Making tracks The magnitude of the effect of anthropogenic aerosols on the formation of clouds is an important unknown about how humans are affecting climate. Studies of stratocumulus cloud tracks that are formed by ship exhaust have been used to estimate the radiative impact of this process, but Glassmeier et al. now show that this approach overestimates the cooling effect of aerosol addition by up to 200%. These findings underscore the need to quantify stratocumulus cloud responses to anthropogenic aerosols to understand the climate system. Science , this issue p. 485