Within a couple of days, water fills New York's subway tunnels. Insects graze our houses down to their foundations, subdivisions vanish under new-growth forests, and wild predators rove our old neighborhoods, picking off the last of our house pets. This chilling prediction was at the heart of Alan Weisman's 2007 bestseller, 'The World Without Us,' which imagines a planet suddenly absent of our influence. His hugely anticipated follow-up, 'Countdown,' out this month, might be read as kind of a prequel: What does the world look like with too many of us?

"In the entire history of biology, every species that outgrows its resource base suffers a population crash – a crash sometimes fatal to the entire species," Weisman writes. 'Countdown' is his bold, troubling, and often inspiring search for ways to save ourselves.

"Until 'The World Without Us,' I hadn't thought about population," Weisman says, "but I did some long division and came up with the fact that every four days or so, we're adding a million people to the planet. That didn't sound very sustainable." In the six years since Weisman wrote that book, the global population has jumped from 6.5 billion to more than 7 billion; we're on target to hit

10 billion by the end of this century. Consider the amount of water it will take to slake the thirst of an extra 3 billion people. Or the impact of increasing, by almost half, the number of humans exhaling carbon dioxide and demanding electricity. Consider the challenge of feeding another 3 billion people – that's nine United States or two Chinas – when there are already a billion going hungry.

In short, says Weisman, either we can find a way to bring our numbers down or nature will do it for us, through famine, drought, disease, and warfare. To write 'Countdown,' he traveled to 21 countries and spoke with dozens of politicians, scientists, and religious leaders (who exercise tremendous influence over family size). He found vivid, real-world portraits of what overpopulation portends – from coastal farms in the Philippines flooded by rising seas, to Mumbai, where families live under tarps strung between "every pair of new skyscrapers" – and models worth emulating, in places like Iran, where voluntary, state-funded birth control and education have averted catastrophe.

So how many of us can the planet hold, with enough fresh air, clean water, and food to go around? Two billion, Weisman says, roughly where we were in 1900. And we could get back to that number in a century, he argues, if we all adopted China's one-child policy. "We're not going into space anytime soon, if ever," says Weisman of the tenacious daydream that if we fill up this planet, we can simply find another. "I don't see us being able to change our lifestyles fast enough. The one thing we can do is contraception. We could change human impact more quickly that way, and give ourselves time to solve these other problems."