In the past century the earth's human population has quadrupled, growing from 1.5 billion in 1900 to about 6 billion today. By 2050, it is estimated that the global population will reach 9 billion. In 1968, a young biologist named Paul Ehrlich wrote a best-selling book called The Population Bomb, which sparked an ongoing debate about the dangers of overpopulation. He argued that population growth was destroying the ecological systems necessary to sustain life. So just how worried should we be? Is population growth a problem or not? And if so, what should we do about it?