In clouds containing both liquid and ice that have temperatures between -3C and -8C, liquid droplets collide with large ice crystals, freeze, and shatter, producing a plethora of small ice splinters. This process, known as Hallett-Mossop rime splintering, can cause clouds to reflect less sunlight and to have shorter lifetimes. Here, we use a novel suite of five global cloud-resolving models, which break up the Earth’s atmosphere into columns with 2-4 km horizontal edges, to show that this microscale process has global implications. Simulations that include Hallett-Mossop rime splintering have reduced cumulus cloud cover over the Southern Ocean and reflect 12 Wm^(-2) less sunlight back to space over the same region, better matching satellite observed radiative fluxes. We evaluate simulated clouds using high-resolution visible images from the Himawari satellite, and radar reflectivities and two-dimensional images of cloud particles from the SOCRATES aircraft campaign. Cumulus clouds from simulations with Hallett-Mossop rime splintering included have more realistic cloud morphology, cloud vertical structure and ice crystal properties. We show that Hallett-Mossop rime splintering is an important control on cumulus cloud cover and cloud radiative effects over the Southern Ocean, and that including it in simulations improves model performance. We also demonstrate the key role that global cloud-resolving models can play in detangling the effects of clouds on Earth’s climate across scales, making it possible to translate the behavior of tiny cloud particles (10^(-8) m^2) to their impact on the radiative budget of the massive Southern Ocean basin (10^(14) m^2).