On the second and final day of a hearing in Missoula, Montana, the plaintiffs' attorneys called Lori Byron -- a former pediatric hospitalist with more than four decades of experience -- to explain how a warming planet is impacting young people.

The case, Lighthiser v. Trump, is emblematic of a growing global trend of using the courts to push climate action amid political inertia or outright hostility. At issue are three executive orders from the president that together seek to "unleash" fossil fuel development at the expense of renewable energy.

Twenty-two plaintiffs represented by the nonprofit Our Children's Trust are also contesting actions they say undermine federal climate science in the United States -- from firing experts to scrubbing reports and reversing the scientific foundation for regulating greenhouse gases.

They are seeking a preliminary injunction that could pave the way for a full trial, while lawyers for the Trump administration and conservative-leaning states want the case thrown out, arguing it is undemocratic.

Byron, who co-authored the state's climate assessment and has served on an Environmental Protection Agency committee, testified that the state faces more extreme heat days by mid-century, longer and more severe wildfire smoke seasons, and an increase in "climate surprises" such as catastrophic floods.

"They breathe more air, they drink more water, they eat more food per pound of body weight compared to adults," Byron said, adding that children are especially vulnerable to illness and injury from heat and extreme weather because of their developing bodies and reliance on adults.

The toll is not only physical but also psychological: "Their brains are still developing, and stability in their life is very important. When you get displaced or lose your home, those experiences can have mental health impacts that last for decades."

Byron's testimony was followed by Isaiah H., a 17-year-old from Missoula, an aspiring athlete who spoke of his love for his home but said it was becoming harder to spend time outdoors and connect with nature as a result of worsening wildfires and declining snowfall.

"When my brother was really little, we had to actually evacuate our house because the smoke was too bad... for his lungs," he said.

"As a 17-year-old, I shouldn't be having to step in like this, and shouldn't have to miss school and make up tests and assignments just to advocate for my health and safety."

Michael Gerrard, an environmental law professor, told AFP: "The plaintiffs are building a strong factual case about the causes and dangers of climate change."

He added: "It would be plowing new ground for a court to say that there is a substantive due process right under the US Constitution to a stable climate system."