I have had this kind of thing happen around me for many years I call it *match* when someone else says or writes the same thing as I do, at nearly the same time. Sometimes it even happens with TV shows lol. Even word for word sometimes. Maybe consciousness is entangled between like minded people?
About your scenarios, this is why many believe we're going to be reduced to pre industrial levels. It's nature having the last laugh, we mess with it, it finds a way to achieve a balance, at our expense of course.
What you outlined in b, I find rather fascinating, it's like being on the boundary of a whole new world isn't it?
What happens after mass extinctions is rapid evolution, completely different from what existed before of course. Remember the K-T event? We went from massive ferns and giant reptiles dominating the planet to flowering plants, feathery birds who evolved from the dinosaurs and tiny mammals who rapidly evolved into brainier ones by consuming high quantities of nuts, protein rich, mind you. These nuts did not exist before the K-T event happened. So mammals would likely have remained tiny rat like creatures if an asteroid did not hit the Yucatan.
By the way, something I find so fascinating about nature regulating the environment, birds still have that gene that made the dinosaurs so huge, but it's been turned off. Why? Natural selection of course; the environment had become inhospitable to creatures the size of dinosaurs so the dinosaurs who survived-- the birds-- had that gigantism gene turned off.
b, diaspora into an alien ecology, which is usually not good for the new ecology ... because it in itself does not posses the capacity to adapt to the change of having hordes of arriving opportunistic climate refugees. And I mean "clime refugees" not just human beings but all migrations up and down the biological kingdom.