Parametric uncertainty effects on aerosol radiative forcing

Among the known radiative forcings, the fourth IPCC assessment report estimates the aerosol radiative forcing to harbor the widest range of uncertainty extending from -1.8 to -0.3 W/m2. The IPCC estimates focus mainly on structural uncertainties, including uncertainties in aerosol sources. Here, we study the uncertainty of the sulfate aerosol radiative forcing due to parametric uncertainty in a state-of-the-art general circulation model (GCM). Numerical experiments were carried out by perturbing seven cloud parameters in the model. We find that the uncertainty due to a single one of these parameters can be as large as 0.5 W/m2, and the uncertainty due to combinations of these parameters can reach more than 1 W/m2. These numbers should be compared with the sulfate aerosol forcing of -1.9 W/m2 for the year 2000, obtained using the default values of the parameters. The uncertainty results from a high sensitivity of cloud optical properties to aerosol concentrations, which can be amplified by changing cloud parameter setting. a, radiative forcing in multi-parameter experiment; symbols are 10-year mean values obtained from each experiment for total aerosol forcing (solid squares) and linear reconstructions (red open squares); dashed line is best linear fit to reconstructed data, error bars correspond to standard deviations of 10-year means; b, Scatter plot of forcing for multi-parameter experiments and linear reconstructions (circles); solid lines indicate default forcing for reference; dashed line is slope expected if forcing results from linear superposition of the effects of individual parameters; bar chart on horizontal (vertical) axis is histogram of multi-parameter (reconstructed) forcing.