Spatiotemporal variations in 3.2 m soil temperature in China during 1980-2017

Soil temperature is an important variable of earth's surface system, and studying it is of great benefit to understanding the energy and hydrological cycles and climate change. Based on soil temperature data at a depth of 3.2 m from 113 stations in China from 1980 to 2017, the spatial distribution and temporal variation of the soil temperatures in China were analyzed by using a fuzzy C-means algorithm (FCM), linear fitting and the Mann-Kendall method. The results show that the average annual soil temperature at 3.2 m in China gradually increased from north to south, with the lowest temperatures in the northeast and Qinghai-Tibetan Plateau (T3.2m <= 5 circle C\documentclass[12pt] and the highest in South China (T3.2m >= 25 circle C\documentclass[12pt]. The country is divided into 10 regions using the FCM method. The soil temperature anomalies show a continuous increase over time, but there are significant regional differences in the warming speeds. The northeast and northwest regions exhibit the largest temperature increases, approximately 0.57 degrees C/10a\documentclass[12pt], North China has the second highest increases, with approximately 0.36 degrees C/10a\documentclass[12pt], and the values in other areas are less than 0.3 degrees C/10a\documentclass[12pt]. Over the 38-year period, the soil temperature increased by approximately 1.4 degrees C\documentclass[12pt], and the temperature increase mainly occurred during the last 20 years. The positive center of the annual soil temperature anomaly at 3.2 m in the Qinghai-Tibetan Plateau during the last 2 decades of the twentieth century became cooling to a negative soil temperature anomaly at 3.2 m during the first 2 decades of the twenty-first century. The negative centers for the annual soil temperature anomaly at 3.2 m in the south of Shaanxi and the middle reaches of the Yangtze River during the last 2 decades of the twentieth century became warming to positive soil temperature anomalies at 3.2 m in the first 2 decades of the twenty-first century. In the other regions, different turning points of the positive-negative temperature anomaly are exhibited at 3.2 m.