There are 5 billion people on the planet today who did not exist, at the dawn of the oil age and they exist only because of oil and natural gas. It is solely the fact that hydro carbon energy, specifically internal combustion engines, amonium based fertilizers made from petroleum, that has allowed the human population to expand in just a little over 130 years.
Human population was fairly stable at just under 1 1/2 - 2 billion people, from the time of Christ until oil was discovered. There was a slight dip for the bubonic plague, a slight increase as the beginnings of the industrial revolution came along with steam and coal.
It was not until the discovery and use of oil that the human population skyrocketed. Current world population sits at approximately 7 billion people. From a simple arithmetic stand point, if you look at the numbers, essentially 5 billion people too many are living on this planet today, who are not sustainable by any other means.
What happens when the oil runs out? First the dwindling supplies will become very expensive. Wars will be fought over what remains. Remember, 2 billion people for 2,000 years. Then WHAM! Oil. The above synopsis is taken from a History Channel presentation with Mike Ruppert.