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 (WUEeco), 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 WUEeco remain poorly understood. Using site observations, satellite data, Earth system models, and wind manipulation experiments, we found a consistent negative sensitivity of grassland WUEeco 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 WUEeco 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 WUEeco increases and drought resilience may exceed current projections for global grassland.