UNCERTAINTY ANALYSIS OF INDIRECT RADIATIVE FORCING BY ANTHROPOGENIC SULFATE AEROSOLS

The effects of aerosols on the optical properties of water clouds (so- called indirect aerosol effects) are a major current source of uncerta inty in the radiative forcing of climate change, Uncertainty in the in direct radiative forcing by anthropogenic sulfate aerosols is analyzed for five different model structures with 20 input uncertain parameter s using the probabilistic collocation method, The structural uncertain ty, measuring the range of the mean responses in different models, is 1.2 to -1.7 W m(-2); and the parametric uncertainty, induced by the un certainties in the input model parameters, is -0.1 to -5.2 W m(-2) wit h 95% confidence, This implies that refining the input parameters may be more important than improving models in order to minimize the uncer tainty in the indirect radiative forcing by anthropogenic sulfate aero sols using these five models, The rankings of sensitivity to and varia nce contribution by the 20 uncertain parameters shows very different p atterns, The variance analysis indicates that the parametric uncertain ty comes mainly from aerosol size parameters and atmospheric transmitt ance, indicating the importance of developing size-resolved global aer osol models, Although the conclusions reached here are subject to the choice of model structures and uncertain parameter distributions, this study introduces a useful framework for computing uncertainty in a cr itical climate issue.