The 1300-year-long Younger Dryas cold reversal (12,900–11,600 cal BP) is seen most clearly in Greenland ice cores. Contemporaneous changes of climate and environment in more southerly regions were variably expressed. The precise timing of the abrupt Younger Dryas onset is still uncertain. It was linked to a sudden increase in ¹⁴C manifest as a “cliff” where ¹⁴C dates drop from 11,000 to 10,600 radiocarbon years BP within a century of real time, followed by a long plateau. Changes in ocean and/or atmospheric circulation or solar radiation, and even a comet impact have been proposed as triggers, but the abruptness and severity of YD onset still elude explanation.