The impact is narrow: it forces the state of Montana to review climate impact as part of environmental review of new projects (eg: opening a new coal mine):

The state’s characterization of the court proceeding as a “MEPA trial” and not a climate trial ignored the fact that the MEPA provision at issue was squarely about climate change. By prohibiting the consideration of greenhouse gas emissions and climate change in environmental reviews, Seeley ruled that provision is at odds with Montana’s constitutional right to a clean and healthful environment. At issue here is not how MEPA works, but how the state’s dismissive treatment of climate change endangers the youth plaintiffs and degrades Montana’s environment.

That's kind of a bare minimum start. It'll take a whole lot more to get to where we need to be.