Alan Weisman is the author of the critically acclaimed New York Times best seller The World Without Us. In his new book, Countdown: Our Last, Best Hope for a Future on Earth? the award-winning journalist traveled to more than 20 countries to ask what experts agreed were probably the most important questions on Earth--and also the hardest: How many humans can the planet hold without capsizing? How robust must the Earth's ecosystem be to assure our continued existence? Can we know which other species are essential to our survival? And, how might we actually arrive at a stable, optimum population, and design an economy to allow genuine prosperity without endless growth?