"In the last 10,000 years, the Northern Hemisphere had the highest insolation (the solar energy received by the Earth from the sun) from 7,000 to 5,000 years ago, and since then insolation there decreased. All climate on Earth is driven by the sun, and so the monsoons were affected by the lower insolation, decreasing in force. This meant less rain got into continental regions affected by monsoons over time."Eventually, these monsoon-based rivers held too little water and dried, making them unfavorable for civilization. Eventually, over the course of centuries, Harappans apparently fled along an escape route to the east toward the Ganges basin, where monsoon rains remained reliable.This change would have spelled disaster for the cities of the Indus, which were built on the large surpluses seen during the earlier, wetter era. The dispersal of the population to the east would have meant there was no longer a concentrated workforce to support urbanism.
"Cities collapsed, but smaller agricultural communities were sustainable and flourished," Fuller said. "Many of the urban arts, such as writing, faded away, but agriculture continued and actually diversified."
LiveScience: Huge Ancient Civilization's Collapse Explained