Estimates of anthropogenic aerosol indirect effects on the planetary energy balance have varied widely. These differences arise from differences in the simulation of multiple factors, including differences in emissions of aerosols and precursor gases, the efficiency of the production of cloud condensation nuclei from the emissions, the sensitivity of cloud droplet number concentration to increases in cloud condensation nuclei concentration, the sensitivity of cloud optical depth to increases in droplet number concentration, and the sensitivity of the planetary energy balance to changes in the cloud optical depth. The relative contributions of each of these factors to uncertainty in the estimated aerosol indirect forcing have not been quantified. We have decomposed the aerosol indirect forcing simulated by a variety of global aerosol models into these factors and will illustrate the contribution of each factor to the uncertainty in the forcing. We will also show examples of how observations can be used to constrain simulated values of each of the factors.