They discovered that periods of warm，wet weather coincided with periods of prosperity，while droughts or varying conditions occurred at times of political upheaval such as the demise of the Roman Empire. To match the environmental record with the historical one，researchers looked at more than 7,200 tree fossils from the past 2,500 years. The study，published in the journal Science，said：The rise and fall of past civilizations have been associated with environmental change，mainly due to effects on water supply and agricultural productivity，human health and civil conflict.