Variations in Global Soil Moisture During the Past Decades: Climate or Human Causes?

As a sensitive climate change indicator, global multi-depth soil moisture (SM) has undergone great variation due to warming trends during the past decades. Here, we investigate the evolutionary pattern of SM and then carry out an attribution analysis from climate and human perspectives. The results reveal an unbalanced surface and rootzone variation trend during 1980-2020. The surface soil had approximately equal proportions of drying and wetting. Nonetheless, the percentage of the wetting region is significantly higher than that of the drying region for rootzone soil. The significantly drying region is mainly distributed in habitable areas, while the remarkably wetting region is usually located in the harsh climate and tropical humid zones. According to the attribution analysis using the Feature Importance Index of the Random Forest model, Pearson correlation coefficient, Maximum information coefficient, Spearman rank correlation coefficient, and Granger causality with 99% statistical significance, both climatic and human factors presented significant impacts on SM. Specifically, air temperature and evaporation are thought to be the primary climatic factors affecting the seasonal and long-term variability of SM, respectively. Irrigation water withdrawal is regarded as the dominant driving force of SM variation among six human water withdrawals, and domestic and electricity water withdrawals play indispensable roles in SM fluctuation. Additionally, SM also provides profound feedbacks on climatic factors and human water withdrawals. The global spatial-temporal SM variation along with warming temperatures could seriously risk food security and sustainable development, which needs to be addressed.