Heat uptake by the ocean is slowing the greenhouse gas-driven warming of the atmosphere, and the ocean represents the dominant long-term sink for the carbon dioxide gas deriving from fossil fuel use. However, these beneficial roles of the ocean are tempered by the slowness with which surface waters are carried into the deep ocean, through a process known as “deep ocean ventilation” that occurs at high latitudes. Moreover, most global climate models have predicted that deep ocean ventilation will slow further in the future as global warming proceeds. The Southern Ocean around Antarctica is active in deep ocean ventilation and thus particularly important in the uptake of fossil fuel carbon dioxide and global warming heat. Evidence will be presented that deep ocean ventilation by the Southern Ocean was slower during past ice ages and faster during warm interglacial periods. These findings raise the possibility that deep ocean ventilation by the Southern Ocean will accelerate into the global warming future, counter to most model-based expectations. The origins and significance of this apparent disagreement will be discussed.