The Uncertainty in the Simulated Climate in Forcing and Prediction Experiments With the NASA GISS GCM II' due to the Uncertainty in the Forcing by Soil Dust Aerosols

The magnitude and even the sign of direct radiative forcing by soil dust aerosols is one of the most uncertain factors in climate change studies, and this contributes to an uncertainty in the response of the simulated climate due to this forcing as well as to other forcings like greenhouse gases. Our objective is to estimate the uncertainty in the simulated climate attributed to imprecisely known quantities such as the dust single scatter albedo and vertical distribution in transient climate forcing and prediction experiments carried out using the National Aeronautics and Space Administration Goddard Institute for Space Studies (NASA GISS) general circulation model II prime (GCM II'). Although various forcings like greenhouse gases, stratospheric aerosols, tropospheric sulfate, black and organic carbon are time dependent in these experiments, a three-dimensional distribution of soil dust aerosols that consists of an unvarying seasonal cycle is prescribed for the whole simulated period from 1951 to 2050. The forcing and prediction experiments with the dust single scattering albedo prescribed based upon measurement of Saharan dust are repeated using values that assume more reflection and absorption, respectively. We also compute the climate effect of concentrating the dust distribution closer to the surface. For each assumed vertical distribution and single scatter albedo of the dust particles, a 5-member ensemble is calculated with different initial conditions to account for the internal variability of the model climate. We analyze the uncertainty of main climate variables such as surface temperature, precipitation, cloud cover, and sea level pressure to the varied parameters and compare the response to the internal climate variability. We present results for the global average as well as for the geographic distribution of these variables.