In her new book The Sixth Extinction: An Unnatural History, Elizabeth Kolbert describes traveling the world to document the mass extinction of species that seems to be unfolding before our eyes. There have been five comparable crises in the history of life on Earth, she writes, but this one is different: It's being caused by us.

Kolbert, a staff writer for the New Yorker, is also a contributor to National Geographic magazine, and her new book is informed by reporting she did for this magazine on the Anthropocene, or "the Age of Man," ocean acidification, and captive breeding in zoos. She is drawn to gloomy subjects—her previous book, Field Notes from a Catastrophe, was on