Last week, with the stroke of a pen, President Trump began the process of undoing the Clean Power Plan.

“With today’s executive action, I am taking historic steps to lift the restrictions on American energy, to reverse government intrusion, and to cancel job-killing regulations,” Trump said.

But experts say it’s going to take time to make big changes to the Obama administration’s initiative to reduce carbon emissions from coal-fired power plants. The EPA will actually have to write a whole new rule to repeal it.

However, Trump’s executive order will force the federal government to re-calculate the so-called “social cost of carbon.” When the federal government produces a new regulation, it has to spell out what the rules cost — to industry, for example — and weigh them against things like benefits to public health. To do that, the government puts a number on everything. And when it comes to the impacts of carbon, Trump wants that number reduced.

“With carbon and with climate change, it can be harder to sort of visualize and understand the damages and the cost to society,” says Janet McCabe, who was one of the architects of the Clean Power Plan and headed the EPA’s air office under Obama. “[With] a localized air pollutant, you have people immediately being affected and maybe having to go to the emergency room for an asthma attack. With climate, the impacts are longer term.”

McCabe says it’s challenging to connect things like droughts, heat waves and flooding back to specific emissions. But the costs are there all the same.

“I’m concerned about going backwards to a time where those costs didn’t need to be considered and not forwards to a time where we’re all thinking about those things together,” McCabe says.

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Reid Frazier contributed to this story.