The Trump administration took the first steps this week to unravel the Clean Power Plan, the Obama-era regulation that attempted to cut coal emissions dramatically in the next 15 years.

But it won’t be a very swift process.

The latest twist in the saga of the Clean Power Plan comes this month as the Environmental Protection Agency begins soliciting public comments for a replacement regulation, according to The New York Times.

The plan, which was enacted by President Barack Obama, was designed to reduce emissions from power plants by 32% in 2030 compared to 2005 levels. If enacted, the plan would force many coal-powered plants to close, and the prospect of the rule has deterred energy companies from investing in coal plants.

In an EPA document titled “October 2017 Tiering List,” and released this week, the agency announced that it will devise a rule “similarly intended to reduce carbon dioxide emissions from existing fossil-fueled electric utility generating units.”

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During the 2016 presidential campaign, President Donald Trump called the Clean Power Plan “stupid” and “job killing” and vowed to eliminate it upon entering office.

But despite an executive directive to fulfill this promise back in March, nothing until now has been done by the administration.

So why, exactly, has it taken so long to address the regulation? And why isn’t it going for a total repeal?

It turns out the Clean Power Plan is one of the most tenaciously contested regulations in modern history, celebrated by supporters for mitigating climate change and excoriated by opponents as government overreach.  

Long before Trump took office, the Clean Power Plan was subject to numerous legal challenges from industry groups and Republican-led states. In fact, the current EPA director, Scott Pruitt, was one of the chief architects of the primary legal challenge to the rule.

That lawsuit ultimately compelled the Supreme Court to freeze implementation of the rule in early 2016, an unusual decision that's still in effect to this day. In August, the U.S. Court of Appeals for the District of Columbia Circuit agreed to continue suspending the rule, but called on the Trump administration to offer monthly updates and to address the ruling in a timely manner.

The current legal entanglement isn’t even the biggest of roadblocks.

Most fundamentally, repealing an environmental regulation is challenging. That’s because all EPA regulations are compelled by official scientific findings, including the one that says greenhouse gases endanger public health and welfare. Repealing a regulation would mean rejecting the science it’s based on.

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The EPA is mandated to regulate carbon emissions because of science showing that they endanger public health; the mandate is known as the endangerment finding. Eliminating the regulation would violate that mandate.

Completely scrapping the Clean Power Plan would be too difficult and invite legal challenges that the EPA is failing on its mandate, according to The New York Times. Devising a suitable replacement could take months and even years of deliberation, public comment, and scientific research. And even then, it could be liable to legal challenges.

And that’s why the Trump administration is just now getting to the Obama-era Clean Power Plan, according to The New York Times.

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Further details on the public comment period are not yet available and it’s unclear how long the process will take.

Meanwhile, the legal challenges to the existing rule will continue to wind their way through the US legal system.

By the time a replacement rule is crafted and implemented, the energy dynamics the rule initially hoped to foster — a transition to renewable energy — will likely be irreversibly underway.  

As Nicole Ghio of the Sierra Club told The Guardian:

“Markets are demanding clean energy, and no amount of rhetoric from Donald Trump will be able to stop the fall of coal in the US and across the globe.”