The mountain valley of the Indus River in the fall.

The Indian subcontinent has been the scene of civilization ebb and flow of diverse peoples, cultures and faiths for centuries. Always one of the most populous regions in the world, many of the societies have relied on stable climatic conditions for farming and food collection.

Now, a new study peering into the isotopes in stalactites and stalagmites in a cave in northern India appears to show the flow of the Indian Summer Monsoon – a powerful natural force that explains the rise and fall of civilizations, report the scientists in the latest Science Advances.

“Our data suggest that significant shifts in monsoon rainfall have occurred in concert with changes in the North Hemisphere temperatures and the discharges of the Himalayan rivers,” they write.

The speleothem oxygen isotope was pulled from the Sahiya Cave, in the state of Uttarakhand, about 200 kilometers north of New Delhi. The geographic location is key, since it is at the cusp of the area where climate is determined by the monsoons.

The cave deposit minerals change with the amount of precipitation falling in each monsoon, especially because of the glacial melt upstream in the Himalayas due to warmer and wetter years. For instance, the latest deposits from the mid-20th century onward show an ongoing declining trend in monsoon rainfall – which matches direct instrument readings of precipitation.

The fluctuations over 1,440 readings of the isotopes spanning 5,700 years was matched with climate records kept by organized peoples globally. The dating was performed at Xi’an Jiaotong University using multicollector inductively coupled plasma mass spectrometers, the Thermo Finnigan Neptune Plus. The isotope measurements were measured using a Finnigan MAT 253 mass spectrometer, in concert with an online carbonate preparation system, the Kiel IV.

The back-dating matches with other historical accounts of lower oxygen isotope values, and stronger monsoons, corresponding to the Medieval Climate Anomaly between 800 and 1,200 years ago, the Roman Warm Period between 1,600 and 2,300 years ago, the Minoan Warm Period between 2,900 and 3,300 years ago, and a phenomenon called the late portion of the mid-Holocene Climate Optimum. The heavier isotopes correspond on the other end of the spectrum with the Little Ice Age, and the Dark Ages Cold Period, they add.

In India, these changes matched with the rise and fall of complex societies. For instance, the Indus Valley peoples were a prominent people in the Bronze Age about 4,000 years ago. Their fall about 3,300 years ago, in which their bigger settlements were broken up and abandoned in favor of pastoral societies, was apparently linked to the change in climate, they found.

“We surmise that reduced (Indian Summer Monsoon) and reduced meltwater discharge from the Himalayas, presumably due to colder temperatures and/or reduced westerly induced precipitation, were responsible for producing multicentennial-scale drier conditions that may have contributed to the Indus Valley deurbanization.”

Similarly, the Vedic period of culture flowered during a period of stable and regular monsoons bringing regular warmth and rains. But the end of the period about 2,500 years ago coincided with a drastic weakening of the monsoons, according to the oxygen isotopes preserved in the caves. The Guge Kingdom in western Tibet, which flourished for centuries, also collapsed around 1620, during the absolute monsoon minimum, bringing the most severe dry even in the millennia of chemical records. 

Taken together, the detailed isotopes in the cave provide nothing less than a “continuous history of the subcontinent-wide changes in the Indian Summer Monsoon rainfall over the last 5,700 years.”

“The close temporal relationship between these large-scale hydroclimatic changes and the intervals marking the significant sociopolitical developments of the Indus Valley and Vedic civilizations suggests a plausible role of climate change in shaping the important chapters of the history of human civilization in the Indian subcontinent,” they conclude.