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A series of sensitivity studies were performed with a spectral radiative transfer model using aerosol data from the Global Aerosol Data Set (GADS, data available at http://www.meteo.physik.uni-muenchen.de/strahlung/aerosol/aerosol.htm) in order to investigate and quantify the relative role of key climatic parameters on clear-sky ultraviolet-visible direct aerosol radiative forcing at the top of the atmosphere (TOA), within the atmosphere and at the Earth’s surface. The model results show that relative humidity and aerosol single-scattering albedo are the most important climatic parameters that determine aerosol forcing at the TOA and at the Earth’s surface and atmosphere, respectively. Relative humidity exerts a non-linear positive radiative effect, i.e. increasing humidity amplifies the magnitude of the forcing in the atmosphere and at the surface. Our model sensitivity studies show that increasing relative humidity by 10%, in relative terms, increases the aerosol forcing by factors of 1.42 at the TOA, 1.02 in the atmosphere and 1.17 at the surface. An increase in aerosol single-scattering albedo by 10%, in relative terms, increased the aerosol forcing at the TOA by 1.29, while it decreased the forcing in the atmosphere and at the surface by factors of 0.2 and 0.69, respectively. Our results show that an increase in relative humidity enhances the planetary cooling effect of aerosols (increased reflection of solar radiation to space) over oceans and low-albedo land areas, whilst over polar regions and highly reflecting land surfaces the warming effect of aerosols changes to a cooling effect. Thus, global warming and an associated increase in relative humidity would lead to enhanced aerosol cooling worldwide. The sensitivity results also demonstrate that an increase in surface albedo due to, for example, a reduction in land vegetation cover, would lead to enhanced atmospheric warming by aerosols leading to a reduction in cloud formation and enhancement of the desertification process. On the contrary, a decrease in surface albedo over polar regions due to, for example, ice-melting associated with global warming, would reduce the planetary warming effect of aerosols over polar areas. Aerosol forcing is found to be quite sensitive to cloud cover, as well as to aerosol optical thickness and the asymmetry parameter, and to the wavelength dependence of the aerosol optical properties.