Reading Richardson Gill's The Great Maya Droughts, I ran across a fascinating discussion of early Europe that shows why climate variability needs to take its place alongside great battles and intellectual ferment in our tellings of history.

Gill talks about the work of Carol Crumley, whose climate reconstruction shows how the Romans and the Celts each dominated large areas of central Europe when the climate was to their liking. The Celtic style of agriculture was more well suited to a cold climate. From 1200 to about 500 BC, much of Europe was cold, with great variability. This was the environment in which Celtic agricultural practices evolved, with a robust flexibility. By about 300 BC, the cold line had shifted north, bringing what we now call a "Mediterranean climate" to much of Europe. The Romans were good in this kind of an environment - few crops, urban living. So when the climatic line shifted north, the Romans went with it, pushing out the Celts and coming to dominate central Europe. Gill quotes Crumley:

The extent and duration of the Pax Romana in Europe was greatly facilitated by climatic conditions that favored Roman - as opposed to Celtic - economic, social, and political organization. Not only were Roman patterns of settlement and land use in marked dstinction to those of Celtic polities, the were especially suited to the mediterraneanized cliamte of Europe.