Humans are fast becoming city dwellers. According to the United Nations, “The urban population of the world has grown rapidly from 746 million in 1950 to 3.9 billion in 2014.” Sixty-six percent of us will likely live in urban environments by 2050. The number of mega-cities (more than 10 million inhabitants) is also skyrocketing, from 10 in 1990 to 28 in 2014—home to more than 453 million people—and is expected to grow to 41 by…