AbstractUncertain climate system response has been raised as a concern regarding solar geoengineering. We explore the effects of geoengineering on one source of climate system uncertainty by evaluating the intermodel spread across 12 climate models participating in the Geoengineering Model Intercomparison project. The model spread in simulations of climate change and the model spread in the response to solar geoengineering are not additive but rather partially cancel. That is, the model spread in regional temperature and precipitation changes is reduced with CO2 and a solar reduction, in comparison to the case with increased CO2 alone. Furthermore, differences between models in their efficacy (the relative global mean temperature effect of solar versus CO2 radiative forcing) explain most of the regional differences between models in their response to an increased CO2 concentration that is offset by a solar reduction. These conclusions are important for clarifying geoengineering risks regarding uncertainty.