Since the Intergovernmental Panel on Climate Change (IPCC) was formed in 1988, it has engaged a substantial proportion of those individuals with relevant scientific expertise in the process of forming reasonable judgments about the effects of aggregate human activity on the composition of the earth’s atmosphere and about the resulting implications for global climate. It is now widely agreed that in concert with other so-called “greenhouse gases,” carbon dioxide (CO2) released from the burning of fossil fuels for energy is causing the earth’s climate to change. Over the last century, the concentration of CO2 in the atmosphere increased from about 300 to 375 parts per million by volume (ppmv), and global average surface temperature increased by 0.4 to 0.8 oC. In the absence of policies designed to substantially reduce global emissions, scenarios developed by the IPCC indicate that CO2 concentrations will reach 550 to 1000 ppmv in 2100 and that global average surface temperature will increase by an additional 1.5 to 6 oC (IPCC 2001a). The consequences of such a temperature increase and associated changes in precipitation patterns and other climate variables are a matter of greater uncertainty and disagreement. At the lower end of the range, it is possible that nothing of global consequence will occur, and that the regional and more localized effects will be moderate enough to be handled by natural adaptation. It also conceivable—particularly at the high end of the temperature range—that abrupt, nonlinear and fundamental changes could be triggered, such as a sudden change in large-scale ocean currents, with truly massive and potentially catastrophic consequences for human societies. The IPCC has identified the possibility of extreme danger, but has been and will remain unable to reach consensus on its exact character, magnitude, probability and timing.