Abstract: A wide variety of scenarios of future economic development and the resulting greenhouse gas (GHG) emissions have played significant roles in climate policy discussions. These scenarios are dependent on many underlying assumptions about future human activity, the pace and shape of political and technological change, and the availability of natural resources. Some scenarios are developed simply as “storylines”, where no attempt is made to assign the likelihood of a particular scenario occurring. Other scenarios try to assign probabilities to specific outcomes. To project the development of human systems for a hundred years is a heroic exercise, but it is a desirable task for informing climate-related decisions.