The key insight of Gaia Theory is that the entire Earth functions as a single living superorganism, regulating its internal environment much as an animal regulates its body temperature. But according to James Lovelock, the theory's originator, that organism is now sick. It is running a fever born of increased atmospheric greenhouse gases. Earth will adjust to these stresses, but the human race faces a severe test. It is already too late, Lovelock says, to prevent the global climate from "flipping" into an entirely new equilibrium that will threaten civilization as we know it. But we can do much to save humanity. In the tradition of Silent Spring, this is a call to action.