I just watched Soylent Green on TV last night. I'm a lot more disturbed by it now then I was when I first watched it in the '70s. It's about overpopulation and how humans are nothing more than a cancer growing on the skin of the Earth. The environment was destroyed with the oceans so toxic not even plankton could grow.
When it happens -- not if -- insane religious belief will be the major cause. How the heck can a Good Book tell people "be fruitful and multiply" and make all the babies they can? Islam teaches that Allah blesses you for every baby you bring into the world. The world population when I first saw the movie was around 4 billion. Now, as I write this, the population approaches 8 billion.
If you never saw the movie try watching it to get a glimpse of what awaits us if ignorant religious fanatics continue to control the world's governments.