Donald Trump has made it perfectly clear that one of his top priorities is to dismantle the climate change regulations that President Obama has put in place over the past eight years. But the details of how he tries to do this matter enormously, and anyone interested in climate policy should pay close attention to the nuances here.

The Trump administration will have a lot of power, on its own, to stall or weaken Obama’s key climate rules, particularly the Clean Power Plan that regulates CO2 from power plants. It's not easy, but Trump has a fair but of leeway here.

The more pressing question, though, is whether Trump and the GOP Congress will pass a bill that will prevent the Environmental Protection Agency from ever regulating carbon dioxide again. If they did that, that would kill the Clean Power Plan entirely — and would prevent any future presidents from tackling climate change the way Obama did.

That latter move is a much, much bigger deal, and it depends on whether Democrats in Congress can stop this from happening. So let’s walk through the scenarios.

1) President Trump can weaken Obama’s Clean Power Plan on his own. This part’s doable.

First, some context: Back in 2007, the Supreme Court ruled that the EPA was required to regulate greenhouse gases like carbon dioxide under the existing Clean Air Act, so long as there was evidence that these pollutants harmed public health and welfare (which, scientists agree, there is).

Obama took that ruling and ran with it, issuing EPA regulations on new power plants, cars and trucks, methane leaks from natural gas drilling, and more. But the centerpiece of his agenda was the Clean Power Plan, which would aim to cut CO2 emissions from coal and gas-fired plants some 30 percent below 2005 levels by 2030. Under the rule, every state has to come up with its own plan for cutting emissions.

The Clean Power Plan is not yet in effect — it’s being challenged in a lawsuit that the DC Circuit Court is still mulling. So Trump has a couple of options here if he wants to weaken or block it, as Nathan Richardson, a legal expert with Resources for the Future, nicely details.

First, Trump could decline to defend the rule and hope it dies in court (remember, he gets to appoint a replacement to Antonin Scalia, so the Supreme Court could well strike the whole thing down).

Second, if the DC Circuit Court hasn’t ruled on the Clean Power Plan by January 20, the Trump administration could ask it to send the rule back to the EPA and then just redo the whole thing, likely making it weaker or toothless. This would be known as a “voluntary remand.”

Third, if the courts did uphold the Clean Power Plan, Trump could order the EPA to start all over and rewrite an entirely new (weak) rule. This isn't easy: it would take at least 12 to 15 months, and would involve a notice, a proposed rule, soliciting public comment, and so on. And the process would likely get bogged down by lawsuits from environmental groups (which are very, very skilled at this sort of litigation). But it’s doable.

Fourth, Trump’s EPA could simply decline to enforce Obama’s Clean Power Plan very rigorously. Again, he might face lawsuits here, but this is totally feasible. He could let recalcitrant states like Texas and West Virginia come up with very, very weak state implementation plans.

"There’s a lot of latitude in the review process," Michael Wara, an expert on energy and environmental law at Stanford, told me earlier this year. "The history of the Clean Air Act shows this. If you have a president who doesn’t like climate policy, they could basically signal to the states that they’re going to give a lot of compliance flexibility and allow states to make assumptions in their plan that reduce their costs."

For Trump, the upside of taking this route is that he could do this on his own. No one can stop him from rewriting EPA rules. The downside is that this would only be temporary. The Clean Power Plan would still technically be floating around, and a future president who cares about climate change could restart the process, forcing emissions reductions anew.

So if Trump and the GOP really wanted to forestall future climate action, they’d have to go through Congress. And that’s when things get trickier.

2) Only Congress can permanently neuter the EPA’s climate powers — unless Democrats stop them

The second, more radical option would be for Trump and the GOP Congress to pass a bill that explicitly forbade the EPA from ever tackling greenhouse gases again. That would override the Supreme Court’s ruling. And it would just require a simple amendment to the Clean Air Act. House Republicans have floated such a bill over and over before, and presumably they’ll try again.

The catch here is that Senate Democrats could try to filibuster this bill — Republicans only have 52 Senate seats, and they need 60 to overcome a filibuster. So that leaves a few possibilities:

It’s possible that Democrats could hold together and block any efforts to amend the Clean Air Act. If successful, that would at least allow the next president to take up EPA rules on climate change and restart what Obama did.

Alternatively, Senate Democrats might just cave. Note that a couple of their key members, like Joe Manchin (WV) and Heidi Heitkamp (ND), are up for reelection in 2018 in Trump-friendly, fossil fuel–heavy states.

Third, Senate Republicans could try to scrap the filibuster altogether and pass their CO2 bill with 51 votes. This isn’t guaranteed, especially with some Republicans strongly in favor of keeping the filibuster, but it’s entirely plausible. See Gregory Kroger’s post for more detail on this.

Fourth, Republicans could try to attach their anti-CO2 regulation amendment to a bigger, more popular bill — like Trump’s proposed infrastructure package, which has already attracted interest from several Democrats. In that case, much would hinge on whether Democrats want to make the EPA’s authority to regulate CO2 an absolute deal breaker.

Fifth, it’s possible that Republicans could decide this legislative battle is simply not worth the political headaches, especially with Trump putting a freeze on EPA rules anyway, and drop any plans to amend the Clean Air Act. This one seems unlikely given how much the conservative base and key industry supporters hate climate regulation, but it’s possible.

Note that a bill to take away the EPA’s right to regulate CO2 would have far-reaching effects. It would undo methane regulations on the oil and gas industry. It would undo the fuel economy standards that Obama has put in place for cars and light trucks. It would undo fuel efficiency rules for heavy trucks. And, most importantly, it would prevent any future presidents from taking unilateral climate action.

A bill like this could also have unintended ripple effects, as Gavin Bade points out at Utility Dive. Back in 2011, the Supreme Court ruled in AEP v. Connecticut that individuals could not file nuisance lawsuits against CO2 polluters because the EPA was already regulating the pollutant. But if the EPA can’t regulate the pollutant, those costly nuisance suits could become an issue again.

It’s also worth observing that scrapping the Clean Power Plan or fuel economy rules won’t kill all efforts to tackle climate change in the United States. Even if Trump gets his way, states like California will keep pushing forward with decarbonization anyway, harnessing the growth of wind and solar and efficiency. Natural gas from the fracking boom will keep killing coal power. But these EPA rules are still extremely significant, and they were intended to accelerate efforts to decarbonize the power sector. If the GOP scraps them, that will almost certainly blunt the place of climate action in the United States.

Which means this is going to be a defining environmental battle going forward. Green groups are likely to make protecting the EPA one of their top priorities for the next four years. It’s the difference, after all, between a mere delay in climate action while Trump is president and a lasting policy shift that will prevent future presidents from switching course.

Further reading:

Here’s a big-picture look at how Trump’s hostility to climate policy — and his plans to abandon the Paris climate treaty — could have repercussions around the world and weaken global efforts to tackle global warming

How Obama’s Clean Power Plan actually works — a step-by-step guide

Watch: A visual tour of the world's CO2 emissions