Weaker-Than-Expected Shift From Snowmelt- to Rainfall-Derived Runoff

Cold region water resources derive from a delicate balance between rainfall-derived runoff and snowmelt-derived runoff. Global warming is expected to disrupt this balance by increasing the former meanwhile decreasing snowmelt-derived contributions. However, significant uncertainties in climate model projections have made it difficult to quantify these changes with high confidence. In this study, we apply observational constraints based on global mean surface air temperature to refine projected runoff changes in snow-affected regions in the Northern Hemisphere. Our findings suggest that the shift from snowmelt-to rainfall-derived runoff will occur gradually, with smaller changes than anticipated. Specifically, under the high-emission scenario through the end of this century, projected decreases in snowmelt-derived runoff are reduced by 13% (from unconstrained 23.8% to constrained 21.0%), while rainfall-derived runoff increases are more moderate by 12% (from unconstrained 43.2% to constrained 37.8%), with associated uncertainties (>66% confidence) narrowing by 40%. This recalibration of runoff components reduces total runoff increases by 16% (from unconstrained 18.4% to constrained 15.4%). In regions with climatological winter temperatures below −15°C, substantial declines in snowmelt-derived runoff are unlikely before 2100, as most precipitation continues to fall as snowfall despite rapid warming. While these refined projections provide sufficient time for adaptation, inevitable shifts in runoff timing and volume still pose flood and drought risks, emphasizing the need for informed water management strategies.