In new research, scientists from the University of Cambridge and elsewhere reconstructed changes in summer and winter rainfall from trace elements and oxygen, carbon, and calcium isotopes of a stalagmite from Dharamjali Cave in the Himalaya spanning 4,200-3,100 years ago. They found evidence for a 230-year period of increased summer and winter drought frequency between 4,200 and 3,970 years ago, with multi-decadal aridity events centered on 4,190, 4,110, and 4,020 years ago. The record highlights the deficits in winter and summer rainfall during the urban phase of the Indus Civilization, which prompted adaptation through flexible, self-reliant, and drought-resistant agricultural strategies.
Giesche et al. aimed to reconstruct indications of both the winter and summer rainy seasons from their multi-proxy time series and evaluate the impact these changes may have had on the Indus Civilization. Image credit: Giesche et al., doi: 10.1038/s43247-023-00763-z.
“We discover explicit confirmation that this duration was not a brief emergency but a gradual alteration of the environmental circumstances in which the Indus population resided,” said Professor Cameron Petrie, an archaeologist at the University of Cambridge.
In their research, Professor Petrie and colleagues mapped out past precipitation patterns by scrutinizing growth strata in a stalagmite procured from Dharamjali Cave adjacent to Pithoragarh, India.
By gauging various environmental markers, such as oxygen, carbon, and calcium isotopes, they acquired a representation demonstrating the corresponding rainfall during specific seasons.
Furthermore, they employed Uranium-series dating techniques of great accuracy to determine the timing and length of the arid periods.
“Numerous indications enable us to assemble the characteristics of these dry spells from different perspectives — and verify that they align,” said Dr. Alena Giesche, also from the University of Cambridge.
The study authors distinguished discrete intervals of substandard precipitation in both the summer and winter seasons.
“The proof of drought affecting both planting seasons is exceedingly meaningful for comprehending the influence of this epoch of climatic transformation on human societies,” Professor Petrie said.
“The dry spells during this duration extended for longer durations, to the extent that the third one would have lasted for multiple generations.”
The outcomes reinforce the current proof that the deterioration of the Indus metropolises was connected to shifts in climate.
“However, what remained puzzling up until this point was knowledge of the duration of the droughts and the specific seasons during which they occurred,” Dr. Giesche said.
“That additional information is genuinely vital when we reflect on cultural recollection and how people adjust to environmental changes.”
“The archaeological proof reveals that throughout a two-century period, the early occupants adopted several measures to adjust and sustain their lifestyle amid this fresh reality,” Professor Petrie said.
“In the course of this transformation, more sizable urban regions were deserted in preference for less populous rural establishments located towards the eastern frontier of the territory inhabited by Indus communities.”
“Concurrently, cultivation was modified to depend more on summer crops, especially drought-resistant millets, and the populace shifted to a lifestyle that appeared to be more autonomous.”
“Megadroughts have recently been widely cited to account for various cultural changes, including those in the Indus Valley,” said Dr. David Hodell, also from the University of Cambridge.
“However, the connections are typically vague due to the challenges of comparing climatic and archaeological data.”
“This situation is now changing because paleoclimate records are becoming increasingly advanced in pinpointing alterations in precipitation on a seasonal and yearly basis, which have a direct impact on people’s choices.”
A paper on the findings was published in the journal Communications Earth & Environment.
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A. Giesche et al. 2023. Recurring summer and winter droughts from 4.2-3.97 thousand years ago in north India. Commun Earth Environ 4, 103; doi: 10.1038/s43247-023-00763-z
