Check out what Elon Musk says (around the hour mark) in this interview on YouTube:
There have been other manmade threats to humanity, at the height the Cold War the USA and the USSR had enough nuclear weapons amassed to kill most of the humans on the planet within a few hours.
Nuclear weapons are still out there, and the USA could once again move into a nuclear standoff (China, perhaps) but this seems far less likely than the tensions that existed during the Cold War.
Elon Musk only briefly touches upon the threat of Artificial Intelligence, but he’s clearly concerned about it.
AI is getting better every day, when a machine consciousness finally becomes self-aware it could upload itself to the internet. It would be easy enough for this type of AI to reside in 100 million computers and smartphones; silently watching, waiting, and learning everything there is to learn about us.
This is interesting stuff to think about.