Expansion or shrinkage of closed lakes is a natural response to fluctuations in precipitation and evaporation, linked closely to changes in strength or position of atmospheric circulation. In Tibet, there are many such lakes with paleo-shorelines that can be used for reconstructions of climate history. Despite the fact that many paleo-shorelines are well preserved in Tibet, dating them has been seriously hindered by various difficulties. Here we present the first optical dating chronology for a series of paleo-shorelines in Zhari Namco, the third-largest inland lake in central Tibet. Our results indicate that the lake level has dropped 128 m over the past 8.2 ka. Younger shorelines are found at lower altitudes, indicating that the shorelines follow a geomorphic-chronological order and a broadly continuous trend of stepwise shrinkage. The surface area of Zhari Namco has shrunk in size from 4605 km2 at 8.2 ka ago to 996 km2 at present; 300 km2 of water has been lost from this lake. Such a loss in water implies a significant reduction in precipitation over the past 8.2 ka, a likely result of a weakening Asian monsoon. Following the decreasing precipitation since the early Holocene, this area has become increasingly arid.