On the Linearity of External Forcing Response in Solar Geoengineering Experiments

Abstract

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 CO2. One key source of uncertainty is the magnitude of solar constant reduction required to offset a CO2 quadrupling. Here, we employ single-forcing simulations with Community Earth System Model to decompose the distinct radiative adjustments associated with solar and CO2 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 CO2 simulations. We also show that net radiative adjustment in G1 cannot be represented linearly with CO2 and solar forcing adjustments, which is primarily traced to reductions in boundary layer clouds.