The last million years have seen the emergence of Homo sapiens, of the complex social fabrics that we call civilization, and of large-scale societies that expect and, to an extent, require some measure of climatic predictability or stability. Intensive food-production began within the most recent major warming cycle in the present ice age (the Holocene epoch); it has supported an unprecedented population expansion in our species. This population expansion has occurred mainly since the end of the “Little Ice Age” in the nineteenth century, in a time of warming and relatively stable climate (Grove 1988). The rate of population growth has steepened dramatically since industrialization, which occurred massively within the early to middle twentieth century, a period of unusually stable climate. Recent droughts and crop losses in Africa, and disastrous rains and flooding elsewhere, force us to realize how precariously balanced we are in depending upon intensive food-production in a world that may be fundamentally inimical to our present ways of doing things. Modern climates are neither typical nor normal in the perspective of world prehistory. Recent instabilities may presage major change.

Climate, as a statistical generalization about temperature and precipitation, varies from place to place, time to time and, especially, with the time duration and the size of spatial unit summed. Climatic statements should be understood as statements about the average and range of weather conditions at a place and time of specified scales.