Every major problem you can think of affecting humanity is caused by overpopulation. There are two solutions to this:- either we will keep growing and the natural world will be utterly overtaken and destroyed and our lives will become completely synthetic. Or nature (which includes human activity) will balance the problem out by getting rid of most/all of us in some way.
Loss of Fresh Water
Depletion of Natural Resources
Less Freedom, More Restrictions
More Intensive Farming Practices
Creation of GMO foods
Increased Global Warming and Climate Change
Elevated Crime Rate
Destruction of Rural Beauty
Increased Habitat Loss
Quality of life in Fastest Growing Countries
Mortality Rate in Fastest Growing Countries
Increased Emergence of New Epidemics and Pandemics
Population growth is directly linked to the type of energy we use. The more easily available and powerful the energy the more we can grow and expand. You can trace this throughout history, the invention of the steam engine sky rocketed the population. Once we sussed out how to use oil the population exploded. If the oil ran out all production and growth would stop and the population would slowly decrease.
No energy is as cheap or as effective as oil - and oil makes everything. Man! You wouldn't believe how much everything is made by oil! Plastic for a start, then all machinery, all technology, computers, clothes, food production, travel, etc, everything is oil.
We will definitely, definitely, definitely, use the oil up too! - humans are greedy and driven to expand. It will probably happen soon too. The more oil we use the greater the population- the more oil we use.
No kind of human intervention can stop the inevitable- You cannot reverse what is happening. Are China and India going to suddenly stop their growth? The only way we can continue to grow is if we find another energy source that does everything oil can do. But even then we will have big problems - for one we will have to feed the expanding population, which will only be possible through GMO foods. If we could continue to grow we would end up becoming completely synthetic. The best case scenario would be something like Aldous Huxley's Brave New World. What is more likely is that nature will re-balance our population.
The population would be brought into balance if the oil ran out, or if we had a nice big nuclear war (I see a nuclear war as an act of nature), some kind of huge natural catastrophe like a comet, or some kind of horrific plague.
I hope beyond hope that the oil runs out in the next ten years. If that happens we will be plunged back into the 1800's. Which would slowly reinstate natural farming techniques and healthier cleaner living. Or another solution is everyone turns gay and stops wanting kids. Maybe homosexuality is a natural evolutionary mechanism that emerges in a species when it grows too big?