Amplified soil moisture variability multiplies summer heat extremes in humid regions

In recent decades, unprecedented summer heatwave events have frequently erupted in humid regions. Our analysis of the period 1980–2022 demonstrates that the surging heat extremes are closely linked to both a progressive drying of mean soil moisture and a concurrent intensification of its intra-seasonal variability. While the drying trend contributes to two-week extreme hot days, amplified intra-seasonal variability multiplies these extremes to four weeks by further enhancing shortwave radiation and high-pressure anomalies. Climate projections indicate that a future characterized by drier and more variable soil moisture shifts summer climates towards an intensified ‘mega hot’ stage, where over two-thirds of summer days in humid regions will experience extreme heat by the end of the 21st century under high-emission scenario. Our findings highlight that amplified soil moisture variability—a marker of intensifying soil moisture-air coupling under global warming—is nonlinearly accelerating the occurrence of extreme dry-hot events, pushing humid climates into a new and unprecedented normal.