Quantifying error in the radiative forcing of the first aerosol indirect effect

Anthropogenic aerosol plays a major role in the Earth's radiation budget, particularly via effects on clouds. The Intergovernmental Panel on Climate Change lists the uncertainty in aerosol modification of cloud albedo as the largest unknown in the radiative forcing of climate change. Common measures of aerosol effects on clouds, Aerosol-Cloud Interaction (ACI = -partial derivative lnr(e)/partial derivative ln alpha, where r(e) is drop size and a aerosol burden), cover an enormous range and, as these measures are now being used as parameterizations in global-scale models, this has large implications for radiative forcing. We quantify the relationship between radiative forcing and changes in ACI over the range of values found in the literature. Depending on anthropogenic aerosol perturbation, radiative forcing ranges from -3 to -10 W m(-2) for each 0.05 increment in ACI. Narrowing uncertainty in measures of ACI to an accuracy of 0.05 would place estimated cloud radiative forcing on a sounder footing.