Assuming of course: - We've "solved" fusion, and can produce near-unlimited amounts of electricity - Per person energy/resource consumption is cut to an absolute minimum. Most tools, machines etc are designed to last virtually forever, instead of having planned obsolescence - We build farming buildings that are 100 stories tall, and produce light from electricity instead of relying on sunlight to grow plants. - Recycling processes and other activities that are labor intensive today can be made 1-3 orders of magnitude more efficient by AI and better access to cheap energy. - We are able to handle the sociological aspects of such population density
Earth's population density is currently at 16/km^2, or 62500m^2/person. If we multiply the population by 100, that's still 625m^2/person.
We clearly have enough space, and probably enough atoms (of each type) to support one billion. So I think if we have enough energy, technology and organize it perfectly, it should be technically possible.
In fact, given the fact that each American already spends almost 100x more energy than most people did 500 years ago, it may be even easier than is assumed above, if we assume that resource consumption is cut to near subsistence levels.
Obviously, such a world would be quite dystopian, making the USSR seem positively cheerful. But impossible?
I mean anything is possible (for additional nightmares: look at factory farming of chickens).
Just not livable.
He was making the argument as a response to being triggered by "liberal" policies about climate change and human footprint.
Now, if on top of the solar energy, we assuem that we will have fusion energy available within a few 100 years, energy available may easily grow way more than 100x of today's production.
Also, if efficiency continues to increase, for instance due to AI development, nanotech, biotech, etc, we may be able to build almost any amount of infrastructure given the atoms we have available in the Earth's crust. Including multi-story farms, feeding electric lighting to power photosynthesis.
In fact, I'm not aware of any hard limits anywhere near this side of 1 billion people. As far as I know, there are billions of billions of kg of most critical elements available on earth, or in the order of 1+ billion kg per person, even if we're 1 billion people. Our ability to organize these atoms into whatever combination we want (including human bodies) is primarily limited by energy.
And even with 1 billion people, it will take a while before we run out of hydrogen for fusion power.
Humans are a species of animals. Recent environmental changes has caused a sharp decline in evolutionary fitness (=reproduction rate) for this population. This has happened to millions of species in the past, and in many (if not most) cases, the population adapted to the new environment, and the fitness grew back to or beyond replacement rate.
Also, this can happen quite a lot faster than some seem to imagine. While it takes a long time for completely new mutations to make it into the gene pool, the gene pool already contains a huge diversity of genes coding for different types of traits, including mental traits. Often, all that is needed to adapt to a new environment, is for the relative frequency of these genes to change.
For instance, many people seem to have a built-in oxytocin response when around young children. A boost in frequency to all genes promoting/strenghtening this mechanism may eventually make the need-to-have-baby emotion in most women stronger than the need-to-have-orgasm drive in the horniest of 18-year-olds.
And if these genes are already in the gene pool, just with a low frequency, they could easily become dominant over 10-40 generations.
In other words, if at some point the kinds of environmental change that cause population decline slows down, I don't think it's likely that the birth rate stays below replacement forever. (If it does, we'll go extinct, of course).
Also, I'm not making predictions about "forever". Rather, my prediction is that the reproduction rate is likely to come back above the replacement rate very quickly, compared to evolutionary time.
My guess is somewhere between 100 to 1000 years, if human societies go on approximately like today, and we don't have some kind of apocalypse, AI takeover or totalitarian world government.