Fifteen years after it opened the door to greenhouse gas regulation, the court is poised to close it.

The Supreme Court issued a pair of seismic rulings last week on abortion and gun control. Now it’s climate change’s turn.

The court is expected to rule this week on whether the Environmental Protection Agency can continue to regulate carbon dioxide and other greenhouse gases using the Clean Air Act.

The decision in West Virginia v. EPA — which comes as scientists warn that the world is running out of time to avert catastrophic global warming — could severely limit the federal government’s ability to combat warming.

And depending on how broad the court’s ruling is, it could also drastically weaken the government’s power to regulate anything.

“Whatever the court does here … it could have long-term, enduring, harmful effect,” said William Buzbee, a law professor and director of Georgetown Law’s Environmental Law and Policy Program.

Any progress the federal government has made on climate change has stemmed in large part from a 2007 Supreme Court case, Massachusetts v. EPA. It established the agency’s right to regulate greenhouse gases including CO2 and methane as “pollutants” under the Clean Air Act. Though it has moved forward in fits and starts, allowing the EPA to set limits on these emissions under the Clean Air Act is, alongside the technological and economic advances of the solar and wind power industries, among the most important developments of the last few decades.

None, some or all of that result is currently at risk.

The current case, West Virginia v. EPA, was brought by several fossil fuel-producing states along with coal and mining companies. The plaintiffs argued that an Obama-era rule called the Clean Power Plan overstepped the agency’s statutory bounds by trying to regulate greenhouse gas emissions from existing power plants.

Interestingly, the rule in question isn’t even a rule anymore; in fact, it never really was.

The Obama administration issued the Clean Power Plan in 2015, but in early 2016, before any actual implementation took place, the Supreme Court issued a stay after 27 states joined a legal challenge. With the election of President Donald Trump, the rule was dead in the water; his administration issued a separate, much more industry-friendly regulation (the Affordable Clean Energy rule), which itself has since been vacated by the courts. As it stands, there is no relevant EPA rule, and the Biden administration has yet to issue its promised replacement.

Improvident Hail Mary

The strange hole at the center of West Virginia v. EPA offers a tiny glimmer of hope for a less-than-dire outcome.

“It’s possible the court is ultimately persuaded that the case isn’t really ripe for resolution until the Biden EPA finalizes a new rule,” said Cale Jaffe, an associate professor and director of the University of Virginia’s Environmental Law and Community Engagement Clinic. “If it goes that route, then it might simply dismiss the case as ‘improvidently granted,’ without reaching the merits.”

Experts say this is rare and, in this case, probably an unlikely outcome, but Jaffe did point out that we have seen one “DIG” — dismissal as improvidently granted — already this term, in a case involving Republican states defending Trump-era immigration policy.

Another possibility is that it could rule that the EPA lacks the authority to institute what are known as “beyond the fence-line” controls, meaning that the agency cannot issue systemwide requirements for emissions but instead can regulate individual facilities and their smokestack emissions — a much more limited mandate, but not one that overturns the Mass. v. EPA result and removes greenhouse gases from the agency’s purview entirely.

A darker possibility lies in what’s known as the “major questions” doctrine — the premise that if Congress intended agencies to make sweeping, economywide changes with their regulations, the relevant legislation must say so specifically and clearly.

“If they really go whole hog, it’s possible they could simply overturn Massachusetts v. EPA, and just say, ‘You know, that was wrong because it’s a violation of the major questions doctrine,’” said Victor Flatt, a law professor at the University of Houston and co-director of the Environment, Energy and Natural Resources Center.

That would send the agency more or less back to square one, without a mandate to regulate greenhouse gases at all. But Flatt said that’s still not even the worst-case scenario.

“The worst thing that can happen is that they say that it violates the non-delegation doctrine,” he said. “[That] it’s a constitutional violation.” That doctrine essentially says that Congress cannot grant its legislative authority to other bodies. If the court says the EPA’s regulatory efforts violate that principle, or if it overturns what is known as the Chevron deference — the idea that if the legislative guidance to the agencies is not explicit, federal courts must defer to the interpretation of the experts at those agencies — the regulatory agencies themselves are in trouble.

“It relegates them to a much more limited role,” Flatt said, though he added that, thankfully, he does not expect the court to invoke the non-delegation doctrine.

A surprisingly positive legacy

But if the major questions-related outcome comes to pass, or even a more limited ruling that shuts off widespread greenhouse gas regulation, it would mark the end of a relatively short but meaningful era in American efforts to combat climate change. The biggest impact of Massachusetts v. EPA was the Obama administration efforts to limit tailpipe emissions from cars and trucks — efforts that clearly worked to a large extent.

Obama-era rules for cars and trucks, which Trump partially rolled back, have now been solidified by a rule President Joe Biden finalized in late 2021. That rule requires that cars and light trucks of model years 2023 through 2026 manage roughly 55 miles per gallon in laboratory emissions testing. The EPA estimates that the rule will prevent more than 3 billion tons of CO2 emissions — an amount bigger than the total annual carbon dioxide output of India, the world’s third-largest emitter — over the next three decades.

Asked about the legacy of Mass. v. EPA, a spokesperson for the agency pointed Grid toward the greenhouse gas data since the ruling. Though the spokesperson cautioned that large trends can’t be pinned to one single event or policy, the results are notable nonetheless: Between 2007 and 2019, greenhouse gas emissions from the U.S. transportation sector dipped by 5.1 percent (2020 data is available, but the pandemic resulted in a significant but short-lived decrease, which unreasonably skews this trend). Over that same period, registered vehicles on American roads increased by 8.7 percent, and passenger-miles driven climbed by 7.6 percent.

“Those [rules] are effective,” Flatt said. “They can do a lot. The automobile thing already has done a lot.”

Taking that bite out of the transportation sector’s emissions was an obvious and important victory — the sector is the biggest U.S. source of greenhouse gases, and there is a definite sense of momentum behind the transition to electric vehicles and dramatically reduced tailpipe emissions in general.

The second-biggest sector for greenhouse gas emissions is the electricity sector, the subject of the Clean Power Plan and West Virginia v. EPA. The economics of coal, natural gas and renewable energy has led to emissions decreases in that sector as well, but achieving the Biden administration’s stated climate goals — about a 50 percent reduction in overall emissions from 2005 levels by 2030 — will require more stringent regulation and enforcement. So the specifics of what the Supreme Court says are enormously important.

“No matter what the Supreme Court does — unless it takes that really eye-opening step of saying [carbon dioxide is] not a pollutant … then there’s going to be some way that EPA can regulate carbon emissions from these sources,” said Brendan Collins, an environmental lawyer and partner in Ballard Spahr’s Philadelphia office. “And it’s better they all get about doing it as quickly as possible, because it’s already two years into Biden’s first term.”