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Schlagwörter:
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Zusammenfassung:
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/m(2). 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/m(2), and the uncertainty due to combinations of these parameters can reach more than 1 W/m(2). These numbers should be compared with the sulfate aerosol forcing of -1.9 W/m(2) 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.
