Highlights What are the main findings? Northern Hemisphere grasslands are experiencing a transition from historical greening-driven drying (GDD) to future greening-driven wetting (GDW). Water availability is transitioning from precipitation-dominated to coupled precipitation-evapotranspiration control, while grassland greening is shifting from a vapor-pressure deficit to temperature regulation. What are the implications of the main findings? Aridity gradient differentiation in driving factors provides a scientific basis for implementing spatially differentiated ecological management strategies. This study quantifies the historical inflection point in grassland greening-water decoupling and reveals systematic transformations in underlying driving mechanisms.Highlights What are the main findings? Northern Hemisphere grasslands are experiencing a transition from historical greening-driven drying (GDD) to future greening-driven wetting (GDW). Water availability is transitioning from precipitation-dominated to coupled precipitation-evapotranspiration control, while grassland greening is shifting from a vapor-pressure deficit to temperature regulation. What are the implications of the main findings? Aridity gradient differentiation in driving factors provides a scientific basis for implementing spatially differentiated ecological management strategies. This study quantifies the historical inflection point in grassland greening-water decoupling and reveals systematic transformations in underlying driving mechanisms.Abstract Grasslands, covering over 40% of terrestrial land surfaces, play a critical role in regional water cycling through their greening processes. However, the decoupling mechanisms between grassland greening and water availability (WA) changes across the Northern Hemisphere, along with their future trajectories, remain poorly understood. Here, we integrated multi-source satellite observations with CMIP6 model ensembles to systematically assess the spatiotemporal evolution and trend divergence of leaf area index (LAI) and WA across Northern Hemisphere grasslands from 2000 to 2100. Our results showed that grassland LAI exhibited sustained growth during 2000-2020, with 55.28% of regions showing significant increasing trends. However, 73.67% of grassland regions experienced declining WA during the historical period, revealing widespread decoupling between grassland greening and water deficit. Future scenario projections indicated a reversal to increasing WA trends, with 57.51% of regions showing significant increases under SSP5-8.5. Furthermore, 61.87% of grasslands exhibited greening-driven drying (GDD) characteristics during the historical period, while greening-driven wetting (GDW) regions were projected to expand to 72.44% in the future. Analysis along aridity gradients revealed that humid zones contributed most prominently to LAI and WA changes. Mechanistic decomposition demonstrated that grassland WA changes shifted from precipitation-dominated control (53.60%) in the historical period toward a regime jointly governed by precipitation dominance and coupled precipitation-evapotranspiration drivers in the future. Concurrently, the dominant factor controlling grassland greening transitioned from vapor-pressure deficit (VPD) to temperature (TEM) control. Additionally, driving factors exhibited pronounced differentiation patterns along aridity gradients during the historical phase: arid zones were dominated by soil moisture (SM) and semi-arid zones displayed dual control by SM and VPD, while humid zones were governed by coupled TEM-VPD regulation. This study reveals the divergent trends between grassland greening and WA and unravels their driving mechanisms, offering important scientific evidence for formulating regionally differentiated ecological water resource management strategies.