With over 50% of the world’s population now living in cities, and with that percentage increasing, along with at least another two billion people projected to be added to global population before it levels off, megacities are destined to rise to complement existing cities.  Some will rise in the middle of empty open space, perhaps offshore or in a desert.  Others will rise from redeveloped sections of exisiting cities.  These new cities will have towers of composites and steel and pervasive photovoltaic and thermal architecture.  The ocean will supply saltwater feedstock for desalinated fresh water, and all waste water will be cleaned and the surplus will be piped to neighboring arid lands, to filter into aquifers or reestablish new forests bringing back moderate and reliable rains.

Future megacities will be like spaceships - self-sufficient, recycling everything, generating surplus water & power

Megacities will be like space colonies on a new planet, completely self-sufficient, and making an imprint calculated to improve the health of the regional ecosystems.  In practice that means desalinating seawater for residential and industrial use, factory farming, then cleaning and recycling all wastewater.  The surplus water from megacities can be used to remoisturize the planet.  The earth is dry because tropical forests are half the extent they once were, and our lakes and water tables are depleted.  The water surplus from new megacities can rewater the planet.

Environmentalists decry mega-projects yet extol the megacity.  New cities have to attract people, which is why cities upon terraced mountains could permit high density while preserving cool, rural ambiance.  Megacities could be more people friendly if multi-family, mid-rise dwellings were designed in a terraced manner to incorporate high-ceilings and panoramic views of multi-story buildings, but with every unit on the terraced structure having a balcony and yard, and only a one story drop to the next yard.  Yards would vary in size.  With a residence above, below and to the side, the interior space of each residential condominium could exist beneath a roof covered with turf that would constitute the yard of the condominium above.  With this design, literally every resident in a huge multistory structure would retain a connection to the land - there could be paths along the turf covering the major structural elements of the massive building, so residents could literally walk up and down on the growing turf of the roof.

Under the ground, and via massive, wide canyons into and out of terraced megastructures, personal vehicle access and parking and transit amenities can be plentiful - freight arrives via a sorting yard not dissimilar to airline luggage processing, hopefully more reliable!  In comes food for the tens of thousands, out goes clean water and usable recycled materials.  The future megacity can produce water and energy surpluses that are exported to the planet.  They can produce virtually zero pollution, and they can house billions.