Snowfall is an important component of Earth's climate system; however, long, continuous high‐resolution records of global snowfall are lacking because of the absence of suitable proxies. In this study, diatom record from the sediments of Yunlong Lake, in the southeastern Tibetan Plateau, was used to reconstruct snowfall during the Younger Dryas (YD). Variations in the abundance of low‐light‐tolerant diatoms and diatom flux indicate that the duration of lake ice cover during the YD was significantly longer than that during the colder Heinrich event 1; this suggests that heavy snowfall, rather than temperatures, was responsible for the greater duration of lake ice cover during the YD. Thus, we conclude that prolonged, heavy snowfall occurred in the southeastern Tibetan Plateau during the YD. In addition, this conclusion is supported by the results of a climate model simulation that also suggest that heavy snowfall occurred at high latitude in the Northern Hemisphere during the YD. We propose that the heavy snowfall intensified cooling in Northern Hemisphere by increasing the albedo and that it increased hydrological variability at low latitudes by increasing the duration of the southward migration of the Intertropical Convergence Zone and by delaying the onset of the Asian summer monsoon. The snowfall would have been a source of continuous freshwater that acted as a positive feedback and resulted in a prolonged weakened state of the Atlantic meridional overturning circulation, which lasted for more than 1,000 years. Overall, our results emphasize the amplification and positive feedback function of heavy snowfall in triggering abrupt climate change.

Yunlong Lake, a subtropical alpine lake in southwest China, was selected for a sediment trap diatom study between September 2013 and August 2015. Combining the results of meteorological records, water temperature monitoring and water chemistry sampling, we draw the following conclusions: water temperature monitoring indicated that the entire upper 7 m of the water column was mixed throughout the year, reflecting the lake’s strong mixing characteristics; this ensured the year-round growth of planktonic species and the high abundance of benthic species. In addition, the diatoms exhibited a pronounced biannual warm-cold seasonal pattern, which may be related to water temperature, Si nutrient concentration and thermal stratification. During the warm season (May–November) when the lake was almost stratified, with overall higher water temperature and Si concentration, the representative species were Aulacoseira granulata var. angustissima and Achnanthidium catenatum. During the cold season (December–April), the water temperature and Si nutrient concentration were lower, but stronger lake mixing occurred, and the blooming species included Aulacoseira ambigua, Discostella asterocostata and Lindavia balatonis. After a detailed analysis, we conclude that the seasonal diatom succession was primarily driven by water temperature, and comparison with other lake records indicates that the strong mixing mechanism and Si were not the determinants of the lake diatom seasonal succession. Our data also show that during the first cold season, diatoms bloomed in the following sequence: A. ambigua – D. asterocostata – Discostella stelligera – L. balatonis – Discostella pseudostelligera, as Si decreased over the season, indicating that smaller species were more competitive in obtaining limited nutrients. Finally, there were interannual differences between diatom assemblages in the second year, especially in the abundance of L. balatonis and the occurrence of Asterionella formosa, which were probably related to the significantly increased lake trophic status.

Although diatom records from lake sediments have been used for quantitative paleoclimatic reconstruction, their validity and sensitivity have rarely been tested rigorously. At Yunlong Lake, an alpine lake in Southwest China, we studied the seasonal succession of diatom assemblages to produce a mean surface water temperature (MSWT) transfer function. In addition, based on the spatial distribution of surface diatom assemblages with water depth, we produced a diatom-water depth (WD) transfer function. Combined with the analysis of diatom assemblages in a sediment core (YL2013-A), changes in surface water temperature and water level over the last ∼100 years were quantitatively reconstructed using the diatom-based transfer functions. Comparison with records of regional meteorology and reservoir water capacity revealed that the diatom-based lake water level reconstruction is a sensitive indicator of short-term fluctuations in precipitation, and it also reflects a long-term stepwise rise in water level caused by the impounding and large-scale extension of the reservoir. In addition, the diatom-inferred MSWT is consistent with the changes in air temperature prior to large-scale human disturbance of the site. However, after the extension of the reservoir, although the regional air temperature continued to increase, the water temperature decreased substantially. This suggests that the large increase in lake water volume in the short term led to a decrease in the average water temperature, which in turn led to the occurrence of a diatom bloom in the cold season. The results demonstrate that diatom transfer functions based on modern observations of the same lake has a high environmental sensitivity and can be used for the quantitative reconstruction of regional climate change. Overall, our findings provide a foundation for the use of lake diatom records for quantitative paleoclimatic reconstruction on various timescales.