We present a shoreline-based, millennial-scale record of lake-level changes spanning 12.8–2.3 ka for a large closed-basin lake system on the southwestern Tibetan Plateau. Fifty-three radiocarbon and eight U–Th series ages of tufa and beach cement provide age control on paleoshorelines ringing the basin, supplemented by nineteen ages from shell and aquatic plant material from natural exposures generally recording lake regressions. Our results show that paleo-Ngangla Ring Tso exceeded modern lake level (4727 m asl) continuously between ~ 12.8 and 2.3 ka. The lake was at its highstand 135 m (4862 m asl) above the modern lake from 10.3 ka to 8.6 ka. This is similar to other closed-basin lakes in western Tibet, and coincides with peak Northern Hemisphere summer insolation and peak Indian Summer Monsoon intensity. The lake experienced a series of millennial-scale oscillations centered on 11.5, 10.8, 8.3, 5.9 and 3.6 ka, consistent with weak monsoon events in proxy records of the Indian Summer Monsoon. It is unclear whether these events were forced by North Atlantic or Indian Ocean conditions, but based on the abrupt lake-level regressions recorded for Ngangla Ring Tso, they resulted in significant periodic reductions in rainfall over the western Tibetan Plateau throughout the Holocene.

14C dating of shoreline deposits of closed-basin lake Baqan Tso in the western Tibetan Plateau shows that lake level regressed from the undated highstand (46 m above modern, 4.3 × modern surface area) of likely earliest Holocene age by 11.5 ka, and remained larger than modern until at least ≈ 5.0 ka. The shoreline record broadly matches other regional climate records, with lake level closely following Northern Hemisphere summer insolation overprinted by sub-millennial lake-level oscillations. A model coupling modern land runoff and lake surface heat closely reproduces estimated modern precipitation of ≈ 240 mm/yr. We estimate that the Baqan Tso basin required ≈ 380 mm/yr precipitation to sustain the maximum early Holocene lake area, a 55% increase over modern. Precipitation increases, not glacial meltwater, drove lake-level changes, as Baqan Tso basin was not glaciated during the Holocene. Our estimate assumes early Holocene insolation (≈ 1.3% overall increase), and mean annual increases of 2°C in temperature, and 37% in relative humidity. We additionally developed a Holocene precipitation history for Baqan Tso using dated paleolake areas. Using the modern and early Holocene model results as end-members, we estimate precipitation in the western Tibetan Plateau which was 300–380 mm/yr between 5.0 and 11.5 ka, with error of ± 29–57 mm/yr (± 12–15%).