Determining the cooling magnitude in the east of China during the Last Glacial Maximum using a degree-day model

There is a great controversy regarding the cooling magnitude and the existence of ancient glaciers during the Last Glacial Maximum (LGM) in the east of China. The cooling magnitude during the LGM can be seen as a basis for determining whether glaciers developed during the Quaternary. In this study, we chose 9 mountain areas where there are long-term meteorological records as the research sites. And there are different glacier-climate conditions at these 9 sites. The degree-day model was conducted to estimate the snow mass and the snowmelt at the altitude of the bottom of ancient cirques (snow line) based on the mean multiple-year-day temperature and mean multiple-year-day precipitation. The results showed that: (1) The altitude of the modern snow line of Tianshan Mountains was similar to the result estimated by the degree-day model, which indicated this model can be used to determine the altitude of ancient cirques, and it can also be used to calculate the cooling magnitude required to meet a condition for cirques to develop at a given altitude. (2) For the sites where ancient cirques existed in the monsoon zone, the cooling magnitude during the LGM was 6.4 °C, 6.4 °C, and 8.0 °C in Garze, Taibi Mountain, and Yu Mountain, respectively. For the site where ancient cirques existed in the non-monsoon zone, the cooling magnitude during the LGM was 3.3 °C in Tianshan Mountains. The mean cooling magnitude during the LGM was 5.9 °C in China. (3) A much greater cooling magnitude was required to develop cirques during the LGM than the average cooling magnitude of China in Huanggangliang, Wutai Mountain, Tai Mountain, Shenlongjia, and Lu Mountain. (4) The cooling magnitude was underestimated by the TraCE-21ka model compared with this study. The differences between the results from the TraCE-21ka model and this study were approximately 3.3 °C, 2.9 °C, and 5.7 °C for Garze, Taibai, and Yu Mountain, respectively, averaging 4.0 °C. (5) There was no sufficient evidence showing that the cooling magnitude during the LGM decreased with latitude.