The Manhattan / New Zealand comparison is interesting to me, having lived and worked in and around NYC all my life. New Zealand is not that much bigger than the

The Manhattan / New Zealand comparison is interesting to me, having lived and worked in and around NYC all my life. New Zealand is not that much bigger than the US Eastern Seaboard metro area, which extends from Washington DC to Boston. If you extend that a bit further, from Virginia Beach VA to Portland ME, and extended it inland about as wide as New Jersey for the whole stretch, that's a pretty good match. A NYC-density city that large would be interesting. It would need rapid mass transit throughout with high-speed lines connecting with local hubs. That would be doable if you're rebuilding the entire area. (Amtrak hasn't been able to build high speed rail because they're stuck using the existing railways which aren't straight enough.) Presumably a lot of traffic could be reduced by replacing all of the highways with mass transit, which would be feasible if the whole area was that densely populated. Again, it hasn't been done now because all public transit around the big cities is designed to get you into and out of the cities; if your destination is anyplace else you have to drive yourself. It's much better within NYC where public transit goes everywhere. NYC, dense as it is, still has lots of open parkland. I think it would be quite livable even at the scale we're talking about. Of course people would still need to be able to get out of the city for a break; I imagine the whole western edge of the giant city would be public nature reserves with plenty of mass-transit access. Of course not everyone would live within the city; there would need to be farms and farmers to feed everyone. Most of the US and southern portions of Canada would be farmland (much as it is today, in fact) and there would be plantations scattered around the rest of the world too wherever the weather is suited to various crops. Much of the world would still be left to nature, I think. If we're at the point technologically and socially where we can move nearly everyone into a megacity, we should also be able to create very high efficiency hydroponic farms that could produce many crops much more densely than a standard farm can. Large areas would only be needed for animals and crops that can't be grown hydroponically. We'd need power too... lots of it. Keeping everyone in a relatively small area would help, but I'll bet the average power usage per-person across the globe is less than the average usage per New Yorker today, and in the megacity everyone would be moved to the New Yorker level. Maybe all of the unfarmable areas in Canada can be filled up with Nuclear powerplants.