ANTHROPOGENIC sulphate aerosols are believed to affect the radiation budget of the Earth in two ways. Through the direct effect they scatter solar radiation back to space, producing a radiative forcing whose global annual mean has been estimated to lie in the range -0.3 to -0.9 W m (-2) (refs 1-3). This is significant compared to the longwave forcing due to increases in anthropogenic trace gases since the beginning of the industrial era, estimated at +2 to +2.5 W m(-2) (ref. 4). Aerosols also have an indirect effect, altering the distribution and concentration of cloud condensation nuclei (CCN) and hence the number density and size distribution of cloud droplets, thus affecting the solar radiative characteristics of Clouds(5,6). This is harder to quantify than the direct effect, because it depends on complex and poorly understood interactions between aerosols, CCN and cloud properties. Here we use sulphate aerosol data derived from a three-dimensional chemical transport model(7) to estimate the indirect radiative forcing by low-level water clouds using a general circulation model. We estimate that the indirect aerosol effect at the top of the atmosphere is approximately -1.3 W m(-2) in the global annual mean. Although this value is subject to a high level of uncertainty, even if the effect is only half as large it would still exceed many estimates of the direct effect, demonstrating its potential importance in climate change.