Climate Change, Not Aryan Invaders, Caused Dramatic Fall of Indus Valley Civilization

The Indus Civilization was one of the most important Bronze Age civilizations. It was comparable in its achievements to the Egyptians and it is often claimed that it was very influential in the development of later societies in the Indian subcontinent. However, the reason why this ancient civilization fell is one of the ancient world’s biggest mysteries. Research on historical monsoon patterns may have solved this puzzle. It now appears that climate change was responsible for the decline and fall of the Indus Valley Civilization, also known as the Harappan culture.

Dr. Nishant Malik, from the Rochester Institute of Technology in New York state, developed a mathematical model. He wrote in the journal Chaos that he and his colleagues developed a ‘hybrid framework appropriate for identifying distinct dynamical regimes and transitions in a paleoclimate time series’. Three different statistical methods were used to understand the climate in ancient times and in particular the level of monsoon rainfall. Sky News reports that Dr. Malik’s ‘ model applied dynamical systems theory to paleoclimate data - for instance, data on rainfall based on the presence of a particular isotope in stalagmites in a cave.’

Lahoot Lamakan (cave). (Umasha79/CC BY SA 3.0) The model applied dynamical systems theory to paleoclimate data - for instance, data on rainfall based on the presence of a particular isotope in stalagmites in a cave.

When the Monsoon Pattern Changed the Ancient Civilization Fell

The model found that around 5,200 years ago a warm period known as the Holocene Climate Optimum ended. This meant that glaciers expanded and cast more sunlight back into space and this cooled the planet. As a result, the differences in temperature between the land masses and the oceans increased and this was critical in the formation of monsoons. The end of the Holocene Climate Optimum meant that the level of monsoon rainfall increased.

The Indus Valley Civilization was dependent on the monsoons, just as succeeding cultures in South Asia were. Dr. Nishant Malik is quoted by the Daily Mail as saying “The region where Indus Valley Civilization bloomed is semiarid, bestowed with several glacier-fed rivers, for example, the Indus river and many of its tributaries.” The people of the Harappan culture often built their settlements and urban centers along the Ghaggar–Hakra river, which is dependent on the monsoons. Dr. Malik explained, “Hence, the Indus Valley Civilization was critically dependent on monsoon driven rainfall, a highly dynamic phenomenon.”

Life in Harappa, one of the main cities of the ancient Harappan culture. (Tejavalli reddy(1830787)/CC BY SA 4.0)

Achievements of the Harappan Culture

As the monsoon brought more rain, agriculture increased and this meant that the people of the Harappan culture could develop a sophisticated society between 3500 and 1300 BC. This ancient civilization reached as far as Afghanistan and covered much of Pakistan and areas of north-west India. Some calculations state that it was home to around five million people. Its two largest cities were Mohenjo Daro and Harappa.

Harappa archaeological site in Punjab, Pakistan. (robnaw /Adobe Stock) This was once one of the largest cities of the Harappan culture.

“This civilization is known for advanced urban infrastructure and technologies, such as systems for measuring length and mass” Dr. Malik said. They had sanitation systems that allowed them to live in big cities. The layouts of their settlements were very uniform and this has suggested to some that it was a very hierarchical society, that was centrally planned. However, much about this remarkable civilization is mysterious because its ancient writing system has not been deciphered.

• Timeline of the Harappan Civilization is Pushed Back 2,500 Years as Researchers Find New Reason for Its Fall
• 5000-year-old skeletons of Harappan Civilization excavated in India
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Climate Change and the Decline of the Ancient Indus Valley Civilization

The study argues that the ancient Harappan culture began to decline because of changes in the size of glaciers due to orbital forcing. This influenced how much light, and therefore heat, reaches a certain area. About 1,300 BC this led to cooler temperatures and influenced the monsoons. As less rainfall fell, the people of the Harappan culture struggled to grow crops’ and this led to the decline of their ancient civilization.

The Daily Mail quotes Dr. Malik as saying that “We here showed that this civilization not only matured but also declined due to transitions in the hydroclimate of this region.” The people of the Harappan culture could not simply cope without plentiful rainfall and it is believed that they left the area, abandoned their cities, and migrated elsewhere, forming small agricultural communities in upland regions.

This Disproves the Aryan Invasion Theory

These findings are also similar to those established by the Woods Hole Oceanographic Institution (WHOI). Based on a study of fossils, they discovered that the monsoon pattern had changed by around 1800 BC. As reported on Ancient Origins, they ‘found that winter monsoons seemed to become stronger - and summer monsoons weaker - towards the later years of the Harappan civilization, corresponding with the move from cities to villages.’

Sky News reports that ‘Other theories explaining the civilisation's decline have suggested it was shattered by earthquakes or by an invasion of nomadic Indo-Aryans’. It was once widely believed that the Aryans invaded the Indus Valley and caused the collapse of the ancient civilization. However, there is little physical evidence in the remains to show that the culture fell because of invasion or war. The latest research shows that climate change was most likely responsible for the collapse of this magnificent yet mysterious civilization.

Top image: Diorama of people in the ancient Harappan culture (Indus Valley civilization). Source: Biswarup Ganguly/CC BY 3.0

By Ed Whelan