Springer Science+Business Media Dordrecht 2014. This article explains how large-scale climate is defined, how it is established physically, and why it can change. The mechanisms for change are briefly described along with internal processes that can magnify or diminish the response to externally imposed perturbations to the conditions for equilibrium. Delays can also occur among the internal feedback loops because some are very slowcompared to human experience. The study of ancient climates helps put the subject of climate changeinto perspective, since the climate has changed many times in the past. The outlook for the coming century is an unrelenting warming with some shifts in precipitation patterns, some of which are very troubling, including the case of theAmerican Southwest where less moisture seems likely.