All the Bombs
Summary: If all the existing nukes were exploded at once, it would suck. But it would not end civilization. However, if we made even more bombs than currently exist, then we could wipe humanity off the earth. As such, it is by definition an Existential Risk but currently, it cannot end humanity.
Some basic facts:
- It would take most of the 15000 existing nukes to destory all cities (about 4500 cities, >100k people) - using up 13.5k of the nukes. This wipes out about 3 billion people, about half the population.
- The remaining 1500 nukes are by far insufficient to kill off every other location. See Nuclear Destruction Radius.
