Archeologists identified signs of a mega-drought for a millennium between 5,000 and 4,000 years ago in Africa and Asia. The drying of the Green Sahara not only turned it into a desert but also disrupted the monsoon seasons in South and Southeast Asia and caused flooding in East Asia, thereby preventing successful harvest and the development of complex culture. It coincided and may have caused the decline and fall of the Akkadian Empire in Mesopotamia and the Indus Valley Civilization. The dramatic shift in climate is known as the 4.2 kiloyear event.