Not All 1.5°C Worlds Are Equal: Mitigation versus Solar Radiation Modification

The continued failure to achieve emission reductions consistent with the Paris Agreement has intensified interest in Solar Radiation Modification (SRM), particularly stratospheric aerosol injection (SAI), as a potential component of the climate response portfolio. Current debates largely rely on a “risk–risk” framing that contrasts the risks of SAI deployment with those of unmitigated or insufficiently mitigated warming. This framing obscures a critical comparison: how different pathways to the same global temperature target may lead to fundamentally different climate outcomes. We therefore propose a complementary “world–world” framing that compares two distinct 1.5°C worlds: one achieved through greenhouse gas (GHG) mitigation and one through SAI. Using CESM2-WACCM simulations from the ARISE-SAI protocol, we assess differences in climate impacts between these pathways. In the absence of simulations that stabilize at 1.5°C through GHG mitigation, we apply a correction to the transient 1.5°C baseline of the SSP2-4.5 scenario to account for the influence of warming rates on impact indicators.

We focus in particular on four socio-economically vulnerable regions: South Asia, East Asia, South-Central America, and East Africa. While SAI effectively limits global mean temperature, it introduces substantial regional and seasonal imbalances, especially in hydrological variables. In several regions, nighttime extreme heat is exacerbated under the SAI pathway. As a complementary line of evidence, we apply machine-learning classifiers to distinguish between mitigation-driven and SAI-driven 1.5°C climates, supported by explainability analyses identifying the regions and variables driving this separation. Together, these results provide quantitative insight into the “moral hazard” dimension of SRM, highlighting how reliance on SAI may mask—but not resolve—important regional climate risks.