| Summary: | The Asian summer monsoon is one of the largest and most influential of the Earth's climate systems, both in terms of human and climate system interactions. Previous research has focused on interannual and millennial variability of the monsoon. Little is known about century-scale fluctuations. Significant threats to human health and economies could result from unanticipated extremes in monsoon behavior on this timescale, however. Using geochemical proxies, I developed a record of century-scale water-balance fluctuations during the Holocene for Ahung Co (Lake) in Tibet. Conditions became drier at ∼7200 and ∼3700 calendar years BP. The event at ∼3700 calendar years BP coincides with the fall of the Indus civilization, which some researchers believe was caused by a decrease in precipitation. This is the first water-balance record from Tibet to have an age model free from reservoir effects. The age model revealed a gap in sedimentation at the top of the core. Other lake records should be reexamined for similar gaps. To determine the cause of water-balance fluctuations in Ahung Co, I used meteorological observations and evaporation models to reconstruct water fluxes to and from the lake over the past eight years. Water-balance fluctuations reflect changes in summer monsoon strength. Water fluxes during the summer are an order of magnitude greater than during other seasons. In addition to controlling precipitation, the summer monsoon controls evaporation through changes in cloudiness, humidity and temperature. These findings are characteristic of monsoon regions and should be relevant to other lakes on the plateau. I compiled paleoclimate records from the East Asian and Southwest Asian monsoon regions to determine the timing and possible causes of abrupt climate change. Unlike previous studies, I used an objective, statistical analysis to identify abrupt events. Events occurred at ∼11,500 calendar years BP, ∼4,500-5,000 calendar years BP and 1300 AD. The middle Holocene event contradicts the notion of a gradual decrease in monsoon strength through the Holocene. The timing of events suggests connections with the North Atlantic and other climate systems in the tropics.
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