I think that (and I don't mean to argue that you think this these are OK) "a sizeable portion of Earth will be unfit for humans, and human population will have

I think that (and I don't mean to argue that you think this these are OK) "a sizeable portion of Earth will be unfit for humans, and human population will have decreased a great deal" is a pretty terrible outcome. Those people will be starving to death, dying in migration related conflicts, or choking to death on wildfire smoke. Not pretty. You're right, it may not cause civilizational collapse, but I find those other outcomes totally unacceptable and we ought to be doing more about it right now.