DRENCHED: Girls walk through rain in Dhanbad. File picture

New Delhi, April 23: An international research team has reconstructed a 700-year history of the South Asian monsoon, discovering fresh evidence on four historical mega droughts, including a six-year-long famine that devastated India in the 1790s.

Scientists from Columbia University in New York and the National Taiwan University used chemical signatures locked in trees to analyse patterns of drought and wetness over hundreds of years during the past millennium.

Their study, which appeared in the US journal Science today provides details of some well-known historical monsoon failures and reveals the occurrence and the severity of previously unknown mega droughts.

While providing a peephole into the past, researchers believe the study may also lead to a better understanding of recent observed trends in the monsoon.

“This reconstruction gives climate modellers an enormous data set that may produce some deep insights into the causes of the monsoon variability,” said Edward Cook, lead author of the study at Columbia University’s earth observatory.

The analysis of signatures on trees from 300 forest sites across India, Indonesia, China, northern Australia and — the South Asian monsoon zone — has helped the scientists show the footprints of four known historical droughts — the Ming Dynasty drought from 1638-41, a particularly long drought from 1756-68, eastern India drought from 1790-96, and the so-called great drought from 1876-78.

Some of these droughts changed history.

Cook and his colleagues said the 1638-41 drought in China may have contributed to the fall of the Ming Dynasty — after a peasant rebellion in 1644. The 1878 drought prompted a rebellion against the French in Vietnam.

Historical records suggest that the eastern India drought led to mass famine that left millions dead and many villages deserted.

But Cook and his colleagues have shown that the drought may not have been uniformly severe across the country, indicating that as yet unidentified non-climatic factors may have contributed to the severity of its consequences on society.

Researchers have also found signatures of droughts in the mid-14th century, the late-16th century, between 1682 to 1699 and a shift towards dry conditions after the mid-1970s.

“Understanding the causes of these earlier droughts may help explain why a late-20th century trend toward drier conditions and weaker monsoons has occurred over India and Southeast Asia,” Cook and his colleagues wrote in their paper.