Until 10000 BC the world lived as hunter gatherers. Some view the hunter gatherer lifestyle as more in tune with nature.
[Hunger Math] The land needed for each hunter gather ranges from 40 hectares per person in an ideal ecosystem, to 150-250 hectares per person for a moderately favorable ecosystem. In unfavorable ecosystems, the number can go very high, to over 1,000 hectares per person. Let’s assume an overly-optimistic 100 hectares per person for hunting and gathering. This means 136 million people on earth.
So returning to being hunter gathers would merely cost 7.1 billion lives.
So Nog could have drawn on his cave wall and realized that there was a hunter gatherer population problem and started killing other hunter gatherers. This would have been especially the case if he was on an island in the ocean and had not boats.
Of course the development of agriculture and other technology changed the calculation.
Greenhouse technology from the 16th century can enable the feeding of a global population of 100 billion. Molten Salt nuclear reactors or aneutronic nuclear fusion or really good solar power could allow for the clean energy for all those people as well.
According to a report and database from Pike Research, total commercial and residential floor space will grow by 26% between 2010 and 2020, increasing from 142 billion square meters (1.5 trillion square feet) to 178 billion square meters (1.9 trillion square feet) during that period.
So less $65 trillion for 1 million square kilometers of greenhouses that would double global agricultural yield is doable over a couple of decades. Greenhouses are far cheaper and simpler to build than regular buildings. This would be 8% of global arable land area. However it would take less than the more than 100 years to double current world population.
The Solar System alone has, according to different estimates, enough material and energy to support numbers of humans anywhere from several thousand, to over a billion times that of the current human population. About 5 quintillion people (5 * 10^18).
Over 100 million are flown across the ocean every year.