At least the ratio of supply with respect to demand. When supply actually begins to fall in earnest then all of the major economies of the world will ultimately be forced to contract as will the world's population. It is predicted that it will be a time of severe upheaval with epic disease, famine and just about everything bad you can imagine. So much of our world relies so heavily on (cheap) energy that it cannot be sustained as we know it without it. And no politician is going to be able to do much of anything about it, either.
In the oil crisis of the '70s, it happened that a 5% decrease in supply effected a 400% increase in price for a barrel of oil. In an economy expanding at a modest percentage per month, you don't actually have to see as much of a real decrease in supply to cause the problem because the supply has to stay in step with the economy except where efficiencies are increasing. That's kinda' difficult to accurately compute for our purposes. In any case, if you see that yearly average for the daily production rates start going down by real percentages, it'll be a very good bet that life as you know it will be over.
Pidgey