Wind stilling shapes grassland water use efficiency by enhancing soil moisture retention

Covering more than 40% of Earth's vegetated surface, grasslands critically regulate terrestrial carbon and water cycles. Their ecosystem water use efficiency (WUE_eco), the ratio of carbon uptake to water loss, governs drought resilience in these water-limited ecosystems. While global terrestrial wind speed declined substantially from the 1960s to 2000s followed by a recovery in the subsequent decade, the extent and mechanisms of its influence on grassland WUE_eco remain poorly understood. Using site observations, satellite data, Earth system models, and wind manipulation experiments, we found a consistent negative sensitivity of grassland WUE_eco to wind speed. Mechanistically, declining winds reduce evaporative water loss, improve soil moisture, and promote stomatal opening, thereby enhancing carbon uptake. Wind speed changes accounted for 7.7 to 25.7% of WUE_eco increases under historical and future climates, making wind the second most important climatic driver after atmospheric carbon dioxide. Since Earth system models underestimate observed wind speed declines, future WUE_eco increases and drought resilience may exceed current projections for global grassland.