PNNL

Abstract

The effective radiative forcings (including rapid adjustments) and feedbacks of ten Earth System Models associated to an instantaneous quadrupling of the pre-industrial CO2 concentration (4xCO2) and a counterbalancing reduction of the solar constant (Solar) were investigated in the context of the Geoengineering Model Intercomparison Project (GeoMIP). The forcing and feedback parameters of the net energy flux as well as its different components at the top-of-atmosphere (TOA) and surface were examined to better understand the impact of solar radiation management on the energy budget. In spite of their very different nature, the feedback parameter and its components at the TOA and surface are almost identical for the two forcing mechanisms, not only in the global mean, but also in their geographical distributions. The conclusion holds for all of the models despite model-to-model differences in how feedbacks affect the different components of the energy budget. This indicates that the climate sensitivity is independent of the forcing (when measured as effective radiative forcing). We also show the existence of a large contribution of the cloudy-sky component to the shortwave (SW) effective radiative forcing at the TOA suggesting rapid cloud adjustments to an change in the solar irradiance. In addition, models present significant diversity in the spatial distribution of the SW feedback parameter in cloudy regions pointing to persistent uncertainties in how clouds feed back onto climate.