In his 2007 book "The World Without Us," author and journalist Alan Weisman explored some harrowing questions about what Earth would look like without people. In his new book, "Countdown: Our Last, Best Hope for a Future on Earth?", he focuses on the perils of overpopulation. The population of Earth is nearing 7.5 billion. How healthy does the Earth need to be to ensure human existence? What is an optimum population for the world?
Journalist Alan Weisman Warns of Overpopulation Crisis
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Alan Weisman, journalist and author of "Countdown" and "The World Without Us"