongoing litigation over the rule.

It is possible for Trump’s EPA/DOI/DOE, etc. to slow-walk any new or proposed regulations, even if they are required by statute, and to likewise seek to hobble enforcement. This is a “rope-a-dope” strategy through which the agencies would appear to be implementing the environmental laws while not in fact seriously doing so. For example, look for a Trump administration to try to abandon or slow-walk unfinished EPA efforts to regulate methane emissions from existing oil and gas infrastructure (on both private and public lands), and to delay such routine things as updating DOE appliance efficiency standards.

It is possible for Trump to try to weaken regulations through centralized oversight of agency rules. Under longstanding Executive Order, the Office of Information and Regulatory Affairs (part of the White House Office of Management and Budget) oversees agency cost-benefit analysis. A Trump OIRA could tighten its grip on the agencies.

Trump could, on his first day, issue a new Executive Order on cost-benefit analysis, imposing additional requirements, to strengthen centralized review, making the regulatory process more time-and resource-intensive, and exerting even more WH control over it.

Pushing the envelope, Trump could seek to extend these centralized review requirements to independent agencies (including the Federal Energy Regulatory Commission, which regulates the electricity sector and has adopted climate-friendly policies of late). No president has sought to do this, fearing a backlash in Congress and a legal fight over the constitutionality of exerting presidential control over independent agencies. But some scholars have argued that requiring these agencies to produce economic justifications for their rules is constitutionally permissible. And there might be a limited backlash from a new Republican Congress. Moreover, a newly comprised Supreme Court with a 9th Trump-appointed Justice might well be deferential to expansive executive power.

A Trump OIRA could also reconsider its policy that federal agencies must calculate a “social cost of carbon” as part of standard cost-benefit analysis (a calculation that seeks to capture the significant societal benefits from rules that reduce greenhouse gases). A single federal court of appeals has held that all federal agencies must include a social cost of carbon calculation in their cost-benefit assessments; a Trump White House could ignore that ruling as inapplicable nationally.

Trump could also withdraw OIRA’s “guidance” to federal agencies on how to incorporate greenhouse gases into their environmental impact analyses (which are required under the National Environmental Policy Act for all major federal actions with significant impacts on the environment). This document was never finalized by the Obama administration.

The Better News

It is not as easy as Trump and others have made it sound to rescind and replace agency rules. This does not happen with the stroke of a pen. A President cannot do it, for example, by Executive Order.

History suggests that while new administrations make a grand show of reviewing their predecessor’s rules — looking particularly closely at those still pending — or those rushed through at the last minute, they rescind very few. According to one study, President Clinton repealed only 9% of George H.W. Bush’s midnight regulations, and George W. Bush repealed only 3% of Clinton’s. (Indeed, 82% of Clinton’s midnight rules were not even amended by Bush.)

At a minimum, rescinding and replacing any rule takes some time. And litigation would be inevitable. The Agency responsible for the rule—a Trump EPA, say— would need to go through a notice and comment process, which typically takes at least a year, often two, and sometimes longer). Challengers inevitably would sue over these actions, and the agency would need to defend them in litigation, mustering enough evidence to persuade a reviewing court that the agency is not being “arbitrary or capricious.”

While it is true that agencies are allowed to change their minds, and may do so in part for political reasons, they still must defend any new rule as rational. (The applicable case is FCC v. Fox, which requires the agency to defend the new rule, just as they would the original.) This can be difficult to do, especially when the new rule does a complete about-face, and where there is a substantial or lopsided record supporting the original rule. It would be extremely difficult, if not impossible, for example, to rescind the “endangerment finding” (the determination that greenhouse gases endanger health and welfare, and the legal predicate for regulating these pollutants under the Clean Air Act), given the comprehensive scientific record EPA assembled to support the finding, which it made in 2009 and which the D.C. Circuit has upheld and the Supreme Court declined to review.

It might be easier to rescind other rules, however—where, for example, a Trump EPA can argue there is room for discretion, and can defend its new interpretation as at least as reasonable as the prior administration’s. This might be Trump’s strategy for revisiting the Clean Power Plan. (See more on that below).

Still, past experience with incoming administrations seeking to revoke already-final rules (as the George W. Bush administration did with the “roadless” rules issued by the Forest Service under Clinton), suggest a bumpy road ahead if Trump were to try the same strategy. Inevitably, the agencies seeking to rescind these rules would be sued, and they would run the risk of being blocked by the courts.

Moreover, some rules are mandated by statute. They are non-discretionary and the agency cannot simply choose not to regulate. This is the case with the proposed rule to control methane emissions from existing oil and gas infrastructure. The Clean Air Act compels it.

Rescinding rules that are already final and partially implemented can be especially difficult to the extent the affected industry already has invested in compliance. In such circumstances, the industry itself may resist change because it would suffer additional expense from the uncertainty or volatility posed by rescission. An example of such a rule might be the historic standards adopted by the Obama Administration doubling the fuel efficiency of cars and trucks by 2025, which, after extensive negotiations, garnered the support of the entire auto industry. Vehicles designed to comply with these rules have for years now been rolling off the assembly lines, and product plans for later years are already underway. (It is possible to imagine some easing of standards for the outer-most years, however, since the agency already has committed to do a mid-term evaluation).

