On Tuesday, President Donald Trump signed an executive order that will begin the long, hard process of dismantling President Barack Obama’s climate policies — including, most prominently, the Clean Power Plan.

Trump’s executive order won’t, by itself, repeal the Clean Power Plan, which is a major Environmental Protection Agency regulation that aims to reduce carbon dioxide emissions from existing power plants 32 percent below 2005 levels. Instead, Trump will ask his new EPA head, Scott Pruitt, to replace Obama’s rule with … something else.

There are lots of options for what “something else” could look like: Pruitt might try to scrap the requirement for CO2 cuts altogether, or he might try to soften the rule considerably, easing the burden on coal plants. But crafting a new rule will take many months, if not years, and Pruitt will face a slew of procedural and legal hurdles in trying to undo Obama’s plan. “Procedurally, it’s hard to do,” says Richard Lazarus, a professor of environmental law at Harvard.

This is likely to be one of Pruitt’s biggest — and most consequential — moves as head of the EPA. So let’s walk through, step by step, what he’d actually have to do.

Repealing Obama’s Clean Power Plan will be a tricky, years-long process

When federal agencies like the EPA issue new regulations, they are required to go through a formal rulemaking process. First the EPA proposes a rule, laying out detailed legal and technical justifications for its actions. Next, the EPA solicits public comments on its proposal. Then the EPA has to read through all of the substantive comments and either take them into account or explain why it’s ignoring them. Finally, that final rule is subject to judicial review.

This whole process can take years. The Obama administration originally proposed its Clean Power Plan in June 2014 and received more than 4 million comments, many of them quite critical. The EPA didn’t finish the final rule — which used a complex formula to set emissions targets for each state and gave utilities flexibility in how to meet them — until August 2015. Once that final rule came out, industry groups and red states challenged it in court, and the Supreme Court put the rule on hold. The DC Circuit Court is currently mulling the Clean Power Plan’s legality and will issue a verdict at some point.

If Pruitt wants to repeal and replace the Clean Power Plan, he’ll have to go through this whole laborious process all over again. The EPA will have to write an entirely new power plant rule, along with a detailed and legally persuasive explanation of why it’s changing its mind. It will have to respond to millions of comments. And environmental groups are certain to challenge any final rule in court.

“The agency can’t just ignore the previous rule,” explains Richard Revesz, a law professor at New York University. “It has to make a sound argument for why its new approach is superior — and prove to the courts that it’s not just acting in an arbitrary or capricious manner.” Otherwise, the courts will knock down Pruitt’s attempts to rewrite the rule.

Making things even more complicated, we don’t know yet what the DC Circuit Court has to say about the Obama Clean Power Plan’s legality. That decision could come next week — or might not come for months. But the opinion handed down by that court will shape how Pruitt can proceed.

There are two big ways Pruitt may try to rewrite the Clean Power Plan

I asked legal experts across the political spectrum what Pruitt could do to roll back the Clean Power Plan, and the answers converged on two main options:

1) Don’t regulate CO2 from existing power plants at all. The most drastic step Pruitt could take would be to repeal the Clean Power Plan and replace it with … nothing. The government wouldn’t regulate carbon emissions from existing power plants at all. (The EPA would still regulate CO2 from other sources however.) This is a bit of a legal gamble, however, and it deserves more explanation.

When Obama’s EPA wrote the Clean Power Plan, it claimed authority to do so under Section 111(d) of the Clean Air Act, which allows the agency to set standards for existing sources of pollution (as opposed to new sources). But there was a weird, troubling loophole here.

See, back in 1990, the House and Senate each approved slightly different wordings of Section 111(d) — and, due to a clerical oversight, never reconciled them. According to critics, the House’s version basically implies that the EPA can’t regulate CO2 from existing power plants under 111(d) because it’s already regulating mercury pollution from those same plants under a different section of the law, section 112. The Senate’s version basically says it’s fine. And it’s not clear which version should prevail.

This is one of the key legal disputes over the Clean Power Plan being heard before the DC Circuit Court right now — and we’re still awaiting a decision. The court could either rule that a) the EPA definitely has the authority to regulate CO2 from existing plants under Section 111(d), b) it definitely does not, or c) the law is ambiguous and it’s up to the agency to decide which interpretation is correct.

