Reconstructing 20th Century Summer Precipitation on the South-Central Tibetan Plateau

The Tibetan Plateau has experienced significant increases in temperature over the 20th century, but trends in precipitation are less clear, as station precipitation records are sparse and satellite observations only extend back to 1979. Here we use the sediment record from Ngamring Co, a closed-basin, freshwater lake in south-central Tibet, to assess summer precipitation over the last century. Ngamring Co is located in a watershed without glaciers, so recent changes in runoff and lake level are independent of the influence of glacial ice volume. The first principal component of the Ngamring Co grain size dataset is highly correlated with median grain size and covaries significantly with local July-August precipitation from the CPC Merged Analysis of Precipitation (CMAP). From 1979 to 2007, median grain size decreases with increasing July-August precipitation and increases with decreasing July-August precipitation. There is prominent multidecadal variability in the 20th century grain size record, including a gradual decline in median grain size from 1900 to 1930, a gradual increase in median grain size from 1930 to 1990, and a rapid decline in median grain size since 1990. Median grain size values from 2000-2007 are the lowest in the 107-year record, suggesting the most abundant monsoon precipitation in the last 107 years has occurred in the last decade. Satellite images of the lake also confirm an increase in lake area since the early 1990s, but also show the greatest lake area occurred in the 1970s. Thus, although summer precipitation and lake area do covary, precipitation does not seem to control grain size by influencing lake area. We presently hypothesize that increased July-August precipitation causes enhanced erosion of the fine-grained sediment (likely paleolake sediments and loess) that blankets the slopes surrounding the lake. The ensuing runoff and deposition of this sediment into the lake then results in a decline in median grain size within the lake sediment record. It is also possible that human activity exerts an influence on Ngamring Co, as there are several towns as well as agricultural activity within the lake watershed. For example, increased water usage may have limited lake area expansion in the last two decades despite the large increase in monsoon moisture.