Since the beginning of recorded time, climate-forced migrations have reshaped civilization. Four thousand years ago a prolonged drought and the resulting famine in Canaan drove Jacob and his sons to Egypt, setting the stage for the famous exodus led by Moses. Three millennia later a prolonged dry period and lack of grazing lands helped to push Mongol armies out of Central Asia as far west as Europe, where many settled and intermarried. And in the 20th century the American Dust Bowl, an ecological catastrophe precipitated by drought and compounded by bad land-management policies, displaced 3.5 million people from the Midwest.