Abrupt Changes in the Asian Summer Monsoon Winds During the Holocene

The Asian summer monsoon affects climate and society throughout a large part of the tropics. Abrupt changes in the monsoon had potentially dramatic effect on human societies during the Holocene. Using fossil evidence of Globigerina bulloides abundance in the Arabian Sea, we reconstructed the history of the Asian summer monsoon winds during the past 10,000 years. G. bulloides provides a reliable index of the monsoon, its abundance correlated with the cool upwelling conditions produced each summer by the monsoon winds. The Arabian Sea sediments are nannofossil-rich foraminifer oozes, and the low oxygen content of the Arabian Sea minimizes the bioturbation that would otherwise smooth the fossil record. In addition to the well-known decrease in the monsoon winds since a maximum in the early Holocene, we found a series of smaller, millennial-scale oscillations between strong and weak monsoon intervals throughout the Holocene. Periods of weaker monsoon winds correlate with cool conditions in the circum-north Atlantic during the Holocene, just as they did during the larger Dansgaard-Oeschger events of the last glacial, evidence of a link between low and mid-latitude climate. The changes in the monsoon winds were probably accompanied by changes in rainfall over India, and we hypothesize that rain harvesting structures built in India since 5000 year BP were societal adaptations to climate change. Widespread evidence exists for the construction of ponds, tanks, and artificial reservoirs during the late Holocene when the monsoon reached its Holocene minimum, and we also find correlation between heightened historical human efforts for adaptation and the most recent minima in the monsoon winds that occurred 1600 AD. The monsoon record supports an emerging paradigm that at least in the tropics, the most societally-important climate changes were driven by changes in precipitation rather than surface temperature. The cause of the monsoon oscillations, and their links to other aspects of the tropical circulation and higher latitude climate are still poorly understood and require improved and more extensive quantitative records of the tropical circulation.