With each drop, the minerals contained in the water accumulate on the floor below, gradually turning into calcium carbonate towers known as stalagmites.
These stalagmites are more than just a geological marvel: like tree rings, their layers record the region’s rainfall history. They also carry a warning of the possibility of catastrophic multi-year droughts in the future.
By analyzing the geochemistry of these stalagmites in a new study published September 19, 2022 in the Proceedings of the National Academy of Sciences, scientists have been able to create the most accurate chronology of the Indian summer monsoon in the past millennium.
It shows that the Indian subcontinent has often experienced prolonged and severe droughts unlike any reliable monsoon rainfall measurement seen in the past 150 years.
The periods of drought we have found are strikingly synchronous with historical evidence of droughts, famines, mass deaths, and geopolitical changes in the region.
They show that the decline of the Mughal Empire and India’s textile industry in the 1780s and 1790s coincided with the worst 30-year drought in a millennium. The depth and duration of the drought must have caused widespread crop failures and the level of famine described in the written records of the time.
Another long drought covers the Deccan Famine of 1630-1632, one of the most devastating droughts in Indian history. Millions of people died due to crop failure. Around this time, the elaborate Mughal capital of Fatehpur Sikri was abandoned, and the kingdom of Guge collapsed in western Tibet.
Our findings have important implications for water resource planning in a warming climate, especially for India, which, with its vast monsoon-dependent agriculture, will soon become the most populous country on the planet.
Why Monsoon history matters
Scientists began systematically measuring monsoon rainfall in India with instruments around the 1870s. Since then, India has experienced about 27 widespread regional droughts. Among them, only one – from 1985 to 1987 – was a three-year drought in a row or worse.
The apparent stability of the Indian monsoon in these data may lead to the assumption that neither prolonged droughts lasting several years nor frequent droughts are integral aspects of its variability. This seemingly reassuring opinion currently defines the modern infrastructure of the region’s water resources.
However, the stalagmites that testify to the long and severe droughts over the past 1,000 years paint a different picture.
This indicates that the short instrumental period does not reflect 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 prolonged droughts in the future.
How do the stalagmites reflect the history of the monsoons in the region?
To reconstruct past fluctuations in rainfall, we analyzed stalagmites from Mawmlukh Cave, located near the city of Cherrapunji in Meghalaya, one of the wettest places in the world.
Stalagmites are cone-shaped structures that slowly grow out of the ground, usually at a rate of about one millimeter every 10 years.
In the layers of their growth are the smallest particles of uranium and other elements, which were obtained as a result of the penetration of rainwater into the rocks and soil above the cave.
Over time, uranium trapped in stalagmites decays into thorium at a predictable rate, so we can determine the age of each layer of stalagmite growth by measuring the ratio of uranium to thorium.
Oxygen in rainwater molecules consists of two main types of isotopes – heavy and light. As the stalagmites grow, they fix in their structure the ratio of oxygen isotopes in the rainwater seeping into the cave. Subtle changes in this ratio can be caused by different climatic conditions at the time the rainwater originally fell.
Our previous research in this area has shown that changes in the oxygen isotope ratio in rainwater, and hence in stalagmites, track changes in the relative abundance of various moisture sources that contribute to summer monsoon rainfall.
In years when the monsoon circulation is weak, precipitation here is mainly due to moisture evaporating from the nearby Arabian Sea. However, during the years of strong monsoons, atmospheric circulation brings here a large amount of moisture from the southern Indian Ocean.
These two sources of moisture have completely different isotopic signatures of oxygen, and this relationship is exactly preserved in the stalagmites. We can use this clue to learn about the overall strength of the monsoon intensity during the formation of stalagmites.
We pieced together the history of the monsoon rains by extracting minute amounts of calcium carbonate from the growth rings and measuring the oxygen isotope ratio. To tie the climate record to exact calendar years, we measured the ratio of uranium to thorium.
Paleoclimatic data usually allow us to determine what happened, where and when. But often they themselves cannot answer why or how something happened.
Our new research shows that prolonged droughts have been common over the past millennia, but we don’t have a good understanding of why the monsoon didn’t work during those years. Similar studies using Himalayan ice cores, tree rings, and other caves have also found prolonged droughts, but faced the same problem.
In the next phase of our study, we will team up with climate modelers to conduct coordinated proxy modeling studies that we hope will provide a better understanding of the climate dynamics that caused and sustained such long periods of drought in the past millennium.
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