Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change?

Many previous studies have shown that a solar forcing must be greater than a CO2 forcing to cause the same global mean surface temperature change but a process-based mechanistic explanation is lacking in the literature. In this study, we investigate the physical mechanisms responsible for the lower efficacy of solar forcing compared to an equivalent CO2 forcing. Radiative forcing is estimated using the Gregory method that regresses top-of-atmosphere (TOA) radiative flux against the change in global mean surface temperature. For a 2.25% increase in solar irradiance that produces the same long term global mean warming as a doubling of CO2 concentration, we estimate that the efficacy of solar forcing is similar to 80% relative to CO2 forcing in the NCAR CAM5 climate model. We find that the fast tropospheric cloud adjustments especially over land and stratospheric warming in the first four months cause the slope of the regression between the TOA net radiative fluxes and surface temperature to be steeper in the solar forcing case. This steeper slope indicates a stronger net negative feedback and hence correspondingly a larger solar forcing than CO2 forcing for the same equilibrium surface warming. Evidence is provided that rapid land surface warming in the first four months sets up a land sea contrast that markedly affects radiative forcing and the climate feedback parameter over this period. We also confirm the robustness of our results using simulations from the Hadley Centre climate model. Our study has important implications for estimating the magnitude of climate change caused by volcanic eruptions, solar geoengineering and past climate changes caused by change in solar irradiance such as Maunder minimum.