SULPHATE aerosols can act as nuclei for cloud formation, thereby cooling the climate by increasing the Earth’s albedo(1-5) : however the magnitude of this effect is very uncertain(3,6). Recently, Langner et al.(7) calculated that at most 6% of the anthropogenic sulphur emission forms new particles, while 44% adds mass to existing sulphate particles activated in clouds. It was therefore suggested(7,8) that previous studies(1,2,9) had overestimated the effect of sulphate aerosols on climate. Although it has been proposed that sub-CCN-size particles can grow to CCN-size in clouds(7,10), this was thought to require the large supersaturations present in cumuliform clouds, rather than the smaller values characteristic of marine stratiform clouds, which are most important for radiative forcing. Here we show that natural variability of even low average supersaturations allows particles as small as 0.015 mu m to grow to become CCN. This process can quadruple the CCN concentration and significantly increase the corresponding aerosol effect on climate.