We have known for hundreds of years that burning coal and burning oils will lead to climate change.
We know that our planet is subject to Meteorite Strikes , Massive Volcanic eruptions , Coronal Mass ejections , Canfield Oceans all leading to mass extinctions 
Our population is over 8 billion people  and the actual number of people in some level of comfort living in harmony with other species is about 1 billion people
We are headed for a collapse.  What will that collapse be like as the worlds poulatoin drops from 8 or 9 billion to a billion or 500 million people.
We know of many civilizations that have failed.
When civilizations fail due to external pressures we see the following
~Breakdown of civil society
~Emergence of Warlords
~Civil unrest and rioting 
~Starvation and Cannibalism 
There is no reason to think that could not happen today.