Sustainability
In the movie "The Matrix" agent Smith was telling Morphius that humans were more like viruses that multiply and use up all the resources instead of being in balance. We have used technology to extend resources further than could have been used in its natural state.
There have been many experiments with population growths. In one case was a group of rats kept in a finite space with infinite food and water available. The population booms and at a critical point the rats started getting stressed and reacting by killing and raping each other as well as stealing or hoarding resources like bedding. Shortly after this the population has a huge die off; after the die off happens then the behaviors of the rats become less aggressive. The lessons from this experiment indicate that human populations would also act this way.
I don't think humans will become extinct in 100 years but there would be a die off of massive proportions. Humans are very adaptable and can live and thrive in many environments from extreme heat of the deserts to the extreme cold of the artic.
A more realistic expectation would be that the global civilization would fall apart and spread to the national and local civilization. World wide surviving populations of people would probably be reduced to more primitive conditions with legends of our current time used to inspire and caution. This has happened before but not on the scale we have now.
Easter Island is a good example of what happens when a population exceeds the resources available. It is also a scary example of what happens when religion fails.
