When all is said and done, environmentalists are guaranteed to be disappointed. Nature is fucked.
Your recent ancestors who lived in the earlier half of the 20th century may have farmed land that nature has now reclaimed. What happened? Labour was cheap then. People were willing to work for whatever money they could get. Since then, wages have risen with sustained economic growth whereas the population has stagnated owing to the modern fertility transition. Massive immigration of people who are used to a Malthusian way of life confounds the trend, since they are willing to work for what is to us a piss-poor standard of living, hence the alacrity with which some companies have embraced pro-migration policies. Notwithstanding that, if current selection pressures in the First World push population growth out ahead of economic growth again, wages will decline and the “denaturing” of the Earth will resume.
It may be that advances in artificial intelligence combined with our expanding the remit of natural resources to the whole solar system, planetary and asteroid mining etc, will enable extraordinarily fast economic growth. The economy might double at a rate of months or weeks rather than decades. In this case, we need not worry for a while about returning to subsistence-level living. Regardless, selection pressures will favour high fertility in our post-Malthusian economy, so the population will keep growing. The work, such as agricultural work, which is no longer done by humans will be done by robots, and land that is potentially useful for food production or whatever will be used – particularly so as technology allows humans to bring ever more of nature under control. So the decisive factor is population growth. History has been a series of temporary “fixes” and “setbacks” to it, but it has never stopped, and there is no precedent for humans’ needing something from their environment, understanding how to get it, and then simply ignoring it. Nature will be eaten away as we turn ever more of it over to human consumption, and this will be a secondary-to-nonexistent concern for most people in comparison to averting the Malthusian collapse.
The only true “solution” in the end, to prevent global Malthusian collapse outright, will be to stop human reproduction entirely. Not limit or restrict it, but stop it altogether, and from then on any genetic engineering one might want will have to be done through somatic gene therapy rather than through the germline.
How to implement this is not exactly clear, but there are things that could buy us time to work it out. The best one by far is to use every genetic trick we know to max out global IQ to its theoretical limit, which could be 10 standard deviations above the white norm, perhaps 20 or more. One method would be to expunge mutational load, i.e. give everyone the functional analogues of their dysfunctional genes, since perhaps as much as half of genetic variation in intelligence is accounted for by such “rare variants.” Another would be to max out all the common IQ-related alleles. This may necessitate “decomposing pleiotropy,” so as to avoid many of the foibles and disabilities of “spergs,” which although a technical challenge is by no means infeasible once complete genome synthesis is on the cards.
This would have all the obvious good effects of high intelligence: improved co-operation and ability to co-ordinate globally on complex matters, not to mention the fact that high IQ is probably the most effective natural contraceptive known. However, also note that any fertility at all is eventually replacement fertility, or over-replacement fertility, when we have done away with ageing successfully. It is hard to say how much time this would buy before we reached the Earth’s carrying capacity, but it would give us the space and acumen necessary to get to work on a host of engineering problems about halting future growth and housing future populations.
Of course, how far off we are from hitting carrying capacity depends on how large one takes that capacity to be. Based on models using all current arable lands for growing super-high-yield crops, you can get numbers on the order of 100-200 billion people, but the actual number is most probably at least an order of magnitude greater than that.
If one could somehow turn the Earth’s whole surface into a potato field, including the oceans, the resultant produce could feed ~3.1 trillion people with current yields. As it happens, you cannot grow potatoes on the oceans, but this is the number you get if you assume that the calorific yield of algal aquafarming is the same as potato-farming. I could not find the requisite numbers for algal yield, but it is generally believed to be higher than that of conventional staple crops by a wide margin. So even if it is just over double, in a world of algae and potatoes you could easily feed 10 trillion people or more.
The simple way to prevent overcrowding on the ground and maximise its productive use (for food, fuel etc) is to shift all the living space above the ground, into preposterously gigantic multi-level mega-arcologies similar to the X-Seed 4000. The most natural place to put farms and aquafarms would be on the ground level, or subaquatic levels if built on the ocean, of these megastructures – but assuming we have the technology to make such huge structures, I can’t imagine it would be that much trouble to simply build big greenhouses outside the arcologies. Population density would cease to be an issue quickly: 70 X-Seed 4000s would accommodate the entire current population of the UK while using ~0.8% of its total land area, and if you want bigger living spaces, just add more storeys and/or floorspace. There is a LOT more potential floorspace above the Earth’s surface than there is on it.
Indeed, if such structures were only one storey high and covered the Earth’s surface, a total population of 40 billion would feel rather like wandering through a series of dead malls, occasionally bumping into another person every few weeks and pausing to take a selfie with them out of the shock.
After all this, you just have to imagine the walkways and tunnels connecting these structures, and you’ve got an ecumenopolis. The levels of this world-city could take on a countless and manifold shapes and forms; some could be reserved for animals if need be. Some could have ceilings several tenths of a kilometre high and have installations in place to replicate any kind of environment that you could wish for: think of an O’Neill cylinder except on Earth instead of in space. In our newly designed habitats, we could design and cultivate our own winds, our own forests, our own sun and stars through sophisticated artificial lighting, if need be.
The major objections to this come down to two things, broadly: the energy problem and the overheating problem. The best as-yet-untapped way to get effectively unlimited cheap energy is by the Sun. The Earth receives a meagre one two-billionth of the Sun’s light, and even that we only harvest with depressing inefficiency. We could start by using self-replicating collectors on asteroids to harvest the Sun’s light before moving onto the more ambitious project of completely disassembling the planet Mercury with self-replicating automata and encompassing the Sun with a Dyson swarm. This video by Kurzgesagt gives a fairly comprehensive explanation of how it could be done and how much of a game-changer it would be for the human species.
After that, there is the overheating problem. The energy that the Sun directs at the Earth at any given time comes to ~173 quadrillion watts. The human body at rest produces ~100 watts, and given that even the fanciest farming and hydroponic techniques are only about 1% efficient at converting electricity into calories, that must be multiplied by about 100 (assuming no improvement in efficiency): 10,000 watts. Then assume that the average person will need another 10,000 to cover all their personal and recreational pursuits. 173,000,000,000,000,000/20,000 = 8,650,000,000,000: 8.65 trillion people. More surface area makes it easier to radiate heat, so you would want the individual arcological “units” of the world-city to be as anti-spherical as possible: complex polyhedra or fractal balls would be ideal, perhaps combined with antennas, reflectors, reducing human body size, and other nifty tricks to reduce heat. Taken together, the 10 trillion figure for maximum population looks comfortably doable.
Matters of identity, character, and peoplehood will presumably be brought into sharper focus when literally 100% of living space on Earth is designed: everything about the environment will reflect the wishes and aspirations of its inhabitants. One thus must think carefully about how one is to go about the interior design process of the ecumenopolis, or your corner of it. The issue makes me reflect on the vast indoor spaces of the Portal games, which are set in a huge underground facility whereas arcological environments will be equivalently large or larger but suspended high in the air.