Lighthiser v Trump is emblematic of a growing global trend of legal action as a tool to push action on planetary warming amid political inertia or outright hostility.
At issue are three executive orders that together seek to "unleash" fossil fuel development at the expense of renewable energy.
They are also contesting the government's actions undermining federal climate science, from firing scientists to removing critical reports.
A two-day hearing opened in a federal courtroom in Missoula, Montana where Julia Olson, lead attorney for the 22 plaintiffs, framed the dispute as a constitutional test.
"Does the United States Constitution guard against executive abuses of power by executive order that deprive children and youth of their fundamental rights to life and to their liberties?" she asked.
Michael Sawyer, representing the Trump administration, countered that the case itself undermined democracy.
"This is, at its core, an anti-democratic lawsuit," he argued.
"We just had an election. One of the major issues in that election was a dueling perspective on emissions, energy policy, and they are now stepping in and asking the court to overrule the results."
- Witnesses grilled -
The spotlight then shifted to the young plaintiffs, represented by the nonprofit Our Children's Trust, who described how climate change is reshaping their lives.
J.M., a teenager from Livingston, Montana, said that even in her short life she has seen snowfall decline, wildfire seasons lengthen, and flooding worsen.
One blaze forced her family to evacuate, and she remembers packing her stuffed toys and worrying about the family's animals.
"Just experiencing that from a young age put the fear of wildfire in me," she said.
Another plaintiff, Joseph Lee, 19, recalled wildfires in California last year that destroyed the home of a friend.
"I don't know if I'm going to be next -- are my parents going to be safe?" he told the court.
Asked why he chose to participate in the lawsuit, Lee, who has been hospitalized for heat stroke that nearly caused organ failure, said: "A better future is possible."
The young people faced tough questioning from government lawyers, who grilled J.M. over her family's decision to keep three horses -- arguing that raising them contributed to greenhouse emissions and implying she was being hypocritical.
- Long odds -
Expert witnesses also took the stand Tuesday, including renowned climate scientist Steven Running, who shared the 2007 Nobel Peace Prize for being a co-author on a major UN climate report, and former senior White House official John Podesta.
"In your expert opinion would their injuries get worse if more fossil fuels were to be unleashed under these executive orders?" Olson asked Running.
"Unquestionably," he answered.
The plaintiffs are seeking a preliminary injunction that could open the door to a full trial.
The federal government, joined by 19 conservative-leaning states and the territory of Guam, wants the case thrown out, but has not called their own witnesses.
The plaintiffs hope to build on recent state-level wins: a 2023 Montana ruling that oil and gas permits violated the state's constitutional right to a clean environment, and a 2024 Hawaii settlement mandating faster decarbonization of its transport sector.
On the federal level, however, the record is bleak. The landmark 2015 Juliana v. United States case was dismissed after the Supreme Court declined to hear an appeal earlier this year.
During cross-examination, government lawyer Sawyer pressed Podesta for previously arguing against Juliana when he served in the government.
But Podesta countered that while Juliana was too broad, and would have required reversing five decades of policy, in the new case the "remedy is direct and narrow" -- "To pull back on specific actions that are heavily burdening these kids and will have direct impact on their lives."
It remains to be seen whether this central argument, key to the plaintiff's case, plays.
Judge Dana Christensen, an Obama appointee with a record of pro-environment rulings, is presiding -- but even if the plaintiffs notch a win, the case could eventually land before the conservative-dominated Supreme Court.
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