Of all the environmental issues that have emerged in the past few decades, global climate change is the most serious, and the most difficult to manage. It is the most serious because of the severity of harms that it might bring. Many aspects of human society and well being – where we live, how we build, how we move around, how we earn our livings, and what we do for recreation – still depend on a relatively benign range of climatic conditions, even though this dependence has been reduced and obscured in modern industrial societies by their wealth and technology. We can see this dependence on climate in the economic harms and human suffering caused by the climate variations of the past century, such as the “El Niño” cycle and the multi-year droughts that occur in western North America every few decades. Climate changes projected for the twentyfirst century are much larger than these twentieth-century variations, and their human impacts are likely to be correspondingly greater.