The retreat of the southern Greenland Ice Sheet (GIS) during the last deglaciation (Termination I: TI) is poorly dated by conventional means; there is even greater uncertainty about the penultimate deglaciation (Termination II: TII), leading to the assumption that the southern GIS has a significant lag in its response to deglacial warming. Here we use geochemical terrestrial sediment proxies ([Fe] and [Ti]) from a well-studied southern Greenland marine sediment sequence to examine the behavior of the southern GIS during TI and TII. Our records show that during TI and TII the southern GIS response was essentially synchronous with deglacial North Atlantic warming, implying greater climate sensitivity than previously assumed. During TI, elevated ablation lasted ~5 k.y., whereas ablation remained elevated for ~12 k.y. during TII, suggesting a reduced southern GIS during TII that contributed a significant fraction of the higher sea level during the subsequent interglacial.