A landmark climate change lawsuit reached Montana’s Supreme Court on Wednesday, with the justices hearing arguments that a state law promoting fossil fuel development violated Montanans’ constitutional rights.

The case was heard in a Helena courtroom filled to capacity with spectators, as temperatures outside closed in on 100 degrees.

At issue was the appeal of a decision last year, when a Montana judge blocked a state law that prohibited agencies from considering climate impacts when deciding whether to approve fossil fuel projects such as new power plants, pipelines or mining. The ruling, by District Judge Kathy Seeley, was prompted by a lawsuit filed by 16 youths who argued that the law violated Montana’s constitutional right to a “clean and healthful environment.”

It was the first ruling in the United States to effectively establish constitutional rights to a stable climate, said Patrick Parenteau, professor of law emeritus and senior fellow for climate policy at Vermont Law and Graduate School, who has been tracking the case and similar lawsuits for years.

“That’s historic,” Parenteau said. “That’s a breakthrough.”

Youth plaintiffs have brought similar cases in other states and on the federal level, assisted by the nonprofit group Our Children’s Trust, but the other lawsuits have been dismissed or have yet to reach trial. Last month, Hawaii Gov. Josh Green announced a settlement with youths who had sued that state, with the government agreeing to take steps to cut climate pollution.

At the hearing in Montana, a lawyer for the plaintiffs spoke in sweeping terms, telling the justices that “this case is about Montana’s climate, Montana’s constitution and Montana’s children.” Roger Sullivan said the plaintiffs had testified during the trial last year about facing extreme heat and wildfire smoke as they worked on family ranches and about disruption to tribal traditions caused by a changing climate. He noted that the state had never denied a fossil fuel permit, adding, “We are in a climate emergency, and additional greenhouse gas emissions will cause additional heating and additional injuries to plaintiffs.”

Lawyers for the state did not dispute the science of climate change, the role of fossil fuels in driving it or even that the constitutional right to a healthy environment extended to include climate change.

“But that doesn’t mean we feel that this problem can be influenced in any way by a state district judge in Montana,” said Mark Stermitz, a lawyer for the state.

Members of the audience applauded the plaintiffs’ arguments at times while booing the state’s attorneys when they questioned the ability of Montana to phase out fossil fuels.

Stermitz and a colleague repeated arguments they had made at trial that Montana’s climate pollution is insignificant on a global scale and that the court therefore cannot possibly issue a ruling that would address the harms the plaintiffs are facing from extreme weather. Yet much of the hearing focused on the far narrower and more procedural question of whether the lawsuit should have been allowed to proceed even though it did not challenge a specific permit.

Several justices appeared skeptical of the state’s arguments, at points calling them “circular.” They also did not seem interested in the argument about whether Montana’s climate pollution is significant, said Michael Gerrard, faculty director of the Sabin Center for Climate Change Law at Columbia University, who watched a livestream of the hearing.

Gerrard said the ruling’s impacts will reach far beyond the state’s borders.

“The case is being closely followed around the world and is being cited in cases in jurisdictions inside and outside the U.S.,” Gerrard said. While a ruling from Montana’s Supreme Court will not be binding anywhere else, he said its decision could prove persuasive. “Most judges don’t want to be first in making a ruling. Most would like to be second.”

Still, even if the justices uphold the lower court ruling, it could have little practical impact. While Montana would no longer be able to prohibit agencies from considering greenhouse gas emissions in its permit reviews, it is unclear whether agencies would be required to do so. And even if regulators were to study climate impacts of individual projects, they could continue to approve the projects anyway.

Parenteau said the case’s importance is heightened in light of recent rulings by the U.S. Supreme Court that could drastically limit the federal government’s ability to regulate climate pollution or other environmental challenges.

“Any environmental case that gets to the Supreme Court is dead on arrival,” Parenteau said. “That’s why people are going to the states.”

Our Children’s Trust is pursuing similar cases in other states. And dozens of cities, counties and states are pursuing climate liability lawsuits against oil companies in state courts.

Some people who had come to support the plaintiffs said the case had importance beyond any practical impacts.