The Birbal Sahni Institute of Palaeosciences (BSIP), an autonomous institute under the Department of Science and Technology, recovered a well-dated sediment core from Deoria Tal in the Garhwal Himalaya to build a high-resolution pollen-based record of vegetation change, hydroclimate and Indian Summer Monsoon (ISM) variability from the mid-Holocene to the present. Pollen and spores are established proxies for reconstructing past vegetation and climate. The researchers found an abrupt increase in the Oak/Pine ratio around 4,250 cal yr BP, corroborating earlier elemental/sedimentological evidence of a short, sharp dry spell at 4,200 cal yr BP — the globally recognised '4.2 ka event'. They attribute the Harappan Civilization's shrinkage — and the migration of its population along the Indus and Ghaggar-Hakra rivers to the east Ganga plains — to this Abrupt Climate Change, as reliable seasonal flooding around the edge of the Thar desert disappeared. The study was carried out in collaboration with researchers from the University of Georgia (USA), University of Lucknow and Kumaun University.

The 4.2 ka event is linked in the palaeoclimate literature not just to the Harappan/Indus Valley decline but also to the collapse of the Old Kingdom in Egypt, the fall of the Akkadian Empire in Mesopotamia, and the end of the Liangzhu culture in China's lower Yangtze region — making it a globally significant Bronze Age climate disruption studied across multiple ancient civilizations.