A Comparison of the Relative Contributions of Solar Variability and CO2 Radiative Forcing to Global Warming

At present, there is considerable debate concerning the effect that fluctuations in the solar flux may have on the climate forcing of the earth. There are two main uncertainties associated with the solar effect; the magnitude of the change in the solar flux, and how it is related to the radiative trapping at the tropopause, which in turn is used to predict climate change on Earth. The latter uncertainty is the focus of this paper. The problem with comparing the radiative forcing associated with the change in the solar flux with the traditional greenhouse radiative forcing is that one does not know where the solar energy component is deposited within the atmosphere/Earth system. For instance, solar energy that is absorbed by clouds rather than reaching the earth's surface may be less effective at causing global warming compared to modelled estimates which are based on the radiative trapping at the tropopause. Hence, we propose that a better comparison can be made with the radiative forcing at the surface, whether it is solar or terrestrial in origin. Surface radiative forcing quantities allow a direct comparison between the solar and terrestrial components since both fluxes traverse the same atmospheric conditions and reach the same destination. Since the pre-industrial period, CO₂ has contributed the most of any trace gas to climate forcing. It is, therefore, interesting to compare the radiative forcing associated with the solar variability to the surface forcing associated with increased levels of CO₂ in the atmosphere since 1850. For this comparison, an increase of 0.3 W/m² was assumed for the solar forcing component. We have calculated the corresponding increase in the surface forcing due to CO₂ to be 0.70 W/m². Others have shown that the increase in the CO₂ forcing at the tropopause is 1.5 W/m². Therefore, based on the CO₂ forcing at the tropopause, the solar contribution is only about 20%. However, based on the surface forcing, which really is the only valid comparison, the solar contribution reaches about 45% of the CO₂ forcing since pre-industrial times. The fractional contribution has been calculated as a function of latitude.