Sexton et al. (2003) did find some evidence for a nonlinear interaction between the effects of greenhouse gases and the indirect effect of sulphate aerosols in the atmosphere - only version of HadCM3 forced by observed SSTs; the additional effect of combining greenhouse gases and indirect aerosol effects together was much smaller than each term separately but was found to be comparable to the warming due to increasing tropospheric ozone. (ipcc.ch)
Mueller remains entirely silent about the potential role of sulphate aerosols in causing the drought and the subsequent greening. (desmogblog.com)
A planned series of further trials is envisaged, eventually answering the question of whether it would ever be practical to put large quantities of sulphate aerosols into the stratosphere this way. (bbc.com)