The Younger Dryas—an intrinsic feature of late Pleistocene climate change at millennial timescales

A box model of the North Atlantic Ocean exhibits self-sustained oscillations of the large-scale ocean circulation, which are reminiscent of Dansgaard–Oeschger (DO)-style oscillations. The freshwater forcing of this ocean model depends on mean climate state, represented by global ice volume. This is computed by a one-dimensional ice-sheet model, subject to changes in high-latitude northern hemisphere summer insolation. At low/large ice volume, the ocean–ice system stays in a permanent interstadial/stadial mode. Millennial scale DO-style oscillations result at intermediate ice-volume values, which thus define a “DO window”. During an interglacial-to-glacial transition, this DO window is crossed sufficiently slow to allow for sustained DO-style oscillations to develop. In contrast, during a glacial termination, the system moves relatively rapid through the DO window, resulting in an intermittent re-appearance of DO-style oscillations, which resemble the sequence of events surrounding the Younger Dryas (YD). When forced with modeled ice-volume evolution over the last 800 thousand years, our model predicts Younger-Dryas-type cooling events for each major glacial termination. Accordingly, the Younger Dryas does not appear to be a one-time event.