Hydrologic Sensitivity of Snow and Streamflow Dynamics to Climate Forcing With and Without Stratospheric Aerosol Intervention

Snowpack accumulation and melt critically regulate freshwater availability across many regions. Under global warming, the dominant control on snowpack variability shifts from cold-season precipitation (Pc) to cold-season temperature (Tc), altering snow–rain partitioning, snowmelt timing, and runoff generation. Here, we use the Community Earth System Model in two versions (CESM1 and CESM2) to evaluate whether stratospheric aerosol intervention (SAI) scenarios—GLENS and Geo SSP5-8.5 1.5 (hereafter Geo-SAI)—can offset this transition under their corresponding high-emission pathways (RCP8.5 and SSP5-8.5). These SAI deployments maintain global mean surface temperature at 2020 levels (GLENS) or 1.5°C above preindustrial levels (Geo-SAI). A moving-window partial-correlation framework shows that both SAIs effectively mitigate the shift from Pc- to Tc-dominance for maximum snow depth (SDmax) and warm-season runoff (Qw) across the Northern Hemisphere induced by greenhouse gas forcing. Under high-emission warming, the emergence of Tc-dominance in regions with minimum Tc ≥ −16°C occurs progressively later in colder climates, with rates of −4.26 ± 1.29 (RCP8.5) and −4.08 ± 0.35 yr °C⁻¹ (SSP5-8.5) for SDmax, and −2.65 ± 0.91 and −4.06 ± 1.03 yr °C⁻¹ for Qw, respectively. In contrast, SAI largely stabilizes these transitions near zero, particularly within transitional thermal regions (−4°C < Tc < 0°C) across 45–70°N in North America and central-to-north Eurasia, where modest cooling preserves snowfall. Runoff responses are weaker and more spatially heterogeneous than snowpack responses. These findings demonstrate that moderate cooling can preserve snowpack predictability and runoff seasonality while highlighting the importance of regional hydroclimatic thresholds for water-resource resilience under future climate interventions.