Also called "farmscrapers," vertical farms could make cities self-sustainable. A 30-story vertical farm the size of one Manhattan city block could theoretically provide food year-round to 50,000 people.

By mid-century, 80 percent of the world's population is expected to reside in urban centers. Properly situated, vertical farms could eliminate the need for long-distance crop transport and refrigeration, reducing fossil fuel use and greenhouse gas emissions. They'd even scrub CO2 from city skies, giving us a much-needed breath of fresh air.