Regional and Seasonal Hydrological Changes With and Without Stratospheric Aerosol Intervention Under High Greenhouse Gas Climates

Stratospheric aerosol intervention (SAI) is being explored for its potential to reduce greenhouse gas (GHG) induced climate damages. We assess the effectiveness of two SAI experiments, G6Sulfur and Geo SSP5-8.5 1.5 (here called Geo-SAI), using the CESM2(WACCM6) model to reduce hydrological changes under high-emission SSP5-8.5 (no mitigation) pathway. Geo-SAI stabilizes near surface global temperatures at 1.5 degrees C above preindustrial levels, whereas G6Sulfur limits temperature rises to those under the SSP2-4.5 scenario. In our findings, Geo-SAI reverts many, but not all, hydrological changes induced by SSP5-8.5 restoring global and regional means, seasonal amplitudes, and peak timings. G6Sulfur delivers smaller restorations, as expected, due to its smaller prescribed forcing. In hyperarid regions such as the Middle East, both SAI scenarios improve water storage compared with both SSP5-8.5 and present conditions. However, in wetter or cooler climates, such as the Amazon, middle and southern Africa and east Europe, they only partly reverse the reductions in available water (AW) and runoff caused by high GHG emissions. Residual warming and snowmelt dynamics play an important role in runoff at mid-to-high latitudes. Additionally, SAI does not completely suppress GHG-induced vegetation expansion and so over-reduces global runoff in three latitude bands: 45 degrees-65 degrees N, 45 degrees-65 degrees S, and 30 degrees S to 0 with end-of-century decreases of 4.1% under G6Sulfur and 7.3% under Geo-SAI despite mean AW levels remaining close to present-day. These findings emphasize that although SAI mitigates many climate-driven hydrological disruptions, its unintended effects on runoff, vegetation feedback, and regional water availability warrants study-especially in regions heavily dependent on surface water resources.