By 2050, 2.5 billion more people are projected to leave the countryside for the city; in the United States alone, urban land will more than double by 2100. Faced with what scientists call “the urban heat island effect,” cities around the world are encouraging the development of roof gardens. These blankets of wildflowers, grasses, and sometimes even vegetables reduce water runoff, absorb carbon dioxide, and lower temperatures. Chicago is home to the world’s largest rooftop farm: The two acres of land atop a soap factory supply a million pounds of vegetables a year.