Emerging global freshwater challenges unveiled through observation-constrained projections

Abstract. Future hydrological projections exhibit significant discrepancies among models, undermining confidence in the predicted magnitude and timing of hydrological extremes. Here we show that observation-constrained changes in global mean terrestrial water storage (TWS), excluding Greenland and Antarctica, could be approximately 83 mm lower than raw projections from the Inter-Sectoral Impact Model Intercomparison Project phase 3b by the end of this century under both low- and high-end future forcing scenarios. Notably, the 95th percentile upper bounds are substantially reduced from 2 mm to −96 mm under the low-emissions scenario and from 8 mm to −105 mm under the high-emissions scenario, revealing a notable overestimation of global freshwater availability in the raw model projections. Global models are intricate process representations, making it challenging to isolate causes of their differences with observations. However, by leveraging the emergent constraint (EC) methodology and inter-model spread to empirically adjust biases against observations, we produce more robust projections of future TWS changes than conventional, unconstrained approaches. EC-corrected projections indicate a significant decrease in freshwater availability, further exacerbating existing water stress worldwide if global water demand remains stable or continues to rise. Our findings pinpoint the urgent need to reduce model uncertainties and enhance the reliability of future hydrological projections to better inform water resource management and climate adaptation strategies.