Dickson Despommier’s hydroponic metropolis would squeeze sprawling farmland into skyscrapers, feed millions, and cool the Earth. Nearly 41 percent of Earth’s land is now used for agriculture, yet we’re on the brink of vast population growth, from 6.7 billion people today to an estimated 9.2 billion by 2050, with the majority living in cities. The only way to make room for enough carbon-sequestering trees to reverse global warming, Despommier argues, is to change the way we farm. Radically. Despommier envisions blocks of vertical farms in the world’s biggest cities, each structure 30 stories high, providing enough food and water for 50,000 people a year, with no waste.