Trump efforts to expand White House control over agency rules could easily overreach, and would invite litigation. While some of the things OIRA might do to tighten its grip over agency regulations would be perfectly lawful, there are limits. Where a cost-benefit or other requirement directly conflicts with an agency’s statutory mandate, the statute governs. Courts are reluctant to adjudicate such conflicts, and generally try to give effect to both executive orders and statutes wherever possible, but any effort to override environmental laws in this indirect way would clearly invite legal challenge and run a significant risk of a rebuke in the courts.

So too any effort to extend White House regulatory review over independent agency rulemaking—even a Republican Congress might bristle at a new president’s effort to exert more control over agencies that Congress purposely designed to be insulated from presidential control. And members of Congress know they will one day again face a Democratic President, who would retain this power. A lawsuit is guaranteed.

It is obviously possible for a Republican Congress to try to weaken environmental statutes, but any such effort is unlikely to succeed. With only 51 members, the Republican Senate does not have a filibuster-proof majority. And the difference between now and even 6 years ago is that just about all of the moderate Democrats (who might cross the aisle to vote with Republicans on climate and environmental issues) are now gone.

It is obviously possible for a Republican Congress to try to defund agency programs or initiatives. (For example, Congress could prohibit agency expenditures for anything related to greenhouse gas regulation.) Such measures, too, are subject to filibuster, although they may be easier to slip into appropriations bills, or omnibus packages, which democrats may have a hard time opposing.

The Fate of the Clean Power Plan in Particular

The D.C. Circuit court of appeals is currently considering the legality of the Clean Power Plan. The Court could issue a decision any time, but realistically given the complexity of the issues is unlikely to do so before December or January. If the Court has not decided the case before Trump’s inauguration, it is possible that, immediately afterward, the new Trump Department of Justice could ask the D.C. Circuit not to decide the case, and instead to remand the case back to the new Trump EPA. If the Court were to grant this request, the new Trump EPA could then rescind and modify the rule.

If the D.C. Circuit already has decided the case by the time Trump assumes office (or if the Court has not yet issued its ruling, but rejects the request for remand) and if the Court strikes down the Clean Power Plan, a Trump Department of Justice may choose not to appeal it, signaling that it wishes to leave it dead. However, all of the state and environmental intervenors who participated in the litigation on EPA’s side would appeal it. The Supreme Court would decide whether to grant review.

If the D.C. Circuit were to decide the case and uphold the Clean Power Plan, and if the state and industry petitioners who oppose the rule were to appeal that decision to the Supreme Court, a Trump Department of Justice may opt not to defend the rule. In that instance, the state and environmental intervenors who support the rule would defend it. At the same time, those intervenors might ask the Supreme Court to lift the stay it has imposed on the rule’s implementation in light of the D.C. Circuit’s ruling — but this is something that a Court still tied at 4-4, or even a newly complete Supreme Court, with a Trump-appointed 9th Justice, would be unlikely to grant.

Ultimately, a new Supreme Court with a Trump-nominated conservative 9th Justice would likely have the votes to strike down the Clean Power Plan. Not to mention that a Trump EPA could just rescind the rule—see more on that possibility immediately below—rendering any Supreme Court decision moot.

To rescind the rule, a Trump EPA would claim that it has the discretion to adopt a less stringent approach than the Obama EPA, because the Clean Air Act is ambiguous in key respects. For example, a Trump EPA could define “best system of emissions reduction” for greenhouse gases more narrowly than the Obama EPA has done, requiring only modest efficiency upgrades to electric generating units on-site, rather than setting higher standards based on the variety of measures that could be taken on the interconnected grids to substitute cleaner natural gas- for coal-fired power, and renewables for both. To support this change, EPA would argue that the statute leaves room for more than one interpretation, and that its newly adopted (and less stringent) view is at least reasonable, thus entitling the agency to deference. This is far from a slam-dunk argument.

Setting aside legal constraints, there are other, real-world limitations on a complete retrenchment of the Clean Power Plan. The utility industry is already investing in a transition to cleaner energy because of separate state and federal regulations, as well as independent market conditions e.g., cheap natural gas. In addition, Congress has already locked in tax incentives for renewable energy—to 2019 for wind, and into the 2020s for solar. Plus, many states intend to push forward with a variety of plans to reduce greenhouse gases and other air pollution, promote renewable energy, and invest in energy efficiency. The diversity and complexity of these measures might ultimately nudge the utility industry to ask the federal government to start regulating again, to promote uniformity and predictability. Thus, in view of all of these dynamics, it may be hard to reverse the trajectory of the Clean Power Plan, even if the rule itself is struck down or rescinded.

In Sum

A determined and united effort by Trump and a Republican Congress could certainly deal a substantial blow to the Obama legacy on climate, energy and environment. Most immediately, Trump singlehandedly could renounce U.S. leadership on international climate negotiations, which would stall momentum for the Paris Agreement. But any effort to fully unravel the substantial and meaningful regulatory initiatives of the last 8 years will be long, complicated, difficult—and in the end likely only partial and arduous because of the significant legal, political, and practical barriers to doing so.

There will be many twists and turns to come in the months and years ahead, as this process plays out in the White House, Congress, the agencies, and the courts. The country is very likely looking at a partial rollback, but the reality is not as bad as all the bluster suggests.

Jody Freeman is the Archibald Cox Professor of Law and the Founding Director of the Environmental Law Program at Harvard Law School. She served in the Obama White House as Counselor for Energy and Climate Change.

This post was originally published on the Environmental Law and Policy Program blog.