What could we do if human population kept going up and did not level off ?

I have already detailed how humanity has not overshot the carrying capacity of the earth and how humans will be able to feed and provide clean water for drinking agriculture and industry for over 15 billion people.

Yes the population projections now are for population to level out by about 2050 at about 8.5 to 9.5 billion people. but population projections out to 2100 get super dicey with population between 5 billion and 19 billion (UN forecasts) The reason for the big variance ? The people who will be doing the breeding have not been born yet. So we do not really know what they will do. We can look at per capita wealth and likely health. Plus people argue about what about too many people. So I say ok. Let us go there and see that there are options that could be used if we had to and doom and collapse would not be what happens.

Business as usual improvements to agriculture (better crops with genetically guided plant breeding, precision agriculture and other measures) we can reduce water usage in half and increase crop yields by double over 2-3 decades.

Greenhouses can use 1/6th the water and 1/10th the land to produce the same amount of crop.

Hydroponics can use 1/20th the water. Aeroponics used 65% less water than hydroponics. NASA also concluded that aeroponically grown plants requires ¼ the nutrient input compared to hydroponics

There have been proposals for large scale hydroponics adoption.

Simplified hydroponics was developed in the early 1980’s in Colombia.These projects were a success as on average a single garden made a profit of $90 in two to three months with the initial investment being $355. Since 1984, projects have been implemented in 12 Latin American and African countries, mostly funded by the UNDP and the UN FAO.

Costs for building gardens for Asia tropical areas

Of the estimated 790 million people classified as hungry in 2000, 525 million lived in Asian nations (UN FAO, 2000).
This is a rough estimate of what Bradley and Marulanda (2000) estimated would be the cost of setting up a hydroponic garden in Tropical Asia.

The total number of beneficiaries would be 232 million for 50 million gardens, each garden accommodating an average family unit of 4. Initial cost of setting up a garden amount to $355 including a year supply of nutrients and seeds. The total cost of implementing this plan in Tropical Asia is 20 billion dollars but it generates a revenue of 135 billion per year. Income from each garden would be about $329-1405 per year.

Indoor and vertical farming have been proposed. Indoor farming can produce crops year-round. All-season farming multiplies the productivity of the farmed surface by a factor of 4 to 6 depending on the crop. With some crops, such as strawberries, the factor may be as high as 30.

The factory mass produced China Broad Group skyscrapers could drastically reduce the cost of buildings for vertical farming as well. Broad Group buildings can be seven times cheaper than the square foot cost of skyscrapers like the Burj Kahlifa (current world’s tallest building.

Despommier suggests that, if dwarf versions of certain crops are used (e.g. dwarf wheat developed by NASA, which is smaller in size but richer in nutrients), year-round crops, and “stacker” plant holders are accounted for, a 30-story building with a base of a building block (5 acres (20,000 m2)) would yield a yearly crop analogous to that of 2,400 acres (9.7 km2) of traditional farming.

A 200 story SkyCity (about $2 billion each) with 4 million square meter (1000 acres) of floor space would be like 480,000 acres of traditional farming.

So 3.78 billion acres of traditional farmland could be replaced with about 8000 SkyCity vertical farms. The actual footprint would be 40,000 acres.

This would be about $16 trillion, but if humanity had to enclose agriculture and radically densify and reduce the agricultural footprint it could be done for about 20-25% of the world’s current GDP in one year. If this was spread out over ten years then it would be affordable if absolutely needed to reduce water and land usage.

The World could sustainable scale up nuclear fission technology with deep burn modular factory mass produced nuclear reactors and achieve 100 times the current world energy production.

One hundred billion people could live at a Western standard of living (10 kilowatts) if they had clean energy, enough food, water and the only concern was the level of waste heat.

A lot of people are worried about global warming.

From now until 2040, if you want to have a 0.75 degree celsius increase instead of a possible 1.25 degree celsius temperature increase then soot mitigation should be targeted. Currently discussed measures against CO2 would not have a noticeable effect until around 2070. Soot mitigation is about ten to twenty times cheaper than CO2 mitigation.

If Global Warming got to be serious we have Cheap Failsafe geoengineering options

The cost to construct a Stratospheric Shield with a pumping capacity of 100,000 tons a year of sulfur dioxide would be roughly $24 million, including transportation and assembly. Annual operating costs would run approximately $10 million. The system would use only technologies and materials that already exist—although some improvements may be needed to existing atomizer technology in order to achieve wide sprays of nanometer-scale sulfur dioxide particles and to prevent the particles from coalescing into larger droplets. Even if these cost estimates are off by a factor of 10 (and we think that is unlikely), this work appears to remove cost as an obstacle to cooling an overheated planet by technological means.

Spaceplanes, Resusable rockets, inflatable space stations and air travel

The number of people that fly each in the USA are 2 million people. As of 2008, it was estimated that close to 800 million flew in the US. The world has about three times the airplane activity. There are between 15 million and 18 million airline flights in the world annually, which makes it about 40,000 to 50,000 flights daily, carrying nearly a million passengers per day.

A large Spacex reusable rocket would bring the cost of launching to space to about $10 per pound.
200 passengers could fit in a passenger pod.

Inflatable space stations and lunar cement can be used to reduce the cost of colonizing space.

For serious space colonization of a billion people would require having about ten thousand space planes or reusable rockets. However, as I have shown with the agriculture and enclosed agriculture there is time to develop serious space colonization transportation and other capabilities.