Uncertainty analysis for estimates of the first indirect aerosol effect

The IPCC has stressed the importance of producing unbiased estimates of the uncertainty in indirect aerosol forcing, in order to give policy makers as well as research managers an understanding of the most important aspects of climate change that require refinement. In this study, we use 3-D meteorological fields together with a radiative 5 transfer model to examine the spatially-resolved uncertainty in estimates of the first indirect aerosol forcing. Uncertainties in the indirect forcing associated with aerosol and aerosol precursor emissions, aerosol mass concentrations from different chemical transport models, aerosol size distributions, the cloud droplet parameterization, the representation of the in-cloud updraft velocity, the relationship between effective radius 10 and volume mean radius, cloud liquid water content, cloud fraction, and the change in the cloud drop single scattering albedo due to the presence of black carbon are calculated. The cloud fraction is found to be the most important source of uncertainty and causes an overestimation of the indirect forcing by almost 0.8Wm−2 in the reference case. Uncertainties associated with aerosol and aerosol precursor emissions are the 15 next most important uncertainty followed closely by uncertainties in the calculation of aerosol burden by chemical transport models and uncertainties in the representation of the aerosol size distribution (including the representation of the pre-industrial size distribution). There are significant regional differences in the uncertainty associated with the first indirect forcing with largest uncertainties in regions associated with the 20 major biomass burning regions followed by uncertainties in Asia and Europe.