Our protagonist moves to the big city, seeking a better life. It's a classic—and increasingly common—tale. More than half the world's population now lives in a metropolis, and by 2050 that figure will very likely jump to two thirds. China's megacities in particular are fueling the trend, with more than 10 million new residents every year. Historically, urbanization has brought about stupendous changes—the Renaissance, the industrial revolution, globalization. Yet this urban migration represents one of the most dramatic environmental shifts human beings have ever undertaken. So one might be tempted to ask: How are we adapting to our new digs?