Humanity is risking the health of the natural environment through a myriad of interventions, including the atmospheric emission of trace gases such as carbon dioxide, the use of ozone-depleting chemicals, the engineering of massive land-use changes, and the destruction of the habitats of many species. It is imperative that we learn to protect our common geophysical and biological resources. Although scientists have studied greenhouse warming for decades, it is only recently that society has begun to consider the economic, political, and institutional aspects of environmental intervention....