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The South Asian summer monsoon (SASM) is a major atmospheric system affecting nearly a quarter of the human population.Climate proxy data derived from tree rings,ice cores,speleothems,and other sources can contribute to an understanding of SASM variability prior to the instrumental period.By proposing an optimal information extraction (OIE) method,we reconstructed the SASM index (SASMI) for the last millennium using 15 tree-ring chronologies that is statistically correlated with the instrumental SASMI at a 90% confidence level (r = 0.7); this correlation is higher than that obtained in previous studies.The reconstructed SASMI captures 18 of 26 (69%) famine events in India over the last millennium; notably,11 of 16 short events with durations of 1–3 years are accurately depicted in our reconstruction.These results indicate that the reconstructed SASMI characterizes detailed climatic variations in the SASM over the last millennium on annual timescales.Moreover,the reconstructed SASMI is positively correlated with variations in total solar irradiance (TSI) on multi-decadal timescales implying that variations in solar activity may directly influence SASM variability.