What governs the spread in shortwave forcings in the transient IPCC AR4 models?



[1] The coupled global atmospheric-ocean models used for transient simulations in the IPCC AR4 report differences in the present-day shortwave forcing of more than 2 W/m2. We show here that about 1.3 W/m2 of this spread could be explained by the different methods used to calculate cloud droplet number concentration (CDNC) from aerosol mass concentrations. Although we cannot rule out that other forcing agents could yield comparable uncertainties, this strongly points to the aerosol indirect effect as the main contributor to the wide spread in the shortwave forcing reported in IPCC AR4.