I grew up in the Soviet Union. In the 80s. A nuclear attack was something we were being prepared for in school. There were nuclear shelters around the city, and we all had to take a class or two about what to do in the event of a nuclear war. I remember watching The Day After and being utterly horrified (can you believe it -- they showed it on Soviet TV in the late 80s!?).
So what's up with the atomic bomb now?
Can the world still be destroyed with a push of a button or two?
Evidently, other people think the same paranoid thoughts. The Guardian just regurgitates a timeric graph. This is other than satisfying to the information is beautiful blog, so the folk(s) over there decided to figure out how many nuclear weapons would it take to destroy the world and whether we have enough.