Detailed image of stalagmite in India’s Mawmluh cave
Researchers studied oxygen isotopes extracted from speleothems in Mawmluh cave, India, to conduct a more robust analysis of the history of the summer monsoon in the region. Credit: Gayatri Kathayat

Between 1627 and 1630, very little rain fell in what is now India, according to historical accounts. Then came the famine. Peter Mundy, a merchant with the East India Company, described traumatic scenes of starvation, mass mortality, and even cannibalism in his travelogue.

The drought was associated with a failure of the annual summer monsoon, a phenomenon India had faced before and would face again. In the early 1600s, a series of such events led to the abandonment of Fatehpur Sikri, once capital of the Mughal Empire and one of the largest cities of its time. Another famine tied to summer monsoon failure occurred in the 1790s. Indian folklore describes this as the “skull famine” because people recalled seeing the “bones of the victims which lay unburied, whitening the roads and the fields.

“I was awestruck by how close the similarities are between the proxy data and the historical data.”

But could India’s summer monsoons really fail for years in a row? Were there really protracted drought-induced famines that dragged on as long as 12 years?

On the basis of modern scientific rainfall measurements, the answer would appear to be no. Those data, going back to 1871, show a remarkably stable rainfall pattern with only one 3-year drought, which occurred in the mid-1980s.

Now, however, a group of researchers has reconstructed a record of the summer monsoon that goes back much further: 1,000 years. By analyzing oxygen isotopes in stalagmites from a remote cave in northeast India, the scientists found that rainfall evidence hidden in the speleothems synchronizes with many of the written accounts of drought and famine. Their results were published in the Proceedings of the National Academy of Sciences of the United States of America.

“I was awestruck by how close the similarities are between the proxy data and the historical data,” said Ashish Sinha, one of the study’s authors and a professor of Earth science at California State University, Dominguez Hills.

Climatic Context for Social Changes

During the summer monsoon, warm, moist air from the southwest Indian Ocean blows across the Indian subcontinent. Sinha explained that when the monsoon is weaker, the moisture it carries comes predominantly from the Arabian Sea. When it’s stronger, the moisture comes from the southern Indian Ocean. Those two sources have different isotope signatures, allowing Sinha and his colleagues to see in which years the monsoon was strong and in which years it was weak.

Researchers led by Gayatri Kathayat, a geoscientist at Xi’an Jiaotong University, China, chose to take stalagmite samples from the Mawmluh cave in the state of Meghalaya in northeast India. The site was chosen in part because the water there drips quickly and accumulates rapidly—within a year or two—into the cave’s stalagmites. Kathayat said Mawmluh’s accumulation process allows for a fairly accurate representation of annual rainfall data. (In contrast, some caves drip water slowly, and rain from several years can mix together, muddying the signal.)

“Our monsoon drought history…provides important climatic context against which the key political and societal changes can now be assessed,” said Kathayat in a statement.

Himalayan ice cores have also shown evidence of protracted drought in the region, as well as greater monsoon variability than what researchers have measured with modern equipment over the past 150 years. Together, the proxy evidence demonstrates that “the pattern of monsoon failure is pretty visible,” Sinha said.

Modern Data Sets Don’t Tell the Whole Story

Both Sinha and Kathayat said their study serves to inform the Indian government, which may be overlooking the odds of drought on the subcontinent. “If such protracted droughts were to reoccur in the future,” said Kathayat in the statement, they could “easily overwhelm the adaptive capabilities of modern societies unless a longer-term and holistic understanding of monsoon variability is incorporated into the region’s drought management and mitigation planning.”

“A lot of scientists think that it’s a very stable system. And if you only look at the past 150 years, it does appear to be a very stable system. But this study highlights what the monsoon is capable of doing if you broaden your horizons.”

Scientists, too, may be overlooking the concept of consecutive monsoon failures. “A lot of scientists think that it’s a very stable system,” Sinha said. “And if you only look at the past 150 years, it does appear to be a very stable system. But this study highlights what the monsoon is capable of doing if you broaden your horizons. And what it’s capable of doing is going into a drought-prone mode, where it can get stuck for 5, 10, 15 years.”

Brendan Buckley, a researcher with the Tree Ring Lab at the Lamont-Doherty Earth Observatory of Columbia University, said the Mawmluh cave study illustrates the advantage of having extended data sets. “You can look at things in the past that show major changes in the climate that are outside the variability that we see in the modern period.”

Buckley pointed to research he published in 2010 that involved using data from tree rings to reconstruct 750 years of the climate in northwestern Cambodia. Results showed that decades-long drought, mixed with heavy monsoons in the 14th and 15th centuries, contributed to the downfall of Angkor, the capital of the Khmer Empire in Cambodia.

—Nancy Averett (@nancyaverett), Science Writer

Citation: Averett, N. (2022), Stalagmites show evidence of prolonged droughts in India, Eos, 103, Published on 24 October 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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