There are a lot of reports coming out lately that say we have 100 or 500 year supply of resource x, y, z, and especially oil "at current consumption". Which really means, at the current rate of consumption. Say, n units per year. It also assumes that the n units of consumption per year will remain constant, and never change. Which of course is not realistic at all, as the world population grows and demand for oil and everything else increases.
As Dr. Albert A. Bartlett notes in "The Most Important Video You Will Ever See", this totally ignores the fact that consumption rates are going up exponentially.
In finance, there is the "Rule of 72". It's a simple finance rule to determine how quickly money will double at a given compound interest rate.
72/interest rate = years required for the money to double
We can use the Rule of 72, for some quick calculations. If you have a population growth rate of 7%, in ten years, the population has doubled. And in 20 years, the population has quadrupled! If your city's sewer system in inadequate now, think how bad it will be in 20 years!
I was thinking about this, and decided to model it in Excel. Just how much remains, and how quickly does it run out? It's amazing how fast things do run out.
Even if there is only a 1% increase in consumption, the "500 year supply" will only last 181 years! Less than half the time of the original 500 year prediction.