> If anyone has pointed this out, I've missed it. Area to support plants
> is not the only limitation; sunlight, or energy, is equally important.
> The Earth intercepts only so much sunlight. A skyscraper won't change
> that. It casts an area in shadow, and piles up an equivalent area. The
> benefit is that you get climate control; the drawback is that you have
> to build the building. That's it. Agri-Manhattan can't grow much more
> than it could as a conventional farm. What would the extra plants grow
> on?
>
I pointed this out, but not as concisely. You only win if you use
a non-solar power source to generate artificial light. You can use
fossil fuels or nuclear power. This would be a horrible waste of
fossil fuels: there is likely to be a better way to convert them to
food. This leaves nuclear, which most people reject for emotional
reasons.
You can of course place your farms on top of the city and still capture
the city's area of sunlight, but the city would be a pretty gloomy
place if you did that.