Spatially uneven impacts of stratospheric aerosol injection on compound dry-heat wave events in the GeoMIP G6sulfur experiment

Abstract. Compound dry-heat wave (CDHW) events, characterized by the joint occurrence of extreme heat and dry conditions for at least three consecutive days, pose substantial risks to ecosystems, agriculture, and human societies. However, the implications of stratospheric aerosol injection (SAI) for such compound extremes, particularly at regional scales, remain poorly understood. Here, we investigate changes in CDHW frequency, duration, and severity under the GeoMIP G6sulfur scenario relative to the high-emission SSP5-8.5 scenario using simulations from three Earth system models. We find that SAI substantially offsets the projected intensification of CDHWs over global land, with marked reductions in their frequency, duration, and severity. However, this mitigation is highly heterogeneous: Central Africa emerges as a prominent regional hotspot where CDHWs become more persistent in all three models and, while their frequency also increases under the percentile-based dry-day criterion, primarily in MPI-ESM1-2-LR. The enhancement is driven mainly by the dry side of the compound: dry spells lengthen in all three models (by 0.2–0.9 days per event) and the number of hot-and-dry days per summer rises, even though the total number of heat wave days falls in two of the three models, whereas the intensity of the individual compound events is reduced, consistent with the overall cooling. Comparison with SSP2-4.5 scenario, which has a global-mean radiative forcing comparable to that G6sulfur, indicates that the enhanced CDHW response over Central Africa is not driven by the greenhouse gas forcing component alone. The reduction in greenhouse gas forcing alone produces little change in CDHW exposure over Central Africa, whereas SAI reduces surface shortwave radiation and evaporation over tropical land, promoting mid-tropospheric subsidence and reductions in cloud cover, precipitation, and soil moisture over the region. The contrasting response over the Amazon Basin, where CDHWs decrease, is largely explained by the greenhouse gas component. Reduced greenhouse gas forcing substantially alleviates the projected drying over the Amazon but provides no comparable benefit over Central Africa, where aerosol-specific drying dominates. These hydroclimatic changes are further coupled with declining soil moisture and reduced latent heat flux, partly offsetting the SAI- induced cooling and favoring persistent heat extremes. Our results demonstrate that reducing global mean warming through SAI does not necessarily translate into uniform reductions in compound climate extremes, highlighting the importance of distinguishing the removal of greenhouse gas forcing from aerosol-specific effects, and of land-atmosphere interactions when assessing the climate risks and regional trade-offs of solar climate intervention.