Trying to remember here ... I think when Clinton was President, he attended a G-5 (?) Summit in Rio, and though not much was said about it, it seems that they felt that the maximum population for Earth should be around 3.5 Billion people.
Well, we are twice that now ... Hmmm
Another source that I read tonight puts that figure around 19 Billion.
I feel it should be the lower figure, but how?
There's no way to know how many people the Earth can sustain, because this depends partly on how much agricultural or mining and energy technologies advance. However, there seemingly isn't gonna be enough room eventually for everybody to live on Earth. Therefore, man-made biospheres will need to be constructed beyond Earth. Mars is likely the first place beyond Earth in our solar system to get a man-made biosphere that is an appreciable fraction in size comparable to Earth's biosphere.
The first step towards the terraforming of Mars is the deployment of a magnetic shield that protects Mars against the solar wind stripping of its atmosphere. This magnetic shielding would subsequently allow the planet's atmosphere to reacquire its former density that'd be high enough to allow for sustainable surface liquid water.
Reference:
https://phys.org/news/2017-03-nasa-magne...phere.html
An effective artificial magnetosphere placed at Langrangian point 1 from Mars is very achievable with foreseeable technology. This magnetic shielding apparatus could weigh less than a few hundred tonnes which is within the load capacity of a big Falcon 9 rocket. I'm guessing the cost of protecting the Martian atmosphere with an artificial magnetosphere would probably be similar to the cost of a small nuclear reactor.
Some few billion tonnes of sulfur hexafluoride gas (SF6) could increase Martian atmospheric surface temperatures by over 20 degrees Celsius.
Sulfur hexafluoride - Simple English Wikipedia, the free encyclopedia
The SpaceX interplanetary transport system could deliver this super greenhouse gas to Mars at a cost of less than $2,000/kg.
https://en.wikipedia.org/wiki/SpaceX_Mar...astructure
A few hundred thousand tonnes of SF6 delivered annually to Mars would cost just approximately $1/2 trillion yearly. This is less than a fraction of a percent of the global economic output value. An accumulation of a few billion tonnes of SF6 at an annual rate of a few hundred thousand tonnes would take less than ten thousand years. The annual cost of less than $100 per person per year on Earth would be totally worth transforming Mars into a world with triple its current atmospheric pressure and a warmer Mars with average surface temperatures greater than typical summer Antarctic temperatures.