Climate change may have been one of the reasons of the downfall of the Indus Valley Civilization

In the 19th and 20th centuries, archaeologists discovered traces of India's earliest civilization, one that developed in the fertile Indus River Valley between 3000 and 1900 BCE. Larger than either the Egyptian or Mesopotamian civilizations of the same period, the population of the Indus Valley (or Harappan) Civilization is estimated at anywhere between two and five million people. Among the civilization's 2000 major settlements were the planned cities of Harappa and Mohenjo-daro, trading and craft production centers where craftspeople and villages wrought pottery and intricate beads made of gold, copper, and ivory.
Archaeological evidence shows that after 700 years of stability, the civilization declined. Most of the Indus settlements had been abandoned or had shrunk in size by about 1800 BCE. Many factors contributed to the end of the Indus civilization, but climate change is emerging as a primary reason for its gradual demise. Geological evidence shows that the region's climate grew colder and drier, in part perhaps because of a weakened monsoon. By 1800 BCE, the Ghaggar-Hakra River, a river in the region that paralleled the Indus system and that some scholars suggest is the Saraswati, the lost sacred river of Rig Veda, was severely diminished. As a result, cities were abandoned and though some of the population remained, many migrated to more fertile lands in the east around the Ganges and Jumna River