Not all 1.5°C worlds are equal: a world-world framing for comparing solar radiation modification and greenhouse gas mitigation outcomes

Solar radiation modification (SRM) through stratospheric aerosol injection (SAI) is typically assessed using a risk-risk framing, which compares a world with SAI against one with unmitigated warming. This framing cannot quantify what is different if SAI substitutes for, rather than complements, emission reductions. Here we propose a world-world framing, which compares two worlds at the same global mean temperature: one reached through greenhouse gas mitigation and one through SAI. We apply it to CESM2-WACCM ARISE-SAI and GeoMIP peak-shaving simulations at 1.5°C and 2°C, using transiencecorrected mitigation baselines. Despite similar global mean temperatures, SAI and mitigation worlds differ regionally, and the extent of these differences depends on the variable. For annual mean temperature, SAI lies close to the mitigation world, and its departures from it are mostly smaller than the difference between 1.5°C and 2°C mitigation worlds. Night-time heat extremes are similar in the global mean, but parts of the tropics and subtropics experience warmer nights under SAI than under mitigation. Precipitation differs more strongly: over more than half of the land area, including the Southern Hemisphere monsoon regions, SAI departs from the 1.5°C mitigation world by more than a 2°C mitigation world does. A machine-learning classifier based on regional patterns distinguishes the SAI and mitigation 1.5°C worlds almost perfectly for temperature and with high skill for precipitation. These results show that a common global temperature reached by different means does not imply a common climate. They further suggest that studying “peak-shaving” SAI applications under overshoot, peak-and-decline pathways requires careful evaluation of regional differences between peak-warming and SAI futures. Our proposed world-world framing complements risk-risk assessments by quantifying the climatic consequences of pathway choice, thereby informing assessments of the implications of substituting mitigation with SAI.