What if everyone jumped at the same time?

You know those questions that seem strange, but deep down you want to know the answer to them? Like this one...

Cartoonist and math guy Randall Munroe gives a prefectly sound response to the question, "what would happen if everyone on earth stood as close to each other as they could, and jumped, everyone landing on the ground at the same instant?" Then, he begins to talk about the greater ramifications of the whole experiment if it were to be undertaken. The results are terrifying.

Source