To be clear I don't think anthropogenic climate change will cause human extinction. But a terrible series of volcanos could absolutely turn Earth into a colder Venus for, say, a couple hundred years - pelagic vertebrates and certain microbes would be ok, but I am not sure humanity would survive more than a few generations, even in underground sci-fi bunkers.
https://en.wikipedia.org/wiki/Permian–Triassic_extinction_ev...