1. Over Engineered/Population Bomb/Resource Crash

You think this would be obvious

Seven billion people and they all want is an ocean front, 3,000 sq ft home and a Lincoln, a Lexus and a Jeep, plus an endless supply of sushi and free range beef. Mormons, Muslims and Mandarins all want 10 children. Neverending suburbs and malls as far as the eye can see. And every government gets to buy advanced weapons, all they can horde.

Everything is maxed out. The oceans are dying, the last forests are being mowed, all the plains are plowed and washing away. At some point this hyper-engineered house of cards will fall. A single variable can collapse a system (see Easter Island).

Quality of life is going to go down, Americans are scared and reacting irrationally. 

http://www.everythingconnects.org/overpopulation-effects.html