C. G. Fletcher

As an idealized proxy for solar geoengineering, the GeoMIP G1 experiment forces Earth System Models with prescribed reductions in solar radiation to balance increases in atmospheric CO 2 . One key source of uncertainty is the magnitude of solar constant reduction required to offset a CO 2 quadrupling. Here, we employ single‐forcing simulations with Community Earth System Model to decompose the distinct radiative adjustments associated with solar and CO 2 forcing in the G1 experiment. We find that radiative adjustments to both single forcings have a net positive effect on top of atmosphere energy balance such that they both increase the net G1 forcing. Stratospheric temperature and shortwave cloud adjustments are the main sources of positive adjustment in both Solar and CO 2 simulations. We also show that net radiative adjustment in G1 cannot be represented linearly with CO 2 and solar forcing adjustments, which is primarily traced to reductions in boundary layer clouds.