(CN) — A landmark climate change trial in Montana entered its second week on Monday, after 16 young plaintiffs, ages 5 to 22, sued in an effort to force the state to take steps to cut greenhouse gas emissions.

The state of Montana began — and rested — its defense that same day, having called just three witnesses.

The suit, believed to be the first of its kind in the nation to go to trial, is premised on a clause in Montana’s constitution.

That clause, Article IX, reads, “The state and each person shall maintain and improve a clean and healthful environment in Montana for present and future generations.” The lawsuit argues that House Bill 971 — a state law recently signed by Republican Governor Greg Gianforte, which prohibits regulators from taking greenhouse gas emissions into account when approving a new project — violates Article IX.