Significance Polar regions are foci of climate change, because of more-than-expected warming, problematic remote-sensing retrievals, and large uncertainties about cloud effects on radiation budgets. Antarctica is the world’s most remote, coldest, and driest location. Until recently, researchers have assumed that low-level clouds over the frozen Antarctic Plateau consist mainly of ice crystals. Now, measurements with a unique tethered balloon system and a ground-based lidar show that nearly 50% of clouds in the austral summer contain supercooled water which has a significant impact on the radiative properties of Antarctic clouds. Modifying a global climate model to relax the freezing below −20 °C results in a strong simulated radiative (cooling) effect, affecting the entire Antarctic Continent and extending out into the Southern Ocean. Precious little is known about the composition of low-level clouds over the Antarctic Plateau and their effect on climate. In situ measurements at the South Pole using a unique tethered balloon system and ground-based lidar reveal a much higher than anticipated incidence of low-level, mixed-phase clouds (i.e., consisting of supercooled liquid water drops and ice crystals). The high incidence of mixed-phase clouds is currently poorly represented in global climate models (GCMs). As a result, the effects that mixed-phase clouds have on climate predictions are highly uncertain. We modify the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM) GCM to align with the new observations and evaluate the radiative effects on a continental scale. The net cloud radiative effects (CREs) over Antarctica are increased by +7.4 Wm ⁻², and although this is a significant change, a much larger effect occurs when the modified model physics are extended beyond the Antarctic continent. The simulations show significant net CRE over the Southern Ocean storm tracks, where recent measurements also indicate substantial regions of supercooled liquid. These sensitivity tests confirm that Southern Ocean CREs are strongly sensitive to mixed-phase clouds colder than −20 °C.