North Atlantic late Pleistocene climate (60,000 to 11,650?years ago) was characterized by abrupt and extreme millennial duration oscillations known as Dansgaard-Oeschger (D-O) events. However, during the Last Glacial Maximum (LGM) 23,000 to 19,000?cal years ago (23 to 19?ka), no D-O events are observed in the Greenland ice cores. Our new analysis of the Greenland ?18O record reveals a switch in the stability of the climate system around 30?ka, suggesting that a critical threshold was passed. Climate system modeling suggests that low axial obliquity at this time caused vastly expanded sea ice in the Labrador Sea, shifting Northern Hemisphere westerly winds south and reducing the strength of meridional overturning circulation. The results suggest that these feedbacks tipped the climate system into full glacial conditions, leading to maximum continental ice growth during the LGM.