Deficient rainfall led to the collapse of the Mansabdari system, started by Mughal emperor Akbar, in the late 17th century. Similarly, drought interspersed with violent monsoon rains sounded the death knell for the Khmer empire of south-east Asia in the 15th century.

A recent study by researchers at Indian Institute of Technology, Kharagpur (IIT-KGP) has revealed that abrupt changes in the Indian monsoon in the last 900 years decided the course of human history in the subcontinent. A paper titled Abrupt changes in Indian summer monsoon strength during the last 900 years and their linkages to socio-economic conditions in the Indian subcontinent by Anil K. Gupta, professor at the geology and geophysics department of IIT-KGP, highlights that decline of Indian dynasties was linked to weak monsoon and reduced food production.

Several dynasties, such as the Sena in Bengal, Solanki in Gujarat in the mid-13th century and Paramara and Yadav in the early to mid-14th century all of which flourished during abundant rainfall declined during the dry phases of Indian summer monsoon (ISM), suggesting role of the climate in the socio-political crisis, the study revealed.

The paper published in international journal PALEO 3 highlights three phases in the 900-year stretch Medieval Climate Anomaly from 950 CE to 1350 CE, Little Ice Age from 1350 CE to 1800 CE and Current Warm Period from 1800 CE till today. The paper highlights strong monsoon during Medieval Climate Anomaly and Current Warm Period and phases of weak and strong monsoon in Little Ice Age.

There can be no doubting the profound impact of the abrupt shifts of rainfall on human history a fact we need to constantly remind ourselves in this day and age of irretrievable climate change. Abrupt shifts in the ISM precipitation has similarly impacted history in India, Prof. Gupta said.

For the study on long-term spatio temporal variability of the ISM, a group of researchers, which also included experts from Wadia Institute of Himalayan Geology, looked at palaeoclimatic records using oxygen isotope proxy record from speleothems (a structure formed in a cave by deposition of minerals from water) at the Wah Shikar cave in Meghalaya.

We took samples from every half millimetre or sometimes even one-third of a mm, and we dated using uranium-thorium time series. Such fine sampling of less time interval means we were covering data at two-three years interval while most researches collect data at 20-30 years interval. We even captured the drought events of last few centuries, Prof Gupta said. The results showed abrupt shifts in the ISM, he added.

For more recent phases of human history the study suggests that from the beginning of the 19 century, the changes in the ISM became more abrupt with a rise in atmospheric temperature that coincides with the dawn of the Industrial Revolution.

An increase in the frequency of abrupt shifts in the ISM during the last three centuries, coincidental with a rise in atmospheric temperature, suggests occurrence of more climatic surprises in future consequent to future rise in the global temperature and subsequently more precipitation in the form of rain at higher altitudes. the paper said.

Prof. Gupta said that they were doing similar work extending their Palaeoclimatic study to 6000 years ago to see the impact of climate change on Indus Valley civilization and on population migrations.

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Monsoon decided history of Indian subcontinent: study - The Hindu