So Pruitt could try to argue that the EPA has no authority to regulate CO2 from existing power plants — and just repeal the Clean Power Plan entirely. (The EPA’s endangerment finding would stay intact and the agency would still be required to regulate CO2 from new power plants and cars, since those are separate legal issues.) But this only works if the courts agree with his interpretation of the underlying law. If they don’t, Pruitt won’t get very far with this approach and will have to try something more subtle.

2) Rewrite the Clean Power Plan to be much weaker. As an alternative, Pruitt could say, okay, the EPA has the authority to regulate CO2 from existing power plants. But the specific way Obama went about it was inappropriate — and EPA should scale it back.

Here’s how this would work. In order to regulate existing power plants, the EPA had to identify a “best system of emissions reduction,” setting state targets based on what’s technically feasible and cost-effective. Obama’s EPA got creative with this and set emissions targets by assuming that states could a) improve the efficiency of their existing coal plants, b) shift from coal to cleaner natural gas, and c) add more renewable energy to their grids.

This allowed Obama’s EPA to require sweeping emission cuts. But it was also controversial, because the EPA was assuming that utilities could reduce emissions at individual power plants by taking actions outside of those power plants (e.g., reduce emissions at coal plants by replacing them with wind farms and gas turbines elsewhere). Opponents challenged this feature in court, arguing that the EPA should only look at measures that can be undertaken at the plants themselves (i.e., actions “within the fenceline”), which would lead to a much weaker rule.

So Pruitt could try to replace Obama’s Clean Power Plan with a more modest version that stays within the fenceline. This new rule might assume that utilities can upgrade the heat rate or efficiency of individual coal plants but wouldn’t have to do anything else. This would allow the EPA to set much weaker CO2 targets for states — allowing them to make a few modest tweaks to their coal plants rather than embark on the wholesale shift away from coal envisioned by the Obama administration.

Would this be legally defensible? That’s unclear. Environmental groups and other opponents might argue that focusing solely on coal plant tweaks isn’t actually the best system of emission reduction — and Obama’s Clean Power Plan is a superior approach. It’d be up to the courts to decide. But if Pruitt wanted to scale back Obama’s climate policy and slow the decline of the coal industry, this would be his best bet.

How much would repealing the Clean Power Plan matter for climate change?

It’s hardly certain that Pruitt will succeed in taking apart the Clean Power Plan. Environmental groups will be challenging him at every turn — and they’re very skilled at this sort of litigation. “We’re going to be watching closely to see how they justify any changes,” says David Goldston, director of government affairs at the Natural Resources Defense Council.

But let’s assume Pruitt does succeed. In that case, how much will scaling back the Clean Power Plan actually matter for climate change?

On the one hand, you could make an argument that it might not be a huge deal in the short run. After all, many states are already switching from coal to gas and adding more renewables anyway, even without an EPA mandate. One analysis by the Environmental Defense Fund found that dozens of states — including Texas, Michigan, and Florida, in addition to California and New York — are currently on track to meet the Clean Power Plan’s 2030 targets. Even if the Clean Power Plan dies, state policies and simple economics will still propel clean electricity forward.

On the other hand, there’s a good case that the death of the Clean Power Plan would very much matter at the margins. After all, there are still plenty of states that aren’t really thinking about decarbonization, like West Virginia or Missouri, and the Clean Power Plan would have prodded them in that direction. What’s more, the solar industry has long said that Obama’s rule would help expand solar power into states that don’t currently have much of it. Killing the rule could also slow the coal industry’s decline somewhat.

Additionally, the Clean Power Plan would’ve been a policy that future administrations could build on to require even deeper emissions reductions post-2030 — the sort of thing that will ultimately prove necessary if we want to halt global warming. If the plan gets repealed or significantly weakened, deep decarbonization becomes much harder and slower. In that sense, repeal is a pretty big deal.

Ultimately, though, the Clean Power Plan was just one slice of Obama’s broader efforts to tackle global warming — and only accounted for about one-fourth of the cuts necessary to hit US targets on greenhouse gas emissions under the Paris climate deal. The broader climate plan also involved carbon standards for new coal plants, fuel economy rules for cars and trucks, regulations around methane leaks from oil and gas drilling, efficiency standards appliances, and much, much more.

So a lot depends on what the Trump administration does to all those other regulations — and also how other countries around the world, especially China and India, react to any slippage in US climate action. Crafting a climate policy is a slow, painstaking process. Dismantling it will be too.

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