A landmark climate change trial opened on Monday in Montana, where a group of young people are contending that the state’s embrace of fossil fuels is destroying pristine environments, upending cultural traditions and robbing young residents of a healthy future.

The case, more than a decade in the making, is the first of a series of similar challenges pending in various states as part of an effort to increase pressure on policymakers to take more urgent action on emissions.

Rikki Held, 22, a plaintiff who was among the first witnesses to testify on Monday, described how her family’s 3,000-acre ranch in eastern Montana had been threatened by droughts, wildfires and extreme weather, including heat waves and floods. At times she grew tearful talking about working through those conditions while trying to maintain the family’s livelihood.

“I know that climate change is a global issue, but Montana needs to take responsibility for our part of that,” Ms. Held said. “You can’t just blow it off and do nothing about it.”

The case revolves around the contention from 16 young residents — who range in age from 5 to 22 — that the state government has failed to live up to its constitutional mandate to “maintain and improve a clean and healthful environment in Montana for present and future generations.”

State leaders have fought the accusations, calling the proceedings a show trial and a “gross injustice.”

“Montana’s emissions are simply too minuscule to make any difference,” ​​Michael Russell, an assistant attorney general, said during the state’s opening statement. “Climate change is a global issue that effectively relegates Montana’s role to that of a spectator.”

The two-week trial in a courtroom in Lewis and Clark County will feature both the accounts of young people dealing with climate change and the testimony of climate experts. At the end, Judge Kathy Seeley will be asked by the plaintiffs to declare that the state’s support for the fossil fuel industry is unconstitutional.

Environmental advocates believe such a finding could put pressure on government leaders in Montana and elsewhere to take action on curbing emissions. They are also hopeful that the judge could order the state to consider climate impacts when approving new projects.

The effects of a warming climate are already spreading across Montana, including shrinking glaciers at Glacier National Park and a lengthening wildfire season that threatens the state’s treasured outdoor pastimes. The plaintiffs in the case have said that the state’s inaction on climate change threatens their ability to access clean water, sustain family ranches or continue hunting traditions.

“Montana’s warming climate will have cascading environmental and economic impacts,” Roger Sullivan, a lawyer for the young residents, said in opening statements.

The young people have personally experienced daunting signs of the future, not only the smoke from wildfires but also the flooding at Yellowstone National Park.

Julia Olson, the executive director of Our Children’s Trust, the environmental nonprofit that helped bring the Montana lawsuit, said the case had the potential to set a new course for a healthier and more prosperous future for the generations to come. Many of the young plaintiffs planned to testify.

Montana, whose unofficial nicknames include the “Treasure State,” has long had its fortunes yoked to the mining industry. Helena, the state capital, where the climate case is being tried, was founded in the 1860s by gold prospectors. Montana is the nation’s fifth-largest coal-producing state and the 12th-largest oil-producing state.

Earlier this year, continuing to demonstrate the state’s support of fossil fuels, Republican lawmakers approved a law that prohibits state regulators from considering the effect on climate when assessing large projects like new power plants or factories.

However, the state has also long treasured its unspoiled landscapes and crystal-clear lakes, embracing another unofficial nickname, “The Last Best Place.” The state added the language to its Constitution about the right to a clean and healthful environment in 1972 in response to growing concern about protecting those assets. Only a handful of states establish clear environmental rights in their constitutions.

The first witness called by the plaintiffs was Mae Nan Ellingson, who was the youngest delegate at the 1972 constitutional convention. She testified about how environmental protection was a key issue for many who were involved in the process.

“We wanted an environment that was clean and healthful, so it was a fairly long and contentious debate to ultimately get the words ‘clean and healthful’ included as descriptors of the environment,” she said.

The first day of the trial also featured an extensive review of charts and scientific reports, exploring the history of rising levels of carbon dioxide in the atmosphere, how the trend is linked to fossil fuels, the ways in which it contributes to a warming planet and the effects on Montana.

But some of the scientific details became a point of conflict. When the plaintiffs introduced the most recent climate assessment from the Intergovernmental Panel on Climate Change, convened by the United Nations, which warned that there was “a rapidly closing window” to secure a “livable” future, the state objected, calling it “hearsay.” When the plaintiffs contended that the report was a government document based on government data, the Montana lawyers retorted: “I don’t think it’s our government.”

The judge allowed the report to be introduced.

State leaders have resisted the climate lawsuit, which had its roots in an unsuccessful effort in 2011 that pressed the state Supreme Court to force the state to take action on climate change. As part of the case, state officials have disputed the overwhelming scientific consensus that the burning of fossil fuels is changing the global climate and denied that severe weather events in the state were linked to rising air temperatures.

Our Children’s Trust has undertaken legal action in every state on the climate issue. While judges have dismissed most of the cases, several of the group’s lawsuits are pending. The group won another preliminary victory on June 1 when a judge ruled that a youth case in Oregon, aimed at the federal government, could go to trial.