Consider a city of roughly 1 million people, Adelaide, Australia for example — Calgary, Canada; Bonn, Germany; Tuscon, Arizona; or Bristol in the UK would do equally well.
Adelaide has two Australian Football League teams, a pro soccer team, two professional basketball teams, three Universities, a cathedral, numerous hospitals, many shopping malls, around 440 schools, an International airport, and a zoo.
There are over 400 suburbs arranged around a CBD that has high-rise office blocks that provide a common destination for a metropolitan public transport system that includes a fleet of over 1,000 buses.
There are doctors, dentists, lawyers, Artisans and actors; and enough skilled tradesmen to build or engineer almost anything.
In short, Adelaide is a self-contained community surrounded by enough farmland to feed everyone.
If it were possible to gather all the people who live in Adelaide into one, standing room only location it would be quite a spectacle. It is hard to imagine what it would look like.
There would people as far as the eye could see. Lay them down head to toe and the line would stretch 1,800 km — 400 km further than a road trip from Adelaide to Sydney.
Stand them in single file and the line would be 30 km long, similar to the queue at the post office.
Now having conjured the image of so many people in your mind’s eye put them all onto commercial aircraft.
Because 1 million is roughly the number of human beings who are, at any one time, airborne in commercial airliners making vapor trails around the globe.
This is both staggering and scary at the same time.
It is enough just to illustrate the scale of the challenge to provide life support to all the people we have made and still retain some environmental integrity.