No serious climate projections that I'm aware of even reduce Earth's carrying capacity under 10 billion over the next couple of centuries. While very serious (to the tune of trillions of dollars and tens of millions of excess deaths), it is not an extinction level threat, unlike something that is to us what we are to chimpanzees (humans have made many animals extinct on accident). Does such a thing exist right now? No. Could it exist this century? Maybe.