Austin Contrarian, who has a nice habit of finding numbers to support the random stuff I say, ran the figures on vertical farming last year. The results?

A 30-story building means steel-and-concrete construction, especially if each floor will be loaded up with plants and elaborate irrigation systems. Around here, that kind of building costs $300+ per square foot, without the fancy finish-out. Equivalent high-rises in NYC go for $1,000+ per square foot. That would be the market price — or opportunity cost — of one square foot of this “urban farmland.”

Let’s see. There are 43,560 square feet in an acre. That means this “urban farmland” would cost between $13 million and $43 million per acre. I don’t know exactly how productive this super-duper advanced hydroponics system would be, but I doubt it’s 4,000-14,000 times as productive as $3,000/acre farmland in Indiana. That locally-grown broccoli had better taste really, really good.

Update: There’s a lively discussion of vertical farming over at Slashdot. Some people actually think this will make sense for New York when gas gets real expensive.

I’m always surprised that people don’t have a better sense of relative magnitudes. If you assume that an acre of hydroponic “land” can support 10 people per year (a generous estimate), then 8,000 acres could support 80,000 people per year — or 1% of New York City’s population.

8,000 acres is roughly 348 million square feet, which, coincidentally, is almost exactly the size of Manhattan’s entire inventory of office space. So to meet just 1% of NYC’s annual food needs, you would have to find space in NYC to duplicate one of the planet’s great concentrations of skyscrapers.

Farming is just incredibly land intensive. There’s just no way to make farming productive enough for it to be worthwhile to grow food in places where land is really expensive. And if you did it anyway, through subsidies and similar, the cost would be enormous.