Frank Fenner: "DOOM!"

Back in June 2010, eminent Australian scientist Professor Frank Fenner, who helped to wipe out smallpox, predicted that humans will probably be extinct within 100 years on account of overpopulation, environmental destruction and climate change.

I think he may be right that we'll be extinct in 100 years, but for literally none of the reasons he has given. It's arguable if "overpopulation, environmental destruction and climate change" are existential threats. They may certainly send civilization back to the Dark Ages, but I'm not so sure they would wipe us out completely.

On the other hand, full-scale nuclear war, molecular nanotechnological warfare, or Singularity-gone-horribly-wrong do qualify as existential threats.