1,000-year-old stalagmites from a cave in India show the monsoon isn’t so reliable – their rings reveal a history of long, deadly droughts

In a remote cave in northeast India, rainwater has slowly dripped from the ceiling in the same spots for over 1,000 years. With each drop, minerals in the water accumulate on the floor below, slowly growing into calcium carbonate towers known as stalagmites.

These stalagmites are more than geological wonders – like tree rings, their layers record the region’s rainfall history. They also carry a warning about the potential for catastrophic multiyear droughts in the future.

By analyzing the geochemistry of these stalagmites in a new study published Sept. 19, 2022, in the Proceedings of the National Academy of Sciences, we were able to create the most precise chronology yet of the summer Indian monsoon over the past millennium. It documents how the Indian subcontinent frequently experienced long, severe droughts unlike any observed in the last 150 years of reliable monsoon rainfall measurements.

The drought periods we detected are in striking synchrony with historical accounts of droughts, famines, mass mortality events and geopolitical changes in the region.

They show how the decline of the Mughal Empire and India’s textile industries in the 1780s and 1790s coincided with the most severe 30-year period of drought over the millennium. The depth and duration of the drought would have caused widespread crop failures and the level of famine discussed in written documents at the time.

Another long drought encompasses the 1630-1632 Deccan famine, one of the most devastating droughts in India’s history. Millions of people died as crops failed. Around the same time, the elaborate Mughal capital of Fatehpur Sikri was abandoned and the Guge Kingdom collapsed in western Tibet.

Our findings have important implications today for water planning in a warming world, particularly for India, which, with its vast monsoon-reliant agriculture industry, is on pace to soon be the most populous country on the planet.

Why the monsoon’s history matters

Scientists began systematically measuring India’s monsoon rainfall with instruments around the 1870s. Since then, India has experienced about 27 regionally widespread droughts. Among them, only one – 1985 to 1987 – was a three-year consecutive drought or worse.

The apparent stability of the Indian monsoon in that data might lead one to surmise that neither protracted droughts lasting multiple years nor frequent droughts are intrinsic aspects of its variability. This seemingly reassuring view currently informs the region’s present-day water resource infrastructure.

However, the stalagmite evidence of prolonged, severe droughts over the past 1,000 years paints a different picture.

It indicates that the short instrumental period does not capture the full range of Indian monsoon variability. It also raises questions about the region’s current water resources, sustainability and mitigation policies that discount the possibility of protracted droughts in the future.