Climate change is a collective action problem; the fact that people don't want to solve it with voluntary donations doesn't imply that they think the problem is unimportant.

It would be equally reasonable to propose that roads or police or militaries should be funded solely with voluntary donations, and then use support for that proposal as a measure of public sentiment towards the necessity of those functions. Of course, these functions are used as canonical justifications for the existence of the state in the first place.