GEOFF BENNETT: A first-of-its-kind trial is under way in Montana, where a group of young people argue the state's lawmakers aren't doing nearly enough to address climate change.

Montana is one of the nation's biggest coal producers, and the industry there has strong political support.

William Brangham reports on how the plaintiffs are using a unique clause in the state Constitution to try and force a change.

WILLIAM BRANGHAM: It was back in middle school when Claire Vlases first asked some adults to do something for the environment.

Her middle school in Bozeman, Montana, was undergoing a major renovation, so she thought, why not put solar panels on all that new roof?

She presented the idea to the project managers.

CLAIRE VLASES, College Student: I was immediately shut down.

They just told me that the idea wasn't feasible and it was way too expensive.

Went home, cried about it a little bit, and then thought about it more.

And I realized, if the only barrier to putting solar panels on the school was the cost, how it wasn't a big deal.

I could just raise the money myself.

WILLIAM BRANGHAM: And this is middle school you having this idea?

(LAUGHTER) CLAIRE VLASES: Yes, it worked well because I didn't have any idea of what money was.

I didn't know.

I didn't know that $125,000 was a big amount.

(LAUGHTER) WILLIAM BRANGHAM: So the teenager and some other classmates held fund-raisers, and they raised the money.

Those panels now supply a quarter of the schools's power, saving the district an estimated $8,000 a year.

But Vlases, who's now going to be a junior in college, has set her sights on an even bigger issue.

She and 15 other young people from across Montana, including multiple members of Native American tribes, are suing their state.

They say Montana's support and permitting of oil, coal and gas projects is polluting the atmosphere, exacerbating climate change, and harming the state's environment, which, they argue, their state Constitution explicitly prohibits.

Their case, known as Held v. Montana, is the first such state case to ever go to trial.

CLAIRE VLASES: One reason why I love Montana is because, in our Constitution, it says that every person here has a right to a clean and healthful environment, which isn't in almost all the constitutions in the United States.

WILLIAM BRANGHAM: Right.

That is pretty rare.

CLAIRE VLASES: Mm-hmm.

That's not a political issue here in Montana.

We all are here because of the land.

And we have a right to protect that.

And so, as I have learned about climate change, and I have seen what our lawmakers have done promoting fossil fuel industries, I have realized that that is unconstitutional acts.

WILLIAM BRANGHAM: Montana's Constitution has this unusual clause because, back in the 1970s, some citizen conservationists who were concerned with mining and clear-cutting wanted to enshrine a more overt environmental protection right near the top of their Constitution.

Sandra Zellmer, who teaches and researches environmental law at the University of Montana's law school, says these young plaintiffs are testing the strength of that protection.

SANDRA ZELLMER, University of Montana: This allows the plaintiffs to put on evidence that the state's decision-making is exacerbating greenhouse gas omissions and leading to climate change, warmer, dryer, temperatures with all of the adverse effects heat waves, wildfires, depleted fisheries, destruction of the stability of our agriculture.

So to be able to introduce experts and to get that before a judge in a courtroom at trial is really quite phenomenal.

WILLIAM BRANGHAM: But the plaintiffs are facing strong opposition.

This year, the Republican supermajority in Montana's capitol and the governor's mansion passed a series of laws promoting further coal and gas development and hampering renewable energy.

It barred state agencies from considering the impact of greenhouse gasses when issuing permits.

And it repealed the state's official energy policy, which narrowed the scope of the youth lawsuit.

Republican Senate Majority Leader Steve Fitzpatrick, who supported these legislative moves, points out that Montana already uses a lot of renewable energy and argues any further changes will have no bearing on climate change.

STATE REP. STEVE FITZPATRICK (R-MT): Just because there's something going on doesn't mean that we necessarily need to take steps that are going to be expensive, that are going to cost people jobs.

We are one million people in a country of over 300 million people.

We're one million people on an Earth with 7.9 billion people on it.

I think there's very little that we're going to do at the state level that's going to have an impact on Earth's temperatures.

WILLIAM BRANGHAM: The young plaintiffs are being represented by Our Children's Trust, an Oregon-based environmental law firm that has tried, mostly unsuccessfully, to bring similar cases in every state and against the federal government.

Montana's being represented by it's attorney general, Austin Knudsen.

His office described the suit as a meritless publicity stunt and, in a statement to the "NewsHour," said these young plaintiffs are being manipulated -- quote - - "Our Children's Trust is a special interest group that is exploiting well-intentioned Montana kids, including a 4-year-old and an 8-year-old, to achieve its goal of shutting down responsible energy development in our state."

CLAIRE VLASES: Even though I'm a younger person, it doesn't mean that I'm manipulated or that I don't know the decisions that I'm making.

He knows the decisions that he makes every day, and I believe it is our decisions that define us as people.

And so there shouldn't be a question to my decision just because I'm a couple of years younger than him.

WOMAN: Claire, you spend a ton of time moving around outside, apparently.

WILLIAM BRANGHAM: Last week, Claire Vlases took the stand at the courthouse in Helena, testifying how a shrinking snowpack impacted her job as a ski instructor and how smoke from wildfires made it hard for her and her cross-country team to run in high school.

CLAIRE VLASES: Knowing that there is something I can do to protect the landscape and the world that's given so much to me and allowed me to be a strong individual, that's the most important thing.

WILLIAM BRANGHAM: Vlases and her fellow plaintiffs hope the judge will rule that state that the state's support for fossil fuels is harming the environment, and thus is unconstitutional.

And even though that kind of ruling won't immediately change Montana's energy or environmental policies, they see a longer-term goal.

CLAIRE VLASES: We hope that it will be the check that our legislature needs to keep them back on the right track, but it, moreover, would set an example to the other states that this is something that people want, especially young people, and that we can make a difference.

WILLIAM BRANGHAM: The trial is scheduled to finish by the end of this week.

For the "PBS NewsHour," I am William Brangham in Bozeman, Montana.