In 1956, M. King Hubbert warned that new oil supplies would not last forever. He predicted that at some point, our extraction of oil would peak and then begin to decrease, following a bell-shaped curve (The Hubbert Curve). As we exhausted our one-time inheritance of oil, it would cost more and energy and money to extract oil from the ground, leading to reduced returns. However, the oil demand would continue to rise for a while after the supplies begin to fall.
This pattern is similar to predator populations still rising after their prey populations begin to decline. The drop in predator populations happens later, allowing the prey to start their recovery. However, with oil, no recovery is possible because the oil cannot reproduce.
Based on what was known at that time, Hubbert predicted that oil use would peak in the 1980s. New finds have pushed his peak further forward, and new sources have led to a variation on his theme: curves that are moving downward with peaks and valleys rather than a constant (monotonic) decline. . Think about this: the cost of extracting oil from shale is far beyond that of digging traditional wells. There are just fewer wells we can now dig.
Still, it has not nullified his point: we will eventually exhaust our one-time inheritance of this precious material, leaving our descendants in a resource-impoverished world. Others have applied his ideas to other resources, from coal and uranium to the overuse of animal resources.