Planet Earth is headed for a tipping point. Midway through this century, there will be more than nine billion people on the planet. Already we are using most of the arable land that exists and overfishing the oceans. Water, too, is becoming scarce in many places. And the services that humans depend upon ? clean drinking water, steady energy supplies, and protection from disease are under threat. The grand challenge of the 21st century is to change the endgame from one that looks like a train wreck, to one to a brighter future. Barnosky and Hadly offer sensible solutions to our most pressing problems.