Representational image.

A study by researchers at the Indian Institute of Tropical Meteorology (IITM) has revealed that climate factors, strikingly similar to the interplay of those affecting today’s Indian monsoons, most likely led to the collapse of the Indus Valley Civilization over 4000 years ago. Analysing ancient cave formations (speleothems) from the Gupteswar caves in Orissa and the Kadapa caves in south India, the study found how reduced solar radiation, El Nino, southward migration of Intertropical Convergence Zone (ITCZ), and a negative phase of Indian Ocean Dipole (IOD) collectively weakened the monsoon, triggering the downfall of the ancient civilisation. The study has been published in Quaternary International journal and cited by a Times of India report.

The research team excavated a 7000-year climate record from cave deposits, shedding light on the region’s past climate fluctuations. The findings revealed a deadly combination of reduced solar radiation, El Niño events, and a southward shift of the Intertropical Convergence Zone (ITCZ)—a vital weather system dubbed the “rain belt.” This convergence of factors crippled the monsoon, spelling doom for the ancient civilization. The Indus Valley Civilization, which boasted major urban centres like Harappa and Mohenjo-Daro, was left high and dry by the weakened monsoon. Prolonged droughts, triggered by the climate conspiracy, ultimately led to the downfall of this ancient powerhouse.

“What is fascinating is that the climate patterns we see affecting monsoons today were at play thousands of years ago, shaping the fate of entire civilization,” according to Naveen Gandhi, Lead Researcher from the IITM, the TOI report quotes.

The study found that around 4200 years ago, the ITCZ — a critical weather system often described as a “rain belt” — shifted southward, depriving the Indus region of crucial rainfall. Simultaneously, a negative phase of the IOD cooled the eastern Indian Ocean, further weakening the monsoon by reducing moisture transport to the subcontinent.  The IOD is a climate pattern affecting the Indian Ocean. These factors, combined with a period of reduced solar radiation and increased El Niño events, led to prolonged droughts in the Indus Valley, the TOI report points out. An earlier study by Maynooth University, Ireland, states that Climate change in the shape of two major droughts brought an end to the world’s largest civilization. Published in the Quaternary Science Reviews, the research shows that 4250 years ago, the Harappan civilisation in the Indus Valley battled against a changing climate. Facing a major drought, the Harappan adapted by switching crops and moving to different cities. But their cities grew into disrepair and were abandoned, their trade networks dismantled, and the world’s largest civilisation ended. The role of climate change in the Harappan decline has long been debated.

Scientists from the Maynooth University pieced together the history of past rainfall in the region. They have used the changing chemical composition in layers of stalagmites growing at the same time as the Harappan to uncover a double climate disaster. The Harappan were hit by not one, but two consecutive major droughts, writes one of the authors of the study, Dr Nick Scroxton, Department of Geography at the Maynooth University in a research spotlight.

In the research spotlight published on the University’s website, Dr Scroxton adds that the Harappan civilization rose to power in and around the Indus Valley in what is now Pakistan and Western India, reaching its peak 4550 to 4250 years ago. The Harappan built their civilisation on the back of two different sources of rain. The summer monsoon and the winter rainfall, which contributes to the winter snowpack and spring thaw that feeds the Indus River.

This study found that first the drought was due to a reduction in winter rainfall. But just as this winter drought was ending, there was a dramatic change in the Indian Ocean. 4000 years ago, tropical climate underwent a profound shift, a change in sea-surface temperatures and in atmospheric circulation. In the Indus Valley this caused a dramatic reduction in summer monsoon rainfall and another drought. However, climate change for the civilization was not an insurmountable challenge that caused instantaneous civilisation collapse. Their societal decisions and governance had a significant bearing on the impacts of a changing climate.