“As long as there’s been life on earth, there have been parasites,” says Dickson Despommier, emeritus professor of Public Health and microbiology at Columbia University. And, he says, a lot of us live in one: Cities — which feed off the earth’s resources without replenishing them — basically function as giant parasites. “They take advantage of what’s there and use it for their purposes,” Despommier says.

And they’re growing rapidly. By 2050, nearly 80 percent of the world’s population will be living in cities, and we’ll need a sustainable way to grow enough food to feed everyone. To that end, Despommier proposes vertical farms. Basically greenhouses stacked on top of each other, vertical farming produces food more efficiently to “ease the parasitism of cities on food production.”

These farms in buildings already exist in the real world. Japan, South Korea, Singapore, China and Taiwan, have hundreds of vertical farms, as do several U.S states, including New Jersey and Illinois. And that’s a good thing, since already fully one-seventh of the earth’s land mass (or the entire continent of South America) is devoted to producing food for the 7.3 billion people who live here.

If we don’t keep working to implement sustainable ways to feed our cities, Despommier says, “The parasite will exceed its capacity and collapse under its own weight.”