What Is Climate?



Most people have an intuitive understanding of the weather as referring to the state of the atmosphere at a given time and place and of the climate as a kind of average weather. A popular expression of this dichotomy is “the climate is what you expect, the weather is what you get” (Heinlein [1973, p. 352], although often attributed to Mark Twain). Implicit in this belief is the notion of climate as a kind of constant natural state to which the weather would converge if it were averaged over a long enough period. A corollary is that climate change is a consequence of “climate forcings,” which are external to the natural climate system and which tend to prevent averages from converging to their true values. In this framework, past climate change may be attributed to orbital changes, variations in solar output, volcanic eruptions, etc. For the recent period, anthropogenic forcings can be added.