Aeolian sand cover affects the soil hydrothermal state and permafrost degradation on the Qinghai-Tibet Plateau

•Aeolian sand affects hydrothermal state of the active layer significantly.•Aeolian sand dries shallow ground and accumulates water in soil below sand.•Sand layer thickening increases water accumulation in the soil below sand.•Effect of aeolian sand on permafrost varies with variation in sand layer thick.•Aeolian sand has a dual-effect on permafrost degradation.