In his bestselling book The World Without Us, Alan Weisman considered how the Earth could heal and even refill empty niches if relieved of humanity’s constant pressures. Behind that groundbreaking thought experiment was his hope that we would be inspired to find a way to add humans back to this vision of a restored, healthy planet-only in harmony, not mortal combat, with the rest of nature. But with a million more of us every 4 1/2 days on a planet that’s not getting any bigger, and with our exhaust overheating the atmosphere and altering the chemistry of the oceans, prospects for a sustainable human future seem ever more in doubt. For this long awaited follow-up book, Weisman 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?