Pollens Help Trace Why The Harappan Civilization Shrank

Key Highlights:

• Research links the shrinking of Harappan Civilization to a weak Indian Summer Monsoon (ISM) between ~5100 and 4000 cal yr BP.
• The 4.2 ka event, an abrupt dry climate condition around 4250 cal yr BP, caused population migration from Indus and Ghaggar-Hakra rivers to the east Ganga plains.
• The study was conducted by the Birbal Sahni Institute of Palaeosciences (BSIP), an autonomous institute under the Department of Science and Technology (DST).
• Researchers used high-resolution pollen-based records from Deoria Tal in Garhwal Himalaya, dated using AMS 14C dating.

Detailed Insights:

• The abrupt climate change at 4.2 ka BP led to the disappearance of reliable seasonal flooding, forcing Harappan populations to seek new agricultural areas.
• Weakening of the ISM is attributed to a southward shift of the Inter Tropical Convergence Zone (ITCZ), stronger El-Niño phases, and a strong negative state of the Indian Ocean Dipole (IOD).
• The study also identified a strong ISM during the Roman Warm Period (RWP) and Medieval Climate Anomaly (MCA), linked to increased solar insolation and northward ITCZ shift.
• Conversely, the ISM was weak during the Little Ice Age (LIA), associated with a southward ITCZ shift and intensified Asian westerly jet stream.
• This research improves understanding of present and future ISM-influenced climatic conditions, aiding policymakers in developing informed strategies.

Scientific/Technical Concepts Involved:

• Indian Summer Monsoon (ISM): A seasonal wind system bringing heavy rainfall to India, crucial for agriculture.
• 4.2 ka event: A globally recognized abrupt, short-lived dry climate event occurring around 4200 years Before Present.
• Inter Tropical Convergence Zone (ITCZ): A low-pressure belt near the equator where trade winds converge, influencing global weather patterns.
• El-Niño: A climate pattern describing the unusual warming of surface waters in the eastern tropical Pacific Ocean, impacting global weather.
• Indian Ocean Dipole (IOD): An irregular oscillation of sea surface temperatures in the Indian Ocean, affecting rainfall patterns in surrounding regions.