On the Linearity of External Forcing Response in Solar Geoengineering Experiments.

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. Plain Language Summary: Solar geoengineering refers to the modification of incoming sunlight upon the earth as a means to offset warming from greenhouse gases. Studies using climate models investigate solar geoengineering in an idealized framework by directly tuning the amount of incoming sunlight within a model. The amount of adjusting necessary to achieve energy balance from a given increase in carbon dioxide varies widely from model to model. Here, we run a series of idealized experiments using a state of the art climate model where CO2 is increased and sunlight is decreased in order to investigate the rapid climate responses to both forcings applied one at a time. We find that rapid climate responses to both reduced sunlight and increased CO2 are distinct and feed back onanother when forcings are applied together. Furthermore, we find such rapid responses to be the main factor in determining the amount of sunlight reduction needed to balance increased CO2. Key Points: The solar constant reduction to balance 4xCO2 is dependent on radiative adjustments that are distinct for solar and CO2 forcingRadiative adjustments as a response to solar and CO2 forcing cannot be linearly separated using single‐forcing experimentsNon‐linear adjustments to both forcings can primarily be traced to boundary layer clouds [ABSTRACT FROM AUTHOR]