That environmental inequality could change direction, however, if we find ourselves with more electricity than we know what to do with in the future. A few timely breakthroughs in cheap photovoltaic film might allow us to plaster solar cells over every window and glass skyscraper. Before we even get close to replacing all fossil fuels with renewables, solar farms will pump out way more electricity than we can use—whenever the sun is unobscured and directly overhead­—because the dirty secret of renewables is that their inconstancy forces us to massively overbuild capacity. (Already, wholesale electric prices go negative in California when the sun is high, as grid operators scramble to find customers to take excess solar power.)

A similar embarrassment of riches could occur if engineers figure out fusion energy or better nuclear power and if we replace all our polluting generators with reactors. We could keep those plants humming at night, when demand is low, and shunt the clean, cheap electrons into vertical farms.

In the meantime, says Leo Marcelis, a vertical-farming researcher at Wageningen University in the Netherlands, indoor farmers could rig their grow lights to switch on and off to take advantage of periodic dips in electricity prices. Their plants would take a little longer to mature, but they would still thrive.

Bomford, however, is unpersuaded by such scenarios. “I’m concerned about the opportunity cost,” he says. “If the vertical farm is using low-carbon energy, it means there is some other area that doesn’t have the opportunity to use it”—for example, recharging electric vehicles, running automated factories, or making hydrogen to power fuel cells.

Good for Veggies, Not for Grains

Setting energy aside, the environmental pros of vertical farms do seem to outweigh their cons, but only for high-value, fast-growing crops. Peek inside most vertical farms or food computers, and there is an excellent chance you’ll see leafy greens such as lettuce or herbs sprouting inside.

The technology shines brightest for such crops. Dan Blaustein-Rejto, an agriculture analyst at the Breakthrough Institute, has calculated that indoor farms in Sweden, where nuclear and hydroelectric power generate 90 percent of the electricity, can already grow lettuce with a smaller carbon footprint than conventional farms produce. He showed that outdoor farms fall behind in the carbon race when you factor in transportation and waste. Indoor lettuce isn’t vulnerable to hailstorms or armyworms, and it’s not as likely to go bad in transit since it won’t be moving from California to New York. Indoor farmers also need less fertilizer because the nutrients can be recycled. One-third of the nitrogen applied to fields, in contrast, is converted to polluting gases or washed away to turn ponds into pools of green slime. Over the longer term, cropland put out of work by indoor farms could even be used to grow carbon-storing forests, just as has occurred after previous booms in agricultural productivity.

The math may work out for lettuce and certain other vegetables, when energy is cheap and green enough. But the numbers don’t add up for grains, which account for the vast majority of farming. To clean up the oxygen-deprived dead zone at the mouth of the Mississippi, for example, we’ll have to figure out new ways of growing corn. Or, as Bomford puts it: “The Gulf of Mexico Hypoxia Task Force was not formed in response to rapacious arugula expansion in Iowa.”

Though it might be technically possible to move our amber waves of grain indoors, Marcelis says, “it will take a long time—let’s prove it with vegetables first.” A blade of wheat needs five months to produce a kernel, whereas microgreens can be harvested every few days. And dry, hard grains are a breeze to store and transport compared to spring mix, which is little more than water packed into millions of gossamer cells.