I don't believe in nuclear extinction. It has never been proven that it is possible. Yes you could theoreticly kill every person on earth with nuclear weapons, but they would have to stand in big circles and the device dropped in the center of each circle.
Now granted nuclear weapons are bad news, just not as much as the media would have us believe.
Hopfully it is understood that a bomb twice the size of the ones dropped on Japan, would not do twice the damage.