2.4.4 Estimates of Aerosol Direct Radiative Forcing

Unless otherwise stated, this section discusses the TOA direct RF of different aerosol types as a global annual mean quantity inclusive of the effects of clouds. Where possible, statistics from model results are used to assess the uncertainty in the RF. Recently published results and those grouped within AeroCom are assessed. Because the AeroCom results assessed here are based on prescribed emissions, the uncertainty in these results is lowered by having estimates of the uncertainties in the emissions. The quoted uncertainties therefore include the structural uncertainty (i.e., differences associated with the model formulation and structure) associated with the RF, but do not include the full range of parametric uncertainty (i.e., differences associated with the choice of key model parameters), as the model results are essentially best estimates constrained by observations of emissions, wet and dry deposition, size distributions, optical parameters, hygroscopicity, etc. (Pan et al., 1997). The uncertainties are reported as the 5 to 95% confidence interval to allow the uncertainty in the RF of each species of aerosol to be quantitatively intercompared.