Unit 3　Back to the past

　　古罗马帝国不仅仅在政治上强大，版图上辽阔，它所创造的古罗马文化在哲学、文学、建筑等方面，也是全人类巨大的精神财富，是世界古典文化中的瑰宝。让我们来了解一下罗马帝国是怎样瓦解的吧。

　　Scientific assumption：the real cause of

　　 the Roman Empire collapsing

　　Climate change could have been responsible for bringing down the Roman Empire，scientists believe.

　　Researchers studied ancient tree growth rings to show links between climate change and major events in human history such as migrations，plagues(瘟疫)and the rise and fall of empires.

　　They discovered that periods of warm，wet weather coincided with periods of prosperity，while droughts or varying conditions occurred at times of political upheaval (剧变) such as the demise (消亡) 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.

Wet and warm summers occurred during periods of Roman and Medieval