Global Terrestrial Water Storage Projections and Uncertainty Decomposition Under Multiple Warming Levels

Terrestrial Water Storage (TWS) is a key integrator of the global hydrologic cycle, yet its future response to climate change remains uncertain due to divergent climate forcings, hydrologic model structures, and parameter choices. This study provides a global, global‐warming‐level (GWL)‐based assessment of future TWS changes at 1.0°C, 2.0°C, and 3.0°C above pre‐industrial conditions. We construct a large ensemble that combines multiple climate scenarios, five CMIP6 global climate models (GCMs), four global hydrologic models (GHMs), and 50 hydrological parameter realizations per model. Our results indicate that evaluation against Gravity Recovery and Climate Experiment (GRACE) mascon observations confirms that the ensemble reasonably reproduces historical large‐scale TWS variations. Projections show that TWS changes become more widespread, stronger in magnitude, and more consistent across models as warming intensifies. High‐latitude regions generally exhibit TWS increases linked to increased precipitation, whereas many tropical and subtropical regions, including the Amazon, the Sahel, and South Asia, experience TWS declines driven by rising evaporative demand. A four‐way analysis of variance reveals that GCMs dominate global TWS uncertainty, particularly in high‐latitude and cold‐climate regions, while hydrologic‐related uncertainty from GHMs and their parameterizations becomes increasingly important in monsoon, subtropical, and semi‐arid regions. These results underscore that developing robust, region‐specific adaptation strategies requires multi‐GCM, multi‐GHM, and multi‐parameter ensembles that explicitly account for both climate and hydrologic uncertainties.