Answered Jul 5, 2019
First, you can forget about the “expand to other worlds” idea.
Space travel is hugely expensive, in money, technology, and time. It is a major set of problems just to get a car-sized machine to the Moon or to Mars.
For humans, space travel is also dangerous, from launching a rocket, to landing, to the reliance on life-support systems, to the long-term effects of low-gravity, radiation, and living inside vehicles, stations, etc.
If you go to the Moon or Mars, you need to bring all of your resources with you, to last your entire stay. Hugely expensive to get it all there, even for just a few days visit. Uncrewed resupply craft are expensive, take a long time to reach Mars, and might crash or otherwise be lost.
Changing the atmosphere on the Moon or Mars (so you could go outdoors without a spacesuit) would take enormous physical resources, and centuries of time, and may be physically impossible. Temperatures and radiation may also be insurmountable issues. Living permanently inside protective structures would be extremely expensive.
As I understand it, chemical analysis of Martian soil suggests that it might be impossible to ever grow plants or fungi in it.
There are fantasies of geological mining of the Moon and Mars. Which would require hugely expensive machinery and other resources, which make Earthly colonisation and resource-extraction (e.g. centuries-past North America and Africa) look cheap and easy by comparison.
Forget about the other planets and their moons. Heat, radiation, pressures, toxic gasses, planets made entirely of gasses, or solid but really cold planets and moons.
Forget about ever visiting any planet outside our solar system. The distances are too great, the expenses too high, and the timeframe too long.
So, back to the real world…
Maximum population is contextual, in place, time, and technology.
This was described back in 1798, by Thomas Malthus. The idea is that, population control is a natural, universal mechanism, affecting all life-forms.
Malthus observed that, population numbers increase based on food supply. This can be plants with plentiful space/water/nutrients, herbivores with plentiful plants available, or carnivores with plentiful prey available. It can also be humans with increasing agricultural efficiency and technology.
While microbiology wasn’t understood in Malthus’s time, the principle includes single-celled organisms, which have a common growth curve of increasing exponentially, and then leveling off.
The population will increase to the contextual maximum, where everyone is just barely obtaining/producing enough to survive.
Then, there are two suppressing scenarios:
- Something reduces the food supply at some level of the food chain or food web. It could be an especially harsh winter, or an extended drought, or a disease of plants or animals. Starvation ensues.
- The food supply remains stable, but the population compulsively keeps reproducing/expanding, and overshoots the supply. Starvation ensues.
This may be cyclic. Food-suppressing events like weather and diseases will occur either yearly, or repeatedly over a longer timeframe. Humans improve technologically, overshoot the population, and suffer until the next big improvement.
There may be hard, regional or global upper limits on food production, fresh waters, etc. Where humans will never be able to improve efficiency past the Earth’s “carrying capacity”.
Another suppressor is communicable diseases.
Increased population leads to increased crowding, leads to increased disease transmission. Anything from the medieval plagues to the 1918 influenza could replay. Global warming may increase the geographic range of malaria-carrying mosquitoes.
High-intensity farming (to feed the current overpopulation) may increase the risk of microbial diseases of plant crops and animal livestock, leading to famine, and starvation of humans.
Another pressure is interspecies and intraspecies competition. A slightly more efficient (for the particular environment) species may out-compete other species, who may then starve. Animals – either individuals or groups – may compete with others of their same species.
The highest level of this is human warfare, which is always (either overtly or thinly disguised) economically motivated. Humans fight over farmland, water, or natural resources (oil, mining, forestry, fishing) which can be used or sold. Which ultimately leads to food and other means to support the population numbers of a particular country or tribe.
Even losing sides can do great damage. This includes ancient retreating armies pouring NaCl salt onto the ground, to impair future farming use, up to modern retreating armies setting oil wells on fire.
Conflicts which are ostensibly about religion or ethnicity are really just using those characteristics to organise and motivate groups for economic competition over natural resources.
In warfare, both sides will have many people killed, or die from degraded conditions, including starvation, diseases, etc. The losers may be completely genocided.
Warfare can be a means of increasing one’s own tribal population. However, it can also result in mass burning-off of surplus impoverished young males (i.e. “cannon fodder”).
Another aggressive angle is China’s “one-child policy”. It seems abusive and micromanaging, but the alternative is, ultimately, mass starvation. China has a mismatch of a high percentage of the world’s people, and relatively low percentage of the world’s farmland.
A gentler angle is advancements in contraception technology and distribution of it. Personally, I strongly support government efforts for free, easy contraception to anyone and everyone who will accept it. Anything from handing out condoms to offering cash payments for welfare recipients to get long-term implants or permanent sterilisation.
All of this sounds somewhat unpleasant to some people. But those people cannot imagine how unpleasant things would be with 20 billion humans on the only planet we will ever have.
And, lest anyone think to accuse me of any kind of hypocrisy or snobbery… I am childfree, never wanted any sprogs, and going to stay that way. At an age and condition where I actually do know what I am talking about on that.