After 30 years on the Supreme Court bench, Justice Anthony Kennedy will leave the nation's highest courthouse at the end of July.

With Kennedy's departure comes much uneasiness. One cause for concern is over the paramount climate decision Massachusetts v. EPA, in which Kennedy proved to be the deciding swing vote, as he often was. The worry is that with him gone, the ruling will be left imperiled.

The case occurred after the EPA decided, in 2003, that it could not regulate heat-trapping greenhouse gases. Twelve states, including Massachusetts, sued the agency. They argued that these gases were pollutants and a danger to the public. Eventually, the case found its way to the Supreme Court.

Settled by a five to four vote in 2007, Massachusetts v. EPA ruled for the first time that heat-trapping greenhouse gases are pollutants, and that the Environmental Protection Agency (EPA) can regulate them, just as the agency reins in pollution emitted by cars and trucks.

"I think Massachusetts v. EPA is the most important environmental decision the Supreme Court has ever decided," Ann Carlson, the director of the Emmett Institute on Climate Change and the Environment at the UCLA School of Law, said in an interview.

President Donald Trump will select the next Supreme Court nominee, and it's almost certain this individual will, at minimum, find Massachusetts v. EPA flawed or bad law. Trump is openly hostile to widely accepted climate science, and appears not to have even an elementary understanding of how climate works.

Just how important has Massachusetts v. EPA been?

Before the decision, the EPA did not consider greenhouse gases — notably the invisible, potent gas carbon dioxide — an air pollutant. So it wasn't regulated as one.

Kennedy's swing vote changed that.

"I think it’s a highly significant case because it opened what had been a locked door," Joseph Goffman, executive director of Harvard University's environmental and energy law program, said in an interview.

"The case was tantamount to the opening gun in a race."

Justice Anthony Kennedy (on right) administers the judicial oath to Judge Neil Gorsuch in April 2017. Image: Chip Somodevilla/Getty Images



Specifically, the case enabled the EPA to use a powerful law, the Clean Air Act, to rein in greenhouse gas emissions from cars, trucks, power plants, and industry. This same law had already proven hugely effective in limiting other air pollutants, like the nitrous and sulfur oxides expelled from cars.

"The Clean Air Act has been exceptionally successful as law in making huge changes in public health," said Goffman. "All you have to do is look at L.A. in the 1970s, and then look at it today."

"I remember visiting L.A. in the '70s, and after a day or two, having a sore throat because of smog," he added.

Following the Massachusetts v. EPA decision, the Obama Administration could use the power of the Clean Air Act to regulate greenhouse gases, which didn't cause sore throats and coughing fits, but stoked accelerated warming and disruption of the global climate.

Cars, trucks, power plants, and fossil fuel operations all became regulated. In 2012, for example, the Obama Administration finalized ambitious gas efficiency standards for cars in the U.S. The goal was to slash fuel use in half by 2025, and accordingly, greenhouse gas emissions, too.

President Obama, though, chose to focus on something all Americans could get behind.

"By the middle of the next decade our cars will get nearly 55 miles per gallon, almost double what they get today," Obama said.

But the most far-reaching effort was the Obama Administration's Clean Power Plan, which finalized EPA standards that intended to slash the carbon dioxide produced by U.S. power plants (notably from coal burning) substantially, to less than 30 percent of 2005 levels by the year 2030.

Smog-filled Los Angeles in the late 1970s Image: Nick Ut/AP/REX/Shutterstock

The Trump administration now seeks to overturn both the historic Clean Power Plan and fuel efficiency standards.

Will a new court overturn Massachusetts v. EPA?

Instead of simply overturning the verdict, it's possible that a new court could take it apart piece by piece.

"I think they’d chip away at it instead of overturning it outright," said Carlson.

For instance, in 2014 the Supreme Court ruled, following conservative Justice Antonin Scalia's opinion, that the EPA couldn't regulate greenhouse gas emissions from some smaller sources of air pollution.

If Kennedy's replacement proves wary or opposed to Massachusetts v. EPA, each time an EPA greenhouse gas regulation is brought to court, the conservative majority may find another way to narrow the scope of when these gases can be reined in.

"This is really where I think people who view the Clean Air Act as the ongoing tool for limiting green gases in the future should be focusing their anxiety," said Goffman.

It's also still plausible that the court could overturn the decision completely.

The Sago coal mine in West Virginia Image: Jeff Swensen/Getty Images



"It’s unlikely, but not impossible," said Carlson, noting that just this week the court overturned a 40-year-old labor union decision.

Two current justices, Samuel Alito and Clarence Thomas, already believe the decision should be overturned, said Carlson.

Massachusetts v. EPA embraced climate science

The landmark case didn't just allow the federal government to mitigate greenhouse gases, it also signaled an early embrace of climate science.

"It recognized that the science of climate change is strong and indisputable," said Carlson.

"Massachusetts v. EPA needed to determine if greenhouse gases endangered health and welfare," just like other pollutants, said Carlson.

The court found it did. These risks included "increases in temperatures, changes in extreme weather events, increases in food- and water-borne pathogens" and were supported by evidence from both the U.S. Global Climate Research Program and the Intergovernmental Panel on Climate Change.

The Clean Air Act, on which the decision rests, certainly didn't always regulate gases like carbon dioxide, but it left ample room for the sciences to progress and reveal new threats to the public.

"It's hard to overstate how much the Clean Air Act was driven by science and an understanding of how science works," said Goffman, noting that Congress overhauled the law in 1977 and 1990 to account for advances in environmental and public health sciences.

"It’s almost as if Congress knew there would be a new class of pollutants," he said.