Global civilization collapse
Over the decades and centuries, modern human civilization has gotten more complex, with more and more dependencies between people, and with more and more complex systems and structures necessary to support it.
For example, if you were to somehow, in a single day, remove all the fossil fuels from the planet; global civilization would collapse. This, because everything depends on these: for energy, and as raw materials (plastics and etc).
People would be suddenly thrust back into the pre-coal, pre-oil, pre-natural gas era — burning trees for heat and energy, and using horses for transportation and labor.
But our infrastructure is not designed or suited for this kind of lifestyle; certainly not for the current number of people on earth, nor for the population densities of cities and suburbs.
Horses are slow, and the distance you could efficiently travel would be radically reduced. The number of people an acre of farmland could support would be radically reduced.
. . . . .
The question is: will fossil fuels suddenly run out one day? This, without a suitable replacement?
For global civilizational collapse to not occur, the following are needed:
Many hope that new technological breakthroughs will solve the problems.
Some say the oil will never run out because we will leave it in the ground once it becomes too expensive to extract. But when this occurs, the oil has run out. The phrase "oil runs out" refers to usable oil.
There have been times in history when an essential natural resource was depleted.
One example is: trees. Many parts of the world were once forested, but the remaining trees are not adequate to use as fuel and materials; the populations moved or died.
Another example is: water. In certain places the non-rechargable aquifiers were depleted.