You’ve probably heard of vertical farms—growing crops indoors stacked as high as the entrepreneur builds. Indoors allows year-round growing, growing near where people live, saving transportation costs and pollution. It allows hydroponics and related techniques, reducing resource use. They light plants with LEDs with frequencies optimized for each plant.

Despite these advantages, they face several limitations. So far, only high water, low calorie plants like lettuce, tomatoes, and herbs have proven economical. Grains and legumes cost far less to grow in traditional farms. They require a lot more energy, which the sun provide free.

Do vertical farms provide a net gain? The answer requires engineering details still being worked out. Here’s one line of inquiry, to give you a flavor. If the LEDs are powered by fossil fuels, the vertical farm is not sustainable, even if it pollutes less for the other efficiencies. Of course, we can power it with renewables like solar and wind. But using solar means converting sunlight into electrical power (with some loss) and turning it back into light (more loss), though concentrating the sun’s broad spectrum into the LEDs optimized frequency (gain). Stacking solar panels doesn’t work since they can’t collect power in shadow, so using solar requires more space, undoing the gain in stacking the plants, though you could put the solar collectors in places you can’t farm. You can use renewables besides solar, though it would take power from the rest of the economy. Completing and quantifying all this engineering suggests vertical farms based on renewable power show long-term profitability for only some specialty crops with little sustainability benefit relative to traditional farming, if any.

Fusion—if it delivered on its promise of energy almost too cheap to meter, no greenhouse emissions, no radioactive waste, and no intermittency on cloudy days or when the wind isn’t blowing—would change everything. People joke that fifty years ago they said it would be ready in twenty years and today, after dozens of nations have spent billions of dollars developing it, they say it will be ready in thirty. Even so, theory suggests it can work. Making it work seems only a matter of engineering.

Let’s say it worked and we resolved issues of terrorism risk and so on. It would enable building vertical farms as high as engineering allowed, growing any crop.

In today’s world, without fusion, we have long found it profitable to convert farmland to suburbs, exurbs, malls, and McMansions. Fusion-powered vertical farms would render farmland worthless for farming. On the contrary, wherever people lived, it would be more economical to build vertical farms. People now live near natural resources like rivers and coastlines, but fusion could bring resources anywhere.

The assumptions driving our expectations of population leveling off would no longer hold. On the contrary, all that energy would portend a bright, clean future of local fresh food anywhere, along with fresh, desalinated water and clean air. Fusion would fuel an inflection in population growth like the Industrial and Agricultural Revolutions’. Cities would expand and grow denser as buildings grew to engineering limits. Exurbs once sparsely populated would grow as dense.

Where I live, Manhattan, has already expanded the island’s shoreline to build, as have many cities and nations. Current zoning laws limit building heights, but not much below engineering limits and developers already find ways to relax those regulations. Fusion would reduce those limits and increase developers’ motivations. One main reason for zoning is to allow sunlight to reach people, but with fusion-powered LEDs growing plants, we could provide ourselves with as much light as we wanted, probably improved to provide vitamin D but not sunburns. Unless we somehow stopped ourselves, market forces would accelerate building outward and upward. I love Central Park, but I can’t see it surviving being surround by mile-high buildings blocking sunlight.

On a larger scale, if you’ve seen pictures of Honolulu from above, you’ve seen the trend of buildings encroaching mountain, forest, and beach. The trend would accelerate there too. Either we would regulate our behavior we’d expand until we built everywhere we could.

On a yet larger scale, New York’s western exurbs growing denser would lead to their developing their western exurbs and so on. What would limit buildings as high and populations as density as engineering allowed? Ultimately running into Chicago’s exurbs advancing eastward, then San Francisco’s.

If we don’t choose by our values a different way to live, the forces driving vertically farming plants leads to vertically farming us. I’m not suggesting good or bad, nor a Matrix-like dystopia. Fusion would provide all the amenities of life, indoors. A life always indoors but with power to meet all our needs might lead to a life everyone prefers.

Fusion doesn’t remove all constraints, though. We can’t build taller than materials allow. Even with fusion we have to stop growing our population. Hawaii’s population of 300,000 before Captain Cook figured out how. Some nations today have. Like so many elk, the global population hasn’t.

Fusion would replicate the Green Revolution on a greater scale. Norman Borlaug’s pleas to address population fell on deaf ears and his predictions are happening—we’re replicating the problems he knew he was only temporarily relieving. The Green Revolution delayed the problems a few decades he alleviated and grew them from affecting millions to billions. Fusion would delay them a few more generations and grow them to affecting far more.

Even with fusion, we have to stop growing at some point. If we don’t believe we can stop ourselves, fusion would lead to suffering orders of magnitude beyond what Borlaug saw. If we believe we can with fusion at a vastly greater population, then we can today, when we’re living beyond what Earth can sustain.

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