Man does not live by bread alone; he needs a healthy ecosphere too, as well as spiritual liberty. Some ecologists have said the carrying capacity of the Earth is two billion people. Yet there is no reason to panic immediately (save that for later). Human prospects are limited by their lack of good sense, foresight and engineering rather than necessity. Malthus regarded people as one might regard bacterial growth in a Petri dish that exhausts its food supply. Hopefully people are a little smarter than bacteria and can do better at sustaining their existence and ecosphere-medium culture.
It would be possible to convert the Earth into an ecosphere park for humanity. Humanity would live mostly off-world in space constructs. Inorganic material could be harvested from other planets and moons for industrial purposes. Projects to build vast new ecospheres in various solar system niches could go ahead. Imagination and determination is all that is lacking.
In the meantime as much of the Earth that can be restored to ecospheric health as possible should be attempted. Because pragmatic scientists like James Lovelock have said that the Earth may tolerate human economic development in its present form just another century or two before breaking down (from a human life sustainability point of view), it might be good to limit new city development to forms that would survive ecospheric breakdown better than steel-glass box cities. Humans could build artificial mountain ranges and develop a pollution free electric transportation infrastructure with clean energy. Wildlife could be tagged and tracked so large mammals could return to wander freely. Even so, if global atmospheric and oceanic heating crashes world agriculture human survival might fare better within temperature-controlled hollow mountain ranges.
Agriculture on places like the moon should be productive. In low gravity one would expect vegetables to grow 600% to 1800% larger than on Earth. In zero gravity there may be no real size limit to vegetable growth…maybe potatoes could grow the size of domed football stadiums. Sealed growing micro-ecospheres would recycle and conserve water on Earth and in space. Space products could be grown, dehydrated and sent to Earth with the water mostly remaining at the source.
Ecological economic policies will be requisite throughout the solar system in the centuries ahead (and sooner hopefully). The primitive unsustainable and wasteful historical ways just won’t work on a tight environmental budget.
Humanity need apply reason, knowledge and common sense to solve the worlds’ problems to its survival. The problems are within us rather than in the stars.