Changes in global land surface frozen ground and freeze‒thaw processes during 1950–2020 based on ERA5-Land data

Frozen ground (FG) plays an important role in global and regional climates and environments through changes in land freeze‒thaw processes, which have been conducted mainly in different regions. However, the changes in land surface freeze‒thaw processes under climate change on a global scale are still unclear. Based on ERA5-Land hourly land skin temperature data, this study evaluated changes in the global FG area, global land surface first freeze date (FFD), last freeze date (LFD) and frost-free period (FFP) from 1950 to 2020. The results show that the current FG areas (1991–2020 mean) in the Northern Hemisphere (NH), Southern Hemisphere (SH), and globe are 68.50 × 10⁶, 9.03 × 10⁶, and 77.53 × 10⁶ km², which account for 72.4%, 26.8%, and 60.4% of the exposed land (excluding glaciers, ice sheets, and water bodies) in the NH, SH and the globe, respectively; further, relative to 1951–1980, the FG area decreased by 1.9%, 8.8%, and 2.8%, respectively. Seasonally FG at lower latitudes degrades to intermittently FG, and intermittently FG degrades to non-frozen ground, which caused the global FG boundary to retreat to higher latitudes from 1950 to 2020. The annual FG areas in the NH, SH, and globe all show significant decreasing trends (p < 0.05) from 1950 to 2020 at −0.32 × 10⁶, −0.22 × 10⁶, and −0.54 × 10⁶ km² per decade, respectively. The FFP prolongation in the NH is mainly influenced by LFD advance, while in the SH it is mainly controlled by FFD delay. The prolongation trend of FFP in the NH (1.34 d per decade) is larger than that in the SH (1.15 d per decade).