OBSERVATIONS from satellite and ground-based instruments1–3 indicate that between 1979 and 1990 there have been statistically significant losses of ozone in the lower stratosphere of the middle to high latitudes in both hemispheres. Here we determine the radiative forcing of the surface–troposphere system4–6 due to the observed decadal ozone losses, and compare it with that due to the increased concentrations of the other main radiatively active gases (CO2, CH4, N2O and chlorofluorocarbons) over the same time period. Our results indicate that a significant negative radiative forcing results from ozone losses in middle to high latitudes, in contrast to the positive forcing at all latitudes caused by the CFCs and other gases. As the anthropogenic emissions of CFCs and other halocarbons are thought to be largely responsible for the observed ozone depletions1, our results suggest that the net decadal contribution of CFCs to the greenhouse climate forcing is substantially less than previously estimated.