Via Schweeb (hat tip arslinux) comes an interesting discussion of the Celt and the Romans and climate change:
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.