Two books that give you a few choices.
Bangs and Whimpers: Stories About the End of the World (Roxbury Park Books) (Paperback)
by
James Frenkel
(Editor)
http://www.amazon.com/gp/reader/0737302712/ref=sib_dp_pt/102-4903242-2069723#reader-link
The late author Isaac Asimov in his book: (The Choice of Catastrophes: The Disasters That Threaten Our World) listed and expounded at least 15 dangers that could jeopardize human survival. Many of these potential global disasters, including nuclear war, have arrived at our door only in the last few decades.
http://www.amazon.com/gp/reader/0671227017/ref=sib_dp_pt/102-4903242-2069723#reader-link