ANTHROPOGENIC depletion of ozone in the lower stratosphere has been of global environmental concern for two decades, but the environmentally relevant quantity-the flux of solar ultraviolet radiation (UVR) reading the Earth’s surface-remains poorly quantified on a global basis. The three most important parameters governing surface UVR fluxes and trends are solar elevation, total vertically integrated ozone abundance and cloud opacity. Here we use global satellite measurements of total ozone abundance and cloud reflectance to examine how the trends in UVR resulting from established trends in total ozone abundance(1,2) compare with the potentially large natural variability in UVR that results from variations in cloud opacity. We find that throughout many temperate regions-including large parts of continental Europe, North and South America, New Zealand, Australia and southern Africa-interannual variability in cloud opacity is sufficiently small that by the end of this century, trends in summer average local-noon UVR dose rates relevant to mammalian skin cancer or plant damage should be significant with respect to cloud variability.