Observed Changes in Seasonal Surface Soil Freeze‒Thaw Cycles in China and Subregions From 1981 to 2017 and Their Relationships With Meteorological Elements

Given the considerable influence of surface soil freeze‒thaw cycles on the surface energy balance, hydrological processes, and ecosystems, there is significant interest in exploring changes in surface soil freeze‒thaw cycles in the context of climate warming. In this study, we investigated changes in the duration of seasonal surface soil freeze‒thaw cycles across China and subregions divided by climate and ecosystem types (temperate and warm‒temperate deserts of northwestern China, temperate grasslands of Inner Mongolia, temperate humid and subhumid zones of northeastern China, warm‒temperate humid and subhumid zones of North China, and high‒elevation and cold zones of the Tibetan Plateau) from 1981 to 2017 and examined their relationships with meteorological elements using both homogenized weather station data and gridded observations. The results showed that the freeze start date has been delayed by 8.6 days and that the freeze end date has advanced by 8.6 days, resulting in a shortened freeze duration by 17.2 days in China. This change was most pronounced in the high‒elevation and cold zones of the Tibetan Plateau, with a shortened freeze duration by 25.2 days, and the weakest change was present in the temperate humid and subhumid zones of northeastern China. Nationwide, the decreasing trend of the freeze duration first increased but then decreased with increasing elevation, and it consistently decreased with increasing latitude. Changes in the freeze duration are significantly correlated with the following factors: air temperature in spring, autumn and winter, snow depth in spring, autumn and winter; and vegetation in autumn. Distinct regional differences exist in these relationships. These results provide a new understanding of surface freeze‒thaw cycle changes and their causes in China.