Maybe because it's a ridiculously vague and abstruse question that presupposes a set of ideas that libertarianism, or any political philosophy for that matter, never set out to directly answer?

Libertarianism is simply a political framework which we can use to see and interpret the world. It can be combined with other frameworks to help us reach even deeper levels of understand (or perhaps not).

The right-libertarian could argue that the best solution is a competitive marketplace that will fuel hyper-economic growth, and accelerate the rate at which we develop technologies that allow us to ween off of fossil fuels.

A left-libertarian, maybe someone with a strong civil libertarian persuasion, could argue that the non-aggression principle is being violated by the emissions of greenhouse gasses. They would then establish an "emissions rights" framework, and perhaps a marketplace to address the issue.