Our planet is approaching a critical environmental juncture. Across the globe we continue to deplete the five pools of carbon - soil, wood, coal, oil, and natural gas - at an unsustainable rate. We've burned up half the planet's known reserves of oil - one trillion barrels - in less than a century. When these sources of energy-rich carbon go into severe decline, as they surely will, society will follow. Former archeologist and Sierra Club activist...