First articulated by the British chemist James Lovelock in the 1970s, the Gaia hypothesis (named for the Greek goddess who personified the earth) proposes that the biosphere, atmosphere, oceans, and surface rocks make up a single, self-regulating, homeostatic system (Lovelock 1979). Key observations that Lovelock used in support of Gaia include the long-term stability of chemical disequilibria in the atmosphere and oceans despite both high fluxes of many chemicals within the earth system, and the fact that these persistent (in some cases for billions of years) yet nonequilibrium conditions are particularly well-suited for life as it has evolved. To Lovelock, the implication of these and related observations is that the biosphere must actively modulate the chemical make-up, temperature, pH, and other attributes of the earth system in order to maintain conditions under which life can flourish. In particular, the composition of the atmosphere must be regulated by the biosphere...