Palaeoceanographic evidence indicates that there was more complete nutrient consumption in Antarctic surface waters during the last ice age^1,2, but lower biological production¹. These results suggest that the Antarctic was stratified during glacial times, reducing the transport of sequestered nutrients and CO₂ into the Antarctic surface. By sequestering CO₂ in the ocean interior, this change could explain the observation of lower levels of atmospheric CO₂ during the ice age³. Geological data offer two possible causes for this stratification. First, the Southern Hemisphere westerly winds apparently shifted northwards during glacial times⁴, which would have reduced Ekman-driven upwelling in the Antarctic⁵ (a 'wind-shift' mechanism). Second, the Antarctic sea-ice cycle intensified during glacial times⁶, which may have allowed a low-salinity lid to accumulate in the open Antarctic, thus reducing vertical mixing and open-ocean overturning (a 'sea-ice' mechanism).