The seeds of an agricultural revolution are taking root in cities around the world—a movement that boosters say will change the way that urbanites get their produce and solve some of the world's biggest environmental problems along the way.

It's called vertical farming, and it's based on one simple principle: Instead of trucking food from farms into cities, grow it as close to home as possible—in urban greenhouses that stretch upward instead of sprawling outward.

Intriguing idea. Economically viable?