The key features of the climate change debate are large degrees of uncertainty and a long time horizon. Although it is fairly well-established that the Earth’s atmosphere has warmed somewhat (one degree Fahrenheit) during the past century, it’s not clear why this happened. The warming may have been due to human impositions (the burning of fossil fuels and other incidents of industrial growth), or to natural solar or climate variations, or to some of each. Whatever the causes, we don’t know if future warming trends will be large or small, or whether the net environmental and economic consequences (including both beneficial and harmful effects) may be large or small.