The Industrial Revolution was likely responsible for the first appreciable fraction of the world's population consuming more than what Nature could provide circa 1848. By 1979, the average person had passed this limit, and half the population was converting into waste what the other half depended on to produce what it needed, which also happened to be Earth's life support system.
In 2001, some of us reached the pinnacle of happiness, with a life expectancy to match; at that point, the only way to continue the average growth in wellbeing was to increase what the poorest of us could consume. The fraction of the population with the highest life satisfaction continued to grow while the fraction in the upper-middle part of that spectrum, corresponding to the West's middle class, stayed roughly constant.
By 2021, when humanity has consumed 90% of the natural world and its life expectancy has nearly doubled what it was at the beginning of the Christian era, there won't be enough resources left to maintain its growth and the population will begin to rapidly die off.
These are the lessons of my most recent simulation of world history, which up to the present time matches closely with reality.
Now more than ever, it is clear to me that the trajectory of our population is intimately tied to our perception of our ecological environment, which makes sense from an evolutionary perspective since that's what our biology's development would have been most attuned to. It may well be that our population growth will always be offset by the diminishing of life around us, perhaps even to the point that we stop reproducing altogether when a certain threshold is reached, detectable by something as innocuous as the doubling of life expectancy. If these speculations are true, then how much fossil fuel we find won't matter nearly as much to our future as the ecological damage done by its extraction and use, and the same goes for every other technology we might imagine.