Special Encore Presentation: Countdown, a new book by Alan Weisman

With a million more of us every 4½ days on a planet that's not getting any bigger, prospects for a sustainable human future seem ever more in doubt. Alan Weisman traveled to more than 20 countries to ask what experts agreed were the 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? Tune in as we talk with Alan about what he encountered on his global search for answers.