“In the weeks after the election of Donald Trump, friends and journalists called Deborah Sivas with roughly the same question: How bad could things get?

Sivas is a professor of environmental law at Stanford University, and she has decades of experience working as a litigator for environmental-rights groups. She knows how hostile new presidents can overturn green protections and she knows how lawsuits from friendly states and nonprofits can shore up those rules.

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“We hear a lot about Paris and the Clean Power Plan, but this [waiver] is a big part of it too,” Sivas told me. “These were the years—2017 and going forward—when the curve was supposed to bend a lot on car emissions.”

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“The car companies said, ‘Well, let us ramp up slowly, and then the curve will bend up.’ We’re now at the point where the curve is bending up. They’ve brokered a deal where they got the benefits of the early, slow ramp-up, and now when they really have to dig in and do it, they say, we really need relief from this,” Sivas told me.

If he did so, it would tee up a legal fight between the Trump administration and the state of California that would have deep and long-lasting consequences for the planet, no matter its outcome. “The state of California is gearing up for fighting all this stuff. They’re almost giddy to do it. They’re like, let’s go. Bring it on,” said Sivas.

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“It feels like this very technical thing about, ‘blah blah, waivers, blah blah blah,’ but it’s a very important part of the climate policy,” Sivas told me. “People aren’t going to stop driving, really. And transportation’s 40 percent of carbon emissions. The only way to get [those emissions] down is to get [fuel] mileage up. If the feds are going to take their foot off the gas—and to fight the states who are doing it—it could be a huge setback,” said Sivas.”