ATMOSPHERIC aerosols can affect climate by scattering and absorbing solar radiation(1-3). Most recent studies of such effects have focused largely on anthropogenic sulphate aerosols, which are believed to exert a substantial cooling influence(2). Mineral dust aerosols have been largely ignored, because it was thought that their scattering efficiency and concentrations were too low to have a substantial effect on climate. Here we report measurements of the light-scattering properties of North African dust delivered to Barbados by the North Atlantic trade winds. Although the mass scattering efficiency of the dust is only about a quarter of that of non-seasalt sulphate over the North Atlantic(5), the annual-mean dust concentration in Barbados trade-wind air is 16 times that of non-seasalt sulphate(6). The net scattering by mineral dust is therefore about four times that by non-seasalt sulphate aerosols. African mineral dust should therefore be the dominant lightscattering aerosol throughout the tropical and subtropical North Atlantic region, Our observations suggest that mineral dust could be an important climate-forcing agent over this ocean region and in other regions where dust concentrations are high(7,8).